StormWater Management and Runoff Mitigation
StormWater Management and Runoff Mitigation
(OP)
Are there any suggestions to solve the following problem? I am working on a site, RV park, that will disturb approximately 2.7 ac without mass grading (drainage area is about 6.5 ac). The problem is with the new pervious area my 10yr post exceeds the 10 yr pre which now requires detention. Unfortunately the area is confined and the required area is not available for a pond,swale or any other permanent structure. I am looking at porous concrete for Pads but am having difficulty quantifying the reduction of runoff. Any suggestions are greatly appreciated.





RE: StormWater Management and Runoff Mitigation
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RE: StormWater Management and Runoff Mitigation
Assign a c value of say 0.4 for the pervious concrete or pavers, then calculate your post construction runoff. This way you could back into how much pervious concrete you needed vs. traditional impervious surfaces. Of course your local regulators would have to accept the C value you ultimately assign to the concrete/pavers. You may find some documentation to back up your assigned value on the net.
Just to point out the obvious, you need good sandy soils under the pervious concrete in order to get any benefit. I'd look for a permeability of at least 0.5 in/hr and a few feet of separation from SHWT.
RE: StormWater Management and Runoff Mitigation
http://www.stormcon.com/sw_0607_voodoo.html
RE: StormWater Management and Runoff Mitigation
So, now that I got that off my chest, I found some guidance on the web from Fairfax County, Virginia that may help you:
-Using the NRCS method (which is a better method then the modified rational these calculations) they recommend a CN number of 40 for porous pavement.
-For rational, they suggest C=(I-kp)/I, where I =rainfall intensity and kp = 4.0 in/hr coefficient of permeability.
Check it out at:
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Look near the end of the document.
These recommendations sound reasonable to me. Good luck.
RE: StormWater Management and Runoff Mitigation
RE: StormWater Management and Runoff Mitigation
In that case, I'd look at the underground options. If you only have to detain the difference between the pre and post on the 10 year event, this may be able to be done with pipes (as opposed to concrete vaults or prefab chambers).
Frankly, detention as it has been approached over the last couple of decades does nothing to really help water quality and actually can increase flooding if multiple detention systems move peaks around too much. I agree that it can be terribly ridiculous public policy. But it takes time to educate folks on the need for change. What we really want to see is the matching of the hydrographs for pre and post in both total volume, highest peak, etc. Hello LID!