Heh, well, if it's Georgia my opinion is follow all the rules and minutia and call it a day, for CYA reasons.
No matter what sort of back-up you find for not having silt fence now, you only have two defenses in court if someone comes after you:
1) Turbidity tests showing no increase, or
2) Proof that you followed the checklist.
If you're a Georgia PE then you know how out of whack those turbidity tests can get, due to no fault of your own or of the contractor. I know our natural tendency as engineers is to find the 'best' solution, but the best way to stay out of court in modern litigious society is often to find the 'safest' one.
When the Georgia NPDES construction permit first came out, there was a lot of language in it that made engineers criminally liable for erosion failures even if their plans met every code. Like, jail-liable. A lot of guys spent a long time campaigning for revisions so that as long as the plans were done to a particular standard, the engineer was covered. Your safest bet is to follow the standard.
Now, beyond that, your post does bring up some interesting academic / engineering questions. All erosion control design for construction I've seen personally followed sets of pretty basic design standards, such as the Georgia Green Book, which crafts its criteria in the style of "always have Type XYZ silt fence at ABC location" and "all basins bigger than PQR must have a sediment pond" etc, without actually trying to model sediment discharge itself.
South Carolina is one of the few states in the southeast I haven't done any work in, but I heard a rumor that they require a SEDCAD model for construction plans, which is the sort of thing that would calculate what you're talking about, because it's a treatment-train calculation software for sediment loading. I'm not familiar with SEDCAD personally, but if they've got a treatment efficiency listed for undisturbed buffer, and it varies with width, and you can compare the SEDCAD predicted efficiency of the buffer with the SEDCAD predicted efficiency of Type C silt fence, you're well on your way. Go through their documentation and find out where they got their numbers from, and cite that to the EPD. But you absolutely ABSOLUTELY must get a signed hardcopy letter from someone reasonably big at the EPD saying they buy your approach. Again I stress, the important thing here is having the ability to deflect liability if it comes up.
Then again, by the time you've done all that you could have done the plans to standard. I also think you'll find that regulators aren't in the business of assuming liability, so the moment you try and get them to hang their hat on a decision that departs from a design standard, they usually balk and refer you back to the standard. Regulators never get fired unless they screw up a hard decision, which means the smart ones can get infinite job security by never making any decisions. It's just an unfortunate artifact of the business model.
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