Ephemeral Stream as Stormwater management ?????

[jgd2]

I did a land development for a single family house. The area for the project is 1.72 ac approx. The runoff from the new development is about 50cfs. There is an ephemeral stream on the site which I thought could be used for drainage. So, I have graded my site such a way that all the water gets dumped on the stream by overland flow. I am trying not to have a structured BMP for this project coz the runoff generated is really small. Now, the problem is county wants me to show that it meets state and local Stormwater management requirement. I am wondering if any one of you has any idea about incorporating drainage on ephemeral stream through overland flow. This project is in Montgomery County, Maryland. Any suggestions will be highly appreciated.

 

[GoldDredger]

How are you getting 50 cfs from a 1.7 acre site? That translates to a rainfall intensity of like 32 inch/hr under a worse case C value of 0.9.

Thats probably your problem right there, the county guy hears 50 cfs, which is like the 100 year storm event for a 6 acre commercial development in Texas.

 

[jgd2]

50 cfs is the excess runoff generated by the proposed impervious area of the development. That was calculated as per guidelines for control of water runoff on small lots for montgomery county. It says 100 sq. ft of impervious area generates 12.5 cfs runoff...

 

[bpattengale]

jgd2, something is wrong with that number. The rational method with a C of 0.9 and an intesity of 7 in/hr produces a peak flow of 10.7 cfs for a 1.7 acre site. 7 in/hr is roughly a 50 yr storm with a 10 min Tc for central MN.

Common stormwater regulations require treatment of the peak runoff from the 2 yr storm.

Why are they designing the BMP to treat a hurricane?

 

[jgd2]

thank you guys for pointing that out. It is actually 12.5 cubic feet of water generated from 100 sq. ft of impervious area....

 

[bpattengale]

That number makes more sense, that works out to be 1.5 in of rainfall. That is a pretty common treatment amount for volume based design.

I'm not familiar with the exact stormwater requirements for Maryland, but some pretty common ones are no increase in the peak runoff rates for the 2, 10, and 100 year storm events and BMP should be designed to treat the runoff from a 2yr storm or the volume of runoff from a 1 to 1.5 inch rainfall.

The rules vary by location, but it sounds like you'll need to do some basic hydrology calcs to show that the increased runoff from your site isn't affecting anything downstream.

 

[GoldDredger]

Something still strikes me as peculiar about this.

jgd2, maybe post a pdf of the county ordinance pertaining to this issue. I'd be curious to read it.

 

[jgd2]

here is the pdf....

 

[GoldDredger]

I read through the ordinance, the intent is for new lot designs to convey stormwater away from adjacent lots and properties. It doesn't offer specifics (or discussion for that matter)on treatment of the stormwater or detention requirements.

It really only says you need to convey additional water from new impervious areas away from adjacent properties so as not to adversely affect them.

In your case, I would indicate that you will utilize overland release (sheet flow/swale flow) across landscaped area into a natural drainage course. Then certify (if you can) that flows from the new impervious areas will not go to adjacent properties. (If some must go to the ROW, you'll need another permit per the odinance).

This allowance comes under the 'natural topography/buffer' on pp8. As long as the lot does not exceed 5% slope (does it?)

 

[TerryScan]

Another thing to note is the applicability of this ordinance. These are "requirements to manage water runoff ...on residential lots of less than 15,000 square feet". It was stated in the original post that the project area is 1.72 ac. (about 5x larger than the ordinance scope). Be sure to check that you are not suppose to be using completeley different management criteria.

 

[jgd2]

Thank you TerryScan for pointing that out. I thought the guideline was for 15,000 sq ft or less of addition. Now, I am realizing that the slope above the driveway doesn't meet the 5% slope criteria. I guess I have to put some structural BMP in that case.