2015 journal article
Influence of Drawdown Period on Overflow Volume and Pollutant Treatment for Detention-Based Stormwater Control Measures in Raleigh, North Carolina
Journal of Sustainable Water in the Built Environment, 1(2), 05015001.
The drawdown rate of detention-based stormwater control measures (SCMs) influences both the treatment capability of the device and the volume of runoff that becomes untreated overflow. Currently, hydrologic design goals set by the North Carolina Department of Environmental and Natural Resources and other mid-Atlantic U.S. state agencies (e.g., those in Maryland, Virginia, and Pennsylvania) require stormwater practices to treat, e.g., 90% of stormwater runoff (or similar). This equates to a 10% overflow volume, defined as the percentage of inflow volume that bypasses the SCM and therefore receives minimal treatment. For detention-based SCMs, this design goal is met by sizing the outlet to release runoff generated from the water quality event (in North Carolina, either 25 or 38 mm) over a required minimum period of 2 days. Previous studies have not shown whether rainfall patterns in North Carolina and other east coast United States states justify a 2-day drawdown rate of the water quality event to limit the annual average overflow volume to 10%. To investigate this, rainfall patterns in Raleigh, North Carolina, were evaluated from Years 2001 to 2010 to determine the fraction of untreated overflow that would result from various design configurations and their associated drawdown periods. Overflow volumes were calculated on an hourly basis by a simple model routing stormwater runoff through a theoretical constructed stormwater wetland (CSW) sized to release the water quality event over drawdown periods of 12 and 18 h, 1 day, 36 h, and 2–5 days. A 2-day drawdown period resulted in an annual average overflow volume of 11.7%, exceeding the design goal of 10%, although not significantly (p=0.1702). A more simplistic approach quantifying overflow volume based on rainfall data determined that 20.1% of rainfall depths are associated with storm events in the Raleigh–Durham area exceeding 25 mm. Both methods suggest that for the Raleigh–Durham area, if a 2-day drawdown is used for the 25-mm water quality event, less than 90% of stormwater runoff will be treated. To ensure that 90% of all runoff received some treatment, the 25-mm water quality event drawdown time would need to be reduced to 36 h. Any reduction in drawdown time, while creating storage space in the SCM for subsequent events, necessarily reduces treatment efficiency by decreasing time for sedimentation and other biogeochemical nutrient removal processes to occur. If a 15% average overflow volume was acceptable, analyses indicate that drawdown periods up to 3 days could be used. Lengthening the drawdown period to 3 days enables more treatment of captured runoff and in a CSW could yield an additional 3–5% nitrogen and phosphorus load reduction, despite the slightly higher bypass volume.