The effect of emergent vegetation on convective flushing in shallow wetlands: Scaling and experiments

Abstract

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 (ϳ17% by volume) on convective flushing of shallow wetlands with low-Reynolds number (ϳ100) 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 ϳ1-10 mm s Ϫ1 were 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 (m 2 s Ϫ1 ) predicted by the developed parameterization compared favorably (R 2 ϭ 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.Convective circulation occurs in aquatic systems when shallow waters heat or cool more rapidly than deeper waters, causing horizontal gradients in water temperature and density. The horizontal density differences drive convective currents, which allow increased flushing of the littoral regions. In wetlands, these littoral zones are typically characterized by dense emergent vegetation, yet the effect of vegetation on convective circulation has rarely been addressed. This paper develops parameterizations to describe that effect, then presents some preliminary laboratory and field data to test the parameterization.Convective currents have been frequently observed in deep lakes (e.g., Monismith et al. 1990;Nepf and Oldham 1997), and they have also been measured in shallow wetlands. Arnold and Oldham (1997) measured the distinctive signals of cool convective currents inserting along the bottom boundary of a shallow (2 m deep) wetland (Fig. 1) for Ͼ30% of the year. These bottom currents are typically 0 (1-10) mm s Ϫ1 (Sturman et al. 1999). Although this is low compared to currents measured in lakes dominated by river in-1

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