Influence of Boundary Condition on the Modified 2D Shallow Water Model near the Flow–Structure Interaction Zone: A Case Study in Brahmaputra River

Abstract

AbstractThis work proposes a modified form of two-dimensional shallow water (2D-SWD) equation with water surface gradient at the source term. This modification was carried out as the conventional 2D-SWD equations often require special treatment when applied to complex braided rivers. The modified model is then applied to two different flow domains of the Brahmaputra river viz. at the braided reach near Umananda Island and reach with a series of spurs dyke near Majuli island. The stability of the model is checked using Courant–Friedrichs–Lewy condition. The maximum flow depth in the braided reach is observed as 15–18 m, and near the spur dykes, it is found as 35–37 m. Model simulated flow depth and velocity are compared with the field measured data. The model’s computational accuracy near the flow-hydraulic structure interaction zone is evaluated with two different boundary conditions: no slip condition and reflecting boundary condition. Based on root mean square error (RMSE) and Nash–Sutcliffe Efficiency (NSE), the reflection boundary condition shows superior in simulating the velocity profile (RMSE = 0.11, NSE = 0.60) as compared to no slip condition (RMSE = 0.28, NSE = 0.22). These findings suggested that the water surface gradient in the modified 2D-SWD model gives more flexibility for application in the braided river. It also stresses the importance of considering reflecting boundary conditions while applying the model near the fluid–structure interaction region.KeywordsShallow water equationHydraulic structureComplex topographyBoundary conditionExplicit scheme