Stormwater Water Quality Volume and Drawdown

The water quality volume is a DIFFERENT SIZING QUESTION from detention, and confusing the two is how a site ends up with a pond that controls floods and treats nothing. Water quality sizing captures the frequent SMALL storms -- the first inch or so, which carries most of the annual pollutant load and which falls many times a year -- while detention and flood control size on events that happen once in ten or a hundred years. A facility sized only for the large event passes the small storms straight through, and those are the ones doing the pollutant work. The volumetric runoff coefficient is the term that carries the site, and its form is worth reading: a base of 0.05 plus 0.009 for each percent of impervious cover means a completely pervious site still runs off five percent of the rain, and a fully paved one runs off ninety five. It is a VOLUMETRIC coefficient for a small storm and it is not the Rational method's peak-flow C, which is a different number for a different purpose -- using one where the other belongs is a common and quiet error. THE IMPERVIOUSNESS LEVER IS THE POINT OF THE CALCULATION. Because the coefficient is linear in impervious cover, taking a site from sixty five percent impervious to forty cuts the volume to be treated by about a third -- and that reduction comes from the site plan rather than from the facility. Disconnecting roof leaders, breaking up parking, and treating runoff at the source are all far cheaper than building the pond the alternative requires, and the comparison here is meant to be run before the grading plan is fixed rather than after. Drawdown is the requirement people meet last and it decides the outlet. A treatment facility has to empty over a stated time -- often twenty four to forty eight hours -- so the volume is available for the next storm and so the settling that does the treatment actually happens. An outlet sized to drain it in two hours does not treat; one that never drains is a pond. A volume, and the release rate that empties it. It does not design a facility: sizing a bioretention cell, a wet pond, a sand filter or an underground unit takes the media, the geometry, the underdrain, the pretreatment and the planting, and each has its own criteria. It does not compute peak flow, route a hydrograph, or size an outlet structure; it does not address channel protection or the extended detention volume, which are separate requirements in most manuals; and it does not evaluate infiltration feasibility, which needs measured rates and a seasonal high water table. The rainfall depth, the coefficient form, and the drawdown requirement all come from the applicable manual and differ between states. The state or local stormwater manual, the reviewing authority, and the engineer of record govern.

WQV = rainfall depth x the volumetric runoff coefficient x area, with Rv = 0.05 + 0.009 x percent impervious and 3,630 cubic feet per inch-acre (43,560 / 12); the release rate that empties it is the volume over the required drawdown time.

The water quality volume relation by name. The Rv is VOLUMETRIC for a small storm and is not the Rational method's peak-flow C. The rainfall depth (commonly 1.0 to 1.5 in), the coefficient form, and the drawdown requirement (often 24 to 48 hours) come from the applicable manual and differ between states. It sizes a volume, not a facility. The state or local stormwater manual, the reviewing authority, and the engineer of record govern.

One multiplication on a depth and an imperviousness the reader has; no state manual is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: rainfall_depth_in, impervious_percent, area_ac, alternative_impervious_percent, drawdown_hours, runoff_coefficient, wqv_cf, wqv_ac_ft, wqv_gal, alternative_wqv_cf, volume_saved_cf, volume_saved_pct, release_rate_cfs

• The rainfall depth is entered commonly 1.0 to 1.5 in, set by the state or local manualthe applicable stormwater manual
• Rv is volumetric, not the Rational C different numbers for different purposesthe applicable stormwater manual
• A volume, not a facility media, geometry, underdrain and pretreatment each have their own criteriathe reviewing authority