Modelling approaches for bedform evolution in sand-bed rivers

Modelling approaches for bedform evolution in sand-bed rivers

The formation of bottom dunes is a typical phenomenon of sandy river beds. In addition to its purely scientific interest, it also has practical and environmental management importance for rivers, as it affects bathymetry (and therefore navigability) and the river's flow capacity. The formation and evolution of these bottom features are highly variable and, in some cases, still unclear. In addition to migrating downstream and changing in shape and size, they sometimes (due to certain characteristics of the motion, e.g. certain flow rates) tend to disappear, evolving into the so called Upper Stage Plane Bed. The causes of this phenomenon are not yet clear. Certainly, at high flow rates, the transport of suspended solids increases. This causes two phenomena that seem to cause the bed to flatten:

• being distributed vertically, suspended transport is not affected by the local erosion and deposition mechanisms that led to the formation of the dunes, and therefore tends to level out the geometry;

• the high presence of suspended sediments modifies the characteristics of the fluid (in particular its density), reducing the turbulent and vortex motions that cause the formation of dunes.

  
  However, how these mechanisms influence the evolution of the seabed is not known in detail, and further and more precise investigations are needed.

The purpose of this work is to better investigate the phenomenon of the formation of river dunes and their subsequent evolution into the upper stage plane bed.

This work will employ both numerical modelling, using three-dimensional hydraulic modelling software that allows turbulence phenomena to be simulated (with various approximations), and physical modelling, in particular in collaboration with Wageningen University (NL).