RELATIONS BETWEEN DEBRIS-FLOW VOLUMES GENERATED FROM RECENTLY BURNED BASINS AND BASIN MORPHOLOGY, TRIGGERING STORM RAINFALL AND MATERIAL PROPERTIES

Debris flows are frequently produced from recently burned basins in response to heavy rainfall. Hazard assessments can benefit from the ability to predict potential debris-flow volumes generated from burned basins. This study aims to develop a model to predict the potential volume of a wildfire-related debris flow as a function of basin morphology, burn severity, material properties and triggering storm rainfall characteristics. For 36 basins in eight burned areas located in Colorado, Utah and California, debris-flow volumes were estimated by quantifying the amount of material eroded from a channel by the passage of a debris flow. Studies of debris-flow producing basins throughout the intermountain west indicate that the bulk of material in a fire-related debris flow is derived from channels rather than from landslides or hillslope erosion. Debris-flow volume estimates were derived from measurements of cross sections spaced between 50 and 200 feet apart in each scoured channel within a basin. The pre-event channel configuration was reconstructed for each cross section by projecting the form of undisturbed hillslopes into the channel. The total volume of channel scour was estimated by taking the average scour of two adjacent cross section and multiplying this value by the distance between the cross sections. The sum of all channel segment volumes in the basin represents the total debris-flow volume. Measures of basin gradient, shape and channel network were calculated from digital elevation models (DEMs), and maps of burn severity were used to quantify the area of each basin burned at high, moderate, and low severities. Grain-size distributions were determined from field samples of burned soil. Networks of rain gages installed throughout the burned areas provided rainfall amounts and intensities for debris-flow triggering storms. A series of multiple regression analyses indicated that the parameters that are most strongly related to debris-flow volume are average storm rainfall intensity, basin area burned at high severities and average gradient. A model that predicts potential debris-flow volumes generated from burned basins will provide a useful tool to guide hazard assessments in burned areas and the construction of debris retention structures.