Hydrology Standards

Mean Seasonal Isohyet Map
The 1977 Mean Seasonal Isohyet Map is the valid Contra Costa County Flood Control & Water Conservation District (FC District) Standard for mean seasonal precipitation (MSP). It is based on a July to June rain year.

Drawing B-166 was used to create a GIS raster of the MSP. This raster is being made part of the Contra Costa County GIS library. The procedure documenting the development of the MSP raster is provided here (14.9 MB KB).

Rainfall Distribution Curves
The following document provides information on the sources of the FC District's rainfall distribution curves and dimensionless unit hydrograph (S-Curve). 

This link is to a spreadsheet of the rainfall distribution curves.

IDF Curves
Intensity Duration Frequency Curves (IDF) are used in Rational Method calculations. The following document explains how to create the IDF curves using the Duration-Frequency-Depth curves (above).

S-Curve in xls format
The Contra Costa County S-curve can be obtained through the following link:

  • S-Curve_for_HMS (zip - 33 KB)(Right click this link and choose "Save As.")
Storm Depth Estimation for Frequent Return Intervals
The following draft guidelines present a method for estimating storm depths for frequent return period storms. For this guideline, frequent storms are defined as those with recurrence intervals of 5-year and less (e.g. 2-year) or annual exceedance probabilities over 20%. (e.g. 2-year storm = 50% annual exceedance probability.)