Sedimenttransport op de Merwedekop tijdens de hoogwaterperiode van 2004

Abstract

High discharges in the Dutch Rhine branches have initiated large 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 the downstream area. The present study focuses on the heavily engineered Merwedekop bifurcation, the location where the Bovenmerwede splits into the rivers Benedenmerwede and Nieuwe Merwede. The Merwedekop is situated in the SW part of the Netherlands, and is characterised by sandy bed sediments (median grain size about 0.5 mm) and tidal water level variations. The aims of this study were to determine the influence of discharge variations, tides and riverbed 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 the discharge wave of January 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 suspended load transport, which accounted for most of total transport, strongly varied during the discharge wave. It was largest during the rising part of the discharge wave, and lowest thereafter. Also the composition of the suspended load changed during the discharge wave: at the peak discharge the suspended load sediment had a median grain size of 0.34 mm, while at the end of the discharge wave the median grain size was only 0.15 mm. There are indications that the suspended load transport in the Bovenmerwede was slightly larger in the left part of the river (inner bend) than in the right part of the river (outer bend), probably because the relatively coarse bed material in the outer bend (D50 about 0.6 mm) could not easily be transported in suspension. The suspended sediment in the outer bend was also slightly coarser than the suspended sediment in de inner bend. The distribution of suspended load sediment over the downstream branches of the bifurcation could not be determined, because no measurements were performed in the downstream branches. The bed load transport at the Merwedekop closely followed the discharge variations, with slightly larger transports during declining flow. The median grain size of the bed load transport in the Bovenmerwede ranged from 0.4 (inner bend) to 0.6 mm (outer bend). Though detailed information on the local riverbed composition was lacking, it seemed that all grain size fractions in the riverbed were mobile during the 2004 discharge wave. The highest transport values therefore occurred on the location with the highest flow velocities (outer bend), not on the location with the finest bed sediment (inner bend). There were no indications for the occurrence of supply-limited transport or sediment waves, like at the IJsselkop and Pannerdensche Kop bifurcations. The total bed load transport in the Bovenmerwede was markedly larger than the summed transports of the Nieuwe Merwede and the Benedenmerwede. This can result from methodological errors, but also from sedimentation around the bifurcation. The distribution of bed load sediment over the Nieuwe Merwede and the Benedenmerwede was about 1:2 at low discharges and 2:1 at high discharges. This is because the Haringvliet sluices (downstream of the Nieuwe Merwede) limit the water and sediment discharge through the Nieuwe Merwede at low discharge conditions. Tidal water level variations caused changes in the suspended load transport and the bed load transport at the Merwedekop of about 50%. In the Benedenmerwede and the Bovenmerwede, the highest transports occurred during low tide and during outgoing tide, when the flow velocity was highest. The lowest transport occurred during incoming tide, when the flow velocity was lowest. In the Nieuwe Merwede the situation was exactly reverse, because of the Haringvliet sluices. While the water discharge through the Benedenmerwede and the Bovenmerwede is hampered during incoming tide, resulting in low flow velocities and low transports, the water discharge of the Nieuwe Merwede is promoted, causing high flow velocities and high transports. The dunes that were present on the riverbed were relatively small, with a width-averaged height of about 0.2 m and a length of about 6 m. The dimensions of the dunes hardly varied spatially, but the dunes became slightly larger during high discharges. Because of the tidal influence, the average migration rate of the dunes was relatively low