Laboratory experiments have been carried out in a large laboratory flume us
ing a strongly bimodal sediment mixture in a range of flow conditions such
that the initially flat bed of the flume became unstable enhancing the deve
lopment of alternate bars. In order to elucidate the effect of grain sortin
g, the present experiments are compared with a previous set of experiments
performed in the same flume under similar hydraulic conditions but using a
nearly uniform sand with a mean geometric grain diameter equal to that char
acterizing present experiments. The comparison suggests that sediment heter
ogeneity may appreciably affect bed form characteristics. The development o
f small-scale (ripples) and mesoscale (dunes) sediment waves tended to be i
nhibited thus allowing a decrease in flow resistance. Owing to the bimodal
character of the adopted mixture at low values of the bed shear stress a co
ndition of partial transport was attained for which only the finer-grained
portion of the mixture was observed to move, while the coarser-grained frac
tions remained essentially immobile throughout the experimental run. Howeve
r, complete mobilization of all size fractions was observed to occur in run
s carried out at higher slopes (i.e., at higher bed shear stress) in order
to generate an alternate bar pattern. Selective transport of individual gra
in size fractions, coupled with the characteristic bar topography pattern l
ed to an intense longitudinal sorting which accreted the coarser particles
on bar crests. Furthermore, bar migration caused, through scour and fill, a
significant vertical sorting. As a consequence of these sorting processes
and in accordance with previous experimental observations, bar height turns
out to be invariably damped with respect to uniform sediment experiments.
The trend exhibited by the wavelengths is less clear and suggests that in t
he present experiments sorting effects do not enhance the clear shortening
of bar wavelengths typically observed in other series of flume experiments
carried out with weakly bimodal mixtures.