Flow Based Vs Volume Based BMPs

[PKAST23]

Hello Guys,
I am tasked with preparing a Water Quality Management Plan (WQMP) for a project here in Riverside County CA. I am looking at bio-retention basins vs Contech Filterra or Bio-Clean MWS Systems for SW treatment. We usually propose a bio-retention basin for treatment. But this time we are also proposing either a Filterra or Modular Wetland System (MWS). Can anyone enlighten me the difference between Flow based (Qbmp) Vs Volume based (Vbmp)?? Are they same, just how you interpret is different? Also, does anyone know which one is cheaper (Filterra Vs MWS)
Thanks.

 

[beej67]

Typically, "Flow vs Volume" are just two different ways of quantifying the first flush. You've got a target treatment storm, say 85% of storms in a given year or similar. Then you either figure out how much volume would only be exceeded by 15% of storms, or what flowrate would only be exceeded by 15% of storms, depending on how your BMP works.

State agencies usually drastically simplify this analysis, by taking regional variation out of the mix and simply assigning a target storm, regardless of where your project site is in the state.

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[PKAST23]

Thank you beej67. What I found out is when you use volume based, you can get away with smaller Filterra or MWS System. Flow based design yields one size bigger in most cases. I do not quite sure how they came up with the sizes.


Here are links to the sizes if anyone is interested
MWS System

http://www.modularwetlands.com/design-assistance/treatment-flow-sizing-table/

http://www.modularwetlands.com/design-assistance/treatment-volume-sizing-table/

 

[TerryScan]

Volume based management involves recharge or infiltration. Flow based management does not rely on infiltration or recharge.

 

[beej67]

Not always, Terry. There are cases of volume based BMPs in many states up the east coast that do not account for infiltration or recharge at all. They operate on the presumption that residency time in a wet pond, or delayed drawdown through a bleeder orifice, allows time for the BMP to treat the runoff. Two treatment methods cited are reuptake through border wetland vegetation, and settling of solids via stokes law between events.

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[PKAST23]

Yes, We use Volume based bio-retention basins in Riverside County CA with no infiltration option. Storm water gets treated in Engineered soil media under the basin and drains out through perforated PVC underdrain pipes. We do not used infiltration basins mostly because the soils here don't perc well.

 

[TerryScan]

Not always, Terry. There are cases of volume based BMPs in many states up the east coast that do not account for infiltration or recharge at all. They operate on the presumption that residency time in a wet pond, or delayed drawdown through a bleeder orifice, allows time for the BMP to treat the runoff. Two treatment methods cited are reuptake through border wetland vegetation, and settling of solids via stokes law between events.

One could argue this is not really volume based. While there may be calculations that involve volumes for a wet pond and other detention bmps, detention (delayed drawdown) reduces the peak and extends the hydrograph. It does not change the volume (except perhaps increased evaporation during detention).

I am not familiar with Riverside County, CA regulations. Sometimes the language used is misused or misleading. The only ways I know of reducing runoff volume would be infiltration or evaporation.

The current regulations in Delaware do now have a volume reduction (infiltration) component. Since a good portion of the state is a coastal plain, there are many areas that have water tables that prohibit infiltration and/or there are plenty of poor soils. In these cases, it severely limits development options or one pays a penalty for what one cannot manage.

 

[LincolnPE]

I agree with Terry.

Extended detention basins (i.e 40 hour, etc.) is not a volume based approach. For example, treating a new proposed development that has an increase in imperviousness will still deliver an increase in total volume downstream.

 

[PKAST23]

I am not talking about detention here. Let me explain. The bio-retention basin treats the high frequency storm volumes and in case of low frequency storms i.e 10 yr and higher, we provide a outlet riser with grate inlet (can be sized for 100 yr Q) to drain the water to downstream storm drain structures.
This is the case when there is no HCOC concerns (Hydromodification) for the site (e.g. your site is just upstream of a lake or river or no considerable change in imperiousness b/w pre and post conditions etc). If not then yes we may need to design a detention basin in addition to bio-retention basin or if possible combine them into one.

 

[beej67]

One could argue this is not really volume based. While there may be calculations that involve volumes for a wet pond and other detention bmps, detention (delayed drawdown) reduces the peak and extends the hydrograph. It does not change the volume (except perhaps increased evaporation during detention).

The hydrograph extension is not the treatment method claimed by the BMP. The treatment method claimed by the BMP design procedure is that the BMP itself treats the first flush through mechanisms within it, so its design is predicated on volume, not on flow rate. In Florida, wet ponds can count for nutrient treatment based purely on a residency time calculation, without considering event discharge rate at all. You figure average inflows and outflows to determine how many weeks or months a water droplet will stay in the wet pond, and cross reference that to a best fit relationship out of the Harper study, for example. The same overall premise is assumed by the Georgia state guidelines, which say a pond with the first flush in permanent storage will have the clean water leave when the dirty water enters, and then the pollutants in the dirty water will settle out between storms. They're not as sophisticated as Florida, but honestly nobody in the country is AFAIK.

In practice, for extended detention ponds outside Florida, the hydrograph mitigation and subsequent reduction of downstream erosion is probably the actual thing benefitting the watershed, but it's still not intended to be a flow rate based BMP. Sizing a BMP based on flow rate is usually reserved for either a proprietary system, or a system based on Stokes Law (falling velocity of spheres) presumptions.

As often is the case on the internet, it all comes down to how you define your terms in your premise.

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[LincolnPE]

"the hydrograph mitigation and subsequent reduction of downstream erosion is probably the actual thing benefitting the watershed"

This is a common misconception... Where one thinks that simply attenuating the peak flow release will solve all the downstream erosion issues.

One has to be aware that there will be increased erosion effects downstream due to longer than historical/existing flow times due to the increase in total volume being released.

 

[beej67]

I totally agree. But extended drawdown of frequent storms to try and mirror base flow is a lot better than purely managing peak discharges of very infrequent storms, and also certainly providing more aid to the watershed than running the runoff from a parking lot through an oil grit separator.

There's a major disconnect still between many state agencies understanding of the source of TSS. They see graphs relating impervious coverage to TSS and jump to the conclusion that the TSS comes from the parking lots. Anything that can be done to restore even an approximation of the hydrologic response of a watershed to predevelopment conditions, including extended drawdown, is better than nothing.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com