CGRG Bibliography of Canadian Geomorphology
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Author : Giles, T.
Date : 2006.
Title : The Stemwinder Mountain debris floods-- an unusual event.
Publication : Mass Wasting in Disturbed Watersheds. 2nd Shlemon Specialty Conference in Engineering Geology. Association of Environmental and Engineering Geologists. May 3-5, 2006. Durango, Colorado.
Issue :
Page(s) :
Abstract
On August 16, 2004, a series of debris floods blocked Highway 3 and reached the Similkameen River just west of Hedley, British Columbia. Three small grassland watersheds experienced a short but intense rainstorm event which caused sheetwash, rilling, gullying and channel bank erosion, and initiated debris floods through the channels and onto the fans. A weather station 2 kilometres west along the Similkameen River valley recorded 30.7 mm of rain in a one hour period just prior to the events. Intense precipitation exceeded the infiltration capacity of the thin grassland soils in the upper Stemwinder Creek watershed, causing overland surface flow which rapidly concentrated water into small rills. Headward gully erosion occurred on steeper, convergent slopes. In the upper channel, scouring between 2 and 5 metres was common for 1000 metres. In the lower reaches, the mainstem channel runs for 1000 metres through a slaty-shale bedrock-rimmed canyon and undercuts thick, loosely consolidated talus. Where the channel emerges from the canyon, the debris flood spread out and sediment began depositing on the 10 degree gradient fan. The majority of the sediment, estimated to be as much as 30,000 cubic metres, is angular pebble-cobble sized fragments of slaty-shale bedrock sourced from the talus slopes. There are several large alluvial fans along the Similkameen River in this area and these were generally thought to be inactive relict deposits. The Stemwinder Creek channel was charged with coarse sediment and this event is termed transport-limited (sediment supply unlimited). This is common in the dry southern interior of British Columbia where active colluvial processes occur on slopes, but the long return period of intense precipitation events restricts the occurrence of catastrophic events. Observations in the watershed, along the channel and on the fan suggests that this event may not be as unusual as first anticipated. A better understanding of the frequency and magnitude of this type of occurrence will be instrumental in developing guidelines for future land-use activities on these fans.
Bibliography of Canadian Geomorphology