Ashley River bed level investigation Report No. R09/71 ISBN 978-1-877542-12-1 A J Boyle M R Surman September 2009 Report R09/71 ISBN 978-1-877542-12-1 58 Kilmore Street PO Box 345 Christchurch 8140 Phone (03) 365 3828 Fax (03) 365 3194 75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636 Ashley River bed level investigation Executive summary This report explores the history of flooding and human responses to that flooding with particular emphasis on sedimentation in the Ashley River from the Okuku confluence to the coast. The Ashley River has a long history of flooding combined with substantial sediment movement and erosion. The Ashley River stopbanks have not been breached since 1953 although break-outs almost occurred in 1986 and 2002. The report is part of the implementation of the Waimakariri District Floodplain Management Strategy. The stopbanks were last upgraded in 1976 to contain floods up to 2400 m3.s-1 with a freeboard of 600mm. Recent survey and modelling suggest the river has more than that capacity (3000 to 3500 m3.s-1, or more, throughout most of the river). The revised 100 year flood estimate recommended for design purposes is 3250 m3.s-1. The changes in mean berm, mean channel (fairway) and channel invert (lowest) levels between 1960/62, 1976, 1997 and 2008/09 are recorded and analysed. A trend of degradation of the fairway which has accelerated markedly since the 1997 survey is evident from the survey record. In the period since 1997 degradation far exceeds the reported gravel extraction. A range of indicators have been tabulated in Section 21.1 which together provide a record of how the capacity and erosion risk have changed over time. Recommended target mean bed levels are set out based on 4 criteria related to the indicators. The criteria are intended to: Effect a minimum flood capacity to safely pass a flow of 3250 m3.s-1 to the sea. Limit berm edge heights to enable erosion risk to be managed and to protect existing infrastructure such as bridges and intakes. Reduce the variability of mean bed slope without introducing large changes in slope to assist with river bed stability and consistency of bedload transport capacity. Maintain a consistency of flood capacity over the length of the scheme. The report concludes with the following recommendations: Adopt the levels set out in Section 22 as minimum bed levels with the exception that bed levels be allowed to drop up to 200mm further in the reach downstream of SH1. Gravel extraction should be limited to the few locations where drops in bed level have been recommended. The volumes involved are small in relation to historic extraction rates and current applications in process. The Ashley River Rating District consider adopting 3250 m3.s-1 (plus 600mm freeboard) as the design flow for the river. Environment Canterbury Technical Report i Ashley River bed level investigation It is apparent that significant changes can occur in a relatively short time, therefore, where large scale excavation is occurring the cross sections upstream of, through and downstream of, the excavation areas should be surveyed annually. In the absence of extraction the reach downstream of SH1 be surveyed at least 5-yearly so that the capacity of the reach can be managed. Elsewhere, a maximum of 10 years between surveys (or more frequent if large floods appear to have altered the bed substantially) is considered adequate. This is longer than the period recommended in the Waimakariri District Floodplain Management Strategy but when considered along with the annual surveys recommended above is a more strategic approach. Allowing extraction of gravel and sediment down to the proposed minimum bed levels would yield about 440,000 m3 (as at the most recent survey of 2008/09) of which 240,000 m3 is available for current applications. ii Environment Canterbury Technical Report Ashley River bed level investigation Table of contents Executive summary....................................................................................................i 1 Introduction .....................................................................................................1 2 Context.............................................................................................................1 3 Methodology....................................................................................................1 4 Physical setting of the Ashley catchment ....................................................1 5 Structural setting of the catchment...............................................................4 5.1 Geomorphology ..............................................................................................................6 6 History of flooding ..........................................................................................9 7 Previous schemes ........................................................................................60 7.1 Population, land-use, and future development .............................................................60 7.2 Perception of risk and public participation ....................................................................61 7.3 Structural integrity of the existing system .....................................................................61 8 Hydrology ......................................................................................................63 8.1 July 2009 review ...........................................................................................................63 9 Sedimentation ...............................................................................................64 10 Climate change .............................................................................................69 11 Survey comparisons.....................................................................................70 12 Hydraulics......................................................................................................72 13 Mean fairway levels ......................................................................................75 14 Invert levels ...................................................................................................77 15 Channel widths .............................................................................................79 16 Mean berm level comparisons.....................................................................81 17 Fairway capacity relative to mean berm levels ..........................................83 18 Mean berm level minus mean fairway level comparisons.........................84 19 Mean berm level minus channel invert comparisons ................................85 20 Slopes ............................................................................................................87 20.1 Fairway slopes..............................................................................................................87 20.2 Channel invert slopes ...................................................................................................90 21 Criteria ...........................................................................................................90 Environment Canterbury Technical Report iii Ashley River bed level investigation 21.1 Indicators ......................................................................................................................91 22 Recommended bed levels ............................................................................92 23 Sediment budget...........................................................................................93 24 Implications for gravel extraction consents ...............................................95 25 Conclusion ....................................................................................................96 26 Recommendations........................................................................................96 27 Peer review ....................................................................................................97 28 Acknowledgements ......................................................................................97 29 References.....................................................................................................97 Appendix 1 Bed level decision matrix........................................................99 iv Environment Canterbury Technical Report Ashley River bed level investigation List of photographs Photo 6-1 March 1902 Flood............................................................................................................14 Photo 6-2 March 1902 Flood............................................................................................................14 Photo 6-4 March 1902 Flood............................................................................................................16 Photo 6-5 March 1902 Flood............................................................................................................17 Photo 6-6 May 1923 Flood...............................................................................................................19 Photo 6-7 The long deep lagoon, 9/09/09........................................................................................22 Photo 6-8 The Café Bank, 9/09/09 ..................................................................................................22 Photo 6-9 February 1945 Flood .......................................................................................................33