Eigentijds rapport - Faculteit Geowetenschappen - Universiteit Utrecht

Abstract
High discharges in the Dutch Rhine branches have initiated large explorative studies to suggest
alternative ways of river management. These studies need detailed information on the
morphological behaviour of rivers: especially information on the sediment transport dynamics at
river bifurcations is wanted, for possible changes in the sediment distribution at river bifurcations
have implications for all parts of the downstream area. The present study focuses on the heavily
engineered IJsselkop bifurcation, the location where the Pannerdensch Kanaal distributes into the
rivers IJssel and Nederrijn. The IJsselkop is situated in the eastern part of the Netherlands, and is
characterised by bimodal bed sediments (modes at 0.7 mm and 7 mm). The aims of this study
were to determine the influence of discharge variations, weir management and river bed
composition on the amount and size composition of sediment transport, as well as the distribution
of sediment transport over the river width and over the downstream branches of the bifurcation. A
sub-aim was to determine the characteristics of the dunes that caused the bed load transport.
To reach these aims, sediment transport measurements were carried out during two discharge
waves in 2002 and 2004, and during a low-flow period in 2004. The bed load transport was
measured directly using a Delft Nile Sampler (DNS), but also indirectly using multibeam echo
soundings and a dune tracking technique. The suspended load transport (excluding wash load) was
measured using Acoustic Sand Transport Meters (ASTMs). During the DNS and ASTM
measurements samples of the transport load were taken, from which the size composition of the
sediment transport was determined. Together with the sediment transport measurements also
detailed data on the flow velocity, water levels and water discharge were collected.
The transport measurements at the IJsselkop showed that the suspended load transport, which
accounted for most of total transport, strongly varied during a discharge wave. It was largest
during the rising part of the discharge wave, and lowest thereafter. This suggests a declining
source of suspended sediment during the discharge wave. The measurements just upstream of the
bifurcation (in the Pannerdensch Kanaal) showed that the suspended load was almost uniformly
distributed over the river width. This means that the distribution of suspended load sediment over
the downstream branches of the bifurcation almost equalled the water distribution: the percentage
of the total sediment load that entered the river IJssel was about 43% during high discharges and
about 90% during low discharges. The suspended sediment distribution thus strongly differed
between low-flow and high-flow periods, which must be attributed to the weirs in the Nederrijn,
which are closed during low-flow periods and divert the water into the IJssel. Not only the sediment
distribution was different between low-flow and high-flow periods, also the composition of the
suspended load sediment: during high-flow periods the suspended load sediment had a median
grain size of 0.35 mm, while the median grain size during low-flow periods varied about 0.14 mm.
The amount of bed load transport at the IJsselkop closely followed the discharge variations,
except for the river Nederrijn. At the beginning of the Nederrijn the maximum transport occurred
just before the peak discharge, while a few hundred metres downstream the maximum transport
occurred two weeks after the peak discharge. This suggests the presence of a sand wave that
slowly migrated downstream during the discharge waves. The exact origin of the sand wave
remains unclear, but the occurrence of a sand wave clearly illustrates that the sediment transport
at the IJsselkop was supply-limited. Namely, if there were no supply-limitation, the maximum
sediment transport would have occurred at the same moment everywhere in the Nederrijn.
The grain size of the bed load transport material at the IJsselkop was much smaller than the
local bed material at most measurement locations, which implies that the coarsest part of the local
bed material was immobile during the discharge waves. The coarseness of the bed sediments at
the IJsselkop thus has a suppressing influence on the amount of sediment transport and is the
cause of the supply-limited transport conditions. During the discharge wave of 2004, the
distribution of bed load transport over the river width of the Pannerdensch Kanaal was highly
asymmetric, with the lowest values near the right bank, where the bed sediments are very coarse
due to bend sorting processes. This uneven distribution of bed load transport over the river width
also leaded to an uneven distribution of bed load transport over the downstream branches of the
bifurcation. The IJssel, which bifurcates from the right part of the Pannerdensch Kanaal, only
received 11% of the total bed load transport, while the Nederrijn received about 89%, probably
resulting in a higher degree of supply-limitation in the IJssel than in the Nederrijn. During the lowflow
period in 2004 the bed load distribution over the bifurcation was totally different, with only 2%
flowing into the Nederrijn, due to the closure of the weirs.
The dunes that were present on the river bed were relatively small, with width-averaged
heights of 0.1-0.4 m. At the end of the discharge waves small dunes developed on top of the
existing dunes, especially in areas with coarse bed sediment. In all branches, the dune zone was
smaller than the river width, which is in accordance with the idea of supply-limited transport.
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