Baffle Design 1

[amicka]

I have several detention basins with forebays on site that will be utilized as sediment basins during construction. I want to use baffles (essentially rows of silt fence to elongate the flow path) in place of the forebay berms for erosion control. Does anyone know if there is some sort of calculation to figure out (1) how far from the inlet structure I can begin placing rows of silt fence and (2) how many rows I will need. I'm guessing there has to be some calculation based on the flow coming in to the basin. Then again, I'm new at this, so there may not be such a calculation.

I plan to keep 20' separation between each row and allow 30' between the end of each row and the basin embankment, so a small tractor may come in and clean out the sediment.

Thanks in advance for you help!

 

[pittsig]

This link to North Carolina DENR's design manual has design information on baffles. Go to Chapter 6. It's a 25+ MB pdf file so will take a little while to download. Baffles are on page 365 of the file.

http://www.dlr.enr.state.nc.us/pages/manualsandvideos.html

In general this manual recommends 3 baffles per basin. Space the baffle rows based on surface area of the basin so that 35% of the basin surface area is upstream of the first baffle, another 25% of the surface area before the next baffle, another 25% before the last, leaving 15 of the surface area between the last baffle and the outlet. If the basin is 20 feet or less in lenghth, then go with two baffles.

The majority of sediment will collect in the first chamber. There are differnt schools of thought on flow patterns. Some create a serpentine flow pattern through the basin by notching to create a weir that is lower than the rest of baffle, and then doing the same on the opposite side of the basin on the next baffle, etc. A method that seems to work better is to run a contiuous row of silt fence backed by "hog wire" (wire fence) across the basin. The horizontal slits are cut with a utility knife in the silt fence in a pattern.

Here is a link to a smaller file that has more information:

http://www.soil.ncsu.edu/publications/Soilfacts/AGW-439-59/AGW_439_59.pdf