Possible effects of outlet dam break lahar

Ruapehu Bulletin, 24 June 1997, Page 6

Possible effects of outlet dam break lahar

Modelling the rapid collapse of the ash barrier suggests it will result in a lahar with a peak flow of 1 500- 1 600 cubic metres per second at the end of the Whangaehu Gorge^ compared to 540-970m3/s for the Tangiwai Lahar. Attenuation down the Whangaehu River reduces the peak flow of the hypothetical lahar to about 1050-1 150m3/s at the Tangiwai rail bridge, which is almost twice the peak flow (650m3/s) of the Tangiwai Lahar based on field measurements at that point. (The report states that the Tangiwai Disaster lahar destroyed five bridges on the Whangaehu River including the Tangiwai Rail Bridge.) It is apparent, therefore, that rapid collapse of the ash barrier to the reformed Crater Lake poses a greater hazard downstream than has occurred in previous historical lahars. The resulting damage to bridges, roads, and possibly power lines and the Rangipo Power Station is likely to be greater than has occurred in the past. There are likely to be significantly greater environmental damage effects down the Whangaehu Valley, and probably also in the Tongariro River catchments and Lake Taupo in the event of lahar overflow from the Whangaehuchannel.