A HYDROLOGICAL AND SEDIMENTOLOGICAL STUDY OF THE POINTE AT JAMESTOWN DETENTION POND, JAMES CITY COUNTY, VIRGINIA

Impervious surfaces increase stormwater runoff and sediment flows to recipient water bodies. Efforts to control runoff and sediment transport include the implementation of stormwater detention ponds. To determine the effectiveness of detention ponds, we measured stormwater inflows and outflows as well as sediment concentration from a detention pond capturing water from a suburban development in James City County, Virginia, during summer 2006. Design of this pond is based on two regulatory requirements. First, the State of Virginia has mandated that the two-year peak outflow from a development site cannot exceed its pre-development flow. Second, James City County, VA, mandated a minimum 24-hour pond detention time for runoff from the one-year, 24-hour storm. The Pointe at Jamestown detention pond collects water from a 34-acre residential housing lot which began construction in 2001. Field results demonstrate the pond's inability to reduce the two-year peak outflow to the pre-development level as specified in its engineering designs. Detention times for twelve storms measured in 2006 were less than 24 hours, although storm rainfall did not reach or surpass the one-year, 24-hour level. Entering pond design data into modeling software allowed for a quantitative comparison of a modeled detention pond to an actual pond that was built with the same specifications. Generated peak inflow, peak outflow, and detention times were significantly lower (p≤0.05) than those measured in the field. Three storms were measured during 2006 for suspended sediment input and output, and the Pointe at Jamestown detention pond captured approximately 91% of the 1849 kg of suspended sediment entering the pond through the inflow culvert. This estimate was approximated by integrating a sediment concentration-discharge curve over the course of the storm period. The data show that with increasing storm size, a greater mass of suspended sediment is transported into the pond, and a larger percentage of this sediment is captured and retained in the pond. Sediment mass values may decrease in the future, as Pointe is still under construction. Sediment concentrations, and ultimately mass yields, tend to increase during the construction phase and decrease following completion of the development.