Laboratory Alluvial Rivers

Abstract:

By which physical mechanisms does a river select its shape and size? We investigate this question using small laboratory rivers formed by laminar flows.

In its simplest form, this experiment consists in a flow of glycerol over a uniform layer of plastic sediments. After a few hours, a channel forms spontaneously, and eventually reaches a stable geometry. This equilibrium state corresponds accurately to the force balance proposed by Henderson (1961).

If we impose a sediment discharge at the inlet of the experiment, the river adjusts to this boundary condition by widening its channel. Observation suggests that this new equilibrium results from the balance between gravity, which pulls the entrained grains towards the center of the channel, and bedload diffusion, which returns them towards the banks. This balance explains why experimental rivers get wider and shallower as their sediment load increases.

However, to test quantitatively this theory against observation, we need to evaluate independently the effect of transverse slope on bedload transport. We propose to use an instability generated by bedload diffusion to do so.