EVALUATION OF THE EFFECTIVENESS OF A RETENTION POND FOR CHANNEL PROTECTION DOWNSTREAM OF A SUBURBAN DEVELOPMENT, JAMES CITY COUNTY, VIRGINIA

Land development increases the amount of impervious cover and associated stormwater runoff. The increased runoff may lead to incision and widening downstream of urbanizing regions as channels adapt to the greater water volume. Retention ponds are the primary strategy for mitigating these impacts, and are designed to hold and release stormwater in a manner that protects channels. In this study, we investigate the effectiveness of retention ponds at protecting stream channels through measurements of discharge during storm events and by comparing channel morphology in watersheds of varying land use. Stream morphology differences were assessed by measuring cross sectional area (normalized to drainage areas), bank slope, and width-to-depth ratio. Generally, channel morphology downstream of retention ponds is not significantly different from upstream. The width to depth ratios of cross-sections below the ponds are closer to measurements of sections upstream, ranging from 2.7-3.5, than to the ratios of pristine streams, all greater than 4.1. In addition, lowered bank integrity of downstream sections is shown in steeper bank slope, greater than 0.7, compared to pristine streams, less than 0.4. These results are paired with a study on stream cross sections in a suburb of Boston, Massachusetts. One specific retention pond in Virginia is capturing water from a 21.08-acre suburban development in James City County (Mulberry Place). Automated measurements of pond elevation were made every 5 minutes and used to determine pond inflow, outflow, and volume change. 15 storms events occurring between January and August 2006 were analyzed for storm performance. To determine whether this retention pond helps to return flows to a pre-development state, we compared the discharges to ones measured in a heavily wooded area nearby. The pristine stream, with a drainage area twice that of Mulberry Place, had lower discharges in monitored storm events. In a 1.48-inch storm, the pristine stream and the retention pond had peak discharges of 0.09 and 0.372, respectively. We conclude from these measures of channel morphology and retention pond outflow that this retention pond is only minimally effective at protecting channels from the impacts of development.