Wharekawa coast 2120 - wider river flood assessment

Report: TR 2021/18

Authors: Whitney Mills, Alex Blunt, Adam Munro, Kenny Growden (Waikato Regional Council) and Adrian de Laborde (Hauraki District Council)

Wharekawa Coast 2120 aims to bring the Wharekawa communities together to define their path for the future while enabling flexibility to respond to changing conditions such as projected climate change. This Wider River Flood Assessment is part of that process, with the purpose of this report to inform the Wharekawa Coast 2120 Community Panel, Technical Advisory Group and Joint Working Party to understand the historical impacts of past river flooding events in the project area.

Information within this report was collected from a range of sources, including council documents on past flooding event impacts, and community eyewitness accounts and stories. Flooding event information presented in this report stretches back to the 1960s through to the present day (June 2022), including 16 river flooding and/or coastal inundation events that have been documented. A wide range of small to significant impacts are documented across the four wellbeing’s and environments, e.g. impacts to building, property, and cultural impacts.

This report recommends priority work areas for further investigation based on our new understanding of the impacts of historical riverine flood events. A better understanding of the drainage capacity and flood dynamics in these priority areas will improve our understanding of the hazard and in turn the risk these streams pose to the community, particularly in the vicinity of the Whakatīwai and Waharau Streams.

The River Management Work Programme is a key piece of work developed conjointly with the Community Panel and technical experts from the representative councils, Hauraki District Council and Waikato Regional Council. The Programme identifies site specific low cost stream, catchment and drainage maintenance options that can be implemented relatively quickly to reduce the flooding risk in the project area in the short term, though these options are not intended to reduce the flood risk completely.

The River Management Work Programme is part of the community panel’s Recommendations Report and will be used to prioritise work to guide future funding options to inform both councils’ long term plans commencing 2024/2034.