Gravel Wedge Progradation in Sand-Gravel Laboratory Experiments: New Insights on the Gravel-Sand Transition

Abstract:

The authors conducted physical experiments on the effects of the progradation of a gravel wedge on bed elevation and bed surface texture. The temporal and spatial changes of the bed surface grain size distribution were measured during the runs with a technique based on coloring sediment classes and image analysis. The measurements indicate that the coarse wedge induces bed degradation and a decrease of the bed slope in the downstream reach as the coarseness of the wedge reduces the sediment supply to the downstream reach. This results in a sharp transition in bed slope and grain size, which is characteristic of gravel-sand transitions. Numerical runs emphasize how at field scale such a gravel wedge migrates at an extremely slow pace and how the progradation of its front can be limited to peak flows. An analytical model illustrates how the time scale of wedge progradation compares to the De Vries time scale of morphodynamic river changes. The research illustrates how the combination of field data, physical experiments, numerical modelling, and analytical analysis adds to our understanding of the physical behavior of river systems.