OC Flood Hydrology
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 flood plain 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 and 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 contact the Hydrology Section at
 714.834.3785
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