River erosion by groundwater seepage

The shear stress exerted by rivers on their banks is well known to drive erosion, over time sculpting the landscape through which they flow. These rivers are ultimately fed by groundwater flow from their surrounding catchments, and as this flow enters the river through the bank it exerts a pressure force on the sediment, further enhancing the erosion. Here we demonstrate the interplay between river and seepage driven erosion from the source of a river and along its length through a series of idealised, 2D experiments with a monodisperse granular bank. These experiments are interpreted both through a classical analysis of erosion, and through a boundary-layer analysis treating the granular bank using the mu(I) rheology. We use these approaches to understand the sediment flux, and to show that for a given water flux the model river attains an ultimate steady state in which the seepage and river shear stresses are equal to the critical (or yield) stress along the river profile.