Risk of Debris‐Basin Failure

Debris basins are a common engineering structure used to control debris flows. Knowledge of the risk of failure as a function of important design variables can improve decision making and can be used as a basis for minimizing the total annual cost (i.e., construction and maintenance plus risk cost). The failure risk was computed for four policy elements: the rainfall frequency, the interval between significant watershed burn, construction and dredging accuracy, and the regularity of maintenance of the debris basin. The burn interval and the rainfall magnitude are the two most important variables associated with the failure risk, with the expected annual risk varying from less than 1% to as'much as 65% for different burn intervals and rainfall frequencies. A failure to maintain the basin can double the risk or failure. The risk of failure does not appear to change much for typical construction and dredging‐volume accuracy. The risk estimates were made using a model developed from data from the southern California area and the conditional expectation variance reduction technique.