FIELD MEASUREMENTS OF RETENTION POND EFFECTIVENESS AT STORMWATER CONTROL IN A SMALL SUBURBAN DEVELOPMENT, JAMES CITY COUNTY, VIRGINIA

Increased impervious surface cover in suburban developments can substantially increase the rate and volume of stormwater runoff to receiving streams. One of the most widely used engineering strategies for mitigation of increased runoff is the retention pond. Despite their ubiquity, the effectiveness of retention ponds at controlling stormwater runoff has been sparsely assessed at best, and never to our knowledge through a systematic measurement program in the field. We present measurements of pond inflow and outflow during storm events in James City County, Virginia, and compare the results to engineering design for pond operation to assess their success in the field. James City County adopted regulations mandating that the inflow to outflow centroid lag be 24 hours for runoff produced by a 1-yr, 24-hr storm event, a standard considered stringent relative to most localities. We installed a monitoring station consisting of a pressure transducer and datalogger to collect surface elevation in a pond collecting water from a 10.84 acre suburban development (Kensington Woods). Pond elevation was collected at 5 min intervals, and was used to estimate pond inflow, outflow and volume change. Rainfall was collected using an automated tipping bucket raingage. Data were analyzed from 16 storms occurring between January and September 2006. The largest storm (8.03 in) had a centroid lag of 1.9 hr, and no storm was held for the required 24 hr standard. The storm most similar to a 1-yr, 24 hr storm (requiring 2.8 in) totaled 2.48 in, and the inflow to outflow centroid lag was ~11 hrs, less than half the regulatory standard. In addition to field data, we used a standard stormwater modeling package calibrated to site conditions to recalculate inflow and outflow hydrographs for the 1-yr, 24-hr storm. The model predicts a centroid lag of ~8 hours, demonstrating that the design is not adequate to meet regulations. The results suggest that this retention pond does not effectively retain stormwater. As it was designed with widely used engineering practices, it is likely that many other retention ponds show similar behavior.