Effect of roughness on velocity distribution in diverging compound channels using experimental and numerical approaches

Rivers in their natural course encounter different slopes, morphology, and physical conditions. Response of rivers adapting to those conditions alters their course, cross-sections, resistance, and other hydraulic properties. Terrains with gentle slope are seen in urban areas, estuaries, etc. In these areas, flow area expands as velocity decreased and become a non-prismatic diverging compound section. When river flow enters its floodplain, the flow structure is influenced by momentum exchanges between subsections. Due to the velocity difference between the main channel and the floodplain, eddies will generate at interface, making flow complex. Hence, velocity distribution is one of the primary interests of river engineers because of its importance in many engineering applications. Effect on hydraulic parameters under the influence of both bed roughness and channel geometric properties is one of the complex situations to prepare analytical models. In this study, velocity is measured in both smooth bed (Perspex sheet) and rough bed (gravel bed) using Micro ADV. Experiments are designed to understand the flow behavior in transition in flume and roughness of bed and interaction between both. Computational fluid dynamics is a prominent technique which is widely used by researchers among the world for its ability in reproducing the physics of flow. Numerical modeling is carried out using Ansys Fluent software. Numerical model results helped in the better understanding of flow structure with better visualization methods. It is known that numerical simulations are susceptible to error due to coarse mesh size, so mesh convergence studies are conducted to produce grid-independent results, and the velocity predicted from numerical simulation tends to be precise with mean absolute percentage error (MAPE) of 3.96%. It can be concluded that numerical simulations gave accurate results in predicting the pattern and magnitude of velocity.