Jeff,

BMPs can be sized by volume or by flow. If say a sand filter is being sized by volume then it will be sized to hold the particular volume of water (say 1/2-inch across the impervious acreage of the site) prior to the sand media with the rest of the storm event left to bypass the filter. The flow of water through the sand media is so slow that routing calculations are not typically performed through the media. Although the intensity will be important for conveyance it can be overlooked for the actual BMP sizing.

Other BMPs, like hydrodynamic BMPs, can be sized by using a flow rate. This is typically done by using a storm that represents maybe 80% of storm events in a year for the local. In Alexandria, VA they use a 3-month storm event. By the Rational Method (or other method) a peak flow rate can be calculated and used as the design flow rate for the BMP. The assumption here is that by capturing the more frequent storm event the BMP will capture completely most all of the runoff producing storms throughout the year and for those events not entirely captured the BMP will at least capture the early portion of the storm (typically the dirtiest portion) will be captured completely.

Depending on flow capacity of the BMP a combination of the two methodologies can be used by routing calculations through the BMP.

Bill Hicks, P.E.
e-mail: bhicks@novaregion.org