The fluvial grain‐size gap: Experimental confirmation of hydraulic origin
Michael Church, Marwan A. Hassan
Abstract
Abstract 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.