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Detention pond volume

Detention pond volume

(OP)
Picking up a project to help an Architect we've worked with before. The project dragged out long enough that the original Engineer (also a small shop) picked up a big project and now can't set aside the time to complete. I'm completing a grading plan for the lot and trying to layout a detention pond. The project SWMP calcs are all provided, as well as storage volume.

The storage volume is 3050 cu. ft. for a 0.52 ac lot, peak inflow = 5.6 cfs, peak outflow = 3.7, storm type II. This strikes me as high, but without much familiarity in the calculations, I can't verify the numbers. The principle will be in next week (unreachable), but I would like to know if I should proceed with the work or wait until he can verify.

General opinion, if possible, would be fine. Thank you. If more data is needed, I can provide it.

RE: Detention pond volume

The actual storage requirement will also depend on the exact shape of the inflow hydrograph and the design of the outlet control. You need all the details in order to assess the design.

Note that the performance of a detention pond (i.e. peak reduction) is dependent on outlet configuration as well as storage capacity. A good outlet design will reduce storage requirements and resulting costs.

I've see so many projects where the designer has used lots of (expensive) storage with almost no regard for the role of the outlet control. (Not saying that's happened here, but it's worth checking.)

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Detention pond volume

The volume is going to fluctuate wildly based on the design criteria. Some places only detain the 10 year storm, some the 100. Some places have a 100 year storm that's twice as heavy as other places. And then you have the project itself - some projects are wall to wall impervious, some aren't. Then there's soils. There's not remotely enough information in your post to answer the question.

As a reference point, stormwater ponds for high density development in central to northern Georgia tend to range from 9,000 cf per acre of watershed to 13,000 cf per acre of watershed, depending on the reg and the stormwater management method chosen. So by that benchmark you're low. If your site is only half buildout, maybe you're not too far off. If the site is in New Mexico, maybe you're high.

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

RE: Detention pond volume

Seems high for Western Washington. Will have to set aside about 5% of the lot area for the pond w/3:1 sides, less if vertical sides. We have fisheries considerations as well as retaining the 100 year. Usually have a triple staged orifice with an overflow.

Mike McCann, PE, SE (WA)


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