Mt Adams rock avalanche of 6 October 1999 and the subsequent formation and breaching of a large landslide dam in Poerua River, Westland, New Zealand

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Hancox, G.T.; McSaveney, M.J.; Davies, T.R.; Hodgson, K. 1999 Mt Adams rock avalanche of 6 October 1999 and the subsequent formation and breaching of a large landslide dam in Poerua River, Westland, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 99/19 22 p.

Abstract: The Mt Adams rock avalanche occurred at around 3 am on the morning of 6 October 1999. Between 10-15 million m3 of schist and colluvium fell a maximum of almost 1800 m from the northern peak of Mt Adams into the Poerua River valley. Slide debris formed a landslide dam more than 100 m high, and impounded a lake 1200 m long and 80 m deep, with an estimated volume of 5-7 million m3. Seismographs show that the avalanche caused an earthquake of local magnitude ML 3.2, and that the main collapse began at 2.35 am, and lasted about 25 seconds. Because of the potential dam-break flood hazard, residents in the Poerua valley were evacuated from their homes just before the dam overtopped at about 8 pm on 7 October. After an inspection by technical experts the following day, the dam was declared safe during the present fine weather, and residents were allowed to return to their homes. The landslide dam breached shortly before 8 am on the morning of 12 October, during a storm in which at least 80 mm of rain fell overnight. There is little risk of a significant dam-break flood in the future, unless further landslides reform the dam. However, debris from the landslide dam is already inundating farmland and has raised the Poerua River bed in the vicinity of the SH6 bridge and elsewhere. The formation and breaching of the Poerua landslide dam has provided valuable information on the performance and effects of a large landslide dam in a large river in Westland. The event has also drawn attention to the vulnerability of many West Coast communities to dambreak flood hazard. Planning for managing landslide dam-break floods is needed if loss of life from such events is to be avoided in the future. Preparations should include improved control and communications for managing the response. There should also be further geotechnical studies, including monitoring the aggradation along the river valley, and modelling of the Poerua dam-break. A large earthquake on the Alpine Fault is likely to result in the formation of landslide-damned lakes in a number of West Coast valleys, which will eventually be breached. A reconnaissance of sites potentially susceptible to landslide dam-break flooding is needed to identify the areas of greatest risk from dam-break flooding and promote awareness. In moderate risk areas this may help minimise the hazard of future landslide dams by allowing flood prevention measures to be considered. However, in some cases there will be a sufficiently high probability of flooding that no practical precautions can be taken to reduce the risk, except relocation of the threatened buildings. (auth)