Some observations of sorting of river‐sediments

Abstract

Various observations have shown quite definitely that sorting takes place during transportation of sediment by flowing water. This phenomenon is often overlooked in experimental studies, and accounts, at least in part, for inconsistent data obtained in bed‐load transportation experiments. The writer observed the results of this sorting process on the Missouri River on various occasions when the sedimentary characteristics of the River were being investigated, and also in the course of sediment‐transportation studies in the hydraulics laboratory at the University of Minnesota. The laboratory experimental work was carried on using materials of various mechanical compositions. In one case, the material, a fine gravel, was as uniform in size as one might expect to obtain commercially. No sorting was to be expected. It was the material retained on a No. 8 sieve and passing a No. 4, resulting in an average size of grain (according to mechanical analysis) of 4 mm (Fig. 1). The material was fed into the headwater end of a flume at such a rate as to maintain the slope constant; it was collected at the downstream end of the flume and weighed periodically. A progressive change in the rate of sediment‐discharge, at least for a time, was observed, although all conditions were maintained as nearly constant as practicable. That this change was, at least in part, due to sorting was evident from subsequent studies. A typical example is shown by Figure 1.