COMPARISON OF DEBRIS-FLOW VOLUMES FROM BURNED AND UNBURNED AREAS

The goals of this work are to show the range of debris-flow volumes and watershed characteristics for several different locations, and to show the differences in flow volumes for events triggered soon after wildfire. A dataset of 929 events was assembled into divided into five main groups: Italian Alps unburned, Pacific Northwest unburned and burned, and Western United States unburned and burned. The Western U.S. burned group was further subdivided based on the elapsed time between the wildfire and the debris-flow event. The three unburned locations show significant differences. Flows in the Italian Alps tended to be larger in volume and emerge from less steep and larger drainage basins. Flows in the Pacific Northwest had the highest area yield (volume of debris divided by source basin area) and highest length yield (volume of debris divided by length of flow channel). Flows in the Western U.S. had the lowest area yields and length yields, and emanated from steeper basins with longer source channels. For the Western U.S. events, burned areas produce debris flows that are typically 200% the area yield and 125% the length yield as unburned areas. These differences decrease with time after the wildfire to the point that events after five years are indistinguishable from unburned areas. It should be noted, however, that the dataset for debris-flow events less than three years after wildfire contains smaller and steeper basins with shorter channels than the dataset of events in unburned areas. Comparison of events between burned and unburned areas in the Pacific Northwest was inconclusive because of the small size of certain dataset elements and the bimodal nature of others. Burned areas in the Western U.S. produced higher area yields than those in the Pacific Northwest, indicating that wildfire may have a stronger effect in the Western U.S.