I noticed a comment from somewhere besides Maryland
that mentioned a 5000 sq ft rule there. Is it some kind of national
standard? How was it arrived at?
There are no national standards, because stormwater is different everywhere you go. (different soils, different rainfall characteristics, different topography, different land use stressors, different pollutants of concern) Everything is either local, or at best regional.
5000 sf was pulled out of a hat by somebody, and many other places stole it because they considered it to be intuitively reasonable. Some places use different thresholds. I've seen 500 sf. I've seen straight exemptions for single family residential properties, not for any scientific reason, but mostly just to keep residents from raising a stink. It all just depends on where you are. If there's not a residential exemption specifically spelled out in the rules, then the land use argument is a dead end and not worth pursuing. The rain doesn't care whether it lands on a shopping center or a house.
lincoln said:
Gravel(packed) surfaces get a 40% imperviousness value for runoff calculations. They are not pervious...
They're not
impervious either, by that very same rationale. And the 5000 sf threshold isn't calculated by weighted runoff coefficients, it's calculated by adding up the total amount of (completely) impervious area on your site. They don't count grass as "slightly impervious" for reg applicability even though it has a higher runoff coefficient than trees.
If his regulator is counting the gravel driveway as 100% impervious for the purposes of determining applicability of the code, and then turning around and counting it as "40% impervious" for the purposes of hydrology, then he's talking out of both sides of his mouth. If he's counting it as "100% impervious" for both reg applicability and hydrology, then he's ignoring science.
..which, sometimes regulators do, because regulators aren't just worried about what makes sense, they're worried about what makes precedent. They may be worried that if they let you count your driveway as pervious for SWM reg applicability, that some developer will come along with a 4900 sf retail store and 15,000 sf worth of gravel parking lot, and claim to be exempt from the regs. That's probably what's on their mind.
What if you did your driveway in pea gravel? Oyster shells? Sand? Ask the regulators if there is a driveway material they would permit to be considered "pervious."
Another idea is to use something like this:
Image came up on a search for "grasscrete" but I think the product shown in the image is actually Strataweb. They can't possibly tell you that's impervious.
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