Under natural conditions stormwater can seep into the ground, whereas in an urbanized environment impervious surface cover prevents water infiltration, increasing the rate and volume of stormwater runoff. Retention ponds are a common method to control this runoff, and municipalities have developed regulations to promote their effectiveness at controlling both the quantity and quality of the water they discharge. The state of Virginia mandates that retention ponds must return the post-development peak outflow for a 2-year, 24-hour storm event to pre-development levels. A local municipality, James City County, Virginia, mandates that the inflow to outflow centroid lag time of a 1-year, 24-hour storm event must exceed 24 hours. To determine if regulations were met for a local pond at Kensington Woods development, I monitored rainfall, water elevation, and discharge, from which centroid lag times were calculated. Discharge rates were taken from engineering plans and corroborated with three salt dilutions taken during storm events. I evaluated observed parameters in light of water quantity regulations and predicted parameters generated from pond modeling software. Data were used from 15 storms occurring between January and August 2006. Although no 1-year, 24-hour storm (requiring 2.8 inches of rain in 24 hours) was observed, the inflow and outflow peaks from several smaller storms exceeded Virginia state regulations, and no storm had a centroid lag approaching 24 hours, as required by James City County. The largest storm, with 2.48 inches of rain, had a centroid lag of 11.04 hours. A standard pond modeling system was calibrated to return the same hydrographs for ideal storms as the Kensington Wood design plans. Field rainfall data were input into the modeling system to see if the plans correctly predicted observed hydrographs. The modeling system adequately predicted hydrographs for standard storms, but failed to predict hydrographs for storms with unusual intensities or durations. The hydrograph for the ideal 1-year, 24-hour storm, moreover, had a centroid lag of 7.94 hours, demonstrating that the design plans themselves were not adequate to meet county regulations.
Summer rain is warmed when it falls on hot surfaces, and retention ponds may act as heat sinks for this water. A second study, therefore, evaluated the effect of retention ponds on downstream temperature. Temperature was continuously monitored upstream, downstream, and in the outflow pipes of two James City County ponds, Ironbound Village and Mulberry Place, from mid-June to August 2006. Water discharged from Ironbound Village was significantly warmer (reached 30.31o C) than the upstream (24.4o C at the same time), and during times of discharge downstream temperatures were frequently elevated several degrees Celsius. At Mulberry Place, upstream and downstream temperatures differed little during times of discharge. Temperature probes were installed upstream and downstream of Longhill Grove, a retention pond directly upstream of Mulberry Place. The temperature downstream of Longhill Grove (reaching 31.12o C) was warmer than the upstream (24.79o C at the same time) during storm events, indicating that the thermal effects of ponds upstream might override effects of ponds downstream.
For additional documentation Lauren Hallett provided a PowerPoint Presentation entitled "Effectiveness of Stormwater Retention Ponds in Controlling Urban Runoff" provided here in PDF form.