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 ve getation in the Littoral zones, no significant riverine inflow and minimal circulation. Recent research on the hydrodynamics of such wetlands has iden tified convective circulation as being important for flushing of the littor al zones. To quantify this process, a parameterization of the convective di scharge per unit width, which had been previously developed for nonvegetate d systems, was extended to include a drag coefficient dependent on Reynolds number and vegetation density. The drag coefficient also included the effe ct of anisotropic permeability of the vegetation. The effects of relatively dense emergent vegetation (similar to 17% by volume) on convective flushin g of shallow wetlands with low-Reynolds number (similar to 100) flow was th en investigated using experiments in a laboratory convection tank (0.5 by 2 by 0.1 in) and in a wetland mesocosm (5 by 15 by 1 m). Bottom convective c urrents of similar to1-10 min s(-1) were measured in both the laboratory an d the mesocosm. These currents resulted in the shallow, vegetated regions o f the mesocosm being flushed in 4 h. The discharge per unit width (ml s-1) predicted by the developed parameterization compared favorably (R-2 = 0.7) with the discharge per unit width measured in both the laboratory and the m esocosm. The short timescales of convective flushing, even in the presence of reasonably dense vegetation, indicate the Likely significance of this me chanism in sheltered wetlands.