Whakatane Waimana Floodplain Management Strategy - Stage 1 Review of the Current Flood Hazard from the Whakatane and Waimana Rivers and Existing Responses
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Whakatane Waimana Floodplain Management Strategy - Stage 1 Review of the current flood hazard from the Whakatane and Waimana Rivers and existing responses Prepared by Robbin Britton, Project Engineer Environment Bay of Plenty Operations Publication 2008/09 June 2008 (updated from January 2007) 5 Quay Street P O Box 364 Whakatane NEW ZEALAND ISSN 1176 - 5550 Working with our communities for a better environment E mahi ngatahi e pai ake ai te taiao Environment Bay of Plenty i Foreword Concern for the security of the Whakatane and Waimana floodplains is the reason behind Environment Bay of Plenty’s preparation of a floodplain management strategy. This floodplain management strategy reviews what the flood hazards are in the floodplains, what flood protection is provided to people and property living on the floodplains and what, if any, additional protection needs to be provided. Floodplain management strategies are not the same as Civil Defence Emergency Management (CDEM) Plans. Under the Civil Defence Emergency Act 2002 each regional and local council must prepare plans detailing how they will manage catastrophic events such as volcanic eruptions, tsunami storms, earthquakes and fires. Construction of flood defences began in the catchment in 1965 and this was complete by the early 1980’s. Since that period components of the flood defence system have been reviewed but not all together. This floodplain management strategy reviews the flood protection system for the Whakatane Waimana catchment only. It identifies potential weaknesses in flood protection and plans for the ongoing security of future generations. The executive summary provides an overview of the report. Part one describes the nature of the floodplain, and the measures put in place to best manage floods of date. The resulting issues for future floodplain management are collated in the conclusions section at the end of Part one. Part two identifies the flood problem by summarising a number of studies that have been undertaken to quantify the potential flood risk and impacts. Alongside these studies are the actual recorded impacts which resulted from July 1998 and July 2004 floods. The issues from Part one and Part two are summarised at the end. Part three summarises all the issues identified in the previous two sections. Part three outlines what subsequent stage two and three aim to achieve. The floodplain management strategy reports make reference to floods in terms of say the ’50 year flood’ or ‘100 year flood’. These terms refer to the chances of a flood occurring of a certain size over a given period of time. Hence for example if a flood flow of a certain size occurred once every 50 years on average, the chances of another flood of this size or greater in any one year is said to be 1 in 50. Predicting random events such as flooding is an approximate science. The following examples may help put risks of flooding of the Whakatane Waimana floodplains into context: Burglary in any one year 1 in 16 Drawing the ace of spades in any one draw 1 in 52 Peak discharge in Waimana River on 2 July 1998 was 1,100 cumecs 1 in 60 Infant mortality 1 in 119 Peak discharge in Upper Whakatane River, July 2004 was 2,893 cumecs 1 in 120 Contracting heart disease in any one year 1 in 1000 (Source: non italic data from Wellington Regional Council, 1996) Operations Publication 2008/09 Whakatane-Waimana Floodplain Management Strategy Environment Bay of Plenty iii Contents Foreword ................................................................................................................ i Executive Summary............................................................................................. v Part 1: Managing the Whakatane Waimana Floodplains ........................1 1.1 Introduction.......................................................................................................1 1.2 Objective...........................................................................................................2 1.3 Planning Framework.........................................................................................2 1.4 Stages of Floodplain Management Planning ....................................................3 1.5 The Output........................................................................................................4 1.6 The Whakatane Catchment ..............................................................................5 1.7 Land use...........................................................................................................8 1.8 Hydrology .......................................................................................................11 1.9 Historical Flooding ..........................................................................................14 1.10 Response to Flooding: Structural Works ........................................................24 1.11 River Management & Planning .......................................................................31 1.12 Response to Flooding: Non Structural Measures ...........................................33 1.13 Current Non- Structural Measures..................................................................36 1.14 Recommended Floodplain Management Practices ........................................39 1.15 Conclusions ....................................................................................................40 Part 2: Identifying the Flood Problem .........................................................41 2.1 Introduction.....................................................................................................41 2.2 Flooding in the Lower Whakatane River.........................................................41 2.3 Flooding in the Upper Whakatane River.........................................................44 2.4 Flooding in the Waimana River.......................................................................45 Operations Publication 2008/09 Whakatane-Waimana Floodplain Management Strategy iv Environment Bay of Plenty 2.5 Lower Whakatane Stopbank Security ............................................................ 45 2.6 Lower Whakatane Stopbank Failure .............................................................. 46 2.7 The Flood Hazard .......................................................................................... 52 2.8 Potential Damages......................................................................................... 67 2.9 Summary of Key Issues ................................................................................. 68 Part 3: Future Management: The Way Forward....................................... 77 Appendices .......................................................................................................... 81 Appendix 1 – Summary of Whakatane Riverbank Capacity Reviews ....................... 83 Appendix 2 – Description of Utility and Infrastructure Damages............................... 85 Appendix 3 – Whakatane Waimana Flood Maps ...................................................... 87 Appendix 4 – Potential Stopbank Breach Flood Maps............................................ 111 Appendix 5 –Potential Damages and Annualised Damages................................... 123 References ......................................................................................................... 127 Glossary of Terms ........................................................................................... 131 Whakatane-Waimana Floodplain Management Strategy Operations Publication 2008/09 Environment Bay of Plenty v Executive Summary The underlying aim of floodplain management planning is to reduce the susceptibility/exposure to flooding to persons and property that exist within the scheme catchment. The aim of this stage one report is to identify and describe existing flood issues within the Whakatane and Waimana catchments, outline existing flood protection measures, identify potential hazards and elements at risk and identify what mitigation options are needed. This stage one study revealed that flood protection service level requirements are in general being met for the scheme. Plans for managing flood risk already exist within the catchment and these are being implemented by Environment Bay of Plenty through the schemes asset management plan. The purpose of this floodplain management strategy is to review current practices and plan for the future flood protection needs of the catchment. Several risk areas were identified within the scheme and stakeholders need to consider whether the level of risk is acceptable. Several tasks need to be completed in stage two before the questions raised in stage one can be answered. Questions raised are the basis of several recommendations outlined in Table 3.1 found in Part 3 of this report. Table 3.1 also nominates the stakeholder responsible, the output required and implementation timeframe. It includes but is not limited to confirmation of assumptions made in stage one, review and provision of flood protection measures provided by utility owners and further geotechnical investigations on stopbanks. For stakeholders in Lower Whakatane and protected by the 100-year urban stopbanks, the main question to be answered is: are stakeholders satisfied with the current service level provided by the urban stopbanks? Currently the probability of at least one stopbank breaching from a total of 8 potential breach sites on the right (urban) stopbank is 11%. This assumes the low point in the Yacht Club