StormWater Management and Runoff Mitigation 4

[armyengr]

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.

 

[PELS]

There are underground detention systems available that allows you to minimize impacts to surface area uses:

http://www.contech-cpi.com/stormwater/technologies/detention/detention/104

 

[jartgo]

I've seen people do this with a lowered C value as well.
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.

 

[RWF7437]

Please read:

http://www.stormcon.com/sw_0607_voodoo.html

 

[655321]

I believe detention on such a small scale to attenuate such a large storm event is terrible public policy. It's a complete waste of money, and doesn't accomplish anything or solve any problems. Remember Hydrology is a guess about rain; how much, how long, how often, where, how much soaks in, how long it takes to get form point a to point b, all based on statistics and more guesses.........the "Voo Doo" article is great, be sure to read it.

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:

http://www.fairfaxcounty.gov/dpwes/publications/lti/08_01.pdf

Look near the end of the document.

These recommendations sound reasonable to me. Good luck.

 

[armyengr]

I appreciate all of the responses. I am going to re-run the model incorporating some of your thoughts and a little Voo-Doo of my own. Thank you

 

[MissouriPE]

You never mention whether or not you have good infiltration in your soil. If you are in any kind of clays, pervious concrete may not really be a very feasible option. It then becomes a "filtration" BMP as opposed to a "infiltration" BMP. It is still an option, but not as good of one.

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!