Tracing and hydraulic modelling to assess the hydraulic performance of a constructed wetland

Abstract

Constructed wetlands are widely used to protect sensitive water resources from non-point source pollution from agriculture. Their potential to remove nitrate and pesticides increases with the water residence time and a uniform distribution of the inflow over the wetland area. Over the hydrological season, inflow variations greatly modify the theoretical residence time. The knowledge of the corresponding variations of the hydraulic performance constitutes a gap for the better management of treatment wetlands, especially for wetland with heterogeneous vegetation implementation. The aim of this work is to investigate how the hydraulic performance changes with the flow rate in a partly vegetated wetland. The study site, a 0.5 ha wetland, is located in an area of intensive cereal crop production in Northern France. The three-dimensional hydrodynamic model Delft3D-Flow was used to simulate flow through vegetation, forced by observed meteorological conditions. It was calibrated on continuous outflow concentration measurements and unmanned aerial vehicle (UAV) images during a 13 d tracing experiment with rhodamine WT. The simulated hydraulic performance indicators matched satisfactorily with observed values thanks to the detailed description of the vegetation. Simulations for the locally usual flow range and for a fixed water depth showed a limited increase of the hydraulic performance with the flow rate. This shows that conducting a tracing at low flow is sufficient to assess the average hydraulic performance of a wetland.