April 17, 2018,
3:30 p.m. - 4:30 p.m.
The frequency, intensity, and areal extent of wildfires in the Western United States are increasing. Changes imposed on watersheds during and after burning result in a dramatic increase in debris-flow occurrence and magnitude, which in turn poses risks to human life and property, especially as the urban-wildland interface advances into mountainous areas. Flows can be initiated following wildfire by low intensity rainfall events, and they grow substantially in volume through channel scour while in transit. On average, flow volumes are 3-5 times larger immediately after wildfires, and the volume magnification effects linger for 1-3 years before the watershed recovers. Volume is difficult to predict accurately, but it can be estimated using multiple linear regression models that rely on GIS-friendly inputs such as area burned, rainfall totals, and watershed slope characteristics. A new probabilistic model has been shown to be more accurate and it relies on even fewer terms. Discharge rates and velocity are also difficult to predict, so reasonable ranges are derived using databases of previous field measurements. Damming and avulsion of the flows make runout patterns unpredictable, but recent studies have helped to quantify the process.