EPISODIC SEDIMENT-YIELD RESPONSES TO SHORT-RECURRENCE RAINSTORMS ON RECENTLY BURNED BASINS

Debris flows generated during rain storms on recently burned areas have taken lives and destroyed property throughout the Western U.S. The debris flows frequently erode and entrain materials that have been stored in channel networks over time periods that are relatively long when compared with the recurrence of the storms that trigger the response. Field measurements of closely-spaced channel cross-sections from 46 post-wildfire debris-flow events in southern California, Utah, and Colorado showed average yield rates for each channel ranging from 0.3 to 9.9 m3 of debris produced for every meter of channel length, with an overall average value of 2.5 m3/m. Yield rates for short reaches of channels (up to several hundred meters) ranged as high as 22.3 m3/m. These channel yield rates generated debris flows that range in volume from 170 to 60,000 m3. The storms that triggered the debris flows were either short-duration convective thunderstorms or long-duration frontal storms, but all showed recurrence intervals of two years or less.

Data from basins burned in recent fires throughout the intermountain west were used to identify the factors that control this episodic debris-flow response to frequently-occurring storms. The probability of debris-flow generation was found to be a function of a combination of aerial burned extent, soil properties, basin gradient, and average storm rainfall intensity. The volume of material that might issue from a basin mouth was found to vary with a combination of basin gradient, aerial burned extent, and total storm rainfall. The presence of the aerial burned extent in both models indicates that wildfire plays an important role in the evolution of channels by allowing frequently-occurring storms to episodically remove stored materials.