What is the design storm for my project?

The County Title 9 (Title 9: 911-2.010) sets the design storm based on the watershed area draining to the creek of drainage facility as follows:

  • Area < 1.0 sq. mi. Contain, with sufficient freeboard, a ten year frequency of average recurrence interval (10-year)
  • 1.0 sq. mi. ≤ Area ≤ 4 sq. mi. Contain, with sufficient freeboard, a 25-year frequency of average recurrence interval (25-year)
  • 4.0 sq. mi.≥ Area Contain, with sufficient freeboard, a 50-year frequency of average recurrence interval (50-year) OR Contain the 100-year frequency of average recurrence interval (100-year)

Show All Answers

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?