Magnitude and frequency of debris flows

Debris flows periodically result in the loss of lives and property. Engineering structures designed to control debris flows are often inadequate because of lack of knowledge of the magnitude of debris events. The objective of this study was to develop a method that could be used to estimate the magnitude and frequency of debris flows. The data base for the study included 29 watersheds in the Los Angeles area, with drainage areas < 3 mile². Assuming a log-normal distribution, prediction equations for 2-, 5-, 10-, 25-, 50-, and 100-year return periods were developed as a function of relief ratio, hypsometric index, the interval between burns, and drainage area. Principal components and correlation analyses were used to select the predictor variables. Numerical optimization was used to calibrate the model. The prediction equations can be used to estimate the magnitude of debris flows for ungaged watersheds where estimates are required for debris basin and channel design, protection of culverts and roads, land use planning, and zoning and establishing insurance rates.