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ISSUES OF BEST MANAGEMENT PRACTICE DESIGN TO IMPROVE WATER QUALITY
Citation:
O'Connor*, T P. AND R Field*. ISSUES OF BEST MANAGEMENT PRACTICE DESIGN TO IMPROVE WATER QUALITY. Presented at Ecological Engr. for Integrated Water Mgmt.: Designing Urban and Industrial Watersheds, Cambridge, MA, 10/30-11/2/2003.
Description:
Today, many municipalities are implementing low-cost best management practices (BMPs). Structural control BMPs involve building a structure of some kind to store stormwater until it can be discharged into a nearby receiving water. Commonly used structural treatment BMPs include ponds (detention or retention), swales, buffer strips and constructed wetlands.
Historically, structural BMPs were employed to capture peak flows, assist in local drainage and manage the quantity of runoff produced during wet-weather flow (WWF), i.e., flood control. Current design typically still focuses on rainfall events that range from 2 to 10 inches of daily rainfall and occur at much longer return periods ranging from 2 years to 100 years. These storms may contain significant pollutant loads, but their contribution to the annual average pollutant load is really quite small due to the infrequency of their occurrence. In addition, longer periods of recovery are available to receiving waters between larger storm events, allowing receiving water systems to recover by flushing themselves and the aquatic environment.
Water quality control of urban runoff is still a relatively new and developing technology. The addition of water quality considerations in the design of BMPs has introduced a new dimension to the traditional hydrologic considerations for BMP design. Water quality considerations have created a shift from flood events to a continuous long-term rainfall-runoff BMP design volume approach and the pollutant loads associated with these volumes.
To treat the bulk of the pollutant loads from stormwater runoff, many states and municipalities specify a treatment volume that is designed to capture the initial component of the stormwater runoff. BMPs that encompass both peak discharge hydrology and small storm hydrology would optimally use a system that incorporates on-site treatment and storage of stormwater for smaller storms while protecting downstream from floods.
Extended detention, a concept introduced to overcome the limitations of early detention pond strategies, provides more and better control of the smaller and more frequent storm events that are unaffected by peak flow design. Basically, extended detention refers to designing or retrofitting the outlet so that smaller storms that pass through detention ponds without being detained are now detained for longer periods. With relatively simple modifications to the outlet control structure, trapping of particles with significant settling velocities can be enhanced. The extended detention approach can provide detention of 24 to 48 hours, which provides longer holding times, increased removal for particulates with lower settling velocities, and thus higher pollutant removal performance.
By including supplemental measures that employ either distributed and/or centralized controls, existing peak discharge controls can theoretically be upgraded to perform water quality control. This paper discusses the importance of properly designed outlet control structures and the issue of regional versus localized controls.