  This paper presents an experimental investigation of moving grains over
subaqueous barchan dunes that consisted of spherical glass beads of known
granulometry. Prior to each test run, a pre-determined quantity of grains was
poured inside a closed conduit, and the grains settled on its bottom wall
forming one conical heap. As different turbulent water flows were imposed, each
heap evolved to a barchan dune, which was filmed with a high-speed camera. An
image processing code was written to identify some of the moving grains and
compute their velocity fields and trajectories. Our results show that the
velocity of grains varies along the barchan dune, with higher velocities
occurring close to the dune centroid, and that grains trajectories are
intermittent. Depending on the region over the dune, we found that the velocity
fields present values within 1 and 10 % of the cross-sectional mean velocity of
the fluid. Considering the average trajectories of grains moving over a given
dune, their mean displacement varies within 30 and 60 grain diameters and their
characteristic velocities within 10 and 20 % of the cross-sectional mean
velocity of the fluid. The displacement time varies between 30 and 90 % of the
settling time, and it seems to have two asymptotic behaviors: one close to bed
load inception and other far from it. When compared with bed load over a plane
bed, we observe that grains have the same mean velocity, but they travel
distances up to 5 times larger, with higher densities of moving grains.
