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Chapter 2 - State of the art
2.2.2 Recent research on sediment transport related parameters
Some recent works deal with parameters related to the sediment transport, like e.g. the shear
stress, the bed surface roughness.
MCEWAN, JEFCOATE & WILLETTS (1999) established a model to calculate explicitly the modification
of the velocity profile and the reduction of the fluid shear stress due to moving grains. Their
laboratory tests showed that moving grains can significantly contribute to flow resistance if there
is no static roughness. They further showed the limitation of the sediment transport rate by the
availability of the sediments.
MASSON & MARTINEZ (2001) observed the shear stress in a mixture of 1050 cylinders at different
densities. The analyzed movements (horizontal and vertical translation and rotation) showed on a
macroscopic scale that the plasticity is independent of the initial density, of the peak stress and of
the dilating or contracting behavior. On a microscopic scale a different behavior between dense
and loose samples was observed.
A rather academic study was performed by PAPANICOLAU ET AL. (2001). They analyzed the influence
of surface roughness on near-bed turbulence and conclude that the time-averaged flow characteristics
(Reynolds stresses and mean flow velocities) do not completely explain the roughness
effect on flow characteristics. Furthermore they found a clear correlation between the sediment
transport and the streamwise velocity for super critical flow conditions.
Considering a large data set of medium and large alluvial rivers, MOLINAS & WU (2001) pointed
out the difficulty of laboratory measurements to obtain flow depth of more than 50 cm and very
small slopes. These difficulties lead to rather low Reynolds numbers (~50’000) compared to field
measurements (~500’000) and to higher Froude numbers compared to field data. To account for
this fact, they developed an energy concept for sediment transport, which is based on the gravitational
theory of VELIKANOV (1954), on the stream power theory by BAGNOLD (1966) and previous
works of their research team (YANG, 1973 and YANG & MOLINAS, 1982). Their formula
mainly applies for sand-bed rivers.
page 8 / November 9, 2002
Wall roughness effects on flow and scouring