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Abstract:The behaviour of suspended sediment in rivers is often a function of energy conditions, i.e. sediment is stored at low¯ow and transported under high discharge conditions. The timing of maximum sediment transport can, however, also be related to mixing and routing of water and sediment from dierent sources. In this study suspended sediment transport was studied in the River Rhine between Kaub and the German±Dutch border. As concentrations decrease over a runo season and as the relationship between water discharge and suspended sediment concentrations during most¯oods is characterized by clockwise hysteresis, it is concluded that sediment depletion occurs during a hydrological year and during individual¯oods. However, analyses of the sediment contribution from the River Mosel indicate that clockwise hysteresis may result from sediment depletion as well as from early sediment supply from a tributary. Thus, although the suspended sediment behaviour in the downstream part of the River Rhine is partly a transport phenomenon related to energy conditions, mixing and routing of water from dierent sources also plays an important role.Suspended sediment transport during¯oods was modelled using a`supply-based' model. Addition of a sediment supply term to the sediment rating curve leads to a model that produces better estimates of instantaneous suspended sediment concentrations during high discharge events. A major constriction of the model is that it cannot be used to predict suspended sediment concentrations as long as the amount of sediment in storage and the timing of sediment supply are unknown.

This paper presents the result of measurements of floodplain sedimentation using sediment traps. The study was carried out on two embanked floodplains along the Rivers Rhine and Meuse in The Netherlands during a 3 day flood in January 1993. Raster maps of sediment accumulation were made by interpolating the measurements from the traps using block kriging. The sediment maps show clear patterns in sediment accumulation, together with the estimated interpolation errors. Average sediment accumulation ranges between 0.57 and 1.0 kgm-2. High sediment accumulation is found on the levees (4kgmp2 or more) and on low lying areas (1.6 kgmp2); sediment accumulation decreases with distance from the main channel. The sedimentation patterns are related to floodplain topography and sediment transporting mechanisms. Sediment transport by turbulent diffusion as well as by convection can be recognized. Also, flood duration and the process of sediment settling out in ponding water in closed depressions are important. The applied method allows comparison of the results with raster-based sedimentation models.

Abstract:The objective of this study was to estimate the potential effects of changes in climate and land use on the mobilization of fine sediment and the net transport of wash load from the upstream basin to the lower Rhine delta. For this purpose, a suite of geographical information system-embedded models was developed that simulates the production, and transport of wash load through the drainage network and deposition on floodplains along the lower river reaches. The model results indicate that if climate changes in accordance with the UKHI climate-change scenario, in combination with land use changes, erosion rates will increase in the Alps and decrease in the German part of the basin. Averaged over the entire basin, erosion will increase by about 12%. However, due to inefficient sediment delivery, increasing erosion in the Alps will have little effect on the sediment load further downstream. In the delta area, sediment loads are expected to decrease by 13%. When changes in river discharge are accounted for, it appears that, although very high discharges are expected to occur more frequently, sedimentation on floodplains tends to decrease. This is caused mainly by reduced sediment loads at discharges during which the floodplains are just inundated and trapping efficiencies are high.

This article addresses spatial variability of comtemporary floodplain sedimentation at the event scale. Measurements of overbank deposition were carried out using sediment traps on 11 floodplain sections along the rivers Waal and Meuse in The Netherlands during the high-magnitude flood of December 1993. During the flood, sand sheets were locally deposited behind a natural levee. At distances greater than 50 to 100 m from the river channel the deposits consisted mainly of silt-and clay-sized material. Observed patterns of deposition were related to floodplain topography and sediment transporting mechanisms. Though at several sites patterns were observed that suggest transport by turbulent diffusion, convection seems the dominant transporting mechanism, in particular in sections that are bordered by minor embankments. The average deposition of overbank fines ranged between 1·2 and 4·0 kg m −2 along the river Waal, and between 1·0 and 2·0 kg m −2 along the river Meuse. The estimated total accumulation of overbank fines (not including sand sheets) on the entire river Waal floodplain was 0·24 Mton, which is 19 per cent ot the total suspended sediment load transported through the river Waal during the flood.