河口砂州自然開削の実測・実験と解析

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(1) The sand spit at the mouth of the Nanakita River was flushed twice by floods. The water level measured upstream the river mouth showed quite similar time-variation in these two flushing processes : at the initial stage of floods, river water overflowed with a depth of several ten centimeters, and flushing was completed within 3 to 4 hours.(2) The k-epsilon turbulence model is applied to a rapidly varied flow over a two-dimensional sand bar. The numerical results are compared with the experimental data obtained by LDV. The water surface profile, the mean velocity profiles and the bottom shear stress are simulated reasonably.(3) The bedload formula of Meyer Perer-Muller is extended to include the effects of local slope and pressure gradient, by using the experimental results. The equation of motion is derived to describe the sand motion under non-equilibrium conditoin. The numerical computations based upon these relationships simulate very well the temporal change of the measured profiles of sand bars and corresponding variation of the water surface.(4) Equations of shallow-water theory and sediment movement in equilibrium condition are solved numerically by means of a finite difference method to predict flow field and topography change at a river mouth during flood. First, the comparison is made with the data of a model experiment. The predicted water surface profiles show excellent agreement with the experimental results.Second, the comparison is made with the field data measured at Nanakita River Mouth. It is found that the shape of the eroded cross section could not be predicted well. This is mainly due to the fact that the non-equilibrium condition of sand movement is not included in the present calculation.