Effects of wetland depth and flow rate on residence time distribution characteristics

The residence time distribution (RTD) representing the hydraulics of a wetland is an important tool for modeling and designing treatment wetlands for optimal constituent removal. To correctly use RTD results, it is necessary to understand the conditions under which this distribution remains stable. Dye tracer experiments were conducted on a stormwater treatment wetland to investigate hydrologic factors affecting RTD characteristics. Dye was introduced into the inflow under normal flow conditions and during simulated storm flows, providing a range of flow rates and water levels. Dye distribution in the outlet was measured using an in situ fluorometer. Results indicate that flow rates did not have a significant effect on RTD characteristics. The RTDs normalized for volume and flow demonstrated a greater amount of short-circuiting and a larger mixing scale when water depth increased, demonstrating that water level can have a direct impact on the RTD of a wetland. This effect suggests that more than one RTD may be necessary for analyzing a wetland subject to changing water levels. For the wetland in this study, increasing the water depth elicited a decrease in hydraulic efficiency. Understanding such factors that affect hydraulic efficiency will aid in the design and management of wetlands.