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?