Flowing water at the land surface - Fundamentals of the Physical Environment - page 448

and relative rock hardness. Channel potholes containing smoothed pebbles exemplify the
general process, known as evorsion through its dependence on a fluid vortex (Plate 14.9).
However, the ability of large entrained boulders to strike off angular bedrock fragments
in turbulent, high-velocity, high-discharge flows is probably more effective in
maintaining angular profiles.
The erosivity of river flow itself by fluid stressing depends on the fluid shear stress
directed towards the channel boundary in turbulent flow and bedrock shear strength. This
is sufficient to erode softer or well fractured rocks and is enhanced by cavitation , when
bubbles or small cavities in rapid turbulent flows implode. When implosion occurs on
contact with the channel walls, microjets of water fill the cavity and can generate stresses
of up to 60 MN m −2 . Bedrock is less susceptible to fluid erosion but unconsolidated
sediments are readily eroded in valley floors, where they form a continuum between the
alluvial toeslope , channel wall and bed (see Chapter 13). Sediments are found
throughout the catchment, as isolated pockets in upper, steeper segments and continuous
sheets across the flood plain. Erosion occurs in two ways. At all discharges, subaqueous
erosion occurs where bed shear stress exceeds shear strength in cohesionless, granular
sediment. Bank failure at low discharges occurs by slumping after both wetting and
drying cycles, or toppling when removal of the supporting effect of water generates
tension cracks (Plate 14.7). Soft-sediment erosion is a partial recycling process, as the
river reworks material it previously deposited, but net downstream sediment transfer is
assured by the influx of mass wasting debris and continuing denudation.