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Mangaroa River Flood Hazard Assessment

SUMMARY REPORT

GW/FP-G-06/65

Final

29 May 2006

Mangaroa River Hazard Assessment

SUMMARY REPORT

GW/FP-G-06/65

Final

29 May 2006

Sinclair Knight MerzLevel 12, Mayfair House 54 The TerracePO Box 10-283 Wellington New Zealand Tel: +64 4 473 4265 Fax: +64 4 473 3369 Web: www.skmconsulting.com

Executive Summary

This report draws together the analysis of the hazards associated with the Mangaroa River to assist

in the preparation of planning and policy controls for the Mangaroa Valley.

A coupled one and two dimensional hydraulic model of the major channels in the valley was

developed to predict and quantify the flooding hazards, further information about the model can be

found in the Hydraulic Modelling Report GW/FP-T-06/63. The model was also used as part of a

geological and morphology study to identify areas of the floodplain at risk of erosion by the river.

Further detail of this investigation is contained within the Erosion Report GW/FP-T-06/64.

Where these hazards have a high impact on property or endanger life a damage assessment has

been carried out to provide an indication of the levels of risk associated with the river. The

hydraulic model was then used to investigate the potential for a flood warning systems on the river.

This investigation has identified extents and depths of flooding for a range of storm events and has

established recommended building levels based on the 100 year flood. An “erosion hazard

corridor” has been developed from the setbacks determined for each “at risk” area in the

geomorphology and erosion investigation. Flood and Erosion Hazard Plans, included in this report,

collate the findings of these investigations.

The major findings and recommendations from this investigation are summarised below:

In large flooding events the ponding behind Wallaceville Hill Road helps reduce the severity

of flooding downstream of the road. This area should be maintained as a storage zone. Greater

Wellington may wish to take this a step further and formalise the overflows into this area as

long-term flood mitigation.

The UHCC District Plan makes all building within the expected 100 year flooding zone of the

Hutt River a discretionary activity this policy should be brought through to the Mangaroa

River Valley to control development.

A number of existing residential buildings were found to be at risk of flooding. These locations

include the Upper Mangaroa area around Johnson’s and Russell’s Road, the Huia Homestead

area, the house upstream of the Mangaroa Hill Road and the Maymorn Road Residential area.

These areas should be investigated further to identify possible protection measures that could

be implemented to reduce the risk.

The Whiteman’s Valley Road Bridge at MIKE11 chainage 9010 was found to be a potential

location for a flood warning system that could provide warning to many of the at risk areas in

the flood hazard zone. Evacuation procedures could be developed for the at risk areas.

Furthermore a gauging station at this location would assist in future calibration of flood flows

as it is will be unaffected by the water levels of the Hutt River.

SINCLAIR KNIGHT MERZ

I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE i

Erosion hazard setbacks have been recommended for development around the river channel.

No new development should occur in these zones unless justified by site specific analysis of

the erosion hazard.

The Earthworks restrictions in the UHCC District Plan should be applied to the erosion hazard

zone developed in this study.

Further investigation of the stability and risk should be carried out in specific locations where

buildings are currently within the erosion hazard zones.


I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE ii

Contents

1. Introduction 4

1.1 Project Summary 4

2. Flooding Hazards Summary 5

2.1 Background 5

2.2 Model Hydrology 5

2.3 Model Description 7

2.4 Model Results 7

2.5 Design Scenarios and Freeboard 7

2.6 Flood Mapping 8

2.7 Modelling Results 8

3. Erosion Hazards Summary 17

3.1 Geological Mapping 17

3.2 Hypsometeric relief maps 17

3.3 Risk Assessment and Planning Constraints 17

4. Flood Damage Assessment 20

4.1 Method 21

4.2 Analysis for the without freeboard condition 23

4.3 Analysis for with freeboard condition 25

4.4 Results 28

5. Flood Warning Critical Level 29

5.1 Potential of the Te Marua Gauging Station for Flood Warning 29

5.2 Whiteman’s Valley Road Bridge 31

5.3 Flood Warning Recommendations 31

6. UHCC District Plan Provisions 33

6.1 Generic provisions in relation to flooding and erosion 33

7. Environmental Database 36

8. Planning and Recommendations 39

8.1 Hazard Area Maps 39

8.2 Recommendations 39

9. Reference 41


I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 2

Document history and status

Revision Date issued Reviewed by Approved by Date approved Revision type

Final 29/05/06 BHF BHF 29/05/06 Final

Distribution of copies

Revision Copy no Quantity Issued to

Final 1 10 Philip Purves (GW)

Printed: 19 September 2006

Last saved: 20 July 2006 10:15 AM

File name: I:\Aenv\Projects\AE02482\Deliverables\AE02482W0004.doc

Author: Jahangir Alam

Project manager: Benjamin Fountain

Name of organisation: Greater Wellington Regional Council

Name of project: Managaroa River Flood Hazard Assessment

Name of document: Summary Report

Document version: Final

Project number: AE02482



1. Introduction

This report seeks to provide the technical basis for the analysis of the existing hazards associated

with the Mangaroa River. Hazard management plans have been developed through this study and

these have been used to discuss policy for managing the risks associated with both the river

flooding and erosion hazards.

This report is supported by two documents that cover in greater detail the investigation

methodology and results. These reports are;

The Hydraulic Modelling Report (GW/FP-T-06/63)

The Erosion Report (GW/FP-T-06/64)

1.1 Project SummaryThe hazards investigated in this study of the Mangaroa River, fall into the two categories, these are

flooding and erosion.

A coupled 1D-2D hydraulic model of the major channels in the valley was developed to predict and

quantify the flooding hazards. This model was also used as part of a geological and morphology

study to identify areas of the floodplain at risk of erosion by the river.

Where these hazards have a high impact on property or endanger life a damage assessment has

been carried out to provide an indication of the levels of risk associated with the river. The

hydraulic model was then used to investigate the potential for a flood warning systems on the river.

The Upper Hutt City Council District Plan has been summarised and a review of the literature on

the environmental issues associated with the river has been carried out to identify other

considerations that could influence the development of hazard management policy.

To aid in the development of policy to manage the flood and erosion hazards associated with the

Mangaroa river the information obtained through this study has been collated into combined Flood

and Erosion Hazard Plans.



2. Flooding Hazards Summary

2.1 BackgroundThe Mangaroa catchment has an area of 103 square kilometres. The Mangaroa River is

approximately 20 km long from its headwaters around Russells Road to its confluence with the

Hutt River, see Figure 2-1. Many of the sub-catchments that feed into the main river channel are

very steep with falls of up to 500 meters over three to four kilometres. 15 – 20 % of the catchment

is rural alluvial floodplain with the rest of the catchment made up of a mixture of indigenous forest,

regenerating scrub and exotic forest.

2.2 Model Hydrology and Boundary ConditionsAn assessment of flood hydrology was completed by Greater Wellington’s Resource Investigation

Department. The investigation is detailed in the report Investigation of Flood Hydrology of the

Mangaroa Catchment (Watts, 2005).

This study involved rainfall analyses for the Mangaroa Catchment including calibration and

validation of a rainfall runoff model, modelling of design rainfall events and flood frequency

analyses. The six hour storm events developed through this study were provided as inputs for the

hydraulic modelling.

The Hutt River was found to influence the water levels over the lower reaches of the Mangaroa

River. A conservative 20 year water level in the Hutt River was used at the downstream boundary

of the model.



Figure 2-1 Location map of Mangaroa River

Hutt River

Mangaroa River Russells Road



2.3 Model DescriptionA coupled 1D-2D hydraulic model was chosen to simulate the flooding in the river. The DHI

software packages MIKE11 and MIKE21 were used.

During the initial stages of the investigation the significance of the Black Creek tributary was

identified. The scope of the study was expanded to include the creek in the model.

Cross section data was obtained from a survey carried out for this investigation. 167 cross-sections

were used to construct a model of the main channels in MIKE11. One dimensional modelling

provides a platform for effectively simulating structures, such as culverts and bridges, as well as

channel processes.

The left and right banks of the river channels were then linked to a MIKE21 two dimensional

model of the floodplain. Greater Wellington made their LIDAR information available for the

construction of the 2D model. The spot elevation information was used to generate a TIN in GIS

and this was used to obtain the topographic levels of the floodplain.

During the simulation of flood flows, water escaping the main channels modelled in one dimension

in MIKE 11overflowed onto the floodplain modelled in two dimensions in MIKE21.

2.4 Model ResultsThe model was calibrated by comparing the results with observed events at the Te Marua gauging

station and with eye witness accounts of recent flooding. While the results obtained from this

comparison demonstrated that the model would produce results of the accuracy required for this

investigation, a number of issues were identified. These include:

The gauging station at Te Marua is influenced by the Hutt River, this could influence the

development of the hydrological model as well as the calibration of the hydraulic model.

The hydrological model appears to underestimate the storage within the catchment.

2.5 Design Scenarios and FreeboardSix different return period floods (Q5, Q10, Q20, Q50, Q100 and the QExtreme) have been simulated using

the calibrated model. The model was then altered to reflect the impacts of potential blockages

around structures and sedimentation within the river bed. The impacts of the blockages and

sedimentation were used along with the QExtreme to conduct a sensitivity analysis that was used to

develop appropriate levels of freeboard.



2.6 Flood MappingThe modelled peak water levels for the 100 and 50 year storms were extracted for mapping. Maps

have been developed to aid in the setting of Recommended Building Levels. An overview of the

Flood Hazards Maps are included as Figure 2-3, Figure 2-4, Figure 2-5 and Figure 2-6.

2.7 Modelling ResultsTo summarise the major findings of the modelling the river can be divided into a number of

sections. These sections are shown in Figure 2-2.

Upper Mangaroa

Huia Stream Area

Wallaceville Road

Residential Area

Lower Mangaroa

Whitemans Valley Road

Figure 2-2 Flood Hazard Areas ofDiscussion



P.MGRREVIEWNo DATE

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The concepts and information containedin this document are the copyright ofSinclair Knight Merz Ltd. Use orcopying of the document in whole or inpart without the written permission ofSinclair Knight Merz Ltd. constitutesan infringement of copyright. DESIGNED

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MANGAROA RIVER MODELLING INVESTIGATIONQ50 AND Q100 FLOOD OVERVIEW

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MangaroaFlood Hazard Maps 2005

- Reduced levels are in terms of MeanSea Level 1953- The levels are basedon a design Q100& Q50 year return stormevent, taken as occurring over the entire catchment in its existingstate.- RBL is theminimum recommended building floor level forhabitable buildings. The RBL includes a freeboardallowance added to the TWL. The freeboard accounts foruncertainties such as, obstructions,aggradation,waveaction etc.- Sensitive or high value developments may warrant floodrisk assessments of greater detail.

The flood Information contained within these plans has beenderived from informationand techniques available at the timeof the study. It is provided to assist in reducing the flood riskfor development in thearea. However theagencies andindividuals involved in theassessment of the flood hazardassumeno responsibility for any action by any agency orindividual that is based on the information provided.

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2.7.1 Upper Mangaroa (Chainage 0m - 2000 m)

The flooding in this area is typical of an unconstrained rural stream. The model demonstrates that

many sections of the channel does not have the capacity to convey even a 5 year event.

Furthermore structures in this area are overtopped or cause the channel flows to overtop the bank.

In this area half a dozen residential properties are inundated in the 50 and 100 year events, see

Figure 2-3.

2.7.2 Whiteman’s Valley Road (Chainage 2000 m -7000m)

The narrow valley in this area confines the flooding to the flat areas adjacent to the channel. While

the floodplains surrounding the main channel are inundated, in some places by over 1 m in depth,

there are very few buildings built in the flooding hazard zones in this area.

The model results demonstrate that the bridge in this region, at 408 Whiteman’s Valley Road is

overtopped in a 50 and 100 year flooding event. The structure of the bridge should be checked to

ensure that the bridge can withstand this occurrence, see Figure 2-7.

Figure 2-7 Bridge at 408 Whiteman’s Valley Road



2.7.3 Huia Stream (Chainage 7000 m - 8000 m)

Just down stream of the confluence with the Huia Stream a low point in the bank allowed the

modelled flows to escape the main channel and inundate a number of homesteads in this area, see



Figure 2-4. The morphological study of the river identified evidence of an ancient course of the

Mangaroa that passed through this area. The model demonstrated that the banks at this location just

overtopped in a 10 year event. The model indicates that the homesteads in these areas are at risk of

flood levels exceeding floor level in events greater than the 10 year event.

2.7.4 Wallaceville Road (Chainage 9000 m - 11000 m)

The model predicts that the main channel between Katherine Mansfield Drive and Mangaroa

Valley Road / Wallaceville Hill Road is unable to convey the 20, 50 or 100 year flows. Water

spilling from the main channel is likely to flow overland and pond behind the Wallaceville Hill

Road, which is raised above the surrounding land, see Figure 2-5. Until the road is overtopped the

only release for this ponding water is through the Black Creek Box Culvert. Because of the limited

capacity of the culvert the water is constrained in this area. The model indicates that the controlled

release of the water through the Black Creek Culvert helps prevent worse flooding in Black Creek

downstream of Wallaceville Road. See the photo of the culvert in Figure 2-8.

Figure 2-8 Black Creek Twin Box Culvert under Wallaceville Hill Road

2.7.5 Lower Mangaroa (Chainage 12000 m - 18000 m)

Downstream of the confluence with Black Creek the Mangaroa becomes incised. This constrains

and deepens the flooding extents. The bridge on Mangaroa Hill Road increases the flooding depths

upstream and the model predicts that the residential property upstream of the bridge will be



inundated even in a 5 year event. Other than this house there appears to be little other affected

development in this area, see Figure 2-6.

2.7.6 Maymorn Road Residential Area (Chainage 18000 m - 19180 m)

The residential area around Maymorn Road near the confluence of the Mangaroa and Collins

Stream has a number of properties at risk of flooding. The flooding in this area is also influenced

by water levels in the Hutt River. The risks to this area are further analysed in Section 4 of this

report.



3. Erosion Hazards Summary

3.1 Geological MappingGeological maps were produced for the Mangaroa River Valley. The maps provided much of the

base information to identify the constraints on river morphology and locate areas of erosion risk.

The Geology of much of the Mangaroa Valley floor is made up of alluvium. The alluvium that has

been active recently was assessed as being highly susceptible to erosion, while the older alluvium

deposits appear to be more stable.

The steeper hills surrounding the valley are generally comprised of greywacke and argillite. These

geological formations are considered to have a low susceptibility to erosion.

3.2 Hypsometeric relief maps To obtain an understanding of the historic erosion and geomorphology of the river system the

LIDAR information, provided by Greater Wellington, was used to develop hypsometeric maps.

The hypsometric and geological maps were combined and used to identify a “corridor of erosion”

surrounding the river. The corridor encompasses all the identified erosion features and significant

at-risk landforms.

An overview of the maps are included in Figure 3-1 and Figure 3-2.

3.3 Risk Assessment and Planning Constraints A site investigation, aerial photographs and the hypsometeric and geological maps were used to

identify areas at risk of erosion or landslides. To aid in the developing of planning constraints a risk

matrix that compares the likelihood and the consequences of failure was used as a means to identify

appropriate setbacks in the at risk areas. These setbacks are shown on the overview maps in Figure

3-1 and Figure 3-2.



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EThis map shows interprete d geology fromexamination of aerialphotographs andsurface exposu res.It should not be used for si te specific works,which require a specific investigation.Reference has been made to GNS 1996'Geology of the Wellington Area' forcross checking purposes.

Disclaimer:

The concepts and information contain edin this document are the copyright ofSinclai r Knight Merz Ltd. Use orcopying of the document in whole or inpart wi thout the written permission ofSinclai r Knight Merz Ltd. constitute san infringement of cop yright.

Data Notes:

MANGAROA VALLEYPARTIAL GEOLOGICAL MAP

Figure 3.1

LegendErosion Hazard AvoidanceArea

Significant Areas at Risk from Erosion

Building Exclusion Offset

Geo logic al Un it

fa Holocene Alluviumfs Holocene Swamp/Peatlg Pleistocene Alluviumtr Triassic Rimutaka Belt Greywacketw Triassic Wellington Belt Greywacke

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fa Holocene Alluviumfs Holocene Swamp/Peatlg Pleistocene Alluviumtr Triassic Rimutaka Belt Greywacketw TriassicWellington Belt Greywacke

LEGEND

This map shows interprete d geology fromexamination of aerialphotographs andsurface exposu res.It should not be used for si te specific works,which require a specific investigation.Reference has been made to GNS 1996'Geology of the Wellington Area' forcross checking purposes.

Disclaimer:

The concepts and information contain edin this document are the copyright ofSinclai r Knight Merz Ltd. Use orcopying of the document in whole or inpart wi thout the written permission ofSinclai r Knight Merz Ltd. constitute san infringement of cop yright.

Data Notes:

MANGAROA VALLEYPARTIAL GEOLOGICAL MAP

Figure 3.2

LegendErosion Hazard AvoidanceArea

Significant Areas at Risk from Erosion

Building Exclusion Offset

4. Flood Damage Assessment

A flood damage assessment has been carried out for affected residential buildings in the residential area around Maymorn Road. For this analysis the tangible direct losses have been estimated in terms of monetary values. The damage analysis is based on the depth damage relation set out for the Hutt River in 1990 (Source: GWRC). This report identified a schedule of damages based on four different levels of flood inundation, which are:

Level 1: Just below floor level

Level 2: Just above floor level but under 500 mm

Level 3: between 500 and 2000 mm above floor level

Level 4: above 2000 mm

To calculate the net worth of the damages it was necessary to adjust the previous costingsinto present day values. Considering its construction period was predominantly after 1960, the buildings in this area are assumed to fall into the “above average” category. The following percentage increase was found for a representative house of above average category, see Table 4-1.

Table 4-1 Percentage Increase of Cost in 2005

1990* 2005 % increase Remarks for 2005 situation

Construction cost 120000 260000 116.67% For 140 m2 house

Chattels withreplacement cost

116000 132000 13.50% Based on CPI index of Statistics New Zealand

*Source: GWRC

Based on the above percentage rate of increase the depth damage relation is increased linearly for all levels of inundation into present day values as shown in Table 4-2. Thedamage estimation has been done for combined buildings and contents.



Table 4-2 Adjusted Depth Damage Relation for 2005

1990* 2005

FloodInundation

BuildingLoss

ContentLoss

Buildingand ContentLoss

BuildingLoss

ContentLoss

BuildingandContentLoss

Level 1 $4000 $1000 $5000 $8667 $1135 $9802

Level 2 $9000 $12000 $21000 $19500 $13620 $33120

Level 3 $49000 $46000 $95000 $106167 $52210 $158377

Level 4 $63000 $106000 $169000 $136500 $120310 $256810

* Source: GWRC

4.1 Method The damage analysis has been carried out using the modelled flooding results both with and

without freeboard. Freeboard at this location is 1m above the top water levels and is likely to

present a conservative approach to damage assessment, particularly in the smaller flooding events.

The affected buildings have been identified for different flooding events from the flood hazard

maps as shown in Figure 4-1 for both the with and without freeboard situations. For this analysis it

has been assumed that floor levels are 0.5m above ground for these dwellings.



Figure 4-1 Buildings at Risk in the 100 Year Flooding Event



4.2 Analysis for the without freeboard condition The flood inundation has been calculated for the buildings at risk. The following table shows the

number of buildings affected for different flood inundation depths.

Table 4-3 Number of Buildings at Risk for Different Flood Events (Without freeboard)

Number of Buildings FloodedDepthLevel

Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5

Level 0 -500 0 0 0 0 0Level 1 -500 - 0 7 5 4 2 0Level 2 0 - 500 3 2 0 0 0Level 3 500 - 2000 0 0 0 0 0Level 4 2000+ 0 0 0 0 0

Based on the numbers of affected buildings the damage at different inundation level was

calculated, this is shown in Table 4-4 below.

Table 4-4 Damage Value for Different Flood Events (Without Freeboard)

Total Value of Damage DepthLevel


Level 0 -500 0 0 0 0 0Level 1 -500 - 0 $68,612 $49,008 $39,207 $19,603 0Level 2 0 - 500 $99,360 $66,240 0 0 0Level 3 500 - 2000 0 0 0 0 0Level 4 2000+ 0 0 0 0 0

Figure 4-2 Damage Value for Different Flood Events (Without Freeboard)

Flood Damage at Different Flood LevelWithout Freeboard( )

0

20000

40000

60000

80000

100000

120000

0.000 500.000 2000.000 >2000

Flood Depth above Floor (mm)

Dam

age

($)

Q100

Q50

Q20

Q10

Q5

Damage Value for Different Flood Events

(Without Freeboard)



Accumulated losses have been estimated for each flood event. The losses are summed from the

lowest level to highest level flooding (Table 4-5). These figures are illustrated in a Damage vs.

Probability curve as seen in Figure 4-4.

Table 4-5 Accumulated Damage (Without Freeboard)

Accumulated Total Damage ($) DepthLevel


Level 0 -500 $0 $0 $0 $0 $0Level 1 -500 - 0 $68,612 $49,008 $39,207 $19,603 $0Level 2 0 - 500 $167,972 $115,248 $39,207 $19,603 $0Level 3 500 - 2000 $167,972 $115,248 $39,207 $19,603 $0Level 4 2000+ $167,972 $115,248 $39,207 $19,603 $0

Figure 4-3 Accumulated Flood Damage (Without Freeboard)

Accumulated Flood Damage(Without Freeboard)

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

0.000 500.000 2000.000 >2000


Dam

ag

e (

$) Q100

Q50

Q20

Q10

Using the maximum accumulated losses a Damage vs. Probability function has been drawn in

Figure 4-4.



Figure 4-4 Damage vs. Probability Curve (Without Freeboard)

Damage vs. Probability(Without Freeboard)

0

20000

40000

60000

80000

100000

120000

140000

160000

180000

0% 5% 10% 15% 20% 25%

Percentage Exceedance

Dam

ag

e (

$)

5 Year Flood

10 Year Flood

20 Year

100 Year Flood

50 Year Flood

4.3 Analysis for with freeboard condition The same analysis was carried out using the flooding with the freeboard added.

Table 4-6 Number of Buildings at Risk for Different Flood Events (With Freeboard)

Number of Buildings FloodedDepthLevel


Level 0 -500 0 0 0 0 0Level 1 -500 - 0 1 1 4 6 7Level 2 0 - 500 5 7 7 6 5Level 3 500 - 2000 10 7 4 2 0Level 4 2000+ 0 0 0 0 0

Table 4-7 Damage Value for Different Flood Events (With Freeboard)

Total Damage Value ($)DepthLevel


Level 0 -500 $0 $0 $0 $0 $0Level 1 -500 - 0 $9,802 $9,802 $39,207 $58,810 $68,612Level 2 0 - 500 $165,600 $231,840 $231,840 $198,720 $165,600Level 3 500 - 2000 $1,583,767 $1,108,637 $633,507 $316,753 $0Level 4 2000+ $0 $0 $0 $0 $0



Figure 4-5 Damage Value for Different Flood Events (With Freeboard)

Flood Damage at Different Flood Level(With Freeboard)

0

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0.000 500.000 2000.000 >2000


Dam

age

($)

Q100

Q50

Q20

Q10

Q5

Damage Value for Different Flood Events

(With Freeboard)

Accumulated losses were estimated for each flood event. The losses are summed from the lowest

level to highest level (Table 4-8). These figures are illustrated in a Damage vs. Probability curve as

seen in Figure 4-6.

Table 4-8 Accumulated Damage (With Freeboard)

Accumulated Total Damage ($) DepthLevel


Level 0 -500 $0 $0 $0 $0 $0Level 1 -500 - 0 $9,802 $9,802 $39,207 $58,810 $68,612Level 2 0 - 500 $175,402 $241,642 $271,047 $257,530 $234,212Level 3 500 - 2000 $1,759,168 $1,350,278 $904,553 $574,283 $234,212Level 4 2000+ $1,759,168 $1,350,278 $904,553 $574,283 $234,212



Figure 4-6 Accumulated Damage (With Freeboard)

Accumulated Flood Damage(With Freeboard)

0

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1600000

1800000

2000000

0.000 500.000 2000.000 >2000


Dam

ag

e ($

)

Q100

Q50

Q20

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Q5

Using the maximum accumulated losses a Damage vs. Probability function has been drawn in

Figure 4-4.

Figure 4-7 Total Damage vs. Probability (With Freeboard)



Damage vs. Probability(With Freeboard)

0

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0% 5% 10% 15% 20% 25%

Percentage Exceedance

Dam

age

($)

5 Year Flood

10 Year Flood

20 Year Flood

100 Year Flood

50 Year Flood

4.4 Results The following table shows the damages associated with direct flooding to the residential buildings

and its contents for the with and without freeboard condition.

Table 4-9 Comparison of damage for without and with freeboard condition

Flood EventDamages (without

Freeboard condition)Damages (with

Freeboard condition)

Q100 $ 167,972 $ 1,759,168

Q50 $ 115,248 $ 1,350,278

Q20 $ 39,207 $ 904,553

Q10 $ 19,603 $ 574,283

Q5 $ - $ 234,212

This comparison demonstrates the high increase in the estimation of flood damage value if

freeboard is applied. Freeboard was derived from a sensitivity analysis and is included to account

for many factors and variables in the modelling process, including model assumptions and data

limitations. The freeboard in this area also includes an allowance for high flows in the Hutt River

causing increased water levels in the Mangaroa. The likelihood of high flows in the Hutt River

occurring during an extreme event in the Mangaroa has not been quantified but is likely to reduce

the probability of flood damage occurring. This suggests that the damages with freeboard are likely

to be conservative estimates.

The results of the flood damage assessment are able to be used for cost-benefit analysis for flood

protection or flood warning systems on the Mangaroa.



5. Flood Warning Critical Level

5.1 Potential of the Te Marua Gauging Station for Flood Warning The hydraulic model has identified the areas of flooding on the Mangaroa for a variety of storm

events. From these areas key locations have been identified as potential high risk areas to property

or life. These locations are shown in Figure 5-1 and have been discussed in further detail in the

Hydraulic Modelling Report.

The model was used to predict the approximate level at which the onset of significant flooding

occurs and the minimum storm event at which this was recorded. At the time when the onset of

flooding occurred the model was used to obtain the water level at the Te Marua Gauging station.

Table 5-1 shows these results, where the water levels are taken direct from the model without the

addition of freeboard.

Table 5-1 Flood Warning Critical Level

Location Water Level atthe Onset ofFlooding (m)

MinimumFlooding Event

Approximate Levelat Te Marua Gauge

at Onset ofFlooding (m)

Upper Mangaroa

(M11 Chainage 1595)

210.9 Q5 89.0

(Peak WL in Q5: 91.9)

Bridge at #750 Whiteman’s ValleyRoad (M11 Chainage 3057)

202.9 Q5

(Road is inundated)

90.3


Huia Stream Area near the Homestead

(M11 Chainage 7120)

172.5 Q10 91.4


Wallaceville Road Area

(M11 Chainage 9630)

152.2 Q20 92.2


Homestead Upstream of the MangaroaHill Road Bridge

(M11 Chainage 13570)

123.6 Q5 89.9


Maymorn Road Residential Area

(M11 18860)

93.0 Q20 91.8


The variability of the levels at the Te Marua Gauge indicates that timing of the flood flows is an

important issue. The model predicts the travel time for flows between Russells Road and the Hutt

River to be over 2 hours. This is likely to be too long for the Gauging Station to provide a viable

location for flood warning in the upper catchment.

It is appears that the Te Marua gauging station would be useful for flood warning only for the

residential area around Maymorn Road. For the other flooding areas a site further up the catchment

is likely to be more suitable.



Figure 5-1 Location Map of Key Flooding Areas

Mangaroa Hill Road Bridge

Residential Area

Bridge at #750 Whitemans Valley Road

Whitemans Valley Road Bridge


Huia Stream Area

Upper Mangaroa



5.2 Whiteman’s Valley Road Bridge One potential gauging site is at Whiteman’s Valley Road Bridge at MIKE11 chainage 9010. To

investigate this location the water levels at this site were recorded at the time when the model

predicted the onset of flooding at key locations.

Table 5-2 shows the results.

Table 5-2 Flood Warning Critical Levels at the Whiteman’s Valley Road Bridge

Location Water Level atthe Onset ofFlooding (m)

MinimumFlooding Event

Approximate Level atthe Whiteman’s

Valley Road Bridge at Onset of Flooding (m)

Upper Mangaroa

(M11 Chainage 1595)

210.9 Q5 155.04


Bridge at #750 Whiteman’s ValleyRoad (M11 Chainage 3057)

202.9 Q5

(Road is inundated)

156.4


Huia Stream Area near the Homestead

(M11 Chainage 7120)

172.5 Q10 156.9

(Peak WL in Q10: 157.1)


(M11 Chainage 9630)

152.2 Q20 157.2

(Peak WL in Q20: 157.3)

Homestead Upstream of the MangaroaHill Road Bridge

(M11 Chainage 13570)

123.6 Q5 156.2


Maymorn Road Residential Area

(M11 18860)

93.0 Q20 157.1

(Peak WL in Q20: 157.3)

Table 5-2 demonstrates a number of flooding locations where a gauge at this location could provide

a useful flood warning tool.

The onset of flooding at three sites, the Huia Stream Homestead, the flooding around the

Wallaceville Road area and the Maymorn Road residential area, all demonstrate similar water

levels at the Whiteman’s Valley Road Bridge. The model indicates that when the water levels

under the bridge exceed 156.9m RL (approximately 3.1m above the river bed) then flooding is

likely to occur in these three locations.

5.3 Flood Warning Recommendations The Te Marua gauging station appears to have little use as a flood warning tool other than for the

residential area around Maymorn Road. It is recommended that a suitable site for a gauging station

be found further up the valley. One potential site that appears to provide better flood warning

results could be a gauge around the Whiteman’s Valley Road Bridge, see Figure 5-1. A gauge at



this location has the potential to provide warning for the Huia Homestead, the area flooded around

Wallaceville road and the Maymorn Road Residential Area.

The Whiteman’s Valley Road Bridge is also well situated about midway down the catchment and

approximately 11km from the residential area on Maymorn road. The trigger levels at this location

could be set to provide sufficient warning to mobilise evacuation or temporary flood protection.



6. UHCC District Plan Provisions

The Upper Hutt City Council District Plan, September 2004, has been reviewed to identify any

generic provisions in relation to flooding and erosion hazards and also any specific provisions and

development information that relates to the Mangaroa Valley.

6.1 Generic provisions in relation to flooding and erosion The following rules and standards with respect to flooding and erosion are outlined in the District

Plan.

Chapter 33 of The UHCCDP covers the Rules for Flooding and Fault Band Hazards

Flood mitigation works undertaken or approved by the local authority are a permitted activity.

Buildings and Structures erected within the 1% (1 in 100 year) flood extent of the Hutt River are

discretionary activities. This impacts only on the area around the confluence of the Hutt a and a

short way up the Mangaroa.

Any new habitable building or structure to be erected within the fault band identified within the

Planning Maps is a discretionary activity. This does not impact on the Mangaroa.

Chapter 23 of The UHCCDP covers the Rules for Earthworks and Vegetation Clearance

Earthworks activities are permitted in the residential, business and special activity zones where

existing ground levels shall not be altered by cutting a vertical height of more than 1.5m or by

filling by a vertical height of more than 0.5m. Earthworks out with these standards are limited

discretionary activities.

Earthworks shall not be undertaken on erosion prone land, identified as land with a gradient steeper

than 28 degrees, with a 10m buffer zone at the top of the slope.

Earthworks identified on a site of Heritage or in a location with a notable tree are discretionary

activities.

Clearance of indigenous vegetation is permitted on sites of up to 500m2 in total area on any one

site within a continuous 5 year period provided the activity meets the requirements of section 23.11

of the District Plan.

Clearance of indigenous vegetation is permitted on sites of up to 1ha in total area on any one site

within a continuous 5 year period where the vegetation is predominately Manuka or Kanuka, which

has a canopy height of no more than 4m and provided the activity meets the requirements of

section 23.11 of the District Plan.



Any indigenous vegetation clearance, which exceeds the above permitted activities or does not

meet the requirements of Section 23.11 of the District Plan are discretionary activities.

6.2 Specific provisions and development in relation to Mangaroa ValleyThe Mangaroa Valley is predominately zoned as Rural with a small area of Residential, Business

Industrial and Open Space near the Mangaroa River confluence with the Hutt River. Refer to

Figure 6-1 below which details the District Plan zoning.

Figure 6-1 District Plan Maps of the Mangaroa Catchment

Development is permitted within the Mangaroa Valley in accordance with the Rules and Standards

for each Zone as detailed within the District Plan in the following sections,

Section 18 – Residential Zone Rules Section 19 – Rural Zone Rules Section 20 – Business Zone Rules Section 21 – Open Space Zone Rules



The majority of the upper valley is zoned as Rural Hill or Rural Valley Floor with pockets of Rural

Lifestyle blocks within the valley. The rules and standards for these areas are covered by section 19

of the District Plan.

There are topographical features identified within the Plan which includes the protected ridgeline to

the North West of the Valley and the ridgeline to the East of the Valley above the Mangaroa River

and its tributaries.

Towards lower end of the valley there are areas designated as Residential Hill on the fringes of the

urban area and the valley. There are sections of the river corridor designated as Open Space

towards the lower valley and an area of residential beside the river near to the confluence with Hutt

River.

The state highway crosses the Mangaroa River just upstream of the confluence with Hutt River.

There is a local purpose amenity designation on Sierra Grove/ Seymour Way, Southern Hills.

The District plan identifies a number of sites within the Mangaroa Valley which have significant

importance for Wellington Regional Council (WRC), Upper Hutt City Council – Reserves and

community facility (UHC), Upper Hutt City Council – Utilities (UHCC), Minister for Education

(ED), Telecom New Zealand and Minister for State Owned Enterprises (TZR). These are detailed

as follows,

UHC3 – Proposed Scenic Reserve (Southern Hills Ridgeline, Wallaceville Road)

UHC17 – Civic Purposes Pound (Mangaroa Hill Road)

UHC51 – Recreation (Local Purpose Esplanade)

UHC84 – Scenic Upper Plateau Road

UHC88 – Recreation (Whitemans Valley Road)

UHCC Utility No.5 (water Storage Seymour grove)

UHCC Utility 19 – Maymourn Treatment Pumping Station

ED6 – Mangaroa Primary School (Flux Road)

TEL2 (Mount Climie Telecommunications and Radio Communications)

TZR1 – Wellington to Woodvile Railway including tunnel 1 and 2.

WRC3 – Proposed Water Catchment (Part of Pakuratahi River lying with Mangaroa River

catchment)

WRC4 – Proposed Water Catchment (Pakuratahi River)



7. Environmental Database

The main intention of the preparation of an environmental database for the Mangaroa River Valley

is to outline the environmental issues in the catchment. The following paragraphs describe the

major findings from the literature review.

Greater Wellington (2005), Selecting Catchments for Streamside Management Assistance,

Publication No. GW/RP-G- 05/05

This report covers the process by which GIS can be used as a tool to determine which streamside

areas can be eligible for financial assistance for rehabilitation. This tool adopts 5 criteria to

determine a ranking for catchments in the Wellington region. The five criteria are:

High quality aquatic habitat; High potential for rehabilitation;Functioning ecosystem potential; Ecological links and corridors;Range of stream types.

Criteria application was undertaken in 3 stages:

Stage One – all catchments in the region investigated for high quality aquatic habitat and high

potential for rehabilitation.

Stage Two – The top sixteen catchments, from stage one, were investigated for functioning

ecosystem potential, ecological links and corridor and range of system types region-wide.

Stage Three – The top twelve catchments from Stage Two were checked against relevant GIS

information.

The Mangaroa River was found to have met the criteria in this investigation and therefore would be

eligible for funding for streamside management assistance.

Outline Summary of VUW Honours Thesis: The Effect of Land Use on Water Quality in the

Mangaroa River, E J Trotter, Research Essay 3/95

This research had three key aims:

1) assess the water quality of the Mangaroa river and any changes through time2) identify and quantify relationships between land use and water quality; and 3) compare the water quality of the Mangaroa river in 1976 as indicated by Shirley (1976), and in

1995.



To meet the first two aims of this study, samples were collected at 15 different sites, once a month

for four months1. The research used the following measures for water quality:

Indicator Measure Assessment

temperature Declined over the time period – consistent with

seasonal variations and other measures (e.g. Smith

and Maasdam 1994)

pH Fluctuated with seasonal variations but results

were consistent with other measures.

Physical quality of water

quality of watercourses

Dissolved Oxygen Was at 100% saturation and exceeded the value of

5mg/l for Class D water. This is lower than the

Wellington average.2

BOD One median reading was 4.2g/m3 which was

attributed to a piggery. The remaining readings

ranged between 0.5g/m3 and 1.7g.m3.

Nitrate nitrogen

(NO3-N)

Concentrations of NO3-N along the Mangaroa

river reflect impacts from tributaries, surface run-

off and point sources. The readings are high in

comparison to the Wellington region (median is

0.4mg/l) but low compared to the national average

(median 32mg/l).

nutrient loadings caused by

non-point and point source

run-off

Dissolved reactive

phosphorus

Normally less than 0.3mg/l except at sites that had

run-off from the piggery (median for that site

4.2mg/l). Readings here are less than the national

average of 5mg/l.

Conductivity Generally low overall but high values related to

contaminated water from various land-uses or

point source pollution. Higher conductivity than

other Wellington rivers / streams.

content of dissolved

organic and inorganic

material total dissolved

solids

Generally less than 0.007g/l but some sites reached

as high as 0.15g/l due to high level of organic

waste from piggery.

aesthetic quality and in-

stream biota

Turbidity Increased with flow with high levels from three

sites. Readings were 10x national average and

was attributed to errors in measurements.

disease producing

organisms from animals

(including humans)

discharged into a waterway

Total coliforms The total number of coliforms varies along the

river depending on the point source pollution (e.g.

dairy sheds, manure dump, grazing animals).

1 March, April, May and July.2 Wellington Regional Council’s readings are taken in autumn and winter when readings are generally high.



The general conclusions of the comparative study in this thesis indicate that an increase in fertiliser

usage in the Mangaroa River Valley is the likely cause of the deteriation in the stream water

quality.

Various websites with relevant information

www.fishandgame.org.nz/SITE_Default/SITE_your_region/SITE_Wellington/Fishing/access.asp

Renowned for it’s free rising trout the Mangaroa fishes best early in the season when a number of

Hutt River spawners are still loitering before returning to the Hutt. For later season fishing there is

a healthy population of resident fish in most areas of deep water, though high water temperatures

and nuisance algal growth, inhibit fishing. The lower reaches are distinctively stained from the

large Te Pango swamp, making spotting fish difficult. South of the Wallaceville Hill Road, the

river is clear but fish are not as plentiful as in the stained reaches downstream.

http://www.wellingtonflyfishers.org.nz/html/rivers.html

The stream and its tributaries are spawning sites for Hutt River fish. Difficult terrain in the lower

section at Te Marua, but becomes easier in the farmland above. Levels drop in late summer and

most fish move down to the Hutt River. Early to mid summer dry fly fishing using caddis patterns.

http://open-site.org/Regional/Oceania/New_Zealand/Wellington/Upper_Hutt_City/Whiteman’s-

Mangaroa_Valley/

http://www.trainweb.org/valleysignals/tunnels/cruickshanks.html

http://www.rimutaka-incline-railway.org.nz/route/upper-hutt-maymorn.html

District Plan is available in the following website

http://www.upperhuttcity.com/Documents/District_Plan/index.html

Various Datasets available through GW mapping tool

Ecology:

“point-click-critter” predictive modelling of native fish probablilities by streamNIWA river classification – classifies stream network segments by landform, bed type, general environment etc Regional ecodomains – broad scale environment classification for region Landcare land Environment of NZ – land environment classification based on 15 climate / soil / landform variables NZ land resource inventory – soil types, land capability, slopes etc DoC ecosites – sited of ecological significance Land cover database 2



8. Planning and Recommendations

This project has sought to identify, quantify and analyse the flooding and erosion hazards

associated with the Mangaroa River. The results discussed in this report are to assist in the

development of planning and policy for the management of the catchment.

8.1 Hazard Area MapsTo further aid in the development of management strategies the hazards identified in the flooding

and erosion studies have been collated into one set of plans. These are attached as Plans 1-7.

The plans contain the following information:

Modelled flooding extents of the 100 year flood.

The recommended erosion hazard setback

The “erosion hazard corridor” has been developed from the setbacks determined for each “at risk”

area in the geomorphology and erosion investigation. The result are continuous lines running down

each bank that enclose the flood erosion hazard zone.

8.2 Recommendations The flooding and erosion investigations have identified a number of recommendations to assist in

managing the hazards and developing planning constraints.

8.2.1 Flooding Hazard Recommendations

Many of the structures on the Mangaroa, particularly in the upper reaches of the catchment, are

constraints to the system that have the potential to increase adjacent flooding. Maintenance of

the structures is important to help reduce the flooding risk.

In large flooding events the ponding behind Wallaceville Hill Road helps reduce the severity

of flooding downstream of the road. This area should be maintained as a storage zone. Greater

Wellington may wish to take this a step further and formalise the overflows into this area as

long-term flood mitigation.

The UHCC District Plan makes all building within the expected 100 year flooding zone of the

Hutt River a discretionary activity this policy should be brought through to the Mangaroa

River Valley to control development.

A number of existing residential buildings were found to be at risk of flooding. These locations

include the Upper Mangaroa area around Johnson’s and Russell’s Road, the Huia Homestead

area, the house upstream of the Mangaroa Hill Road and the Maymorn Road Residential area.



These areas should be investigated further to identify possible protection measures that could

be implemented to reduce the risk.

The Whiteman’s Valley Road Bridge at MIKE11 chainage 9010 was found to be a potential

location for a flood warning system that could provide warning to many of the at risk areas in

the flood hazard zone. Evacuation procedures could be developed for the at risk areas.

Furthermore a gauging station at this location would assist in future calibration of flood flows

as it is will be unaffected by the water levels of the Hutt River.

8.2.2 Erosion Hazard Recommendations

Erosion hazard setbacks have been recommended for development around the river channel.

No new development should occur in these zones unless justified by site specific analysis of

the erosion hazard.

The Earthworks restrictions in the UHCC District Plan should be applied to the erosion hazard

zone developed in this study.

Further investigation of the stability and risk should be carried out in specific locations where

buildings are currently within the erosion hazard zones.



9. Reference

Austroads, 1994. Water Way Design: A Guide to the Hydraulic Design of Bridges, Culverts and

Floodways, Sydney, Australia.

Chow, V. T., Maidment D.R., Mays, L. W., 1988. Applied Hydrology, McGraw-Hill International

Edition, New York.

DHI- Water &Environment, 2003. MIKEFLOOD-automated flood modelling and mapping, User

Guide, Denmark.

D M Hicks and P D Mason, 1998. Roughness Characteristics of New Zealand Rivers: A handbook

for assigning hydraulic roughness coefficients to river reaches by the “visual comparison”

approach, National Institute of Water and Atmospheric Research Ltd, Christchurch, New

Zealand, Water Resources Publication, LLC.

Trotter ,E. J., 1995. The Effect of Land Use on Water Quality in the Mangaroa River, Research

Essay 3/95, Research School of Earth Science, Victoria University of Wellington, New

Zealand

Greater Wellington Regional Council (GWRC), 2000. Selecting Catchments for Streamside

Management Assistance, Publication No. GW/RP-G-05/05

Greater Wellington Regional Council (GWRC), 2000. Wainuiomata River Flood Hazard

Assessment report, (Hydraulic Modelling and Floodplain Morphology), Greater Wellington

City Council, New Zealand, pp28.

Greater Wellington Regional Council (GWRC), 2003. Flood Procedures Manual, Flood Protection

Department, Greater Wellington City Council, New Zealand

Watts, L., 2005. Flood Hydrology of the Mangaroa River, Greater Wellington Regional Council,

New Zealand.



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Data Notes:MANGAROA VALLEYFLOOD AND EROSION

HAZARD AREASSHEET 1

LegendFlood Erosion Hazard Area

Q100 Flooding

FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Flood&ErosionHazardZones\FloodErosionHazardZone_Sheet1.mxd

PLOTDATE:10/5/200613:34:38

Disclaimer:

This map shows inte rpreted geolog y fromexa min atio n of a erial photographs an dsurface e xpo sures.It should not be used for site specific works,which requ ire a specific in vestigation.Reference has b een made to GNS 1996'Geolo gy of the Welli ngton Are a' forcross checking purposes.

The flood Information contained within these plans has beenderived from informationand techniques available at the timeof the study. It is provided to assist in reducing the flood riskfor development in the area. However the agencies andindividuals involved in the assessment of the flood hazardassume no responsibility for any action by any agency orindividual that is based on the informationprovided.

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The co ncep ts a nd information containedin this document are the copyright ofSin clair Knigh t Merz Ltd. Use orcopyin g of the docume nt in whole or inpart with out the written permission o fSin clair Knigh t Merz Ltd. constitu tesan in fringement o f cop yrig ht.

Data Notes:

This map shows in terpreted g eology fromexamina tion of aerialp hotograph s andsurface exposures.It should not be used for site specific works,which requi re a specifi c investigation.Reference has bee n made to GNS 1 996'Geolog y of the We llington Area' forcross checking purposes.

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