Experiments on Gravel-Sand Transitions: Behavior of the Grain Size Gap Material

River bed sediments often lack fine gravel between 1 and 5 mm, a phenomenon referred to as the grain size gap. The gap corresponds to the rapid reduction in grain size associated with the gravel-sand transition (GST), where median bed material grain size reduces from similar to 10 mm gravel to similar to 1 mm sand. Fine gravel grain sizes are often present in hillslope sediment, so it is not clear why they are absent on riverbed surfaces. We present a phenomenological laboratory experiment examining changes in sediment dynamics across a GST to examine the behavior of grain size gap material when included in the bed and feed grain size distributions. Our observations indicate that where sand falls out of washload, forming persistent surficial deposits at the GST, grain size gap material experiences enhanced mobility. This is due to hydraulic smoothing by sand that occurs because of a geometric effect, where medium sand bridges interstitial pockets in fine gravel bed surfaces. Our experiments show that gap gravel flux is enhanced by sand deposition, making gravel beds mobile at the threshold of motion. We are unable to maintain an immobile grain size gap gravel bed when sand is fed which explains why gravel beds composed of 1-5 mm particles are so rare on Earth. We hypothesize that in natural systems, grain size gap particles are either buried in the diffuse extension of GSTs or transported into coastal and marine environments where they are more commonly observed. There are very few river beds made up of 1-5 mm gravel particles. That is not to say that these grain sizes do not exist, but that they rarely form the dominant grain size in river beds. This grain size gap is most obvious where river beds rapidly transition from gravel (similar to 10 mm) to sand (similar to 1 mm) at the gravel-sand transition. We undertook a flume experiment looking at the behavior of grain size gap particles at gravel-sand transitions. We tried to build a gravel-sand transition with 2-8 mm particles in the gravel bed, but when we fed sand onto the gravel bed, the previous static bed started moving and was transported out of the flume. We also tried to mix grain size gap particles into a coarser gravel bed and found that gap particles became disproportionately mobile when sand settled on the bed. Our experiment suggests that gap particles are transported across the transition where they may then be buried in the sand reach. As such, stable gravel beds of this grain size are rarely observed on Earth. Gravel beds composed of 2-8 mm particles cannot maintain a stable slope when sand is added to the bed materialStable gravel-sand transitions cannot be produced by washload deposition where gravel beds are composed of 2-8 mm particlesEnhanced relative mobility of grain size gap particles occurs when medium sand falls out of washload at the gravel-sand transition

Automated summary

River bed sediments often lack fine gravel between 1 and 5 mm, a phenomenon referred to as the grain size gap. This study investigated the behavior of grain size gap material during the gravel-sand transition. It was found that the presence of sand deposition enhanced the mobility of gap gravel, suggesting that gap particles may be transported to other environments.