First flush volume to flow conversion 1

[erikhorton]

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?

 

[daddymem]

Here in Mass. we run into this problem all the time, and the answer always comes down to how the reviewer or regulatory agency views the issue. To sum it all up, the first flush calculations involving a 1-inch or 1/2-inch depth is used for the treatment methods that required volume to work. With the new flow-based treatment units such as the Downstream Defender, Vortechnics, etc. sizing is based on flowrate. Chapter 2 in the following document is what we use to reinforce our calculations when using flowrate devises:

http://www.ceere.org/ees/EES_Publications/Environmental Research/99_6_319_Final_report.pdf

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.

 

[cemorway]

We have similar regulations in NC. Typically systems are designed as off-line as well, where the treatment system is designed to handle the runoff from 1" (or more) of rainfall (merely a volume calc). This may be oversized to comply with more strict local regs, or excess is diverted to a seperate system for peak flow control if need be.

 

[daddymem]

I found another paper regarding the conversion from volume to peak flow:

http://www.vortechnics.com/assets/04 Design SW Qual for Vl Based Requirements.pdf

 

[erikhorton]

daddymem,

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?

 

[daddymem]

Well, in MA we use the 1" that falls on the impervious surface only since this is what would produce the first flush typically. We also discount roof areas since this is considered clean as far as TSS removal. Does your entire site drain to the WQ unit? It seems sizing the unit for flows that don't even drain to the units would not make sense. Ultimately, it will be the agency that makes these calls and that will have a lot to do with whether this requirement is a guideline or a regulation.

 

[erikhorton]

We are required 1" for sites with dry ponds, 1/2" for sites with wet ponds over the entire site. The county hasn't told me which they would consider an underground unit with a permanent water volume. They do consider roofs here due to fecal materials.

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.

 

[ALBUD1962]

Erik. are you using the rational method or SCS? I would look at using CN's , since most of the infiltration practices are tied to TR-55 and the SCS method.

 

[sam74]

I agree with the above post you should be using the SCS method.

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.

 

[erikhorton]

That is exactly what I've been doing, I just think the flows that I'm getting are high for what I should be expected to treat.

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.

 

[LHA]

I agree with daddymem. The concern for WQ is impervious, usually just blacktop, but if you know the agency wants rooftops, then include them also. As far as the method, just pick which ever you trust most. For small, homogenous watersheds (such as the impervious cover from a 7 acre site), I believe rational would be fine. But so would SCS.

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