Retrofitting Detention Basin with Water Quality Control Pool

Many detention basins built before 1990 are not equipped with stormwater quality control device. With the latest developments in low impact development (LID) for storm water management, these existing detention basins need modifications on their outlet structures to increase on-site runoff treatment and disposal. An outlet shall be designed to have, at least, three levels of release, including water quality release over 12 to 48 hours, low flow release for 10-yr event, and 100-yr high flow release. All these efforts are to aim at the full spectrum runoff treatment that is not only to capture the minor and major events but also to store micro events. Over the years, the empirical methods under different assumptions have been developed for determining the design stormwater quality control volume (WQCV). To improve the consistency in stormwater detention designs, this paper presents a mathematical model that produces the synthetic runoff-volume capture curves normalized by the local average rainfall event-depth. A runoffvolume capture curve defines the relationship between WQCV and runoff capture ratio on a long term basis. A higher runoff capture ratio requires a larger storage volume. Using the runoff capture curve as the basis, the WQCV can be consistently determined for the pre-selected runoff capture target such as 80% recommended by US EPA (1986). A case study illustrates how to retrofit an existing outfall concrete vault with a perforated plate and a micro pool for WQCV. With a three-level release control, the outfall box can have a slow release for micro events and a fast release for extreme events. This procedure has been recommended for designing a new basin and retrofitting an existing for the metro Denver area, Colorado. Details can be found in the UD-DETENTION computer model available at www.UDFCD.org at no cost to download.