When do I need a sedimentation basin?

[JD144]

I'm preparing a SWPPP for a project at the base of the Coast Ranges in Northern California. A tributary area of 40 acres drains to a storage tank site conveniently located in the little valley's flow channel, and we want to build a second tank on the same site. The new site when finished will be 5 acres. Using the state's RUSLE calculator the Total Predicted Sediment Loss is 117.1 tons per acre which is classed a High Sediment Risk. Surface soils per the geotech report are fat to lean clays (CL), silt (ML), clayey silt-silty clay (ML-CL). While I'm specifying the erosion and sediment control BMPs, the run-on is being routed around and through the site as much as possible, and the run-on and runoff are being conveyed away by an existing drainage channel, I have 3 good places to trap sediment if I need to. The CASQA BMP for Sedimentation Basins (SE-2) says they should be considered for use at the outlet of disturbed watersheds between 5 and 75 acres and evaluated on a site-by-site basis. Can anyone suggest the considerations I should use to make such a determination? Thanks for your input.

 

[Twinkie]

I mean you pretty much have it all outlined in your post. I will typically go to the sed basin if the recommended slope lengths for silt fence or filter socks are exceeded.

 

[JD144]

Thanks for your response, Twinkie. I had to go back to the BMP Fact Sheets to review those slope length limits. This looks like a good rational approach. I can see ranks of silt fences trying to deal with the runoff (about 12 cfs altogether in a 10 yr/24 hr storm) so maybe I'll just make a big basin or two. Thanks again.

Jim

 

[Twinkie]

A good rule of thumb ...... if you have to put silt fence in a series (to deal with slope length) you need a basin. In PA, we are not even allowed to place the silt fence in series.