HR: 1105h
AN: H52D-04    [Abstracts]
TI: Direct measurement of storm hydrographs from wet retention ponds: How effective are they at protecting streams from urbanization?
AU: * Hancock, G S
EM: gshanc@wm.edu
AF: Dept of Geology, College of William and Mary, Williamsburg, VA 23187, United States
AB: Wet stormwater retention ponds (SRP) have become the method of choice for controlling storm runoff, and are a significant component of suburban hydrology. Despite widespread use, little data has been collected to assess their success in controlling stormwater quantity. We have collected data on flows from four SRPs and a forested watershed in James City County (JCC), Virginia. Our goal is to assess whether these SRPs function as designed and release flows comparable to natural conditions. All monitored SRPs are in suburban developments of seven to 34 acres, and are designed to meet 1) JCC criteria requiring 24 hr retention of runoff produced by the one year, 24 hr storm and 2) Virginia regulations requiring the reduction of the two year, post- development peak discharge to pre-development conditions. We have analyzed data for from July 2004 through September 2007. Storms with recurrence intervals of one to two years consistently generate peak inflows into the SRPs greater than predicted by engineering design, resulting in outflows to streams greater than the maximum allowed under Virginia stormwater regulation. In addition, during storms similar to the 1 yr, 24 hr design storm, none of these SRPs retained runoff for the required 24 hr period, the primary requirement for stream protection in JCC. Comparison of outflow hydrographs from SRPs to a forested watershed during high-frequency storm events indicates peak flow reduction to predevelopment conditions is sometimes achieved, but total flow volume is much greater. During a near 100-year storm, SRPs passed runoff with only minor attenuation, resulting in peak and total flows into receiving streams substantially greater than natural conditions. The results suggest SRPs receive and release higher peak discharges than intended, often release stormwater over periods shorter than required, and are far from mimicking natural flows. Underprediction of developed runoff, inadequate pond design, and lack of maintenance contribute to this poor performance. As these ponds were designed using standard engineering methods, our results suggest that these shortcomings in SRP performance are widespread.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1834 Human impacts
DE: 1880 Water management (6334)
SC: Hydrology [H]
MN: 2007 Fall Meeting