Water Quality Flow Calulations

[orceng]

A jurisidiction requirement for water quality treatment is "0.5 in of post developed runoff from each storm". How would you go about determining flow rate from that to use to size a conveyance ditch? They also say to "provide a minimum water quality treatment rate of 0.05 inches per minute". I am assuming this would be the minimum infiltration rate?

 

[amb2002]

I would check to see if the jurisdiction has specific design guides for your WQ calcs. In the Kansas City metro area, the APWA guide has the following calculations:


"Two methods can be used to estimate the WQv for a proposed development—the Short-Cut Method and the Small-Storm Hydrology Method. Use the Short-Cut Method (Claytor and Schueler 1996) only for sites with one predominant type of cover and a drainage area less than 10 acres:

WQv = P * Rv

where:

WQv = Water Quality Volume (inches)
P = Rainfall event in inches (the Water Quality Storm of 1.37 inches or other appropriate amount, with the approval of the city engineer)
Rv = Volumetric runoff coefficient = 0.05 + 0.009(I)
I = Percent site imperviousness (%)

The Small Storm Hydrology Method (Claytor and Schueler 1996) is based on the volumetric runoff coefficient (Rv), which accounts for specific characteristics of the pervious and impervious surfaces of the drainage catchment. This method may be used for all drainage areas. Rv’s used to compute the volume of runoff are identified in Table 7. The Small Storm Hydrology Method is:

WQv = P * Weighted Rv

where:

Weighted Rv = ?(Rv1*Ac1)+(Rv2*Ac2)+…(Rvi*Aci)/Total Acreage
Rvi = Volumetric runoff coefficient for cover type i
Aci = Area of cover type i (acres)
Total Acreage = Total area of the drainage area (acres)"


Note: in the above caculations WQv is in watershed inches. To get a volume to size your BMP, multiply it by the watershed area (usually acres). You will need to watch the units:

WQv * Acreage * 43,560 sf/ac * 1 ft/12 in = Treatment Volume in cubic feet.

Also note for the impervious percent I above, an 85 percent impervious would be input as "85" in the equation, not "0.85".

In those calculations above, P would be 0.5 inches per your "0.5 in of post developed runoff from each storm" statment instead of the 1.37 inches noted in the text.

The "water quality treatment rate of 0.05 inches per minute" would be a release rate from your BMP, which as you indicated would be handled preferrably by infiltration if the site soils have the capacity. Otherwise I assume you would have to covert that to a flowrate and convey that flow through a weir or similar structure, to provide the detention time in the BMP.

These calculations are detailed here:

http://kcmetro.apwa.net/chapters/kcmetro/specs/APWA_BMP_Manual_Mar08.pdf

and are based on similar proceedures I've seen used in Maryland, North and South Carolina, and St. Louis. But again, I would check with the jursidiction to see if the have specific quidelines for your area.

Hopefully this info is helpful, as opposed to just more confusing.

Good luck!

 

[cvg]

the method above is fine for calculating the "volume" but does not calculate the flow "rate" which was the original question. For flow rate calculations see the method in the attached pdf.

 

[jartgo]

I always enjoy seeing how various jurisdictions word their definition of "water quality volume."

In the oringinal poster's statement, the jurisdiction requires to treat, "the first 0.5 inches of runoff from each storm."

This is a very onerous requirement on the designers and developers and I imagine they didn't actually intend what it says. ...of each storm...where do you stop? 25 year? 100 year? PMP? The flowrate at which that volume occurs is different for every storm.

I'm a little confused by cvg's suggested example. I've not seen this method before and have a few questions. The original statement is concerned with the first 0.5 inches of runoff. It appears that cvg's method is perhaps trying to caculate the first 0.5 inch of rainfall.

I'm also having trouble getting the units to balance out. The rational formula converts in/hr and acres to cfs with an additional (usually ignore) coefficient of 1.008. The suggested method introduces a 1/hr unit which mathematically gives a result of cfs/hr. I'd be interested in seeing more on this method, as it cerainly appears simpler that the method I usually use.

The method I used to use is to take the hydrograph of say the 1 year storm and then integrate to find the water quality volume under the curve...whereever that point is will correspond to the flowrate. It's really a simple exercise if you have a math program like maple or mathcad. However, in order to meet the original poster's requirement you would essentially need to do that for "each" storm.

It's been about 5 years since I've submitted this calculation for review but it certainly impressed some folks in its day. In the recent years, all the BMPs I've designed were adjacent to the main flow channel. In this manner there is no reason to constrict the flow to the flow rate of the water quality volume.

 

[cvg]

given that the runoff coefficient is 1.00, then 0.5 inches of rainfall will all runoff and must be captured in a retention basin. Generally, this amount is retained and allowed to percolate / evaporate / transpirate. This "volume" is required and it is irrelevant if it comes from a 2-year or 100-year storm. The rainfall to be captured is still 0.5 inches.

 

[amb2002]

I see I did miss the "rate" in the original question. The method I described above utilizes the following:

The Rational Method is defined as follows:
Q = K•C•i•A
where:
Q = Peak rate of runoff (cfs)
C = Runoff Coefficient
C = 0.3 + 0.6•I where I is percent impervious divided by 100
i = Rainfall intensity from Table 9 at the calculated time of concentration (inches/hr)
K = Dimensionless coefficient to account for antecedent precipitation
K = 1 (Water Quality Storm which is 90% Event)

TABLE 9:
Rainfall Intensity for Water Quality Rainfall Event (1.37 inches)
[only a portion of the table values are listed below]
Time of Conc. (min) i (in/hr)
5 1.90
10 1.68
> 15 1.40

I like that cvg's method calcs intensity by an equation. I find it irritating to have to look up values from a table or graph when all of the rest of my calcs are in a spreadsheet or piece of software.

I agree with jartgo that the units in cvg's method seem ambiguous. But I guess regulatory guidelines don't have to make sense, you just have to figure out how to use them to complete your project.

 

[psmart]

Wouldn't the ditch be sized for the storm peak, making the 0.5" flow rate irrelevent? (Unless somehow the flow is restircted so the ditch will never see a higher flow rate.)

If you really want to know the flow rate for 0.5" of runoff:
1) Calculate a complete runoff hydrograph (try the 2-year event) using whatever H&H program you're using.
2) Determine the time at which 0.5" of flow has occured. Some programs will tabulate this value vs time. Otherwise you will need to integrate the hydrograph until you get 0.5"
3) Lookup the flow rate at that time.

As jartgo points out, this flow rate will be different for each event, supporting my belief that you don't really need this value. I've never seen it used before.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

I have not seen it used either since "treatment" of the first flush around here consists of simply retaining the stormwater in a basin. Release of that flow is generally limited to a bleedoff within 36 hours. To a storm drain it is also restricted to no more than 1 cfs. And treatment during a storm requires a device which can handle large peak flow rates unless you have a detention basin.

 

[jartgo]

"given that the runoff coefficient is 1.00, then 0.5 inches of rainfall will all runoff and must be captured in a retention basin. Generally, this amount is retained and allowed to percolate / evaporate / transpirate. This "volume" is required and it is irrelevant if it comes from a 2-year or 100-year storm. The rainfall to be captured is still 0.5 inches."

I think you meant to say "runoff" to be captured, but
I agree with what you said (although if volume is all you were after you could just multiply 0.5 inch of runoff times your drainage area), but the original poster wasn't questioning how to determine the volume, he was asking at what flow rate would that volume occur.

In most designs, this question is irrelevant because as several posters have suggested, just size the inline BMP to treat the quality volume and it must also handle the peak flows.

I interpreted the original poster's questions as if he were placing the BMP offline and only needed to convey the water quality volume to the BMP...then the question is what flow rate would you need to design for to capture the first 0.5 inches of runoff.

This question is relevant if you are sending water quality volume some distance in one direction through a small pipe and then excess volumes would go over a weir and on downstream. You would then need to know what the flow rate would be at the water quality volume and then would make sure the "small pipe" could flow that rate before water overtops the weir.

It's especially relevant when you are pumping the water quality volume. That is, all you want to pump is the water quality volume...so you need to know what the flow rate is when the water quality volume occurs for any particular design storm.