The fluvial grain-size gap: Experimental confirmation of hydraulic origin

The origin of the well-known 'grain size gap' within the size range 1-10 mm in fluvial gravels is explored in a laboratory experiment to ascertain the effect of hydraulic sorting. A widely graded sand/gravel mixture was fed into a flume containing a bed composed of the same sediment mixture. The rate of sediment feed was varied but the water flow remained constant throughout the experiment at a rate that sustained size-selective bedload transport with negligible suspension. We observed persistent deposition of the coarsest sediments (+16 mm) due to lack of competence to mobilize these sizes, partial entrapment of the finest sizes (-1 mm) in interstices of the gravel bed, and preferred transport of the intermediate sizes, including uptake of grains from the bed, establishing the conditions for development of the grain size gap.