混合砂礫の限界掃流力に関する研究

Abstract

In studying the problems on sediment transport in a stream and stabilities of astream channel, the criterion for mvement of sands and gravels by action of fljw, theso-called critical tractive force, becomes generally important. The present paper dealswith the hydraulic approach to solve the mechanism of the critical tractive force ofgraded sand gravels.In the first part of this paper, the results of some basic experiments on the critical trac-tive force of graded sand gravels, especially the mechanism of sediment transport nearthe critical condition for movement and the sorting effect of flow, are described. It isconcluded from the experiments that the practical definition for the criterion for move-ment of sand gravels proposed by the author in the previous paper can also be appliedto the critical tractive force of graded sand gravels, and the sorting effect of flow nearthe critical condition is expressed by a function of size frequency distributions of gradedsand gravels.In the second part, a theory for the critical tractive force of graded sand gravels isdeveloped, based on the author's procedure to solve the criterion for movement of sandgravels and the existence of wakes behind sand gravels. It is clarified from the com-parison of the theoretical curve with the results of experiments performed by the authorand some other authorities that the critical tractive force of graded sand gravels expressedin the dimensionless form is a function of the standard deviation of graded sand gravels, and the theoretical curve is in good agreement with the experimental results within therange of the standard deviation less than about two. Based on the above consideration, an empirical formula for the critical tractive force of graded sand gravels is proposedand compared with some other formulas proposed by authorities.