It's about volume. Even though you're preserving the peak flows of the pre-developed conditions, the resulting volume is actually greater. This increase in volume, post-construction, is known to erode downstream channels. Hope this helps.

Terry Stringer
Hydraulics & Hydrology Software
http://www.HydrologyStudio.com

volume is half the answer, the other half is water quality. Stormwater retention basins capture sediment and pollutants from the first flush. Storing those sediments in a small basin would result sediment being washed out downstream when you get a storm with volume greater than your retention basin. so the retention basins are often sized for the 100 year, 2 hour storm.

Thanks for the comments, understood HydrologyStudio & cvg. I'm still unsettled in mind why a municipality would want to be more restrictive on peak discharge than the state.

All the draft legislation the the DNR provides to municipalities, and even the extremely urban areas (this particular project is not one) are all the same as the DNR requirements.

Wisconsin DNR seems unusual, most states do not regulate stormwater, that is generally reserved for the county or city. The only exceptions to that are generally for state owned land or right of way.

Quote (rwcardinal)

Without knowing details of a single municipality, what would be some reasons behind restricting this?

Some variation on Robbing Peter to Pay Paul. I know some areas which require a reduction percentage beyond your predevelopment condition for your pre/post analysis. I know many which force you to pretend your site is wooded predevelopment even when it's not. I know some areas I've worked in where they had a flat discharge limit per acre for a given storm regardless of predevelopment condition, as a regional flood control measure. Any traditionally urbanized area in the US, which was developed prior to stormwater management regulations, is going to have a flooding problem that they're ill equipped to fix universally, so the hope is that it gets fixed through restrictions on redevelopment. And since the land, and land development, is so different by region, each region cooks up its own solution on how to best rob peter to pay paul.

Quote (cvg)

Wisconsin DNR seems unusual, most states do not regulate stormwater, that is generally reserved for the county or city.

This varies widely by state. I'm not sure I can think of any one thing that I'd say "most" states do, when it comes to stormwater.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

Thanks beej67. That helps.

Peak shaving using an optimized inflow weir is your friend in these cases. Design your BMP so it doesnt start filling up until you hit the peak of the target storm.