EFFECTS OF VEGETATION ON FLOW-THROUGH FREE-WATER SURFACE WETLANDS

The one-dimensional Saint-Venant equations are modified to account for stem drag and volumetric displacement effects of dense emergent plant s on free surface flow. The modified equations are solved with an impl icit finite difference method to give velocities and depths for shallo w flows through a vegetated wetland channel. Estimated flow profiles a re used to investigate how vegetation density, downstream boundaries a nd aspect ratio affect detention time, an important parameter in deter mining nutrient and pollutant removal efficiencies of wetlands constru cted to treat wastewater. Results show that free water surface wetland s may exhibit static, neutral or dynamic behavior. Under static condit ions, the wetland behaves like a pond in which displacement effects ca used by submerged plant mass invariably decrease detention times. Unde r dynamic conditions, stem drag induced by aquatic plants predominates and wetland detention times increase with vegetation density. These o pposing responses are separated by a narrow neutral condition where th e presence of vegetation has virtually no net effect on detention time . For a given flow rate and surface area, detention times and hence tr eatment efficiencies in vegetated free water surface wetlands can be m anaged to some degree by adjusting the downstream control or by changi ng the aspect ratio.