Sediment transport processes associated with biogenic structures.

Predicting the generation of ripples downstream of bluff obstacles, such as sediment
mounds, is critical to estimating current speed from bedforms and predicting
interfacial exchanges of solutes and fine particles. Herein, observations from
previous studies of sediment transport downstream of bed roughness on flat sand beds
are condensed into a single threshold for the generation of ripples. This threshold
is based on two dimensionless numbers: one describing the impact of the roughness on
near-bed turbulence and another comparing the bed shear stress to the threshold for
sediment motion. Results from additional flume experiments confirmed this threshold
for sediment mounds of both fine and coarse sands. Measurements of the velocity
field near sediment mounds revealed changes in the vortices shed from bed roughness
depending on the flow and roughness scales. When vortices begin to coalesce, the
magnitude of velocity fluctuations downstream of the mound increases. Even though
mean conditions are below Shields’ threshold, sediment is transported when the local
bed shear stress due to vortex shedding exceeds the critical shear stress for
initiation of sediment motion. Including the critical shear stress in the threshold
for ripple generation makes it applicable to more complex boundaries, such as
biologically bound and heterogeneous sediments. The threshold for ripple generation,
and the mechanisms behind it, describe a potentially important sediment transport
process.