Flood Hazard Assessment of Potential Growth Areas Palmerston North City: Ashhurst

Transcription

1 Flood Hazard Assessment of Potential Growth Areas Palmerston North City: Ashhurst Report prepared for Palmerston North City Council by Philip Wallace River Edge Consulting Limited May 2013

2 Table of Contents 1 Introduction Ashhurst Stream Flood Scenarios Hydraulic Model Results Discussion Conclusions... 8 Appendix A Ashhurst Street Map... 9 Appendix B Summary of Ashhurst Stream Upgrade Works Appendix C Hydrological Assumptions i

3 1 Introduction Palmerston North City Council is investigating the suitability of areas for urban expansion. Two semi-rural areas have been identified in Ashhurst: around 19 ha on the northern side of North Street and 8.5ha to the west of Winchester Street (Figure 1). (Refer to Appendix A for street locations.) Flood hazard is one of the issues the Council must consider in assessing whether these are suitable areas for urban development. This report describes the modelling of the flood hazard in these areas, particularly the hazard posed by the Ashhurst Stream. Figure 1 Ashhurst and proposed city growth areas 2 Ashhurst Stream The primary source of historic flooding in Ashhurst has been the Ashhurst Stream. In response, a flood protection scheme for the stream was initiated in the 1960s and various channel works were subsequently carried out 1,2. More recently, Horizons Regional Council has been undertaking an 1 Ashhurst Comprehensive Stormwater Management Plan, PNCC report 29 January

4 upgrade of the channel and constructing stopbanks, with the aim of providing 1% AEP ( 100 year ) flood protection. 3 The principal components are stopbanking and deepening the Ashhurst Stream between Custom St and 350 m upstream of North St (Appendix B). The works are now substantially complete. 3 Flood Scenarios The flood scenario considered for this current assessment is a 1% AEP event with allowance for climate change to This is considered an appropriate minimum standard for long-term landuse developments such as are under consideration here. It is noted that in recent years Horizons has required new developments to be flood-free in 0.2% AEP ( 500 year ) events without the need for additional stopbanking. A lack of on-site hydrological records would make any flow and rainfall estimates of that scenario subject to a large uncertainty and it has not been modelled. The 1% AEP flow (North Street) has previously been estimated to be 15 m 3 /s, and increasing to 19 m 3 /s at SH3. These estimates were derived using the Rational Method. For this current assessment, the 1% flow figures have been reviewed, using a range of methods. Results for North Street range from 9.1 m 3 /s (TM61 Method) to 20.6 m 3 /s (Regional Flood Frequency). The Rational Method gives a value of 16.7 m 3 /s, with the reasons for the variation from the previous figure being a re-estimation of catchment area, use of a slightly larger runoff coefficient and use of revised rainfall estimates (HIRDS v3). Considering the various results, a value of 18 m 3 /s is recommended. The same methods have been used to provide an estimate of the 1% AEP flow with allowance for climate change to The resulting value was 21.5 m 3 /s. However this has been reduced by 2 m3/s on the advice of Horizons, to allow for a natural overflow to the Pohingina River near Ulysses Road (upstream of Ashhurst). 4 The shape of the flow hydrograph has been derived using the HEC- HMS software program. Inflow hydrographs for relevant subcatchments downstream have been derived using the same range of methods. Further details on the hydrological assumptions are included in Appendix C. 4 Hydraulic Model To assess the vulnerability of the sites, a computer hydraulic model was developed to simulate flooding in design flood events. The principal modelling tool used is MIKE FLOOD, a software program developed by DHI. It dynamically links river/stream channel flow and overland flow during a simulation. This program is 2 Ashhurst Stream Scheme Review, Horizons Regional Council, November Ashhurst Stream Scheme Upgrade Design Report, Horizons Regional Council, February s from Peter Blackwood (Horizons Regional Council) to Daniel Batley (PNCC) dated 23 October 2012 and to Philip Wallace (River Edge Consulting) dated 6 December

5 well suited to floodplain modelling and has been used in several studies commissioned by Horizons Regional Council. The channel component of the model is based on a model of the Ashhurst Stream supplied by Horizons. The model covers the stream from about 350 m upstream of North Street (in line with the northern boundary of the North Street Growth Area) to about 500 m downstream of State Highway 3. Modifications made to the model supplied include more incorporating more design details (as in Appendix B). The North Street Drain, from 150 m west of the Ashhurst Stream to Pohangina Road, has also been added to the model, using survey data provided by Horizons. No survey of the Ashhurst Stream has been carried out since the recent works, and unfortunately design drawings provided by Horizons were not very detailed, requiring some interpretation. Thus the stream model contains some idealised and assumed channel dimensions and features. The floodplain representation is based on Lidar data, supplemented by ground survey carried out by Horizons on the north side of North Street. There are no data for a portion of the North Street Growth Area however (Figure 4). Aerial photographs were also used in setting up the model, and a site visit was carried out on 16 January It is important to note that the model is not calibrated, there being no calibration data available for either the pre-works or the post-works state of the stream. 5 Results Results for the 1% AEP (climate change) simulation show water spilling along much of the length of the Ashhurst Stream (Figure 2), as might be expected for a channel designed for a lesser flow. The main points of overflow are on the upstream sides of Oxford Street and Wyndham Street and at the western-most corner of the Winchester Street Growth Area. 5.1 Winchester Street Growth Area These overflows eventually make their way into and through the Winchester Street Growth Area (Figure 3). Flows then drain away to land south-west of the site. The predicted peak level in the site is 400 mm, near the western corner adjacent to the stream. Flow depths are however for the most part fairly shallow; less than 200 mm. Slightly higher ground along a line of trees on the northern boundary of the site appears to protect the site from further inundation. (Note that the floodmaps shown below do not include any freeboard allowance.) 3

6 Figure 2 Peak flow predictions, 1% AEP flood with climate change allowance 4

7 Figure 3 Peak flow predictions, Winchester Street Growth Area,1% AEP flood with climate change allowance 5

8 5.2 North Street Growth Area The scenario modelled is predicted to cause very little inundation of the North Street Growth Area (Figure 4). Minor spilling from the North Street drain affects a small portion of the site, while shallow overland flow is predicted on the southern side of North Street. Although not modelled, overflow from the Ashhurst Stream north of the site would be blocked by the recently constructed stopbank that goes eastward from the stream towards Pohangina Road along the northern boundary of the site. (It is understood that the overflow passed through the north-west corner of the growth area site in the February 2004 flood event. 5 ) Inundation in the portion of the site on the eastern side of Pohangina Road has not been modelled, but as discussed below the flood risk to that area is considered minimal. Figure 4 Peak flow predictions, North Street Growth Area, 1% AEP flood with climate change allowance 6 Discussion 6.1 Winchester Street Growth Area It may be possible to raise stopbanks in the future to compensate for climate change (although costs tend to increase non-linearly with additional height). Containing the flow at road crossings 5 Warren Wheeler, Horizons Regional Council, pers. comm.. 6

9 however would be more difficult. Thus it is reasonable, and conservative, to assume that overflows as modelled will occur in the 1% AEP event with climate change. The predicted flood depths are generally shallow enough in the scenario modelled so that it would be practical to raise building platforms and to develop most of the site. If the area is rezoned for urban growth however, it would be important for secondary flow paths to be maintained (or enhanced), to allow flow through the site without adversely affecting properties on Winchester Street. The western-most corner of the growth parcel is only about 4 m from the Ashhurst Stream centreline. It is unclear exactly where the new stopbank sits in relation to this, but Horizons should require a minimum set back from the landward toe of the stopbank. (It is also understood that from this corner the stopbank follows the growth parcel boundary for around 80 m rather than following the stream bank.) No contribution from the urban catchment between Winchester Street, Wyndham Street and Cambridge Avenue has been included (Figure C.1, Appendix C). An examination of stormwater network records indicates that this area is piped eastwards towards the Pohangina River. However, in a 1% AEP climate change scenario, it can be expected that the network would be overwhelmed, in which case the ground contours show any resulting surcharges would flow overland towards the Ashhurst Stream. Although with further work this could be quantified, the Ashhurst Stream overflow would dominate and be more significant. Neither have stopbank breach scenarios been modelled. No assessment has been made of the geotechnical stability of the stopbanks, however it is assumed that the probability of stopbank breach is small. Nonetheless, final building areas, platform levels and secondary flow paths (assuming the area is rezoned) should allow for the possibility. Breach scenario assumptions would need be to discussed with Horizons, and modelled once an updated survey of the stream channel and stopbank works has been completed. 6.1 North Street Growth Area The North Street Growth Area does not appear to be subject to any significant flood risk. It is on a flat plateau with Ashhurst Stream overflows only expected to approach the north-western corner of the Area (and now prevented from flowing into the Area by a new stopbank). Local runoff in the portion to the west of Pohangina Road is not predicted to cause any concerns, and contours suggest that any overtopping of the new Ashhurst Stream stopbanks in over-design events would not flow into the Area. Topography data are not available upstream of the Growth Area but it appears that catchment area of the Pohangina Road roadside drains is fairly limited, while the drains themselves are substantial. Inundation in the portion to the east of Pohangina Road has not been modelled, but there is nothing to suggest that flooding would be an issue there. Although again topography data are not available for some of that portion, it appears to be fairly flat with a small slope towards the intersection of Pohangina Road and North Street. From that point, roadside drains take flow towards the Pohangina River valley. 7

10 7 Conclusions A review of the Ashhurst Stream hydrology indicates that the 1% AEP design flows previously adopted are low. A value of 18 m 3 /s at North Street is considered more appropriate. Incorporating climate change to 2090, a value of 21.5 m 3 /s is recommended. A lack of detailed stream and stopbank survey data (or detailed design drawings) for the Ashhurst Stream has meant that the hydraulic model developed for this assessment is not as refined as would be ideal. Furthermore the model is uncalibrated. Nonetheless, the model is sufficient to show that the North Street Growth Area is not subject to any significant flood hazard and that the Winchester Street Growth Area is subject to a moderate but manageable flood hazard. If the Winchester Street Growth Area is rezoned for urban development, more detailed modelling is recommended in order to set aside secondary flow paths within the area, and to ensure that there are no adverse effects on surrounding properties (mainly along Winchester Street). Such modelling would require a stream and stopbank survey, and consideration of breach possibilities. The modelling could also incorporate surcharging from the stormwater pipe network in the block bounded by Wyndham Street, Winchester Street and Cambridge Avenue. An adequate set back of development from the Ashhurst Stream stopbanks would need to be agreed with Horizons for the Winchester Street Growth Area. 8

11 Appendix A Ashhurst Street Map 9

12 Appendix B Summary of Ashhurst Stream Upgrade Works The following are extracts from the Ashhurst Stream Scheme Upgrade Design Report prepared by Horizons Regional Council in February 2010 and included in the resource consent application for the works. 10

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14 Appendix C Hydrological Assumptions Contributing flows from the catchments identified in Figure B.1 are included in the hydraulic model. Figure C.1 Catchments incorporated in modelling Catchment Area (ha) Table C.1 Catchment area 12

15 22 20 Flow (m 3 /s) /01/20153:00 1/01/20159:00 1/01/201515:00 Figure C.2 1% AEP (climate change) inflow hydrographs, by catchment High Intensity Rainfall System V3 Depth-Duration-Frequency results (produced on Monday 25th of June 2012) Sitename: Ashhurst (centre) Coordinate system: NZTM2000 Easting: Northing: Extreme rainfall assessment with climate change Projected temperature change: 2.1 degree Celsius Rainfall depths (mm) Duration ARI (y) aep 10m 20m 30m 60m 2h 6h 12h 24h 48h 72h Table C.2 HIRDS rainfall estimates, 1% AEP (climate change) 13

16 Notes: Catchment 1 Method 1% AEP 1% AEP + CC comment TM Rational, C = Rational, C = Regional FF 20.6 may be conservative interpolation of contours HEC-HMS 6hr storm may be conservative, as nested all peak intensities HEC-HMS 12hr storm may be conservative, as nested all peak intensities suggest use Ratio 21.5/18 = 1.194, ie 19% increase - ok Table C.3 Peak flood estimates, main Ashhurst Stream catchment 1 Initially used 12 hr HEC-HMS storm hydrograph for shape, scale to 21.5 m 3 /s peak. However, Horizons advice is that the flow should be reduced by 2 m 3 /s to account for some natural diversion to the Pohingina River upstream of Ashhurst. Hence final results assumed the hydrograph was scaled to 19.5 m 3 /s peak. Catchment 2 1% AEP CC flows: TM61 4 m 3 /s, Rational (c= 0.35) 11 m 3 /s, HEC-HMS 13 m 3 /s (6hr) 15 m 3 /s (12hr) (1% AEP: Regional FF, 8.1 m 3 /s) Likely to be conservative as channel obstructions, road crossings Use 8 m 3 /s peak (use 12 hr HEC-HMS storm hydrograph for shape) Catchment 4 1% AEP CC flows: HEC-HMS 5 m 3 /s (12hr) Seems high. Rational (c= 0.45, developed), 2.5 m 3 /s. Seems more appropriate, especially considering some of flow may also pass under Pohangina Rd to drain eastward. (Use 12 hr HEC-HMS storm hydrograph for shape) Apply half of the hydrograph to eastern end of North St drain in model, other half to point 400 m to the west. Catchment 3 1% AEP CC flows, Rational (c= 0.35) 1.5 m 3 /s Assume similar t c to Catchment 4 (similar travel distance, steeper but not developed). Use the Catchment 4 hydrograph, scaled to 1.5 m 3 /s peak 14

17 Catchment 5 1% AEP CC flows, Rational (c= 0.35) 0.4 m 3 /s Use the Catchment 4 hydrograph, scaled 0.4 m 3 /s peak (for simplicity, even though t c will be slightly shorter) Catchment 6 1% AEP CC flows, Rational (c= 0.4) 5.2 m 3 /s, HEC-HMS 7 m 3 /s (12hr) Use HEC-HMS, (conservative, also some contribution likely from unmodelled catchment Winchester-Wyndham-Cambridge block) Catchment 7 1% AEP CC flows HEC-HMS 1.8 m 3 /s (12hr). May be conservative, but model assumes it gets into Ashhurst Stream whereas it may be held back due to high water levels in stream Catchment 8 1% AEP CC flows HEC-HMS 9 m 3 /s (12hr). May be conservative, but peak occurs early on compared to main stream peak flows. Other catchments Area of village to the east of Cambridge Av drains towards the Pohangina River valley. Area bounded by Winchester/Wyndham/Cambridge discussed elsewhere primary flow is piped to Pohangina River valley. Areas south of Mulgrave Street have no impact on areas of interest. 15