The fatal Cleft Peak debris flow of 3 January 2002, Upper Rees Valley, West Otago

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McSaveney, M.J.; Glassey, P.J. 2002 The fatal Cleft Peak debris flow of 3 January 2002, Upper Rees Valley, West Otago . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2002/03 28 p.

Abstract: About mid-morning on 3 January 2002, Alan Buxton, 28, was killed by a debris flow while attempting to cross an unnamed headwater tributary of the Rees River, west Otago. The event occurred during heavy rain. The high-intensity rain triggered many shallow landslides in the thin layer of loose, weathered rock debris (regolith) overlying the steeply dipping schist bedrock in many of the tributary headwaters in the area. These, in turn initiated debris flows in many streams. Mr Buxton and Bevan Thrower had the misfortune to be in a stream channel and about to cross one of these streams at the time a debris flow approached them at high speed. Mr Thrower heard it approaching and escaped it by the narrowest of margins. The fatal debris flow came from one or more of a group of unnamed tributary drainage basins on the north flank of Cleft Peak (2250 m). Probably originating from a small shallow regolith failure, the debris flow grew rapidly in volume as it accelerated down the steep, narrow, and flooded mountain torrent. Travelling faster than the water flow, it picked up further water and sediment from the channel. Within minutes of initiation, a rapidly moving wall of sediment and water about two metres deep and with the consistency of very sloppy wet concrete, had descended the steep mountain slope and poured across the fan at the foot of the slope. After about five minutes it settled down to a raging torrent but left thick, concrete-like debris on the banks. The fine-grained schist of the area is easily weathered to a fine sandy regolith, that easily gives rise to debris flows during storms. There were no special circumstances of the fatal debris flow. Any of the more than twenty or so debris flows in the upper Rees valley that morning could have been fatal had people been in their way. (auth)