Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR

Transcription

1 Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR NIWA Client Report: CHC June 2010 NIWA Project: WCR10501

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3 Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR Maurice J Duncan Jo Bind NIWA contact/corresponding author Maurice Duncan Prepared for West Coast Regional Council NIWA Client Report: CHC June 2010 NIWA Project: WCR National Institute of Water & Atmospheric Research Ltd 10 Kyle Street, Riccarton, Christchurch 8011 P O Box 8602, Christchurch 8440, New Zealand Phone , Fax All rights reserved. This publication may not be reproduced or copied in any form without the permission of the client. Such permission is to be given only in accordance with the terms of the client's contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system.

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5 Contents Executive Summary i 1. Introduction 1 2. Modelling methods Background 1 3. Model calibration Further recalibration 2 4. The flood modelling Introduction Boundary conditions Digital elevation model Hydraulic resistance Input hydrograph Tides and sea level Hot starts Determining the extent of inundation for flood models Determining the location and minimum height of stop banks for mitigating the 1% AEP event 9 5. Results Discussion General assumptions The role of the Orowaiti overflow and the Nine Mile Road Railway embankment The consequences of stop bank failure Global climate change: sea level rise and increased rainfall intensities Risk of a 1% AEP event Summary Disclaimer References 21 Appendix 1: Appendix 2: The role of the Orowaiti overflow and the Nine Mile Road Railway embankment Files used for the simulations

6 Reviewed by: Approved for release by: Graeme Smart Roddy Henderson

7 Executive Summary The West Coast Regional and Buller District Councils wish to quantify the extent of Westport s flood hazard with 2% and 1% AEP (i.e., annual exceedance probabilities of inundation with average recurrence intervals of 50 and 100 years respectively) and revisit mitigation options for the 1% AEP event, because more extensive elevation airborne laser survey (ALS) data has become available. The ALS data were also used to estimate hydraulic roughness and gave more varied and spatially detailed hydraulic roughness than in the previous study. The modelled inundation shows more water going down the Buller River than the earlier study resulting in more inundation of Westport than previous studies. Flooding in Westport could be substantially reduced by five kilometres of low stop banks. Overtopping of the railway embankment at Nine Mile Road and across the Orowaiti Estuary is a concern. The effect of erosion of the stop banks was not modelled. The largest uncertainty with the modelling is the amount of river bed scour at the flood peak. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR i

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9 1. Introduction The West Coast Regional and Buller District Councils wish to quantify the extent of Westport s flood hazard with 2% and 1% AEP (i.e., annual exceedance probabilities of inundation with average recurrence intervals of 50 and 100 years respectively) subsequent to an airborne laser survey (ALS) of the Buller River flood plain. They also wished to revisit mitigation options for the 1% AEP event. NIWA was engaged to carry out the study. The approach was to use a two dimensional (2-D) hydrodynamic model, calibrated using data from the 1970 flood, to model the floods. NIWA has already reported on flood extents caused by 2% and 1% AEP events (Duncan et al. 2005) based primarily on ground based GPS data and a coarser model grid than used here. Duncan (2005) suggests locations and nominal elevations for stop banks to mitigate flooding from a 1% AEP event in Westport. This study uses the scour and grain-size-based hydraulic resistance information gained from earlier studies (Duncan 2004, Duncan and Smart 2004). This study differs from earlier ones because selection of hydraulic resistance coefficients for the flood plain is guided by the ALS data which gives more spatial variation information than that of the blanket vegetative cover data used in previous models. There are many possible combinations of sea-level and river flood that result in 1% and 2% AEP inundation events and they may result in different spatial distributions of inundation. The combinations of sea-level and river flood that are most likely are used here. All references in the report to sea levels are with respect to mean sea level Lyttelton 1937 datum. 2. Modelling methods 2.1. Background Details of the modelling methods are given in Duncan et al. (2003) and are summarised here: 1. A digital elevation model (DEM) of the Buller River flood plain downstream of Te Kuha was derived from ALS (Duncan et al. 2010) and for areas without ALS coverage, from contour maps of the land and sea bed, riverbed crosssection and river-bank surveys and an echo-sounder survey of the Buller River. (Duncan et al. 2003). Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 1

10 2. The DEM was sampled on a 4.8 m square grid (cf. 7 m for the previous models) to provide the bed boundary for the 2 dimensional hydrodynamic model Hydro 2de (Beffa and Connell 2001). The extent of the model is shown in Figure The model was re-calibrated, (Duncan et al. 2010) primarily by adjusting the amount of general riverbed and Orowaiti Estuary scour, so that water levels observed in Westport during the August 1970 flood were reflected by the predicted levels in the model. 3. Model calibration 3.1. Further recalibration Details of the model recalibration subsequent to the ALS are contained in Duncan et al. (2010). However, since that report further calibration runs were carried out where the scour in the Buller River bed was increased to the same level as reported in Duncan (2004) and the hydraulic roughness of the Orowaiti overflow channel reduced in the vicinity of the Nine Mile Road Bridge. The outcome of this further calibration was to achieve modelled water-levels in the Menzies/Roebuck Street and Chamberlain/Russell Street neighbourhoods (Figure 2) to within m and m of the average levels of the most reliable flood-level information. These results can be considered to be very good. The water-level at Rintoul Street was too low but marginally higher than reported in Duncan et al. (2010) (Table 1). Table 1: Comparison of measured and modelled levels for the 1970 flood at the flood peak for the February 2010 and current calibrations. Difference of simulated level minus measured level (mm). Site February 2010 calibration difference (mm) 1 Current calibration difference (mm) 1 Menzies/Roebuck Gladstone/Russell Rintoul Street Negative indicates model level is too low The flooding pattern has most flooding originating from the Buller River with floodwater entering the town via the Domain and over the wharf in the vicinity of Chamberlain Street (Figures 1, 2 and 3). Water from the Domain floods the Menzies/Roebuck Street area (Figure 3). Water from near Chamberlain Street flows east and ponds. Water from Mill Creek flows via Mill Street to Colvin and Rintoul Streets and the depth of flooding there is very sensitive to the duration of the overflow period. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 2

11 Wharf Domain Orowaiti Overflow Figure 1: Location map showing the approximate extent of the 2-D model. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 3

12 Chamberlain St. Figure 2: Westport street map (not to scale). Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 4

13 * Gladstone/ Russell Wharf Rintoul/Colvin * Domain * Menzies/Roebuck Figure 3: Westport showing the modelled depth of flooding from the calibration. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 5

14 High water levels in the Orowaiti Estuary back up into the creeks that drain the town, but are not a major source of flooding in this calibration event. 4. The flood modelling 4.1. Introduction The purpose of the flood modelling was to show the extent of flooding for 2% AEP (for Building Act purposes) and 1% AEP events (for exploring mitigation options). In doing this work we have assumed that the sea level and river flow peak at the same time during a flood, i.e. we have worked with coincident daily maximum river flows and daily maximum sea levels. This assumption will maximise flooding in Westport from those events. The peaks of large Buller River floods have a relatively broad shape, so it is quite likely that at some time during the river flood peak the tide will also peak Boundary conditions Digital elevation model The DEM for the determination of the flood extents differed from that of the DEM used for calibration in the flowing ways: The Buller River Bridge and approaches as they were in 1970 were digitally removed and replaced by the modern bridge and approaches as determined by the LIDAR. A railway bridge spanning the Orowaiti River overflow channel was replaced by an embankment. The steepness of sections of the true right river bank between Pakington and Henley streets was reduced for computational reasons Hydraulic resistance The hydraulic resistance of that part of the Orowaiti River Overflow channel to the east of the Lidar coverage was reduced to reflect the reduction in tall willow cover between 1970 and the present time as indicated by aerial photographs. The effect of this reduction will be to increase the proportion of the flood flow travelling down the Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 6

15 Orowaiti River overflow channel. However the increase will be limited by the reduction in waterway area as a result of the replacement of a bridge by an embankment as indicated in Section Input hydrograph The input hydrograph for the flood modelling was different to that used by Duncan et al. (2005). To obtain a hydrograph for this model the hydrographs of the five largest river floods from the Buller at Te Kuha record were centred on their flood peaks and the arithmetic average at each hour before and after the flood peak was calculated (Figure 4). This average flood was then scaled to the peaks required for the simulations. The flood peaks used for the 2% and.01 AEP inundation events were 8800 m 3 s -1 and 9400 m 3 s -1 respectively (Duncan et al. 2005). These flood peaks are similar to the 2% and 1% AEP river flood peaks of 8540 m 3 s -1 and 9340 m 3 s -1 determined by McKerchar (2004). Real floods with the same recorded peak as those used in the simulations but with peaks sharper than those used in the simulations will result in less inundation of Westport than is given in this report. Real floods with broader peaks than those used in the simulations would result in more inundation of Westport. 10,000 9,000 8,000 Discharge (m3/s) 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0 Model durations Aug-70 Jul-93 May-88 Oct-83 May-79 Mean 0.02 AEP 0.01 AEP Time since nominal start of flood (h) 0.02 AEP 0.01 Figure 4: The largest five flood hydrographs on record for the Buller River at Te Kuha, their mean and the scaled 2% and 1% AEP floods. The mean hydrograph was scaled up to the required peak for flood simulations Tides and sea level The DEM for the model was surveyed to mean sea level Lyttelton 1937 vertical datum. For both the calibration and the modelling it has been assumed that the elevation of mean sea level at Westport was 0.0 m with respect to that datum. This Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 7

16 assumption has been discussed with Glen Rowe of Land Information New Zealand who states that mean sea level at Westport is -025 m with respect to mean sea level Lyttelton 1937 datum based on mean sea level at Westport for 1918 to 1922 and carrying precise levelling from Lyttelton to Westport. We conclude that while our assumption was not strictly correct this difference in mean sea level is unlikely to have any material effect on the outcome of the study. The peak tide levels and the peak flood flows (see Section 4.2.3) chosen for the simulations are the most likely combinations of peak tide levels and peak river flows to cause flooding of Westport for 2% and 1% inundation events. See Duncan et al. (2005) for a fuller explanation. For the peak tide levels used here of 1.24 m and 1.35 m above mean sea level for the 2% and 1% inundation events respectively, the tides used to obtain those peak levels are less than the peak levels for mean high water springs of 1.49 m above mean sea level (LINZ standard port tidal levels) and so there was no need to add storm surge to achieve the required sea level peak, i.e. no storm surge was used. Figure 5 shows the floods and tide levels used for the simulations Flood discharge (m3/s) Tide level (m MSL Lyttelton) 0.02 AEP flood 0.01 AEP flood 0.02 AEP tide 0.01 AEP tide Time since start of flood simulation (h) Figure 5: The flood hydrographs and tides used for simulating the 2% and 1% AEP Westport inundation events. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 8

17 Hot starts Model runs were hot started from a model with a constant flow of 6500 m 3 /s and tide levels of 1.24 m and 0.4 m for the 2% and 1% AEP events respectively as these were the river flows and tide levels for the start of the flood simulations (Figure 5). At these flows and tide levels the model shows water flowing down the Buller River and the Orowaiti overflow, but no flooding in Westport. The model equilibrates with the inflow and tide level within one hour, that is, about the time that the flood wave takes to move from the upstream boundary of the model to Westport. There are a further 12 to 14 hours to the tide peak and the peak inflow to the model. One consequence of this timing is that the flood peak arrives at Westport about one hour after the tide peak. This may result in the inundation extent being less than it would have been had they coincided exactly, but will be close to the worst case inundation for the particular combination of river flood and sea level peak Determining the extent of inundation for flood models The program saves the maximum water-level, depth and velocity calculated for each cell and this is the value illustrated in the results. The maximum velocity and depth calculated for any cell may not occur at the same time. The maximum depths within the model may occur at different times as the flood wave moves downstream and as the tide floods and ebbs Determining the location and minimum height of stop banks for mitigating the 1% AEP event The 1% AEP model output was examined to determine where water was moving from rivers and channels on to the flood plain. Flow velocity vectors were used to help determine the direction of flow. Virtual stop banks were inserted into the DEM at locations indicated by the examination of the model outputs. The stop banks were two cells wide (9.5 m) and were made high enough to prevent overtopping. The model was rerun with the virtual stop banks in place. The water levels on the river side of the stop banks were extracted from the file containing the maximum water levels during the model run to determine the minimum stopbank heights to prevent overtopping. The DEM containing the stopbanks was interrogated to determine the lowest ground elevation along the stopbank line. This elevation was subtracted from the stopbank elevation to obtain the maximum bank height to stop overtopping. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 9

18 The stopbank locations will only be approximate as the model DEM comprises 4.8 m square grids where elevations have been averaged. Definitive locations should be obtained after detailed site investigations to determine foundation conditions, the line to minimise earth works while minimising interference with existing infrastructure, and allowing for free board and so on. Where stopbanks span existing drainage channels, e.g., Hunters Creek, it has been assumed that the relevant bridges or culverts have been fitted with well-maintained and effective flap valves to prevent flooding. 5. Results Figure 6 shows the extent of inundation for the most likely river flood and sea-levels for a 2% AEP inundation event. Figure 7 shows the extent of inundation for the most likely river flood and sea-levels for a 1% AEP inundation event. Comparison of these floods with those shown in Duncan et al (2005) shows that less water appears to go down the Orowaiti overflow and more is returned towards the Buller River with a portion of that flowing down the flood plain between the Buller River and the railway embankment alongside Nine Mile Road. Some of this water gets returned to the river, but some continues on and into Westport. Water from the Orowaiti River also overtops the terrace near McKenna and Soapworks Road and runs towards the town. The topography and hydraulic roughness in Duncan et al (2005) were less well defined than for this study. The difference in topography is the most likely cause of the different flooding pattern shown in this study. The 2% AEP inundation event shows much more inundation of Westport than was shown for the calibration event. This is attributed to the flood peak level being slightly larger and the coincidence of high tide and river flood peak whereas for the calibration flood the flood peak was at low tide. For both the 1% and 2% AEP floods the modelling shows that the railway embankments at Nine Mile Road and at the Westport to Ngakawau line as it crosses the Orowaiti estuary are both overtopped. This overtopping is likely to cause erosion of the banks which was not modelled. However, overtopping has implications for inundation of Westport. Erosion of the Nine Mile Road embankment would tend to increase Westport flooding from the Orowaiti System and reduce that from the Buller River. Overtopping of the Orowaiti embankment may reduce inundation in the south east of the town. The Nine Mile Road railway embankment is also overtopped with the virtual stopbanks in place. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 10

19 Figure 6: The Buller River flood plain showing an example of flooding for a 2% AEP inundation event. The river flood and sea level peaks used in this simulation were 8800 m 3 /s and 1.24 m. Red arrows indicate flow paths. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 11

20 Figure 7: The Buller River flood plain showing an example of flooding for 1% AEP inundation event. The river flood and sea level peaks used in this simulation were 9400 m 3 /s and 1.35 m. Red arrows indicate flow paths. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 12

21 Table 2 lists the stopbanks and their approximate lengths, minimum reduced level (RL) of the stopbank top to prevent overtopping and approximate stopbank height. The exact location of any stop banks needs to be checked by on ground survey to determine foundation condition, and exact location and alignment to avoid existing infrastructure. Design stop bank height needs to include a safety factor and allowance for wave action in addition to the modelled minimum stopbank height. The Hidden Lagoon stopbank height is the height above the existing stopbank. It is possible that the maximum height for this stop bank is overestimated as the height of the current stopbank may not have been properly captured in the model because it is so narrow. The listed stopbank lengths are maximum lengths and actual lengths would have to be confirmed by ground surveys. Table 2: List of stopbank locations approximate lengths, RL of top to prevent overtopping and maximum bank height. Location Length (m) RL of bank top (m) Height (approx. m) Easting (TM 2000)(m) Northing (TM 2000)(m) Hidden Lagoon Wharf d/s 4.62 u/s Esplanade d/s 4.78 u/s Domain d/s 5.30 u/s Stafford d/s 5.87 u/s Stafford Victoria Road d/s 9.78 u/s McKenna Road u/s 5.24 d/s Soapworks Road u/s 4.93 d/s Figure 8 shows the locations of the proposed stop banks and Figure 9 shows they were effective in reducing flooding in Westport. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 13

22 Hidden Lagoon Wharf Esplanade Domain Stafford 2 Stafford 1 Soapworks Road McKenna Road Victoria Road Figure 8: The Buller River flood plain showing the proposed locations of stop banks to reduce flooding of Westport by a 1% AEP inundation event. Red lines indicate the locations of the proposed stop banks. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 14

23 Figure 9: The 1% AEP flood with stopbanks in place showing they are effective in reducing flooding on the flood plain and in Westport. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 15

24 Figure 10 shows flooding from the 1% AEP event for Westport in more detail and Figure 11 the effectiveness of the stopbanks in reducing the extent of flooding. Figure 10: Flooding in Westport from the 1% AEP event. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 16

25 Figure 11. Westport showing the reduced extent of flooding from the 1% AEP event with the stopbanks in place. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 17

26 6. Discussion 6.1. General assumptions The aim of this study was to determine the extent of inundation of Westport with 2% and 1% AEP inundation events and to determine the locations of stop banks to prevent flooding of Westport from the most likely 1% AEP inundation event. It is clear from Duncan et al that there are many combinations of sea level peak and river flood peak that can result in 1% AEP inundation events in Westport. This report uses, as an example, a single combination close to the most likely 1% AEP event (9400 m 3 /s and 1.35m sea level), to illustrate the inundation mitigation options. This is referred to as the mitigation event. Mitigation measures were not checked against other combinations of river floods and peak sea level. However, stop banks proposed for the same mitigation event in an earlier study (Duncan 2005) provide protection for the most likely 90% of cases. Specifically, the peak flows and sea level combinations used in Duncan (2005) were: 9525 m 3 /s and 0.96 m, 9400 m 3 /s and 1.35 m, and 9095 m 3 /s and 1.98 m. The levels of inundation are based on models with the time of high tide almost coinciding with annual peak river levels using a flood hydrograph that is more peaked than the highest continuously recorded flood (the August 1970 flood). The assumption that the tide, storm surge and river flow peaks coincide during the flood is a conservative one that maximises flooding. It is unlikely that they will coincide, but the river flood peaks are relatively flat and there is about a 50:50 chance that a high part of the flood peak will coincide with the higher parts of a tide. The modelled river flood peaks at Westport reported here occur about one hour after the sea level peak. Examination of the timing of the modelled peak inundation of Westport shows that small river floods and large sea levels cause the inundation to peak at high tide. As the river floods increase, peak inundation levels are delayed to up to three hours after high tide. The delay is primarily because it takes some time for water to move from where it overflows to where it ponds. Three hours after the peak river flood, flood levels are likely to be dropping slowly in other parts of Westport. The comments in this paragraph apply where there are no stop banks. A further issue that affects the extent of inundation is the volume of water spilling over a bank and that is determined by both the water level in the river and the duration that it is above bank level, e.g. a flood with a high sharp peaked hydrograph could cause less flooding than a flood with a flat-topped, lower-peaked hydrograph. The main uncertainties with the modelling relate to the allowance for flood scour, and its variation down the river. Gravel build up varies with time and with dredging of the river. Flood scour will vary with flood duration and flow rates, but was kept constant Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 18

27 for the modelling exercises. A previous study (Duncan 2004) has shown that the extent of flooding is very sensitive to the amount of flood scour. The likelihood is that larger and/or longer flood peaks will result in more scour and less inundation than modelled and vice-versa for smaller floods. The amount of scour is also likely to vary with the tide phase, i.e., scour will be maximised when the flood peak coincides with low tide The role of the Orowaiti overflow and the Nine Mile Road Railway embankment There is a useful discussion on the role of these features in Duncan (2005). It is not repeated here but an amended version can be found in Appendix The consequences of stop bank failure Should the Victoria Road stop bank fail water would move north down the flood plain between the railway line and the Buller River and some of the water would flood into the town. Widespread failure of this bank would probably have a more serious effect on Westport than failure of any of the other stopbanks. Failure of either the McKenna or Soapworks Road banks is less serious as much of the water would be captured by Mill Creek and returned to the Orowaiti Estuary. If the Stratford 1, 2, Domain or Esplanade stopbanks fail, this would result in small area of the town being flooded and the degree of flooding would depend on the extent and duration of the failure of the bank. A Wharf bank failure would result in flooding of the north of the town with the deepest parts in the Gladstone/Russell Street neighbourhood. Failure of the Hidden Lagoon bank would inundate parts of the north part of the town, but the seriousness of the break would depend on its location and size Global climate change: sea level rise and increased rainfall intensities. This report is about mitigation for a 1% AEP inundation event assuming the current climate. The concept of global warming is now accepted by the scientific community. The most likely consequences are that rainfall intensities will increase causing larger floods, and sea levels will rise. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 19

28 Some relevant principles prescribed in the Resource Management Act for achieving the purpose of the Act include: having particular regard to:. the effects of climate change (Section 7 (i)). Thus in considering whether or not to adopt the mitigation measures suggested in this report particular regard should be given to their effectiveness in the light of projected climate change effects in New Zealand (Ministry for the environment 2008) Risk of a 1% AEP event A question arises as to the likelihood of exposure to a 1% AEP event. While an event as large as a 1% AEP can be expected, on average, once every 100 years, the probability of a 1% AEP event being exceeded at least once in a 10 year period is 9.6%, in 50 years 39.5% and in 100 years 63.4% (Beable and McKerchar 1982). 7. Summary 1. Inundation modelling of Westport and the Buller River flood plain has been undertaken using ALS based elevation and hydraulic resistance data using a smaller calculation grid than previous studies. 2. The most likely combinations of river flow and sea-level data from a previous study were used to indicate the extent of flooding of Westport for 1% and 2% inundation events. 3. The results show a greater relative flow down the Buller River and increased flooding of Westport compared with previous studies. 4. It appears that flooding in Westport could be substantially reduced by a series of low (<1.8 m high) stopbanks with a total length of 5.1 km. 5. River peak levels are primarily responsible for the inundation rather than high sea levels. However, as sea levels increase due to higher tides and/or storm surges the balance changes, but such combinations become less likely with higher sea levels. 8. Disclaimer Some of the mitigation suggestions will result in more water in the Buller River adjacent to the wharf at Westport. As part of any detailed design process Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 20

29 investigations should be made as to whether the extra water is likely to cause more scour in this area and or cause failure of the wharf structure and adjacent bank. Such concerns here and elsewhere in the modelled area have not been addressed in this report and NIWA accepts no responsibility or liability should the report recommendations be implemented and such failures occur. 9. References Beable, M.E., McKerchar, A.I Regional flood estimation in New Zealand. Water and Soil Technical Publication No. 10. MWD, Wellington. 132 p. Beffa, C.; Connell, R.J. (2001). Two-dimensional flood plain flow. 1:Model description. Journal of Hydrologic Engineering 6(5): Duncan, M.J. (2005). Potential mitigation measures for 1% AEP flood extents at Westport. NIWA Client Report CHC p. Duncan, M.J. (2004). The sensitivity of inundation in Westport to general scour in the Buller River. NIWA Client Report CHC Duncan, M.J.; Bind, J.; Smart, G.M. (2005). Calibration of the Westport 2D model based on Lidar. NIWA Client Report CHC p. Duncan, M.J.; Image, K.; Woods, R.A. (2005). Determining 1% and 2% AEP flood levels for Westport. NIWA Client Report CHC Duncan, M.J.; Smart, G.M. (2004). River bed grain size distributions in the Buller River downstream of Te Kuka. NIWA Client Report CHC p. Duncan, M.J.; Shankar, U.; Smart, G.M., Willsman, A. (2003). Westport flood mapping study. NIWA Client Report CHC p. McKerchar, A.I. (2004). Revision of flood frequency for the Buller River at Te Kuha. NIWA Client Report CHC Ministry for the Environment Coastal Hazards and Climate Change. A Guidance Manual for Local Government in New Zealand. 2nd edition. Revised by Ramsay, D, and Bell, R. (NIWA). Prepared for Ministry for the Environment. viii+127 p. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 21

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31 Appendix 1: The role of the Orowaiti overflow and the Nine Mile Road Railway embankment The Orowaiti overflow For the purpose of this discussion the Orowaiti overflow system starts in the true right channel of the Buller River around Organs Island, from where water flows northeast over-bank towards the Nine Mile Road railway line embankment. At the embankment water flows northwest beside the railway line and most then flows southwest back to the river. The remainder overtops the south west trending terrace and continues north beside and to the west of the railway line. Some water flows back into the Buller River upstream of Westport and the remainder floods Westport. Some water flows under the rail bridges and culverts into the Orowaiti River and into the Estuary after passing under the road and rail bridges of, and along side, Stephen Road. In a large flood (e.g., AEP <4%) the railway embankments alongside Stephen Road may be overtopped. The role of the Orowaiti overflow in relieving flooding in Westport is somewhat contentious. There is no doubt that it reduces the flow in the main stem, and thus reduces flooding in Westport from the main stem, but there are a number of issues associated with it which caution against inducing too much change to the overflow system. The issues are: 1. Its capacity is limited, e.g., during the 1993 flood (7830 m 3 /s at Te Kuha) with an AEP of 4% (average return period 25 years) the railway line embankment adjacent to Stephen Road was overtopped and damaged. Thus any moves that increase the flood flows to the Orowaiti overflow could threaten this important piece of infrastructure. 2. The division of flow between the Buller River and the overflow is currently delicately balanced so as to minimise flooding in Westport, e.g., removing the willows from the overflow in the area immediately northwest of and adjacent to the Organs Island loop would increase the flow down the Orowaiti and increases the possibility of flooding of Westport from the Orowaiti. 3. The amount of water flowing in the Organs Island channel is dependent on the locations of gravel bars at its diversion with the Buller River, the location of groynes in the main stem of the Buller (such as those currently being constructed to protect State Highway 67), and the density of vegetation in and on the banks of the Organs Island channel. So, even if the willows were cleared from the overflow land adjacent to the channel and the weir reinstated, a variable amount of water may pass over the weir for the same flow in the Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 1

32 main stem of the river upstream of the diversion. Thus it is debatable as to whether the Orowaiti overflow upstream of the Nine Mile Road rail embankment should be engineered to maintain or to increase its capacity. 4. Modelling of the 1% AEP mitigation event peak flow (9400 m 3 /s) shows that in the area of the overflow much of the water leaving the Organs Island loop via the overflow channel is prevented from flowing down the Orowaiti River by the Nine Mile Road railway embankment. Instead some of the water flows northwest beside the railway line and then southwest back to the river or continues north towards Westport. Thus the railway embankment acts as the main hydraulic control on the amount of water entering the Orowaiti River. 5. Because some of the floodwaters turned back by the Nine Mile Road railway embankment flow towards Westport a stopbank is proposed between Nine Mile Road and Reedys Road to prevent flood waters from 1% AEP inundation event river floods from reaching Westport. The Nile Mile Road railway embankment Given that flows into the upstream end of the Orowaiti overflow may vary for the same flow in the Buller River upstream of Organs Island and that more flow in the downstream portion of the overflow is likely to increase Westport flooding, then consideration should be given to controlling the flows passing beyond the railway embankment. This control could take the form of floodgates incorporating a concrete floor designed to control scour. The modelling indicates there is overtopping of the lowest part of the embankment when subject to a main stem flow of 9400 m 3 /s (approximate AEP 1%). Overtopping may cause rapid failure of the bank because of the 1 to 1.5 m water elevation difference from one side of the bank to the other. A breached bank would most likely result in a much increased flood flow down the Orowaiti River with serious implications for the Stephen Road railway line and flooding in Westport. Accordingly, some consideration should be given to raising part of the railway embankment to give some free board. Further, consideration should be given to the design AEP that is appropriate for this structure. The modelling indicates that during the 1% AEP mitigation event river flood, hydraulic conditions are very harsh under and immediately downstream of the Nine Mile Road railway embankment bridges, with water velocities over 4 m/s, Froude numbers over 1.5 and bed shear stresses over 180 N/m 2. Such flows are capable of moving large boulders (>0.2 m median diameter) and could create large scour holes that threaten the integrity of the bridge piles and abutments should such flows Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 2

33 continue for too long. Their failure has the same implications for flooding in Westport as overtopping of the railway embankment. This study has not investigated the integrity of the embankment with respect to its ability to cope with scour from the discharges under the bridges or from spill over the top. Given the implications of failure of the embankment by either of these modes the integrity of the bank should be investigated. Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 3

34 Appendix 2: Files used for the simulations. Simulation Dain file DEM file z0 file Results file Hot start file Hot start time (h) Calibration daincalibl.txt dem100210_70s.asc z asc WPcalibla WPcalibl % AEP inund dainwp01aep.txt dem100430_rec.asc z0_100423_rec.asc WP01AEP Hotstart01i 3.2 2% AEP inund dainwp02aep.txt dem100430_rec.asc z0_100423_rec.asc WP02AEP Hotstart102a 2.1 1% AEP stopbank dainwp01aepstb_e. txt dem100603_stb.asc Z0_ asc WP01AEPstb_e Hotstart01n 2.59 Flood extents for 1% and 2% AEP floods and potential mitigation measures for 1% AEP flood extents at Westport based on LiDAR 4