Associate Professor, Department of Civil and Environmental Engineering, Graduate School of Engineering, Hiroshima University, Japan

Speech Title: Bottom Velocity Calculation method for multi-scale simulation of flood flows and sediment transport in rivers

Abstract: Flood flows and sediment transport with bed variations in rivers include a wide range of phenomena scales from the sediment diameter to the reach scale. Although simulations of complex turbulence flows and local scouring have been made possible by recent advanced three-dimensional turbulence models, those applications are still limited to small scale phenomena such as flows and bed variations in experimental channels, because of large computational time, many memories, and the huge computational task. As water flows in rivers are confined by the water depth in the vertical directions and many practical calculation domains are larger than the water, depth integrated models including one- and two-dimensional calculation methods have been widely applied to flows and bed variation analysis in rivers. However, three-dimensional flow structures induce important sediment transport phenomena including local scouring, depositions and sand waves. In addition, smaller scale phenomena than the water depth with three-dimensional flow structures can affect larger scale phenomena through bed variations and flow resistances. It is highly required to enhance the applicability of the depth integrated models to depth-scale three dimensional motions for the simulations of flow and sediment transport in rivers.

We clarified the applicability range and limitations of two-dimensional calculation models both calculations for flow and sediment transport with theoretical point of view with the shallowness parameter and then highlighted the need of a new depth integrated model with the ability to calculate velocity and pressure distribution. The present integrated simulation method, which is known as Bottom Velocity Computation (BVC) method, was proposed to calculate bottom shear stress and pressure acting on sediment particles without calculating vertical velocity distributions with three dimensional models. The BVC method employs several equations which includes equations for depth integrated vorticity, water surface velocity, vertical velocity and bottom pressure equations, coupling with the depth integrated continuity and momentum equations. We have applied and verified the method to flows and bed variations through the comparisons with experimental results laboratory channels, which were difficult phenomena to be calculated by previous depth integrated models, including secondary flow and bed variation in a sharp curved channel and a compound meandering channel, horseshoe vortexes and local scouring around a vertical cylinder and submerged groins and boulders. The above results indicate many three dimensional flow structures can be explained by the BVC method with calculating depth scale vortex motions. Recent advancements of the BVC method have succeeded in employing a dynamic wall law and a two-phase quasi-three dimensional model. It allows us to calculate dam-break flow over movable bed in a channel with an enlargement section of the width.