Mangaroa River Flood Hazard Assessment
HYDRAULIC MODELLING REPORT: VOLUME 1 GW/FP-T-06/63
Final
20/3/07 Mangaroa River Flood Hazard Assessment
HYDRAULIC MODELLING REPORT: VOLUME 1 GW/FP-T-06/63
Final
20/3/07
Sinclair Knight Merz Level 12, Mayfair House 54 The Terrace PO Box 10-283 Wellington New Zealand Tel: +64 4 473 4265 Fax: +64 4 473 3369 Web: www.skmconsulting.com Hydraulic Modelling Report
Contents
Executive Summary
1. Introduction 1 1.1 General 1 1.2 Scope of Modelling 1 2. River and Catchment Description 2 2.1 General Description 2 2.2 River Description 2 3. Hydraulic Modelling Concept 4
4. Survey and Data Collection 5 4.1 Topographic Data 5 4.2 Hydrologic Data 5 4.3 Water Level Boundary 5 4.4 Observed Water Level and Rating discharge 6 4.5 Assessment of Mangaroa Gauge Rating and Influence of Hutt Water Level 8 5. Hydraulic Modelling 10 5.1 Model Construction 10 5.2 Model Calibration and Validation 18 5.3 Design Scenarios 22 5.4 Results and Discussions 24 6. Flood Hazard Mapping 51 6.1 Mapping Introduction 51 6.2 Mapping Methodology 51 7. Summary, Conclusion and Recommendation 52 7.1 Recommendations 52 8. References 53
Appendix A Raw Model Results 54
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Document history and status
Revision Date issued Reviewed by Approved by Date approved Revision type A 29/5/2006 CMM CMM 8/12/2005 Final Final 20/3/07 CMM CMM 20/3/07 Final (Updated RBL)
Distribution of copies Revision Copy no Quantity Issued to Final 1 1 Phillip Purves Final 1 10 Sharyn Westlake
Printed: 22 March 2007
Last saved: 20 March 2007 05:34 PM File name: I:\Aenv\Projects\AE02482\Deliverables\AE02482W0006_ver3(ls).doc
Author: Jahangir Alam
Project manager: Benjamin Fountain
Name of organisation: Greater Wellington Regional Council
Name of project: Mangaroa River Flood Hazard Assessment
Name of document: Hydraulic Modelling Report
Document version: Final
Project number: AE02482
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Executive Summary
A combined one and two dimensional hydraulic model was constructed of the Mangaroa River to assist in the development of hazard plans. These plans are intended to assist in the preparation of planning controls to address the hazards in the valley. This report details the technical background to the construction and calibration of this model.
Comparison of the results of the hydraulic modelling with historical flooding records, including gauged levels, historical flooding records and eye witness accounts, found that the model produced results that were acceptable for the purposes of this investigation.
In many locations the bridges and culverts were found to be restrictions that could contribute to flooding, however the majority of flooding out of the main channel is due to the under-capacity of the river. While regular inundation of the flood plain is likely the modelling only identified 4 major locations of flooding that endangers residential buildings. These areas are:
Upper Mangaroa, near the intersection of Russell Road and Whitemans Valley Road.
The breakout point downstream of the Huia and Mangaroa confluence.
Upstream of the Mangaroa Hill Road Bridge.
The Residential properties on Maymorn Road near the confluence of the Mangaroa and Collins Stream.
The Recommended Building Levels and Flood Hazard Maps developed for the 100 year flood event will provide the basis for the development of flood hazard planning controls within the Mangaroa River Valley.
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1. Introduction
1.1 General The main focus of the Mangaroa River Floodplain Hazard Assessment is concerned with the development of planning controls, and their implementation. Much of the work in this project has gone into the construction and testing of a hydraulic model to provide the tools to accomplish this. This report focuses on the technical aspects of the construction and calibration of this model, and the basis of decision making for the development of Hazard Plans. SKM has developed a hydraulic model using a combined one dimensional (1D) and two dimensional (2D) flood modelling approach using MIKEFLOOD. This approach is compatible with Council software and is aimed to be transferable to Greater Wellington at the completion of SKMs involvement.
The purposes of the modelling activities in the project are:
to develop a model that can provide an understanding of the dynamics of floods as well as the hydraulic functionality of the river and flood plains.
use the model for prediction of flood extent, depth and velocity on the flood plains for different scenarios.
a high level assessment of flood damages to residential development and infrastructures on the floodplain.
identification of critical water levels for use by the Flood Protection Department (FPD) in carrying out their flood warning functions and duties.
1.2 Scope of Modelling The initial brief was to simulate flooding from the main channel of the Mangaroa between the confluence with the Hutt River at Te Marua to the floodplains at the headwater catchments as shown in Figure 2.1.
Following the initial coarse model runs, the model was extended to include the lower portion of Black Creek (up to Wallaceville Hill Road), and the Huia and Narrow Neck stream between Mangaroa Valley Road and the Mangaroa River. These extensions were both included to improve the model detail in areas where significant overflows were identified from the main River channel.
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2. River and Catchment Description
2.1 General Description The Mangaroa catchment, with an area of 103 square kilometres, is characterised by the cluster of small catchments and streams that contribute to the main river channel. These small catchments are very steep with falls of up to 500 meters over three to four kilometres. The catchment comprises approximately 15 – 20 % of alluvial floodplain (a smaller percentage is now active floodplain) with the balance in indigenous forest, regenerating scrub and exotic forest. The Mangaroa River is approximately 21 km long from Johnson’s Road at its headwaters to its confluence with the Hutt River (Figure 2-1).
2.2 River Description The Mangaroa River and floodplain is broadly characterised by three reaches, as shown in Figure 2.1.
The lower reach (approximately 7.5 km long) is entrenched from SH2 up to approximately one kilometre above the Mangaroa Valley Road Bridge. The lower 2.5 kilometres of this reach, near Plateau Road, has outcropping rock features and parts of the adjacent residential development may be floodable. The section adjacent to Maymorn Road runs through a short gorge. The final section from above the gorge to upstream of the Mangaroa Valley Road Bridge (approximately five kilometres long) runs through alluvial floodplan; land-use is predominantly rural and pastoral. The Black Creek tributary (which drains the expansive swampy area behind Katherine Mansfield Drive) joins the Mangaroa in this rural section.
The middle reach of the Mangaroa runs through Whitemans Valley from the Mangaroa Valley Road Bridge almost up to Russells Road. The floodplain is generally narrow and there are approximately 10 lateral tributary catchments north and south of the main Mangaroa River channel. The channel is relatively shallow and mobile through this reach.
The upper “reach” comprises the cluster of headwater catchments and small tributaries at Russells Road, Johnson’s Road and Blue Mountains Road. These smaller streams have, in their flatter sections, been modified by channelisation and the construction of access culverts and bridges.
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Figure 2-1 Location Map of Study Area
Hutt River
Mangaroa River
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3. Hydraulic Modelling Concept
Modelling of valley floodplains using two dimensional practices provides a number of advantages over traditional one dimensional modelling. Dynamic as opposed to static modelling of storage, greatly improved assessment of overflow paths, an improved understanding of floodplain velocities, and a reduction in the time required to prepare floodplain maps are all benefits of the 2D modelling system.
In a typical floodplain model these benefits need to be balanced against the limitations to accurately model structures, a requirement for quality digital terrain models, the necessity for some areas to be modelled at coarse grid spacings, and long model run times.
For this project we have developed a coupled mike11/Mike21 model for the Council that optimises the benefits of both packages, and provides Greater Wellington with a high quality, functional model that minimises run times to acceptable levels for future manipulation and testing of options.
Technically MIKEFLOOD preserves hydraulic momentum through its links between the 1D and 2D model. These lateral links allow a string of MIKE21 cells to be laterally connected to a given reach in MIKE11. The flow through the link is dependent upon a structure equation and water levels in MIKE11 and MIKE21. The structure is typically a weir that represents overtopping of riverbank or levee. Momentum preservation is maintained through the links.
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4. Survey and Data Collection
4.1 Topographic Data A LiDAR (Airborne laser scanning) survey of the Mangaroa floodplain was commissioned in July 2004. This survey was used to create a Digital Elevation Model (DEM) of the floodplain that formed the basis of the Mike21 model. In order to accurately represent the hydraulics of the channel an engineering survey was proposed to complement the LiDAR information in this project. Based on requirement of the hydraulic model a detailed cross section survey along the main channel as well as few tributaries was carried out.
Approximately 130 cross-sections were surveyed on the open channel of the Mangaroa River with a further 40 cross-sections covering the tributaries being Black Creek, Huia, and Narrow Neck Streams. These cross sections are located in the flood hazard management plans, see Volume 2 (Plans) of this report.
4.2 Hydrologic Data The hydrological information was provided by the Greater Wellington Regional Council. An assessment of flood hydrology was completed through Greater Wellington’s Resource Investigation Department. The outcome is a report on investigation on flood hydrology of the Mangaroa Catchment (Watts, 2005). The investigation involves rainfall analyses for the Mangaroa Catchment, calibration and validation of a rainfall runoff model, modelling of design rainfall events and flood frequency analyses. The result from this study has been used as input for calibration and validation of hydraulic modelling, and design flow simulation. Six hour storm events were considered for design flood events. A rainfall runoff model TimeStudio developed by Hydstra was used to generate runoff for different sub-catchments (refer study report, Watts, 2005 for detailed information).
The Mangaroa catchment has been subdivided into 17 sub-catchments (A-Q) for modelling purpose, see Figure 4.1. The inflow from each subcatchment was provided as a discharge time series for entry into the hydraulic model. The rainfall runoff model has been calibrated to the gauging station located at the downstream extent of the river (Figure 2.1).
4.3 Water Level Boundary The Hutt River confluence is the downstream boundary for Mangaroa River Model. While there is no level recorder at this location on the Hutt River, Greater Wellington maintain a MIKE11 River model of the Hutt River, that has been used to provide water levels at the Hutt Confluence with the Mangaroa River for various design events as well as for two observed events of 21 and 28 October 1998. These water levels were used as downstream boundary conditions in the simulation.
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Figure 4-1 : Catchments used by Greater Wellington in the Development of the Hydrology.
4.4 Observed Water Level and Rating discharge Water level data for the Mangaroa River has been recorded by Greater Wellington on a continuous basis in the Mangaroa River at Te Marua (29830) since May 1977. Data for the site was collected in accordance with the Resource Information Quality Procedures, which meet the ISO: 9002 Standard and is audited on an annual basis by TELARC registered auditor (Watts, 2005). The Regional Council provided observed water level and rated discharge data, used for the calibration
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and validation of their Rainfall and Runoff model, for subsequent calibration and verification of the hydraulic model.
Table 4-1 : Time series data available for observed events at Te Marua (29830)
Year Date of Occurrence Remarks 1984 18 October Calibration 1985 19 August Validation 1991 7 August Calibration 1994 8 November Validation 1997 4 October Calibration 1998 21 October Validation 1998 28 October Calibration 2000 2 October Validation 2003 3 October Calibration 2004 16 February Validation 2005 6 January Calibration
Figure 4.2 provides the current rating curve for the Mangaroa River at Te Marua station. The rating is checked every 6 weeks and adjusted as necessary. The rating is adequate for high-stage flood level comparisons, as suggested by the Regional Council.
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Figure 4-2 : Rating Curve of Mangaroa Gauge at Te Marua
4400 Graph 17 Drawn on 15-Sep-2005 08 4200
4000
3800
3600
3400
3200
3000
2800
2600
2400 Stage (mm)
2200
2000
1800
1600
1400
1200
1000 775 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Flow (m³/sec)
Mangaroa River at Te Marua
(Source: Greater Wellington)
4.5 Assessment of Mangaroa Gauge Rating and Influence of Hutt Water Level It is recognised by Greater Wellington that the gauging of Mangaroa at Te Marua is likely to be influenced by the water levels of the Hutt River at the confluence particularly in higher flow events. To ascertain the degree of influence it is necessary to analyse the water level profile between the two locations. Unfortunately observed data at the Confluence of the two rivers is not available to carry out such analysis accurately.
To simulate the effects of the Hutt River on the Mangaroa gauging station, we have run a series of design flow events on the Mangaroa River using modelled design tailwater levels in the Hutt River as taken from Greater Wellingtons MIKE11 model.
The results of this analysis, as shown in figure 4.3, have identified that tailwater levels in the Hutt River can influence the gauging station across the full range of design flow events in the Mangaroa. As would be expected the influence of the Hutt River becomes less as the size of the design event increases in the Mangaroa.
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Figure 4-3 Hutt River Water Level impacts on the Mangaroa Gauging
Peak Flows in the Mangaroa
Hutt River Peak Water Levels at the Confluence
Greater Wellington may wish to give some consideration to assessing an alternative site for long term gauging that is not influenced by the Hutt River.
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5. Hydraulic Modelling
5.1 Model Construction Model construction involved the development of a 1D river model in MIKE11 and a 2D floodplain model in MIKE21. To allow overbank flow between the river and floodplain, the river model was coupled with the floodplain model using the lateral link functionality of MIKEFLOOD.
5.1.1 Channel Network The Mangaroa River has been schematized from the headwaters near Johnson Road to the confluence of the Hutt River at Te Marua for the open channel (MIKE11) model. In addition three tributaries were included in the MIKE11 model following preliminary assessments of flooding. These were Narrow Neck, Huia, and Black Creek. Table 5-1 describes the river set up for the MIKE11 model. The schematized river network is shown in Figure 5-1.
Table 5-1 : MIKE11 River Set-up
MIKE 11 Chainage River Connection Surveyed River Name Cross Remark US DS US DS Section Mangaroa 0 19250 Open Open 128 Not named (near Trib_3000 0 1300 Open Mangaroa 1640 8 Johnson Road Not named Trib_4000 0 675 Open Mangaroa 1895 5 (near Russel Road) Mangaroa Black Creek 0 2153 Open 20 12126.02 Mangaroa Narrow Neck 0 390 Open 3 6298.63 Huia Stream 0 450 Open Mangaroa 7040 3 * US - Upstream, DS – Downstream
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Figure 5-1 : Schematised River Network
Untitled 6011000
6010000
6009000
6008000
6007000
Bl ac 6006000 k_ cre ek 0-2 15 6005000 3
6004000 0 5 4 0 -
8 0
1 9 m 1 a 6003000 - 0 re t s a _ o a r i a u g H 6002000 n a N arro m w _n eck 0- 390 6001000
6000000 0-675 t rib_ 30 00 000 0-1 _4 30 5999000 b 0 tri
2678000 2680000 2682000 2684000 2686000 2688000 2690000 269200
On average the cross sections incorporated in the model are in the order of 150m apart. In key areas cross section densities are higher. This spacing was chosen to achieve the modelling accuracy required to generate the desired outcomes of this project. To further investigate specific hydraulic features greater accuracy could be achieved with more survey detail.
5.1.2 Structures Structures such as bridges, culverts and weirs have been incorporated into the MIKE11 model. The bridge/weir method was used to model these constrictions to the flow. For each structure a cross sections was surveyed upstream and downstream of the feature, also surveyed were the overflow heights over the structure and, in the case of bridges, the underside of structural abutments. The list of the culverts and bridges are given Table 5-2 and shown in Figure 5-2.
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Table 5-2 : List of Bridges and Culverts
River M11 Chainage Structure ID
Mangaroa 1430 Bridge 913 Whitemans Valley Road Mangaroa 1595 Whitemans Valley Road Mangaroa 3057 Bridge #750 Whitemans Valley Road Mangaroa 6485 Bridge #408 Whitemans Valley Road Mangaroa 9010 Bridge Whitemans Valley Road Mangaroa 10860 Bridge Mangaroa Valley Road Mangaroa 13580 Bridge Mangaroa Hill Road Mangaroa 19140 Bridge over SH2 Trib_4000 640 Whitemans Valley (Trib. Stream) Trib_4000 412 #13 Russell Road Black Creek 479 Bridge #280 Wallaceville Road (Gun Club) Black Creek 674 Box Culvert (Wallaceville Road) Black Creek 1309 Under Gorrie Road 1 Black Creek 1464 Under Gorrie Road 2 Black Creek 1777 #85 Under Gorrie Road
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Figure 5-2 : Modelled Structures Locations
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5.1.3 MIKE 21 Model Set up Construction of the MIKE21 model grid is often a compromise between the computational time required for simulation and model resolution (grid size). For Mangaroa a 10m grid has been chosen with 1484800 cells and a run time of 10-15 hours. This allows for a single day run time which is preferable. The model was tested for anomalies with this grid size by comparison of the grid levels with surveyed levels. In general the accuracy was very good. In some steeper areas there was found to some discrepancy however this is unlikely to have an impact on the desired outcomes of this project.
Figure 5-3 : Bathymetry of the MIKE21 Model ArcView Grid Data
1200
1100
Palette 1000 Above 393.7 387.4 - 393.7 381.1 - 387.4 374.8 - 381.1 368.5 - 374.8 362.2 - 368.5 900 355.9 - 362.2 349.6 - 355.9 343.3 - 349.6 337 - 343.3 330.7 - 337 800 324.4 - 330.7 318.1 - 324.4 311.8 - 318.1 305.5 - 311.8 299.2 - 305.5 700 292.9 - 299.2 286.6 - 292.9 280.3 - 286.6 274 - 280.3 267.7 - 274 600 261.4 - 267.7 255.1 - 261.4 248.8 - 255.1 (Grid spacing 10 spacing (Grid meter) 242.5 - 248.8 236.2 - 242.5 500 229.9 - 236.2 223.6 - 229.9 217.3 - 223.6 211 - 217.3 204.7 - 211 400 198.4 - 204.7 N 192.1 - 198.4 185.8 - 192.1 179.5 - 185.8 173.2 - 179.5 300 166.9 - 173.2 160.6 - 166.9 154.3 - 160.6 148 - 154.3 141.7 - 148 200 135.4 - 141.7 129.1 - 135.4 122.8 - 129.1 116.5 - 122.8 110.2 - 116.5 100 103.9 - 110.2 97.6 - 103.9 91.3 - 97.6 85 - 91.3 Below 85 0 Undefined Value 0 100 200 300 400 500 600 700 800 900 1000 1100 (Grid spacing 10 meter) 1/01/2000 12:00:00 a.m., Time step: 0, Layer: 0
The model bathymetry has been prepared from the LiDAR information. Since river channels are modelled in MIKE11 the grids along the river are omitted in the 2D calculation to avoid duplication. These areas were excluded by defining those grids as land where no flow occurs see Figure 5-3.
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5.1.4 Coupling between MIKE11 and MIKE21 model The left bank and right bank of the river channel have been linked with the corresponding grid cells in Mike21 model. The most accurate method of modelling the overflow from the main channel was to use the higher of the two levels of the Mike 11 bank or the connected Mike 21 cell. This allows for the modelling of stop banks or levees not accurately identified in the grid.
The HGH and Weir1 type formula in MIKEFLOOD was used to model the overflow. The Table 5-3 shows the linking of MIKE11 and MIKE21 model.
Table 5-3 : MIKE11 and MIKE21 Coupling
Link Coupling River M11 Chainage Total Cell in Type Type US DS M21 Lateral HD only Mangaroa 0 910 88 Lateral HD only mangaroa 1280 1650 31 Lateral HD only Mangaroa 0 1060 102 Lateral HD only Mangaroa 1060 1895 75 Lateral HD only Trib_3000 0 360 37 Lateral HD only trib_3000 560 860 32 Lateral HD only Trib_3000 0 1300 125 Lateral HD only Trib_4000 77.5 670 51 Lateral HD only Trib_4000 250 670 38 Lateral HD only Mangaroa 3330 4290 89 Lateral HD only mangaroa 4580 5130 61 Lateral HD only mangaroa 5360 5580 30 Lateral HD only mangaroa 7810 8350 49 Lateral HD only mangaroa 8470 8670 19 Lateral HD only mangaroa 9020 10120 104 Lateral HD only mangaroa 10120 11180 107 Lateral HD only mangaroa 11180 12100 84 Lateral HD only mangaroa 12120 12200 8 Lateral HD only mangaroa 12360 12640 28 Lateral HD only mangaroa 13080 13570 46 Lateral HD only mangaroa 13730 14340 57 Lateral HD only mangaroa 14630 16460 172 Lateral HD only mangaroa 16730 16870 12 Lateral HD only mangaroa 16970 17290 33 Lateral HD only mangaroa 17420 17670 23 Lateral HD only mangaroa 17960 18220 27 Lateral HD only Mangaroa 18780 19180 40 Lateral HD only mangaroa 1895 3050 114
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Lateral HD only mangaroa 3070 3720 66 Lateral HD only mangaroa 4170 4760 61 Lateral HD only mangaroa 4970 5440 48 Lateral HD only mangaroa 5540 6230 66 Lateral HD only mangaroa 6310 7020 62 Lateral HD only mangaroa 7045 7660 63 Lateral HD only mangaroa 7660 7810 18 Lateral HD only mangaroa 7810 9300 142 Lateral HD only mangaroa 9630 9860 19 Lateral HD only mangaroa 10120 11520 133 Lateral HD only mangaroa 11520 12640 120 Lateral HD only mangaroa 12640 14260 155 Lateral HD only mangaroa 14260 15650 119 Lateral HD only mangaroa 15740 16310 67 Lateral HD only mangaroa 16460 17540 108 Lateral HD only mangaroa 17820 19180 134 Lateral HD only black_creek 0 595 56 Lateral HD only black_creek 1719 2153 41 Lateral HD only black_Creek 0 2153 206 Lateral HD only Huia_stream 0 450 41 Lateral HD only Huia_Stream 0 450 43 Lateral HD only Narrow_neck 0 390 34 Lateral HD only Narrow_neck 0 390 34
5.1.5 Roughness Parameter The roughness parameter has been estimated by applying sound engineering judgement based on industry best practice and physical observation of the watercourse. The roughness of the Mangaroa River Channel is considered to have a Mannings coefficient of 0.04–0.045 in the middle and lower reaches due to the rough bed that includes boulders and vegetation/trees on the banks. The upper reaches were assigned a Mannings coefficient of around 0.035. The flood plain roughness is based on landuse type. The landuse has been subdivided into four types: pasture, vegetation, swamp and residential area. The majority of the Mangaroa catchment is covered in pasture for rural and semi- rural landuse. The values of Manning’s n are shown in Table 5-4 and Table 5-5 below.
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Table 5-4 : Manning’s Roughness used in MIKE11 Open Channel model
River Chainage Manning's n Mangaroa 0 0.035 Mangaroa 2000 0.035 Mangaroa 2100 0.04 Mangaroa 15000 0.04 Mangaroa 15100 0.045 Mangaroa 19180 0.045 Narrow Neck 0 0.04 Narrow Neck 390 0.045 Huia Stream 0 0.04 Huia Stream 450 0.04 Black Creek 0 0.035 Black Creek 689.38 0.035 Black Creek 1069.75 0.04 Black Creek 1796.36 0.04 Black Creek 2153 0.035
Table 5-5 : Manning’s Roughness used for different landuse in MIKE21 Overland Flow model
Landuse Manning’s Number Pasture 0.035 Vegetation 0.075 Swamp 0.05 Residential area 0.1
5.1.6 Model Boundaries The channel upstream is open boundary assigned inflow hydrographs from defined subcatchments. The in flow hydrograph has been directly assigned to the Mangaroa River as a point source at the corresponding location as shown in Table 5-6. The downstream control is the water level at Hutt River confluence. However, for the calibration and verification of the model the downstream boundary was simulated at the location of Mangaroa Gauge due to the unavailability of known water level data at the confluence. The exception to this rule was the events of 21 October and 28 October of 1998. These events had been calibrated within the MIKE11 model of the Hutt River enabling simulation of water levels within the Hutt River at the models downstream boundary.
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Table 5-6 : MIKE11 Model Boundaries
Catchment River Chainage Description Boundary type ID/Source Mangaroa 0 Open Inflow A Mangaroa 19250 Open Water Level Hutt water level at confluence Trib_3000 0 Open Inflow C Trib_4000 0 Open Inflow D Mangaroa 1280 Point Source Inflow B Mangaroa 2120 Point Source Inflow E Mangaroa 4580 Point Source Inflow F Mangaroa 4580 Point Source Inflow G Mangaroa 8470 Point Source Inflow J Mangaroa 12080 Point Source Inflow K Mangaroa 13180 Point Source Inflow M Mangaroa 15830 Point Source Inflow N Mangaroa 17540 Point Source Inflow O Mangaroa 18520 Point Source Inflow P Mangaroa 18860 Point Source Inflow Q Narrow Neck 0 Open Inflow H Narrow Neck 0 Open Inflow I Narrow Neck 0 Open Inflow L
5.2 Model Calibration and Validation It is necessary to calibrate and verify the model results against observed water level and discharge. Calibration of the model can be achieved through alteration of the rainfall-runoff profile ,or through details within the hydraulic model. Both can significantly influence the final calibration. The rainfall-runoff model defines the total volume of a storm, rainfall losses associated with storage and infiltration, the ‘shape’ of a storm event, and timing of the storm peak. The hydraulic model can influence the timing of a storm, and storm peaks, through channel alteration, the roughness and storage capacity of the floodplain, and through detention behind structures. In the case of our model early model runs overestimated periods, underestimated storage, and responded more quickly than recorded peaks.
Some refinements in model schematization were made in the hydraulic model by incorporating Black Creek, Huia stream and Narrow Neck channel with surveyed cross-sections after this initial assessment. This added a substantial amount of storage into the model which had a positive impact
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on calibration. A sensitivity analysis was also undertaken for channel roughness, but this provided little improvement to calibration results.
Should Greater Wellington Regional Council consider refinement of the hydrology particular attention should be given to the catchment based storage losses, and concentration times for design events. Results for observed events of October 1998 and January 2005 are as follows.
Figure 5-4 : Observed and Simulated Water Level at Mangaroa Gauge Te Marua for 28 October 1998 Flood
[meter] Time Series Water Level 91.8 91.6
91.4
91.2
91.0
90.8 90.6 Observed Water Levels 90.4 90.2
90.0 89.8
89.6 89.4 Modelled Water Levels 89.2
89.0
88.8
88.6
88.4
88.2 88.0
20:00:00 00:00:00 04:00:00 08:00:00 12:00:00 16:00:00 20:00:00 00:00:00 04:00:00 27-10-1998 28-10-1998 29-10-1998
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Figure 5-5 : Observed and Simulated Discharge at Mangaroa Gauge at Te Marua for 21 October 1998 Flood
[m^3/s] Time Series Discharge
260.0
240.0
220.0
200.0
180.0 Modelled Discharge
160.0 Observed Discharge 140.0
120.0
100.0
80.0
60.0
40.0
20.0
0.0
18:00:00 21:00:00 00:00:00 03:00:00 06:00:00 09:00:00 12:00:00 15:00:00 18:00:00 21:00:00 00:00:00 03:00:00 27-10-1998 28-10-1998 29-10-1998
Figure 5-6 : Observed and Simulated Water Level at Mangaroa Gauge Te Marua for 21 October 1998 Flood
[meter] Time Series Water Level
91.5
91.0
90.5 Observed Water Levels 90.0
89.5
89.0 Modelled Water Levels
88.5
88.0
87.5
12:00:00 16:00:00 20:00:00 00:00:00 04:00:00 08:00:00 12:00:00 16:00:00 20:00:00 20-10-1998 21-10-1998
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Figure 5-7 : Observed and Simulated Discharge at Mangaroa Gauge at Te Marua for 21 October 1998 Flood
[m^3/s] Time Series Discharge
240.0
220.0
200.0
180.0 Modelled
160.0 Discharge
140.0
120.0
100.0
80.0 Observed 60.0 Discharge 40.0
20.0
0.0
12:00:00 16:00:00 20:00:00 00:00:00 04:00:00 08:00:00 12:00:00 16:00:00 20:00:00 20-10-1998 21-10-1998
Figure 5-8 : Observed and Simulated Discharge at Mangaroa Gauge at Te Marua for January 2005 Flood
[m^3/s] Time Series Discharge Discharge MANGAROA 18987.50 320.0 External TS 1 300.0 Obs_Q_Te Marua
280.0
260.0 Modelled 240.0 Discharge 220.0
200.0
180.0
160.0
140.0
120.0 100.0 Observed Discharge 80.0
60.0
40.0
20.0
0.0
12:00:00 15:00:00 18:00:00 21:00:00 00:00:00 03:00:00 06:00:00 09:00:00 12:00:00 15:00:00 18:00:00 21:00:00
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Figure 5-9 Observed (Dot) and Simulated Discharge (Line) at Mangaroa Gauge at Te Marua for February 2004 Flood
[m^3/s] Time Series Discharge 320.0
300.0
280.0
260.0
240.0 Modelled Discharge 220.0 Observed Discharge 200.0
180.0
160.0
140.0
120.0
100.0
80.0
60.0
40.0
20.0
0.0
18:00:00 20:00:00 22:00:00 00:00:00 02:00:00 04:00:00 06:00:00 08:00:00 10:00:00 12:00:00 14:00:00 16:00:00 15-2-2004 16-2-2004
From the comparisons of modelled and recorded water levels and discharges it can be seen that the model provides conservative predictions. While the model predicts similar shaped graphs to the recorded events, the model peaks more rapidly suggesting that initial catchment losses are being underestimated. This is also likely to be one of the reasons why in some scenarios, such as the 28 October 1998 event, the hydrologic model appears to over estimate the initial runoff volumes.
For the purposes of this investigation the hydrological model provides acceptably conservative comparisons with observed events.
5.3 Design Scenarios The six hour design storm has been considered for flow simulation of a range of design flood events (Q5, Q10, Q20, Q50, Q100, Qextreme). These storm events were used in both the base scenario using existing channel cross-section geometry and the calibrated model and also in a sensitivity scenario including a combination of sedimentation at critical locations, 50% blockage of culverts and double head loss for the bridges.
Further sensitivity investigations were carried out using the Qextreme event. This event is
calculated as 1.5 x the Q100 storm flood event. The sensitivity analysis was used to develop appropriate freeboards for the modelled results.
A summary of the model scenarios is listed below:
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Table 5-7 Model Scenarios
Design Flows Existing Sediment & Design Condition Blockage Option Scenarios (Base) (investigation) (Design)
Q5 ¥
Q10 ¥
Q20 ¥
Q50 ¥¥¥
Q100 ¥¥¥
Qextreme ¥¥
5.3.1 Consideration in Downstream Boundary Condition The sensitivity analysis of the boundary water level at the Hutt River confluence indicates that the Hutt water level has a considerable impact on the downstream reach of the Mangaroa River. To model the effect of the Hutt influence on Mangaroa a Hutt River Q20 water level has been used for downstream boundary condition for all design scenarios. The use of Q20 water level for lower events (Q5 and Q10) is a conservative approach in simulating low flow flood depths. The boundary conditions are discussed further in section 4.5.
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5.4 Results and Discussions Much of the Mangaroa was found to overtop the banks even in lower return periods. Section 5.4.1 discusses the results of the modelling investigation over the length of the river and section 5.4.2 concentrates on the flooding associated with the various structures crossing the main channels.
The inundation model results shown in the figures in this section do not contain freeboard and are only indicative of the potential flooding. The raw top water levels and peak discharges for the Q50 and Q100 have been extracted from the model and are contained in Appendix A.
5.4.1 Flooding in Mangaroa The model demonstrated flooding over nearly the entire length of the river, low lying areas are inundated, culverts and bridges create back water effects causing flooding upstream, and secondary flowpaths inundate the flood plain. For the purposes of this report the analysis of the flooding has been divided into areas exhibiting similar flooding causes or characteristics. The geographic flooding areas are detailed in Figure 5-11.
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Figure 5-10 Flooding Areas Described in this Report.
Residential Area
Lower Mangaroa
Wallaceville Road
Huia Stream Area
Whitemans Valley Road
Upper Mangaroa
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Upper Reaches of Mangaroa (from Chainage 0m - 2000 m): Figure 5-11 and Figure 5-12 show the flood inundation for 5 Yr and 100 Yr return period flood, respectively. The model demonstrates that much of the flooding in this area is due to the under-capacity of the stream channel. Furthermore there are a number of culverts that cannot convey even a 5 year storm flow. These culverts create back water effect upstream contributing to the flooding.
Figure 5-11 Flood Inundation map for 5 Yr return period at Upstream location
Figure 5-12 Flood Inundation map for 100 Yr return period at Upstream location
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The Velocity profile (Error! Not a valid bookmark self-reference.) gives an indication of the dominant flow paths in this area. The flooding in this area endangers approximately 10 residential buildings in this area, especially around the intersection of Russell Road and Whitemans Valley Road.
Figure 5-13 Velocity Profile at Peak Flow for 100 Yr return period Flood at Upstream location
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Narrow Floodplain adjacent to Whitemans Valley Road: The floodplain in this area is constrained by the steep slopes of the hills on each side of the stream. Overtopping of the banks can occur even in a 5 year flood event (Figure 5-14). However because the valley is narrow in this section of the river the 100 year flood event only slightly increases the flood extents (Figure 5-15).
Figure 5-14 Flood Inundation for a 5 Yr return period flood near Whitemans Valley Road
Figure 5-15 Flood Inundation in 100 Yr return period at the same location
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There are almost no residential buildings within this narrow flood plain area.
Flooding around Huia Stream: The model identified that there was a high risk of flooding just downstream of where the Huia Stream combines with the Mangaroa. The LiDAR survey identified an ancient course of the river breaking away from the main channel at its current location.
While there unlikely to be flooding in a 5 year event the model indicates that it may be possible for the river to overtop the banks at this location in a 10 year event. This endangers a number of residential buildings. The flooding depths and extents from the model are shown for a 10 year and 100 year events in Figure 5-16 and Figure 5-17.
Figure 5-16 Flood Inundation around Huia Homestead for 1 in 10 Yr flood event
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Figure 5-17 Flood Inundation around Huia Homestead for 1 in 100 Yr flood event
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The secondary flow path at this breakout is clearly seen in the velocity profile for 100 year event Figure 5-18.
Figure 5-18 Velocity profile for 100 Yr event near Huia Homestead area.
Ponding behind Wallaceville Road: Another location of the river that the model indicated was susceptible to breaking out was the low lying floodplain area between Wallaceville road and Katherine Mansfield Drive. The model results suggested that even in a 20 year flood event the river can spill out onto the flood plain and flow overland, eventually ponding behind Wallaceville Road, see
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Figure 5-19. In a 100 year flood event the model predicts that the flooding could cover wider extent but is unliklley to be much deeper because of the flat nature of the floodplain, Figure 5-20.
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Figure 5-19 Predicted flood inundation near Wallaceville Road in a 20 year event
Wallaceville Road
Katherine Mansfield Drive
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Figure 5-20 Predicted flood inundation near Wallaceville Road in a 100 year event
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The velocity profile of the flow over the floodplain in a 100 year event is shown in Figure 5-21.
Figure 5-21 Velocity profile demonstrating the secondary flowpath near Wallaceville Road
Lower Mangaroa Reach: In general, downstream of Wallaceville Road the river is incised and the flooding is confined to the low lying river terraces as shown in Figure 5-22 and Figure 5-23. The model indicates that there is no significant breakouts in this region. Only properties and buildings adjacent to the main channel are likely to be at risk of flooding from the Mangaroa. The residential house upstream of the Mangaroa Hill Road Bridge appears to be at risk.
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Figure 5-22 Predicted flood inundation near the Mangaroa Road in 10 year event
Figure 5-23 Predicted flood inundation near the Mangaroa Road in a 100 year event
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Residential Area Near the Confluence of the Mangaroa and the Hutt River: The Mangaroa River at this location steepens as it drops down towards the Hutt River. It is also cut into the valley and has steep banks. As a result the model indicates that there is little flooding through this reach even in a 100 year event (Figure 5-24). However, where Collins Stream joins the Mangaroa the model predicts that the banks could be overtopped in this location endangering a number of residential properties.
Figure 5-24 Flooding in downstream area in 1 in 100 Yr event
Collins Stream
Hutt River
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5.4.2 Structures Structures often represent constriction point to flood flows within a river network and care must be take to accurately model the impacts of bridges and culverts.
One of the main reasons the 1D-2D coupled model approach was selected was because of its ability to provide adequate modelling of structures. The main channel and the structures within it were modelled using the 1 dimensional Mike 11 model. One dimensional models have the ability to allow for more accurate modelling of structures. Water building up behind the structures and over topping the banks would flow out over the floodplain in the coupled Mike 21 model.
The culvert/weir method was used to model the bridges and culverts. This methodology is an industry accepted practice and it is also recommended by DHI, the software producers.
A Mike 11 “culvert structure” was constructed in the model with the same cross-section shape as that surveyed under the bridge. The ‘culvert soffit’ is set at the height of the lowest point of the bridge. The culvert will act as an ordinary cross-section until the flow depth reaches the ‘soffit’ of the culvert (the underside of the bridge). At the same chainage as the culvert a Mike11 “weir structure” is also inserted into the model. The invert of the weir is set to the height of the top of the bridge, usually the top of the slab or, if the railing is solid, the top of the railing. A weir of appropriate width, determined by site observation was used to model the overtopping of the bridge structure.
This method allows for flood flows to overtop the structure within the Mike11 model as well as bypass around the structure in Mike21 should the flood flows overtop the banks. The following table,
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Table 5-8, shows the depth of water above the deck of the bridges and culverts in the model in the different flooding events. Figure 5-2 shows the location of these structures. The depths in the table are taken from the peak top water levels and do not include freeboard.
The model results shown in the table demonstrate that many of the culverts at the top end of the model, around Russells Road are undersized, even in a 5 year event. There are three culverts in this area that are over topped, these are the culvert at number 13 Russell Road, Whitemans Valley bridge(Trib. Stream) and the Whitemans Valley Road bridge see Figure 5-25. All three of these structures appear to have a high blocking potential.
The model demonstrates that while all three of these culverts contribute to increases in localised inundation depths, the major cause of flooding in the area is the under capacity channel.
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Table 5-8 Flood water depth over the decks of Bridges and Culverts
Bridge Name/IDChainage Modelled water depth overtopping the structure in MIKE11 Model (m) (Without freeboard) Q100 Q50 (m) Q20 Q10 Q5 (m) (m) (m) (m) #13 Russell Road Trib_4000 0.894 0.871 0.854 0.821 0.777 412 Whitemans Valley (Trib. Stream) Trib_400 0.341 0.339 0.334 0.298 0.298 640 Bridge 913 Whitemans Valley Road Mangaroa not over not over not over not over not over 1430 topped topped topped topped topped Whitemans Valley Road Bridge Mangaroa 0.712 0.641 0.537 0.437 0.351 1595 Bridge 408 Whitemans Valley Road Mangaroa 0.217 0.122 not over not over not over 6485 topped topped topped Bridge Whitemans Valley Road Mangaroa not over not over not over not over not over 9010 topped topped topped topped topped Bridge Mangaroa Valey Road Mangaroa not over not over not over not over not over 10860 topped topped topped topped topped Bridge Mangaroa Hill Road Mangaroa not over not over not over not over not over 13580 topped topped topped topped topped Bridge SH2 Mangaroa not over not over not over not over not over 19140 topped topped topped topped topped Bridge_#280 Wallaceville Road Black Creek 0.844 0.677 0.302 0.094 not over (Gun_club) 479 topped Box Culvert (Wallaceville road) Black Creek 0.036 not over not over not over not over 674 topped topped topped topped Under Gorrie Road 1 Black Creek 0.321 0.205 0.146 0.077 not over 1309 topped Under Gorrie Road 2 Black Creek 0.873 0.648 0.548 0.444 0.32 1464 #85 Under Gorrie Road 3 Black Creek 0.072 not over not over not over not over 1777 topped topped topped topped
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Whitemans Valley Road #13 Russel Road
Whitemans Valley (Trib.)Stream Vegetation and Fence demonstrates blockage potential
Figure 5-25 Under- sized Culverts and Bridges in the Upper reaches of the Mangaroa Russells Road
The Bridge at 408 Whitemans Valley Road, see Figure 5-26, is overtopped in the Q100 and the Q50. In the 50 year flooding event this constraint increases flooding depths upstream of the structure by over 1.3m. While the increase in flooding depth caused by this bridge appears not to endanger any habitable dwellings upstream, the bridge should be checked to ensure it can withstand the force of the water acting on it in high rainfall events.
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Figure 5-26 Bridge at 408 Whitemans Valley Road
Figure 5-27 is a photo of the Twin Box culvert under Wallaceville Road. While the model predicts that the Wallaceville Hill Road above the culvert at this location will only just be overtopped in a 100 year rainfall event the culvert forms a major constraint in the system.
The model predicts that the main channel between Katherine Mansfield Drive and the Mangaroa Valley Road Bridge is not able 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 5-29. 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 backs up Black Creek and behind Wallaceville road. The ponding depth at this location in the 10-100 year events drowns out the bridge to the gun club, see Figure 5-28 and Figure 5-30.
The model demonstrates the significance of the Black Creek Box Culvert during heavy flooding events. By allowing the storage and controlled release of the flood flows ponding behind Wallaceville road it reduces the flood depths down stream.
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Figure 5-27 Black Creek Twin Box Culvert under Wallaceville Hill Road
Figure 5-28 Gun Club Bridge
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Wallaceville Hill Road
Gun Club Bridge Black Creek Box Culvert
Ponding Area Mangaroa Valley Road Bridge
Katherine Mansfield Drive
Figure 5-29 100 Year Flooding Extent around the Wallaceville Hill Road
[meter] 1-1-1990 08:31:00
142.0 0 686 201 486 490 665 595 141.0 470 760
140.0 368 952 1064
139.0 1237 1300 1316 1427 138.0 1473 1455 137.0 Gun Club Bridge 136.0 1783 1770 Black Creek Box Culvert 1719 135.0 1869 134.0
133.0 2153 132.0
131.0
130.0 BLACK_CREEK 0 - 2153
0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 1000.0 1100.0 1200.0 1300.0 1400.0 1500.0 1600.0 1700.0 1800.0 1900.0 2000.0 2100.0 2200.0 [m]
Figure 5-30 Longitudinal Profile of Peak Water Levels along Black Creek for the 50 Year Flooding Event
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Gorrie Road Culvert 3
Gorrie Road Gorrie Road Culvert 2
Gorrie Road Culvert 1 Figure 5-31 Gorrie Road Culverts
There are three triple barrel culverts on Black Creek that pass under Gorrie Road, see Figure 5-31. While Gorrie Road above the third culvert is only just overtopped in the 100 year event, the road over the first Gorrie Road Culvert overtops in
a Q10, and the second culvert in a Q5, see
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Table 5-8.
Although the flood flows overtop the road the model indicates that there is unlikely to be significant flooding as the road above the culverts forms the secondary flowpath allowing the waters to re-enter the creek.
The model indicates that deeper flooding is prevented by the Black Creek Box Culvert under Wallaceville Hill Road which forms a constraint on the flows entering the lower reaches of Black Creek. Should the Wallaceville Hill Road be overtopped during extreme flooding this lower Black Creek region could be more heavily inundated.
The model indicates that the remainder of the bridges within the study area are not overtopped even in the Q100 event.
5.4.3 Freeboard When setting levels for development it is normal to add a freeboard margin to the levels derived by the analysis. Modelled top water levels (TWL’s) plus a freeboard allowance make up the given plan levels designated as recommended building levels (RBL’s). The freeboard covers such variables as:
Data limitations and modelling approximations
Parts of the stream and floodplain are modelled by only a limited amount of survey information, e.g. limited LIDAR information.
Availability (or lack) of historical runoff records.
Storm runoffs are derived based on assumptions as to rainfall patterns, ground soakage and saturation.
Assumptions as to hydrograph shape.
Assumptions as to ground and channel roughness.
Physical considerations
Wave action caused by wind or motor vehicles.
Silting of the stream or debris or slips occurring during a storm which may affect channel capacities.
Effect of obstruction on flows
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Buildings need to be adequately above water levels so that obstructions to moving water do not cause local waves and resulting ingress. This is of less impact in large ponding areas than in sloping, high velocity flow areas.
House construction limitations
If water gets to within 100 to 150 mm of a slab or timber framed floor over any length of time water can be absorbed into the structure enough to cause flooring problems (e.g. carpet damage).
The economic and social impact of water ingress
The freeboard would normally be set higher where a large number of high value improvements are affected.
In addition to these considerations a sensitivity analysis was undertaken to further identify appropriate freeboard allowances. The model was used to identify the impacts of:
Debris being caught on bridges and at culvert inlets.
Sedimentation
Extreme flows (1.5 x Q100)
The model was modified from the existing situation by applying a 50% blockage to all culverts and doubling the headloss of the flow through the bridges. In selected areas the bed level of the Mangaroa was increase by half a meter to model the impacts of sedimentation. These areas were selected because they were either known locations of sedimentation or they were locations of significant changes of grade.
The inflows for the 100 year and 50 year rainfall events were run through the model with these modifications and the flooding results were then extracted and compared to the model results of the existing situation.
A summary of the sensitivity analysis is illustrated in Figure 5-32 and Figure 5-33 which compare the sensitivity results with the same storm events in the existing catchments.
Figure 5-32 demonstrates that there is very little impact on the flooding depth as a result of the sedimentation and blockages. In most areas the impact is less than 0.2m increase in flooding
depths. Much of the Q100 flood flow has already broken the banks and drowned out constraints such as the culverts, as a result the sedimentation and blockages make little difference to the flood depths. However at one location, upstream of the Mangaroa Hill Bridge, the flooding was increased by almost 0.4 m.
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Unlike the sedimentation and blockage results, the extreme event (1.5 x Q100) results demonstrate a
considerable increase in flooding in comparison with the Q100 (Figure 5-33). Over much of the lower reaches of the Mangaroa the increase in the flooding depths are between 0.6 -1.0m. Above Wallaceville Hill Road the flood difference is less severe, generally between 0.2-0.6m.
Furthermore in the extreme case the extents of flooding are altered, most notably the water ponding behind the Wallaceville Hill Road reaches a depth where it overtops the road and floods the rural land between the road and the confluence with Black Creek.
The sensitivity analysis and freeboard considerations suggest that, for the development of Recommended Building Levels, as a minimum 0.5m freeboard should be applied to the top water levels on both the 100 and 50 year floods. This is considered appropriate on a river system of this size within a predominantly rural catchment and based on the techniques employed to derive the top water levels.
While 0.5m freeboard appears to be appropriate over much of the upper Mangaroa catchment there are a number of considerations that indicate a greater freeboard should be applied in the lower catchment.
Between Wallaceville Hill Road and the confluence with the Hutt River, much of the channel is incised and is therefore subject to greater fluctuations in water level with increased discharge. Slips in the steep embankments could also cause localised ponding and flooding and there are also a number of structures, particularly on the Black Creek and the Mangaroa Hill Road Bridge, that demonstrated considerable increases in depth during sedimentation and blockage runs. Furthermore, in the assessment of the influence of the Hutt River on the Mangaroa water levels (Section 4.5) the impact of high water levels in the Hutt River was found to increase water levels in the Mangaroa up to Collins Stream, at the bottom end of Maymorn Road.
The analysis indicates that a 0.8m freeboard would be appropriate on the lower reaches of the Mangaroa, below Wallaceville Hill Road. This should be increased to 1m near the confluence to account for the influences and uncertainty associated with high water levels in the Hutt River influencing the Mangaroa flooding. In the Flood Hazard Maps (Volume 2) below chainage 18780 1m of freeboard has been applied.
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Figure 5-32 Plan detailing the difference in flood depths between the Q100 in the existing condition and the Q100 with Sedimentation and Blockages.
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Figure 5-33 Map showing difference of flood depths between Qextreme and Q100 flood in the existing conditions.
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6. Flood HazardMapping
6.1 Mapping Introduction Two sets of flood hazard maps have been developed from the results of the hydraulic modelling. These maps are included in Volume 2 of this report.
The first set of plans (Mangaroa River Flood Modelling Investigation: Q100 Flooding Extents and Depths) details the flood extents and inundation depths extracted from the modelled results. These plans are a summary of the raw modelled results from the model without any allowance for freeboard.
The second set of plans (Mangaroa River Flood Hazard Assessment Q100 and Q50 Extents) includes the freeboard allocations discussed in this report. These plans are intended to be used as a guide to the establishment of Recommended Building Levels (RBLs) in the Mangaroa Valley.
The Building Act requires that the floor level of residential dwellings are built above the peak water level expected in a 50 year return period storm. The flood hazard maps define the 50 year flood extent and recommended building levels.
For subdivision of land it is recommended under the Upper Hutt City Council Code of Practice, that new dwellings are constructed above the 100 year return period storm. The 100 year extents and levels have also been mapped in the plans.
6.2 Mapping Methodology The process for the creation of the Mangaroa flood hazard maps in GIS involved the conversion of the WSL (Water Surface Level) ASCII output file from Mike21 into a raster layer in GIS. This WSL raster layer was then overlaid onto the aerial photos and contours of the Mangaroa River. The raster was queried to determine WSL along the rivers length. The contours that corresponded with the WSL were traced to produce the Q50 and Q100 flood maps. Freeboard, as described in 5.4.3, has then been incorporated into the modelled results to provide recommended building levels.
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7. Summary, Conclusion and Recommendation
Comparison of the results of the hydraulic modelling with historical flooding records, including gauged levels, historical flooding records and eye witness accounts, found that the model produced results that were acceptable for the purposes of this investigation. In some locations the model was found to over estimate the peak water levels and discharges. One of the reasons for this is that it is likely that the catchment storage has been underestimated in the hydrology. However the modelled results, while conservative, closely correlate to historical flooding locations, where records exist.
The causes of flooding over the length of the modelled channel have been identified. While in many locations bridges and culverts form restrictions that can contribute to the flooding, the majority of flooding out of the main channel is due to the under-capacity of the river. The banks over much of the river are overtopped in a 5 year storm event.
While regular inundation of the flood plain is likely the modelling only identified 4 major locations of flooding that endangers residential buildings. These areas are:
Upper Mangaroa, near the intersection of Russell Road and Whitemans Valley Road.
The breakout point downstream of the Huia and Mangaroa confluence.
Upstream of the Mangaroa Hill Road Bridge.
The Residential properties on Maymorn Road near the confluence of the Mangaroa and Collins Stream.
7.1 Recommendations
Refining of the hydrological model is recommended for future use of the model and for improvement of the present calibration and validation of the model.
The regional Council may look to consider a second gauging station on the Mangaroa that is not affected by the Hutt River. A possible location could be Whitemans Valley Road Bridge.
A combined Mangaroa and Hutt river model could be used to provide better accuracy of the flooding near the confluence of these two rivers.
The Recommended Building Levels for the 100 year flood event should be used as the basis for developing flood planning controls in the Valley.
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8. References
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.
Watts, L., 2005. Flood Hydrology of the Mangaroa River, Greater Wellington City Council, New Zealand.
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Appendix A Raw Model Results
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