The Effect of Emergent Vegetation on Convective Flushing in Shallow Wetlands: Scaling and Experiments

Many wetlands around the world are characterized by shallow water, dense vegetation in the littoral zones, no significant riverine inflow and minimal circulation. Recent research on the hydrodynamics of such wetlands has identified convective circulation as being important for flushing of the littoral zones. To quantify this process, a parameterization of the convective discharge per unit width, which had been previously developed for nonvegetated systems, was extended to include a drag coefficient dependent on Reynolds number and vegetation density. The drag coefficient also included the effect of anisotropic permeability of the vegetation. The effects of relatively dense emergent vegetation (∼17% by volume) on convective flushing of shallow wetlands with low-Reynolds number (∼100) flow was then investigated using experiments in a laboratory convection tank (0.5 by 2 by 0.1 m) and in a wetland mesocosm (5 by 15 by 1 m). Bottom convective currents of ∼1-10 mm s-1were measured in both the laboratory and the mesocosm. These currents resulted in the shallow, vegetated regions of the mesocosm being flushed in 4 h. The discharge per unit width (m2s-1) predicted by the developed parameterization compared favorably (R2= 0.7) with the discharge per unit width measured in both the laboratory and the mesocosm. The short timescales of convective flushing, even in the presence of reasonably dense vegetation, indicate the likely significance of this mechanism in sheltered wetlands.