Run-up of two avalanches on the deflecting dams at Flateyri, Northwestern Iceland

Abstract

Two deflecting dams were constructed above the village Flateyri, northwestern Iceland, after the catastrophic avalanche accident on 26 October 1995 when 20 people were killed. Both deflectors have since been hit by moderately large avalanches, in each case with a volume of > 100 000 m3 and an estimated return period of 10-30 years. The avalanches hit the deflectors with deflecting angles of 20-25° and a maximum run-up of 12-13 m. They flowed about 500 m along the deflectors after the initial impact and terminated in the ocean on the respective sides of the reef where the village is situated. Back-calculated impact velocities are of the order of 30 m s-1. In both cases, the impact channelized a part of or the whole of the width of the avalanche into a stream 20-80 m wide, the run-out of which is estimated to have been increased by > 100 m by the deflection of the avalanche by the dam. Although the avalanches are much smaller than the design avalanches of the deflecting dams, they have provided avalanche professionals and the public in Iceland with much-welcomed direct evidence of the effectiveness of the defence structures at Flateyri against moderately sized events. They also provide unique direct observations for further scientific investigations of the deflection of avalanches by man-made deflecting dams.