Development and Application of Calculation Method for Amount of Suspended Sediment Entrainment under Non-equilibrium Conditions of Flows and Sediment Transports

Suspended sediments are dominant for bed variations during flood in rivers composed of fine uniform sands. Therefore, it is important to calculate accurately suspended sediment entrainments from the river bed for estimating bed variations during floods in sandy rivers.

However, in the conventional method for estimating suspended sediment entrainments from the river bed, it is assume that flood flows and sediment transports near river beds are equilibrium conditions.

The objective of this study is to develop depth integrated calculation model for flood flows and bed variations which was able to calculate three-dimensional flows, non-equilibrium motions of bed loads and suspended sediment entrainments by applying two-phase model in bed load layers.

We explained the concept of this important problem and showed governing equations of our developed model. The calculation domain was divided into three layers for vertical directions, bed load layer, transition layer and main flow layer. In the calculation domain for the main flows, we applied quasi-three-dimensional flow calculation method (GVBC Method) developed by Uchida and Fukuoka for calculating three-dimensional flows. The transition layer which was the second lowest region connected with the main flow and the bed load layer. In bed load layer which was the lowest layer, the flow and the sediment transport were calculated by treating as the two-phase motion. Flow velocities and sediment particle velocities in the bed load layer were calculated by equation of motions of fluid phase and sediment phase for horizontal directions. These interactions were evaluated by drag force term acting on sediment particles. And, vertical flow velocities in the bed load layers were estimated by continuity equation in this layer. In addition, since suspended sediment entrainments were closely related with turbulences near the bed, we calculated turbulent intensities by their transport equations in the bed load layer. The amount of suspended sediment entrainment was calculated by equation of motions of sediment particles for vertical direction considering non-equilibrium motions of flows, turbulences and sediment transports in the bed load layers.

The developed model is applied to the experiment of local scouring around a cylinder by Fukuoka et.al. (1997). The calculation model is able to explain the bed scouring in front and side of the cylinder compared with the conventional model which the flow and sediment transport is in equilibrium conditions. It is examined that how the thickness of the bed load layer influences the estimation of flow and bed variation.