Massey CI, Potter SH, Leonard GS, Strawbridge G, Rosser BJ, de Vilder SK. 2020. Awatarariki catchment debris flow early warning system framework. Lower Hutt (NZ): GNS Science. 54 p. (GNS Science report; 2019/77). doi:10.21420/8D3K-HD78.
In 2005, a large debris flow occurred in the Awatarariki catchment, above the township of Matatā, in the North Island of New Zealand. The debris flow damaged homes and infrastructure in Matatā. Risk assessments have been carried out by Tonkin & Taylor, and the calculated annual individual fatality risk to dwelling occupants in the area affected by debris flow hazards has been assessed as being intolerable by Whakatāne District Council (WDC) and most of the local community. In November 2019, GNS Science was commissioned by WDC to scope out the potential design and effectiveness-evaluation framework for a public-facing Early Warning System (EWS) for debris flows on the Awatarariki Fan, Matatā. As part of this work, GNS Science was asked to identify any initially obvious issues that could reduce the effectiveness of the EWS (i.e. ‘show stoppers’) and whether an EWS would be suitable/unsuitable for the Council to consider as an option to manage the risk. This report evaluates a multi-staged debris flow EWS based on the proposed design and effectiveness framework. Results from the three scenarios discussed indicate that a multi stage EWS is unlikely to allow all potential people present in the hazard zone at the time that a debris flow event is initiated to evacuate to safe areas. Therefore, people who do not or cannot evacuate would continue to be exposed to the risk levels calculated by Tonkin & Taylor, depending on their location on the fan. Given the uncertainties associated with a debris flow EWS, as listed in this report, adopting such a system as the means to mitigate the risk to people living on the fan is not aligned with taking a precautionary approach, as stated in Section 1.7 of the Bay of Plenty Regional Council’s Regional Policy Statement. In writing this report, our thoughts are with everyone effected by the debris flow in 2005. This was a traumatic event, and many have experienced ongoing difficulties in recovery. We acknowledge these past and current residents and property owners, as well as any future potential users of the land. (auth)