Contra Costa County, CA Official Website

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Public Works Hydrology

1. Where can I find your design standards?: The District is preparing to bring all its standards into one document. Until then, contact District office at (925) 313-2000 and ask for someone in Flood Control. We will be happy to explain, e-mail, or main you information about our standards.

2. Do you have the flow rate at a specific location on a creek?: The Flood Control District has over 2,000 data points where the flow rates have been calculated. Most of these are the “design” flow rates based on General Plan build out2. Depending on the watershed size, the flow rate could be for the 10-year, 25-year, 50-year or 100-year flood. For information on the flow rates that the Flood Control District has for these data points call the
County Public Works Department, Flood Control Division at (925) 313-2000.

3. There is no Flood Control data point where I need it or I need to know the post development design flows for my project. How do I calculate those flows?: If the watershed is small, you can use the Rational Method to calculate the design flow rate. If the area is large or you need hydrographs for designing a detention basin, the Flood Control District can calculate the hydrographs for you. You need to request a HYDRO6 submittal package. There is a fee for the District to calculate the flows for you. Contact the Hydrology Section at the number below and they can e-mail the HYDRO6 instructions and submittal package to you. You may also purchase the submittal package at the front counter of the Public Works Department office.

4. How do I determine the 25-year 24-hour storm depth: Find your project location on the Mean Seasonal Isohyet Map that can be found at the Hydrology Standards link (below) and determine the mean seasonal rainfall for your site.
Then, use the 25-year Precipitation Duration-Frequency-Depth Curve found at the same link. Find 24-hours (1-day) on the x-axis, go up parallel to the y-axis on the graph to the mean seasonal rainfall for your site. Go horizontal parallel to the x-axis to the y-axis to read off the rainfall depth for the 25-year -24-hour storm.
Hydrology Standards