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 Mean Seasonal Precipitation. 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) (PDF).

Duration-Frequency-Depth Curves

The Duration-Frequency-Depth curves are used in conjunction with the Mean Seasonal Isohyet Map to determine rainfall characteristics at any location in Contra Costa County.

Rainfall Distribution Curves

The following document provides information on the sources of the FC District’s rainfall distribution curves.

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

Intensity Duration Frequency Curves and Spreadsheet

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). The Excel Workbook can be downloaded to calculate the IDF curver per the "IDF Curve Development" document.

S-Curve in Excel (xls) format

The Contra Costa County S-curve can be obtained through the following link:

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.)