BED MORPHOLOGY AND SEDIMENTOLOGY AT THE CONFLUENCE OF UNEQUAL DEPTH CHANNELS

Spatial and temporal variations in the bed geometry and bed material size of a sand-bed river confluence with unequal-depth channels were monitored during a sequence of floods. Additionally, two other sand-bed confluences were surveyed to test the replicability of these observations. The confluences studied here have only one avalanche face, which corresponds to the front of a tributary mouth bar in the shallower channel, and do not exhibit a marked scour zone. Three distinct morpho-sedimentological zones are present: (1) an area around the upstream comer of the junction where the sediments are generally finer than the mean, (2) a maximum depth zone with coarser than average particles and (3) a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream. Changes in relative discharge between the two channels had little effect on the grain size of the downstream junction comer bar, but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar. The downstream junction comer bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels. The contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone.