Detention Pond Question
Detention Pond Question
(OP)
We are designing an industrial site where detention is required up to the 100 year storm. The rear of the site is a loading dock. We are allowed to pond water in the loading dock area up to 18", and are thinking of using this as the detention pond.
When I model the pond using a 4" orifice 1' below the loading dock area the results are "insufficient volume". But when I add a storm drain box and lower the invert of the 4" orifice to 6' below the loading dock area the detention calculations work out. Intuitively this makes sense to me, but if someone asked me to explain it I wouldn't be able to do it. Why does the pond work when I lower the outlet?
When I model the pond using a 4" orifice 1' below the loading dock area the results are "insufficient volume". But when I add a storm drain box and lower the invert of the 4" orifice to 6' below the loading dock area the detention calculations work out. Intuitively this makes sense to me, but if someone asked me to explain it I wouldn't be able to do it. Why does the pond work when I lower the outlet?





RE: Detention Pond Question
You may also want to post this question in the HydroCad forum.
good luck
RE: Detention Pond Question
Mike McCann
McCann Engineering
RE: Detention Pond Question
RE: Detention Pond Question
RE: Detention Pond Question
In this case it is detention. In this area we have policies in the works where you are required to retain the first inch of runoff, above that detention is required.
RE: Detention Pond Question
-we are required to retain the runoff from the 1st inch of rainfall.
RE: Detention Pond Question
" 4" orifice 1' below the loading dock area "
It doesn't matter how far below the bottom of the "pond" you put the orifice. You should put this orifice at the elevation corresponding to the bottom of the pond.
As far as designing for the 100-year storm, its unlikely if your site is over 1 acre that you will able to detain it completely with only 18" of storage height. The problem will occur just as the water starts to pass over the weir, the flows will be highest at this point.
Most likely, that is why the program said insufficient volume for storage. The explanation of why the pond worked when you merely lowered the orifice in a control box?? Well it DIDN'T work!! just because you didn't get a message doesn't mean it worked.
Generally, in metro atlanta, you need around 8-10 thousand cubic feet of pond volume per acre. Around 12k cubic feet for areas which require additional storage for water quality and channel protection.
-Jeff
www.civiltips.com
RE: Detention Pond Question
It doesn't matter how far below the bottom of the "pond" you put the orifice. You should put this orifice at the elevation corresponding to the bottom of the pond. "
It DOES matter how far below the maximum water surface elevation the orifice is placed. The greater this distance, the higher the outflow rate will be, assuming free outfall conditions.
RE: Detention Pond Question
RE: Detention Pond Question
Well cvg has a point, the additional head would increase the flow. I just haven't personally designed anything using that methodology. Go ahead with that if that first 1" storm volume is let out over the required length of time and not before, and also you can reduce the flows for the required frequencies. I still doubt 4" is small enough though.
cheers
-jeff
www.civiltips.com
RE: Detention Pond Question
RE: Detention Pond Question
Also, I should apologize - I should have been more clear. The 4" orifice works with a 3' weir above it...And the site IS roughly 1.5 acres.
RE: Detention Pond Question
Not to pander on, but you said:
"a 4" orifice I can guarantee you even at a 50' depth below the HWE isn't going to carry a 100 year storm.. It will only carry roughly 3.5 cfs, which is considerably less than the 100 year flow from a 1 acre impervious site ( IN Atlanta that would be about 8.75 cfs)"
Yeah, well that's the point of having detention ponds