Flow Based vs. Volume Based BMPs

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)


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.

Above is a snippet.