N T 6.13 - Planning Inspectorate... · Engineering, 2014) Figure 4-6 Particle size distribution...

6.13.3

Appendix 13.3: Hydrogeological Impact Assessment

River Humber Gas Pipeline Replacement Project

Under Regulation 5(2)(a) of the Infrastructure Planning (Applications: Prescribed Forms and Procedure) Regulations 2009

D O

C U

M E

N T

Application Reference: EN060004

April 2015

Environmental Statement Volume 6 Environmental Statement Document 6.13.3

The River Humber Gas Pipeline Replacement Project Page i

CONTENTS

Tables ....................................................................................... iv Figures ...................................................................................... v DCO Documents Referenced .................................................... ix Abbreviations ............................................................................ x Glossary .................................................................................... x EXECUTIVE SUMMARY .......................................................... xii 1 Introduction ....................................................................... 1

1.1 Scheme Description ................................................. 1 1.2 Scope of Works ........................................................ 2

2 The HIA Methodology ....................................................... 3 2.1 Introduction .............................................................. 3

3 Step 1: Establish the regional water resource status ........ 5 3.1 Catchment Abstraction Management Strategy

(CAMS) status .......................................................... 5 3.2 Water Framework Directive status ........................... 5 3.3 Step 1 Outcome ....................................................... 6

4 Step 2: Develop a conceptual model for the abstraction and the surrounding area .................................................. 7 4.1 Regional conceptual model ...................................... 7 4.2 Site conceptual model ............................................ 10 4.3 Physical configuration of excavations ..................... 47 4.4 Reinstatement of the pits ........................................ 52 4.5 Groundwater Management ..................................... 53 4.6 Summary ................................................................ 54 4.7 Assessment of impacts from the tunnel .................. 60 4.8 Numerical model setup ........................................... 62

5 Step 3: Identify all potential water features that are susceptible to flow impacts ............................................. 65 5.1 Drive Pit .................................................................. 65 5.2 Reception pit .......................................................... 66


The River Humber Gas Pipeline Replacement Project Page ii

6 Step 4: Apportion the likely flow impacts to the water features ........................................................................... 69 6.1 Groundwater abstraction flow rates ........................ 69 6.2 Baseflow impacts ................................................... 69

7 Step 5: Allow for the mitigating effects of any discharges, to arrive at net flow impacts ......................... 71

8 Step 6: Assess the significance of the net flow impacts .. 72 8.1 Drive Pit .................................................................. 72 8.2 Reception Pit .......................................................... 73

9 Step 7: Define the search area for drawdown impacts ... 75 9.1 Drive pit .................................................................. 75 9.2 Reception pit .......................................................... 76

10 Step 8: Identify all features in the search area that could be impacted by drawdown .............................................. 78 10.1 Introduction ............................................................ 78 10.2 Water features ........................................................ 78

11 Step 9: For all these features, predict the likely drawdown impacts .......................................................... 82

12 Step 10: Allow for the effects of measures taken to mitigate the drawdown impacts ....................................... 88

13 Step 11: Assess the significance of the net drawdown impacts ........................................................................... 93 13.1 Introduction ............................................................ 93 13.2 Derogation of existing abstractors .......................... 93 13.3 Environmental impacts on water bodies and

wetlands ................................................................. 95 13.4 Settlement .............................................................. 97

14 Step 12: Assess the water quality impacts ................... 101 14.1 Discharge of abstracted groundwater ................... 101 14.2 Point sources ........................................................ 103 14.3 Diffuse pollution .................................................... 105 14.4 Dilution of poor quality surface water being

adversely affected ................................................ 106 14.5 Saline intrusion ..................................................... 107

15 Step 13: If necessary, redesign the mitigation measures to minimise the impacts ................................................ 110

16 Step 14: Develop a monitoring strategy ........................ 111 16.1 Baseline monitoring .............................................. 111 16.2 Construction phase monitoring ............................. 111 16.3 Post-construction monitoring ................................ 111

17 Overall Summary .......................................................... 112 18 Bibliography .................................................................. 118 19 2D GROUNDWATER MODELLING ............................. 120

19.1 Introduction .......................................................... 120 19.2 Approach to 2D numerical modelling .................... 122 19.3 Scope and purpose .............................................. 122


The River Humber Gas Pipeline Replacement Project Page iii

19.4 Model setup .......................................................... 122 19.5 Distribution and properties of hydrogeological units124 19.6 Model boundaries ................................................. 124 19.7 Model calibration .................................................. 126 19.8 Results of numerical modelling............................. 127

20 Response to EA Comments.......................................... 132 20.1 HIA methodology .................................................. 132


The River Humber Gas Pipeline Replacement Project Page iv

Tables

Table Title

Table 3-1 Current status of Water Framework Directive parameters

Table 4-2 Lithological descriptions of layers immediately above the Chalk

Table 4-3 Estimates of hydraulic properties

Table 4-4 Estimates of Storage Coefficient

Table 4-5 Estimates of Coefficient of Compressibility (Mv)

Table 4-6 Summary of groundwater level monitoring

Table 4-7 Boreholes sampled for groundwater chemistry (Soil Engineering, 2014)

Table 4-8 National grid references and elevations relative to ordnance datum for selected points along the tunnel based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

Table 4-9 Adopted hydraulic parameters used in the 2D model

Table 6-10 Average groundwater abstraction flow rates for various scenarios

Table 6-11 Modelled baseflow response to dewatering


Table 8-13 Summary of the Water Framework Directive assessment for East Halton Beck

Table 8-14 Summary of the Water Framework Directive assessment for Thorngumbald Drain

Table 10-15 Water features around the drive pit

Table 10-16 Water features around the reception pit

Table 11-17 Summary of drawdown effects around the drive pit

Table 11-18 Summary of drawdown effects around the reception pit

Table 12-19 Summary of mitigated drawdown effects around the drive pit

Table 12-20 Summary of mitigated drawdown effects around the reception pit

Table 13-21 Private water supplies and other potential abstractors around the drive pit

Table 13-22 Private water supplies and other potential abstractors around the reception pit


The River Humber Gas Pipeline Replacement Project Page v

Table Title



Table 13-25 Summary of settlement estimates

Table 14-26 Summary of groundwater chemical analyses

Table 19-27 Adopted hydraulic parameters used in 2D model

Table 19-28 Model Setup – Boundary Conditions

Table 19-29 Modelled drawdown - Drive Pit

Table 19-30 Modelled Drawdown - Reception PIt

Table 19-31 Average Dewatering flow rate (alternative scenarios)

Figures

Figure Title

Figure 1-1 Tunnel long section showing vertical alignment based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

Figure 4-2

Schematic hydrogeological cross-section illustrating regional conceptual model (adapted from Entec, 2011)

Figure 4-3 Location of ground investigation boreholes (Capita 2014b)

Figure 4-4 Geological cross section along the tunnel route (Capita 2014b)

Figure 4-5 Summary borehole log descriptions around the drive pit (adapted from Capita 2014a; Soil Engineering, 2014)

Figure 4-6 Particle size distribution plot for superficial sediments in boreholes L01, L02, L03 and L08 (Soil Engineering, 2014)

Figure 4-7 Particle size distribution plot for superficial sediments in boreholes M12, M13, M14, M19 and M20 (Soil Engineering, 2014)

Figure 4-8 Summary borehole log descriptions around the reception pit (adapted from Capita 2014a; Soil Engineering, 2014)

Figure 4-9 Particle size distribution plot for borehole L15 from 11.5 m to 33.5 m bgl (Soil Engineering, 2014)


The River Humber Gas Pipeline Replacement Project Page vi

Figure Title

Figure 4-11 Summary borehole log descriptions for tidal flat deposits (adapted from Capita 2014a; Soil Engineering, 2014)

Figure 4-12 Particle size distribution plot for tidal flat deposits from boreholes L04, L05 and L06 (Soil Engineering, 2014)

Figure 4-13 Summary borehole log descriptions for borehole M01 to M04 (adapted from Capita 2014a; Soil Engineering, 2014)


Figure 4-15 Particle size distribution plot for estuarine alluvium from boreholes M01 to M04 (Soil Engineering, 2014)

Figure 4-16

Particle size distribution plot for estuarine alluvium from boreholes M06 to M11 (Soil Engineering, 2014)

Figure 4-17 Chalk groundwater contours on the South Humber Bank (Entec, 2011)


Figure 4-19 Chalk groundwater hydrograph at East Halton (data provided by EA)

Figure 4-20 Chalk groundwater hydrograph at Saltend (data provided by EA)

Figure 4-21 Groundwater levels near the drive pit

Figure 4-22 Tidal variations in groundwater levels near the drive pit

Figure 4-23 Groundwater levels near the reception pit

Figure 4-24 Tidal variations in groundwater levels near the reception pit

Figure 4-25 Chalk groundwater chloride concentration on the North Humber Bank (Gale and Rutter, 2006)


Figure 4-27 Piper diagram of groundwater in superficial deposits within boreholes near the drive pit



The River Humber Gas Pipeline Replacement Project Page vii

Figure Title

Figure 4-29 Average chloride concentrations (mg/l) in boreholes near the drive pit (adapted from Capita 2014a, 2014b)

Figure 4-30 Piper diagram of chalk groundwater in boreholes near the reception pit

Figure 4-31 Piper diagram of groundwater in superficial deposits within boreholes near the reception pit

Figure 4-32 Average chloride concentrations (mg/l) in boreholes near the reception pit (adapted from Capita 2014a, 2014b)

Figure 4-33 Spot flows in a wet and dry period (taken from Entec 2011)

Figure 4-34 Survey across drainage ditch near the drive pit (taken from Capita Drawing No. H160/BH/02/01/F9/102. Rev A)

Figure 4-35

Survey across the Thorngumbald Drain and a drainage ditch near the reception pit (taken from Capita Drawing No. H160/BH/02/01/F9/101. Rev A)

Figure 4-36 Proposed location of the drive pit

Figure 4-37 Drive pit long section showing key features based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

Figure 4-38

Tunnel long section showing vertical alignment based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

Figure 4-39 Long section of the reception pit showing key features


Figure 4-41 Hydrogeological long section along the tunnel route

Figure 4-42 Hydrogeological cross section through the drive pit

Figure 4-43 Hydrogeological cross section through the reception pit

Figure 5-44 Water features around the drive pit highlighting East Halton Beck



The River Humber Gas Pipeline Replacement Project Page viii

Figure Title

Figure 5-46 Groundwater velocity vectors during dewatering of the reception pit with the proposed groundwater control

Figure 4-47 Water features around the reception pit highlighting Thorngumbald drain

Figure 9-48 Modelled drawdown and radius of influence at Drive pit

Figure 9-49 Drawdown response and indication of radius of influence at Reception pit (no engineering controls)

Figure 9-50 Modelled drawdown and radius of influence at Reception pit

Figure 10-51 Water features around the drive pit

Figure 10-52 Water features around the reception pit

Figure 11-53 Effect of dewatering on land drains – Drive Pit

Figure 11-54 Effect of dewatering on land drains – Reception Pit

Figure 11-55 Groundwater mounding response (up-hydraulic gradient) at drive pit

Figure 11-56 Groundwater mounding response (up-hydraulic gradient) at reception pit

Figure 12-57 Modelled drawdown with uniform lower permeability glacial deposits around the reception pit



Figure 14-60 Landfill sites near the reception pit

Figure 14-61 NVZs near the drive pit and reception pit


Figure 14-63 Effect of dewatering on saline interface (particle tracking) – no groundwater control

Figure 19-64 Line of drive pit cross section model

Figure 19-65 Line of reception pit cross section model

Figure 19-66 General setup of 2D models, long section (top), drive pit cross section (middle), reception pit cross section (bottom)

Figure 19-67 Calibration of groundwater levels


The River Humber Gas Pipeline Replacement Project Page ix

DCO Documents Referenced

DCO Document Reference

Title of Document

6.8 Chapter 8: Geology and Soils

7.3 Initial Construction Environmental Management Plan


The River Humber Gas Pipeline Replacement Project Page x

Abbreviations

BGS British Geological Survey

CAMS Catchment Abstraction Management Strategy

COD Chemical Oxygen Demand

EA Environment Agency

EPBM Earth Pressure Balance

EQS Environmental Quality Standards

HIA Hydrogeological Impact Assessment

IDB Internal Drainage Board

NVZ Nitrate Vulnerable Zone

OD Ordnance datum

PSD Particle size distribution

SAC Special Area of Conservation

SSSI Site of Special Scientific Interest

SPA Special Protection Area

VSM Vertical Shaft Sinking Machine

Glossary

Term Description

Abstraction Removal of water from surface water or groundwater, usually by pumping.

Aquifer A body of permeable rock that is capable of storing significant quantities of water; is underlain by impermeable material, and through which groundwater moves.

Auger A tool, resembling a large corkscrew, for boring holes (e.g. in earth or wood).

Bathymetry The study of underwater depth of lakes, rivers or ocean floors.

Caisson A watertight structure within which construction work is carried out below groundwater levels.

Construction Environmental Management Plan

A site specific plan developed to ensure that appropriate environmental management practices are followed during the construction phase of a project.


The River Humber Gas Pipeline Replacement Project Page xi

Term Description

Dewatering The removal of groundwater/surface water to lower the water table or to empty an area, such as an excavation, of water.

Groundwater Defined by the European Commission groundwater Directive (80/68/EEC) as "all water which is below the surface of the ground in the saturation zone and in direct contact with the ground or subsoil".

Hydrogeology The branch of geology that deals with water below the ground surface.

Hydrostatic Test

A way in which pressure such as pipelines can be tested for strength and leaks. The test involves filling the vessel or pipe system with a liquid, usually water to a pressure exceeding that seen during operation.

Outfall End of a temporary or permanent pipe from which water (or other effluent) is discharged. Can refer either to the end of a length of pipe or to a dedicated structure.

Pillow tank A tank used to store liquids which when full takes the shape of a large pillow.

Silt The generic term for particles with a grain size of 4-63mm, i.e. between clay and sand.

Source Protection Zone

Designated protection area around drinking water supplies.

Surface Water Water that appears on the land surface that has not seeped into the ground, i.e. lakes, rivers, streams, standing water, ponds, precipitation.

Tunnel Boring Machine

A machine used to excavate tunnels through varying types of strata.


River Humber Gas Pipeline Replacement Project Page xii

EXECUTIVE SUMMARY National Grid Gas is proposing to construct and operate a replacement high pressure transmission gas pipeline beneath the River Humber from Goxhill to Paull (the River Humber Gas Pipeline Replacement Project). This document describes the findings of a hydrogeological impact assessment following relevant Environment Agency guidance.

The drive pit includes a 13 m deep section which would penetrate through low permeability glacial deposits into the Chalk aquifer. Groundwater levels in the glacial deposits and Chalk are within 2 m of ground level so groundwater control would be required during excavation.

Within the drive pit and reception pit, groundwater control is likely to be achieved by combining four approaches; cut-off walls (secant and sheet piling), deep well dewatering, sump pumping and passive relief wells within the base of the pit.

Prior to excavation of the pits, piles would be installed around the perimeter and to a depth designed to minimise groundwater seepage into the pits and then deep wells installed.

The proposed groundwater control for the deep section of the drive pit includes installation of secant piles to a depth of approximately 28 m. It would take an estimated 41 days to complete the excavation and cast a base slab to effectively seal the pit, after which the dewatering would be stopped and groundwater levels outside the pit would be allowed to recover.

The relatively low pumping rate (70 m3/d) predicted for the drive pit dewatering and the short duration (41 days) is considered to be negligible in comparison to the 77,500 m3/d abstracted from this catchment, mostly for public water supply and industrial use. The drive pit dewatering would also take place in March and April when water resources are less stressed after winter recharge. However, water resources on the Goxhill side are in deficit and abstraction is generally not permitted so this would need to be discussed with the Environment Agency following its review of this report.

A groundwater model has been developed that predicts groundwater level would be lowered by 3.1 m at the closest abstraction borehole, Fir Tree Farm located 300 m north of the drive pit. Impacts would lessen with distance away from the pit with drawdown of 0.5 m at the nearest private water supplies, 2 km away. Given the depths of boreholes, where known, it is unlikely this would interrupt use of these water supplies. However, this would need to be confirmed by a detailed water features survey as part of further investigation prior to detailed design.

Dewatering of the drive pit is predicted to cause an order of magnitude reduction of baseflow to East Halton Beck. Although the current rate and significance of baseflow is not well understood, and the duration of impact would be short, the impact may affect the Environment Agency’s objective of achieving good ecological potential by 2027.

The model indicates that impacts can be significantly reduced by using deeper piles that effectively seal out the Chalk aquifer. The depth of piles required to reduce impacts is estimated to be 50 m but this is highly dependent on the vertical hydraulic conductivity variations in the fractured chalk and glacial deposits, and the thickness of the fractured chalk. The model is conservative (impacts are likely


River Humber Gas Pipeline Replacement Project Page xiii

to be less than predicted) and better characterisation of these ground conditions would enable a revised impact assessment and inform detailed design.

Given the short duration of impacts, additional mitigation measures such as providing alternative water supplies and stream support may be considered, although the latter would require consent from the Environment Agency supported by additional monitoring and assessment.

The reception pit would be constructed entirely within glacial deposits and the extent of impacts around the reception pit is largely controlled by a zone of higher permeability (sand and gravel) material that has not yet been fully characterised in the field. Again, if required, sealing out these deposits by piling to a depth of approx. 36 m could be used to significantly reduce impacts.

On both the Goxhill and Paull sites groundwater is at risk of failing to meet Water Framework Directive objectives, largely due to saline intrusion into the strategically important Chalk aquifer. The model indicates that the proposed groundwater control for the drive pit would not significantly increase the risk of saline intrusion. The risk of saline intrusion would be temporarily increased at the reception pit with the proposed pile depths. Again, piling to a depth of approx. 36 m could be used to significantly reduce impacts.

Analysis has been undertaken to estimate the effects of dewatering on ground surface settlements. At the flood levees on both sides of the estuary, the range of groundwater levels caused by tidal action within these areas exceeds the levels of dewatering required for the works. The stress changes that would occur due to dewatering are therefore within the ranges already 'experienced' by the ground and would therefore have negligible effect on ground movement.

There is a risk of significant settlement associated with groundwater control required for excavation of the reception pit. This settlement analysis is based on the calculation of drawdown (lowering of groundwater level) from the hydrogeological model. There is uncertainty about the hydrogeology around the reception pit, namely the thickness and extent of a body of permeable sand and gravel, and whether this is hydraulically connected to adjacent interbedded sand, silt and clay layers that are prone to settlement. The hydrogeological model is based on a conservative interpretation of the available data, i.e. actual impacts are likely to be less than predicted by the model.

A subsequent investigation aims to reduce uncertainty by conducting a pumping test. It is also recommended that the Phase 2 ground investigation include a provision for strategic settlement monitoring before, during and after any pumping testing undertaken, to provide an indication of the actual response to dewatering effects.

Should the results from the Phase 2 investigation not reduce uncertainty to an acceptable level and/or the revised hydrogeological impact assessment still predicts significant settlement would occur around the reception pit, there are a number of engineering options that could be used to mitigate settlement impacts. These include recharge wells and/or trenches, ground treatment at the base of the reception pit prior to excavation e.g. ground freezing, and excavation by Vertical Shaft Sinking Machines, which are a relatively new technology that can be used to create a shaft without the need for lowering the groundwater table.


The River Humber Gas Pipeline Replacement Project Page 1

1 Introduction

1.1 Scheme Description

1.1.1 National Grid Gas (the Applicant) is proposing to construct and operate a replacement high pressure transmission gas pipeline beneath the River Humber from Goxhill to Paull - the River Humber Gas Pipeline Replacement Project (Hereafter referred to as the ‘Scheme’).

1.1.2 The preferred solution is a minimum 3.65 m internal diameter tunnel driven from Goxhill on the south Humber bank to Paull on the north bank. The tunnel drive will commence from a drive pit near Goxhill with dimensions of approximately 200 m x 10 m x 13 m deep. The reception pit would be rectangular with approximate dimensions of 50 m long, 10 m wide and 15 m deep. The pit walls would be supported by 1.2 m diameter secant piles around the perimeter of the pit. The base slab would be approximately 3 m thick.

1.1.3 In plan view, the proposed tunnel route is a straight line directly between the drive pit and the reception pit. The proposed vertical alignment of the tunnel is plotted on the interpreted geological section in Figure 1-1.


1.1.4 The drive face is situated at the boundary between the superficial deposits and the Chalk. The tunnel drops at a gradient of 2.5% through the Burnham Chalk Formation, followed by a shallower section (1% gradient) passing through the Burnham Chalk into the Flamborough Chalk Formation. The tunnel then begins to rise, initially at a shallow gradient of 0.25% and then steepens to a 1.67% gradient passing up into the superficial deposits to meet the reception pit. The reception pit would be constructed solely within the superficial glacial deposits and would not penetrate into the Chalk.

1.1.5 The tunnel would be constructed using a tunnel boring machine (TBM). For the geology likely to be encountered, the choice of TBM is limited to an Earth

Tunnel

Drive pit

Reception pit



Pressure Balance Machine (EPBM) or a Slurry TBM. Both TBMs include methods of limiting groundwater inflow during construction and the tunnel would be lined with reinforced precast concrete segments as tunnelling progresses.

1.1.6 The activities that could affect groundwater during the construction and operation of the Scheme are as follows:

Groundwater control during construction of the Goxhill drive pit;

Groundwater control during construction of the Paull reception pit;

Groundwater control during tunnel construction;

Discharge of groundwater abstracted during all groundwater control operations;

Drive pit and reception pit during operational phase; potential groundwater mounding behind piled walls; and

Tunnel during operational phase.

1.2 Scope of Works

1.2.1 This document describes the findings of a Hydrogeological Impact Assessment (HIA) following the Environment Agency (EA) guidance document titled Hydrogeological impact appraisal for dewatering abstractions. Science Report – SC040020/SR1 (EA, 2007).

1.2.2 The scope of works was defined in the method statement (Reference No. 0019-UA006029-UP31R-02-HIA-MethodStatement). This is reproduced below:

Collate relevant information including ground investigation data, EA information, British Geological Survey maps and records;

Liaise with the EA and Anglian Water. Review the need for a site visit and/or meeting with the EA during the Scheme;

Develop a detailed conceptual understanding of the hydrogeology (aquifer hydraulic properties, groundwater flow regime);

Identify water features at risk of being impacted and display these on a ‘water features map’;

Build a numerical model to simulate groundwater flow and the dewatering and tunnelling activities;

Assess the impacts described above from the dewatering and tunnelling activities;

Assess mitigation options (if required) including changes to pile lengths and provide a groundwater monitoring strategy to verify impacts (or lack of) during the Scheme construction; and

Deliver a technical report on the findings of the above that may be used to inform the Environmental Statement.



2 The HIA Methodology

2.1 Introduction

2.1.1 Assessment of significance of impacts has been guided by the HIA methodology (EA, 2007). The HIA methodology can be summarised in terms of the following 14 steps:

Step 1: Establish the regional water resource status;

Step 2: Develop a conceptual model for the abstraction and the surrounding area;

Step 3: Identify all potential water features that are susceptible to flow impacts;

Step 4: Apportion the likely flow impacts to the water features;

Step 5: Allow for the mitigating effects of any discharges, to arrive at net flow impacts;

Step 6: Assess the significance of the net flow impacts;

Step 7: Define the search area for drawdown impacts;

Step 8: Identify all features in the search area that could be impacted by drawdown;

Step 9: For all these features, predict the likely drawdown impacts;

Step 10: Allow for the effects of measures taken to mitigate the drawdown impacts;

Step 11: Assess the significance of the net drawdown impacts;

Step 12: Assess the water quality impacts;

Step 13: If necessary, redesign the mitigation measures to minimise the impacts; and

Step 14: Develop a monitoring strategy.

2.1.2 The steps are not intended to be prescriptive, and the level of effort expended on each step can be matched to the situation. The methodology depends heavily on the development of a good conceptual model of the dewatering operation and the surrounding aquifer. The steps of the methodology are followed iteratively, within a structure with three tiers, and the procedure continues until the required level of confidence is achieved.

2.1.3 Tier 1 (Basic) uses average groundwater levels and simple analytical equations to assess impacts. Tier 2 (Intermediate) uses more detailed data such as seasonal changes in groundwater level and simple numerical models. Tier 3 (Detailed) includes time-variant and spatially-distributed numerical groundwater models to assess impacts.

2.1.4 It is considered that a Tier 2 risk assessment is required because the potential impacts would have significant consequences e.g. settlement of flood levels would increase the risk of flooding, saline intrusion and/or mobilisation of



contamination could render public water supply boreholes unusable for several years, and impacts on watercourses (reduced river flows) could compromise the EA’s objectives of the Water Framework Directive.

2.1.5 In accordance with comments received from the EA to National Grid Gas and previous discussions with the EA, the study area would be a 5 km buffer around the tunnel route.



3 Step 1: Establish the regional water resource status

3.1 Catchment Abstraction Management Strategy (CAMS) status

3.1.1 The starting point for the HIA is to establish the CAMS status for the area in which the dewatering operation is located.

3.1.2 The south Humber bank is covered by the Grimsby, Ancholme and Louth CAMS (EA, 2013a). The Scheme lies in Area B Barrow Beck and Skitter Beck (East Halton Beck). The CAMS status is no water available for abstraction except at extremely high flows. Groundwater resources in the Lincolnshire Chalk are fully committed to existing users and the environment.

3.1.3 The north Humber bank is covered by Hull and East Riding CAMS (EA, 2013b). The resource status is ‘water available’ at very low flows (Q95) and higher flows i.e. consumptive abstraction is available at least 95% of the time. However, conditions may also be written into licences to ensure that abstraction stops if groundwater levels in the Chalk aquifer fall too low. This would help to prevent saline water being drawn into the aquifer and reducing the quality of the groundwater.

3.2 Water Framework Directive status

3.2.1 The study area lies across two groundwater waterbodies delineated by the Water Framework Directive. These are the Grimsby Ancholme Louth Chalk on the south Humber bank and the Hull and East Riding Chalk on the north Humber bank. Table 3-1 lists the current status of Water Framework Directive parameters for these waterbodies.



Water Framework Directive Parameter

Grimsby Ancholme Louth Chalk

Hull & East Riding Chalk

Current quantitative status

Poor Poor

Current chemical status Poor Poor

Groundwater dependent terrestrial ecosystems (quantitative impacts)

Good Good

Impact on surface waters Good Good

Saline or other intrusions Good Poor

Water balance Poor Good

Overall risk of failing to meet Water Framework Directive objectives

At risk At risk

Table 3-1 Current status of Water Framework Directive parameters

3.2.2 Both waterbodies have been determined to be at risk of failing to meet Water Framework Directive objectives of having a good quantitative status and good chemical status by 2015. In the case of the Grimsby Ancholme Louth Chalk this is due to a poor resource status (as reflected by the CAMS status described above). The Hull and East Riding Chalk is impacted by saline intrusion, and therefore is unlikely to achieve good chemical status by 2015.

3.3 Step 1 Outcome

3.3.1 The water resource (CAMS) status for the Grimsby, Ancholme and Louth Chalk is ‘no water available’ except at very high flows. The Hull and East Riding Chalk has water resources available most of the time (Q95 flows or higher) but groundwater quality is impacted by saline intrusion and abstractions may need to be reduced or stopped at times of low groundwater levels.

3.3.2 Both waterbodies are considered to be at risk of failing to meet Water Framework Directive objectives to be at good quantitative status and good chemical status by 2015. Therefore, in accordance with Table 4.1 of the HIA methodology, the assessment would need to demonstrate that that the proposed dewatering abstractions would not exacerbate the regional water resources problem.



4 Step 2: Develop a conceptual model for the abstraction and the surrounding area

4.1 Regional conceptual model

4.1.1 A number of sources of information have been reviewed to gain an understanding of the regional hydrogeology including hydrogeological maps (Institute of Geological Sciences, 1967; 1980) and reports (Smedley et al., 2004; Gale and Rutter, 2006; Whitehead and Lawrence 2006).

4.1.2 In addition, recent conceptual and numerical groundwater modelling of groundwater and surface water resources provides an understanding of the hydrogeology on the south Humber bank (Entec, 2011) and north Humber bank (ESI Ltd, 2010).

4.1.3 The main features of the conceptual model of the south Humber bank are:

Groundwater recharge to the Chalk aquifer occurs on the Lincolnshire Wolds and there is almost no runoff;

Groundwater flows down dip, in a northeast direction towards the Humber Estuary;

Springs discharge groundwater from the Chalk to small streams (Barrow Beck, East Halton / Skitter Beck) at the boundary with overlying glacial deposits. Flow from these springs is much reduced during drought periods. The amount of water discharged at these springs is small compared with the quantity of water that flows down dip through the confined chalk;

The most active flow in the confined Chalk occurs in the top 40 m of the formation beneath the glacial deposits. The term “Chalk bearings” is commonly applied to the deposits of broken chalk and chalk gravel that are found between the competent chalk and overlying drift deposits. The chalk bearings can be up to 12 m thick but are more typically 2-3 m thick. This layer has a high porosity and transmissivity. The PWS borehole at Goxhill indicated a transmissivity of 3500 m2/day;

Beneath the active Chalk aquifer permeability decreases significantly;

The glacial till has variable thickness. It is thinnest along East Halton Beck. During wet years groundwater in the Chalk aquifer may seep upwards into East Halton Beck in places where low permeability glacial till deposits are relatively thin and more permeable sand and gravel deposits occur;

Glacial deposits and estuary sediments are eroded to expose the Chalk in the Humber Estuary in a narrow strip south of Immingham. Fresh groundwater is able to discharge to the estuary in this area but similarly water from the estuary may flow into the Chalk aquifer if head gradients permit, leading to saline intrusion;

There is a very large rate of groundwater abstraction (77.5 Ml/d) from the Chalk aquifer in the study area (Table 6.2 Entec, 2011). Large springs (e.g. Barrow Blow Wells) and groundwater dependant wetlands e.g. Barton and Barrow Clay Pits, Killinghome Haven Pits Site of Special Scientific Interest



(SSSI), are located down gradient of the groundwater abstraction boreholes;

East Halton Beck and Barrow Beck are the two main rivers within the catchment. Both outfall to the Humber Estuary;

There is a buried glacial valley that has eroded the Chalk aquifer and extends approximately 8 km from the Lincolnshire Wolds at Kirmington to the Humber Estuary at Immingham. This channel is estimated to be 2 km wide and up to 60 m deep at Immingham. Evidence indicates that the buried valley contains low permeability glacial sediments. This buried valley is located approximately 6 km south of the proposed drive pit at Goxhill; and

The surface and groundwater catchment boundary to the south is associated with a geological boundary (Caistor Monocline) and the position of the flow divide does not appear to change significantly with groundwater level.

4.1.4 The main features of the conceptual model of the north Humber bank are:

Groundwater recharge to the Chalk aquifer occurs on the Yorkshire Wolds to the west of the study area;

Regional groundwater flow is towards the east although there is thought to be very little flow across the Holderness Peninsular where the Chalk is confined by relatively thick glacial deposits;

Groundwater water levels remain near ground level with very little variation through time implying little recharge in this region;

Despite the presence of significant thickness of low permeability superficial deposits, it is considered that interaction between surface waters and the chalk aquifer potentially occurs, primarily via two mechanisms:

1 From the presence of artesian heads within the Chalk providing slow upward vertical flow through the confining low permeability glacial deposits; and

2 Where windows of higher permeability glaciofluvial sand and gravel deposits penetrate the full thickness of the till, artesian springs known as ‘blow wells’ may be present.

An extensive network of channels serves to drain the Holderness Plain with water collected in these features being pumped to higher level systems or the River Hull; and

Discharge from the Chalk aquifer to the Humber Estuary is known to have historically occurred via freshwater springs such as Hessle Whelps. Such springs ceased flowing following the onset of heavy abstraction in the Hull area in the early twentieth century. Analysis of the saline intrusion beneath Hull indicates that hydraulic connection is now limited as a consequence of the clogging of pore space in the estuary bed material.

4.1.5 Figure 4-2 is a schematic illustration of the conceptual understanding of regional hydrogeology adapted from Entec (2011).



Figure 4-2 Schematic hydrogeological cross-section illustrating regional conceptual model (adapted from Entec, 2011)



4.2 Site conceptual model

Ground investigation

4.2.1 The ground investigation undertaken during summer – autumn 2014 (Soil Engineering, 2014; Capita, 2014) forms the basis of the geological and hydrogeological model along the tunnel route. A total of 28 boreholes were drilled through the superficial deposits and into the underlying Chalk Formations.

4.2.2 Figure 4-3 shows the location of boreholes constructed as part of the ground investigation undertaken for the Scheme.

Figure 4-3 Location of ground investigation boreholes (Capita 2014b)

4.2.3 Full details of the ground investigation including borehole logs and geotechnical testing are provided in the factual report (Soil Engineering, 2014) and interpretative report (Capita, 2014). In summary the boreholes comprise the following:

Seven boreholes were drilled on the south Humber bank at or near the location of the proposed drive pit; L01, L02, L03, L04, L05, L06 and L08.

Five boreholes were drilled on the north Humber bank at or near the location of the proposed reception pit; L14, L15, L16, L16A, and L18

Sixteen ‘marine’ boreholes drilled on the River Humber; M01, M02, M03, M04, M05, M06, M07, M08, M09, M10, M11, M12, M13, M14, M19 and M20.

Tunnel route

Drive pit Reception pit



4.2.4 Details and interpretation pertinent to the hydrogeological impact assessment are discussed below.

Superficial deposits

4.2.5 Capita (2014) divided the superficial deposits into two facies, namely glacial deposits and marine and estuarine alluvium. Figure 4-4 shows the distribution of superficial deposits along the tunnel route.

Figure 4-4 Geological cross section along the tunnel route (Capita 2014b)

4.2.6 Superficial deposits should be divided into four hydrogeologically distinct units on the basis of lithological descriptions from the borehole logs and particle size distribution (PSD) data from geotechnical testing. These deposits are:

Glacial till;

Glacial sand and gravel;

Tidal flat deposits; and

Estuarine alluvium.

4.2.7 The distribution of these deposits is shown in Figure 4-40. In addition, the Chalk bearings are discussed below.

Glacial till

4.2.8 Glacial till is widespread over the Lincolnshire Marsh, the low-lying land between the Lincolnshire Wolds and the Humber Estuary. This area of till is characteristically clay-rich and considered to be low permeability, less than 1 x 10-6 m/s, with lenses of more permeable sand and gravel (Entec, 2011).

4.2.9 At and around the drive pit site, boreholes L01, L02, L03 and L08 contain around 9 m thickness of sandy gravelly clay (Figure 4-5). PSD data indicates these are



poorly sorted sediments with a mix of grain sizes, but predominantly clay and silt (Figure 4-6).

Figure 4-5 Summary borehole log descriptions around the drive pit (adapted from Capita 2014a; Soil Engineering, 2014)

Figure 4-6 Particle size distribution plot for superficial sediments in boreholes L01, L02, L03 and L08 (Soil Engineering, 2014)

4.2.10 Marine boreholes M06 to M11 located under the central part of the River Humber indicate a wedge of glacial till thickening to the northeast and overlain by alluvium. Further to the northeast the alluvium thins to less than 1 m and the glacial till reaches a thickness of 15 m in borehole M20. Lithological descriptions of the glacial till in boreholes M06 to M20 are similar to the glacial deposits at the drive pit i.e. silty clay and sandy gravelly clay. PSD data indicates these are poorly sorted sediments with a mix of grain sizes, but predominantly clay and silt (Figure 4-7).

200 m



Figure 4-7 Particle size distribution plot for superficial sediments in boreholes M12, M13, M14, M19 and M20 (Soil Engineering, 2014)

4.2.11 Rising head permeability tests undertaken within glacial deposits from the ground investigation gave a minimum of 6.8 x 10-8 m/s, maximum of 3.3 x 10-5 m/s and geometric mean of 7.7 x 10-6 m/s.

4.2.12 The range of permeability values calculated from PSD data using Kozeny Carman formula is between a minimum of 1.7 x 10-9 m/s and maximum of 5.0 x 10-5 m/s with a geometric mean of 2.7 x 10-7 m/s.

Glacial sand and gravel

4.2.13 Close to the reception pit the borehole logs indicate an increase in the proportion of sand and gravel (Figure 4-8). Note that this drawing contains reference to reception shaft which is currently not the preferred option.

Figure 4-8 Summary borehole log descriptions around the reception pit (adapted from Capita 2014a; Soil Engineering, 2014)

200 m



4.2.14 Lithological descriptions in borehole L18 include several layers of sand, gravelly sand and sand & gravel. Each layer is around 3 m to 5 m thick alternating with silty clay and sandy clay. In borehole L15, closest to the reception pit, there is approximately 24 m of gravelly sand lying directly above the Chalk.

4.2.15 The evidence for distinguishing glacial sand and gravel as a separate hydrogeological unit beneath the reception pit is supported by PSD data (Figure 4-9). Samples from 11.5 m bgl to 33.5 m bgl in borehole L15 are predominantly composed of fine to coarse sand with a notable proportion of gravel in some samples.

Figure 4-9 Particle size distribution plot for borehole L15 from 11.5 m to 33.5 m bgl (Soil Engineering, 2014)

4.2.16 The range of permeability values calculated from PSD data using Kozeny Carman formula from L15 and L18 is between a minimum of 3.9 x 10-7 m/s and maximum of 4.0 x 10-2 m/s with a geometric mean of 1.0 x 10-4 m/s.

Tidal flat deposits

4.2.17 Tidal flat deposits are characterised by the occurrence of peat amongst layers of clay and silt. Regionally, these deposits are considered to be slightly more permeable than the underlying glacial till (Entec, 2011).

4.2.18 Tidal flat deposits are found to the northeast of the drive pit in boreholes L04, L05 and L06. Sediments from these boreholes are generally described as clay, clayey silt, silty clay and peat (Figure 4-10). A thin wedge of tidal flat deposits also occurs in shallow ground on the northeast bank of the River Humber. Sandy clay, clay



and sandy silt are described in boreholes L14, L16 and L16A, all within 5 m of ground level (Figure 4-8).

Figure 4-10 Summary borehole log descriptions for tidal flat deposits (adapted from Capita 2014a; Soil Engineering, 2014)

4.2.19 PSD data from tidal flat deposits within boreholes L04, L05 and L06 can broadly be divided into two groups of sediments; predominantly clays and silts, and predominantly gravels (Figure 4-11). Gravel is identified in boreholes L04 and to a lesser extent in L05. No gravel was identified in PSD data from borehole L06.

4.2.20 Only two rising head permeability tests were undertaken within Tidal flat deposits from the ground investigation. A test in L04 gave a value of 9.4 x 10-6 m/s and a test in L06 gave a value of 4.0 x 10-6 m/s. The average of these two is 7.7 x 10-6 m/s.

4.2.21 The range of permeability values calculated from PSD data from L04, L05 and L06 using Kozeny Carman formula is between a minimum of 2.1 x 10-9 m/s and maximum of 7.4 x 10-5 m/s with a geometric mean of 1.3 x 10-7 m/s.

4.2.22 Taken together, the data suggests that low permeability layers of clay and silt are interbedded with relatively permeable lenses of gravel.

200 m



Figure 4-11 Particle size distribution plot for tidal flat deposits from boreholes L04, L05 and L06 (Soil Engineering, 2014)

Estuarine alluvium

4.2.23 The alluvial sediments directly beneath the River Humber form a separate hydrogeological unit to the tidal flat deposits. The evidence for distinguishing estuarine alluvium from tidal flat deposits is a predominance of fine to coarse sand, gravelly sands and gravel in lithological descriptions from marine boreholes M01 to M11 (Figure 4-12 and Figure 4-13). This contrasts with predominantly clay, silt and peat in boreholes L04, L05 and L06 on the south Humber bank.


200 m




4.2.24 In addition, PSD data indicates that the estuarine alluvium is characterised by well sorted sand size sediments. Figure 4-14 shows PSD data from boreholes M01 to M04. Figure 4-15 shows PSD data from boreholes M06 to M11 (Figure 4-15). Sediments from borehole M05 are predominantly clay and silt and have been omitted.


200 m




4.2.25 No permeability tests were undertaken in the marine boreholes during the ground investigation.

4.2.26 The range of permeability values calculated from PSD data from M06 to M11 using Kozeny Carman formula is between a minimum of 2.6 x 10-8 m/s and maximum of 2.7 x 10-5 m/s with a geometric mean of 2.4 x 10-6 m/s.

Chalk bearings

4.2.27 The term “Chalk bearings” is commonly applied to the deposits of broken chalk and chalk gravel that are found between the competent chalk and overlying drift deposits. The Chalk bearings can be up to 12 m thick but are more typically 2-3 m thick. This layer is hydrogeologically important because it has a high porosity and transmissivity. Entec (2011) report that a pumping test at the PWS borehole at Goxhill indicated a transmissivity of 3500 m2/day for the Chalk bearings and a transmissivity of 1800 m2/day for the underlying Chalk.

4.2.28 Chalk bearings has not been delineated in the geological cross section produced by Capita (2014a). Table 4-2 lists the thickness and lithological descriptions of layers immediately above the Chalk in each borehole from the ground investigation. Some of the borehole logs from the ground investigation do contain descriptions which considered to indicate the presence of Chalk bearings.

4.2.29 For example the lithological descriptions of borehole L01 includes “Dense light brown mottled white very clayey fine to coarse SAND and subangular fine to coarse GRAVEL of chalk and flint.” This occurs between 8.8 m bgl and 9.25 m bgl, i.e. a thickness of 0.45 m.

4.2.30 However, only L01 and L03 are considered to be unequivocally consistent with the Chalk bearings. Other boreholes such as L04, M03, M08 and L18 suggest Chalk bearings are present but the presence of “mixed (igneous, sandstone)



lithologies” could be considered to represent glacial sediments that have been transported and may not have similar hydraulic properties to the Chalk bearings.

4.2.31 Several other boreholes (e.g. L02, L05, L06, M09) are similar to typical descriptions of putty chalk. For example, the lithological description of the layer above the Chalk in borehole L02 is “Firm yellowish brown slightly gravelly sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk.”

Borehole Thickness (m)

Description

L01 0.45

Dense light brown mottled white very clayey fine to coarse SAND and subangular fine to coarse GRAVEL of chalk and flint

L02 0.70

Firm brown slightly gravelly sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk.

L03 0.70

Loose grey mottled white very clayey sandy subangular to subrounded fine to coarse Gravel of chalk and flint. Sand is fine to coarse.

L04 1.30

Medium dense white, grey and brown slightly sandy angular to subrounded fine to coarse GRAVEL of chalk, flint and mixed lithologies.

L05 1.00

Firm yellowish brown slightly gravely sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk.

L06 1.10

Firm yellowish brown slightly gravelly sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk.

L08 4.70

Very stiff brown slightly sandy slightly gravelly CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of sandstone, chalk, mudstone and flint.

L14 5.50

Stiff dark brown slightly gravelly sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk, flint, sandstone, mudstone and mixed lithologies.

L15 9.00

Dense grey brown gravelly fine to coarse SAND. Gravel is angular to subangular medium to fine of chalk.

L16 Assumed zone of core loss - chalk not reached

L16A 4.00

Stiff thinly to thickly laminated slightly gravelly brown CLAY. Gravel is angular to subrounded fine to medium of chalk, chert and mixed igneous lithologies.




Description

L18 3.00

Medium dense brown fine to coarse SAND and subangular to subrounded fine to coarse GRAVEL of flint, chalk and mixed lithologies with low cobble content. Cobbles are subrounded to rounded of mixed igneous lithologies.

M01 8.40

Medium dense dark grey silty fine to medium SAND. Locally with fine to coarse gravel sized pockets of dark grey silt.

M02 3.90 Loose becoming medium dense grey silty fine to coarse SAND.

M03 1.80

Greyish brown slightly silty sandy angular to subrounded fine to coarse GRAVEL of chalk, flint and mixed lithologies. Sand is fine to coarse.

M04 3.10 Medium dense brown mottled orangish brown silty fine to medium SAND.

M05 0.60 Very soft to soft dark orangish brown mottled dark grey slightly sandy clayey SILT.

M06 2.00

Firm to stiff medium strength slightly sandy slightly gravelly CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of chalk.

M07 2.10

Stiff very high strength slightly sandy slightly gravelly CLAY. Sand is fine. Gravel is angular to rounded fine to medium of chalk rarely mixed igneous lithologies.

M08 2.40

Medium dense yellowish brown fine to coarse SAND and angular to rounded fine to coarse GRAVEL of quartzite, flint, chalk, sandstone and mixed lithologies.

M09 4.30

Stiff very high strength dark brown slightly gravelly sandy CLAY. Sand is fine to coarse. Gravel is subangular to subrounded fine to coarse of sandstone, mudstone, chalk, flint and mixed lithologies.

M10 2.10

Stiff thinly laminated brown slightly sandy slightly gravelly CLAY. Sand is fine to coarse. Gravel is angular to subrounded fine to medium of chalk and chert. (GLACIAL TILL)

M11 2.80

Stiff indistinctly fissured dark greyish brown slightly sandy slightly gravelly CLAY. Gravel is angular to rounded fine to coarse of chalk, chert and sandstone.

M13 0.47 Firm greyish brown slightly sandy gravelly CLAY. Gravel is angular to subrounded fine to medium of




Description

chalk, rarely chert and mixed sedimentary lithologies.

M14 4.00

Firm to stiff dark brown slightly sandy slightly gravelly CLAY. Gravel is subangular to subrounded fine to coarse of chalk and flint.

M19 5.10

Stiff medium strength brown slightly sandy slightly gravelly CLAY. Sand is fine to coarse. Gravel is angular to subangular fine to medium locally coarse of chalk.

M20 0.80

Very dense dark brown mottled greyish white clayey silty sandy angular to subrounded fine to coarse GRAVEL of chalk. ('Putty' Chalk) (FLAMBOROUGH CHALK FORMATION)

Table 4-2 Lithological descriptions of layers immediately above the Chalk

4.2.32 Borehole cores obtained from the ground investigation along the proposed tunnel route have been logged in terms of their geotechnical properties (Capita, 2014b). CIRIA grades have been assigned in accordance with C574 (CIRIA, 2002). Figure 4-4 shows the distribution of CIRIA grades in the Chalk.

4.2.33 Typically, the borehole log descriptions of Chalk corresponding to CIRIA grade DC as shown on Figure 4-4 are ‘Structureless white CHALK. Composed of slightly silty slightly sandy angular to subrounded fine to coarse gravel.’ It could be argued that this indicates Chalk bearings. However, several of these lithological descriptions are based on cuttings from rotary drilling using air mist flush rather than drill cores. The drilling technique used outside of the cored sections would have broken up the solid chalk and the lithological descriptions of Chalk outside of the cores cannot be relied on for hydrogeological interpretation.

4.2.34 In summary there is uncertainty regarding the appropriateness of the drilling methodology, and difficulty in distinguishing Chalk bearings and putty chalk (predominantly gravel with some clay versus predominantly clay with some gravel). In view of these difficulties the most appropriate conceptual model to adopt is one that is consistent with regional understanding and conservative in terms of potential impacts i.e. a layer of Chalk bearings that is uniformly 4 m thick and with uniform hydraulic properties (high permeability and high storage coefficient).

Chalk

4.2.35 The area is underlain by the Chalk Group, comprising the Burnham Chalk Formation and the overlying Flamborough Chalk Formation. The Burnham Chalk Formation is regionally characterised by hard, thinly bedded chalk with frequent tabular flints and discontinuous flint bands. The Flamborough Chalk Formation is typically softer with frequent marl bands and negligible flint (Whitehead and Lawrence, 2006). In general, softer chalk tends to have a lower hydraulic conductivity than harder chalk. This is because fractures through which



groundwater flows tend to have smaller aperture in soft chalk owing to its reduced compressive strength.

4.2.36 The regional dip of the Chalk is between 1° and 2° to the northeast. The thickness of the Chalk therefore increases from zero along its base on the western slope of the Lincolnshire Wolds to approximately 130 m thick along the tunnel route based on contour maps by Entec (2011).

4.2.37 Regionally, the depth to the top of the Chalk also increases to the northeast as shown in Figure 4-16. A brief review of borehole records from the BGS GeoIndex website1 indicates that the depth to the top of the Chalk continues to increase to the northeast along the Holderness Peninsular.

Figure 4-16 Contours showing elevation at the top of the Chalk (Entec, 2011)

4.2.38 The ground investigation data are consistent with the regional picture and show that the top of the Chalk is encountered at approximately 9 m bgl (-7 m Ordnance datum (OD)) at the drive pit and approximately 36 m bgl (-34 m OD) at the reception pit (Figure 4-5 and Figure 4-8). The boundary between superficial deposits and the Chalk beneath the River Humber is an uneven surface as a result of erosion and deposition of glacial till and estuarine alluvium (Figure 4-4).

4.2.39 The regional lithological characteristics of the Burnham Chalk and Flamborough Chalk were recognised in the borehole cores from the ground investigation. In particular, the presence of a band of white flints at the change from non-flinty to flinty chalk is a common feature marking the boundary between the Burnham Chalk Formation with numerous flint bands and the Flamborough Chalk Formation above without flints (Capita, 2014b).

4.2.40 Typical descriptions of both the Burnham Chalk and Flamborough Chalk from the ground investigation include “extremely close to closely spaced laminations of marl” (Soil Engineering, 2014). This indicates that the vertical hydraulic

1 http://www.bgs.ac.uk/geoindex/home.html

Drive pit



conductivity of the Chalk is likely to be significantly lower than the horizontal hydraulic conductivity. Vertical flow of groundwater within the Chalk would therefore be impeded.

4.2.41 Borehole core logging indicates that all chalk below CIRIA grade DC horizon has been assigned to CIRIA grade A i.e. discontinuities closed (Capita, 2014b). However, a number of large fractures or fissures are indicated by drilling logs from the ground investigation. During the construction of L05, significant volumes of liquid grout were lost to the ground over the interval 18 m bgl to 30 m bgl. This interval is within the Burnham Chalk and implies a transmissive preferential flow horizon or developed void space.

4.2.42 Also during several packer tests undertaken during the ground investigation it was not possible to achieve pressure (Capita, 2014a):

L14 at a test centre of approximately 40 m bgl;

M04 at 28 m bgl and 38 m bgl; and

M09 at 30.2 m bgl and 31.2 m bgl, M10 at 27.6 m bgl, M20 at 36 m bgl.

4.2.43 This information supports the regional conceptual understanding that groundwater flow through the Chalk is likely to be concentrated through a few large fractures or fissures.

4.2.44 Entec (2011) report that a pumping test at the PWS borehole at Goxhill, 5km west of the drive pit indicated a transmissivity of 1,800 m2/day for the Chalk. This equates to a hydraulic conductivity of 5.2 x 10-4 m/s if the thickness of the fractured chalk is assumed to be 40 m.

4.2.45 Transmissivity values from pumping tests are reported by the BGS for the public water supply boreholes abstracting from the Chalk aquifer. The values are 10,000 m2/d at Barrow pumping station and 3,300 m2/d at Thornton pumping station, located 9 km and 6 km southwest of the drive pit.

4.2.46 The hydraulic conductivity of the Chalk below the fractured Chalk is considered to be low. Entec (2011) report a value of 8.3 x 10-8 m/s for Chalk at 190 m bgl at South Killngholme gas caverns. The conceptual modelling study of the Yorkshire Chalk reported a transmissivity value of 50 m2/d for the Chalk beneath the Holderness Peninsular. This equates to a hydraulic conductivity of 2.9 x 10-6 m/s if the thickness of the Chalk is assumed to be 200 m.

Hydraulic conductivity

4.2.47 Estimates of hydraulic conductivity have been made for the lithologies encountered in the area and are summarised in Table 4-3. For superficial unconsolidated deposits, permeability estimates derived from PSD data using the Kozeny Carman formula were given preference to other permeability test data (e.g. falling head test results). The PSD data are more numerous and target specific units, providing a more robust estimate of permeability for individual units. Falling head test data were fewer in number and derived from boreholes that



typically traverse a range of lithological units, and so averages out the permeability result. PSD results are comparable to other permeability data.

Lithology Hydraulic conductivity (m/s)

Justification

Glacial till 7.0 x 10-8 Combination of professional judgement and permeability calculated from PSD data using Kozeny Carman formula


1.0 x 10-4 Geometric mean of permeability calculated from PSD data using Kozeny Carman formula from L15 and L18

Tidal flat deposits 7.0 x 10-6 Geometric mean of rising head permeability tests in L04 and L06

Estuarine alluvium 2.4 x 10-6 Geometric mean of permeability calculated from PSD data using Kozeny Carman formula from M06, M07, M08, M09, M10 and M11

Chalk bearings 1.0 x 10-2 Equivalent to transmissivity of 3500 m2/d reported from Goxhill PWS pumping test (Entec, 2011), divided by 4 m thickness

Fractured Chalk 5.2 x 10-4 Equivalent to transmissivity of 1800 m2/d reported from Goxhill PWS pumping test (Entec, 2011), divided by 40 m thickness

Chalk below active zone

2.9 x 10-6 Equivalent to transmissivity of 50 m2/d reported from Yorkshire Chalk modelling (ESI, 2010), divided by 200 m thickness

Table 4-3 Estimates of hydraulic properties

Storage Coefficient

4.2.48 Estimates of storage coefficients for the unconsolidated strata were taken from site specific data and interpretation, as presented in the Ground Investigation Report (Capita, September 2014a). Values for chalk units were derived from



pumping testing results as presented in the Entec (2011) report. These values are summarised in Table 4-4.

Lithology Storage Coefficient (m3/m3)

Justification

Glacial Till 0.21

Capita Ground Investigation Report (2014a)


0.21 Capita Ground Investigation Report (2014a)

Tidal flat deposits 0.4


Estuarine alluvium 0.4


Chalk bearings 0.025

Pumping testing interpretation (Entec, 2011)

Fractured Chalk

0.005 to 0.025

Pumping testing interpretation (Entec, 2011) Chalk below active

zone

Table 4-4 Estimates of Storage Coefficient

Coefficient of Compressibility

4.2.49 Estimates for coefficients of compressibility for unconsolidated strata were taken from consolidation test results and their subsequent interpretation, as presented in the Ground Investigation Report (Capita, September 2014a) and summarised in Table 4-5.


Alluvium and Peat

Glacial Deposits

Flamborough Chalk

Burnham Chalk

Mv (m2/MN) 1.10 0.23 - -

Table 4-5 Estimates of Coefficient of Compressibility (Mv)

4.2.50 Consolidation tests were not undertaken on Chalk strata as they were not required.

Regional Groundwater levels

4.2.51 Groundwater levels are routinely monitored at several locations within the Chalk aquifer. Groundwater contours interpreted from these data are shown in Figure 4-17 for typical low and high groundwater levels on the south Humber bank.



Groundwater levels at the drive pit range from a typical high of 1 m OD to a typical low of 0.5 m OD.


4.2.52 The hydrograph from the nearest monitoring borehole at East Halton (TA13962261) is shown in Figure 4-18. The data show groundwater levels generally follow a typical seasonal trend of high groundwater levels of about 1.5 m OD in winter/spring and low groundwater levels of about 1 m OD in late summer. Drought periods such as 1990 to 1992 are marked by groundwater levels dropping to around -0.5 m OD. Wet periods such as March 1980 lead to higher than usual groundwater levels of around 2 m OD.

Figure 4-18 Chalk groundwater hydrograph at East Halton (data provided by EA)

4.2.53 On the north Humber bank, the Holderness Plain, there is very little variation in groundwater level which reflects the reduced effect of recharge due to a thick layer of superficial deposits which confine the Chalk in this area.

4.2.54 The hydrograph shown in Figure 4-27 is from Saltend (TA163275), which is the nearest monitoring borehole to the reception pit. The data show groundwater

500000 510000 520000 500000 510000 520000

Typical lowDrive pit

Drive pit

Typical high



levels generally fluctuate by less than 1 m within each year. Average groundwater level is approximately 0.75 m OD.

Figure 4-19 Chalk groundwater hydrograph at Saltend (data provided by EA)

Site specific groundwater levels

4.2.55 Water level data are available from 10 boreholes and 19 installations located roughly in line with the pipeline route. Water levels are recorded manually and automatically every 20 minutes. Some installations are open standpipes and some are installed with sealed vibrating wire piezometers. Full details are given in Table 4-6.

4.2.56 Only boreholes L01, L02, L04, L06, L15, L16 and L18 are considered to provide the reliable data.

4.2.57 Groundwater levels across the Goxhill site show little long term variation over the monitoring period May to October 2014 (Figure 4-20). Borehole L02 is situated approximately 15 m distance from a drainage ditch and the shallow response zone is between 1 m bgl and 5 m bgl. Therefore groundwater level peaks in L02 shallow in July and August are probably due to rainfall and the influence of the drainage ditch. A smaller peak is seen in L04 shallow groundwater level, which is approximately 45 m distance from a drainage ditch and has a response zone between 5 m bgl and 11.5 m bgl.

4.2.58 Figure 4-21 shows hydrographs from boreholes near the drive pit compared to tide levels for a five day period in September 2014. L02 deep and L04 deep show a slight response from the effect of the tide flowing into and out of the Humber on groundwater levels in the Chalk. The response in L06 shallow and deep indicates that tidal influence increases markedly in both the superficial deposits and Chalk closer to the River Humber.

4.2.59 In boreholes L04 and L06, groundwater level in the shallow installations is higher than groundwater level in the deep installations. This indicates a downward hydraulic gradient from the superficial deposits to the Chalk.

4.2.60 These data were recorded in September when groundwater levels in the Chalk are usually at low levels. It is possible that the situation is reversed (i.e. upward



hydraulic gradient) in winter when groundwater levels in the Chalk are usually higher.

4.2.61 On the Paull side, water levels also show little long term variation (Figure 4-22). Tidal responses are seen in L18 deep and L16 deep indicating the Chalk aquifer is in hydraulic connection with the River Humber. The response is slightly greater in borehole L16, which is closer to the estuary than L18.

4.2.62 Groundwater level in L15 shallow is likely to be influenced by the adjacent drainage ditch. L15 deep shows a greater tidal influence on groundwater level in the glacial sand and gravel deposits compared to glacial till in L18 shallow.

4.2.63 It is also worth noting that the water level at L16 shallow is higher than the levels further inland at L18 and L15. This is presumably caused by water discharging into the shallow sediments when the tide is in. The water level in the superficial deposits on the landward side is controlled by the artificial drainage network.



Figure 4-20 Groundwater levels near the drive pit

Wat

er le

vel (

m O

D)



Figure 4-21 Tidal variations in groundwater levels near the drive pit

Tid

e le

vel (

m O

D)

Wat

er le

vel (

m O

D)

Wat

er le

vel (

m O

D)

Wat

er le

vel (

m O

D)



Figure 4-22 Groundwater levels near the reception pit

Wat

er le

vel (

m O

D)



Figure 4-23 Tidal variations in groundwater levels near the reception pit

Tid

e le

vel (

m O

D)

Wat

er le

vel (

m O

D)

Wat

er le

vel (

m O

D)

Wat

er le

vel (

m O

D)



Borehole Installation Screen section (mOD)

Top Base

Start Date End Date Strata Comments

L01 Standpipe -7.33 -10.33 22/05/2014 30/10/2014 Flamborough Chalk

L02 Shallow: Possible that water levels are affected by

proximity to drainage ditch

L03 Shallow: Data considered to be suspect given that a

very similar response is observed in both boreholes

L05 Shallow: Data are considered suspect

L18 Deep: Data from 9/10/2014 manually corrected because

of an apparent instant reduction in water level. Needs

checking against subsequent manual calibration data.

L02 Shallow Standpipe 1.16 -2.84 22/05/2014 30/10/2014 Glacial Till

L02 Deep Standpipe -18.84 -21.84 22/05/2014 30/10/2014 Burnham Chalk

L03 Shallow VWP -7.28 -9.28 22/05/2014 30/10/2014 Flamborough Chalk

L03 Deep VWP -31.78 -33.78 09/07/2014 30/10/2014 Burnham Chalk

L04 Shallow Standpipe -2.6 -9.1 22/05/2014 30/10/2014 Tidal Flat deposits


L05 Shallow VWP 1.61 -5.89 27/05/2014 30/10/2014 Tidal Flat deposits

L05 Deep VWP 1.61 -17.89 09/07/2014 30/10/2014 Tidal Flat deposits and

Chalk

L06 Shallow Standpipe -2.47 -5.47 22/05/2014 30/10/2014 Tidal Flat deposits


L14 Shallow Standpipe -7.37 -10.67 23/05/2014 09/10/2014 Flamborough Chalk

L14 Deep Standpipe -36.67 -42.67 04/06/2014 09/10/2014 Glacial Till

L15 Shallow Standpipe -0.11 -3.11 12/06/2014 30/10/2014 Glacial Till

L15 Deep Standpipe -24.91 -27.91 01/07/2014 30/10/2014 Glacial Sand and Gravel

L16 Shallow Not stated -6.59 -8.09 23/05/2014 30/10/2014 Glacial Till

L16 Deep Not stated -36.09 -38.09 09/07/2014 30/10/2014 Flamborough Chalk

L18 Shallow Standpipe -14.78 -16.78 28/08/2014 30/10/2014 Glacial Till

L18 Deep Standpipe -33.28 -39.28 26/08/2014 30/10/2014 Flamborough Chalk

Table 4-6 Summary of groundwater level monitoring



Groundwater chemistry

4.2.64 Variations in groundwater chemistry can be used to infer flow paths and mixing of different waters, which in turn can be used to develop the conceptual model.

4.2.65 The south Humber bank Salinity Research Project identified several distinct hydrochemical zones within the Chalk aquifer (University of Birmingham, 1978, reported in Entec, 2011). Figure 4-24 shows that the drive pit would be located with Type IVc zone defined as chalk groundwater that is brackish or slightly saline and also enriched in sulphate. These groundwaters were thought to have been trapped in the Chalk by deposition of estuarine sediments at the end of the last glacial period. However, groundwater level fluctuations in boreholes within Type IVc zones are consistent with tide cycles (University of Birmingham, 1978, reported in Entec, 2011). This suggests that there is mixing of Chalk groundwater with modern estuarine water.

Figure 4-24 Chalk groundwater chemistry zones on the South Humber Bank (Entec, 2011)

4.2.66 Type II groundwater is found to the west of the drive pit although the boundary with the Type IVc zone is uncertain. The major ion chemistry of this groundwater is dominated by calcium and bicarbonate ions, typical for Chalk groundwater. This groundwater has a low chloride concentration i.e. it is fresh water. Type II groundwater is also low in nitrate and sulphate indicating that it was recharged the aquifer prior to extensive fertiliser use by agriculture in the 1940s. In contrast, groundwater Type I further to the west has elevated nitrate and sulphate indicating it is relatively young groundwater.

4.2.67 Saline intrusion has affected parts of the Chalk aquifer on the north and south Humber Bank. Saline intrusion causes loss of freshwater resource for abstraction and the environment. It has historically been caused by over-abstraction,

Drive pit



particularly around Hull on the north Humber bank and around Grimsby on the south Humber Bank.

4.2.68 On the north Humber bank, large areas of the confined Chalk beneath the Holderness Plain contain brackish to moderately saline groundwater. This is considered to be due to a combination of ancient seawater trapped within the aquifer and historic unsustainable pumping, which led to estuary water being drawn into the aquifer (Elliot et al., 2001). Chloride concentrations in the vicinity of the reception pit are indicated as being above 500 mg/l (Figure 4-25).


4.2.69 Groundwater abstractions proposed as part of the construction of the Scheme could exacerbate saline intrusion. This is because abstraction would lower groundwater levels in the Chalk aquifer and potentially cause increased flow from saline water within the estuary and estuarine sediments into the Chalk aquifer.



Brackish to saline water already within the Chalk could be pulled inland towards areas of the aquifer currently containing fresh groundwater.

4.2.70 Site-specific groundwater chemistry is available from the ground investigation (Soil Engineering, 2014; Capita, 2014a). Three rounds of sampling were carried out on 9th and 10th September, 8th and 9th October, and 22nd October 2014.

4.2.71 Table 4-7 lists the borehole sample intervals and strata being sampled.

Borehole / ID

Ground Level (m OD)

Response Zone Top (m OD)

Response Zone Base (m OD)

Strata being sampled

L01 1.97 -7.33 -10.33 Chalk bearings

L02/1 2.16 -18.84 -21.84 Burnham Chalk

L04/1 2.4 -16.1 -21.1 Burnham Chalk

L04/2 2.4 -2.6 -9.1 Tidal flat deposits

L06/1 2.53 -17.47 -22.47 Burnham Chalk

L06/2 2.53 -2.47 -5.47 Tidal flat deposits

L08 1.87 -1.13 -4.13 Glacial till

L14/1 2.33 -36.67 -42.67 Flamborough Chalk

L14/2 2.33 -7.37 -10.67 Glacial till

L15/1 2.09 -24.91 -27.91 Glacial sand & gravel

L15/2 2.09 -0.11 -3.11 Glacial till

L18/1 2.72 -33.28 -39.28 Flamborough Chalk

Table 4-7 Boreholes sampled for groundwater chemistry (Soil Engineering, 2014)

4.2.72 Figure 4-26 is a piper diagram showing the major ion chemistry of groundwater in the Chalk aquifer near the drive pit. The data plot in a line from calcium-bicarbonate type water at the drive pit (L01) to sodium-chloride type near the River Humber (L06). This indicates mixing of normal Chalk groundwater with saline water.

4.2.73 Figure 4-27 is a piper diagram showing the major ion chemistry of groundwater in the superficial deposits near the drive pit. Groundwater within the tidal flat deposits closest to the River Humber (L06 and L04) lie within the sodium-chloride



field. Borehole L08 samples groundwater from the glacial till and indicates chloride type water with no dominant cation.

4.2.74 Figure 4-28 shows average chloride concentrations from the three sampling rounds on the geological cross section. Text boxes showing the chloride concentrations are shown at the sampling level.

4.2.75 Water can be considered to be fresh when it has a chloride concentration equal to or less than 250 mg/l, which is the prescribed limit within UK drinking water regulations. Brackish water can be considered to have a chloride concentration between 250-5,000 mg/l. Water with a chloride concentration greater than 5000 mg/l is generally considered to be saline. Seawater has a chloride concentration of about 19,400 mg/L.

4.2.76 The tidal flat deposits contain saline groundwater, chloride concentration increases towards the River Humber. This indicates water from the estuary is flowing into the tidal flat deposits.

4.2.77 The glacial till deposits contain brackish water indicating that they are in hydraulic connection with saline water in the tidal flat deposits and mixing with relatively fresh groundwater recharged from rainfall and runoff.

4.2.78 Groundwater in the Chalk aquifer at the drive pit location is fresh, with an average chloride concentration less than 70 mg/l. However, the chloride concentration in L02 just to the northeast of the drive pit is around 380 mg/l indicating the fresh water - brackish water boundary is just to the northeast of the drive pit. Higher chloride concentrations are observed in boreholes closer to the River Humber (L04 and L06). This indicates that groundwater in the Chalk aquifer is mixing with saline water. The tidal response in groundwater level observed in L06 and to a lesser extent in L04 and L02 indicates that saline water from the estuary is in hydraulic connection with the Chalk aquifer i.e. estuary water is flowing down into the Chalk aquifer.



Figure 4-26 Piper diagram of chalk groundwater in boreholes near the drive pit




Figure 4-28 Average chloride concentrations (mg/l) in boreholes near the drive pit (adapted from Capita 2014a, 2014b)

4.2.79 Figure 4-29 is a piper diagram showing the major ion chemistry of groundwater in the Flamborough Chalk aquifer near the reception pit. The data plot within the sodium chloride field except for one analysis from borehole L18 that plots within the carbonate field with no dominant cation.

4.2.80 Figure 4-30 is a piper diagram showing the major ion chemistry of groundwater in the superficial deposits near the reception pit. Groundwater within the glacial till at L14 and glacial sand and gravel at L15 plots within the sodium chloride field. The analysis from the glacial till in borehole L15 has significantly less chloride. This is likely to be due to the influence of surface water; borehole L15 is adjacent to a drainage ditch and has a shallow sampling interval (2 m bgl to 5 m bgl).

4.2.81 Figure 4-31 shows average chloride concentrations from the three sampling rounds on the geological cross section near the reception pit. Text boxes showing the chloride concentrations are shown at the sampling level. The difference in average chloride concentrations in the Flamborough Chalk between L14 and L18 is largely due to one analysis from L18 with a low chloride concentration, 180 mg/l compared to 980 mg/l and 1400 mg/l on the two other sampling rounds.



Figure 4-29 Piper diagram of chalk groundwater in boreholes near the reception pit

Figure 4-30 Piper diagram of groundwater in superficial deposits within boreholes near the reception pit



Figure 4-31 Average chloride concentrations (mg/l) in boreholes near the reception pit (adapted from Capita 2014a, 2014b)

Surface water features

4.2.82 The following sections describe the main surface water features within the vicinity of the Scheme including their hydrological behaviour, their elevations in relation to groundwater levels, and their potential interaction with groundwater.

River Humber and associated water dependant sites

4.2.83 The Scheme is adjacent to and crosses beneath the River Humber. The River Humber along this reach is classed in Water Framework Directive terms as a transitional waterbody i.e. an estuary. This waterbody is classed as being “heavily modified” from its natural state, as a result of the flood protection measures that are in place.

4.2.84 The nearest measurement of tide heights to the Scheme is at Humber Sea Terminal approximately 4.5 km southeast (downstream) of the proposed tunnel route. The tidal range at Humber Sea Terminal is approximately 7.5 m. Between May and October 2014, tide level relative to ordnance datum range between -3.6 m OD and +3.9 m OD2. Since groundwater level at the drive pit is around 1.5 m OD and around 1 m OD at the reception pit, water level in the River Humber would be above groundwater levels at high tide and below groundwater level at low tide. There is flow of water between the River Humber and groundwater in both the superficial deposits and Chalk as described above.

4.2.85 The River Humber is designated as a Special Area of Conservation (SAC), SSSI, Special Protection Area (SPA), and Ramsar site. There are a number of sites of

2 http://www.tidetimes.org.uk/humber-sea-terminal-tide-times-20140528#ixzz32QjqGEVu



high conservation value on the south Humber bank e.g. East Halton Saltmarsh (part of the Humber Estuary SSSI) and North Killingholme Haven Pits SSSI. The importance of groundwater to these sites is not well understood (EA, 2002).

4.2.86 East Halton Saltmarsh is part of the Humber Estuary SSSI and is in unfavourable declining condition due to public access/disturbance (off-road vehicles and fly tipping).

4.2.87 Key surface water receptors in the study area also include the Thorngumbald Drain and the East Halton Beck, which are both designated as EA Main River and outfall to the River Humber. In addition, two ordinary watercourses, the South Pasture Drain and Pasture Drain, are also located within the study area. These two watercourses drain to the Thorngumbald Drain immediately upstream of the outfall to the River Humber.

East Halton Beck

4.2.88 East Halton Beck is located approximately 770 m southeast of the drive pit and is the closest EA main river.

4.2.89 Entec (2011) reports that flow in East Halton Beck is not well characterised because there are only approximately 6 months of reliable flow gauge data from 1981. Spot flow gauging data indicate zero flow during a dry period (September 1996) and 10 l/s (864 m3/d) during a wet period (January 1973) as shown in Figure 4-32.

4.2.90 Entec (2011) reports that glacial till is relatively thin under East Halton Beck compared to surrounding catchments and thick gravel deposits are known within the glacial sequence. This suggests that East Halton Beck may receive a small component of baseflow from the confined Chalk when groundwater levels are high.

4.2.91 Results from the regional groundwater model (AMEC, 2012a) indicate that during a typical dry month East Halton Beck has a total flow of between 430 m3/d and 740 m3/d along the reach adjacent to the proposed drive pit. The beck is predicted to receive baseflow of approximately 0.1 Ml/d (100 m3/d) along each model stream cell (200 m). This equates to 0.5 m3/d per metre length of the beck.



Figure 4-32 Spot flows in a wet and dry period (taken from Entec 2011)

Drainage ditch network around drive pit

4.2.92 There is a network of drainage ditches that flow into East Halton Beck. Northeast Lindsey Internal Drainage Board (IDB) are responsible for management of these drainage ditches. Mr. Darren Scott, Area Manager for Northeast Lindsey IDB, has

Drive pit

September 1996 (Dry)



stated that the IDB does not record levels or flows in the drainage ditches but drains do not generally dry out in summer.

4.2.93 Survey data shows that these ditches are approximately 2 m below ground level at their deepest point, which is approximately 0 m OD (mean sea level). Water depth in the ditches at the time of surveying (February 2014) was in the order of tens of centimetres.

Figure 4-33 Survey across drainage ditch near the drive pit (taken from Capita Drawing No. H160/BH/02/01/F9/102. Rev A)

Thorngumbald Drain

4.2.94 A study of the Thorngumbald Drain catchment was undertaken by Flynn and Rothwell (1998), on behalf of the EA. The drain is typically trapezoidal in cross

Drive pit



section with channel base widths of between 1.3 m and 3.5 m. The gradient of the drain is very shallow, approximately 1 in 10,000.

4.2.95 Pumps are installed on the landward side of the Thorngumbald drain to lift water over the flood levees into the River Humber when required.

4.2.96 Water level in the Thorngumbald Drain was -0.93 m OD during the survey undertaken in February 2014. The channel base is approximately -1.5 m OD as shown in Figure 4-34.

4.2.97 Land drainage to Thorngumbald Drain takes place through a series of surface drainage ditches and herringbone pattern sub-surface drains.

Drainage ditch network around reception pit

4.2.98 There is a network of drainage ditches that cross the area. South Holderness IDB are responsible for management of these drainage ditches. Mr. Ralph Ward, spokesperson for South Holderness IDB, has stated that that the IDB does not record levels or flows in the drainage ditches but drains do not generally dry out in summer.

4.2.99 Survey data shows that the bed level of these ditches is slightly below mean sea level. Water depth in the ditches at the time of surveying (February 2014) was in the order of a few centimetres as shown in Figure 4-34.



Figure 4-34 Survey across the Thorngumbald Drain and a drainage ditch near the reception pit (taken from Capita Drawing No. H160/BH/02/01/F9/101. Rev A)

Reception Pit

Thorngumbald Drain



4.3 Physical configuration of excavations

Drive pit

4.3.1 The drive pit would be located on the south Humber bank approximately 3.5 km northeast of Goxhill, Lincolnshire. The NGR for the centre line of the drive face at the point where the tunnel boring would commence is 513565 E, 423286 N. Figure 4.35 shows the proposed location of the drive pit on the 1:25,000 scale ordnance survey base map.

Figure 4-35 Proposed location of the drive pit

Reference design

4.3.2 The reference design for the drive pit is approximately 200 m in length in an approximately southwest - northeast orientation and 10 m wide. The pit would be wedge shaped along its length as shown in Figure 4-36. The step down in the profile at a chainage of 120 m is designed to allow the pipe to be installed into the vertical centre line of the tunnel without having to raise the pipe off the ground.

Drive pit

Tunnel

(Contains Ordnance Survey data ©Crown copyright and database right [2014])



Figure 4-36 Drive pit long section showing key features based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

4.3.3 Secant piles would be installed along the perimeter of the deep section of the drive pit (chainage 120 m to 200 m) prior to excavation. These piles would be 1.2 m diameter. These piles would be installed to a level of -26m OD.

4.3.4 The deepest section of the drive pit beyond 120 m chainage and the tunnel drive face would be excavated through the Chalk bearings and the top of the Chalk. The top of the Chalk was identified at an elevation of -7.3 m OD in borehole L01 from the ground investigation (Soil Engineering, 2014). A layer of coarse sand

Top of base slab

Base of base slab

Existing ground level (approx.)

Start of tunnel (drive face)



and gravel, most likely the Chalk bearings, was logged above the top of the Chalk up to an elevation of -6.8 m OD.

4.3.5 Where the excavation is less than 2 m deep (chainage 0 m to 64 m) the sides of the pit would be re-graded for slope stability.

4.3.6 Sheet piles would be installed along the sides of the middle section of the drive pit (chainage 64 m to 120 m) prior to excavation. These sheet piles would be installed to a level of approximately -7 m OD.

4.3.7 A concrete base slab would be placed along the length of the drive pit to enable a dry working platform. The thickness of the base slab is estimated to be between 2 m and 3 m and would be confirmed at detailed design stage. As a conservative assumption in terms of groundwater impact it has been assumed the base slab would be 3 m thick and that the maximum dewatering level would be -12.6 m OD.

4.3.8 The reference design (by others) for the drive pit would prevent base heave by tying the base slab to the secant piles with post drilled dowels. This method utilises the weight of the piles as well as skin friction between the piles and the ground. A further option would be to hold the base down with tension piles.

4.3.9 Sulphate resistant cement for piles, base slab and ground anchors should be used as groundwater has a high sulphate content.

4.3.10 The following construction sequence for the drive pit has been assumed:

Drill and install secant piles around the perimeter of the deep section of the drive pit;

Excavate deep section of drive pit, groundwater inflow to be controlled by passive relief wells from within the secant piled pit. Dewatering duration would be 41 days;

Cast base slab with ground anchoring to prevent base heave;

After this the drive pit would be sealed and the groundwater level outside would recover. Minimal sump pumping to maintain dry working environment;

Following tunnel construction, the shallow section of the drive pit would be constructed to allow pipe stringing;

Install sheet piles around the perimeter of the shallow section of the drive pit;

Excavate shallow section of drive pit, groundwater inflow to be controlled by passive relief wells from within the pit. Dewatering duration would be 28 days; and

After this the shallow section of the drive pit would be sealed and the groundwater level outside would recover. Minimal sump pumping to maintain dry working environment.



Tunnel

4.3.11 In plan view, the proposed tunnel route is a straight line directly between the drive pit and the reception pit. The proposed vertical alignment of the tunnel is plotted on the interpreted geological section in Figure 4-37. Boreholes from the recent ground investigation are shown along with CIRIA Chalk grades (Soil Engineering, 2014: Capita, 2014).


4.3.12 The drive face is situated at the boundary between the superficial deposits and the Chalk. The tunnel drops at a gradient of 2.5% through the Burnham Chalk Formation, followed by a shallower section (1% gradient) passing through the Burnham Chalk into the Flamborough Chalk Formation. The tunnel then begins to rise, initially at a shallow gradient of 0.25% and then steepens to a 1.67% gradient passing up into the superficial deposits to meet the reception pit.

4.3.13 National grid references and elevations relative to ordnance datum for selected points along the tunnel are listed in Table 4-8.

Easting Northing Tunnel crest (m OD)

Tunnel base (m OD)

513565 423286 -5.95 -9.95

514181 423537 -22.57 -26.57

514246 423563 -23.8 -27.8

515201 423951 -34.11 -38.11

515255 423974 -34.33 -38.33

516977 424674 -29.68 -33.68

517028 424695 -29.22 -33.22

518227 425182 -8.26 -12.26

Table 4-8 National grid references and elevations relative to ordnance datum for selected points along the tunnel based on Capita drawing ref. H160/BH/03/03/F9/101 Rev. A

Tunnel

Drive pit

Reception Pit



Reception pit

4.3.14 The reception pit is located on the north Humber bank approximately 1.5 km southeast of Paull. The NGR for the centre line of the tunnel at the point where the tunnel boring would meet the reception pit is NGR 518227 E, 425182 N. Figure 4-38 shows the proposed location of the reception pit on the 1:25,000 scale ordnance survey base map.

Figure 4-38 Proposed location of the reception pit (Contains Ordnance Survey data © Crown copyright and database right [2014])

4.3.15 The reception pit would be rectangular with approximate dimensions of 50 m long, 10 m wide and 15 m deep. The pit walls would be supported by 1.2 m diameter secant piles around the perimeter of the pit. The base slab would be approximately 3 m thick. Figure 4-39 is a long section of the reception pit showing the elevation of key features.

Reception pit

Tunnel

Fort Paull Battery Private Water Supply

W





4.3.16 The following has been assumed following the construction sequence for the reception pit:

Drill and install secant piles around the entire perimeter of the proposed reception pit;

Excavate reception pit, groundwater inflow to be controlled by passive relief wells from within the secant piled pit. Dewatering duration would be 35 days;

Cast base slab with ground anchoring to prevent base heave; and

After this the reception pit would be sealed and the groundwater level outside would recover. Minimal sump pumping to maintain dry working environment.

4.4 Reinstatement of the pits

4.4.1 The reference design includes secant piled wall around the perimeter of the deep part of the drive pit with a 2 to 3 m thick concrete base slab with ground anchoring to prevent base heave. This would effectively seal the excavation from water ingress. On completion of the pipeline installation the proposal is to retain this ‘concrete box’ in the ground and backfill the excavation with suitable material. The Chalk would therefore remain confined.

Top of base slab -12.7 m OD

Base of base slab -15.7 m OD

Existing ground level 2.1 m OD (approx.)

End of tunnel

Base of piles -18.7 m OD

1.2 m diameter secant piles 1.2 m diameter secant piles



4.5 Groundwater Management

Drive pit and reception pit

4.5.1 Within the drive pit and reception pit, groundwater control is likely to be achieved by combining four approaches; cut-off walls (secant and sheet piling), deep well dewatering, sump pumping and passive relief wells within the base of the pit.

4.5.2 Prior to excavation of the pits, piles would be installed around the perimeter and to a depth designed to minimise groundwater seepage into the pits and then deep wells installed.

4.5.3 During excavation of the pits groundwater would flow up in to the base of the excavation. The base of the pits would be dewatered from sumps set in the base. Pumps would lift the water from the excavation to a system of pipe work which collects the water into settlement tanks and/or lagoons. The schedule of dewatering activities is described below.

4.5.4 Dewatering the 100 m long deep section of the drive pit for 41 days (from 1/3/2018 – 10/4/2018). This is the period during which excavation within the piled wall is planned. After this date the deep section of the drive pit would be sealed and the water level outside would recover.

4.5.5 Dewatering the reception pit for 35 days (from 29/05/2018 – 02/07/2018). Again, this is the period during which excavation within the piled wall is planned. After this date the reception pit would be sealed and the water level outside would recover.

4.5.6 Dewatering the 100 m long shallow section of the drive pit for 28 days (from 16/10/2019 – 12/11/2019). After this date the shallow section of the drive pit would be sealed and the water level outside would recover.

4.5.7 It is proposed to discharge groundwater abstracted during the dewatering phases to the Humber Estuary. For the drive pit this would occur on the estuary side of the sluice gate at the mouth of East Halton Beck. For the reception pit water would be discharged on the estuary side of the flood levee.

Tunnel

4.5.8 The Tunnel Boring Machine (TBM) would be a closed face system. Groundwater inflow into the tunnel during construction would be minimal and dealt with by normal sump pumping. As the TBM progresses the tunnel would be lined with pre-cast concrete rings and grouted in to the rock, so that groundwater would be excluded from the tunnel. The lining specification stipulates the leakage class rating of the tunnel, in this instance class 3; characterised as ‘capilliary dampness’, which is defined as having occasional damp patches on the tunnel lining, but no drops of water. The allowable daily leakage rate is 0.1 litres / metre



squared. This class rating would also prevent any fines / materials from entering the tunnel.

4.6 Summary

4.6.1 A series of hydrogeological cross sections have been drawn to illustrate the conceptual understanding of hydrogeology in the area of the scheme. Figure 4-40 is a hydrogeological long section along the tunnel route from the drive pit to the reception pit. Figure 4-41 is a hydrogeological cross section through the drive pit. Figure 4-42 is a hydrogeological cross section through the reception pit. These two cross sections are based on BGS borehole records as well as logs from the ground investigation.

4.6.2 The key features of the site conceptual model are described below:

The Burnham Chalk is harder and likely to have a higher hydraulic conductivity than the soft Flamborough Chalk;

The elevation on the top surface of the Chalk decreases to the northeast along the tunnel route;

The Chalk is described as having extremely close to closely spaced lamination of marl, which are likely to impede vertical groundwater flow within the Chalk i.e. vertical hydraulic conductivity is much less than horizontal hydraulic conductivity;

Superficial deposits can be divided into four hydrogeologically distinct units on the basis of lithological descriptions from the borehole logs and PSD data from geotechnical testing. These deposits are:

Glacial till;

Glacial sand and gravel;

Tidal flat deposits;

Estuarine alluvium;

Superficial deposits at the location of the drive pit are poorly sorted glacial till to a depth of approximately 8 m bgl. They have low hydraulic conductivity;

Tidal flat deposits adjacent to River Humber are characterised by peat deposits and are variable in thickness. They are poorly sorted and have low hydraulic conductivity;

Tidal fluctuations in groundwater level within the Tidal flat deposits and to a lesser magnitude in the Glacial till indicate they are in hydraulic connection with the River Humber;

At the drive pit groundwater levels in the Chalk are approximately 1 m OD, slightly below those in the superficial deposits 1.5 m OD. Both are within 2 m of ground level;

Estuarine alluvium occurs directly beneath the River Humber which has a higher proportion of sand and gravel, and is considered to be of moderate permeability;

Further northeast under the River Humber glacial tills occur and are overlain by tidal flat deposits on the river bank;



Glacial sand and gravel deposits occur between 10 m and 34 m depth below ground level approximately 100 m northwest of the reception pit. These are relatively well-sorted coarse grained deposits and are likely to have moderately high hydraulic conductivity. The lateral extent of these deposits at depth and connection with glacial sand and gravel mapped at the surface to the southeast of the reception pit is uncertain;

Chemical analyses indicate that groundwater in the glacial till is brackish immediately to the northeast of the drive pit and becomes saline within the tidal flat deposits. Fresh groundwater is present in the underlying Chalk aquifer directly beneath the drive pit but passes into transitional fresh-brackish water and eventually brackish water a few hundred metres to the northeast. This indicates the freshwater – saline water interface is currently within a few hundred metres of the drive pit. There is likely to be flow of saline estuary water into the estuarine alluvium and tidal flat deposits; and

There is no hydraulic barrier (e.g. low permeability clay) between the permeable estuarine alluvium and the Chalk bearings and Chalk directly beneath the River Humber. Therefore there is likely to be mixing of saline water in the estuarine alluvium and fresh groundwater flowing from the Chalk aquifer to the southwest to produce brackish groundwater. It is likely that the current location of the freshwater – saline water interface is maintained by the dynamic equilibrium of the high hydraulic head in the Chalk driving fresh groundwater flow to the northeast and the tidal River Humber driving saline water into the Chalk via the estuarine alluvium.



Figure 4-40 Hydrogeological long section along the tunnel route



Figure 4-41 Hydrogeological cross section through the drive pit



Figure 4-42 Hydrogeological cross section through the reception pit



Potential impacts

4.6.3 Potential effects of the Scheme activities can be categorised into four groups:

4.6.4 Drawdown

Derogation of public and/or private water supplies

Lower water levels in wetlands, ponds etc.

4.6.5 Flow

Reduction in groundwater baseflow to streams (e.g. East Halton Beck)

Loss of groundwater resource from the Chalk aquifer

Interruption of flow paths, cross-connection of aquifers

4.6.6 Water quality

Saline intrusion – saline / brackish water within and beneath the Humber Estuary may be drawn into areas of fresh groundwater;

Mobilisation of existing ground and/or groundwater contamination;

Removal of protective confining layer and opening contaminant pathways to the Chalk aquifer; and

Reduced dilution of contaminants in surface watercourse(s) due to decrease in groundwater baseflow.

4.6.7 Settlement:

Ground settlement due to reduced pore water pressures e.g. flood levees, nearby buildings

Derogation of habitats due to change in flood level

Key model uncertainties

4.6.8 Uncertainties in our conceptual model that are considered to be significant for impact assessment are described below:

Anisotropy of hydraulic properties:

o The vertical hydraulic conductivity of the Chalk and superficial deposits is not well established. The available evidence suggests that vertical hydraulic conductivity is significantly less than horizontal hydraulic conductivity in both the Chalk and superficial deposits. The vertical hydraulic conductivity used in the numerical model would affect the flow and drawdown calculations; and

o The ground investigation has provided information on ground conditions along tunnel route. Ground conditions perpendicular to the tunnel are relatively poorly constrained by BGS maps and borehole records and EA data. The extent of the glacial sand and gravel deposits around the reception pit is of particular



significance because they are likely to have a much higher hydraulic conductivity than the surrounding glacial till.

Thickness and hydraulic properties of the weathered chalk and fractured Chalk:

It has not been possible to distinguish the thickness, extent and variability of the weathered layer between the Chalk and superficial deposits. Chalk bearings (predominantly gravel) have been identified in some borehole logs near the drive pit and these sediments are likely to have high hydraulic conductivity. However, in other locations putty chalk (predominantly clays) may be present, which are likely to have low hydraulic conductivity;

The thickness of the fractured Chalk (the zone of Chalk through which most groundwater would flow) is not well characterised by site-specific data; and

Both the thickness and hydraulic properties of the weathered chalk and fractured Chalk would significantly influence the depth to which piles are required to reduce impacts on groundwater.

Hydraulic connection between surface water and groundwater:

The hydraulic conductivity of sediments in stream beds and the base of drainage ditches would significantly influence the flow of groundwater into and out of these watercourses. This in turn would influence the impact on these watercourses from lowering groundwater levels as part of pit construction.

4.6.9 In view of these uncertainties the most appropriate model to adopt is one that is consistent with regional understanding and conservative in terms of potential impacts i.e.:

Isotropic hydraulic conductivity (Kh = Kv) has been assigned to the superficial deposits. A vertical permeability of Kh/0.05 has been assigned to the Chalk where there is strong evidence (lithological descriptions of closely laminated marl bands and differences in permeability testing results with depth);

A layer of Chalk bearings that is uniformly 4 m thick and with uniform hydraulic properties (high permeability and high storage coefficient);

A 40 m thick layer of fractured Chalk, with hydraulic properties typical of regional data (e.g. Goxhill PWS pumping test results); and

No hydraulic resistance to flow has been introduced along stream beds.

4.6.10 A Phase 2 ground investigation is also planned to address some of these uncertainties.

4.7 Assessment of impacts from the tunnel

4.7.1 The hydrogeological conceptual model and outline tunnel construction described above indicates that the potential impacts of the tunnel can either be assessed



without the need for a numerical model or are beyond the scope of a hydrogeological assessment.

TBM launch

4.7.2 The design and risk control of launching the TBM is beyond the scope of a hydrogeological risk assessment except to note that the drive face where the TBM would be launched is at the boundary between superficial deposits and the Chalk. The Chalk bearings are likely to occur here and are characterised by high hydraulic conductivity and porosity.

4.7.3 At launch, the TBM would pass through a Contractor designed, fit-for-purpose, concrete tunnel eye and synthetic launch seal. This would be arranged in such a way that the operating parameters of the TBM can be tested, controlled and regulated before the TBM advances into virgin soil. The launch seal would prevent pressurized slurry loss back into the launch pit, and also prevent groundwater ingress along the annular space between the TBM shield and the excavated surface. Launch seals are usually manufactured to suit the selected TBM, and are not governed by any particular BS or DIN Standard. Their primary purpose is to facilitate TBM launch by containment of the pressurized slurry circuit. In the event that problems are encountered, the risk of slurry and/or groundwater ingress to the launch pit is wholly mitigated by turning off the slurry pumps. Where necessary, bentonite and/or polymers can be added to alter the engineering properties of the slurry at launch (only), and/or ground stabilization methods (i.e. grouting) can be used to stabilize a zone immediately in front of the TBM. This would aid TBM launch and further contain the slurry circuit, without which it should be noted, the TBM is unable to advance, and unable to commence tunnelling in the normal way.

Cross-connection of aquifers

4.7.4 The TBM would be a closed loop system that would minimise groundwater coming back into the launch pit.

4.7.5 The tunnel would pass through overburden glacial deposits into chalk and out of chalk back into glacial deposits. However, if those glacial deposits are granular then there is already hydraulic conductivity between the two. During construction the annulus between the tunnel segments and surrounding ground would be grouted thereby preventing flowpaths for water. The impact of tunnel excavation on the chalk surrounding the tunnel is expected to be minimal and limited to the immediate environs of the tunnel location. At this stage it is anticipated that the tunnel would be flooded with saline water. There is some possibility of a net inflow or outflow into or out of the tunnel due to minor leakage in the tunnel lining depending on internal and external pressures, but this is likely to be very limited.

4.7.6 During the construction of the pipeline there would be a short time when the tunnel is open. As a result there is a low risk of flooding if, in the unlikely event the tunnel was to collapse. However, the risk of a tunnel collapse would be very low as an appropriate tunnel design would be developed (taking account ground conditions and ground water) using best practice tunnel methods. It is envisaged that the tunnel would be constructed from concrete lined segments and the design would be subject to an independent validation prior to construction. In



addition, detailed analysis would be undertaken to confirm the minimum of the tunnel depth under the estuary (industry good practice typically requires an absolute minimum of 2 tunnel diameters (6m)). With design it is concluded that the risk of a tunnel collapse, during the short construction period, is very low.

Operational phase

4.7.7 Upon completion of the installation of the pipeline, the constructed tunnel would be flooded (using sea water), to protect the pipe from corrosion, and sealed at both ends. Therefore, once the pipeline has been completed the tunnel would not operate as a conduit for groundwater.

4.7.8 Regional groundwater flow within the Chalk aquifer is towards the northeast i.e. parallel with the tunnel alignment. Given the extremely small cross sectional area of the tunnel compared to the aquifer it is highly unlikely that the tunnel would act as a barrier to groundwater flow.

4.7.9 As the tunnel would be sealed with concrete lined segments which would be grouted to the rock, there would be very limited potential for the tunnel to act as a conduit for groundwater flow when the works have been completed.

4.7.10 During the excavation of a tunnel, short term ground settlements would occur due to a loss of material volume at the face and around the annulus. Settlement analysis has been undertaken by Capita (2014c). The results indicate maximum settlement values on the order 3 cm. Settlements of this magnitude are considered unlikely to have any significant impacts on the water environment. The effect these settlements may have on the overlying structures would depend on the sensitivity of the structures.

4.8 Numerical model setup

4.8.1 A detailed description of numerical model setup and results of modelling are presented in Section 19. A summary is provided below.

4.8.2 A 2D (long section) groundwater flow model was developed parallel to the proposed tunnel alignment and the predominant groundwater flow direction. The model has been used to assess the potential effects of construction dewatering on aquifer groundwater levels. Two additional 2D (cross section) groundwater flow models were also produced perpendicular to the tunnel and predominant groundwater flow pathway to assess the effects of construction on water features. Because the 2D sections cannot consider 3D flow effects they tend to overestimate the magnitude and extent of drawdown, the models are therefore conservative.

4.8.3 The 2D models were constructed to replicate the cross sections and hydraulic parameters presented in the conceptual model descriptions, above. The models were initially constructed using parameters estimated from site specific geotechnical, geological and hydrogeological data. Rainfall recharge was not applied to the model.

4.8.4 Groundwater levels derived from initial model runs were then compared against measured groundwater levels taken from a series of monitoring wells installed at



Goxhill and Paull (including L02, L06, L08, L15 and L18), that were chosen based on reliability of data (see Paragraph 4.2.56). Input parameters were then arbitrarily adjusted using sensitivity analysis until a good fit between modelled and actual groundwater levels were obtained (see Section 19.7). The final hydraulic parameters adopted for use in the models are presented in Table 4-9.

4.8.5 Calibration required the following adjustments:

An increase in the hydraulic conductivity (up to one order of magnitude) for superficial deposits. This would seem reasonable given that initial input parameters were based on PSD data.

A reduction in the hydraulic conductivity of the Chalk units. However, this seems reasonable, as site derived data is from a Goxhill PWS pumping test and it is noted that transmissivity tends to reduce in a northerly direction as the Chalk units become progressively confined. This is in agreement with modelling work undertaken by AMEC (2012a).

Estimation of the coefficient of compressibility for the majority of units, given the lack of data. Estimates were based on typical values for the strata type considered (Terzaghi et al., 1996).

4.8.6 Once calibrated, the potential effects of the Scheme were modelled by:

Simulating dewatering conditions without engineering controls; and

Simulating dewatering conditions with engineering (secant piles) controls.

4.8.7 It is noted that the cross section models for Paull and Goxhill predict a larger drawdown response than is evident in the long section model. It is noted that these cross sections are not fully calibrated (due to a lack of data off the tunnel alignment) and are perpendicular to groundwater flow, and likely overestimate the drawdown effects. The results however support the adoption of an engineered solution to limit drawdown effects in the superficial deposits.

4.8.8 It is recognised that the numerical groundwater model is based on site specific data and estimates (where data is limited or absent). It will therefore be an important next step to review the model when new information becomes available, such as that derived from future proposed pumping testing.

4.8.9 It is further recognised that the model has not been directly used to assess the effects of adopting positive relief wells to prevent heave/uplift. However, the model assumes an instantaneous reduction in pit water level to final dewatering level at the outset of dewatering and that inflow to the pit is controlled by pressure relief at/close to the pit base.



Hydrogeological Unit

Initial Input Parameters Goxhill Paull

Hydraulic Conductivity

(m/s)


(kPa-1)

Storage Coefficient

(m3/m3)

Adopted Hydraulic

Conductivity (m/s)

Adopted Coefficient of

Compressibility (kPa-1)

Adopted Storage

Coefficient (m3/m3)

Adopted Hydraulic

Conductivity (m/s)

Adopted Coefficient of

Compressibility (kPa-1)

Adopted Storage

Coefficient (m3/m3)

Estuarine Alluvium 2.0x10-6

n/a 0.4 2.0x10-5

1.0x10-6

0.3 2.0x10-5

1.0x10-6

0.3

Glacial deposits (sandy gravelly clay)

7.0x10-8

2.3x10-4 0.21 2.0x10-7

2.4x10-4

0.21 3.0x10-7

1.0x10-6

0.21

Glacial deposits (sandy gravel) 1.0x10

-4 n/a 0.21 n/p n/p n/p 7.0x10

-4 5.0x10

-7 0.21

Alluvium 7.0x10-6

1.1x10-3

0.4 7.0x10-5

5.0x10-3

0.4 9.0x10-6

2.5x10-5

0.4

Chalk Bearings 1.0x10-2

n/a 0.056 6.5x10-3

2.5x10-5

0.02 6.5x10-3

2.5x10-5

0.02

Fractured Chalk 5.2x10-4

n/a 0.005 to 0.025

5.2x10-5

0* 0.025 5.2x10-6

0* 0.025

Un-fractured Chalk 2.9x10

-6 n/a 2.9x10

-7 0* 0.005 2.9x10

-7 0* 0.005

n/a – none available n/p – not present *Assumed incompressible

Table 4-9 Adopted hydraulic parameters used in the 2D model



5 Step 3: Identify all potential water features that are susceptible to flow impacts

5.1 Drive Pit

5.1.1 The drive pit construction details and hydrogeological conceptual model indicate that the majority of groundwater entering the drive pit during excavation would come from the Chalk bearings and fractured Chalk. Since these layers are confined and due to the relatively short duration of groundwater control, it is likely that most of this water would come from aquifer storage and interception of groundwater flow through the aquifer.

5.1.2 This conceptual understanding is supported by results from the numerical model. Figure 5-43 shows groundwater velocity vectors during dewatering of the drive pit with the proposed groundwater control (secant piles to -26m OD).

Figure 5-43 Groundwater velocity vectors during dewatering of the drive pit with the proposed groundwater control

5.1.3 Flow impacts on the River Humber are considered very unlikely due to its tidal nature and the large volume of water within the estuary compared to the likely dewatering volumes.

5.1.4 East Halton Beck is the closest surface water feature that may be susceptible to flow impacts as shown on Figure 5-44.




5.2 Reception pit

5.2.1 The reception pit construction details and hydrogeological conceptual model indicate that the majority of groundwater entering the drive pit during excavation would come from the Chalk bearings and Glacial sand and gravel. Since these layers are highly porous and due to the relatively short duration of groundwater control, it is likely that most of this water would come from aquifer storage and interception of groundwater flow through the aquifer.

5.2.2 This conceptual understanding is supported by results from the numerical model. Figure 5-45 shows groundwater velocity vectors during dewatering of the drive pit with the proposed groundwater control (secant piles to -18.7 m OD).

Well East Halton

Halton Skitter Haven

East Halton saltmarsh

Flood levee

Chapel Farm Staveley House

Brockhampton

Spring Farm

East Marsh Farm pond

Fir Tree Farm borehole

East Halton Beck

Old Coastguard Station

Elm Tree Farm

Fir Tree Farm slurry pit

Loe Risby Farm


WWII Bomb decoy site

Horsegate Farm



Figure 5-45 Groundwater velocity vectors during dewatering of the reception pit with the proposed groundwater control

5.2.3 Again, impacts on the River Humber are considered very unlikely due to its tidal nature and the large volume of water within the estuary compared to the likely dewatering volumes.



5.2.4 Thorngumbald Drain is the closest surface water feature that may be susceptible to flow impacts as shown on Figure 5-46.

Figure 5-46 Water features around the reception pit highlighting Thorngumbald drain

Boreas Hill Farm pond

Paull Holme Strays

Flood levees

Dem’s Wood pond

Paull Holme moat & tower

Thorngumbald drain

South Pasture drain

Reception pit

Fort Paull Battery

High Paull, Cliff

Paull

Cow Hill landfill

Paull Holme Quarry (landfill)

Unnamed well/spring




6 Step 4: Apportion the likely flow impacts to the water features

6.1 Groundwater abstraction flow rates

6.1.1 Average groundwater abstraction flow rates from each pit and for various groundwater controls are listed in Table 6-10.

Scenario Drive Pit

(m3/d)

Reception Pit

(m3/d)

Drive Pit Shallow

(m3/d)

No piles 190 165 -

Proposed pile solution

70 149 9

Full depth pile solution (base of fractured chalk)

35 7 -

Table 6-10 Average groundwater abstraction flow rates for various scenarios

6.1.2 These values do not include pumping to remove direct rainfall or runoff into the pit.

6.2 Baseflow impacts

6.2.1 It is evident from the potential drawdown response that groundwater levels would be drawn down below surface water features. The cross section models indicate that the Thorngumbald Drain, South Pasture Drain and East Halton Beck receive groundwater contribution under baseline conditions. Estimates of groundwater inflow rates into these watercourses are presented in Table 6-11. Values are presented as rates per metre length of watercourse.

Surface Water Feature

Stream Bed Level

(m OD)

Groundwater Inflow Rate (Baseline)

(m3/day/m)

Groundwater Inflow Rate (Proposed

Groundwater Control)

(m3/day/m)

Thorngumbald Drain -1.5 0.39 -0.1

East Halton Beck 0.0 0.04 0.004


6.2.2 Whilst the baseline value for baseflow predicted by our model is an order of magnitude below the values predicted from the regional groundwater model (AMEC, 2012a) it should be remembered that there was very little flow gauge



data available to calibrate the regional model and the model is considered to significantly overestimate flows in East Halton Beck (AMEC, 2012b)

6.2.3 With the proposed groundwater control, baseflow to East Halton Beck is reduced by an order of magnitude. Negative values for Thorngumbald Drain indicate that this watercourse may temporarily lose water to underlying strata.

6.2.4 With the revised groundwater control (deeper piles) the effect on groundwater impacts on the Thorngumbald Drain, South Pasture Drain and East Halton Beck is reduced and some baseflow is expected to be maintained. Estimates of groundwater inflow rates into these watercourses are presented in Table 6-12.

Surface Water Feature

Stream Bed Level

(m OD)

Groundwater Inflow Rate (Baseline)

(m3/day/m)

Groundwater Inflow Rate (Proposed Solution)

(m3/day/m)

Groundwater Inflow Rate (Full Depth Solution)

(m3/day/m)

Thorngumbald Drain

-1.5 0.39 -0.1 0.15

East Halton Beck 0.0 0.04 0.004 0.03




7 Step 5: Allow for the mitigating effects of any discharges, to arrive at net flow impacts

7.1.1 The Scheme as proposed includes discharge of groundwater from the pits and tunnel to the River Humber.

7.1.2 For the drive pit and tunnel this would occur on the estuary side of the sluice gate at the mouth of East Halton Beck. This has been requested by the EA in order to help alleviate siltation problems at the mouth of East Halton Beck. For the reception pit water would be discharged on the estuary side of the flood levee.

7.1.3 However, a revised groundwater control that effectively seals out the fractured Chalk could potentially be used to reduce flow impacts as presented in Section 6 (Step 4) above.



8 Step 6: Assess the significance of the net flow impacts

8.1 Drive Pit

Groundwater resources

8.1.1 Entec (2011) undertook water balance calculations for the area between Barrow upon Humber and Grimsby where the Chalk is confined by superficial deposits (Immingham water balance domain). This encompasses the proposed drive pit.

8.1.2 The area of the Immingham water balance domain defined by Entec (2011) is 202.68 km2.

8.1.3 Recharge to the superficial deposits is calculated to be 37.2 Ml/d (37,200 m3/d) and all of this is considered to discharge to surface water via springs and baseflow.

8.1.4 Combined inflow to the Chalk is 87.4 Ml/d (87,400 m3/d). This is largely comprised of 71.2 Ml/d (71,200 m3/d) groundwater flow from the unconfined Chalk to the west and 15.9 Ml/d recharge where the Chalk is concealed but not confined by glacial deposits.

8.1.5 Outflow from the Chalk is largely comprised of 77.5 Ml/d (77,500 m3/d) by abstraction and 8.7 Ml/d (8,700 m3/d) discharge to surface water via springs and baseflow. The remainder is made up of outflow to other formations and net outflow of groundwater to the estuary.

8.1.6 In comparison, the groundwater abstraction rate of 68 m3/d for the proposed groundwater control for is considered to be negligible in the overall water resource status, especially as the abstraction would take place in March and April when water resources are less stressed after winter recharge.

East Halton Beck

8.1.7 East Halton Beck is classified as a heavily modified waterbody. It is currently designated as being at poor ecological potential with the target to achieve good ecological potential by 2027. As a freshwater body this assessment is based on macro-invertebrates, fish and a range of chemical parameters. The EA online Water Framework Directive mapping states that the chemical status of East Halton Beck does not require assessment. A summary of the Water Framework Directive assessment for East Halton Beck is listed in Table 8-13.



Water Framework Directive Parameter

Status

Hydrology (Flow) Moderate

Invertebrates Moderate

Fish Poor

Ammonia Good

Dissolved Oxygen High

pH High

Phosphate Moderate

Temperature High

Overall Ecological Potential

Poor

Table 8-13 Summary of the Water Framework Directive assessment for East Halton Beck

8.1.8 Under the proposed groundwater control (piles to -26 m OD) the model predicts an order of magnitude reduction of baseflow to East Halton Beck. Whilst this is unlikely to be acceptable to the EA on a permanent basis, because of the short duration (41 days) of the groundwater abstraction it is considered there would be negligible impact long term on the objective of achieving good ecological potential by 2027. Rather this objective is likely to be achieved by a combination of catchment sensitive farming and reducing large consumptive abstractions.

8.2 Reception Pit

Groundwater

8.2.1 The north Humber bank is covered by Hull and East Riding CAMS (EA, 2013b). The resource status is ‘water available’ at very low flows (Q95) and higher flows i.e. consumptive abstraction is available at least 95% of the time. It is not clear from the CAMS document how Q95 conditions would be assessed as there are no Assessment Points (APs) within the Water Resource Unit.

8.2.2 The groundwater abstraction rate of 149 m3/d for the proposed groundwater control for is considered to be negligible in the overall water resource status, especially as the abstraction is only planned to take 35 days (from 29/05/2018 – 02/07/2018).

8.2.3 In accordance with the CAMS strategy for this catchment it is recommended that baseline groundwater levels are monitored and compared to regional



groundwater levels. This can be used to ensure that abstraction does not occur when groundwater levels in the chalk aquifer are at very low levels.

Thorngumbald Drain

8.2.4 Thorngumbald drain is classified as an artificial waterbody. It is currently designated as being at moderate ecological potential with the target to achieve good ecological potential by 2027. A summary of the Water Framework Directive assessment for Thorngumbald drain is listed in Table 8-14.

Water Framework Directive

Status

Hydrology (Flow) Supports Good

Ammonia Good

Dissolved Oxygen Poor

pH High

Phosphate Poor

Temperature High

Overall Ecological Potential

Moderate

Table 8-14 Summary of the Water Framework Directive assessment for Thorngumbald Drain

8.2.5 Under the proposed groundwater control (piles to -18.7 m OD) the model predicts that Thorngumbald Drain would temporarily be subject to potential loss of water at a rate of 0.1 m3/d/m. Whether this would actually occur is dependent on the permeability of the channel base, which is not known.

8.2.6 Given the short duration of the groundwater abstraction for the Feeder 9 reception pit (35 days) it is considered there would be negligible impact on the objective of achieving good ecological potential by 2027.



9 Step 7: Define the search area for drawdown impacts

9.1 Drive pit

9.1.1 The radius of influence of dewatering during drive pit construction was estimated from 2D groundwater model outputs. The extent of the drawdown cone is highly dependent on whether an engineering (piling) control is implemented or not.

9.1.2 With no engineering controls (Figure 9-47) the radius of influence rapidly expands in a southerly direction and results in partial dewatering of the chalk bearings with full desaturation of the overlying glacial deposits, at approximately 1,200m. The model shows that partial desaturation of the glacial deposits extends further south (to the edge of the model). To the north, expansion of the radius of influence is controlled by water levels in the River Humber.

Figure 9-47 Drawdown response and indication of radius of influence at Drive Pit (no engineering controls)

9.1.3 With adoption of the proposed groundwater control, the associated radius of influence is significantly reduced in all directions. The Chalk Bearings and glacial deposits are no longer dewatered. The model indicates that the radius of influence extends to around 3.0 km when an engineered solution is adopted and greater than this with no engineering control in place (Figure 9-48). The predicted drawdown range at the drive pit varies from approximately 2.0 m to 3.0 m. The predicted drawdown range is taken from the long section (minimum drawdown) and cross section (maximum drawdown).

9.1.4 The model though assumes isotropic conditions in the glacial deposits and likely overestimates the radius of influence extent. The interlayered nature of the glacial



deposits (as described in the conceptual model above) would result in a reduction in the extent of the drawdown cone.

Figure 9-48 Modelled drawdown and radius of influence at Drive pit

9.2 Reception pit

9.2.1 The radius of influence of dewatering during reception pit construction has also been estimated from 2D groundwater model outputs. Again, the extent of the drawdown cone is highly dependent on whether an engineering (piling) control is implemented or not. Modelling shows that a zone of high permeability glacial deposits around the reception pit would need to be fully cased (piled) to minimise the effects of construction dewatering. A detailed understanding of the local geology and hydrogeology would therefore be needed to better estimate the extent of the radius of influence

9.2.2 With no groundwater control (Figure 9-49) the radius of influence rapidly expands in the direction of the high permeability glacial deposits (to the north). The lower permeability units of the glacial deposits limits the extent of the radius of influence in a southerly direction.

Drive Pit Humber Estuary

Predicted drawdown range



Figure 9-49 Drawdown response and indication of radius of influence at Reception pit (no engineering controls)

9.2.3 The radius of influence extends to over 2.0 km to the model (northern) boundary for both an engineered and non-engineered solution. No dewatering effects are evident to the south when groundwater control is adopted.

Figure 9-50 Modelled drawdown and radius of influence at Reception pit

Reception Pit Humber Predicted drawdown range



10 Step 8: Identify all features in the search area that could be impacted by drawdown

10.1 Introduction

10.1.1 Having defined the radius of influence of drawdown, this section identifies water features that could potentially be impacted by drawdown, such as abstractions, protected rights (e.g. boreholes used for agriculture), wetlands, springs and buildings that may be affected by settlement. Our search has involved publically available information, including:

Abstraction licence records held by the EA;

Records of domestic private water supplies from the relevant local authorities;

Records of boreholes and wells held by the British Geological Survey;

Ordnance Survey maps; and

Databases of conservation sites (www.magic.gov.uk).

10.1.2 Our search has focussed on water features within 2 km of the drive pit and reception pit since drawdown would be significantly greater within these areas.

10.2 Water features

10.2.1 Water features that are susceptible to drawdown impacts are shown in Figure 10-51 and Figure 10-52 and listed in Table 10-15 and Table 10-16.

10.2.2 BGS borehole records have been reviewed and a professional judgement made as to the likelihood of each borehole being still in use for water supply. For example the borehole at Fir Tree Farm was drilled in 2013 and a pumping test was undertaken, and consultation with the landowner has confirmed that this borehole is still in use for livestock. Borehole records have been discounted where it is clear that they have been drilled for geotechnical investigation purposes or where the record states (abandoned).



Figure 10-51 Water features around the drive pit

Figure 10-52 Water features around the reception pit

Well East Halton



Flood levee


Brockhampton

Spring Farm



East Halton Beck


Elm Tree Farm


Loe Risby Farm


Paull Holme Strays

Flood levees

Dem’s Wood pond


Thorngumbald drain

South Pasture drain

Reception Pit

Fort Paull Battery

High Paull, Cliff

Paull

Cow Hill landfill


Unnamed well/spring



Horsegate Farm



Name Potential impacts Distance from drive pit (m)

Information source


Derogation of protected right 306

BGS borehole record TA12SW151

WWII Bomb decoy

Mobilise potential contamination from fuel store 334

Chapter 8: Geology and Soils (DCO Document Reference 6.8)

Fir Tree Farm slurry pits

Mobilise potential contamination from slurry pits 366

Identified on OS 1:25k scale map

Loe Risby House



East Halton Beck

Fail to achieve Water Framework Directive objectives 764


Well East Halton Lincs



Elm Tree Farm Derogation of protected right 805


Spring Farm Derogation of protected right 813


Halton Skitter Haven / Goxhill



Flood Levee Settlement and increased flood risk 863


East Halton Saltmarsh Derogation of habitat 969

Identified on OS 1:25k scale map and www.magic.gov.uk

East Marsh Farm

Derogation of pond, amenity? 1001





Chapel Farm Derogation of protected right 1194

BGS borehole record TA12SW34/A/B/C/D

Horsegate Farm



Stavely House Derogation of private water supply 2260

Private water supply registered with NE Lincolnshire Council

Brockhampton Derogation of private water supply 2377

Private water supply registered with NE Lincolnshire Council

Table 10-15 Water features around the drive pit



Name Potential impacts Distance from drive pit (m)

Information source

South Pasture Drain Derogation of habitat 95


Cow Hill Landfill

Mobilise potential contamination 100

EA website http://maps.environment-agency.gov.uk/wiyby

Unnamed well/spring



Paull Holme Quarry (Landfill)

Mobilise potential contamination 211

EA website http://maps.environment-agency.gov.uk/wiyby

Dem's Wood pond

Derogation of amenity feature 253




Thorngumbald Drain

Derogation of habitat, increased flood risk 314


Paull Holme Strays Derogation of habitat 400


Paull Holme moated site and tower

Damage to scheduled monument 445

Identified on OS 1:25k scale map and www.magic.gov.uk


Derogation of amenity feature 597




Paull Derogation of protected right 1168

BGS borehole record TA12NE22

High Paull, Cliff


BGS borehole record TA12NE233

Fort Paull Battery

Derogation of private water supply, damage to scheduled monument 1386

Private water supply registered with East Riding Yorkshire Council, Identified on OS 1:25k scale map

Table 10-16 Water features around the reception pit



11 Step 9: For all these features, predict the likely drawdown impacts

11.1.1 A summary of the modelled changes to groundwater levels for the water features identified in Section 10 is given in Table 11-17 through Table 11-18, for a non-engineered and the proposed engineered solution at the drive pit and reception pit. The results are taken from the long section model which assumes equal radial flow in all directions and the cross section model which are considered to be more conservative (because the River Humber is not included in the 2D cross section models). The tabulated results therefore present a range in likely dewatering effects that are taken from the long section (least conservative) model and cross section model (most conservative) results.

11.1.2 The tabulated results indicate that the majority of water features on both sides of the Humber Estuary are affected by a lowering of groundwater levels caused by dewatering. Adoption of the proposed engineered solution minimises the drawdown effect, although some modelled drawdown at water interest sites is still evident.

11.1.3 Effects on water features located between the Humber Estuary and the drive pit are minimised by the importance of the estuary in providing a constant head boundary control. Between the Humber Estuary and the reception pit this effect is less significant because of the potential to encounter lower permeability deposits there.



Name Distance from

drive pit (m)

Non-Engineered Solution

Drawdown Effect (m)

Proposed Groundwater

Control Drawdown Effect (m)

Fir Tree Farm borehole* 306 4.3 to 10.6 0.4 to 3.1

WWII Bomb Decoy Site 334 6.4 0.7

Fir Tree Farm slurry pits* 366 4.9 to 9.7 0.3 to 2.2

Loe Risby House* 568 7.9 to 9.1 1.4 to 2.4

East Halton Beck* 764 4.2 to 7.8 0.0 to 0.9

Well East Halton Lincs* 778 4.2 to 8.9 0.0 to 2.2

Elm Tree Farm 805 2.9 0.0

Spring Farm 813 8.3 1

Halton Skitter Haven / Goxhill 844 2.3 0.0

Flood Levee 863 1.7 0.0

East Halton Saltmarsh 969 0.9 0.0

East Marsh Farm* 1001 6.0 to 6.3 0.3 to 1.5

Old Coastguard Station* 1041 2.0 to 8.7 0.0 to 2.1

Chapel Farm 1194 7.1 0.8

Horsegate Farm 1688 5.8 0.6

Stavely House 2260 4.8 0.4

Brockhampton 2377 4.6 0.4

*Drawdown range derived from long section and cross section model

Table 11-17 Summary of drawdown effects around the drive pit



Name Distance from

drive pit (m)


Drawdown Effect (m)



South Pasture Drain* 95 9.3 to 11.1 8.1 to11.1

Cow Hill Landfill* 100 11.4 to 15.6 10.9 to 15.5

Unnamed well/spring* 165 10.8 to 15.3 10.3 to 15.3

Paull Holme Quarry (Landfill)*

211 6.8 to 15.1 6.1 to 15

Dem's Wood pond* 253 10.2 to 14.6 9.7 to 14.5


Thorngumbald Drain* 314 1.8 to 7.1 1.7 to 7.0

Paull Holme Strays 400 2.1 2.0

Paull Holme moated site and tower*

445 8.9 to 14.1 8.5 to 14.0

Boreas Hill Farm pond 597 7.5 7.2

Flood Levee 887 2 1.9

Paull* 1168 1.1 to 2.4 0.9 to 2.4

High Paull, Cliff* 1270 0.7 to 5.9 0.6 to 5.9

Fort Paull Battery* 1386 0.5 to 5 0.3 to 5.0

*Drawdown range derived from long section and cross section model

Table 11-18 Summary of drawdown effects around the reception pit

11.1.4 The predicted drawdown effect on land drains within the affected area were considered using the results of the SEEP/W model. Land drains have previously been surveyed by Capita and in general have a bed level that is slightly lower than sea level, approximately -0.3 m OD. When compared to the predicted drawdown curves (Figure 11-53 and Figure 11-54) it can be seen that:

Land drains to the south and within 1 km of the drive pit have the potential to be affected during dewatering when the proposed (-26.0m OD) dewatering control is adopted; and

All land drains within the radius of influence of the reception pit have the potential to be affected during dewatering when the proposed (-18.5 m aOD) dewatering control is adopted.





11.1.5 The SEEP/W model was also used to consider the effects of groundwater mounding on the up-hydraulic gradient side of each of the pits. The mounding effect response was considered following completion of drive pit and reception pit

Drive Pit Humber

Affected Area

Humber Reception Pit

Affected Area



construction and allowing for steady state rebound conditions to be reached. The groundwater mounding response is presented in Figure 11-55 (drive pit) and Figure 11-56 (reception pit).

11.1.6 The results indicate that some groundwater mounding would occur up-hydraulic gradient of the drive pit and that no mounding occurs at the reception pit. Mounding is therefore limited by the permeability of the deposits in proximity to each pit (the deposits at the drive pit having lower permeability than at the reception pit).

11.1.7 The model estimates groundwater mounding of up to around 0.4 m above baseline conditions in close proximity to the drive pit. However, the model assumes no flow in an east-west direction and essentially no flow around the structure. In reality, the degree of mounding is therefore expected to be much less than predicted and likely to be negligible. As ground levels are greater than 2 m OD, any groundwater mounding caused by pit construction is not expected to increase flooding potential in the area.

Figure 11-55 Groundwater mounding response (up-hydraulic gradient) at drive pit

Drive Pit



Figure 11-56 Groundwater mounding response (up-hydraulic gradient) at reception pit

Reception Pit



12 Step 10: Allow for the effects of measures taken to mitigate the drawdown impacts

12.1.1 The results presented in Section 11 (Step 9) show that the effects of dewatering can be minimised by adoption of an engineered solution, although it is noted that some impact may still be evident. The 2D models were therefore updated to include a piled (engineering) solution at variable depths to assess pile depth on dewatering response. A summary of the modelled changes to groundwater levels for the water features identified is given in Table 12-19 and Table 12-20 for the proposed groundwater control and an engineered solution that effectively seals out the fractured component of the Chalk (at Goxhill) and the high permeability sand and gravel superficial strata (at Paull).

12.1.2 The model results on the Goxhill side of the Scheme indicated ‘diminishing returns’ with adoption of progressively deeper piles. However, where the fractured Chalk is effectively sealed out the extent of drawdown is reduced to less than 1.1 m. It would therefore be important to establish the thickness of the fractured chalk during pile design.

12.1.3 Model results on the Paull side of the Scheme show a substantial reduction in drawdown effects, effectively a result of sealing out of the high permeability superficial deposits.



Name Distance from drive pit (m)



Revised Groundwater


Fir Tree Farm borehole* 306 0.4 to 3.1 0.4 to 1.0

WWII Bomb Decoy Site* 334 0.7 0.0

Fir Tree Farm slurry pits* 366 0.3 to 2.2 0.2 to 0.4

Loe Risby House* 568 1.4 to 2.4 0.4 to 0.7

East Halton Beck* 764 0.0 to 0.9 0.0 to 0.3

Well East Halton Lincs* 778 0.0 to 2.2 0.3

Elm Tree Farm* 805 0.0 0.0

Spring Farm 813 1 0.1

Halton Skitter Haven / Goxhill 844 0.0 0.0


East Halton Saltmarsh 969 0.0 0.0

East Marsh Farm* 1001 0.3 to 1.5 0.0 to 0.2

Old Coastguard Station* 1041 0.0 to 2.1 0.2 to 0.6

Chapel Farm 1194 0.8 0.0

Horsegate Farm 1688 0.6 0.0

Stavely House 2260 0.4 0.0

Brockhampton 2377 0.4 0.0

*Drawdown derived from long section and cross section model




Name Distance from

drive pit (m)



Revised Groundwater


South Pasture Drain* 95 8.1 to11.1 0.5 to 0.6

Cow Hill Landfill* 100 10.9 to 15.5 0.8 to 1.1

Unnamed well/spring* 165 10.3 to 15.3 0.8 to 1.1

Paull Holme Quarry (Landfill)* 211

6.1 to 15 0.9

Dem's Wood pond* 253 9.7 to 14.5 0.8 to 1.1


Thorngumbald Drain* 314 1.7 to 7.0 0.0 to 0.7


Paull Holme moated site and tower* 445

8.5 to 14.0 0.8 to 1.0

Boreas Hill Farm pond 597 7.2 1


Paull* 1168 0.9 to 2.4 0.1 to 0.9

High Paull, Cliff* 1270 0.6 to 5.9 0.5 to 0.8

Fort Paull Battery* 1386 0.3 to 5.0 0.0 to 0.8

*Drawdown derived from long section and cross section model


12.1.4 The extent of drawdown around the reception pit is largely controlled by the modelled inclusion of a large zone of higher permeability (glacial) material that has not yet been fully characterised in the field. An additional model was therefore setup that looked at adoption of more uniform conditions attributable to lower permeability glacial deposits identified around the reception pit and when adopting the proposed pile scheme (Figure 12-57). The model results shows a significant reduction in drawdown with only localised effects at the reception pit face.

12.1.5 Improvements in the characterisation of the geology around the reception pit would therefore improve the understanding of dewatering effects around the reception pit.



Figure 12-57 Modelled drawdown with uniform lower permeability glacial deposits around the reception pit

12.1.6 With mitigation the extent of drawdown is reduced and the effects on land drains that traverse the landscape obviously minimised . As discussed in paragraph 11.1.4, land drains are typically encountered at an elevation of -0.3 m OD. It can be seen from the model results (Figure 12-58 and Figure 12-59) that:

land drains are not affected by dewatering at the drive pit when fully engineered groundwater controls are adopted; and

Land drains within a 800 m radius north of the reception pit may still be affected by construction dewatering even when groundwater control is adopted.

12.1.7 As discussed in paragraph 12.1.4, the drawdown extent at the recpetion pit is largely affected by the presence of high permeability sand and gravels deposits. Improved characterisation of these deposits is required to establish a better estimate of dewatering effects at the recpetion pit.





Drive Pit Humber



13 Step 11: Assess the significance of the net drawdown impacts

13.1 Introduction

13.1.1 According the HIA methodology (EA, 2007) the significance of the potential drawdown impacts should now be assessed. These can be described in terms of three categories, derogation of existing abstractors, environmental impacts on water bodies and wetlands, and settlement/desiccation. Taking each of these in turn:

13.2 Derogation of existing abstractors

13.2.1 The EA defines derogation as preventing a person entitled to a protected right from abstracting water to the extent authorised on their licence. Derogation includes cases where pumping water levels are lowered below the current pump intake, but increased pumping costs (which inevitably result from lower pumping water levels, because the pump is working against a greater head) do not qualify as derogation.

13.2.2 Table 13-21 lists the private water supplies and other potential abstractors around the drive pit including the depth of boreholes where recorded.



Name Depth (m) Proposed Groundwater Control Drawdown (m)

Revised Groundwater Control Drawdown Effect (m)


39.5 3.1 1.0

Loe Risby House unknown 2.4 0.4

Well East Halton Lincs

113.4 2.2 0.3

Elm Tree Farm unknown 0 0

Spring Farm unknown 1.0 0.1

Halton Skitter Haven / Goxhill

13.7 0 0


19.8 2.1 0.2

Chapel Farm Unknown 0.8 0

Horsegate Farm unknown 0.6 0

Stavely House unknown 0.4 0

Brockhampton unknown 0.4 0

Table 13-21 Private water supplies and other potential abstractors around the drive pit

13.2.3 There are no details available on pump depths for any of the boreholes, which could be used to assess whether the proposed engineered solution drawdown would cause derogation.

13.2.4 Most of the boreholes are likely to draw water from the Chalk aquifer and therefore would be at least 10 m deep. Since groundwater level in the Chalk aquifer is approximately 2 m bgl, a drawdown of at least 7 m (allowing 1 m for the pump above the base of the borehole) would be needed to draw the groundwater level below the base of the borehole.

13.2.5 Under the proposed groundwater control, drawdown of this magnitude is not predicted for any of the abstractions. Therefore, whilst the potential for derogation still exists if the pumps are at shallow depths, it is likely these impacts (if realised) could be mitigated by lowering pump(s), or deepening a borehole if necessary.

13.2.6 This assessment should be verified by a site-based water features survey i.e. visiting abstractors and measuring groundwater levels, pump depths and borehole depths where possible.

13.2.7 Under the revised groundwater control, drawdown is significantly reduced (maximum 1.1 m at Fir Tree Farm) and it is considered unlikely that there would be derogation.



13.2.8 Given the short duration of the potential impacts (41 days of groundwater abstraction at the drive pit) alternative mitigation measures such as providing bottled water or tankered supply could conceivably be implemented if necessary.

13.2.9 Table 13-22 lists the private water supplies and other potential abstractors around the drive pit including the depth of boreholes where recorded.

Name Depth (m) Proposed Engineered Solution Drawdown (m)

Revised Groundwater control Drawdown (m)

Unnamed well/spring Unknown 15.3

0.8

Paull 8.5 2.4 0.1

High Paull, Cliff 8.5 5.9

0.5

Fort Paull Battery Unknown 5

0.4

Table 13-22 Private water supplies and other potential abstractors around the reception pit


13.2.11 Under the proposed groundwater control some of the drawdown predictions are large e.g. at the unnamed well / spring located 165 m southeast of the reception pit and it is possible that this would cause derogation.

13.2.12 Under the revised groundwater control drawdown is significantly reduced and it this is less likely to cause derogation.

13.2.13 Again, given the short duration of the potential impacts (35 days of groundwater abstraction at the reception pit) alternative mitigation measures such as providing bottled water or tankered supply could conceivably be implemented if necessary.

13.3 Environmental impacts on water bodies and wetlands

13.3.1 Around the drive pit, East Halton Saltmarsh (part of the Humber Estuary SSSI) is potentially subject to drawdown. There is also a pond at East Marsh Farm which may also be adversely impacted if groundwater levels were to be lowered. Table 13-23 lists the drawdown effects for these features.




Revised Groundwater


East Halton Saltmarsh 969 0 0






Revised Groundwater


East Marsh Farm* 1001 1.5 0.2

*Drawdown derived from cross section model and therefore conservative


13.3.2 Drawdown at East Halton Saltmarsh is predicted to be zero under both the proposed groundwater control and revised groundwater control. This is due to the River Humber, which effectively acts as a constant head boundary. Therefore no impacts are predicted.

13.3.3 Under the proposed groundwater control, groundwater level is predicted to be drawn down by 1.5 m beneath the pond at East Marsh Farm. Drawdown under the revised groundwater control is 0.2 m.

13.3.4 The significance of impact at East Marsh Farm pond would depend on whether the pond is fed by groundwater i.e. the elevation of the base of the pond in relation to groundwater level and the permeability of its base. These should be verified during the water features survey. The use and/or amenity value of the pond should also be verified in order to establish the overall impact.

13.3.5 Around the reception pit, there are four sites with protected rights that would experience drawdown as listed in Table 12-20.

Name Approx. Ground

elevation (m OD)


Drawdown Effect (m)

Revised Engineered Solution Drawdown

Effect (m)

Dem's Wood pond* 7 14.4 0.8


Paull Holme moated site and tower* 5 13.9 0.8

Boreas Hill Farm pond 5 4.9 0.4

*Drawdown derived from cross section model and therefore conservative


13.3.6 Baseline groundwater levels around the reception are approximately 1 m OD and the ground elevation of the above sites (except Paull Holme Strays) is well above that. Therefore drawdown of groundwater level at these sites would have negligible impact.

13.3.7 Whist groundwater level may be close to ground level at Paull Holme Strays, the water balance for this site is dominated by the tidal River Humber. Drawdown of



groundwater level would therefore have a negligible impact of the water balance. Potential impacts associated with settlement are discussed below.

13.4 Settlement

13.4.1 A design analysis has been undertaken to estimate the effects of dewatering on ground surface settlements. Nine locations and two dewatering levels were considered. Settlement estimates are summarised within Table 13-25. Design parameters used in the settlement analysis are based on the calibrated SEEP/W groundwater model and its numerical outcomes as presented in Section 4.8 and Section 19.

13.4.2 Due to the variability of the geology encountered in the ground investigation (Capita, 2014a) a number of design assumptions have been made and therefore upper and lower bound settlement predictions have been calculated to provide the range of estimated settlements.

13.4.3 Design assumptions are as follows:

Stiffness constant with depth;

Dewatering effects are instantaneous ;

Settlement is 1 dimensional;

Consolidation settlement stops occurring after 50% ground water recharge;

Change in water level is equal to increase in effective stress at full soil depth (typically 30 m deep);

Dewatering duration plus recovery is assumed to be 50 days; and

Beneficial suctions or negative pore pressures have not been considered resulting in conservative settlement estimation.

13.4.4 Settlement predictions should be used with caution due to the variability of the ground condition and derived settlement design parameters and sensitivity to analysis to changes in permeability.

Site Proposed groundwater control

Revised groundwater control

Lower estimate

(mm)

Upper estimate

(mm)

Lower estimate (mm)

Upper estimate (mm)

Thorngumbald Drain

400 1000 250 600

South Pasture drain

600 1500 300 700

Fort Paull Battery 60 140 20 60

Paul Holme moat and tower

600 1700 300 800

Drive Pit 150 350 100 200



Site Proposed groundwater control

Revised groundwater control

Lower estimate

(mm)

Upper estimate

(mm)

Lower estimate (mm)

Upper estimate (mm)


The range of ground water levels caused by tidal action within these areas exceeds the levels of dewatering required for the works. The stress changes that would occur due to dewatering are therefore within the ranges already 'experienced' by the ground and would therefore have negligible effect on ground movement.

Paul Holme Strays

Flood Levee - Goxhill

Flood Levee - Paull no.1

Flood Levee - Paull no.2

Table 13-25 Summary of settlement estimates

13.4.5 It is noted that the exploratory hole information shows interbedded sand and gravel layers with the upper alluvium and underlying cohesive glacial till. The presence of these layers would have the effect of increasing the rate of consolidation due to increased drainage paths i.e. layers typically bedded 5 m intervals in the alluvium and 2.5 m intervals in the cohesive glacial till. This has been taken into account in the calculations, whereby a high proportion of the total consolidation settlement would occur within the 50 day period allowed.

13.4.6 Moreover, consideration should be given to the potential of recharge of the soils through the interbedded granular layers from the adjacent River Humber.

13.4.7 The analysis indicates significant settlement around the Paull site. Settlement at Thorngumbald Drain, South Pasture drain, Fort Paull Battery and Paull Holme moated site is predicted to be between 2 cm and 1.7 m. Settlement of this magnitude is likely to have a significant impact on flood risk and potential structural damage to the buildings.

13.4.8 It should be noted that this settlement analysis is based on the calculation of drawdown (lowering of groundwater level) from the SEEP/W groundwater model. There is uncertainty about the hydrogeology around the reception pit, namely the thickness and extent of a body of permeable sand and gravel, and whether this is hydraulically connected to adjacent interbedded sand, silt and clay layers that are prone to settlement. The hydrogeological model and the settlement assessment is therefore based on a conservative interpretation of the available data, i.e. actual impacts are likely to be less than predicted by the model.

13.4.9 A Phase 2 ground investigation is planned which would seek to improve understanding of the hydrogeological behaviour and reduce uncertainty. It is also recommended that the Phase 2 ground investigation include a provision for strategic settlement monitoring before, during and after any pumping testing



undertaken, to provide an indication of the actual response to dewatering effects. This would be needed for 2 weeks before the test and for the same afterwards.

13.4.10 The data will subsequently be reviewed using a finite element model to gain a better understanding of the settlement potential caused by proposed dewatering. Following this, the HIA should be revised based on these new data.

13.4.11 Should the results from the Phase 2 investigation not reduce uncertainty to an acceptable level and/or the revised HIA still predicts significant settlement would occur around the reception pit, there are a number of engineering options that could be used to mitigate settlement impacts as described below.

Recharge wells and/or trenches

13.4.12 Artificial recharge may be a viable option to maintain groundwater levels and reduce settlement around key structures such as scheduled monuments. The concept of artificial recharge is that water is returned to the ground around the site to prevent groundwater levels falling below prescribed limits. The recharge water is usually the water abstracted by the groundwater control system, although other sources (e.g. mains water) are sometimes used (CIRIA C515).

13.4.13 Such schemes require detailed ground investigation, careful planning and monitoring. This is because there are number of potential problems and environmental risks with recharge wells and trenches such as clogging by sediment/biofouling and adverse changes in groundwater chemistry. If recharge wells or trenches are positioned too close to the dewatering system, recirculation may also be a problem, and no net benefit would be realised.

13.4.14 The operation of any form of recharge system would require consent from the EA.

Ground treatment

13.4.15 Grouting and ground freezing are two methods of ground treatment that could be used to reduce permeability at the base of the reception pit prior to excavation, thereby reducing drawdown of groundwater levels and associated settlement risks.

13.4.16 Controlled injection of grout using a grouting bit attached to the end of a drill stem could be used to create a ‘horizontal pile’. This process is called jet grouting and, in simple terms, replaces the sediment with an engineered layer with designed strength and permeability.

13.4.17 Ground freezing is a long established and effective method of ground stabilisation for temporary works (CIRIA C537). The process involves refrigeration of the ground to convert pore water to ice, binding together particles to reduce permeability and increase strength. Frozen conditions are achieved by circulating a cold medium through a series of closed system tubes that are positioned close enough to form an ice wall, which would be virtually impermeable. Freezing is a



temporary measure with the groundwater regime restored after thawing of the ice wall.

13.4.18 As with other techniques there are limitations and risks associated with ground freezing. Saline groundwater can affect the freezing point and strength of the frozen ground. Brine pockets can form and clay soils may not freeze leading to seepages. Ground movement can be caused by the freezing process including ground heave during freezing and consolidation after thawing. The application of ground freezing also requires detailed ground investigation, careful planning and monitoring.

Vertical Shaft Sinking Machine (VSM)

13.4.19 VSM are a relatively new technology that can be used to create a shaft without the need for lowering the groundwater table. The shaft is excavated by a cutting drum that can rotate through 360°, swivel up and down and is attached to a telescopic boom. Therefore the entire cross-section of the shaft can be excavated gradually. The excavated material is removed by a submersible pump and transported to the surface. The entire shaft structure including concrete rings can be held and lowered in a controlled manner during excavation. When the desired depth has been reached, the machine is recovered. Subsequently, the shaft bottom is sealed by an underwater concrete plug and the annular gap is filled up with grout, creating a frictional support locking the shaft in place. Once the slurry water is pumped off, the shaft is ready for further use



14 Step 12: Assess the water quality impacts

14.1 Discharge of abstracted groundwater

14.1.1 It is proposed to discharge groundwater abstracted during the dewatering phases to the Humber Estuary. For the drive pit this would occur on the estuary side of the sluice gate at the mouth of East Halton Beck. For the reception pit water would be discharged on the estuary side of the flood levee. There is potential for this discharge to adversely impact the water quality of the receiving water course. In order to assess this potential impact, groundwater chemical analyses from the ground investigation have been compared to Environmental Quality Standards (EQS) values for transitional waters (estuaries). Exceedances of the EQS values are listed in Table 14-26.

14.1.2 Three groundwater samples have been taken from selected boreholes. The first round of sampling was undertaken on 9th and 10th September 2014, the second round on 8th and 9th October 2014, and the third round on 22nd October 2014.

14.1.3 The results indicate that there are exceedances of sulphate, arsenic, copper, zinc and ammoniacal nitrogen EQS values. Sulphate is likely to be naturally occurring and derived from organic matter within the sediments. Most other exceedances appear to be localised, for example, groundwater within glacial deposits adjacent to the reception pit slightly exceeds the EQS value for copper at borehole L14 but is below the EQS value at borehole L15. Chalk groundwater is above the EQS values for arsenic and zinc at borehole L06 but below the EQS values at L02.

14.1.4 Boreholes L04 and L06 are screened within the Tidal flat deposits and both indicate groundwater consistently exceeds the EQS values for arsenic and copper.

14.1.5 The average arsenic value of three samples from L04 is 69 µg/l and the average from L06 is 105 µg/l. The EQS value for arsenic is 25 µg/l.

14.1.6 The average copper value of three samples from L04 is 21 µg/l and the average from L06 is 56 µg/l. The EQS value for copper is 5 µg/l.

14.1.7 The source of this metal contamination is likely to be the River Humber itself which suffers from industrial pollution.

14.1.8 Whilst groundwater within the tidal flat deposits significantly exceeds the EQS values for copper and zinc, the volume of water that would enter the drive pit from these deposits can reasonably be expected to be much less than the volume of water from the Chalk bearings and fractured Chalk. Therefore dilution would likely mean that the groundwater entering the drive pit would not exceed the EQS values for copper and zinc.



Borehole Cl‐ NO3 as N Phosphate SO42‐ As B Cd Cr Cu Pb Hg Ni Se Zn NH3‐N

EQS ‐ 250 25 ‐ 1.5 32 5 7.2 0.07 ‐ ‐ 40 21 Date Sampled Strata mg/l mg/l mg/l mg/l μg/l mg/l μg/l μg/l μg/l μg/l μg/l μg/l μg/l μg/l mg/l

10‐Sep‐14 Chalk bearings L01 66 <0.5 <0.5 28 11 0.03 <0.02 6 <0.5 <0.3 <0.05 4 2.3 4 0.07

09‐Oct‐14 Chalk bearings L01 56 <0.5 <0.5 26 7.3 <0.01 <0.02 3 1.1 0.7 <0.05 4 2.6 3 <0.05

22‐Oct‐14 Chalk bearings L01 56 <0.5 21 9.8 <0.01 0.03 3 1.1 <0.3 <0.05 4 2.2 5 0.07

10‐Sep‐14 Burnham Chalk L02/1 370 <0.5 <0.5 83 3.9 0.18 <0.02 5 <0.5 <0.3 <0.05 4 7.8 4 0.26

09‐Oct‐14 Burnham Chalk L02/1 450 <0.5 <0.5 96 2.7 <0.01 <0.02 4 2.9 1.4 <0.05 4 9.3 5 0.24

22‐Oct‐14 Burnham Chalk L02/1 320 <0.5 83 3.2 0.15 <0.02 4 1.9 0.5 <0.05 3 6.9 9 0.29

10‐Sep‐14 Burnham Chalk L04/1 860 <0.5 1.2 2.5 5.1 0.32 <0.02 4 1.6 <0.3 0.29 3 10 <2 3.5

08‐Oct‐14 Burnham Chalk L04/1 3300 <0.5 6.6 1.8 24 1.3 <0.02 10 12 <0.3 0.14 5 69 <2 16

22‐Oct‐14 Burnham Chalk L04/1 1400 <0.5 7.4 12 0.61 <0.02 7 3.4 <0.3 0.07 3 24 <2 6.6

10‐Sep‐14 Tidal flat deposits L04/2 5800 <0.5 9.1 0.5 64 3.7 <0.02 13 19 <0.3 <0.05 5 88 3 32

08‐Oct‐14 Tidal flat deposits L04/2 6200 <0.5 17 0.5 66 2.2 0.07 14 23 <0.3 0.46 7 93 4 33

22‐Oct‐14 Tidal flat deposits L04/2 5900 <0.5 0.5 78 3.3 0.04 17 22 0.6 <0.05 4 89 4 33

10‐Sep‐14 Burnham Chalk L06/1 1000 <0.5 <0.5 76 43 0.27 0.02 7 1.6 <0.3 <0.05 240 21 85 3.7

08‐Oct‐14 Burnham Chalk L06/1 1800 <0.5 <0.5 110 29 0.13 0.04 6 8.4 0.5 0.1 210 33 65 9.1

22‐Oct‐14 Burnham Chalk L06/1 2200 <0.5 140 32 0.42 0.03 6 14 <0.3 <0.05 130 41 39 12

10‐Sep‐14 Tidal flat deposits L06/2 11000 <0.5 11 1600 95 2 0.09 19 30 <0.3 <0.05 21 48 8 33

08‐Oct‐14 Tidal flat deposits L06/2 12000 <0.5 13 1600 110 2 0.09 17 47 <0.3 <0.05 20 56 9 33

22‐Oct‐14 Tidal flat deposits L06/2 11000 <0.5 1500 110 2.2 0.09 16 91 <0.3 <0.05 20 45 29 32

10‐Sep‐14 Glacial till L08 2100 <0.5 <0.5 1700 11 1.4 0.03 5 6 <0.3 <0.05 45 53 16 0.99

08‐Oct‐14 Glacial till L08 2300 <0.5 <0.5 1600 11 0.94 0.09 5 9.6 <0.3 0.14 29 48 31 0.9

22‐Oct‐14 Glacial till L08 2000 <0.5 1800 17 1 0.04 6 3.6 <0.3 <0.05 25 35 17 1.1

09‐Sep‐14 Flamborough Chalk L14/1 2500 <0.5 <0.5 550 13 0.76 0.03 6 <0.5 <0.3 <0.05 6 48 4 0.46

09‐Oct‐14 Flamborough Chalk L14/1 2000 <0.5 <0.5 360 9.4 0.35 0.05 4 7.4 0.4 <0.05 9 42 3 1.7

22‐Oct‐14 Flamborough Chalk L14/1 1900 <0.5 300 10 0.54 0.03 4 5.6 <0.3 <0.05 9 37 8 1.8

09‐Sep‐14 Glacial till L14/2 1900 <0.5 <0.5 490 11 0.73 0.04 6 5.5 <0.3 <0.05 4 39 3 3.6

09‐Oct‐14 Glacial till L14/2 2200 0.5 <0.5 520 9.2 0.52 0.04 5 9.6 3.7 <0.05 4 41 6 0.28

22‐Oct‐14 Glacial till L14/2 1700 1.2 470 9.1 0.71 0.05 5 8.2 <0.3 <0.05 4 32 9 0.25

09‐Sep‐14 Glacial sand & gravel L15/1 1500 <0.5 <0.5 88 10 0.41 0.02 6 1.7 <0.3 <0.05 6 27 2 2

09‐Oct‐14 Glacial sand & gravel L15/1 1500 <0.5 <0.5 98 7.3 0.14 <0.02 3 4.5 0.5 <0.05 6 31 2 2

22‐Oct‐14 Glacial sand & gravel L15/1 790 <0.5 90 6.5 0.91 <0.02 6 4.8 <0.3 <0.05 6 17 59 1

09‐Sep‐14 Glacial till L15/2 780 <0.5 <0.5 88 7.5 0.72 0.04 9 1.3 <0.3 <0.05 6 16 3 1

09‐Oct‐14 Glacial till L15/2 770 <0.5 <0.5 74 5.4 0.67 0.02 6 4.7 0.7 0.25 7 17 3 0.96

22‐Oct‐14 Glacial till L15/2 720 <0.5 95 7 0.88 <0.02 6 5 <0.3 <0.05 7 15 10 1.1

09‐Oct‐14 Flamborough Chalk L18/1 1400 <0.5 <0.5 200 7.6 0.1 0.03 4 4.1 0.4 <0.05 21 26 5 1.9

22‐Oct‐14 Flamborough Chalk L18/1 180 <0.5 490 3.9 0.23 <0.02 4 1.9 0.3 <0.05 4 5.7 5 0.34

09‐Sep‐14 Flamborough Chalk L18/1 980 <0.5 <0.5 270 7.5 0.27 <0.02 5 1.6 <0.3 <0.05 16 18 6 1.5

Exceedances highlighted in red text

Table 14-26 Summary of groundwater chemical analyses



14.2 Point sources

Landfill sites

14.2.1 The EA website3 shows that two landfill sites occur within the vicinity of the reception pit as reproduced in Figure 14-60.

Figure 14-60 Landfill sites near the reception pit

14.2.2 The EA website states that the landfill site named ‘South West Corner of Cow Hill’ last received waste in 1983 and contains inert waste, which is defined as “Waste which remains largely unaltered once buried such as glass, concrete, bricks, tiles, soil and stones”.

14.2.3 The EA website states that the landfill site named ‘Paull Holme Quarry’ is an authorised landfill i.e. is still active. This landfill site is licenced to accept type A5 waste, which is defined as “Non-Biodegradable Wastes”.

14.2.4 Figure 14-60 shows that ground elevation at these two landfill sites is approximately 5 m OD. Data from the ground investigation show that groundwater level in this area is approximately 1 m OD. Therefore groundwater level is likely to be below the base of these landfill sites and drawdown would not be expected to significantly increase mobilisation of any contaminants.

3 http://maps.environment-agency.gov.uk/wiyby

Reception pit



14.2.5 In view of the fact that these landfill sites are unlikely to contain significant contamination (i.e. they contain inert waste) the risks posed from drawdown of the groundwater level are considered to be negligible.

Other point sources

14.2.6 Two potential point sources of contamination have been identified near the drive pit. The first is Fir Tree Farm slurry pits located approximately 360 m northwest of the drive pit. The second is a World War II bomb decoy site that may have underground fuel storage tanks and is located approximately 330 m south of the drive pit. The locations of these sites are shown in Figure 10-51.

14.2.7 Potential contamination from Fir Tree Farm slurry pits includes microbiological parameters, nutrients (nitrogen, phosphate, potassium) and effluent with a high chemical oxygen demand (COD).

14.2.8 If the slurry pits are constructed with a low permeability lining such as clay or concrete then there is a very low risk that contamination could be mobilised by the dewatering of the drive pit.

14.2.9 If the slurry pits are constructed without a low permeability lining then it is likely that effluent is already entering into groundwater. Drawdown of groundwater level associated with dewatering the drive pit may increase the rate at which effluent drains from the pits into groundwater.

14.2.10 The low levels of nitrogen and phosphate in groundwater samples near the drive pit (Table 14-26) suggests that effluent from the slurry pits is not currently entering groundwater and by inference the pits are likely to be lined.

14.2.11 A requirement to confirm the construction details of the slurry pits including material, thickness and age of any lining should be included as part of the Initial Construction Environmental Management Plan (DOC Document Reference 7.3). If the details cannot be confirmed or the lining is not adequate to prevent significant leakage of effluent, the effluent within the slurry pits should be emptied and disposed at an appropriate facility. The slurry pits should not be used for the duration of the dewatering activities.

14.2.12 The main potential contamination from the World War II bomb decoy site is fuel hydrocarbons that may have leaked from underground fuel storage tanks (if these exist). It is recommended this site be investigated by intrusive methods (e.g. trial pits, exploratory holes) to assess potential sources of contamination.



14.3 Diffuse pollution

14.3.1 The EA website4 shows the locations of nitrate vulnerable zones (NVZs) near the drive pit and reception pit as reproduced in Figure 14-61.


14.3.2 The drive pit would be located adjacent to a surface water NVZ and approximately 800 m down hydraulic gradient of a groundwater NVZ. The reception pit would be located approximately 1.5 km downstream of a surface water NVZ.

4 http://maps.environment-agency.gov.uk/wiyby

Drive pit



14.3.3 Groundwater chemical analyses from the ground investigation indicate that nitrate concentrations are below detection limit in all but two analyses from borehole L14 near the reception pit.

14.3.4 Since the drive pit and reception pit are down hydraulic gradient of these NVZs baseline groundwater samples would be expected to contain elevated concentrations of nitrate if groundwater was actually high in nitrate.

14.3.5 Therefore there is a low risk that groundwater abstracted from the drive pit or reception pit during dewatering would contain significant concentrations of nitrate.

14.4 Dilution of poor quality surface water being adversely affected

East Halton Beck

14.4.1 A summary of the Water Framework Directive assessment for East Halton Beck is listed in Table 8-13.

14.4.2 Reducing groundwater input (baseflow) to East Halton Beck may have adverse impacts on the water quality in East Halton Beck by reducing the dilution of contaminants in surface water.

14.4.3 Under the proposed groundwater control (piles to -26 m OD) the model predicts an order of magnitude reduction of baseflow to East Halton Beck. Whilst this is unlikely to be acceptable to the EA on a permanent basis, because of the short duration (41 days) of the groundwater abstraction it is considered there would be negligible long term impact on the objective of achieving good ecological potential by 2027.

14.4.4 The proposed baseline water quality monitoring would establish current status of East Halton Beck and Phase 2 ground investigation would enable improved assessment of potential impacts. If required the revised groundwater control could be implemented to ensure impacts are reduced to negligible.

Thorngumbald Drain

14.4.5 Thorngumbald Drain is classified as an artificial waterbody. It is currently designated as being at moderate ecological potential with the target to achieve good ecological potential by 2027. A summary of the Water Framework Directive assessment for Thorngumbald drain is listed in Table 8-14.

14.4.6 Under the proposed groundwater control (piles to -18.7 m OD) the model predicts that Thorngumbald Drain would temporarily be subject to potential loss of water



at a rate of 0.1 m3/d/m. Whether this would actually occur is dependent on the permeability of the channel base, which is not known.

14.4.7 Given the short duration of the groundwater abstraction for the (35 days) it is considered there would be negligible long term impact on the objective of achieving good ecological potential by 2027.

14.4.8 The proposed baseline water quality monitoring would establish current status of East Halton Beck and Phase 2 ground investigation would enable improved assessment of potential impacts.

14.5 Saline intrusion

14.5.1 A potential impact of dewatering is a temporary change to the saline water interface. The calibrated SEEP/W numerical model has therefore been used to show how the saline interface is likely to change as a result of dewatering. The numerical model was run with a line of particles at the base of the estuarine alluvium and glacial deposits beneath the Humber Estuary to simulate the movement of saline water under baseline, non-engineered and engineered dewatering conditions, for comparison. The results shown in Figure 14-62 and Figure 14-63 indicate the following:

The exchange of saline water between the River Humber, the estuarine alluvium and the Chalk under baseline conditions i.e. when no dewatering is undertaken (Diagram A);

Saline water is drawn towards the drive pit and reception pit through the higher permeability chalk bearings, during the construction period and when no piles are used (Diagram B and C);

Particles drawn towards the drive pit and into the fractured chalk during dewatering start to track back into the chalk bearings (area around drive pit in Diagram C);

Movement of saline water towards the drive pit is limited when groundwater control is adopted (Diagram D);

As the proposed pile depth for the reception pit does not case out the higher permeability glacial sand and chalk bearings, migration of saline water towards the reception pit is still evident (Diagram E); and

Construction of a hydraulic barrier that effectively seals out the glacial sands and chalk bearing deposits is simulated in Diagram F. It can be seen that the movement of the saline interface is significantly reduced when compared to construction with the proposed pile depth.



A. Baseline condition (simulation run time - 1 year)

B. No piles condition (simulation run time - at end of Drive Pit dewatering phase)

C. No piles condition (simulation run time - at end of Reception pit dewatering phase)

Figure 14-62 Effect of dewatering on saline interface (particle tracking) – no groundwater control



D. Proposed groundwater control, piles to -26 m OD (simulation run time - at end of Drive Pit dewatering phase)

E. Proposed groundwater control, piles to -18.7 m OD (simulation run time - At end of Reception pit dewatering phase)

F. Revised groundwater control, piles to -35 m OD (simulation run time - At end of Reception pit dewatering phase)

Figure 14-63 Effect of dewatering on saline interface (particle tracking) – with groundwater control



15 Step 13: If necessary, redesign the mitigation measures to minimise the impacts

15.1.1 A revised groundwater control (effectively sealing out the fractured Chalk aquifer) has been assessed within earlier steps of this HIA.

15.1.2 Below is a summary of additional mitigation measures.

15.1.3 Using sulphate resistant cement for piles, base slab and ground anchors as groundwater has a high sulphate content.

15.1.4 Install recharge wells/trenches to counteract drawdown at sites potentially affected by settlement e.g. flood levees. Groundwater level monitoring at these sites should be used along with predefined trigger levels that initiate recharge.

15.1.5 A requirement to confirm the construction details of the Fir Tree Farm slurry pits including material, thickness and age of any lining should be included as part of the Initial Construction Environmental Management Plan (DCO Document Reference 7.3). If the details cannot be confirmed or the lining is not adequate to prevent significant leakage of effluent, the effluent within the slurry pits should be emptied and disposed at an appropriate facility. The slurry pits should not be used for the duration of the dewatering activities.

15.1.6 The main potential contamination from the World War II bomb decoy site is fuel hydrocarbons that may have leaked from underground fuel storage tanks (if these exist). It is recommended this site be investigated by intrusive methods (e.g. trial pits, exploratory holes) to assess potential sources of contamination.

15.1.7 Discharge of abstracted groundwater to groundwater dependant features could be considered to minimise flow and/or level impacts. However, baseline monitoring is needed to establish water quality of receiving watercourse(s).



16 Step 14: Develop a monitoring strategy

16.1 Baseline monitoring

16.1.1 A site-based water features survey should be undertaken to verify the location and construction details of boreholes, springs, ponds and other features susceptible to impacts from dewatering.

16.1.2 Groundwater levels should continue to be monitored adjacent to the proposed drive pit and reception pit locations. In addition, groundwater levels should be monitored at water features identified as being susceptible to drawdown impacts.

16.1.3 Flow gauging should be undertaken along East Halton Beck and Thorngumbald Drain in order to characterise the baseline flow regime in these watercourses. The location(s) and method(s) of flow gauging should be determined by a reconnaissance survey.

16.1.4 Baseline water chemical analyses should continue to be undertaken in order characterise groundwater quality including potential contaminants.

16.1.5 Baseline water chemical analyses including potential contaminants should be undertaken to characterise East Halton Beck and Thorngumbald Drain.

16.2 Construction phase monitoring

16.2.1 Groundwater level monitoring, flow gauging and water chemical analyses as described above should be continued into the construction phase.

16.2.2 Groundwater abstraction rates (and subsequent discharge rates) from the drive pit and reception pit should be closely monitored.

16.2.3 Periodic review of the above data should be conducted to ensure system performance and identify any potential for changes in impacts. As a minimum this should include:

Weekly review of groundwater levels and in-situ water quality data (pH, temperature, conductivity) in key boreholes; groundwater abstraction rates (instantaneous and cumulative); flow gauge data; and

Fortnightly review of laboratory water chemical analyses including potential contaminants.

16.3 Post-construction monitoring

16.3.1 The monitoring described above should be continued following construction.

16.3.2 A monthly review of data should be undertaken for the first six months post construction period, followed by quarterly reviews thereafter.



17 Overall Summary 17.1.1 National Grid Gas is proposing to construct and operate a replacement high

pressure transmission gas pipeline beneath the River Humber from Goxhill to Paull.

17.1.2 This document describes the findings of a hydrogeological impact assessment following relevant EA guidance. A summary of the findings for each step is provided below.

Step 1: Establish the regional water resource status

17.1.3 At the drive pit the CAMS status is no water available for abstraction except at extremely high flows. At the reception pit, consumptive abstraction is available at least 95% of the time i.e. except during very dry periods). Both groundwater bodies have been determined to be at risk of failing to meet Water Framework Directive objectives to be at good status by 2015.

Step 2: Develop a conceptual model for the abstraction and the surrounding area

17.1.4 The conceptual model is based on all currently available geological and hydrogeological data sources for the Chalk and superficial deposits. The groundwater contamination with respect to saline intrusion and other potential sources of groundwater contamination have been addressed. The current groundwater level and any variations in the groundwater level in the superficial deposits and the Chalk have been identified. Interaction between surface waters and groundwaters have been assessed.

17.1.5 The Chalk underlies the tunnel route and is confined by superficial deposits including glacial sediments and estuarine deposits. Most groundwater flow is thought to occur in the top approx. 40 m of fractured Chalk and an overlying layer called “Chalk bearings”. The drive pit would be wedge shaped in long section with the deepest part penetrating the Chalk bearings and fractured Chalk. The reception pit would be constructed entirely within glacial sediments, although there is uncertainty in the sediment type and its hydraulic properties that would be encountered.

17.1.6 Tidal fluctuations in groundwater level and chloride concentrations indicate the superficial deposits and Chalk are in hydraulic connection with the River Humber.

17.1.7 Within the drive pit and reception pit, groundwater control is likely to be achieved by combining four approaches; cut-off walls (secant and sheet piling), deep well dewatering, sump pumping and passive relief wells within the base of the pit.

17.1.8 Prior to excavation of the pits, piles would be installed around the perimeter and to a depth designed to minimise groundwater seepage into the pits and then deep wells installed.

17.1.9 The proposed groundwater control for the deep section of the drive pit includes installation of secant piles to a depth of approximately 28 m. It would take an estimated 41 days to complete the excavation and cast a base slab to effectively



seal the pit, after which the dewatering would be stopped and groundwater levels outside the pit would be allowed to recover.

17.1.10 The proposed toe level for the secant piles is -26 m OD for the drive pit and -18.7 m OD at the reception pit. Some of the impacts associated with this proposed groundwater control may be unacceptable to the EA. Therefore a revised groundwater control is assessed which includes pile depths which effectively seals out the fractured Chalk at the drive pit and permeable sand and gravels at the reception pit (as discussed in Step 10 and Step 11).

Step 3: Identify all potential water features that are susceptible to flow impacts

17.1.11 East Halton Beck is susceptible to flow impacts during drive pit construction and Thorngumbald Drain on the reception pit side. Groundwater abstraction could potentially adversely impact the resource status in the Lincolnshire Chalk.

Step 4: Apportion the likely flow impacts to the water features

17.1.12 The pit construction details and hydrogeological conceptual model indicate that the majority of groundwater entering the pits during excavation would come from the Chalk bearings and fractured Chalk.

17.1.13 For the proposed pile depths average groundwater abstraction flow rates are predicted to be 70 m3/d from the drive pit and 149 m3/d from the reception pit. For the revised pile depths these are reduced to 35 m3/d and 7 m3/d, respectively.

17.1.14 With the proposed groundwater control, baseflow to East Halton Beck is reduced by an order of magnitude. Thorngumbald Drain may temporarily lose water to underlying strata.

Step 5: Allow for the mitigating effects of any discharges, to arrive at net flow impacts

17.1.15 The Scheme as proposed includes discharge of groundwater from the pits and tunnel to the River Humber, therefore no mitigation of this type is planned.

Step 6: Assess the significance of the net flow impacts

17.1.16 In comparison to large abstractions for public water supply and industrial use, the short duration and groundwater abstraction rate of 68 m3/d for is considered to have a negligible impact on the overall water resource status, especially as the abstraction would take place in March and April when water resources are less stressed after winter recharge.

17.1.17 Under the proposed groundwater control the model predicts an order of magnitude reduction of baseflow to East Halton Beck and slight loss of water from Thorngumbald Drain. Whilst this is unlikely to be acceptable to the EA on a permanent basis, because of the short duration of the groundwater abstraction it



is considered there would be negligible impact long term on the objectives of achieving good ecological potential by 2027.

Step 7: Define the search area for drawdown impacts

17.1.18 The model indicates that the radius of influence extends to between 2 and 3 km when the proposed pile depths are adopted at the drive pit.

17.1.19 Modelling shows that a zone of high permeability glacial deposits around the reception pit would need to be fully isolated (piled) to minimise the effects of construction dewatering.

Step 8: Identify all features in the search area that could be impacted by drawdown

17.1.20 Our search has focussed on water features within 2 km of the drive pit and reception pit since drawdown would be significantly greater within these areas. A number of water features have been identified from relevant records.

Step 9: For all these features, predict the likely drawdown impacts

17.1.21 Modelling indicates that the majority of water features on both sides of the Humber Estuary are affected by a lowering of groundwater levels caused by dewatering. Maximum drawdown adjacent to the drive pit is approximately 3 m whereas approximately 16 m of drawdown is caused adjacent to the reception pit.

17.1.22 Effects on water features located between the Humber Estuary and the drive pit are minimised by the importance of the estuary in providing a constant head boundary control.

Step 10: Allow for the effects of measures taken to mitigate the drawdown impacts

17.1.23 At the drive pit and for a revised solution where the fractured Chalk is effectively sealed out, the extent of drawdown is reduced to less than 1.0 m. It would therefore be important to establish the thickness of the fractured Chalk during pile design.

17.1.24 Model results for the reception pit show a substantial reduction in drawdown effects, effectively a result of sealing out of the superficial deposits.

17.1.25 The extent of drawdown around the reception pit is largely controlled by the modelled inclusion of a large zone of higher permeability (glacial) material that has not yet been fully characterised by ground investigation.

17.1.26 Improvements in the characterisation of the geology around the reception pit would therefore improve the understanding of dewatering effects around the reception pit.



Step 11: Assess the significance of the net drawdown impacts


17.1.28 It is very likely that boreholes used for water abstraction are likely to draw water from the Chalk aquifer and therefore would be at least 10 m deep. Under the proposed groundwater control, the Chalk aquifer remains saturated. Therefore, whilst the potential for derogation still exists if the pumps are at shallow depths, it is likely these impacts (if realised) could be mitigated by lowering pump(s), or deepening a borehole if necessary.

17.1.29 This assessment should be verified by a site-based water features survey i.e. visiting abstractors and measuring groundwater levels, pump depths and borehole depths where possible.

17.1.30 Given the short duration of the potential impacts (41 days of groundwater abstraction at the drive pit) alternative mitigation measures such as providing bottled water or tankered supply could conceivably be implemented if necessary.

17.1.31 Under the proposed groundwater control some of the drawdown predictions are large e.g. at the unnamed well / spring located 165 m southeast of the reception pit and it is possible that this would cause derogation.

17.1.32 Under the revised groundwater control, drawdown is significantly reduced and it this is less likely to cause derogation.

17.1.33 Again, given the short duration of the potential impacts (35 days of groundwater abstraction at the reception pit) alternative mitigation measures such as providing bottled water or tankered supply could conceivably be implemented if necessary.

17.1.34 The significance of impact at East Marsh Farm pond would depend on whether the pond is fed by groundwater i.e. the elevation of the base of the pond in relation to groundwater level and the permeability of its base. These should be verified during the water features survey. The use and/or amenity value of the pond should also be verified in order to establish the overall impact.

17.1.35 Analysis has been undertaken to estimate the effects of dewatering on ground surface settlements. At the flood levees on both sides of the estuary, the range of groundwater levels caused by tidal action within these areas exceeds the levels of dewatering required for the works. The stress changes that would occur due to dewatering are therefore within the ranges already 'experienced' by the ground and would therefore have negligible effect on ground movement.

17.1.36 There is a risk of significant settlement associated with groundwater control required for excavation of the reception pit. This settlement analysis is based on the calculation of drawdown (lowering of groundwater level) from the hydrogeological model. There is uncertainty about the hydrogeology around the reception pit, namely the thickness and extent of a body of permeable sand and gravel, and whether this is hydraulically connected to adjacent interbedded sand, silt and clay layers that are prone to settlement. The hydrogeological model is



based on a conservative interpretation of the available data, i.e. actual impacts are likely to be less than predicted by the model.

17.1.37 A Phase 2 ground investigation is planned which would seek to improve understanding of the hydrogeological behaviour and reduce uncertainty. It is also recommended that the Phase 2 ground investigation include a provision for strategic settlement monitoring before, during and after any pumping testing undertaken, to provide an indication of the actual response to dewatering effects. This would be needed for 2 weeks before the test and for the same afterwards.

17.1.38 The data will subsequently be reviewed using a finite element model to gain a better understanding of the settlement potential caused by proposed dewatering.

17.1.39 Should the results from the Phase 2 investigation not reduce uncertainty to an acceptable level and/or the revised HIA still predicts significant settlement would occur around the reception pit, there are a number of engineering options that could be used to mitigate settlement impacts. These include recharge wells and/or trenches, ground treatment at the base of the reception pit prior to excavation e.g. ground freezing, and excavation by VSMs, which are a relatively new technology that can be used to create a shaft without the need for lowering the groundwater table.

Step 12: Assess the water quality impacts

17.1.40 Whilst groundwater within the tidal flat deposits significantly exceeds the EQS values for copper and zinc, the volume of water that would enter the drive pit from these deposits can reasonably be expected to be much less than the volume of water from the Chalk bearings and fractured Chalk. Therefore dilution would likely mean that the groundwater entering the drive pit would not exceed the EQS values for copper and zinc.

17.1.41 Two potential point sources of contamination have been identified near the drive pit. The first is Fir Tree Farm slurry pits located approximately 360 m northwest of the drive pit. The second is a World War II bomb decoy site that may have underground fuel storage tanks and is located approximately 330 m south of the drive pit.

17.1.42 A requirement to confirm the construction details of the slurry pits including material, thickness and age of any lining should be included as part of the Initial Construction Environmental Management Plan (DCO Document Reference 7.3). If the details cannot be confirmed or the lining is not adequate to prevent significant leakage of effluent, the effluent within the slurry pits should be emptied and disposed at an appropriate facility. The slurry pits should not be used for the duration of the dewatering activities.

17.1.43 The main potential contamination from the WWII bomb decoy site is fuel hydrocarbons that may have leaked from underground fuel storage tanks (if these exist). The site should be investigated by intrusive methods (e.g. trial pits, exploratory holes) to assess potential sources of contamination.

17.1.44 On both the Goxhill and Paull sites groundwater is at risk of failing to meet Water Framework Directive objectives, largely due to saline intrusion into the



strategically important Chalk aquifer. The model indicates that the proposed groundwater control for the drive pit would not significantly increase the risk of saline intrusion. The risk of saline intrusion would be temporarily increased at the reception pit with the proposed pile depths. Again, piling to a depth of approx. 36 m could be used to significantly reduce impacts.

Step 13: If necessary, redesign the mitigation measures to minimise the impacts

17.1.45 A revised groundwater control (effectively sealing out the fractured Chalk aquifer) has been assessed within earlier steps of this HIA. Additional mitigation may include:

Using sulphate resistant cement for piles, base slab and ground anchors as groundwater has a high sulphate content.

Install recharge wells/trenches to counteract drawdown at sites potentially affected by settlement e.g. flood levees. Groundwater level monitoring at these sites should be used along with predefined trigger levels that initiate recharge.

Discharge of abstracted groundwater to groundwater dependant features could be considered to minimise flow and/or level impacts. However, baseline monitoring is needed to establish water quality of receiving watercourse(s).

Step 14: Develop a monitoring strategy

17.1.46 A monitoring strategy has been outlined comprising baseline, construction phase and post construction monitoring of groundwater level, flow gauging and water chemical analyses.

17.1.47 Periodic review of the above data should be conducted to ensure system performance and identify any potential for changes in impacts.



18 Bibliography AMEC (2012a) Lincolnshire Chalk and Spilsby Sandstone Groundwater Investigation, LCSS Model Map. Project No. 35281, MM_LCSS_LCSS296_March2014_Reduced. 11/12/2014.

AMEC (2012b) Lincolnshire Chalk and Spilsby Sandstone Groundwater Investigation, Phase 2 Model Construction and Calibration Report, Rev. 2. 1 October 2012.

Capita (2014a) Feeder 9 – River Humber Pipeline Replacement Project, Ground Investigation Report, CS/064298/F9/GEO/RPT/101. Rev A. 16th September 2014.

Capita (2014b) Feeder 9 – River Humber Pipeline Replacement Project, Chalk Core Boreholes Report, CS / 064298 / F9 / GEO/ RPT / 102. Rev P1. 14th November 2014

Capita (2014c) Feeder 9 – River Humber Pipeline Replacement Project, Ground Settlement Analysis, CS / 064298 / F9 / C / CAL / 101. Rev A. 14th November 2014

Construction Industry Research and Information Association (2000) Groundwater control – design and practice, CIRIA C515.

Construction Industry Research and Information Association (2002) A guide to ground treatment, CIRIA C573.

Entec UK Ltd. (2011) Lincolnshire Chalk and Spilsby Sandstone Groundwater Investigation, Northern Chalk Conceptualisation Report.

ESI Ltd. (2010) East Yorkshire Chalk Aquifer Conceptual Model Report, ESI Report Ref. 60271R1D1.Elliot T., Darminder S.C., and Younger P. (2001) Quarterly Journal of Engineering Geology and Hydrogeology, 34, 385-398.

Environment Agency (2002) Proforma for Stages 1 and 2 of the Review of Consents under the Habitats Directive, Northern Area, Anglian Region. Humber Flats, Marshes and Coast pSPA/pRamsar: North Killingholme Haven Pits SSSI.

Environment Agency (2007) Hydrogeological impact appraisal for dewatering abstractions. Science Report – SC040020/SR1.

Flynn & Rothwell Ltd (1998) Environment Agency - North East Region Section 105 Survey 1997/1998. Report Ref. 901201/THO/R1/reports/thorn/final/001.

Gale I.N. and Rutter H.K. (2006) The Chalk aquifer of Yorkshire. British Geological Survey Research Report RR/06/04.

Institute of Geological Sciences (1967) Hydrogeological map of North and East Lincolnshire including hydrometric areas 29, 30 and parts of 31.

Institute of Geological Sciences (1980) Hydrogeological map of East Yorkshire including parts of hydrometric areas 26 and 27.

Smedley P.L., Neumann I. and Farrell R. (2004) Baseline Report Series 10: The Chalk Aquifer of Yorkshire and North Humberside. British Geological Survey Commissioned Report No. CR/04/128.



Soil Engineering (2014) Report on a ground investigation for Feeder 9 – River Humber Pipeline Replacement Project, GI. Volume 1. Project No. TA7335. Document No. D01. 31/07/2014.

Terzaghi, K., Peck, R. B. and Mesri, G., (1996) Soil Mechanics in Engineering Practice, 3rd Ed. Wiley-Interscience.

Whitehead, E.J. and Lawrence, A.R. (2006) The Chalk aquifer system of Lincolnshire. British geological Survey Research Report, RR/06/03.



19 2D GROUNDWATER MODELLING

19.1 Introduction

19.1.1 A long section through the Feeder 9 tunnel route and cross sections through the drive pit and reception pit (perpendicular to the tunnel route) were selected to assess the response from groundwater dewatering during construction work, as compared to the observed natural and modelled (existing) groundwater conditions. The alignment of the cross sections are shown in Figure 19-64 and Figure 19-65.

19.1.2 The long section model is constructed parallel and the cross sections models perpendicular to the main flow direction. Hence the models would have a minor component of flow / 3D effects that is not considered in the 2D model. Because the 2D sections cannot consider 3D flow effects they tend to overestimate the magnitude and extent of drawdown (especially when perpendicular to groundwater flow).



Figure 19-64 Line of drive pit cross section model

Figure 19-65 Line of reception pit cross section model


Paull Holme Strays

Flood levees

Dem’s Wood pond


Thorngumbald drain

South Pasture drain

Reception pit

Fort Paull Battery

High Paull, Cliff

Paull

Cow Hill landfill


Unnamed well/spring


Well East Halton



Flood levee


Brockhampton

Spring Farm



East Halton Beck


Elm Tree Farm


Loe Risby Farm


Horsegate Farm



19.2 Approach to 2D numerical modelling

19.2.1 Two-dimensional groundwater modelling was undertaken using the computer software GeoStudio 2012 SEEP/W and C/TRAN (Geo Slope International). SEEP/W is a finite element model formulated on the basis that flow of water through the saturated soil follows Darcy’s Law. For finite element calculation, the SEEP/W model is divided by nodes and the elevation of the water level at each node is calculated.

19.2.2 As a hydraulic conductivity function is defined (where hydraulic conductivity varies as a function of pore-pressure) SEEP/W can model both saturated and unsaturated flow.

19.3 Scope and purpose

19.3.1 2D groundwater modelling was undertaken to assess the likely effects of the Project on the existing groundwater regime.

19.3.2 A series of simulations on three cross-sections were developed for the tunnel alignment, the drive pit and the reception pit. The models are used to assess the potential changes in groundwater levels and aquifer through flow for individual hydrogeological units, resulting from construction dewatering.

19.4 Model setup

19.4.1 Geological sections for each of the models were produced from local borehole data. The central part of the long section was based on the “Feeder 9 – River Humber Pipeline Replacement Project; Ground Investigation Report5” (Capita, September 2014a). The following additions were incorporated into the long section:

Extension to 4.5 km in a southerly direction and 2.0 km in a northerly direction to limit effects of boundary conditions on model outputs;

Inclusion of a 4.0 m horizon below the superficial deposits to represent the higher transmissivity Chalk Bearing layer; and

Division of the Chalk into three units, including two 40.0 m deep units representing the fractured chalk units of the Burnham Chalk and Flamborough Chalk and a basal lower permeability (un-fractured) chalk unit representing the Lower Active Chalk.

19.4.2 A coarse mesh of rectangles and triangles were generated for each model with an element size of 100 m width. This is the lowest mesh size for the given size of model.

19.4.3 The general model setups are shown in Figure 19-66.

5 Drawing No. H160/BH/070/01/F9/101 dated 29 August 2014



Figure 19-66 General setup of 2D models, long section (top), drive pit cross section (middle), reception pit cross section (bottom)



19.5 Distribution and properties of hydrogeological units

19.5.1 Each hydrogeological unit was constructed as a separate region within each of the models. A hydraulic conductivity function was then defined for each hydrogeological unit based on a set of initial hydraulic parameter estimates derived from local pumping testing and a review of published literature (as described in Section 4 (Step 2)).

19.5.2 A summary of the distribution of units and their adopted values is set out in Table 19-27.

19.5.3 Due to the nature of deposition, it is expected that superficial deposit parameters may vary spatially.

Hydrogeological Unit

Goxhill Paull

Adopted

Hydraulic

Conductivity

(m/s)

Adopted

Coefficient of

Compressibility

(kPa-1)

Adopted

Storativity /

Specific Yield

(m3/m3)

Adopted

Hydraulic

Conductivity

(m/s)

Adopted

Coefficient of

Compressibility

(kPa-1)

Adopted

Storativity /

Specific Yield

(m3/m3)

Estuarine Alluvium

2.0x10-5

1.0x10-6

0.3 2.0x10-5

1.0x10-6

0.3

Glacial deposits (sandy gravelly clay)

2.0x10-7

2.4x10-4

0.21 3.0x10-7

1.0x10-6

0.21

Glacial deposits (sandy gravel)

n/p n/p n/p 7.0x10-4

5.0x10-7

0.21

Alluvium 7.0x10-5

5.0x10-3

0.4 9.0x10-6

2.5x10-5

0.4

Chalk Bearings 6.5x10-3

2.5x10-5

0.02 6.5x10-3

2.5x10-5

0.02

Fractured Chalk 5.2x10-5

0 0.025 5.2x10-6

0 0.025

Un-fractured Chalk

2.9x10-7

0 0.005 2.9x10-7

0 0.005

n/p not present

Table 19-27 Adopted hydraulic parameters used in 2D model

19.6 Model boundaries

19.6.1 The model incorporates boundary conditions as described in Table 19-28. Rainfall recharge was not applied to the model. The model is therefore conservative.



Model Description

Boundary Condition

Long Section Constant Head Boundary Chalk horizon – 1.6 m (southern boundary) / 0.9 m

(northern boundary)

Superficial deposits – 1.4 m (southern boundary) / 0.3 m (northern boundary)

Variable Head Boundary River Humber – four-stage low and high tide levels (daily)

Drive Pit dewatering phase 1

Reception pit dewatering


Constant flux boundary Land drains – unit flux (0 m/sec) potential seepage face

Base of model – total flux (0 m3/sec)

Goxhill Cross Section

Constant Head Boundary Chalk horizon – 1.6 m (southern boundary) / 0.9 m

(northern boundary)







Paull Cross Section

Constant Head Boundary Chalk horizon – 1.6 m (southern boundary) / 0.9 m

(northern boundary)



Reception pit dewatering



Table 19-28 Model Setup – Boundary Conditions



19.7 Model calibration

19.7.1 The focus of calibration was on groundwater levels recorded in monitoring wells with a reliable long term record of groundwater levels. The model long section was used for calibration because of the availability of site specific data along the tunnel section. The model was considered to be well calibrated where general trends in water level changes were replicated and where calculated water levels in the majority of wells in close proximity to the alignment were to within +/- 0.3 m of observed water levels. Figure 19-67 shows the calibration of groundwater levels.

Figure 19-67 Calibration of groundwater levels

19.7.2 The long section model was calibrated in the first instance to replicate observed groundwater levels in monitoring wells located at the proposed drive pit and reception pit (including L02, L06, L08, L15 and L18), that were chosen based on reliability of data (see Paragraph 4.2.56). Input parameters were then arbitrarily adjusted using sensitivity analysis until a good fit between modelled and actual groundwater levels were obtained (see Section 19.7). The cross section models



then adopted the hydraulic parameters derived from the calibrated long section model. The final hydraulic parameters adopted for use in the models are presented in Table 19-27.

19.7.3 Calibration required the following adjustments:

An increase in the hydraulic conductivity (up to one order of magnitude) of superficial deposits. This would seem reasonable given that initial input parameters were based on PSD data.

A reduction in the hydraulic conductivity of the Chalk units. This would seem reasonable given that initial input parameters were based on falling head test data and likely to be affected by near well damage caused by drilling;

A reduction in the hydraulic conductivity of the Fractured Chalk unit beneath Paull, compared to Goxhill, to reflect the reduction in transmissivity where the Fractured Chalk becomes increasingly confined to the north; and

Estimation of the coefficient of compressibility for the majority of units, given the lack of data. Estimates were based on professional judgement.

19.7.4 It is recognised that the numerical groundwater model is based on a combination of site specific data and estimates (where data is limited or absent). It will therefore be an important next step to review the model when new information becomes available, such as that derived from future proposed pumping testing.

19.8 Results of numerical modelling

19.8.1 The results are summarised in Table 19-29 and Table 19-30, however some more general comments are provided below.

19.8.2 The long section model indicates that dewatering of the drive pit results in large changes in groundwater level when engineering controls (piles) are not used, i.e. up to 12.0 m. When engineering controls are applied, the effect on groundwater levels is significantly reduced, i.e. to around 1.0 m. The radius of influence of dewatering is also reduced to over 5.0 km when no piles are used and around 2.0 km when a piled solution is adopted. The results suggest that drawdown can be reduced when the pile depth is increased, but it is noted that there are marginal relative gains when the pile depth is increased beyond -26.0 m OD.

19.8.3 The adoption of groundwater control in the long section model on the Paull side of the Feeder 9 replacement tunnel has a lesser effect than that at Goxhill. The model indicates that dewatering of the reception pit results in groundwater level drawdown of up to 12.0 m when no groundwater control is applied, and between 11.0 m and 1.0 m when groundwater control is used. The large variation in drawdown produced by different pile depth solutions is a result of the ability of the piles to seal out the glacial deposits. A significant reduction in drawdown is produced when piles terminate within the fractured chalk.

19.8.4 The cross section models for Paull and Goxhill infer a larger drawdown response than is evident in the long section model. It is noted that these cross sections are not fully calibrated (due to a lack of data off the tunnel alignment) and are perpendicular to groundwater flow, and likely overestimate the drawdown effects.



The results however support the adoption of an engineered solution to limit drawdown effects in the superficial deposits.



Section

Case Effect at Drive Pit

Effect at 100 m Effect at 250 m Effect at 500 m Effect at 1 km Effect at 2 km

(m) (m) (m) (m) (m) (m)

South North South North South North South North South North South North

Dewatering of Drive Pit

Component - Tunnel

long section

without piles 11.5 11.5 11.2 10.6 10.7 9.1 9.9 6.6 8.3 0.3 5.4 0

with piles (-26 m OD) 1.7 1.5 1.7 1.5 1.6 1.3 1.3 1 1 0 0.4 0

with piles (-35 m OD) 1.1 1.1 1.1 1 1 0.9 0.8 0.7 0.5 0 0.1 0

With piles (full depth) 0.9 0.7 0.9 0.7 0.8 0.6 0.7 0.5 0.5 0 0.2 0

Dewatering of Drive Pit

Component - Goxhill

cross section

without piles 13 13 12.4 12.4 11.6 11.4 10.2 9.8 7.5 6.9 2.4 1.6

with piles (-26 m OD) 3.4 3.7 3.4 3.6 3.1 3.4 2.6 2.9 1.8 2 0.4 1.8

with piles (-35 m OD) 2.3 2.7 2.3 2.6 2.1 2.4 1.7 2.1 1.1 1.4 0.2 0.3

With piles (full depth) 0.9 1.4 0.9 1.4 0.8 1.3 0.6 1.1 0.3 0.7 0 0.1

Dewatering of Shallow

Drive Pit Section without piles 0.8 0.8 0.8 0.7 0.7 0.6 0.5 0.5 0.4 0.2 0.1 0

with piles (-26 m OD) 0.2 0.2 0.1 0.2 0.1 0.2 0 0.2 0 0.1 -0.2 0

with piles (-35 m OD) 0.1 0.2 0.1 0.2 0.1 0.2 0 0.2 -0.1 0.1 -0.2 0

With piles (full depth) 0.1 0.2 0.1 0.2 0 0.2 -0.1 0.2 -0.1 0.1 -0.2 0

Table 19-29 Modelled drawdown - Drive Pit

19.8.5 Note that the drawdown levels are based on the lowest water levels recorded and associated with low tide conditions.



Section Case Effect at Reception Pit

Effect at 100 m Effect at 250 m Effect at 500 m Effect at 1 km Effect at 2 km

(m) (m) (m) (m) (m) (m)

South North South North South North South North South North South North

Dewatering of Reception

Pit Component - Tunnel

long section

without piles 11.7 12.4 8.9 11.3 4.7 10.1 3.5 8.4 0.8 5.4 - 0.2

with piles (-18.7 m OD) 10.3 11.8 7.9 10.7 4.3 9.6 3.1 8 0.6 5.1 - 0.2

With piles (full depth) 1.1 3.9 1.1 3.7 0.8 3.3 0.6 2.7 0.1 1.7 - 0

Dewatering of Reception

Pit - Paull cross section without piles 16 15.9 15.4 15.3 14.7 14.6 13.5 8 5.9 5.1 2.2 1.8

with piles (-18.7 m OD) 15.8 15.7 15.3 15.2 14.6 14.5 13.4 8 5.8 5 2.2 1.7

With piles (full depth) 1 1.1 1 1.2 0.9 1.2 0.8 0.9 0.3 0.3 0.2 0.7

Table 19-30 Modelled Drawdown - Reception Pit

19.8.6 Note that the drawdown levels are based on the lowest water levels recorded and associated with low tide conditions.



Scenario Drive Pit

(m3/day/m)

Reception PIt

(m3/day/m)

Drive Pit Shallow

(m3/day/m)

No piles 19.1 16.5 0.9

Proposed pile solution (-26 m OD)

6.6 14.9 -

Full depth pile solution (base of fractured chalk)

3.0 0.7 -

Table 19-31 Average Dewatering flow rate (alternative scenarios)



20 Response to EA Comments

20.1 HIA methodology

20.1.1 Our method statement for this hydrogeological impact assessment was provided to the EA in Hyder technical note reference 0019-UA006029-UP31R-02-HIA-MethodStatement dated 11 November 2014.

20.1.2 Comments made by the EA on our method statement were provided in a letter reference RA/2014/130328/01-L01 dated 05 December 2014.

20.1.3 EA comments are reproduced below and along with our corresponding responses.



EA comment Hyder response

We agree in principle with the proposed scope of works. The hydrogeology of the area needs to be clear and it is important that data obtained through the HIA is used to develop a robust conceptual model which can inform the proposed dewatering methodology and pit installation. We consider this evidence is key to understanding the potential impacts of the proposal on the surface and groundwater environment. Once the HIA has been completed we would be better placed to comment on the proposed approach to groundwater management.

Noted. Data from the Feeder 9 ground investigation has been used to develop a conceptual model; this is described in Section 2 of this HIA document.

1) Secant piling may be an appropriate choice for the sidewalls. However, there needs to be careful consideration to the depth of these. The active zone of chalk is thought to be the top 50m but this could be deeper locally. Can piles effectively isolate the chalk aquifer beneath the pit from the rest of the aquifer?

Results from this HIA have been fed back to the design team and the outline design of the groundwater control method, including pile depths, base slabs and groundwater abstraction period has been developed by an iterative process.

2) The depth of the pile wall compared to the depth of the boreholes pumping groundwater also needs careful consideration. There is the potential that the boreholes’ zone of influence could extend beneath the piles and more groundwater could be pumped than is necessary if the depths are not targeted.

As above.

3) How is groundwater seepage and rainfall into the box to be dealt with? Is there a need for sumps within the box?

Once the pits are established with piles and base slabs they would be effectively sealed from the groundwater inflow. After that, minor seepage and rainfall would be dealt with via sump pumps.

4) How thick does the base of the box need to be so it doesn’t float? Is it feasible to lay a floor to the base to the thickness required? Alternatively can it be anchored to counteract the anticipated groundwater pressure?

The proposed design for the pits includes base slabs with ground anchoring to prevent base heave. Detailed design would be the responsibility of the main works contractor.

5) Is the box to be decommissioned? Would there be consideration of long term maintenance of any remaining sub surface structure

On completion of the pipeline installation the proposal is to retain the deep section of the drive pit in the ground




post construction? Is there a proposal for removing the box and reinstating?

and backfill the excavation with suitable material. The Chalk would therefore remain confined.

At the shallow end of the pit, remove the sheet piles and drill, say, half a dozen holes thorough the base at this end. Thus the 100m long drive pit would be nominally three sided (on the fourth side the secant piles would have been cut off to a level of -5.24m OD when extending the pit for pipe insertion. This would allow the groundwater to return to its natural level. The area could be monitored to ensure that the pit has had no detrimental effect. Should it prove necessary, holes could be post drilled though the pit base slab at a later date.

6) There is no discussion yet on the methodology or the feasibility of break out and break though of any boring machine from the box to the aquifer at Goxhill and to the shaft at Paull.

Hyder understands that the TBM would be a closed loop system that would minimise groundwater coming back into the launch pit.

This is discussed in more detail in section 4.7 of this HIA

7) It is recommended that consideration is given to obtaining background groundwater level data and surface water flow data. For example the single gauge station on East Halton Beck may not be appropriately positioned to assess the potential impacts of dewatering.

Agreed. A monitoring strategy is outlined in Section 16 (Step 14) of this HIA.

8) Likewise it is recommended that background surface water and groundwater quality data is collected.

As above.

9) Dewatering is not currently a regulated activity, however, there is a responsibility on the undertaker of the activity to not impact licensed and/or lawful unlicensed water users. Despite there not being a requirement to be licensed, the undertaker would be liable under civil law for damage. Under the Water Framework Directive, there is a responsibility to prevent deterioration in the status of

Acknowledged. This HIA has followed the EA guidance document for assessing impacts from dewatering. Noted - this guidance is broadly the same process as the EA guidance for assessing impacts from abstractions. Science Report – SC040020/SR2




ground and surface water bodies. While a licence is not currently required for dewatering, DEFRA are working on bringing dewatering into the licensing regime within the next 12 months. Given this and the issue of derogation and Water Framework Directive status, it is recommended that the proposal follows the principles of the licensing regime.

10) It is recommended that discharge arrangements are considered at the earliest opportunity. Discussions should be held as to whether a permit would be required and where discharging may take place.

Discussions with the EA are ongoing.

11) Agree with the recommendation that modelling should be undertaken to assess the potential impacts of the proposal.

Acknowledged. The results of the numerical modelling are presented in this HIA report.

12) Regarding the third bullet point of Section 3 ‘Scope of Works’. It is recommended that lawful unlicensed users of groundwater and surface water are consulted on the proposed activity.

A desk based survey of water features has been undertaken that could potentially be impacted by drawdown, such as abstractions, protected rights (e.g. boreholes used for agriculture), wetlands, springs and buildings that may be affected by settlement. Our search has involved publically available information, including:

Abstraction licence records held by the EA;

Records of domestic private water supplies from the relevant local authorities;

Records of boreholes and wells held by the BGS;

Ordnance Survey maps;

Databases of conservation sites (www.magic.gov.uk).

A detailed site based water features survey and consultation with lawful unlicensed users of groundwater and surface water is proposed for further work.




13) We also consider that revisions in the design of the dewatering system should be included in the HIA. The groundwater conditions found during intrusive works may affect the proposals depth, length or duration of pumping.

As above, results from this HIA have been fed back to the design team and the outline design of the groundwater control method, including pile depths, base slabs and groundwater abstraction period has been developed by an iterative process.

This is ongoing and a Phase 2 investigation is planned for Spring 2015 to address some of the hydrogeological uncertainties identified in this HIA.

Date post:	12-Aug-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times