Detention Pond Question 3

[bws2]

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?

 

[RWF7437]

To get an answer to your question you'll need to provide more information. What software, or method, are you using to model your pond ?

You may also want to post this question in the HydroCad forum.

good luck

 

[msquared48]

More head to get more water out faster = less storage volume to get the same amount of water out.

Mike McCann
McCann Engineering

 

[bws2]

Thanks Mike

 

[cvg]

is your requirement for 100-year "detention" or "retention"? If retention, then you need to provide volume to store the entire storm, not just detain part of it. Around here, detention is usually not allowed, but retention is required.

 

[bws2]

Thanks CVG,
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.

 

[bws2]

Just to clarify, In my previous post I should have said
-we are required to retain the runoff from the 1st inch of rainfall.

 

[jeffcivil2]

Here in metro Atlanta, we are typically required to detain the first 1 inch for 24 hours, so I can be of assistance. If you are required to do this, then I know from experience that 4" sounds a little high. Its usually going to be between 1" and 3".

" 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

http://www.civiltips.com

 

[RWF7437]

" 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. "

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.

 

[cvg]

It sounds like your detention routing calcs are correct and that with an additional 5 feet of head on your orifice, you get much higher flow rates, requiring less storage. putting the orifice and drain outlet in a 6 feet deep box, below the pavement is a good way to drain the water out quicker, if you have a suitable outfall that is that deep.

 

[jeffcivil2]

RWF7437, I was assuming he'd put the orifice at or below the bottom of the pond, otherwise he'd have water sitting there forever. He wasn't clear, but that's the obvious thing to do.

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

http://www.civiltips.com

 

[jeffcivil2]

Not to pander on, but 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). I still highly doubt his numbers really work as he says, but if they do, then either he must be on a really small site, or his WQ calcs don't work.

 

[bws2]

Thanks Jeffcivil2, but as CVG pointed out, this is detention as opposed to retention. Based on new requirements in the Charlotte, NC area (Hi, neighbour)we are required to retain the 1st inch of runoff for 24-72 hrs. However, this site is grandfathered.

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.

 

[bws2]

Jeffcivil2,
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