Gravel bed rivers have complex, porous, and irregular surfaces,
characterized by a range of morphological forms. These topographical
structures determine the flow structures that develop over the river
bed, primarily by the shedding of vortices in the downstream wake of
protruding clasts. Previous research into these flow structures has
come from experimental studies, which have used either flow
visualization or single-point measurements techniques. Here we present
a numerical experiment where large-eddy simulation (LES) is used to
study the generation, evolution, and destruction of these flow
structures over a naturally water worked gravel surface. The numerical
simulations reported in this paper show that there are two distinct
scales of boundary influence upon the shallow flow and emphasize that
the measured flow variability at any one point in a natural river will
contain both locally derived and upstream-inherited flow structures,
according to the range of scales of bed topography present.