Experimental observations on sorting patterns of heterogeneous sediment mixtures in low constrained flows

Field and laboratory investigations indicate that gravel bed rivers with bimodal grain size distribution and low lateral confinement, such as in the case of braided or multi-channel rivers, can present simultaneously active channel variations, both in the planimetric and altimetric directions, together with planimetric and vertical sorting. Such aspects were reproduced in new flume experiments considering three flow confinements with characteristic aspect ratios from about 80 to 5. Three long runs of about 60 hours were carried out under constant feeding rate conditions, with a bimodal mixture of natural sediments, a fixed flume slope of 3.18 %, and width imposed by lateral walls from 0.50 m to 0.12 m. We present here the results obtained with the first run, with a width of 0.5 m. We observed fluctuations of bed surface granulometric composition, bed slope, and outlet solid discharge; each of which with its own periodicity. In particular, the rearrangement of the bed surface texture was rapid. Cyclic bed states were observed: a stage of coarsening and aggradation corresponding to low values of outlet solid discharge; a stage of fining and degradation concomitant to high values of transport rate. The other two on-going runs (narrower configurations) aim at investigate the effect of lateral confinement on the morphodynamics of the system.