Experimental study on flow analysis and energy loss around discontinued vertically layered vegetation

Vegetation acts as a natural protection measure as it reduces inundation depth downstream of it and dissipates the energy of flow like flood and tsunami. The present study performed a series of laboratory experiments in which the flow variations and energy dissipation around discontinuous and vertically double layered vegetation (VDLV) were investigated with changing the vegetation density and water depth under steady subcritical flow conditions. A case of vertically single layered vegetation (SLV) was also considered for comparison purpose. The results showed that incorporating an intermediate (G/d = 0.563, where G is the clear spacing between cylinders in transverse direction, and d is the diameter of cylinder) as well as dense (G/d = 0.041) short submerged vegetation within a sparse (G/d = 2.125) emergent vegetation considerably increased the backwater rise by 22-55% and 26-59%, respectively for the upstream located vegetation model (UM), whereas it raised the backwater rise by 28-53% and 35-63%, respectively for the downstream located vegetation model (DM), as compared to that of sparse SLV forest. The combination of upstream located dense VDLV and downstream located intermediate VDLV offered larger resistance to the flow and gave maximum overall loss of flow energy i.e. 43-61%. It is observed that increasing the density of short vegetation layer in UM increases resistance to the flow by narrowing the passage of fluid flow, due to which the backwater rise upstream of the UM increased and inundation depth downstream of the DM reduced significantly; and hence, consequently resulted in a significant amount of loss in the flow energy.