First flush volume to flow conversion
First flush volume to flow conversion
(OP)
Our regulatory agencies require a treatment of 1" of runoff over the entire project site.
We will be using an ADS underground treatment unit to provide water quality for the project. This unit will be sized to treat a peak FLOW.
My problem is to get my WQ volume translated into some kind of flow rate. Should this be over 24 hours, 6 hours??
Are any of you having the same problems?
We will be using an ADS underground treatment unit to provide water quality for the project. This unit will be sized to treat a peak FLOW.
My problem is to get my WQ volume translated into some kind of flow rate. Should this be over 24 hours, 6 hours??
Are any of you having the same problems?





RE: First flush volume to flow conversion
We typically design with these units off-line unless they are capable of handling flows from our largest design storm. I'd suggest contacting your regional sales representative to see if they have any documentation for your particular state.
RE: First flush volume to flow conversion
RE: First flush volume to flow conversion
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RE: First flush volume to flow conversion
Trying the last link you posted by Thomas R. Adams:
My site:
Tc=15 min.
A=6.86ac
c=0.68
Using the method outlined here, I get a flow of 18.6 cfs, which I feel to be extremely high.
Using Hydroflow Hydrographs, and the hydrographs it generates for the site, I:
1 - found the rainfall amount needed to give 1" of runoff
2 - took this rainfall amount and found the time needed for this amount to fall in a 2-yr 24-hr storm.
3 - took this time and found, by the hydrograph, the Q at that particular time, which came up to be 9 cfs (half of the Adams method)
I guess, ultimately it will be, as cemorway said, up to the regulatory agencies to determine a proper method to be used and to enforce that method on all engineers submitting to them.
Thoughts?
RE: First flush volume to flow conversion
RE: First flush volume to flow conversion
The majority of this site will pass through the WQ unit due to a perimeter road design that encircles the bundle of buildings.
I've just noticed the flows I'm designing for seem to be alot higher than others that I've seen.
Thanks for the help.
RE: First flush volume to flow conversion
RE: First flush volume to flow conversion
1) Take your acreage and convert it to square feet, 6.86 ac = 298822 SF.
2) Take your 1 in of rainfall and convert it to feet, 1 in = .0833 ft.
3) Multiply these numbers to generate the volume of direct runoff that counts as first flush volume.
4) Next take any SCS TR55 hydrology software and input your acerage, CN, and Tc. Then play with your input for rainfall in inches per 24 hours until you get a hydrograph volume equal to the volume of direct runoff you calculated earlier. (Softwares vary but the one I am using calculates volume in CF which is why I converted to ft in 1 and 2 above).
5) The peak flow associated with the hydrograph in 4 is your design flow for sizing your water quality unit.
This has worked for me in the past but it may not qualify in your area.
RE: First flush volume to flow conversion
Secondly, who's to say the rainfall event has to happen over a 24 hr. period. That's my other problem, the regulatory agency I'm working under won't say what they expect to see.
RE: First flush volume to flow conversion
It sounds like your agency is struggling with what ours are here in PA, USA: a total revamping of what we all used to call stormwater management, and no clear guidance on what to require. I have found that, most often, in cases like this, you should just submit something reasonably conservative which you are willing to stand behind. If the reviewer isn't telling you what to do, I'd bet he or she wants you to do just that.
Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve