HOW EFFECTIVE ARE RETENTION PONDS AT MITIGATING THE EFFECTS OF URBAN RUNOFF?

Retention ponds are a common best management practice implemented to mitigate the impacts of urbanization, including the alteration of downstream channel morphology, water temperature, and sediment transport. However, some studies suggest that retention ponds do not reach their design goals of reducing peak discharges and/or reducing the duration of sediment-transporting discharges in post-urbanization flows to that of pre-urbanization sediment-transporting flows. Here we compare the stream downstream of a retention pond located in James City County, Virginia (Stream A) to a stream draining a forested watershed (Stream B). On both of these streams and in the retention pond, we collected discharge, specific conductance, and temperature data. Furthermore, we compare the current longitudinal profiles of both channels, as well as a reconstructed profile of Stream A, to try to quantify the alteration of stream morphology. We installed conductivity and temperature probes in both streams and in the pond to collect data over a number of months. We used survey equipment to measure elevation at various distances from the outflow point of each stream in order to create the channel profiles. Specific conductance values in Stream A ranged from 150µS/cm to 300µS/cm, with a mean value of 211µS/cm and a standard deviation of 35µS/cm (from 328 measurements). In Stream B specific conductance values ranged from 10µS/cm to 250µS/cm, with a mean of 94µS/cm and a standard deviation of 78µS/cm (from 2537 measurements). Water temperatures in Stream A ranged from 20⁰C to 31⁰C, with a mean of 24⁰C and a standard deviation of 2⁰C (from 572 measurements); daily fluctuations in this stream were on the order of 5⁰C. Stream B had water temperatures ranging from 13⁰C to 21⁰C, with a mean of 18⁰C and a standard deviation of 2⁰C (from 3413 measurements). The daily temperature fluctuation in this stream was on the order of 1-2⁰C. From the differences we observe in the channel geometries and in the specific conductance and temperature data, it is apparent that Stream A is different from Stream B. Though retention ponds are meant to protect downstream channels from the impacts of urbanization, it is evident that watershed urbanization has altered the stream downstream of the retention pond.