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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.
SINCLAIR KNIGHT MERZ
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
SINCLAIR KNIGHT MERZ
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
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 3
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 4
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 5
Figure 2-1 Location map of Mangaroa River
Hutt River
Mangaroa River Russells Road
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 6
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 7
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
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 8
P.MGRREVIEWNo DATE
DESIGN REV'DP.DIRAPP'D
AMENDMENT
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
DRAWN
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APPROVED
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Level 12, Mayfair HouseP54 The TerracePPO Box 10-283, WellingtonPNEW ZEALANDPTel +64 4 473 4265PFax +64 4 473 3369
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GREATER WELLINGTONREGIONAL COUNCIL
MANGAROA FLOOD MAPPING
TT 02DEC05
1:15,000 AE02482 Figure 2.3
MANGAROA RIVER MODELLING INVESTIGATIONQ50 AND Q100 FLOOD OVERVIEW
LegendQ50 RBL
Q100 RBL
River
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Q100Overviews\Q100_Overview1.mxd
PLOTDATE:10/5/200612:01:13
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.
Disclaimer
@A3
P.MGRREVIEWNo DATE
DESIGN REV'DP.DIRAPP'D
AMENDMENT
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
DRAWN
PROJECT
CLIENT
DATE
DESIGN REVIEW
REVIEWEDPROJECT MANAGER PROJECT DIRECTOR
APPROVED
SCALE
TITLE
SKM PROJECT No DRAWING No AMDT
CHECKDRAFTING
FILENAME:
PLOTDATE:
COPYRIGHT
F
E
D
C
B
A
A3
87654321
F
D
E
B
C
A
1 2 3 4 5 6 7 8
Level 12, Mayfair HouseP54 The TerracePPO Box 10-283, WellingtonPNEW ZEALANDPTel +64 4 473 4265PFax +64 4 473 3369
0
146
149
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WELLINGTON REGIONAL COUNCIL
MANGAROA FLOOD MAPPING
TT 02DEC05
1:15,000 AE02482 Figure 2.4
LegendQ50 RBL
Q100 RBL
River
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Q100Overviews\Q100_Overview_2.mxd
PLOTDATE:10/5/200611:58:38
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.
Disclaimer
@A3
MANGAROA RIVER MODELLING INVESTIGATIONQ50 AND Q100 FLOODING OVERVIEW
P.MGRREVIEWNo DATE
DESIGN REV'DP.DIRAPP'D
AMENDMENT
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
DRAWN
PROJECT
CLIENT
DATE
DESIGN REVIEW
REVIEWEDPROJECT MANAGER PROJECT DIRECTOR
APPROVED
SCALE
TITLE
SKM PROJECT No DRAWING No AMDT
CHECKDRAFTING
FILENAME:
PLOTDATE:
COPYRIGHT
F
E
D
C
B
A
A3
87654321
F
D
E
B
C
A
1 2 3 4 5 6 7 8
Level 12, Mayfair HouseP54 The TerracePPO Box 10-283, WellingtonPNEW ZEALANDPTel +64 4 473 4265PFax +64 4 473 3369
0
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WELLINGTON REGIONAL COUNCIL
MANGAROA FLOOD MAPPING
TT 02DEC05
1:15,000 AE02482 Figure 2.5
LegendQ50 RBL
Q100 RBL
River
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Q100Overviews\Q100_Overview3.mxd
PLOTDATE:10/5/200612:04:40
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.
Disclaimer
@A3
MANGAROA RIVER MODELLING INVESTIGATIONQ50 AND Q100 FLOODING OVERVIEW
P.MGRREVIEWNo DATE
DESIGN REV'DP.DIRAPP'D
AMENDMENT
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
DRAWN
PROJECT
CLIENT
DATE
DESIGN REVIEW
REVIEWEDPROJECT MANAGER PROJECT DIRECTOR
APPROVED
SCALE
TITLE
SKM PROJECT No DRAWING No AMDT
CHECKDRAFTING
FILENAME:
PLOTDATE:
COPYRIGHT
F
E
D
C
B
A
A3
87654321
F
D
E
B
C
A
1 2 3 4 5 6 7 8
Level 12, Mayfair HouseP54 The TerracePPO Box 10-283, WellingtonPNEW ZEALANDPTel +64 4 473 4265PFax +64 4 473 3369
16310
19180 19020
18930
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180701796017820
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WELLINGTON REGIONAL COUNCIL
MANGAROA FLOOD MAPPING
TT 02DEC05
1:15,000 AE02482 Figure 2.6
LegendQ50 RBL
Q100 RBL
River
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Q100Overviews\Q100_Overview4.mxd
PLOTDATE:10/5/200612:06:59
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.
Disclaimer
@A3
MANGAROA RIVER MODELLING INVESTIGATIONQ50 AND Q100 FLOODING OVERVIEW
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
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 13
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
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 14
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
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 15
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 16
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 17
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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
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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.
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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.
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Figure 4-1 Buildings at Risk in the 100 Year Flooding Event
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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
Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5
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)
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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
Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5
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
Flood Depth above Floor (mm)
Dam
ag
e (
$) Q100
Q50
Q20
Q10
Using the maximum accumulated losses a Damage vs. Probability function has been drawn in
Figure 4-4.
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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 (
$)
5 Year Flood
10 Year Flood
20 Year
100 Year Flood
50 Year Flood
4.3 Analysis for with freeboard condition The same analysis was carried out using the flooding with the freeboard added.
Table 4-6 Number of Buildings at Risk for Different Flood Events (With Freeboard)
Number of Buildings FloodedDepthLevel
Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5
Level 0 -500 0 0 0 0 0Level 1 -500 - 0 1 1 4 6 7Level 2 0 - 500 5 7 7 6 5Level 3 500 - 2000 10 7 4 2 0Level 4 2000+ 0 0 0 0 0
Table 4-7 Damage Value for Different Flood Events (With Freeboard)
Total Damage Value ($)DepthLevel
Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5
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 $0
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Figure 4-5 Damage Value for Different Flood Events (With Freeboard)
Flood Damage at Different Flood Level(With Freeboard)
0
200000
400000
600000
800000
1000000
1200000
1400000
1600000
1800000
0.000 500.000 2000.000 >2000
Flood Depth above Floor (mm)
Dam
age
($)
Q100
Q50
Q20
Q10
Q5
Damage Value for Different Flood Events
(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
Depth above FloorLevel (mm) Q100 Q50 Q20 Q10 Q5
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 $234,212
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Figure 4-6 Accumulated Damage (With Freeboard)
Accumulated Flood Damage(With Freeboard)
0
200000
400000
600000
800000
1000000
1200000
1400000
1600000
1800000
2000000
0.000 500.000 2000.000 >2000
Flood Depth above Floor (mm)
Dam
ag
e ($
)
Q100
Q50
Q20
Q10
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)
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Damage vs. Probability(With Freeboard)
0
200000
400000
600000
800000
1000000
1200000
1400000
1600000
1800000
2000000
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.
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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
(Peak WL in Q5: 91.9)
Huia Stream Area near the Homestead
(M11 Chainage 7120)
172.5 Q10 91.4
(Peak WL in Q10: 92.3)
Wallaceville Road Area
(M11 Chainage 9630)
152.2 Q20 92.2
(Peak WL in Q20: 92.5)
Homestead Upstream of the MangaroaHill Road Bridge
(M11 Chainage 13570)
123.6 Q5 89.9
(Peak WL in Q5: 91.9)
Maymorn Road Residential Area
(M11 18860)
93.0 Q20 91.8
(Peak WL in Q20: 92.5)
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.
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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
Wallaceville Road Area
Huia Stream Area
Upper Mangaroa
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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
(Peak WL in Q5: 156.8)
Bridge at #750 Whiteman’s ValleyRoad (M11 Chainage 3057)
202.9 Q5
(Road is inundated)
156.4
(Peak WL in Q5: 156.8)
Huia Stream Area near the Homestead
(M11 Chainage 7120)
172.5 Q10 156.9
(Peak WL in Q10: 157.1)
Wallaceville Road Area
(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
(Peak WL in Q5: 156.8)
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
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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.
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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.
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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
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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)
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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.
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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.
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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
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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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 39
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 40
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.
SINCLAIR KNIGHT MERZ
I:\Aenv\Projects\AE02482\Deliverables\Summary Report (final).doc PAGE 41
0 100 200 300 400 500m
1:10,000
Document Status:
Name Date
ELEGEND
The co ncepts and info rmation containedin this docume nt are the copyright ofSin clair Knig ht Me rz Ltd . Use orcop ying of the document in whole or inpart without the written pe rmission ofSin clair Knig ht Me rz Ltd . co nstitutesan infring emen t of copyright.
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.
Disclaimer
0 100 200 300 400 500m
1:10,000
Document Status:
Name Date
ELEGEND MANGAROA VALLEY
FLOOD AND EROSIONHAZARD AREAS
SHEET 2
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.
Disclaimer:
LegendFlood Erosion Hazard Area
Q100 Flooding
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Flood&ErosionHazardZones\FloodErosionHazardZone_Sheet2.mxd
PLOTDATE:10/5/200613:38:14
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.
Disclaimer
Revision B BHF 09/06
0 100 200 300 400 500 m
1:10,000
Document Status:
Name Date
ELEGEND MANGAROA VALLEY
FLOOD AND EROSIONHAZARD AREAS
SHEET 3
The co ncepts and info rmation containedin this docume nt are the copyright ofSin clair Knig ht Me rz Ltd . Use orcop ying of the document in whole or inpart without the written pe rmission ofSin clair Knig ht Me rz Ltd . co nstitutesan infring emen t of copyright.
Data Notes:
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.
Disclaimer:
LegendFlood Erosion Hazard Area
Q100 Flooding
FILENAME:I:\Aenv\Projects\AE02482\Gis\ReportMXD's\A3Flood&ErosionHazardZones\FloodErosionHazardZone_Sheet3.mxd
PLOTDATE:10/5/200613:40:11
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.
Disclaimer
0 100 200 300 400 500 m
1:10,000
Document Status:
Name Date
ELEGEND
MANGAROA VALLEYFLOOD AND EROSIONHAZARD AREAS
SHEET 4
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.
Disclaimer:
The concepts and information con tainedin this do cument are the copyright ofSincla ir Knight Merz Ltd. Use orcopying of the document in whole or inpart without the written p ermission ofSincla ir Knight Merz Ltd. constitutesan infringement of copyright.
Data Notes:
LegendFlood Erosion Hazard Area
Q100 Flooding
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.
Disclaimer
0 100 200 300 400 500m
1:10,000
Document Status:
Name Date
ELEGEND
MANGAROA VALLEYFLOOD AND EROSIONHAZARD AREAS
SHEET 5
The concepts and information con tainedin this do cument are the copyright ofSincla ir Knight Merz Ltd. Use orcopying of the document in whole or inpart without the written p ermission ofSincla ir Knight Merz Ltd. constitutesan infringement of copyright.
Data Notes:
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.
Disclaimer:
LegendFlood Erosion Hazard Area
Q100 Flooding
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.
Disclaimer