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Contributing Area for Water Quality Volume

Contributing Area for Water Quality Volume

Contributing Area for Water Quality Volume

This is in NYS. I have a linear highway project that involves reconstructing a portion of the pavement and addition of new pavement over what is now pervious area. The work area drains to a road side swale in which we plan to construct a wet swale to address the Water quality requirements of Phase II. I also have 25 acres of hillside that drain into this swale (not across the project, just into the swale from the other side). I understand that I would need to size any feature to accommodate the contributing area or use flow diverters to intercept the hill side and direct it around the swale, but what I am wondering about is the “A” in the water quality volume required to be treated calculation. {WQv = (P)(R)(A)/12}. Since the offsite area is not going through my site is the "A" the area of my work (disturbed area so to speak) or is it the entire drainage basin going to the practice, according to the manual. You see how a simple 100’ lane widening can result is huge water quality volumes to be treated if the offsite area going to the practice is significant.

Thanks for any help....

RE: Contributing Area for Water Quality Volume

With a swale treatment decreases as flow depth increases and residence time decreases.  If your offsite drainage affects this you may need to look at another option if possible.  Can you use a weir or something to divert flows less than your treatment storm?

Will your regulator give you a little wiggle room?  Infill and road widening sometimes get treated less strictly especially if you might have to condemn land for a treatment option.

RE: Contributing Area for Water Quality Volume

We have limited space and other standard treatment options are not viable in this location. There is some wiggle room so to speak in a Guidance document put out by the agency doing the work. A memo of understanding that they have with the NYSDEC that addresses this somewhat, but the group we are working with wants us to use the Storm Water Manual and ignore their own guidance standards which were developed to address linear type highway projects of this kind. So my long winded questions was really…what is “A” defined as in the manual. Contributing area to the practice, regardless of if it traverses through your project or just the area from the project.


RE: Contributing Area for Water Quality Volume

I'm not familar with NYS criteria but shouldn't the very low imperviousness of the hillside offset the effect of the large area?  The runoff for the hillside in the minor (water quality) event should be very small so the volume needed to be treated should be equally as small.

RE: Contributing Area for Water Quality Volume

In a commonsense world “yes”. The equation we use for WQv is (P)*(R)*(A)/12 which gives you an ans in ac-ft. P is the 90% rainfall number at the project (0.8) in my case. “A” is the area. (In my case the contributing area to the practice is say 25 acres vs a disturbed area of 0.25 acres) and R = 0.05+0.009(I) with "I" being the impervious cover in percent. (In my case about 1% of the area). The catch is that the manual has a min value of 0.2 for R so if you have a low imp. % like 1% you still have to use a higher R than what the calc gives you. In my case that triples the required WQv for my little lane widening project.

RE: Contributing Area for Water Quality Volume

I think the answer is that since you are concerned with water quality coming off the highway right of way, I would use the distance from the centerline to the edge of right of way.  The owner of the hillside property can worry about their own water quality - assuming you don't use some of that land for a temporary construction easement...

RE: Contributing Area for Water Quality Volume


The only problem with that is that the treatment method selected is highly dependent on inflow rate and quantity.  I do not work with the NY reg.s but am familiar with them and much of the East Coast and West Coast reg.s. All of the research that I have been able to find that they base their water quality calculations on are based on the treatment method sized for the total inflow shed.  

RE: Contributing Area for Water Quality Volume

OP said he would divert offsite around his project, so treatment efficiency would not be affected.  Even if not, most agencies I have worked with don't require interception of offsite areas.

RE: Contributing Area for Water Quality Volume


I was a little confused. I read about the diversion but then dmx stated:

"... Since the offsite area is not going through my site is the "A" the area of my work (disturbed area so to speak) or is it the entire drainage basin going to the
practice, according to the manual. ..."

It seemed to contradict the diversion statement.  This is the typical question since sometimes there is a thought that since only some of the water needs treatment,however since residence time is a principal design consideration in wet swales I jumped to the conclusion that all water would enter the "practice" which I defined in my head as the swale. A lot of stormwater manuals call treatment options practices so I have some excuse.  

I like your comment especially because I simply cannot imagine that an agency could make you treat offsite drainage that you divert around your project and your treatment device.  Never even considered that a question before. I need a bigger box.

RE: Contributing Area for Water Quality Volume

The way I read the question (and even some of the subsequent answers), my thought is either:

1. Calculate the WQ volume based on all the flow coming to the BMP
 - or -
2. Divert the flow that you don't want/need to treat around the BMP

I don't think you can just disregard the flow from an offsite area because the volume affects the treatment.  Depending on the topography, it could even cause resuspension of the solids that you are trying to settle out.

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