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.

Automated summary

A laboratory experiment was conducted to investigate the origin of the well-known 'grain size gap' in fluvial gravels within the size range of 1-10 mm, focusing on the effect of hydraulic sorting. A mixture of sand and gravel with varying feed rates was introduced into a flume with a bed composed of the same sediment mixture. The experiment revealed that the coarsest sediments were consistently deposited, fine sediments were partially trapped in interstices of the gravel bed, and intermediate-sized grains were preferentially transported, including uptake from the bed, which led to the formation of the grain size gap.