Portsmouth City Council · Site Investigation Risk Assessment Conclusions Post Adoption Statement...

Portsmouth City Council

Portsea Island Coastal Strategy Study

Coastal Processes

June 2009

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© Halcrow Group Limited 2009


Portsea Island Strategy Study

Coastal Processes

June 2009

Halcrow Group Limited

Halcrow Group Limited Burderop Park Swindon Wiltshire SN4 0QD

Tel +44 (0)1793 812479 Fax +44 (0)1793 812089

www.halcrow.com


Portsea Island Strategy Study

Coastal Processes

Contents Amendment Record This report has been issued and amended as follows:

Issue Revision Description Date Signed

1 0 Draft Aug 04 IAT

1 1 Final Nov

2004

IAT

2 0 For NRG Jun 2009 IAT

Reporting Structure

StARStrategy Appraisal

Report

Overarching Document

Coastal Processes

ReportGeomorphology

Coastal ConditionsSediment Processes

SEAAppraisal Process

Existing EnvironmentEnvironmental Assessment

of OptionsConclusions

Numerical Modelling Report

Wave Modelling

Joint Probabil ity Hydrodynamic

ModellingFlood Propagation

Modelling

Coastal Change

DataBeach Plan Shape

Modelling

Economics ReportMethodology

DamagesCosts

Results

Coastal Defences Report

Existing DefencesIntervention Options

Appropriate Assessment

Assessment of likelySignificant effect


In CombinationConclusions

Contamination RiskExisting SituationSite InvestigationRisk Assessment

Conclusions

Post Adoption

StatementConsultation

Summary

Statement of CaseJustification of ROPI

StARStrategy Appraisal

Report

Overarching Document

Coastal Processes

ReportGeomorphology

Coastal ConditionsSediment Processes

SEAAppraisal Process

Existing EnvironmentEnvironmental Assessment

of OptionsConclusions

Numerical Modelling Report

Wave Modelling

Joint Probabil ity Hydrodynamic

ModellingFlood Propagation

Modelling

Coastal Change

DataBeach Plan Shape

Modelling

Economics ReportMethodology

DamagesCosts

Results

Coastal Defences Report

Existing DefencesIntervention Options


Assessment of likelySignificant effect


In CombinationConclusions

Contamination RiskExisting SituationSite InvestigationRisk Assessment

Conclusions

Post Adoption

StatementConsultation

Summary

Statement of CaseJustification of ROPI

Contents

1 Introduction 1

2 Geology & Geomorphology 3

2.1 General Review of Information 3

2.2 Geology 3

2.3 Geomorphology 7

2.4 Recommendations 12

3 Coastal Conditions 15

3.1 General Review of Information 15

3.2 Assessment of Conditions 15

4 Shoreline Evolution 27

4.1 Introduction 27

4.2 Factors Affecting Shoreline Evolution 27

4.3 Sedimentology and Shoreline Evolution 28

4.4 Current Shoreline Processes 37

4.5 Predicting Future Shoreline Evolution 44

4.6 Scenario Assessment 55

4.7 Summary 57

Doc No 2 Rev: 0 Date: June 2009 1 S:\Projects\DCPSTS\Docs\Docs 2009\StAR\Coastal Processes\Coastal Processes report.doc

1 Introduction

To enable the production of an effective coastal management strategy for Portsea

Island, it is important to understand the issues and extent of information that is

available for the shoreline. This report provides an overview of the current coastal

processes acting along the coast, incorporating both conclusions drawn from

previous studies and work carried out as part of this strategy study.

This report covers the geology and geomorphology, the coastal conditions and

shoreline evolution of the study area.


2 Geology & Geomorphology

2.1 General Review of Information

2.1.1 Requirements

For this Study it was important to assess coastal geology and geomorphology to

understand how the shoreline may develop in the short and long term. Changes in

forcing factors, such as wind and waves, along with geology and surface deposits,

influence coastal realignment, orientation and shoreline morphology. To develop a

feasible management strategy it is essential to understand the general shoreline

behaviour and response, material sources, sediment sinks and areas at risk.

2.1.2 Information Sources

The following geological and geomorphological sources were reviewed:

• The East Solent Shoreline Management Plan (SMP);

• The Pagham Harbour to River Hamble Strategy Study;

• Old Portsmouth Strategy Study;

• Futurecoast;

• Solent Coastal Habitat Management Plan Volume II (CHaMP);

• Langstone Harbour Management Plan;

• EA - aerial photographs;

• PCC – Historic Maps;

• Havant Borough Council – Langstone Harbour Management Plan.

2.2 Geology

Over the last two million years glacio-eustatic sea level changes have repeatedly

exposed the bed of the English Channel to subaerial conditions. These sea level

lowstands correspond to periods of glaciation in the northern hemisphere, the last

such episode reaching a maximum around 18,000 years ago. Southern England at

this time would have been exposed to periglacial conditions and weathering,

leaving thick solifluction and head deposits that extended southwards to the

modern south coast of England (Figure 2.1). The offshore area of the English

Channel was the site of extensive fluvial deposits including river terrace deposits,

which covered a large proportion of the area now occupied by the English

Channel. Fluvial downcutting of the rivers in southern England may have occurred

at this time in response to the lowered sea level. Incision may have been enhanced

by episodic high-energy river discharges in a periglacial environment. The flooding


of the English Channel commenced from the west as sea levels began to rise. The

transgression of the English Channel region probably led to the destruction or

reworking of many of the fluvial terrace deposits to form either beaches, which

rolled onshore, and/or marine bedforms in the shallow sea. As the transgression

continued these newly formed shelf sediments may have moved extensively before

sea levels reached approximately their present level about 5,000 years ago.

The bedrock underlying Portsea Island is Upper Chalk of Cretaceous age, which

inland forms the chalk escarpment of the South Downs. Gently dipping Eocene

and Oligocene beds, soft clay and sands, with a mantle of recent sediments, overlie

this and indent the low-lying coastline. The typical sequence is shown in Table 2.1

and Figures 2.2 and 2.3.

Date (millions

of years ago)

Geological Time

Interval

Solid and Drift Formations Lithological Description

Blown sand Modern deposits

Shingle and sand beaches Modern deposits

River, marine & estuarine alluvium Relict & modern deposits of fine material

River terrace deposits Mainly gravels

0-2 mya Quaternary

Holocene

Pleistocene

Brickearth Mainly loam and clay

Bracklesham Beds Clays and clayey sands

Bagshot Beds Sands and gravels, with seams of clay

London Clay Sandy clays, with occasional pebble beds

2-65 mya Tertiary

Palaeogene

Reading Beds Clays, sand with occasional flint gravels

65-245 mya Mesozoic: Cretaceous

Upper Chalk Thickly bedded chalk with regularly spaced bands of flint nodules

Table 2.1 - Summary of Geological Events that Shaped Portsea Island

The broad shape of Portsmouth and Langstone Harbour was formed during the

flooding of the lower courses of these palaeo-rivers, although the harbours have

since been subject to anthropogenic and natural modification. Portsea Island

represents a gravel-covered island between the Quaternary palaeo-valleys. The

southern end is formed predominantly in Bracklesham Sands that provide very

little resistance to erosion and overlain by storm gravel beach deposits. Further

north the Bracklesham sands are overlain by Quaternary Brickearth deposits. The

area is also highly urbanised, with large areas of reclaimed land along the shoreline.


2.3 Geomorphology

2.3.1 Overview

The end of the last Pleistocene Ice Age produced a substantial, rapid rise in sea

levels between c.15000-5000 years BP (Before Present); the ‘Holocene Marine

Transgression’ period (Figure 2.4). It submerged river valleys formed during the

last Devensian age, (c.75,000–10,000 years BP), when sea levels were up to c.120m

lower than today. The ‘Solent River’ formed what is now the East Solent, whilst

the valleys of Portsmouth and Langstone Harbours were drowned.

Waves and tidal currents along the new coastline reworked and redistributed large

quantities of sand and shingle, which were originally deposited as fluvial terraces.

Some of this material was driven together to form coarse clastic barrier beaches,

initially located several kilometres seaward of the modern coastline and some of

the material was left behind deposited on the new seabed.

Figure 2.4 - Time-Depth Plot of Sea-Level Index Points, S England (Devoy,1987c)

Between c.6,500-6,000 years BP, it is thought that breaches in the barrier(s)

occurred, inducing a saline incursion as well as introducing finer sediments.

Subsequent wave attack caused the barrier to ‘roll-back’ creating the antecedent for

the modern environment. As the rate of sea level rise continued to fall between

5,500-5,000 years BP it caused a shift in the sedimentary dynamics; fluvial

sedimentary inputs were reduced and replaced by large quantities of fine marine

sediments to form mudflats and saltmarsh. By c.3,000 years BP, a marine ‘still


stand’, c.-4.5m OD, was in place and from there on, it is asserted, the natural

outline of the harbour and coastline were attained. The most recent sequence of

storm induced breaches and barrier breakdown occurred c.700-400 years BP.

Currently the instability of the barrier is further exacerbated by a reduction of

longshore sediment supply and/or on-offshore losses induced by storms.

Examples of nearshore and offshore sediment accumulations are evident today at

Eastney beach and Horse and Dean Sand, the latter located offshore and forming a

large area of shallow water which shelters the shoreline. There are also sediment

accumulations at East and West Winner, either side of Langstone Harbour mouth.

2.3.2 Open Coast

(a) Natural Features

Portsea Island is low lying and liable to flooding and erosion. Much of the

open coastline comprises massive shingle accumulations, influenced by

various defence structures and management operations. The nearshore

zone is mostly formed of wide, shallow sand banks, apart from

Portsmouth and Langstone Harbour mouths.

The open shoreline consists of coarse, shingle beaches underlain by

Eocene sand. The shingle acts as the major barrier to wave attack.

However, the coastline is naturally erosive and throughout the Holocene

Marine Transgression the shoreline displayed the classic dynamics of a

coastal barrier migrating across a low-lying hinterland. Management

constraints are today in place to prevent this damaging the urban area.

The natural beach form has been altered through the construction of

various sea defence structures and the addition of recharge materials. the

presence of linear defences has resulted in narrowing of the upper shingle

beach in places, e.g. Southsea Castle. There are some accretional features,

however, such as East and West Winner Sands and Horse and Dean

Sands, all located offshore. Other constructive features include West

Winner Spit, west of Langstone Harbour; which has experienced

considerable changes in size and position since the late nineteenth century.

From map comparisons, between 1896-1972 Eastney Beach accreted

significantly. Site observations indicate that a littoral drift divide exists

around Eastney - this is probably wave induced, originating from the

Langstone Harbour tidal delta.


(b) Anthropogenic Constraints

Until the early nineteenth century (c.1820-1830), much of the backshore of

Portsea Island was a swampy residue of former lagoonal conditions known

as The Great and Little Morasses. From this, and the fact that most of the

frontage is at or below sea level, it is easy to see why regular breaches were

common. Human occupation resulted in much of the coastline becoming

urbanised, with coastal erosion prevented by the provision of extensive sea

defences which have been in place for approximately 400 years. One

management involves the practice taking shingle from the accreting

Eastney frontage, particularly near the Southern Water storm outfall pipe,

and occasionally from in front of the old Royal Marine Barracks, for use as

beach recharge material in other areas, such as Clarence Beach and

Langstone Harbour entrance.

2.3.3 The Harbours

(a) Overview

Both Portsmouth and Langstone harbour are defined as ‘tidal inlets’. They

are characterised by extensive tidal lagoons and barrier beaches. Davis &

Hayes (1984) categorised inlets in terms of tidal range and wave conditions

as either symmetrical or and asymmetrical. Portsmouth and Langstone

Harbours are symmetrical types, with the inlet channel directed straight

out to sea and spits approximately aligned to each other. Symmetrical

inlets are likely to be tide-dominated mixed energy inlets. Sand-bypassing is

thought to be either by shoal migration, outer channel shifting or

deflection of littoral sand transport (Fitzgerald, 2001). The harbours are

under threat of erosion and flooding, caused in part by the dieback of

saltmarsh vegetation causing what may be serious changes in the long-term

stability of the shoreline.

(b) Portsmouth Harbour

(i) Natural Features

The harbour occupies the floodplain cut from a previous tributary

of the Solent. It is postulated that the main harbour channel was

cut during a marine regressions but has since infilled with fluvial,

marine and peri-glacial sediments. Geological exploration

illustrates that these sediments lie predominantly in the northern

part of the harbour and under more recent estuarine deposits.

Between c.15,000-14,000 years BP and c.3,500-3,000 years BP, a

sequence of marine transgression inundated the incised floodplain.


Due to its constricted entrance channel and protection afforded

by Hasler and Camber spits, Portsmouth Harbour is virtually

landlocked. As a tidal basin it is ebb-dominated, although this has

been altered over the past two centuries due to dredging, with

even the small flood tide delta at the harbour mouth routinely

dredged. Most coarse sediment thus has a short residence, but

fine, nominally suspended sediments, are transported into the

harbour providing material for mudflats and marshland. The

sediment budget is one of overall moderate gain as the harbour

acts as a ‘sink’ for silt and fine sand sediments, with this trend

stable since at least the late eighteenth century. The basic

characteristics of Portsmouth Harbour are summarised in Table

2.2.

Area Intertidal

Area (ha)

Marsh Area

(ha)

Shoreline

(km)

Tidal

Range (m)

Cross-sectional

Area (m2)

Mouth

Width (m)

1593 964 181 55.2 4.1 6228 220

Table 2.2 Basic Characteristics of Portsmouth Harbour: (Halcrow, Futurecoast 2002)

(ii) Anthropogenic Constraints:

Portsmouth is a highly developed harbour, with its margins

subject to reclamation for port development and landfill.

Elsewhere, the shoreline has been stabilised and drained, initially

for agriculture and more recently for development/ recreation. Up

to 25% of the intertidal habitat has been lost to development in

the last thirty years (Tubbs years). Sea defences have built up at

the entrance along with large military and commercial port

facilities. All approach channels to and within the harbour are

dredged.

(c) Langstone Harbour

(i) Natural Features

Since c.15,000–400 years BP, Langstone Harbour has experienced

a similar evolution to Portsmouth Harbour, occupying a flat, low-

lying coastal plain cut in tertiary sediments with strongly re-curved

spits of sand and gravel on each flank preventing wave penetration

into the harbour. The confined harbour entrance is the result of

spit growth sustained by convergent pathways of littoral sand and


gravel transport along the adjacent open coastline and it is

probable that both spits acquired their present form in the

thirteenth century. The tidal regime is ebb-dominated, resulting in

the harbour being a sink for fine suspended sediments and leading

to mudflats and saltmarshes colonising shores in the upper

harbour reaches, providing protection from local wave action.

This environment has kept pace with sea level rise for the past

5,000 years BP and is indicative of a system in equilibrium. Flood

dominant sandbanks, eg Sword Sands, show that there is little

scope for further sediment deposition, apart from allowing

intertidal elevations to adapt to sea level rise. East of the harbour

entrance are the large intertidal sandbanks of East and West

Winner. Their evolution is associated with the ebb-tidal delta in

the harbour that pushes material south and westwards towards the

sandbanks resulting in periodic erosion and accretion. At Gunner

Point over the last 400 years there has been steadily accretion

seaward, creating a dune covered shingle foreland. The basic

characteristics of Langstone Harbour are summarised in Table 2.3.

Area Intertidal

Area (ha)

Marsh Area

(ha)

Shoreline

(km)

Tidal Range

(m)

Cross-

sectional Area

(m2)

Mouth

Width (m)

1925 1513 100 43 4.2 4703 160

Table 2.3 - Basic Characteristics of Langstone Harbour: (Halcrow, Futurecoast 2002)

(ii) Anthropogenic Constraints:

Much of the Langstone shoreline comprises reclamation and land

stabilised for the Eastern Road. Embankments have been built to

protect low-lying areas from flooding. The intertidal area is mostly

composed of fine alluvial sediments forming mudflats, with a

fringe of coarser material in places. Maintenance dredging is

undertaken in approach channels to Kendall’s Wharf,

Bedhampton Quay and Southsea Marina. Aggregate dredging of

about 6,000T/year from the flood tide bank opposite the entrance

channel ceased in 1994. The entrance is not dredged for

navigation as tidal currents are sufficient to sweep the channel.

Eastney Spit provides a natural shelter to Eastney Lake, a large

intertidal mudflat area between the spit and main shoreline. Many


defence structures are present here due to reclamation. North of

this is Milton Bund, a man-made structure built across a former

tidal inlet to form a landfill site in 1962. Beyond this, the shoreline

is again stabilised, mainly to protect Eastern Road.

(d) Port Creek

At the north of the island, joining the harbours, Port Creek is all that

remains of the channel separating the island from the mainland. The creek

is tidal, with the dominant flow of the small water exchange being west

towards Portsmouth. The mud bed is extensively exposed at low water.

2.4 Recommendations

The present environment is the result of many factors and processes - some relict,

some ongoing. Evolution occurs over a variety of timescales and the present day

situation must be seen as transient rather than fixed. It is best viewed in this

manner because change to the shoreline may be beneficial as well as detrimental

and therefore should not be restricted.

There are two distinct shoreline environments on Portsea Island, which have the

potential to be broken down further (see Table 2.4). One is the sheltered harbour

environment, and the other the open coast along the south of the island. These

vary greatly in their shoreline dynamics and hence coast defence requirements.

Artificial Coastline (Old Portsmouth to Southsea Castle) The Open Coast

Natural Coastline (Eastney)

Artificial Harbour (Portsmouth)

Natural Harbour (Langstone)

Sheltered Environment

Port Creek

Table 2.4 - Environments on Portsea Island

The following factors should be considered when setting management objectives:

• Much of the open shoreline is liable to erosion due to the soft surface

geology and a lack of available drift material to form stable beaches.

• Large areas of Portsea Island are below predicted extreme sea levels and

are potentially at risk from flooding.

• As much as 20% of the current land area has been reclaimed from the sea

by drainage and land raising activities through time.


• Much of the shoreline is heavily developed and contains important

infrastructure and facilities. As a result, a high proportion is protected and

dependant on continued protection.

• Walls, revetments and embankments defend the island’s perimeter.

Protection is against both erosion and flood risk to commercial, industrial,

military, residential, recreational, infrastructure and historic interests.

• Undeveloped areas and the intertidal zone contain considerable

environmental assets, including large areas protected by international

legislation.

• The contrast between the open coast shoreline and the rest of the island is

dramatic due to the shelter afforded by the narrow harbour entrances,

which restrict wave penetration.


3 Coastal Conditions

3.1 General Review of Information

3.1.1 Requirements

A thorough knowledge of the ongoing coastal processes and their interaction is

essential in developing an effective Strategy Plan for Portsea Island. The natural

forces acting upon the shoreline largely drive these processes and are forces that

defences must be designed to withstand.

This section reviews the data sets that exist and assesses their relevance to the

Strategy.


The key sources of information reviewed have been the existing coastal

management plans, including the SMP, Pagham Harbour Strategy Study and the

Old Portsmouth Strategy Study.

To provide additional data, a survey was undertaken in 2002 by Gardline Surveys

Ltd. The survey comprised a bathymetric survey of the open coast, harbour areas

and Port Creek. Grab samples were taken from the seabed at specified locations

and current metering and tide level measurements were also undertaken at a

number of locations throughout the study area.

3.2 Assessment of Conditions

3.2.1 Water Levels

(a) Tides and Surges

Portsmouth is a Standard Port for which water levels have been measured

over many years. The tidal levels here are presented in Table 3.1. The

difference between Ordnance Datum Newlyn (ODN) and Chart Datum

(CD) is 2.73m. These figures are only directly applicable to Portsmouth

Harbour and there are no correction factors given in the tide tables for

any other parts of the study frontage, i.e. the open Portsea coast or

Langstone Harbour.


Table 3.1 - Tide Levels (source Admiralty Tide Tables, 2004)

Tidal levels for any particular day and time can be predicted using

harmonic constants. However, actual water levels will vary from these

predictions as a result of meteorological conditions (winds or barometric

pressure); this water level difference, known as surge, may be positive or

negative depending upon the weather system. Actual water levels are

measured in Portsmouth Harbour by the Queen’s Harbour Master and

Proudman Oceanographic Laboratory and in Langstone Harbour by the

Harbour Master.

(b) Sea Level Rise

General government guidance on sea level rise is to allow 6mm/year for

this area of the UK. The report ‘The Impacts of Climate Change in the

South East in the 21st Century’ also presents sea level rise scenarios. It

identifies an average rise of 34cm by 2050, or approximately 6.5mm/year

for the English Channel coast, for the medium high scenario. The report

also states that in parts of the Solent the rate may be faster, approaching

10mm/year. The SMP states that Defra (then MAFF) accepted that a

10mm/year allowance for sea level rise should be allowed for in the design

of coastal defences around Portsea Island. This was based on research

undertaken by the University of Portsmouth in 1992. It has, however,

since been shown that the data used in this research was flawed and thus

that such an allowance for sea level rise is not valid.

Therefore the Defra guidance has been followed and the sea level rise rate

of 6mm/year has been applied over 50 and 100 year periods. In addition,

Tidal State Level

(mODN)

Level

(mCD)

Highest Astronomical Tide (HAT) + 2.37 5.1

Mean High Water Springs (MHWS) + 1.97 4.7

Mean High Water Neaps (MHWN) + 1.07 3.8

Mean Sea Level (MSL) + 0.17 2.9

Mean Low Water Neaps (MLWN) - 0.83 1.9

Mean Low Water Springs (MLWS) - 1.93 0.8

Lowest Astronomical Tide (LAT) - 2.63 0.1


in accordance with recent Defra guidance, sensitivity tests have been

undertaken using rates of 10mm/year and 4mm/year as part of the

numerical modelling studies.

(c) Extreme Water Levels

The SMP presents extreme water levels for various locations as shown in

Table 3.2. These are based largely on interpolation from the available

water level records, with only the Portsmouth Harbour figures based on

actual recorded data.

Extreme water Levels mODN

Return Period (years)

Portsmouth Harbour

South Parade Langstone Harbour

200 3.05 - 3.30 50 2.90 3.04 3.14 10 2.78 2.92 3.02

1 2.46 2.58 2.69

Table 3.2 Extreme Water Levels, from SMP (1996)

Extreme water levels were calculated for the Old Portsmouth Strategy

Study using annual maxima data recorded at Portsmouth between 1813-

1998 (Table 3.3). Examination of this data showed an increase in the mean

of these extreme values. Between 1813 and the present day, this equates to

0.92mm/yr - undertaking the same analysis from 1950 onwards, however,

shows a trend of increasing extreme values of 3.4mm/yr.


2000 Water Level (mODN)



200 3.03 3.34 3.64

100 2.98 3.29 3.59 50 2.93 3.24 3.54

20 2.85 3.16 3.46 10 2.77 3.08 3.38 5 2.69 3.00 3.30

2 2.54 2.85 3.15

Table 3.3 - Extreme Water Levels from Old Portsmouth Strategy Study (1999)

Since the analyses performed for the Old Portsmouth Strategy Study, the

EA (Southern Region) has commissioned further research into extreme

water levels along the south coast. This work is currently ongoing but for


the purposes of this commission, the results for Portsmouth from an

interim report produced in March 2003 have been compared against other

available estimates of extreme water levels (Table 3.4).


2000 Water Level, Old Portsmouth Strategy Study (mODN)

2000 Water Level, EA (mODN)

2004 Water Level EA (revised – mODN)

200 3.03 3.57 3.1 100 2.98 3.41 3.1 50 2.93 3.21 3.0 20 2.85 3.11 2.9 10 2.77 2.91 2.7 5 2.69 2.84 2 2.54 2.65

Table 3.4 – Extreme Water Levels from Old Portsmouth Strategy Study (1999) and

EA (2003)

Comparison of Tables 3.2-3.4 shows that the 1 in 200 year extreme water

levels stated in the SMP and Old Portsmouth Strategy Study show very

good agreement, whereas the 1 in 200 year extreme water level given in the

EA report (2000) is 0.54m higher. Investigations into these differences

generated a considerable amount of discussion. As a result, an independent

review of the results of the EA commission was undertaken and revised

water levels were published in 2004. These show a much closer correlation

to those used in this assessment. It is considered that the differences will

have a negligible effect on the assessments undertaken. However, to

determine the potential effect of different water levels on the selection of

the preferred option, appropriate sensitivity tests were undertaken as part

of the economic assessment (see the Economics Report).

(d) Tidal Currents and Sediment Transport

Famous for its unusual tide, the Solent exhibits a double high water and a

young flood stand, i.e. the tide slows in its rise part way towards high

water. Tidal resonance in the English Channel is the root cause of this due

to the Solent lying approximately central in this long basin. Also, a

degenerate amphidromic point lies west of the Isle of Wight, causing the

tidal range to approach zero (a full amphidrome has a range of zero). Tidal

range increases eastward along the Solent, with the range in Portsmouth

Harbour slightly less than that in Langstone Harbour.


The open coast of Portsea Island is made up of shingle with finer

sediments offshore, with sediment transport dominated by littoral (wave-

driven transport) drift. The Harbours contain a range of sediment from

sands to fine silts/muds, the intertidal areas are predominantly made-up of

fine cohesive sediments with some vegetation/algae. Vegetation and algae

has the effect of stabilising the mudflat on which it resides, thus aiding the

reduction of erosion and increasing the potential for deposition.

Portsmouth and Langstone Harbours both exhibit the characteristic

double high water and a young flood stand. The extended period of high

water has the effect of reducing the duration of the ebb tide; it also allows

for a long period of slack water during which time suspended sediment

can be deposited. Much of Portsea Island consists of hard defences

protecting relatively low-lying land.

A hydrodynamic model of the area surrounding Portsea Island has been

set-up and calibrated using the following sources of data:

• Topographic and hydrodynamic data collected during the site

survey.

• Hydrographic survey data for Hayling Island provided by Havant

Borough Council.

• Detailed bathymetry in Portsmouth Harbour provided by the

MoD.

• Portsmouth Commercial Port data provided by PCC.

• Admiralty Charts.

The purpose of this model was to assess how future changes to the mean

sea level may affect the hydrodynamic regimes influencing Portsea Island.

A range of analysis techniques have been used, the results of which are

summarised below, with full details presented in the Numerical Modelling

Report:

• The existing peak speeds were predicted in the harbours during a

mean spring tidal cycle. The results of this show that the highest

speeds occur in the harbour entrances (in excess of 2m/s) and the

main channels (generally less than 1.2m/s). Slower speeds of less

than 0.4m/s occur elsewhere in the harbours in shallower water.


• The differences in peak tidal current speeds between the existing

MHWS and MHWS+6mm pa sea level rise over 50 years were

calculated. Increases in speeds within the two harbour entrances

are of the order of 0.1m/s, as might be expected. In general, the

results show that the majority of current speed changes within the

harbours are increases of less than 5cm/s. Along the foreshore of

the western part of Portsea Island only very small changes are

observed, although some decreases of less than 10cm/s are

observed towards the north-west of the Island.

• Along the eastern shoreline of Portsea Island there are areas of

both increased and decreased peak tidal current speeds. There is

slightly more predominance of increased speeds of <10cm/s

towards the northern half of this shoreline and decreases of up to

20cm/s towards to the middle of the southern half of this

shoreline. Decreases in peak speeds in this area could lead to a

reduction in erosion and/or an increase in deposition.

• There is a general decrease in peak tidal current speeds (up to and

over 20cm/s) towards the eastern and north-eastern portions of

Langstone Harbour. This may lead to a potential increase in

sedimentation in the shallower areas of this region where current

speeds are generally low.

• Non-cohesive sediment reaching the Harbour mouths from the

Solent will be transported offshore due to ebb dominance in the

outer portions of these areas. Thus, little non-cohesive sediment

will enter the harbours. Non-cohesive sediment exists in the main

channels in the southern part of the harbours.

• Bed shear stresses due to tidal currents are unlikely to erode

partially consolidated mud from the intertidal mudflats under

normal conditions.

• The duration of the slack before ebb period increases with sea

level rise, potentially allowing more sediment to settle out of

suspension and deposit onto the bed.

• Rising mean sea levels increase the tidal prism and thus the

amount of suspended material entering the harbours.

• Wave height at the shoreline will increase by only 5cm for a mean

sea level rise of 30cm, showing that the waves are fetch limited

within the harbours and undergo limited changes due to the

increased water depth.


• Bottom shear stress due to waves may increase in some areas but

in general the stress will reduce over the intertidal areas. On the

western shore there is a general decrease in peak bed shear stress

(combined wave and currents) towards the north end of Whale

Island of about 0.5N/m2 and a general increase towards the

middle and southern end of less than 0.5N/m2. The shoreline in

the north-western tip of the Island at the entrance to the Hilsea

Channel undergoes a reduction of 0.5N/m2 or less.

• The eastern shore exhibits both increases and decreases in peak

bed shear stress (combined) but in general the intertidal mudflats

show more reduction.

• A calculation of how the bottom orbital velocity changes with an

increase in wave height of 0.7 to 0.75m and water depth from 2 to

2.3m was performed; an 8% reduction in bottom orbital velocity

resulted. This shows that the increase in means sea level could

reduce the amount of erosion occurring on intertidal shorelines.

The conclusion from these predicted changes is that the two harbours are

likely to undergo accretion as mean sea level rises. There is however some

uncertainty as to whether or not the accretion will be able to keep up with

the increase in water level. If the rate increases over time then the input of

sediment from the Solent may not be sufficient for this to happen; the

suspended sediment input into the harbours is the limiting factor.

Full details of the hydrodynamic and sediment transport modelling that

has been undertaken is given in The Numerical Modelling Report.

3.2.2 Wave Conditions

(a) Normal Wave Activity

Wave data is available for a number of locations around Portsea Island

from the Pagham Harbour Strategy Study. This is based upon 20 years of

hindcast data for April 1971 to March 1991.

For Portsmouth Harbour mouth this indicates that the most waves

(approximately 50%) come from south and west directions, these also

being the largest with waves up to 1.40m high recorded. Harbour

generated waves at the mouth reach 0.80m high. At Langstone Harbour

mouth the majority of waves are again from the south and west directions,

although here the largest waves, up to 2.40m, come from the south-east.


Harbour generated waves at the mouth reach 0.8m high. Within both

harbours the maximum recorded wave height was 1.10m. On the open

coast, data for a point near Southsea Castle indicates that 50% of waves

come from the south and west, with a maximum recorded wave height of

1.60m, over the 20 year period.

Wave periods are not reported for general conditions or marginal

extremes within the above study. Offshore, the relationship between wave

height and wave period (i.e. the wave steepness) usually lies between 0.05

and 0.065 (1/15 to 1/20). However, this relationship alters inshore. The

Old Portsmouth Strategy Study determined the inshore wave steepness to

be constant between different events at approximately 0.09 (or 1/11)

which is to be expected with synthetic data. This value appears valid as

waves propagate inshore and tend towards breaking (which occurs when a

steepness of 1/7 is approached). This relationship was adopted for the

Old Portsmouth Study to approximate wave periods for all conditions

used and is also appropriate for this Strategy Plan.

Linear regression on the 20 year wave data set was also performed as part

of the Pagham Harbour Strategy Study, to see if there were any trends in

wave height increase or directional shift that would need to be taken into

account in the future. It was determined that, for those points relevant to

Portsea Island, there are no significant trends.

For the Old Portsmouth Strategy Study, plane bed refraction analysis of

the inshore wave across the harbour entrance assessed the effect of the

sudden change of depth on the wave train direction. The directions of the

waves were not affected, and the angle of wave approach used in assessing

overtopping was taken as the direction given in the wave data.

(b) Ship-Induced Wave Activity

The effect of vessel movements in harbours is threefold; they induce

waves, currents, and have the potential to cause scour due to ship

propeller action. Studies for the Old Portsmouth Strategy concluded that

these waves were likely to impact on coastal defences, due to the

proximity of the main shipping channel to that frontage. Wave heights and

periods were calculated for the vessel types most frequently passing the

frontage.


(c) Extreme Wave Conditions

Extreme wave heights have been calculated using Halcrow’s wave

modelling suite MWAVE. To perform this analysis, a time-series of wave

and water level data at a range of locations along both the open coast of

Portsea Island, as well as within both Portsmouth and Langstone

Harbours, was required.

For the open coast, time-series wave and water level data at a number of

points was determined by using the MWAVE suite to transform a time-

series of waves and water levels inshore from an offshore point located in

the English Channel south of the Isle of Wight. These inshore time-series

data were then analysed using the MWAVE suite to derive extreme wave

heights at ten specified locations along the open coast.

For points within both Portsmouth and Langstone Harbours, it was not

possible to derive time-series data inshore using the method applied to the

open coast. For these points inside the harbours, hindcasting was

performed using the MWAVE suite and wind data records. Analysis of the

time-series data generated from hindcasting was again carried out using

the MWAVE suite to derive extreme wave heights within the harbours.

The locations used for this analysis are shown in Figure 3.1 and the results

are summarised in Table 3.5. More detailed information about how these

extreme wave heights have been derived is given in The Numerical

Modelling Report.

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Extreme Wave Heights at Various Locations (m)

Return

Period

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1 1.94 1.33 2.41 3.77 4.23 1.27 0.42 0.48 0.58 0.17 0.43 0.42 0.37 0.35 0.36 0.34 0.25 3.71 3.45 0.90

5 2.34 1.56 2.83 4.34 4.85 1.69 0.49 0.56 0.68 0.20 0.50 0.48 0.45 0.41 0.43 0.39 0.29 4.34 4.13 1.07

10 2.51 1.66 3.01 4.58 5.10 1.88 0.52 0.60 0.72 0.21 0.52 0.51 0.49 0.43 0.46 0.41 0.30 4.61 4.42 1.14

20 2.68 1.76 3.18 4.81 5.35 2.08 0.55 0.63 0.77 0.22 0.55 0.54 0.53 0.46 0.49 0.43 0.32 4.87 4.72 1.20

50 2.91 1.88 3.41 5.13 5.67 2.35 0.59 0.68 0.82 0.23 0.59 0.58 0.58 0.49 0.52 0.46 0.34 5.22 5.10 1.29

100 3.09 1.98 3.59 5.63 5.91 2.57 0.62 0.71 0.86 0.24 0.62 0.60 0.62 0.52 0.55 0.48 0.36 5.48 5.39 1.36

200 3.26 2.07 3.76 5.58 6.15 2.79 0.65 0.74 0.91 0.25 0.65 0.63 0.66 0.54 0.58 0.51 0.37 5.73 5.69 1.43

Table 3.5 – Extreme Wave Heights by Location


3.2.3 Wave and Water Level Joint Probability

Joint probability analysis has been carried out using the HR Wallingford JOINSEA

software for a number of points around Portsea Island, both on the open coast

and within the harbours. These points are the same as used to derive extreme wave

heights as shown in Figure 3.1.

This analysis produces a series of joint-probability curves of water level versus

wave heights for a range of extreme return period events. An example of these

results is shown in Figure 3.2. More detailed information about how these joint

probability waves and water levels are derived is given in The Numerical Modelling

Report.

Figure 3.2 – Joint Probability Results for Location 3

Joint Probability Curves from JOINSEA Analysis at Location 3

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 1 2 3 4 5 6 7

Water Level (mCD)

Wa

ve

He

igh

t (m

)

1 2 5 10 20 50 100 200 500


4 Shoreline Evolution

4.1 Introduction

4.1.1 Requirement

The shoreline presents a mobile environment and an understanding of how it is

evolving is essential for effective management. Natural coastal development is

dictated by the physical environment, which can be seriously affected by human

actions. An understanding of shoreline processes (Map 3) and their interaction is

central to the development of appropriate policies that can be successfully

implemented. This section reviews previous studies to establish a contemporary

knowledge of Portsea Island’s shoreline evolution and postulate potential future

evolutionary trends to support the development of effective management options.


Information was drawn from a variety of sources. These included the SMP and the

Pagham Harbour Strategy Study, the latter presenting the best analysis of sediment

transport and historic evolution for the study area. OS maps and the EA’s Annual

Beach Monitoring Survey (ABMS) provided historic and current beach profile and

shoreline evolution information. The Solent CHaMP detailed current and future

management options and their implications, whilst the Langstone Harbour

Management Plan considered sustainable management of the harbour. Halcrow et

al (2001) assessed the impact of climate change on the central south coast of

England. Aerial photographs of the open coast and harbours (1997 and 2001) were

supplied by the EA and examined. Dr M Bray, University of Portsmouth, also

supplied two unpublished technical papers ‘Chichester Harbour Entrance to

Portsmouth Harbour Entrance: Sediment Transport and Sedimentation’ and

‘Portsmouth, Langstone and Chichester harbours: Sedimentology and Sediment

Transport’ which discuss sediment movement, budgets and responses.

4.2 Factors Affecting Shoreline Evolution

The main impetus of long-term shoreline evolution, 15,000 yrs BP to the present,

is discussed in the Geology and Geomorphology chapter. This chapter focuses on

documenting shoreline change along Portsea Island’s coast and harbours on a

medium (150 years-present) to short-term (20 to 10 years-present) time scale.


The over-riding physical factors that temporally influence shoreline evolution are:

• The solid and drift geology (see Chapter 2 : Geology & Geomorphology);

• Beach material type, including upper and lower foreshore (See Chapeter 2 :

Geology & Geomorphology);

• Wave climate (See Chapter 3 : Numerical Modelling);

• Tidal regime (See Chapter 3 : Numerical Modelling);

• Sediment transport pathways, including sources and sinks (See Chapter 2 :

Geology & Geomorphology);

• Flood risk areas (see ?Engineering);

• Nearshore bathymetry and sediment distribution. (See Chapter 3 :

Numerical Modelling);

• Anthropogenic Influence (See ? Economics).

4.3 Sedimentology and Shoreline Evolution

4.3.1 Introduction

Each beach may be inferred as a ‘sediment store’, with material inputs and outputs.

Acknowledging that sediment transfers are frequent, the ‘store’ readily and

regularly adjusts to facilitate this. Sediment size and its ability to withstand attrition

is an influential factor regarding potential offshore losses from the beach store.

4.3.2 Overview of Portsea Island

Both shingle drift rates and potential seabed sediment mobility were examined in

the Pagham Harbour Strategy. The report identifies a westward flow of sand and

shingle along the beaches of the East Solent as a result of the sheltering effect of

the Isle of Wight from larger waves. Entrapment within the harbour deltas and

bank systems results in little material reaching Portsmouth Harbour. Fortunately,

the harbour is very sheltered and therefore acts as a sediment sink for fine material.

The sediment budget for Portsea Island is controlled by two main factors:

• The key contemporary material source is cliff erosion at Selsey Bill;

• A series of complex processes at Langstone Harbour mouth.

4.3.3 Langstone Harbour

Hooke and Riley (1987) infer that the peripheral harbour fringes have experienced

only slight erosion. Mapping of the harbour perimeter shows no significant change

in the position of the high water line occurred between 1870-1965, except for

those areas gained as a result of land reclamation – consequently, erosion has


added little sediment into the harbour system. These results are mirrored by this

study (Figure 4.1), with land reclamation the over-riding mechanism for coastal

change since 1870. In total, around 85ha of land has been reclaimed since 1870,

over half of which occurred between the 1940s-1970s at Milton Common. Other

significant reclamation areas include Kendall’s Wharf and around Eastney Lake.

4.3.4 Eastern Coast (Eastney Point to Southsea Castle):

HR Wallingford (1995) document that there has been a substantial build up at

Eastney Point. Between 1983-1992 the (vertical) rate of build up was as high as

0.6m/year at the northern end of Fort Cumberland. Material is transported north

in ‘pulses’ towards Eastney Spit but research documents show that the spit is prone

to intermittent erosion as a result of south-easterly gales.

From the historic mapping analysis (Figure 4.2), the tip of Eastney Point appears

to have migrated slightly northwards. The magnitude of change is small and may

simply be explained as a technical issue, e.g. an error in the mapping of mean high

water around the spit. Between here and Fort Cumberland, much of the shoreline

is artificially constrained, though slight erosion has occurred round the Fort itself.

Although the general sediment drift direction is westward, a drift divide is known

to exist at Eastney due to refraction at the East Winner Bank. The Pagham

Harbour Strategy Study identifies a unidirectional drift from west to east caused by

locally generated waves that move approximately 6,800m3/year of sediment. This

discovery is supported by the massive sediment accretion at Fort Cumberland

(Webber, 1982 estimates that between 1970-1981 this was c.15,000-

16,000m3/year). There is also believed to be a transfer of shingle of up to

13,000m3/year from Eastney across Langstone Harbour mouth. From the work

done for this study, it has been concluded that the shoreline west of Fort

Cumberland has accreted by over 100m since 1870 – this is by far the most

significant natural change around Portsea Island. The focus of accretion appears to

have moved slightly westwards from Fort Cumberland beach.

West of the Eastney divide, a weak westward drift of about 2,000 or 6,000m3 /year

has been estimated, reducing towards Southsea Castle as the coast becomes

orientated normal to the south-westerly waves. The Pagham Harbour Strategy

showed the annual average westward drift over the 20 year data period to be just

300m3/year probably reducing to almost zero due to the impact of defence

structures. This section of beach has been accreting for many years and provides a

good level of protection. The medium term trend is that the eastern part of the


beach maintains itself whilst the western frontage has experienced storm

overtopping.

The beach that fronts South Parade is a wide, shingle beach. Its levels are relatively

stable due to the frontage being extremely well sheltered from waves. Changes are

so small that no real historic trend can be determined. HR Wallingford (1995)

infers that between 1983 and 1992 the increase was approximately 5,200m3/

600m3/year. Analysis of historic maps (Figure 4.2) indicates that the shoreline has

remained static in this region due to the presence of defences along the back of the

beach.

4.3.5 Western Coast (Southsea Castle to Old Portsmouth):

Since 1870, there has been little shoreline change along this frontage (Figure 4.3).

This is due to its south-westerly aspect, which makes potential drift very low, and

long-term anthropogenic interference. Seawalls and rock groynes punctuate the

natural shingle beach and influence behaviour, i.e. the construction of Clarence

Pier in the 1940’s initiated a zone of shoreline change, nominally accretion,

immediately east of the pier. Elsewhere, the frontage has generally experienced

slight accretion, although there is some erosion towards the eastern end.

Examining each epoch individually, shoreline change along most of the frontage is

slight, with the magnitude of change generally 20m or less between epochs (Figure

4.3). This could be due to small historic mapping discrepancies. The most coherent

shoreline change occurs at the eastern end of the frontage, where the coastline

seems to have retreated by around 40m between 1870-1970 but re-advanced

thereafter, possibly due to a realignment of backshore defences. Generally the deep

water surrounding Southsea Castle separates and limits the net transfer of material

from the wide beach at South Parade to the narrow frontage at Southsea Common.

There is evidence of limited longshore drift between Southsea Castle and Clarence

Pier in the form of accumulations against structures. There is a regular build up of

shingle on the hovercraft slipway - this is cleared to maintain operations and is

probably related to recharge material placed intermittently to the east in front of

the War Memorial during the winter/ after storm events.

4.3.6 Portsmouth Harbour

Only a very small amount of material reaches the harbour due to the sheltering

effect of the Isle of Wight, entrapment of sediment against defences, the drift

divide at Eastney and offshore transfer of material. Some sand and shingle is swept


onto banks either side of the approach channel by the fast ebb-currents with finer

material deposited in the channel, although this is later removed by dredging. The

harbour is a defined sediment sink and thus is a convenient western boundary.

As in Langstone Harbour, mapping of the shoreline demonstrates how land

reclamation has dominated shoreline change in Portsmouth Harbour since 1870

(Figure 4.4). Almost 200ha of land has been reclaimed in the last 130 years - more

than double that in Langstone Harbour. Much of the reclamation occurred

between 1870-1890 when the navel base and port were constructed north of Old

Portsmouth. At this time Whale Island was also enlarged to its current size and

Alexandra Park was created. Land reclamation has been renewed since the 1970s,

creating land at Stamshaw for the M275 motorway and for ferry port expansion.

4.3.7 Port Creek

Port Creek is a brackish channel forming the northern boundary of Portsea Island

and joining Portsmouth and Langstone Harbours (Figure 4.5). Almost 10ha of land

has been reclaimed here, originally in the late 19th century around the railway bridge

and more recently since the 1970s, due to the construction of the M27 motorway.

Port Creek has a negligible contribution to the island’s overall sediment budget.

4.3.8 Offshore Sediment Transport

The dominant flow of sand and shingle along the open coast is westwards towards

Portsmouth Harbour. However, as material moves along the coast, the majority

becomes trapped within the harbour deltas and/or the flanking bank systems

adjacent to the entrances, thus the quantity arriving at Portsmouth Harbour mouth

is small. Material that does enter is flushed out by the rapid ebb flow, sustaining

offshore features such as Hamilton Bank and Spit Sand. Once here the Pagham

Harbour Strategy asserts that material is transported south-eastwards across Horse

and Dean Sands. Harlow (1980) hypothesis’s that some material is then transported

onshore by wave action to sustain accretion at Eastoke Point - he explains this

phenomenon as a process of wave induced mass transport at the seabed.

4.3.9 Summary

The present rapid accumulation of shingle at the eastern end of the open coast of

Portsea Island has not been accompanied by comparable losses elsewhere along

the frontage. Literature and new research infer that the most likely source of

material for these eastern beaches is from either Horse and Dean Sand or the

Langstone Harbour ebb delta.

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4.4 Current Shoreline Processes

4.4.1 Geomorphological Approach

The most effective geomorphological approach to determine current shoreline

processes is to relate ‘form’, i.e. beach profile, to ‘process’, i.e. wave action.

Applying this methodology enables one to:

• Interpret the degree of physical adjustment of the shoreline to prevailing

process regimes;

• Indicate the type and magnitude of process variables involved in creating

the form.

(a) Assessment of ABMS Beach Monitoring Data

Beach profiles are monitored annually by the EA and were assessed in the

Pagham Harbour Strategy Study and in the Old Portsmouth Study.

Individual beach profiles primarily provide a ‘snap-shot’ insight of the

frontage. However, if a sufficient number are collaborated then medium

term systematic changes in the profile, as well as short-term fluctuations,

may be inferred.

(i) Data Interpretation

Figures 4.6 and 4.7 illustrate ten years’ (1989-1999) of ABMS data.

Trends of positive (accretion) and negative (erosion) beach profile

change for Portsea’s eastern (Eastney Point to East Battery) and

western (Southsea Castle to Point Battery) frontage are

documented.

Table 4.1 depicts the 10-year trend that the eastern frontage (c. 4.5km) has

generally experienced, which is supported by Figure 4.6.

Location Outcome

Eastney Point (BP1) to Fort Cumberland (~BP7) Erosion

A drift divide exists in between these two zones (c.BP26-35) Drift Divide

Fort Cumberland (~BP7) to Pavillion (~BP50) Erosion

Pavillion (~BP50) to South Parade Car Park (~BP57) Erosion

South Parade Car Park (~BP57) to East Battery (BP83) Accretion

Table 4.1 - East Coast (Eastney Point to East Battery): 1991-2001 (BP = years

before present)

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Table 4.2 depicts the 10-year trend the western frontage (c.2.25km) generally

experienced which is supported by Figure 4.7.

Location Outcome

Southsea Castle (BP1) to Pavillion (~BP9) No change (v. slight

erosion at BP6-7)

Pavillion (~BP10) to past Portsea Monument

(~BP23)

Accretion (v. slight

erosion at BP12-16)

Portsmouth Monument (~BP23) to Point

Battery (BP38)

Accretion (v. slight

erosion at BP23-25,31)

Table 4.2 - East Coast (Eastney Point to East Battery): 1991-2001

(ii) Cumulative ABMS Trends

The main trends inferred from the EA’s ABMS (1989-1999) data

are:

• On a yearly basis periods of erosion tend to be followed by

periods of accretion and vice versa, ie cyclical change.

• The overall decadal picture shows erosion on the flanks of

the eastern frontage with accretion in the centre, and

predominantly accretion along the western frontage

punctuated by pockets of erosion.

• The drift divide appears to be slightly west of what Bray

(1995) hypothesised. It also has the potential to migrate

alongshore by up to 500 metres on a yearly basis. The littoral

drift divide is associated with wave refraction around East

Winner Bank.

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(b) Assessment of aerial photography/ historical maps

A medium-term perspective on shoreline evolution can be gained from

analysis of historical aerial photographs and maps. The aim was to

develop an understanding of shoreline change over the last 130 years - any

changes found give an insight into sediment dynamics and thus potential

future coastal change.

(i) Method

Historical OS maps were available from 6 epochs: 1870-74, 1898,

1910, 1938-40, 1969-73 and 1990-91. For each epoch, mean high

water was clearly marked and this was digitised using GIS. Aerial

photography was available from ABMS data - the position of

mean high water was digitised from the September 2002

photographs.

(ii) Data interpretation

To interpret changes in the mean high water line position, the

frontage was divided into 5 sections:

• (Open Coast) West Coast – The Point to Southsea

Castle;

• (Open Coast) East Coast – Southsea Castle to Eastney

Point;

• (Non-coastal) Portsmouth Harbour;

• (Non-coastal) Langstone Harbour;

• (Non-coastal) Port Creek.

On the open coast, shoreline evolution assumed to be at least

partly due to natural processes. Transects at 20m spacings were

analysed and direction, magnitude and rates of coastal change

calculated. These results are summarised in Table 4.3 and Figures

4.8 and 4.9.


Average rate of change (m/yr) Epoch

West Coastal East Coast

1870-1874 to 1898 -0.23 -0.17

1898 to 1910 +0.64 +0.50

1910 to 1938-1940 -0.29 -0.01

1938-1940 to 1969-1973 -0.15 +0.78

1969-1973 to 1990-1991 +0.68 +0.08

1990-1991 to 2002 +0.31 +0.34

1870-1874 to 2002 +0.05 +0.24

Table 4.3 - Shoreline Evolution Along Coastal Frontage 1870-2002

(Average rate of change refers to average change calculated for all transects over the entire

study period. Negative values show erosion; positive values accretion).

For the ‘non-coastal’ frontages, namely Portsmouth and

Langstone Harbours and Port Creek, it has been assumed that

most shoreline evolution since 1870 is due to land reclamation/

development. Hence, this area has been calculated using GIS

between each of the epochs, with the results summarised in

Table 4.4.

Figure 4.8 - West Coast (Battery Point to Southsea Castle) 1870-2002

-60

-40

-20

0

20

40

60

0 500 1000 1500 2000

Distance along frontage (west to east, m)

Sh

ore

lin

e c

ha

ng

e (

m)


-120

-80

-40

0

40

80

120

0 1000 2000 3000 4000 5000

Distance along frontage (west to east, m)

Sh

ore

lin

e c

han

ge (

m)

Figure 4.9 - EastCoast (Southsea Castle to Eastney Point) 1870- 2002

Area of Land Reclaimed, m² Epoch

Portsmouth

Harbour

Langstone

Harbour

Port Creek

1870-74 to 1898 1,506,983 85,430 35,068

1898 to 1910 29,860 138,029 1,922

1910 to 1938-40 42,157 76,661 11,197

1938-40 to 1969-73 112,936 448,508 43,485

1969-73 to 1990-91 231,938 7,740 1,037

1990-91 to 2002 69,621 92,049 1,478

1870-74 to 2002 1,993,495 848,417 94,187

Table 4.4 - Area of Reclaimed Land Along Estuarine Frontage 1870-2002


4.5 Predicting Future Shoreline Evolution

4.5.1 Introduction

The coastline is naturally dynamic and despite the many modifications to the

Portsea shoreline, natural processes continue. Therefore to implement any strategy,

or accommodate any change within such a densely populated and heavily managed

area, requires a flexible approach that may have to incorporate further

consideration of the importance and function of areas beyond the confines of the

strategy site.

4.5.2 Conceptual Approach

In predicting future change it is assumed that its general direction and scale will

occur mostly according to historical behaviour, but modified by factors such as

future management and climate change. A major problem is that many trends may

only have become established or identifiable in recent decades. This means that

extrapolation into the future may be based on short historical records which,

although representative of recent past change, may not provide an accurate guide

to the future, particularly over longer timescales.

Future changes are likely to be driven by the following factors:

• Ongoing changes representing continuing readjustments to past sea

level rise and several centuries of human interference.

• Climate change involving:

- Future sea-level rise and an increase in extreme water levels,

primarily due to greater storm surges and a possible increased

frequency of extreme inundations;

- More severe and frequent winter gales and rainfall (UKCIP,

2002).

• Changes due to future shoreline management policies such as

abandonment/ removal of defences, provision of new defences or

maintenance of defences.

(a) The ECUMEN Coastal Classification System

To assess the potential physical impacts of climate change one approach

that may be adopted is the European ECUMEN (1998) generic

morphological ‘coastal behaviour systems’ classification. Composed of

several landforms and linked by processes, the system recognises that the

overall behaviour of the coastal system is a function of the combined

effects of processes, responses of landforms and feedbacks operate


between them. Fundamentally a system will alter its behaviour to develop

a form that optimally dissipates the energy of the forcing agents – a

system in this state is in dynamic equilibrium. If the system is sensitive

and/or the magnitude of forcing too great, however, then modification

will be induced. As climate change involves potential variations in several

key forcing agents, the sensitivity of a system must be known in order to

estimate physical impact and hazard extent.

The ECUMEN system classifies different coastal land types, recognising

that they function in characteristic manners and have inherent sensitivities

that provide a morphological basis for the classification and understanding

of coastal process behaviour. The system also recognises that any given

land type may also exhibit a range of processes and sensitivities to change

according to its environmental setting, management and exposure.

(i) Portsea Island: Open Coast

The open coast acts as a wave dominated sedimentary plain with

barrier type beaches. Defences heavily control the frontage,

however, and it currently acts as a fringing beach. Tables 4.5 and

5.6 show the natural and modified beach sensitivity and response

to changes in sediment mobility and budget, sea level rise and

storm surges.

Beach

Type

Sediment

Mobility

Sensitivity to

Sediment Budget

Sensitivity to

Sea Level Rise

Sensitivity to

Storm Surge

Shingle

Barrier

Low /

Medium

High High, migration

by rollover

High, seepage,

flattening, over

washing,

breaching

Table 4.5 - Portsea Island Open Coast Classification (Undefended) using ECUMEN System


Beach

Type

Sediment

Mobility

Sensitivity to

Sediment Budget

Sensitivity to

Sea Level Rise

Sensitivity to

Storm Surge

Fringing Low /

Medium

Medium Low/Medium –

restrained by

backshore,

tendency for

down cutting

Medium/Low –

profile flattening

without loss

Table 4.6 – Portsea Island Open Coast Classification (Defended) using ECUMEN System

(ii) Portsea Island: Harbours

Harbours are complex to assess as they involve interaction

between estuarine and open coast systems with major sediment

storage and morphological adjustments occurring at the shoreline

and in the nearshore zone. Table 4.7 is a classification of the type,

i.e. inlet, and sub-type, i.e. ebb tidal delta, and response to sea level

rise, storm surges and change in wave direction.

Element Response to

sea level rise

Response to storm

surge

Response to wave

direction change

Inlet Erosion of banks

and/or bed

(permanent)

Erosion of banks

and/or bed (temporary)

Altered spit configuration,

possible altered spit stability

Ebb tidal

delta

Enlargement and

possible migration

seaward

Migration towards inlet,

possible reduction in

size

Longshore migration

Table 4.7 - Portsea Island Harbour Classifications – Type and Sub-Type using ECUMEN System

Regarding the predominant processes and environments

applicable to the harbours, tidal flats and saltmarshes are located

in Portsmouth Harbour but predominantly in Langstone

Harbour. Their dynamic behaviour is controlled by four main

physical factors - tidal regime, wind-wave climate, sediment

supply and relative sea level. The following components of


climate change have the potential to affect tidal flats and

saltmarshes:

• Sea level rise – causing saltmarsh inundation, reducing

the viability of their vegetation. Greater water depths may

cause enhanced wave action and tidal currents by

increasing the tidal prism. This will encourage erosion,

involving cliffing and recession of the seaward marsh

margin and widening of tidal channels. A larger tidal

prism should increase marine sediment inputs and

possibly encourage minerogenic accretion. Where

accretion fails to maintain pace with sea-level rise there is

a natural translation of the profile and its vegetation

communities landward, provided that low hinterland may

be inundated;

• Wave climate variations - may increase the exposure of

marsh margins to erosion;

• Temperature, salinity and precipitation variations - may

affect the viability of saltmarsh vegetation. This is

potentially very significant as most mid and lower

saltmarshes are dominated by a single vegetation species,

Spartina, that is already suffering die-back throughout

much of the region. It may also affect weathering rates

and freshwater runoff from the land and thus alter fluvial

sediment sources.

The potential sensitivity of tidal flats and saltmarshes is affected

by two other key factors:

• Sediment Supply - where supply is sufficient, mudflats

and salt marshes can accrete vertically to maintain their

surface levels with respect to rising sea-level. Analyses of

Holocene sedimentation sequences suggest that UK

saltmarshes receiving an adequate sediment supply can

survive prolonged rates of sea-level rise of up to 4-

5mm/year. Beyond that, inundation generally prevails

leading to deterioration of marsh. The harbours on the

Portsea coast are however relatively deficient in fresh


minerogenic sediment sources so that widespread

acceleration of accretion rates may not be possible.

• Landward Migration - is likely to be the dominant natural

response wherever accretion fails to keep pace with sea-

level rise.

4.5.3 Numerical Approach

(a) Introduction

The ability to predict future evolution is based around the quantity and

quality of the data set. For Portsea Island there is a large amount of data

due to the long-standing anthropogenic involvement and interest in the

site, although there are some gaps in the data record. Two numerical

modelling methods were adopted for the numerical analysis of coastal

change, one considering changes to the prevailing hydrodynamic regime

and the other changes to beach shape.

(b) Hydrodynamic Modelling

(i) Approach

The aim of this modelling exercise was to assess how changes to

mean sea level over the next 100 years may affect the prevailing

hydrodynamic regime, both along the open coast and in the

harbours. Various rates of sea level rise scenarios were examined,

namely 4mm, 6mm and 10mm/year, equating to an increase in

mean sea level of 0.2, 0.3 and 0.5m respectively over 50 years.

Further details of this work are given in the Coastal Conditions

section of this report. (see Chapter 3)

(ii) Results

• Sea level rise will result in increases in peak current

speeds in both harbour entrances. Along the western

foreshore of the island only very small changes are

predicted, with a gentle decrease towards the northwest.

Along the eastern shoreline of the island areas of both

increased and decreased peak tidal current speeds were

observed.

• At both present and future sea levels, the model shows

that offshore of the harbour entrances, sediment

transport is in an offshore, ebb direction, feeding Spit

Sands and Hamilton Bank. There is also a predominant

transport of sediment from inside the entrance of


Langstone Harbour towards Sword Sands in the harbour.

Sinah Sands lies between areas of ebb-and flood-

dominated transport, thus allowing this sand bank to

form. Higher sea levels are unlikely to significantly alter

the general pattern of sand transport, but may increase

transport within the main channels as a greater volume

of water moves in and out of the harbours.

• There are vast expanses of intertidal mudflat in both

harbours that may exist because seabed shear stresses are

too small for erosion to occur. The model shows that

increasing sea levels by 0.3m does alter bed shear

stresses, but only marginally. It should noted that the

model uses the same bathymetry for present and future

sea levels, assuming no changes have taken place.

• The narrow harbour entrances allow only small amounts

of wave energy to propagate through the entrance

channels. The height of locally generated waves inside

the harbours depends on bathymetry, wind speed and

fetch length. With an increase in sea level of 6mm/year

over 50 years (0.3m) the differences in wave heights are:

- Along the western shore of the island, wave heights

increase by 3 - 5cm;

- Along the eastern shoreline, wave height increases

are predicted to lie between 3 - 5cm in some places,

but mostly lie between 1 - 3cm. The eastern shore

exhibits both increases and decreases in peak bed

shear stress (combined) but in general the intertidal

mudflats show more reduction.

Consideration has been made of changes in the slack water period around high

water, known as the Slack Before Ebb (SBE), due to changes in sea level. This is

the period in which sediment transported into the estuary can be deposited onto

the bed as current speeds fall below threshold levels. The analysis has indicated

that within the estuaries over 8 minutes per tide extra potential for deposition

would occur with a 30cm rise in water levels. Over a whole year (710 tides) this

gives 95 extra hours of deposition.

In reality the sea level would not jump up instantly by 0.3m but would change

slowly (6mm/year for instance). What this calculation does show however is that


the harbours have the potential to increase the amount of deposition that can

occur and potentially keep up with sea level rise provided that the input of

sediment also increases. The total volume of water lying between mean spring

high water and mean spring low water (tidal prism) in both harbours is increased

by 7% with a 6mm rise in mean sea level (derived from volume calculations on the

numerical model bathymetry). This increase in tidal prism with increasing water

level will effectively bring an increased mass of suspended sediment from the

Solent. Therefore two mechanisms exist for the mudflats within the harbours to

accrete due to a rise in sea level. An estuary tends towards equilibrium, so that the

least amount of energy is expended. The equilibrium will be disturbed with a rise

in mean sea level, but an increase of suspended sediment entering the harbour and

an increase in the duration of the slack period around the high water will work

against the rising water levels by increasing sedimentation.

(c) Beach Plan Shape Modelling (BPSM)

(iii) Approach

Halcrow’s longshore modelling programme BPSM was used to

predict the long-term response of the open coast beaches to wave

action. Waves were established from Met Office data for the

period July 1992 to June 2002. Further details of this work are

given the Coastal Change Data Report.

(iv) Calibration

Two BPSM models (BPSM East and BPSM West) were

established relating to the varying alignment of the open coast

either side of Southsea Castle. It was not possible to calibrate the

BPSM west model ∴ this was abandoned (see the Coastal Change

Data Report for further details). After successfully calibrating the

BPSM East, the ongoing processes along the coast were

simulated, nominally the erosion and accretion in response to a

series of wave conditions. (Figure 4.10). The model was run to

examine potential future rates of change under a ‘Do Nothing’

management scenario.

(v) Model Outputs

The BPSM East model was run to predict 50 years of beach

evolution from 2003 to 20053. Figure 4.11 illustrates the results


of this assessment, Figure 4.12 shows the same result overlaid on

an OS background.

(vi) Conclusions

• There is very little shoreline change on an annual and

decadal basis, in the both alongshore and cross-shore

sediment movement.

• The drift divide hypothesised by Bray (2003) has been

modelled at being slightly more westwards along the

Eastern frontage. Despite this slight shift in the predicted

location of the divide, this result is coherent with Bray’s

research.

• Over 50 years, the model shows increased erosion at

Lump’s Fort and, to a lesser extent, east of this location.

West of Lump’s Fort the beach accretes, so the beach is

relatively stable on balance. This is coherent with the

smallest annual change mentioned above.

• Sensitivity variables were not included, e.g. sea level rise.

This was because different sea levels would alter the

wave climate and also model sensitivity to different wave

angles would need further calibration - thus comparison

of identical cases would not be possible.


4.6 Scenario Assessment

4.6.1 Introduction

The predicted effects of climate change and sea level rise combined with variations

in other forcing agents present a significant challenge for future coastal

management. It is anticipated that there will be an increased risk to many the assets

on Portsea Island.

The island is low-lying and vulnerable to flooding. One of the aims of the Strategy

Study is to establish the current level and extent of flooding and establish what

induces this and how it may change in the future according to geomorphological

predictions. Figure 4.13 depicts the aforementioned assessment by combining

results from the numerical approach with those from the conceptual approach,

nominally the ABMS data together with engineering appraisal. It is apparent that

there are three potential risk areas - Southsea Common, South Parade Pier and

Fort Cumberland.

4.6.2 Do Nothing Scenario

If nothing is done to maintain or improve the coastal defences that protect the

perimeter of the island then the following impacts are likely:

• Defences remain in place until failure under storm conditions or at end of

residual life.

• Following open coast defence failure, denudation of the foreshore is

initially anticipated with probable breaching of the shingle beach.

• Over time beach will stabilise to become progressively more natural.

Without artificial nourishment, beach volumes will be significantly lower

than today.

• Higher sea levels will result in bigger waves at the shoreline, causing

increased wave forces on existing structures.

• Potential changes in patterns of longshore drift could result in altered

patterns of erosion and deposition.

• There is an increased potential for low-lying areas to experience flash-

flooding due to intense rainfall events.

• The frontage may find its own equilibrium, i.e. the harbours are likely to

lose some intertidal areas but should keep pace (vertically) with sea level

rise, as it has done for the past 10,000 years.


4.7 Summary

4.7.1 Introduction

The ongoing monitoring and research of shoreline processes and the future

evolution of the coast are the key parameters for future, sustainable defence

policies. Collaborating, investigating and assessing material have established the

present understanding of shoreline processes and evolution; a critique of the

findings has been described.

4.7.2 Geomorphological Considerations Applicable to Portsea Island

Coasts controlled by artificial structures cannot adjust naturally to forcing agents

such as sea-level rise, or changing patterns of storminess that will alter the intensity

and direction of shore-normal and shore-parallel sediment transport. Such coasts

are unable to retreat or supply sufficient sediments in response to changing

patterns of erosion and accretion.

• The shoreline and nearshore zone is heavily used for commercial, military,

recreational and residential purposes. This demand puts an unprecedented

strain on the coastline.

• Sediment transport on much of the open coast is subject to drift divides and

coastal defences, making predictions of the sediment budget and future

beach development naturally subject to wide confidence limits.

• As harbour entrance channels dissect the open coastline, the continuity of

littoral transport is affected by tidal influences along with routine

maintenance dredging.

• Long-term climatic changes could reverse drift directions and increase the

risk of breaching, damage shoreline structures and induce overtopping.

• The process interaction of the coastal/harbour boundaries still needs to be

fully investigated.

4.7.3 Conclusion

The main cause of shoreline change around Portsea Island over the past 130 years

has been anthropogenic, with close to 300ha of land reclaimed. Natural shoreline

evolution has been very small due to the presence of hard defences around much

of the coastline. Significant natural change has only been experienced at Eastney,

where the beach has accreted by around 100m. In the future, beach loss and scour

are anticipated in front of defences, resulting in increasing nearshore water depths

and exposure to higher waves. Over time these factors will increase overtopping

and the probability of structural damage - therefore an allowance for future climate

change should be an important factor in the design of all new defences.

References

Bowen, D. Q., Rose, J., McCabe, A.M. & Sutherland, D.G. (1986): Correlation of Quaternary glaciations

in England, Ireland, Scotland and Wales. Quaternary Science Reviews, 5.

Bray M, 2003, Chichester Harbour Entrance to Portsmouth Harbour Entrance: Sediment Transport and

Sedimentation. unpublished paper

Bray M, 2003, Portsmouth, Langstone and Chichester harbours: Sedimentology and Sediment Transport.

unpublished paper

Environment Agency 2003, Solent Coastal Habitat Management Plan Volume II (CHAMP);

Environment Agency, aerial photographs;

Environment Agency, Annual Beach Monitoring Survey Data

Environment Agency, 2003, Extreme Sea Level Analysis – Kent, Sussex, Hampshire and Isle of Wight,

Interim Report

Guardline Surveys, 2002, Field survey data to Portsea Island Strategy Study.

Halcrow 1999, Old Portsmouth Strategy Study, Report to Portsmouth City Council

Halcrow, University of Portsmouth, University of Newcastle and Met Office, 2001, Preparing for the

Impacts of Climate Cheng. Report to SCOPAC.

Halcrow, 2002, Futurecoast. Report to Defra

HR Wallingford 1996, The East Solent Shoreline Management Plan (SMP); 4 Volumes.

HR Wallingford 1995, The Pagham Harbour to River Hamble Strategy Study;

Langstone Harbour Board, 1995, Langstone Harbour Management Plan Review.

Portsmouth City Council, Historic Maps

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