2-D FLOW AND SEDIMENT TRANSPORT MODELING AT CONFLUENCE OF STREAMS

Abstract

Floodplain research is essential for human lives and the economy. It can be demonstrated that most computer models cannot adequately match the positions of buildings impacted by floodings, such as bridges and roads, to the floodplain site in stream floodplain analysis. The goal of this research is to provide an output from the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) hydraulic model that allows for two-dimensional unsteady flow analysis. These models would tell us how the flow behaves in various conditions and how to respond; as a result, to prevent the undesired impacts. There are several ways available in flood simulation models to compute the water surface heights associated with a flood event. A hydrodynamic model at the confluence of two rivers has been developed using unsteady open channel flow code HEC-RAS. A digital elevation model of 1m resolution is generated using available cross-section data for the two rivers in the RAS Mapper. In this study, the model is simulated for the unsteady flow condition for different discharge ratios. This study is divided into two main parts, namely flow simulation at confluence and modeling of sediment transport. DEM represents the topography and is used for displaying the floodplain. The boundary condition is specified as flood hydrograph and normal depth at the upstream and downstream of the river, respectively. For the sediment modeling, the equilibrium load is set to the external boundary condition initially; then, the observed sediment concentration load is used as the input boundary condition. HEC-RAS uses the various sediment transport function in 2D; all are tested in this study. The Wu formula shows a good agreement for analyzing sediment deposition profiles in 2D modeling. The model is run for the various time period and for varying computational intervals, and parameters like velocity, water surface elevation, and bed changes have been studied at different cross-sections along the river reach. The velocity contour map has been plotted for different flow conditions for both channels. The observed maximum velocity in the first flow condition is around 0.92m/s at section one and 1.1m/s in the second flow condition. A change of around 1.1m in water surface level along the main river and 1.5m in tributary is observed during simulated unsteady flow conditions. The erosion in the entire bed is observed during the model run for a duration of 24hr when equilibrium load is given as a boundary condition. The maximum Sediment concentration is reported at 15000mg/l and 12000mg/l in the main and tributary river, respectively.