Origin, effect and implications of a storm-induced debris dam in Black Birch Stream, Mount Cook Village

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Macfarlane, D.F. 1980 Origin, effect and implications of a storm-induced debris dam in Black Birch Stream, Mount Cook Village. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 326 24 p.

Abstract: Erosion of surficial debris by groundwater outflow deposited an estimated 100-150,000m3 of material into the outlet gorge of Black Birch Stream during the December (1979) storm at Mount Cook. A gravel fan which formed in the gorge created a debris dam of an estimated 50,000m3 volume and, combined with up to 6m of aggradation in the stream channel, raised Black Birch Stream to the point that it almost overstopped its protective embankment. As this would have resulted in flooding of the Black Birch Fan, that part of the village was evacuated. Remedial treatment within the debris source area is considered impractical. Emphasis must therefore be placed on protective work to enable control of any future problem. Debris from the stream has been utilised to extend the protective embankments which are now thought to be capable of handling debris influx and stream flows about 50% greater than experienced in the December (1979) storm. The lack of hydrological records for the Black Birch catchment makes it difficult to fully assess future risk. Although the return period for storms of the December (1979) magnitude may exceed 25-30 years a flood even larger than experienced at that time must be regarded as a long term certainty with future planning based on this assumption.