Response to landslide dam failure emergencies: issues resulting from the October 1999 Mount Adams landslide and dam-break flood in the Poerua River, Westland, New Zealand

On October 6, 1999, a large rock avalanche from Mount Adams on the west coast (Westland) of the South Island, New Zealand, fell into the Poerua Valley. The landslide blocked the river valley, damming the Poerua River, and creating a large lake. The potential for overtopping and failure of the landslide dam presented a potential dam-break flood hazard that was assessed as posing a serious danger to Poerua Valley residents located downstream. The dam eventually failed 6 days after it was formed. Fortunately, the resulting flood was largely confined to the river channel and flood-plain areas, causing little damage and no deaths. The Poerua River landslide dam-break flood highlighted a range of issues that should be addressed in managing future landslide dam-break flood emergencies. This paper summarizes the key organizational, community, and response issues arising from a break-out flood such as this. Planning for the management of future landslide dam-break floods may help reduce loss of life from future events. Preparations could include setting aside more resources for assessing the hazard, and improved control and communications for managing the response. From an emergency management perspective, because of the remote and inaccessible location of landslide dam sites in steep mountain valleys on the west coast of New Zealand's South Island, it is important to ensure that the community has direct involvement in the readiness and response process.