Select a language:


The 1986 Orange County Hydrology Manual and its 1996 Addendum have been calibrated to local watershed conditions. They provide guidance in estimating peak discharges and volumes of storm water runoff for the design of flood control facilities and floodplain determination. The primary goal is to provide 100-year flood protection for all habitable structures. The Manual and Addendum are also used to produce discharges and volumes of runoff for more frequent storms (2, 5, 10, 25, and 50-year) that are useful in mitigation of increased runoff due to development and design of local drainage facilities. The design storm is of 24-hour duration. The Manual provides two methods for development of discharges—the Rational Method and the Unit Hydrograph Method.

Rational Method

The Rational Method is used to estimate the peak discharge of stormwater runoff for drainage areas that are less than 640 acres. The traditional formula for the Rational Method is Q=CIA where Q is the peak discharge, C is the runoff coefficient, I is the rainfall intensity, and A is the drainage area. The Manual does not use the traditional formula. Instead, the Manual uses a modified Rational Method which considers factors such as land use, quality of cover, soil type and time of concentration (Tc) to find discharges for areas less than 640 acres.

The Rational Method can be used for drainage areas larger than 640 acres only to estimate the Tc for use in the unit hydrograph studies.

Unit Hydrograph Method

The unit hydrograph method is used for watersheds larger than 640 acres to estimate peak discharges and volumes of stormwater runoff. This method produces a graph of discharge vs. time for the entire length of a storm. Input needed to derive a runoff hydrograph includes lag time (=0.8Tc), drainage area, appropriate S-graph, rainfall depths, soil loss rates, and depth-area-reduction factors. Besides proprietary programs, the single event unit hydrograph option of the U. S. Army Corps of Engineers’ HEC-1 program (combined with the LAPRE-1 program) is publicly available to find the runoff hydrograph. Complex modeling of watersheds that may include retarding basin analyses, streamflow routing, and multi-day storm analyses is possible with this method provided it is prepared in accordance with the Manual—specifically Chapter K.

Hydrology Reports & Studies

The hydrology studies and reports contain forecasted and historical data, i.e. topography maps, storm/rain precipitations, intensities, durations and frequencies; ultimately, storm discharges are resulted from the data. For information regarding specific reports or studies, please fill out a Public Records Act Request via For further questions, please contact the Hydrology Section at (714) 647-3961