Effects of seasonal variation of vegetation on hydraulic residence time distributions in a stormwater pond

Treatment system performance is often evaluated using Residence Time Distributions (RTDs), which will be different for different discharges. However, when normalised for discharge, the shape of the RTDs, for the same system, should be similar. In natural stromwater ponds, the change in vegetation cover throughout the year alters the system’s effective bathymetry and flow field, as summer vegetation has a greater resistance than winter vegetation/free water. This will change the system RTD from summer and winter, which will impact on treatment performance. Fluorescent dye traces were conducted at a stormwater pond field site in Warwickshire, UK, in both summer and winter to evaluate the system RTD. The bathymetry of the site was surveyed and the vegetation distribution was analysed using aerial imagery in summer and winter. The results showed that the change in vegetation had an impact on the system hydraulics. The total vegetation surface coverage varied from ~60 % in summer to 40 % in winter. In terms of the RTDs, first arrival times and peak concentration times were similar for both summer and winter. However, the winter data showed less spread, and had a lower mean residence time than the summer data. The data suggests that the summer flow experienced more mixing, probably as a result of the increased vegetation coverage.