Physical Processes and Sediment Transport in Stormwater Wet Ponds and Constructed Wetlands
Creators: Ahmed, Sherif Salah
Publisher: name: University of Alberta Library
Year: 2023

<p>Wet ponds and wetlands can improve stormwater quality by removing sediment before being discharged into receiving water bodies. Despite a large amount of published information, their optimal design, operation, and management have not yet been determined due to the lack of adequately monitored sites over long periods that span a wide range of environmental conditions. In addition, performing computer modelling alone would provide a simplified picture of reality. As a result, a mixed problem-solving approach of a comprehensive two-year field monitoring program during the ice-free seasons between May and October in 2018 and 2019 and computer modelling investigations were carried out in two stormwater wet ponds and two constructed wetlands in Calgary, Alberta, Canada. Although stormwater wet ponds and wetlands are similar in many aspects, a wet pond has a greater portion of deep water zones, 2-3 m, while wetlands are dominated by shallower water and are often thoroughly and densely vegetated. The study aims to investigate the key factors affecting the water quality, physical processes, sediment transport and fate and the difference in the function of a wet pond and a wetland to point the direction for improved design and operation guidelines. First, field data were used to investigate thermal and chemical stratification in the ponds and wetlands as a fundamental physical process that may impact their water quality. Second, the field data supported the application of the Environmental Fluid Dynamics Code (EFDC) hydrodynamic model to simulate the physical processes and evaluate the hydraulic performance of one wet pond and one wetland. Finally, the sediment transport compartment of the EFDC model was applied to the calibrated hydrodynamic model to evaluate the current design and operation by predicting the annual mass of deposits and sediment removal efficiency. In addition, the model simulated different scenarios of permanent pool depth, thermal stratification, wind speed and vegetation design. <br>
The field data showed that the wet ponds had vertical water temperature differences &gt;1 °C for up to 83% of the time from May to October. In addition, salt-laden inflows from road deicing salts led to a significant densimetric stratification (i.e., at vertical density difference &gt; 0.25 Kg/m3, equivalent to the density change in freshwater between 24 and 25 °C) for up to 96% of the time. Significant densimetric stratification was also thoroughly and intensely present in the wetlands for up to 79% of the time, in contrast to the assumption of urban wetlands being well-mixed. The stratification forced the runoff from the inlet to the outlet to move above or below the pycnocline, creating dead zones. Wind-induced surface currents in ponds/wetlands were insignificant, scaling at 0.3% of the wind speed. Strong densimetric stratification and low wind stress on the water surface caused anoxic conditions near the bed, potentially adversely affecting water quality and downstream aquatic communities. Hence, additional consideration of stratification is required when designing new stormwater wet ponds and constructed wetlands.<br>
The hydrodynamic modelling in the current study simulated water depths, velocities, temperatures and salinities in a stormwater wet pond and a constructed wetland with reasonable accuracy. The calibrated/validated hydrodynamic models represent an essential step toward incorporating hydraulic complexity into sediment transport and fate to explore optimum design and operation guidelines for better sediment removal efficiency. The sediment transport compartment of the EFDC model revealed that sediment removal efficiency during individual inflow events varied between 70-100%. The fluctuation of removal efficiencies was investigated against inflow characteristics, sediment inflow load, vegetation design, wind, stratification, and others. For instance, the sediment removal efficiency was negatively correlated with the inflow duration at R2 up to 0.71. Yet, the sediment removal efficiency was least sensitive to the inflow rate as the submerged berms efficiently dissipated the inflow momentum before transporting sediment to the outlet. Furthermore, stratification significantly influenced the internal hydrodynamic behaviour and the movement of suspended particles in the water column, increasing or decreasing the sediment removal efficiency based on the inflow characteristics and interaction with variations in internal flow paths. The study models are promising tools for predicting sediment removal efficiency and optimizing design and operation by simulating various remediation options. Yet, coupled sediment transport and biogeochemical models are needed in future studies to predict the internal loading of the algal biomass and suspended matter.</p>