PREDICTING DEBRIS-FLOW CHANNEL YIELD RATES IN RECENTLY BURNED, SOUTHERN CALIFORNIA DRAINAGE BASINS
Field measurements of channel yield rates were collected in four drainage basins burned by the 2009 Station fire in southern California. The drainage basins each produced two debris flows and repeat surveys of channel cross sections before and after the passage of each debris flow provided measures for the channel yield rates due to each event. Additional channel yield rates were measured from five drainage basins burned by the Grand Prix and Old fires in California shortly after a debris-flow event using channel cross sections and inferring the original channel shape by projecting non-eroded channel bank slopes into the channel. The debris-flow volume models rely on measures of precipitation, drainage basin morphology and burned extent. Triggering storm rainfall characteristics for the debris flows were measured using tipping bucket rain gages. Debris-flow volumes were predicted for locations immediately upstream and downstream of the measured cross sections and divided by the channel length to generate a predicted yield rate.
The measured debris-flow channel yield rates were generally within the 95 percent confidence interval of the drainage basin scale model predictions. The debris-flow volume model can be used to predict average channel yield rates along the length of the channel, and thus total volume at the mouth of the drainage basin. However, channel yield rates may vary significantly along short channel lengths and models more sensitive to specific channel properties, such as channel confinement and available material, are needed to better predict this local variability.