河口砂州自然開削の実測・実験と解析

(1)七秋田河口的沙嘴被洪水冲了两次。河口上游实测水位在这两次冲刷过程中的时变规律十分相似：洪水初期河水溢出，水深几十厘米，冲刷过程在3 ~ 4小时内完成。(2)本文将k-ε紊流模型应用于二维沙坝上的急变流。数值计算结果与LDV的实验数据进行了比较。合理地模拟了水面线、平均流速线和底切应力。(3)利用试验结果，将Meyer-Perer-Muller推移质公式推广到包括局部坡度和压力梯度的影响。导出了描述非平衡条件下沙粒运动的运动方程。基于这些关系的数值计算很好地模拟了实测沙坝剖面的时间变化和相应的水面变化。(4)本文采用有限差分法数值求解浅水理论方程和平衡条件下的泥沙运动方程，预报洪水期河口的流场和地形变化。首先，与模型试验数据进行了比较。预测的水面线与实验结果吻合较好。其次，与七田河口实测水面线进行了比较。结果发现，侵蚀截面的形状不能很好地预测。这主要是由于在计算中没有考虑泥沙运动的非平衡条件。

(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.