  Experiments on the effects due solely to a mobile granular layer on a liquid
flow are presented (feedback effect). Nonintrusive measurements were performed
in a closed conduit channel of rectangular cross section where grains were
transported as bed load by a turbulent water flow. The water velocity profiles
were measured over fixed and mobile granular beds of same granulometry by
Particle Image Velocimetry. The spatial resolution of the measurements allowed
the experimental quantification of the feedback effect. The present findings
are of importance for predicting the bed-load transport rate and the pressure
drop in activities related to the conveyance of grains.
