Debris flow risk management in practice: a New Zealand case study

Abstract

In 2005 the settlement of Matatā on Awatarariki fan, Bay of Plenty, North Island, New Zealand was devastated by a >3 x 105m3 debris flow; several dwellings were destroyed and many damaged, but no fatalities occurred. In the 7 years following the event, design options for a debris-flow containment structure in the catchment were developed. Following a formal determination by the Government’s building control agency in 2006, building consents were granted for a number of replacement dwellings on the fan. In 2012, the previously chosen containment structure project was cancelled due to effectiveness and cost concerns. Subsequent investigations confirmed there were no viable engineering solutions to manage debris-flow risk from this catchment, and risk analyses have demonstrated that no debris-flow management systems, warning and evacuation systems, or individual dwelling protection mitigation measures, independently or in combination, could deliver a residential environment with tolerable risk-to-life levels. Since 2013, Whakatāne District Council (the Council) has been working towards a non-legislated managed voluntary retreat from the area where the risk to life is greater than about 10-4a-1, which is also the area of boulder deposition in the 2005 event. This has involved many meetings with affected landowners, including legal counsel and experts, but the currently 13-year delay in resolving uncertainty about landowners’ futures has generated considerable stress and even hostility. A parallel legislation-based workstream the Council has undertaken to fulfil its statutory responsibilities has exacerbated tension between the Council and some property owners. From a technical perspective, this study emphasises the danger of lay officials and consultants placing too much confidence in immature technologies to reliably modify debris-flow occurrence. From a public management perspective, it highlights the immaturity of New Zealand’s natural hazard management policy framework, in particular the significant disconnect between policy intent and policy implementation and its polarising effects on a small provincial community.