Hydraulic investigation of flow and bed load transport in diverging compound channels with rigid and flexible vegetation

Abstract

Floodplains are really important for the environment as they provide various functions for waterways, including ecological, hydrological, and geomorphic ones. During overbank flows, there are complex hydrodynamic conditions that occur as water interacts with the rough floodplain vegetation and drag forces due to the vegetation that changes the flow pattern. However, we still don't fully understand how floodplain vegetation influences channel-altering hydrodynamic forces and sediment transport. To address this, we conducted flume experiments and measured flow pattern and sediment transport under varied floodplain geometry and rigid and flexible conditions. For this purpose, a physical model was constructed in the laboratory, in which plastic rods with a diameter of 10 mm will be used as trees, and artificial grass will be grass vegetation. Thus, 36 experiments were conducted in which the sedimentation and hydraulic parameters of the flow were calculated by measuring the flow velocity, water surface, and sediment transport rate. The results show that in the floodplain with dense (rigid and flexible) vegetation, the friction coefficient (f) is even 1.5 times higher than in the floodplain without vegetation. The parameters such as flow velocity, bed shear stress and Darcy Weissbach friction coefficient increase in the vertical plane between the main channel and floodplains due to the difference in roughness between the main channel and floodplains. In addition, in the main channel, the flow resistance is almost constant and does not depend on the vegetation density. The Darcy-Weissbach friction coefficient is greatly increased in the presence of grass and tree vegetation on the floodplain, and the larger contribution of f is due to the grass vegetation. The results also showed the sediment transport rate has a direct relationship with the angle of divergence and density of vegetation and an inverse relationship with the relative depth of the flow. Therefore, as the sediment transport rate decreases in the diverging compound channel, the wavelength and height of the dunes increase. At smaller divergence angles, the dunes tend to the top of the zero line (the level of the sediments before the formation of the bed form). In the presence of vegetation, due to the increase in the roughness ratio of the floodplain to the main channel, the contribution of the main channel to the flow rate increases. As a result, the sediment transport rate increases.