HR: 1340h
AN: B43C-0293    [Abstracts]
TI: Retention deficit: Evaluating retention pond effectiveness at controlling suburban stormwater runoff, James City County, Virginia
AU: * Hancock, G S
EM: gshanc@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187 United States
AU: Popkin, M B
EM: marshall.popkin@earthtech.com
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187 United States
AB: Stormwater retention ponds (SRP) have become a significant component of suburban hydrology. SRPs are designed to protect streams by releasing storm runoff slowly, reducing peak flows generated from developed surfaces. Despite widespread use, little data has been collected to assess their success at controlling stormwater quantity. We present data from two SRPs in James City County, Virginia, to assess whether they manage stormwater as designed and achieve stream protection goals. Pond A collects runoff from a ~7 acre residential development (~23 homes on ~0.25 acre lots), and Pond B collects runoff from a ~22 acre development (~50 homes on ~0.3 acre lots). The SRPs are designed to retain the runoff produced by the one year, 24 hr storm (2.8 inches) for 24 hrs as well as to reduce peak discharge below uncontrolled conditions. At each pond, a pressure transducer connected to a datalogger records pond elevation every 15 min, from which pond outflow and inflow are obtained. A tipping bucket raingage at each site records rainfall totals. We analyzed data for 16 significant rainfall events from July through December 2004. All rainfall was less than the one yr, 24 hr design storm, and pond flows should therefore be less than the flow expected to occur in such an event. However, pond flows frequently exceeded this threshold. At Pond A, flows exceeded that predicted for the one year storm at least four times. Inflow exceeded the predicted 2 yr storm response four times, and outflow exceeded the 25 yr storm response three times. At Pond B, inflow exceeded the one yr storm response five times, while outflow exceeded the one yr storm response nine times, with one exceedence of the 10 yr outflow. In 16 events, Pond A met the 24 hr runoff detention goal only once, with detention ranging from 0.5 -10.5 hrs during other events. These results suggest that these SRPs receive and release higher peak discharges than intended, and often release captured stormwater over periods much shorter than the regulatory goal. The receiving streams are exposed to higher peak discharges and to larger sustained high flows. The failure to perform as designed may result from underprediction of developed runoff in pond design, leading to undersized ponds, and/or to a lack of pond maintenance following construction.
DE: 1834 Human impacts
DE: 1860 Streamflow
SC: Biogeosciences [B]
MN: Fall Meeting 2005