Waimakariri District Council and the Canterbury Regional Council Adopted a Waimakariri District Flood Hazard Management Strategy
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Report R08/23 ISBN 978-1-86937-804-2 58 Kilmore Street PO Box 345 Christchurch Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru Phone (03) 688 9069 Fax (03) 688 9067 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 Environment Canterbury Technical Report EXECUTIVE SUMMARY This study of the Ashley River floodplain uses a combined one and two dimensional hydraulic computer model to estimate flood extent and depths on the Ashley River floodplain. Modelling has indicated the current capacity of the Ashley River stopbanked system is approximately equivalent to the 2% AEP (50 year return period) flood event. Breakouts however, could occur in more frequent events. Stopbank breaches and outflows onto the floodplain could potentially occur anywhere along the stopbanks. Breakout scenarios onto the floodplain have been modelled for the 1%, 0.5% and 0.2% AEP (i.e. 100, 200 and 500 year return period) events at the most likely breakout locations. The modelling indicates significant flooding to large areas of land between the Ashley and Waimakariri Rivers. Kaiapoi and adjacent areas are predicted to be flooded to depths over 1 metres in the 0.2% AEP (500 year return period) event. The impact of future urbanisation in Kaiapoi has also been modelled. It is hoped the floodplain maps and associated depths will assist land use planning within the area and provide information on minimum floor levels for new dwellings located on the floodplain, where appropriate. Environment Canterbury Technical Report 1 January 1953 Flood Looking across Ashley River at flooding between Waikuku Beach and Woodend Beach. Floodwaters extended through to the Pines and Kairaki. Photograph by L Ernle Clark for North Canterbury Catchment Board. Environment Canterbury Technical Report Table of Contents 1. Introduction...............................................................................................................5 2. Background...............................................................................................................5 2.1 Ashley Catchment and Geomorphology.............................................................5 2.2 Historical Flooding ..............................................................................................6 2.3 Ashley River control Scheme ...........................................................................11 3. Flood Investigations...............................................................................................12 3.1 Risk Assessment ..............................................................................................12 3.2 Flood Hydrology ...............................................................................................15 3.2 Climate Change................................................................................................15 3.3 Hydraulic Model Description.............................................................................15 3.4 Model Simulations and Results ........................................................................19 4. Discussion ..............................................................................................................38 5. Glossary ..................................................................................................................43 Appendices Appendix 1: Floodplain Geomorphology Appendix 2: Ashley River Flood Levels Appendix 3: Floodplain Roughness Appendix 4: Ashley River and breakout flow hydrographs Appendix 5: Floodplain Roughness Sensitivity Analysis – Kaiapoi Area Appendix 6: Breakout Flow Sensitivity Analysis Appendix 7: Hazard Category Definition Appendix 8: Flood Probability List of Photos Photo 1: Damage to the Ashley River railway bridge in March 1902 flood. ........................ 7 Photo 2: Surface flooding in High Street, Rangiora – February 1945 ................................ 8 Photo 3: Kaiapoi River (Williams Street) in flood – February 1945 ..................................... 9 Photo 4: Kaiapoi – February 1945 flood.............................................................................. 9 Photo 5: Rangiora – Loburn traffic bridge at the height of the flood on 25 January 1951. Donated by Mrs C Tyler to Rangiora and Districts Early Records Society. ............................................................................................................... 10 Photo 6: The Coldstream area, showing the major overflows, and extent of flooding between Rangiora Golf Course and the sea. Photo by L Ernle Clark for North Canterbury Catchment Board..................................................... 10 Environment Canterbury Technical Report 3 List of Figures Figure 1: Ashley, Waimakariri, and Kaiapoi River Stopbanks ........................................... 11 Figure 2: Potential failure zones and historical breakouts, Ashley River........................... 14 Figure 3: Ashley River Floodplain Topography ................................................................. 18 Figure 4: Potential Flooding in 1% AEP Breakout A ......................................................... 21 Figure 5: Potential Flooding in 1% AEP Breakout B ......................................................... 22 Figure 6: Potential Flooding in 1% AEP Breakout E/F ...................................................... 23 Figure 7: Potential Flooding in 1% AEP Combined Breakout ........................................... 24 Figure 8: Potential Flooding in 0.5% AEP Breakouts A & B.............................................. 26 Figure 9: Potential Flooding in 0.5% AEP Breakouts A & C.............................................. 27 Figure 10: Potential Flooding in 0.5% AEP Breakouts E/F ............................................... 28 Figure 11: Potential Flooding in 0.5% AEP Combined Breakout Flooding........................ 29 Figure 12: Potential Flooding in 0.2% AEP Breakout........................................................ 31 Figure 13: Potential Flooding – Kaiapoi area in 0.5% AEP Event..................................... 33 Figure 14: Impact of Filling – Kaiapoi Growh areas 0.5% AEP Event............................... 34 Figure 15: Potential Flooding – Kaiapoi area in 0.2% AEP Event..................................... 35 Figure 16: Impact of Filling – Kaiapoi Growth area 0.2% AEP Event................................ 36 Figure 17: Flood Hazard Map – 0.5% AEP Event ............................................................. 41 Figure 18: Flood Hazard Map – 0.2% AEP Event ............................................................. 42 Environment Canterbury Technical Report 1. INTRODUCTION The Ashley River floodplain covers approximately 190 km2 and includes two large urban areas, Rangiora and Kaiapoi. A previous investigation on flooding of the Ashley River Floodplain was undertaken in 1995. “Ashley River Flood Plain Management Plan Technical Investigation Report 95(6), 1995.” This investigation highlighted potential breakout risk and predicted overland flow paths and ponding areas, using a one dimensional hydraulic computer model, Mike 11. In 2003 the Waimakariri District Council and the Canterbury Regional Council adopted a Waimakariri District Flood Hazard Management Strategy. Mitigation measures in the strategy included maintenance of the existing stopbank system and land use management measures, such as appropriate development in flood prone areas and appropriate minimum floor levels for new dwellings. The investigations described in this report have been undertaken with the aid of very detailed topographic data, obtained from a LiDAR (Aerial Laser) survey and a more physically realistic two dimensional hydraulic computer model of the floodplain (using the Mike 21 software). This more accurate (c.f. previous investigation) should enable better informed decisions to be made regarding land use management of the floodplain and will also enable a more accurate assessment of appropriate minimum floor levels for new dwellings to be made. The hydraulic computer model has also been used to estimate the effects of various development scenarios (i.e. filling) on the floodplain in the Kaiapoi area. The assessment of the probability of Ashley River breakouts (outflows) and locations is described in a separate report “Ashley Floodplain Hazard Risks Assessment”, Report No. U08/1, January 2008. 2. BACKGROUND 2.1 ASHLEY CATCHMENT AND GEOMORPHOLOGY The Ashley River is a relatively steep braided river and together with its major tributary, the Okuku, drains a catchment of approximately 1200 km2. The catchment is relatively steep with the highest point in the Puketeraki Range at over 1900m. The unconfined floodplain commences on the south bank of the Ashley River, immediately downstream of the Okuku River confluence. Numerous breakouts and change of course here over thousands of years have resulted in alluvial deposition leading to the formation of the adjacent alluvial fans, extending to Kaiapoi and the Waimakariri River. On the north side of the Ashley, however, the river is confined by a prominent 5-10 metre high terrace. This terrace extends downstream from the Okuku River as far as Ashley Township. Downstream of the Ashley Township the river is confined by stopbanks but the floodwaters can break out onto the Ashley Fan. Environment Canterbury Technical Report 5 A full description of the Ashley Floodplain Geomorphology can be found in the earlier Technical Investigation Report on the Ashley, Report 95(6). A map of the Ashley Floodplain geomorphology