The dynamics of a gravel-sand transition

Abstract

A
gravel-sand transition (GST) seems to be the result of a gravel wedge. Such a
wedge can prograde, halt, and even retreat. It is typical of an ungraded or
transient reach (Blom et al., 2016), where profile concavity and downstream fining
can be much stronger than in a graded or equilibrium river reach. The GST
migration speed decreases in time and is likely of the order of a few meters or
less per year. The effect of abrasion on GST migration is limited to mainly
affecting the sand and silt load transported into the sand-bed reach and so its
slope. Our simple GST migration model describes GST dynamics fairly well. It
provides an estimate of the time scale of GST migration and a criterion for the
onset for a GST to halt or retreat. We recommend analysis of temporal change in
bed elevation and bed surface texture in GST zones, and the development of new
techniques to measure the bed surface texture to assess GST dynamics in the
field