Abstract:The catchment of the Dashidaira reservoir located on the Kurobe River has high sediment yield. Because of the sufficient available amount of water in the catchment during flood events, the free-flow sediment flushing operation with full water-level drawdown is employed every year to preserve the effective storage capacity of the Dashidaira reservoir. This paper focuses first on the numerical simulation of a previously conducted free-flow flushing operation in the Dashidaira reservoir using the available in situ obtained data. Afterwards, to improve the flushing efficiency, the effects of water and discharge manipulation and the construction of an auxiliary channel on the total volume of the flushed sediment were studied. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the flow velocity field and simulate the movable bed variations. The outcomes revealed that increasing the average free-flow discharge during the free-flow stage by approximately 56%, in the form of multiple discharge pulses, can enhance the flushing efficiency by up to 13%, and the construction of an auxiliary channel in the wide midstream of the reservoir can locally increase the sediment erosion from this area.

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Shallow flows can play a significant role in sediment management for dam reservoirs (e.g. sedimentation in shallow reservoirs and free-flow flushing operation). When shallow flow emerges with symmetric or asymmetric patterns, the flow domain exhibits complex three-dimensional (3D) features (e.g. helical flows). This study focuses on the numerical modelling of the velocity field in shallow reservoirs with varying geometries and also varying bed conditions (i.e. flat and misshaped beds). A fully 3D numerical model using the finite-volume method was utilised to reproduce the 3D flow velocity field. The experimentally measured surface velocity in all cases and velocity profiles in one case were used to validate the model. The numerical results showed that a slight disturbance in the inflow boundary condition results in a steady asymmetric flow pattern in reservoirs with a higher defined shape factor, but does not affect the flow pattern in reservoirs with a lower defined shape factor. Nonetheless, the simulated and measured flow velocity fields are reasonably consistent in all cases. These results can be used to optimise the design of sand traps or water storage facilities, and also to optimise sediment management in existing reservoirs.

Owing to the interaction between three-dimensional (3D) flow pattern and bed material, the process of local scouring around bridge piers is complex. Simply using the empirical scour formula may be insufficient for the evaluation of pier scour under the unsteady flow condition. In the present study, the evolution of scour depth under unsteady flow has been simulated using a 3D numerical model. The model solves 3D Navier-Stokes equations incorporated with the sediment continuity equation to take into account the interaction between flow and sediment.The simulated scour-depth evolution coincides with the measured one quite well under the steady flow condition.For the unsteady hydrograph, a compounded method is proposed for simulations of scour-depth evolution as well as for scour hole development. The results show the simulated bed elevation contours are in good agreement in front of the pier, while the simulated final scour depth is slightly lower than the measured data by 15%. In addition, it is found that the simulated scour depth at the end of the rising limb is 74-79% of the final scour depth, which implies that the recession period of the hydrograph plays a less effective role than the rising limb in the unsteady scouring process.Notation a reference level set equal to the roughness height C bed sediment concentration close to bed C i temporal variation curve of scour depth evolution around circular pier under boundary condition of Q i D 50 mean size of sediment d sediment particle diameter E average error of numerical simulation results after calibration stage g acceleration due to gravity k turbulence kinetic energy k s roughness equivalent to a diameter of particles on the bed Q i ith step discharge of stepwise hydrograph Q p peak flow discharge Q 0 base flow discharge q s,1 suspended load sediment transport in x direction q s,2 suspended load sediment transport in y direction q t x bed load sediment transport in x direction q t y bed load sediment transport in y direction S ce calculated scour depth by employing numerical model at the end of hydrograph duration S cp calculated scour depth by employing numerical model at the end of time to peak flow S i calculated scour depth by employing superposition methods at the end of time t i on the curve C i S me measured scour depth at the end of hydrograph duration S mp measured scour depth at the end of time to peak flow Sc Schmidt number T i corresponding time for S i t time t d duration time of hydrograph t i time at end of ith step of stepwise hydrograph t p time to peak flow u i flow velocity in i direction u x shear velocity

Sediment flushing is one of the proposed methods for preserving the storage capacity of dam reservoirs. In flushing with water level drawdown, the incoming flood erodes a flushing channel in the deposited sediment. Flow pattern as well as flushing channel formation procedure in shallow reservoirs is complex phenomenon due to the dynamic interaction between flow field and bed changes. In the present study, the flow field and flushing channel formation procedure were investigated in various shallow reservoir geometries using physical experiments and numerical simulation. A fully 3D numerical model which applies Finite Volume Method (FVM) was utilized. Reasonable agreement was found between the numerical and experimental outcomes. The results would be useful to understand the influence of geometry on flow pattern and flushing process to conduct more efficient sediment management strategies.