Abstract

On 11 May 1985 a magnitude 7.1 (Richter scale) earthquake occurred on the island of New Britain, Papua New Guinea. The earthquake caused localized property damage and initiated widespread landsliding up to 30 km from the epicentre. One of the largest landslides, with an estimated volume of 180 x 106m3, occurred in the narrow, steep-sided Bairaman River valley. This landslide started as a rockslide in weathered limestone and rapidly transformed into a debris avalanche, filling over 3 km of the Bairaman River valley to a maximum depth of 200 m and creating a landslide dam which impounded the Bairaman River. By August 1986 the resultant lake had attained a volume of about 50 x 106 m3 and posed a grave threat to downstream inhabitants of the Bairaman valley. By September 1986 the lake was close to overtopping, the people were evacuated and the dam was breached. The failure of the dam took about 3 hours and generated a debris flow with an estimated volume of 120 x 106 m3 and average velocity of 20 km h-1 that travelled 39 km down the Bairaman valley to the Solomon Sea. At the river mouth the flood was 8 m above normal river level and swept away Bairaman village. Due to the evacuation no lives were lost.

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