Wharekawa Coast 2120: Natural hazard risk assessment

Report: TR 2020/08

Authors: Sophie Marsh, Whitney Mills, Phil Mourot, Terry Hume, Rick Liefting, Stephen Hunt

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 natural hazard risk assessment is part of this process, with the purpose to enable the community panel to understand and evaluate the most significant natural hazard risks to the project area, and compare the risks across compartments and impact categories. The results will predominantly inform community panel workshops, an exercise with the wider community to assess community risk thresholds and a stakeholder workshop on risk thresholds.

To achieve this purpose; information was collated on the impacts of recent significant historical natural hazard events, qualitative (coastal erosion and freshwater flooding) and quantitative (coastal inundation and Hauarahi Stream flooding) risk assessments were undertaken, additional information on potential impacts was collated, and vulnerability was assessed.

The results show that in terms of potential impacts, coastal inundation is the most significant natural hazard to the project area, with exposure and estimated damage costs being greatest in sub-compartments 1a (Pūkorokoro Miranda) and 2a (Kaiaua township). Hauarahi Stream flooding is a significant natural hazard for sub-compartment 2a, but events are likely to have lesser impacts than coastal inundation.

The impacts of coastal erosion are not significant for most of the Wharekawa Coast currently; however, they may be in the future, with shoreline retreat likely to be primarily driven by coastal inundation and sea level rise. For the five streams assessed qualitatively for freshwater flooding risk, impacts are generally expected to be low, with the exception of Whakatiwai Stream if the stopbanks breach or overtop. When determining community risk thresholds, vulnerability should also be considered as it has the potential to influence when intolerable thresholds may be reached. Characteristics and vulnerability factors have been assessed for the project area, with any spatial variations described. It is considered that this risk assessment achieves its purpose, but it is recognised that more detailed assessment may be required to evaluate potential adaptation options; this will occur later in the process.