Fate of the grain size gap material in river bed sediments

Abstract

<p>There is a paucity of rivers beds with median surface grain sizes between ~1 and 5 mm, a range also referred to as the grain size gap. The grain size gap corresponds to the rapid reduction in grain size associated with the gravel-sand transition in river systems, where grain sizes reduce from >5 mm to 1 mm over a downstream distance equivalent to just a few channel widths. In existing models and experiments, these grain sizes must typically be omitted to generate the abrupt reduction in grain size across the transition. However, there is evidence that these grain sizes are present in river systems and hillslope sediment supplies. We present a series of new laboratory experiments in a narrow flume, examining the fate of grain size gap material in both sediment feed and bed distributions. Our observations indicate that where sand falls out of suspension at the upstream end of the gravel-sand transition, grain size gap material in gravel beds experiences enhanced mobility. We propose that this occurs through a geometric effect where medium sand is the exact size to bridge interstitial pockets in fine gravel bed surfaces. We hypothesize this effect could enhance grain protrusion of fine gravel and increases the likelihood of entrainment, or that sand deposition smooths the bed, generating fluid acceleration in the near-bed flow region. Grain size gap particles cannot form the dominant mode in river bed surface sediments because sand destabilizes particles of this size. </p>