Surface Cooling of Sloping Water Bodies with Shallow Vegetated Regions

1. Introduction In this work, natural convection, due to constant surface cooling, in sloping water bodies with significant vegetated shallow regions are investigated numerically. The water body consists of (a) a deep region, of length equal to 0.67m, with a horizontal bottom (b) a sloping region, of length equal to 0.67m, with a bottom slope equal to 0.1 and (c) a shallow region, of length 0.66m, with vegetation and horizontal bottom. The vegetation has a porosity equal to 0.85 (typical of aquatic plants found in lakes) and its length is equal to the length of the shallow region. Depending on its structure the vegetation may block the surface cooling partially or totally. 2. Objectives The objectives of this study are: (a) to assess the vegetation effects on the thermal siphon, induced by surface cooling, for water bodies with a sloping region of relatively large bottom slope and a vegetated shallow region with horizontal bed. (b) to evaluate how the water exchange between nearshore and offshore is affected by the presence of vegetation in the nearshore shallow region, (c) to determine the significance of the vegetation shading in the occurrence and intensity of cold downslope gravity currents and the thermal siphon in general. 3. Methods The Volume-Averaged Navier-Stokes equations together with the Volume-Averaged Energy equation are solved numerically in the vegetated region. In the non-vegetated region these equations are simplified to the classical Navier-Stokes and Energy equations. A surface cooling is applied at the top free surface with a varying heat loss rate between the non-vegetated and the vegetated regions of the waterbody. The water body with a non-vegetated shallow region is also considered for validation purposes and computed results are compared with available data. 4. Results Isotherms, streamlines and exchange flow rates are presented for a water body with vegetated and non-vegetated shallow region at various characteristic times which show the development of flow and the large scale circulation (thermal siphon) as well as the effect of the vegetation shading on the flow regimes developed in the deep section of the water body. 5. Conclusions The results (isotherms, streamlines, exchange flow rate) indicate significant vegetation effects on the flow regimes, large-scale circulation and the exchange flow rates especially in the case of no surface cooling of the vegetated shallow region.