Abstract

The magnitude and frequency of floods from gaged and ungaged drainage areas in California, for any recurrence interval from 2 to 100 years, can be estimated by use of the method presented. Equations relating flood magnitudes of selected frequency to basin characteristics such as drainage area, precipitation, and altitude were developed for six regions in the State. Nomographs are included for solution of the equations. The regression equations were developed for streams that have natural flow or flows not substantially affected by storage.

Annual flood peak discharge for more than 700 streamflow stations with records generally for 10 years of more, including more than 340 stations on streams with basins smaller than 10 square miles, were analyzed with respect to several drainage-basin characteristics. The drainage-basin areas ranged in size from 0.01 to 9,020 square miles. Maximum known peak discharges are shown and their relation to drainage area defined. Discharge rates greater then 1,000 cubic feet per second per square mile resulting generally from summer storms were observed in only 25 basins smaller than 25 square miles, most of which are in southern California or in desert areas.

Data on basin characteristics used in the regression analysis, in addition to those applicable to the regional equations, are tabulated to provide pertinent basic information for use in other studies and evaluations. Some adjustments to the estimated peak discharges for urbanization effects, based on flood-peak rations for developed and undeveloped areas, are suggested.

First posted November 29, 2010