Calculating the hydraulic efficiency of a constructed wetland

Abstract

Measurement of the efficiency of constructed wetlands is critical for maintenance of their functions and contribution to improving water quality in urban areas. A fluorescent dye tracer, Rhodamine-WT, was used to track water flow through a stormwater-reliant, free water surface (FWS) constructed wetland in a Mediterranean climate. The mean hydraulic residence time (HRT) was calculated and compared to the design HRT, in order to gain an appreciation of the hydraulic efficiency of the wetland system. The actual HRT (4.0 days) was calculated to be less than the design parameters (7.0 days), giving the system a hydraulic efficiency of 0.57. Water movement was also tracked across the wetland profile to determine movement patterns within the system and identify any areas of short circuiting. A remediation design was then implemented and tested under experimental conditions. The remediation design increased the HRT from 4.0 to 5.6 days. This equates to a hydraulic efficiency of 0.80, which indicates good performance. Improvement to the flow patterns within the wetland was also demonstrated, with the influence of short circuits reduced. Nonetheless, further changes need to be implemented within the wider wetland area to encourage even distribution of water. Wetland processes are highly time dependent, therefore the ability to measure and maintain the efficiency of constructed wetlands is essential. This study demonstrates it is possible to improve the hydraulic residence time of a constructed wetland through changes to vegetation and obstructions to flow. It also demonstrates the benefits of dye tracer studies in tracking the lateral movement of water through the constructed wetland. The use of dye tracers is a costeffective and valuable method to measure the hydraulic parameters of a constructed wetland and should be incorporated into the ongoing management plan for stormwater-reliant free water surface constructed wetlands.

Ecohydrology and flood risk management under climate vulnerability in relation to the sustainable development goals (SDGs): a case study in Nagaa Mobarak Village, Egypt

“…This was considered the optimal alternative from an ecohydrological perspective. Harrington (2019) estimated that the typical required hydraulic retention time (HRT) in a constructed wetland for stormwater management is five days and Blick et al (2004) suggested an average depth of 3 m is required for treatment.…”