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I am looking for a non-wire backed specification for silt fence. I really need guidance on maximum length of slopes draining to any portion of the fence. I do have a CO-based NRCS sheet for wire backed showing a spacing of every 50 feet for slopes 2:1 or steeper and that you can't have more than 1/2 acre of area per 100 foot of fence. Looking for similar guidance but for non-wire backed. My project is in central MT and have to design to the 10yr 24hr which the NOAA I is 2.2" of precip. Maximum slope lengths are like 120' and the acreages are all less than 1.

Also, I am looking for guidance on the rock size (D50) for rock check dams. I have to convince a client our plan is "engineered" or at least conventional industry practice. I am overwhelmed as to how disparate the guidance truly is across the board.

I've looked at EPA guidance (construction site BMPs fact sheets online) and Dr. Fifield's "sediment and erosion control on construction sites" and these items just don't have it quite nailed as far as guidance to convince my client. Any insight would be appreciated.

Thanks,
Christopher Ennes

Answers

Check out:
California Stormwater Quality Association’s Construction BMP handbook:
http://www.cabmphandbooks.com

or CalTrans’ Construction BMP handbook:
http://www.dot.ca.gov/hq/construc/stormwater/details.htm

Both have silt fence and check dam typical details that should have the details you are looking for
____________________
Sandy Mathews

You could try the MnDOT 2005 Standard Specifications for Construction. ttp://www.dot.state.mn.us/pre-letting/spec/2005/div-iii.pdf

Look for Specification 3886 Silt fence

For rock checks: look for 3889 Temporary Ditch Checks (Type 7).

You should find the information you are looking for.

Aaron Mlynek, CPESC

We offer our 180lb fabric for the non supported silt fence (see attached spec). It meets and exceeds most of the non supported fabric specifications I've seen. I don't always have every size listed but I do have a lot of options in stock in Seattle. If you want information on Wireback too; we make this as well.

Cheers,

Rob Emmons

Chris:

Perhaps the ASTM Standards for Silt Fence materials and installation - ASTM D 6461 & D 6462 - would be helpful.

Joel Sprague

Chris,

I don't do work in MT, but I would start at the Montana MPDES website:

http://www.deq.state.mt.us/wqinfo/MPDES/StormwaterConstruction.asp

Another page lists their Erosion Control Manual, which seems old, but should specify silt fence and check dam requirements.

http://www.deq.state.mt.us/wqinfo/WaterDischarge/DischargePublications.asp

For my money, 120 feet on regular silt fence at slopes greater than 2:1, seems like an awful long way. You might consider engaging an engineer who designs E & S in Montana. That way, your plan would be engineered.

---John Lehane

Virginia's silt fence fabric spec is a little outdated in the handbook but they do have some decent design criteria. They attempt to keep silt fence limited to sheet flow; where it belongs. No more than 2:1 slope, no more than 100' of slope length and no more than 1/4 acre of drainage per 100' of barrier. Not to say that it is adhered to. The check dams allow a #1 stone (fist size) for less than 2 acres of drainage and 2-10 acres requires a Class 1 backing on the #1's to prevent blow-outs. The handbook needs to be updated but in it's debut in the mid 90's it was a good one. If you would like more information on these publications visit http://www.dcr.virginia.gov/soil_and_water/e&s.shtml#pubs

Robert Connelly