LAND USE IMPACTS ON STORM HYDROGRAPHS AND THE EFFECTIVENESS OF BEST MANAGEMENT PRACTICES, VIRGINIA COASTAL PLAIN, USA

We have measured hydrograph changes in response to land use change in small (<2 km²) watersheds in the Virginia Coastal Plain. This data is used to test the sensitivity of stream flow to land use and to evaluate the effectiveness of Best Management Practices in preventing volume changes. We have installed several gauging stations near Williamsburg, in the eastern Virginia Coastal Plain. This region is undergoing rapid growth, with population increasing by ~100% in James City County over the past 20 yrs. Study locations are chosen based on varying degrees of development and detention basin implementation. Development ranges from high (60% developed) to low (10% developed) in the basins studied. Each site is equipped with a stilling well, pulley system and datalogger to record stream stage. Rating curves are constructed using the salt dilution technique to determine discharge at multiple stages. Multiple rain gauges, installed in and around monitored basins, were used in conjunction with the two-axis method to calculate average rainfall. Measurements of lag time, peak discharge, event flow, and runoff coefficient were made and compared between basins. When three or more basins recorded data, peak discharge was greatest in the most developed basins 88% of the time. On average, peak discharge was 5 times higher in highly developed basins than it was in lightly developed basins. In six cases, one of the three recorded basins had a BMP. The BMP basin had a lower peak discharge than other developed basins 83% of the time. Lag times in lightly developed basins were shorter than those in highly developed basins 54% of the time by an average of 1.5 hours. For 38% of the recorded storms, the BMP basin had the shortest lag time. A more detailed assessment of detention pond stage prior to rainfall may elucidate the cause of these short lag times. The results suggest BMP’s in the Williamsburg area are effective at reducing peak flow, but not necessarily lag time. However, results have also shown that while peak flow is reduced, the duration of the event flow is increased. We are currently monitoring additional basins to assess the effectiveness of detention basins at reducing these observed impacts.