HYDROLOGIC IMPACTS OF URBANIZATION ON SMALL WATERSHEDS AND THE EFFECTIVENESS OF BEST MANAGEMENT PRACTICES, WILLIAMSBURG, VIRGINIA

The urbanization of watersheds results in substantial changes to stream hydrologic regimes. Natural channels respond to increases in flow volume and runoff rate brought on by urbanization with incision and channel enlargement. To prevent these impacts, best management practices (BMP’s) like detention basins are constructed to reduce flow rates. However, little empirical data is available to assess BMP effectiveness. We have installed gauging stations on streams in small watersheds (1-2 km2) in Williamsburg, VA and surrounding counties. This area is undergoing rapid growth with population increasing up to 111% between 1980-2000. Gauging stations are equipped with stilling wells and dataloggers to measure stage, and rating curves are established for each site. Rain gauges are installed in each monitored basin. Hydrograph measurements include lag time, peak discharge, event flow, baseflow and runoff coefficient. These were compared between basins of differing land use and detention basin implementation. Lag times in more highly developed basins were shorter than in lightly developed basins 82% of the time. Peak discharge was up to 5 times higher in the more developed basins than the lightly developed basins, but some lightly developed basins showed peak flows reaching twice as high as more developed ones during smaller storm events. This discrepancy could be due to differences in baseflow and indicates a storm-size threshold that must be surpassed in order to obtain expected results. We are currently monitoring additional basins to assess the effectiveness of detention basins at reducing these observed impacts.