OVERBANK SAND DEPOSITION IN RELATION TO TRANSPORT VOLUMES DURING LARGE-MAGNITUDE FLOODS IN THE DUTCH SAND-BED RHINE RIVER SYSTEM

Using aerial photographs and field measurements, sandy overbank deposi ts formed by the large-magnitude floods of 1993/94 and 1995 were quant ified along two branches of the Dutch Rhine river system: the Waal (19 93/94 and 1995) and the IJssel (1995). These deposits were laid down i ntermittently all along the length of these rivers on the top and land ward slope of the natural levees, and covered about 4 per cent of the embanked floodplain on the Waal and about 1 per cent on the IJssel. Th e overbanks and transport mechanism is basically convective by nature. The spatial variability of overbank sedimentation points to the impor tant role played by helicoidal currents in determining overbank deposi tion. The presence of embankments and training works appears to influe nce the sand transport to and morphological development of the floodpl ains along the Dutch Rhine river system. Overbank deposition volumes a bout equal present estimates of sand transport during a large-magnitud e flood. It appears that studies on sand transport in the Dutch Rhine carried out so far underestimate sand transport during floods. (C) 199 8 John Wiley & Sons, Ltd.