Alluvial and colluvial fans were studied to determine geomorphic disturbance type, methods of predicting disturbance type and power, and how forest operations can destabilize fans. Fifty-five fans in the southern Coast Mountains, northern Vancouver Island, and south-western Vancouver Island were field traversed and additional watershed data was collected using GIS. Thirty-nine fans had harvesting or roads, although in some cases the harvesting was very little. Forest operations on the study fans occurred from 1957 to 2004. Forty-five fans showed evidence of old debris flows (>50 years old), 5 showed evidence of old debris floods, and 9 showed evidence of old water floods. Only 13 fans had recent (<50 years old) debris flows, 7 had recent debris floods, and 29 had water floods. Best predictors of geomorphic disturbance type are apex slope gradient, the Relative relief ratio, and the Melton Ratio. Forest operations caused destabilization at 22 sites, and included avulsions, channel incision, bank erosion and channel widening.
Thomas Millard, David Wilford, and Marian Oden
Millard, Tom H., Wilford, David J.; Oden, Marian E.. 2005. Coastal fan destabilization and forest management: final report for the 2004/05 project year. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report