The role of debris supply conditions in predicting debris flow activity

Debris flow frequency and magnitude were determined for 33 basins in southw est British Columbia. Basins were first classified as either weathering-lim ited or transport-limited using a discriminant function based on debris-con tributing area, an area-weighted terrain stability number, and drainage den sity. Multiple regression was used to predict magnitude, peak discharge, fr equency and activity (frequency times magnitude) within each group of basin s. Model performance was improved by stratifying the total sample of debris flow basins into weathering-and transport-limited groups. Explained varian ce increased by an average of 15 per cent in the transport-limited sample, indicating that sediment supply conditions in the more active basins are fu ndamental in predicting debris flow activity. An independent test of the re gression models with 11 basins yielded generally good results for debris fl ow magnitude and peak discharge. Prediction of debris flow frequency proved problematical in weathering-limited basins. The methods developed here pro vide estimates of debris flow attributes in basins for which few data on pa st events are available. Copyright (C) 1999 John Wiley & Sons, Ltd.