These maps present preliminary assessments of the probability of debris-flow
activity and estimates of peak discharges that can potentially be generated
by debris flows issuing from basins burned by the Old and Grand Prix
Fires of October 2003 in southern California in response to the 25-year,
10-year, and 2-year 1-hour rain storms. The probability maps are based
on the application of a logistic multiple regression model that describes
the percent chance of debris-flow production from an individual basin
as function of burned extent, soil properties, basin gradients and storm
rainfall. The peak discharge maps are based on application of a multiple-regression
model that can be used to estimate debris-flow peak discharge at a basin
outlet as a function of basin gradient, burn extent, and storm rainfall.
Probabilities of debris-flow occurrence range between 0 and 85% and estimates
of debris flow peak discharges range between 460 and 5,900 ft3/s (13
to 167 m3/s). These maps are intended to identify those basins that are most prone to the largest debris-flow events and provide critical information
for the preliminary design of mitigation measures and for the planning
of evacuation timing and routes.
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