The October 1999 Mt Adams rock avalanche and subsequent landslide dam-break flood and effects in Poerua River, Westland, New Zealand

On 6 October 1999 a very large (c. 10-15 million m(3)) rock avalanche from Mt Adams blocked the Poerua River 11 km upstream from the SH6 road bridge on the West Coast of the South Island. The 120 m high rock debris dam impounded a lake with a volume of 5-7 million m(3) before it overtopped on 7 October. The short survival time of landslide dams in rivers in Westland and around the world suggested a high probability of rapid dam failure and flooding downstream. This was confirmed when the dam breached 6 days later on 12 October 1999, during the first significant rain after the landslide occurred. The resulting dam-break flood deposited considerable coarse gravel from the landslide in the valley downstream of the dam, and (mostly fines) on the alluvial fan below the Poerua gorge exit. The flow inundated farmland in the upper Poerua valley, but otherwise was largely confined to the river channel and did little damage at the time, mainly because of significant flow attenuation (c. 50% or greater) and sediment deposition on the alluvial fan below the gorge exit. Subsequently the remnant lake has been infilled, and c. 75% of the dam material has been transported downstream during floods. About 1.7 million m(3) of alluvium has been deposited in the river channel and across farmland between the gorge exit and the SH6 bridge, changing the river's course and causing lateral erosion of older terraces. These effects are continuing and will cause ongoing problems in the future. The rock avalanche and landslide dam failure in the Poerua valley were significant events which have had a profound local geomorphic impact. The landslide and subsequent downstream effects are typical land scape-forming events. These events and the resulting community response to them have provided valuable information on the hazards, effects, and management of future landslide-dam failures in Westland. Increased resources within local authorities for hazard assessments and response planning during such events would reduce the risk from dam-break floods in the future, especially following the next Alpine Fault earthquake, when further landslide dam failures are expected.