Hi, this may be a silly question. When dimensioning a temporary sediment basin, the CGP says to use either the calculations for the 24 hs- 2 year desing storm or 3600 cubic feet x acre drained. If using the 24hs-2year desing storm, we should be using the figures for the post-development situation, not the pre-development situation, am I right? Particularly when the site more than 50% pervious in the pre-development situation. Sorry, but this is not common practice here. Thanks!

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Michael DeVuono

Regional Stormwater Leader

Arcadis North America

LEEDuser Expert

144 thumbs up

Wed, 04/08/2015 - 20:02

It is not a silly question. If you want to be ultra-conservative, you could use the post-development runoff condition for sed basin design. This can be a moving target as drainage patterns may change as the site is graded. Cover types will definitely change. I typically just use the 3600 cf/ac, but if this becomes and issue, and you need a smaller basin, you may have to put some thought into the sequencing, drainage areas, etc.

Paula Hernandez

MRS.

PAH&A

LEEDuser Premium Member

58 thumbs up

Wed, 04/08/2015 - 20:22

Thank you Michael! The fact is that the 3600 qft/acre is much larger than the actual post development runoff conditions, as it is a mostly flat site with a large pervious landscape, which makes the post-development conditions better than the 3600 cft/acre. We sequencing the project so as not to disturb all the site at once, in fact, most of the site will remain undisturbed until the final phases of the project. So I could be safe using the calculations made for the retention ponds in the post-development conditions for the temporary sediment basin?

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