MODELING OF GRADUALLY-VARIED FLOW THROUGH SUBMERGED SEMI-RIGID BLADE-TYPE VEGETATION

Abstract

The condition of flow through a vegetation patch is gradually-varied. The drag generated by vegetation depends on theshape of each piece of vegetation and the flow interaction among the vegetation elements. Some vegetation is of bladetypewhich has different drag characteristics as compared to the circular-cylinder type vegetation. In the present study experiments are carried out to investigate the hydrodynamic behavior of gradually-varied flow through blade-type vegetation. Particular attention is focused on the effect of distribution pattern of vegetation elements on the total drag generated by a vegetation patch. The values of the density parameter of vegetation (λ = Nbv , N is areal density, bv is the blade width) are 72, 48, 24, 18, 15, 12, 6 and 3 (1/m). The blade Reynolds number Re using the blade width as the length scale ranges from 670 to 1110 and six flow rates are used in each set of experiments. Theoretical longitudinal momentum equation relating the vegetation resistant force, water surface slope and mean velocity in the vegetation layer is used to determine the averaged drag coefficient Cd. Using a regular-array pattern of vegetation elements, the sheltering effect and channeling effect are studied separately by varying the longitudinal element spacing (Sx) and lateral element spacing (Sy) respectively. For a constant lateral element spacing (Sx), Cd decreases with increasing areal density of vegetation due to sheltering effect. For a constant longitudinal spacing (Sy), Cd increases with increasing areal density of vegetation due to channeling effect. The results show that apart from the density parameter λ, the distribution pattern of vegetation elements can exert significant effect on the drag coefficient and the associated flow characteristics.