Downstream variation of grain size in gravel rivers: Abrasion versus selective sorting

Abstract

Most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers. This change in grain size might be due to a) selective sorting, according to which finer grains are preferentially transported downstream, b) abrasion, by which individual grains are reduced in size, or some combination of the two. Here a framework for the simultaneous treatment of both phenomena is developed. The analysis is restricted to bedload transport of gravel. Differential sorting is treated by means of a hiding function embedded in a surface-based sediment transport relation. Abrasion is postulated to be due primarily to binary collisions between moving bedload and stationary bed particles. The model is applied to river systems undergoing wave-like aggradation such that the river profile obeys an appropriate similarity profile. The results indicate that the downstream variation in grain size is mostly due to selective sorting in the case of quartz, but that abrasion and selective sorting are of roughly equal importance in the case of limestone.