Evolution of an overflow channel across the Young River landslide dam, New Zealand

Abstract

New Zealand's Young River was dammed on 29 August 2007 by 11 million m 3 of debris at 44° 080'44.6''S, 169° 06' 46.0''E. The fractured biotite-schist debris travelled up to 1.8 km to form a 74-m high dam. Water flowed from the new 2.5 km-long, 23 million m3 lake after 5 weeks. The slope on the dam face is about 10°. The mean surface particle size is about 1 m (uniformity coefficient, 8). Lake level and rainfall at the dam have been monitored since 10th October 2007. There have been many high flows since then; the largest, 3.4 m deep at the outlet. The outlet had lowered 0.7±0.2 m by January 2011; the channel mostly widened but locally aggraded and degraded as boulders < 1 m across moved. Larger boulders appear stable. Outlet lowering is attributed to 3 outlet flow depths > 2.1 m. The rainfall which put 3.4 m water depth through the outlet had an annual exceedence probability of 0.1 a-1. We estimate the expected (most probable) longevity of the lake as twice the current age, but recognise that this probabilistic estimate will eventually be 2x too large. © Springer-Verlag Berlin Heidelberg 2013.