A distinction between bed-load and suspended load in natural streams

Problems pertaining to the transportation of sediment by flowing water have been extensively studied by engineers and scientists during the last few years. Most of the various experiments have been concerned with either bed-load or suspended load, but seldom have they been studied simultaneously. When bed-load and suspended load occur simultaneously in a flume or stream a distinct difference in their mechanical composition is often observed. A curve fitted to the plotted data from a mechanical analysis of the bed-material shows a distinct break. This break occurs most commonly in the data plotted from samples taken in mountain streams where the bed-material is extremely coarse. For instance, in the Rhine River, Switzerland [see 5 of “References” at end of paper] the bed-material ranges in size from pebbles 5 mm in diameter to boulders 100 mm in diameter and greater. Between boulders and in protected pools, however, sand one mm in diameter and finer is found, whereas particles ranging in size from one to five mm are not present. The explanation for this size-distribution has been that the material greater than five mm in diameter moves as bed-load and the material finer than one mm in diameter moves as suspended load. Particles ranging in size from one ram to five mm in diameter occur rarely for certain geologic or hydraulic reasons [3].