Abstract
Purpose At the land–ocean interface, large river deltas are
major sinks of sediments and associated matter. Over the
past decennia, many studies have been conducted on the
palaeogeographic development of the Rhine delta and
overbank deposition on the Rhine floodplains. This paper
aims to synthesise these research results with special focus
on the amounts and changes
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of overbank fines trapped in
the Rhine delta and their controls at different time scales in
the past, present and future.
Materials and methods Sediment trapping in the Rhine
delta throughout the Holocene was quantified using a
detailed database of the Holocene delta architecture.
Additional historic data allowed the reconstruction of the
development of the river’s floodplain during the period of
direct human modification of the river. Using heavy metals
as tracers, overbank deposition rates over the past century
were determined. Measurements of overbank deposition
and channel bed sediment transport in recent years, together
with modelling studies of sediment transport and deposition
have provided detailed insight in the present-day sediment
deposition on the floodplains, as well as their controls.
Results and discussion Estimated annual suspended sediment
deposition rates were about 1.4×109 kg year–1 between 6,000
and 3,000 years BP and increased to about 2.1×109 kg year–1
between 3,000 and 1,000 years BP. After the rivers were
embanked by artificial levees between 1100 and 1300 AD,
the amount of sediment trapped in the floodplains reduced to
about 1.16×109 kg year–1. However, when accounting for reentrainment
of previously deposited sediment, the actual
sediment trapping of the embanked floodplains was about
1.86×109 kg year–1. Downstream of the lower Waal branch
an inland delta developed that trapped another 0.4×
109 kg year–1 of overbank fines. Since the width of channel
was artificially reduced and the banks were fixed by a regular
array of groynes around 1850, the average rates of deposition
on the embanked floodplains have been 1.15×109 kg year–1.
Scenario studies show that the future sediment trapping in the
lower Rhine floodplains might double.
Conclusions The variations in amounts of sediment trapped in
the Rhine delta during the past 6,000 years are largely
attributed to changes in land use in the upstream basin. At
present, the sediment trapping efficiency of the floodplains is
low and heavily influenced by river regulation and engineering
works. Upstream changes in climate and land use, and direct
measures for flood reduction in the lower floodplains, may
again change the amounts of sediments trapped by the lower
floodplains in the forthcoming decades.
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