Numerical calculation for bed variation in compound-meandering channel using depth integrated model without assumption of shallow water flow

•The BVC method can compute the bottom velocity acting on a bed surface in 3D flow.•Bed variation calculation based on the BVC method is presented.•We reproduce bed topographies in a compound meandering channel by overbank flow.•The drop in sediment transport rate due to overbank flow is examined. In a compound meandering channel, patterns of flow structures and bed variations change with increasing water depth owing to complex momentum exchange between high-velocity flow in a main channel and low-velocity flows in flood plains. We have developed a new quasi-three-dimensional model without the shallow water assumption, i.e., hydrostatic pressure distribution; our method is known as the general bottom velocity computation (BVC) method. In this method, a set of depth-integrated equations, including depth-integrated momentum and vorticity equations, are prepared for evaluating bottom velocity and vertical velocity distributions. The objective of this study is to develop a bed variation calculation method for both single and compound meandering channels by using the BVC method coupled with a sediment transport model. This paper shows that the BVC method can reproduce the pattern change of bed variation in a compound meandering channel flow with increasing relative depth. The variation in sediment transport rate due to overbank flow is explained by experimental and computational results.