Estimating % Sediment Removal for 50' Buffer Requirements

[kc27315]

I am putting together a SWPPP for a site that will have disturbed portions within 20' of a Surface Water. I have estimated the % removal efficiency as if I had retained a 50' natural buffer - 26%. Now I need to design BMPs capable of achieving equivalent sediment removal efficiency. I downloaded RUSLE and managed to input my site conditions and also modeled a silt fence. I have attached a copy of the results from RUSLE. My problem is I don't know how to interpret them. The results give numbers with units of tons per acre per year. I need to know the Percent Removal Efficiency suggested by the results. Can anyone help me determine the Percent Removal Efficiency suggested by the RUSLE results? Thank you!

Ryan

 

[Twinkie]

RUSLE does not estimate sediment yield it computes the average annual erosion expected (tons/ac/year), and is estimated by the equation:

A=(R)(K)(L)(S)(C)(P)

I'm not familiar with silt fence (or any other BMPs other than seeding and cover management being used to manipulate this equation. What you are seeing in this is, I believe, the sediment yield at your area of deposition (the silt fence).

Don't you have guidance from your local conservation district as to how you need to sequence various BMPs to obtain the level of filtration they need?

 

[kc27315]

Thank you for your reply. The only guidance I can find is in Appendix D of the Utah Construction General Permit UTRC00000. Step 2 on Page D-11 suggests the use of a model such as RUSLE for determining the Percent Sediment Removal Efficiency of storm water controls.

 

[cvg]

a silt fence should stop nearly 100% of the silt that reaches the fence, that is until the fence fails, or is overtopped or is outflanked. at that point, nearly 100% of the silt will continue past the fence.

 

[kc27315]

When I was first looking at this problem, I was hoping to find some literature somewhere that shows a % Sediment Removal Rate for different BMPs such as a silt fence. However, I spoke with someone more familiar with the requirements to comply with the 50' buffer section of the General Permit, and they told me that regardless of the BMP chosen, the Percent efficiency would be a function of the unique site characteristics like soil type and slope - not just a function of which BMP was chosen... So I'm still scratching my head with this one.

 

[Twinkie]

Well that doesn't make sense, because if you did a sed basin with a skimmer that picked up the entire drainage area, you would essentially achieve 100% efficiency, the soil type and slope have nothing to do with it.

Soil type and slope are factored into the RUSLE, but that equation is really to be used for estimating soil loss.

Can't you call whomever will be reviewing this plan?

 

[kc27315]

I'm not anticipating that anyone will review this plan in the near future. I'm just worried that the County or the EPA will show up on-site and want to see which "model or other type of calculator" I used to come up with the % sediment removal efficiency number that I came up with. I am required to provide the number in the SWPPP per the Utah Construction General Permit. I agree that it seems obvious that a sed basin would have a 100% efficiency if built large enough for the required design storm. But, I have just never been in a situation where my site was within 50' of a surface water.

The verbiage in the EPA NPDES Construction General Permit is virtually identical to the Utah Permit. in the EPA permit in Section G.2.2.2.b, it says...

"b. Step 2 - Design Controls That Match the Sediment Removal Efficiency of the 50-
foot Buffer
Once you have determined the estimated sediment removal efficiency of a 50-
foot buffer for your site in Step 1, you will be required to select stormwater controls
that will provide an equivalent sediment load reductions. These controls can
include the installation of a single designed control, such as a sediment pond,
additional perimeter controls, or other type of device. Alternatively, you may
elect to install a combination of stormwater controls and to retain some amount
of a buffer. Whichever control(s) you select, you must demonstrate in your SWPPP
that the controls will provide at a minimum the same sediment removal
capabilities as the 50-foot buffer (Step 1). You are allowed to take credit for the
removal efficiencies of your required perimeter controls in your calculation of
equivalency, because these were included in calculating the buffer removal
efficiencies in tables G - 8 through G - 15. (Note: You are reminded that the
controls must be kept in effective operating condition until you have completed
final stabilization on the disturbed portions of the site discharging to the surface
water.)
To make the determination that your controls and/or buffer area achieve an
equivalent sediment load reduction as the 50-foot buffer, you will need to use a
model or other type of calculator. As mentioned above, there are a variety of
models available that can be used to support your calculation, including USDA’s
RUSLE-series programs and the WEPP erosion model, SEDCAD, SEDIMOT, or other
models. A couple of examples are provided in Attachment 3 to help illustrate
how this determination could be made."

Notice that it wants me to use a "model" or "calculator" to make the determination. Has anyone ever used a model or calculator to make determination of Sediment Removal Efficiency?