Study on the reduction in grain sizes of river bed material being in downstream transport

Abstract

In most rivers, grains of sediment in transported downstream are smaller in size in the downstream direction. In the field of hydraulics for sediment transport, it is considered that the reduction in grain size in the downstream direction is due to selective transport. The abrasion of grains then is not taken into consideration.
However, in mountain streams with geologically fragile rock grains, grain abrasion might need to be considered.
The Yoshino River was chosen as the object of this study. First, the reduction in the grain size of the longitudinal bed material was investigated there. Secondly, we assumed that the grain size of the longitudinal bed material would be reduced by the collision of saltating bed-load grains with stationary bed grains. We executed free fall impact tests on three types of rocky boulders. Then, on the basis of the tests, the mechanism of the reduction in grain size was considered.
The summary of the test results suggests that: 1) in mountain streams with geologically fragile rock grains, individual grains possibly collided repeatedly with each other and were consequently reduced in grain size during their downstream transportation, and 2) as a result of the reduction in grain size, wash load was produced and possibly became the source of turbid water.