Deposition of sediment and associated heavy metals on floodplains

Abstract

In recent years, floodplains have become widely acknowledged as important natural sinks for sediments and associated substances like nutrients, PAHs, PCBs and heavy metals. The character of floodplains will change in the near future because of landscaping measures (excavation of secondary channels, lowering floodplains and removing minor embankments) and the rehabilitation of floodplain forests. In addition, climate and upstream land-use change will change the discharge frequency distribution and the suspended sediment concentrations. These three groups of changes affect the patterns and processes of sediment and associated heavy-metal deposition on floodplains.

We studied the key factors behind the process of floodplain deposition, the present deposition patterns and how the patterns and processes will change due to the environmental changes. We firstly measured patterns, amounts and characteristics of floodplain deposition during inundations of several floodplains along the River Rhine branches in The Netherlands. The results of these measurements showed the strong influence of inundation hydrodynamics, river works and floodplain morphology on the deposition patterns. Next, we measured settling velocities of suspended matter during these inundations using a LISST-ST (a laser particle sizer and settling tube), which delivered an effective settling velocity of 0.000067 m/s. We subsequently used this settling velocity in a floodplain deposition model (MoCSED) that uses the ‘Method of Characteristics’ to simulate transport and deposition of sediment on floodplains. Since the model results were promising, we used the model to calculate scenarios for two selected floodplains experiencing floodplain rehabilitation in the near future. In one of them, a floodplain lake will be excavated, while in the other a secondary channel will be constructed. In addition, we studied the effect of set-aside practices in the Rhine basin and more frequent peak discharges due to climate change on the deposition of sediment and associated heavy metals.

We conclude from this work that the conveyance loss of sediment on the Waal River floodplains will rise under climate change scenarios but will decrease when set-aside is included. The combination of climate change, set-aside and floodplain rehabilitation will result in the highest increase in conveyance losses (now 5 % for metals and 7 % for sediment). The sediment (now 1.5 kg/m2 per year for Waal River floodplains and 2.3 kg/m2 per year for IJssel River floodplains) and metal deposition rates will not change under that combination. This shows that when assessing effects of environmental changes on floodplain deposition both conveyance losses and deposition rates should be studied.