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 in the Public Works Hydrograph Standards webpage 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

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1. How do I calculate the Flood Control flow data point? Do I need to know the post development design flows for my project?
2. How do I determine the 25-year 24-hour storm depth
3. What are Pollutants of Concern (POCs)?
4. What are Stormwater Best Management Practices (BMPs)?
5. What is a Watershed?
6. What is an illicit discharge and who do I call to report it?
7. What is an NOI and what permits does it cover?
8. What is C.3?
9. What is NPDES and who oversees it?
10. What is the design storm for my project?
11. What is the difference between a Stormwater Pollution Prevention Plan (SWPPP) and a Stormwater Control Plan (SWCP)?
12. What is the difference between point source and non-point source?
13. What is the difference between the County Watershed Program and the Contra Costa Clean Water Program?
14. What is the difference between the storm drain system and the sanitary sewer?
15. What is the drainage easement across my property? Can I build on it? What can I do on it?
16. Who do I contact if I have flooding on my street?