Follow the sediment
Researchers at the Water, Climate and Future Deltas hub closely follow the sediment along the river in the Rhine-Meuse delta, from river bifurcations on the delta plain, along the floodplains, through the estuary and along the coast.
Sediment history of the Rhine-Meuse delta
The Rhine-Meuse delta, as existing today, is the product of 9000 years of development. By creating an extremely high-resolution database of the Rhine-Meuse delta, researchers at the hub have reconstructed its sediment history and development over thousands of years.
Splitting the river in two
River bifurcations, such as the point where the Pannerden Canal divides into the Lower-Rhine and the IJssel, are so-called hotspots as they are important points in the system where the distribution of the incoming water and sediment over the delta channels is determined. They therefore also influence the distribution of flood risk in the delta. Maarten Kleinhans, professor of rivers and estuaries, explains, "A bifurcation becomes unstable if one channel receives less sediment than its transport capacity, so that it erodes, and the sediment feed into the other channel exceeds its transport capacity, so that it shallows". The Pannerden Canal was dug in 1709 to bypass one such blocked channel.
Stability of bifurcations
PhD researcher Arya Iwantoro studies the stability of river bifurcations in river-dominated vs tide-influenced delta systems. He says, “Tidal bifurcations are in double trouble: their stability depends on the upstream river and on how strong the tidal influence is relative to river discharge”. The tidal influences are not only determined by the amplitude of tide coming from the sea but also by the downstream depth and width of the estuary. Deltas with strong tidal influence, such as the Mahakam delta in Indonesia, have more stable bifurcations. New modelling results show that tides stabilise bifurcations due to a subtle balance between river sediment and tidally driven transport.
Sediment deposition over time
Floodplains are areas of land adjacent to rivers which experience flooding during high discharge events. After the embankment of river branches between 1100 and 1300 AD to protect against flooding, the amount of sediment trapped in the floodplains was about 1.9 million tonnes per year (including re-entrainment of previously deposited sediment). Since the width of channels 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 reduced to 1.2 million tonnes per year. The present-day trapping of sediment by the embanked floodplains has decreased to 0.30 million tonnes per year which is only 18% of the sediment load entering the delta. Sedimentation rates on the Waal floodplains are generally higher than along the two lower Rhine branches, with the largest trapping of suspended sediment occurring in the Rhine estuary.
River branches and estuary
Sediment enters the Rhine-Meuse delta via the Rhine and Meuse rivers, and it roughly follows the direction of the river flow from east to west. In the upstream sections, the sediment is relatively sandy, whereas towards the river mouth (Maasmond) the sediment composition changes to muddy, silty sediment. The sandy sediment is often removed and used as a building material, leaving only muddy sediment in the rivers, which flows towards the sea. The rest of the muddy sediment comes in with the tides from the North Sea. The sediment ends up in the ports and harbours in the mouth area near the Port of Rotterdam, in the south-western part of the Netherlands. This sediment is then removed by dredging for navigation of large ships, leading to lowering of the beds through time.
Net loss of sediment
Extensive, year-round dredging is undertaken in the Rhine-Meuse estuary to keep the network of inland ports and harbors open to shipping. In the Rotterdam port area and access channels alone, ~15-20 million m-3 is dredged annually. This dredging trend is driven by increasing global ship size which require deeper channels, ports and harbors. “Despite a potential increase in the flux of coastal and riverine sediments to the Rhine-Meuse delta, dredging will overwhelm both these components if current dredging trends continue”, explains Jana Cox, PhD researcher who studies the response of sediment to climate change and sea-level rise in the lower Rhine and Meuse rivers, “a trend that we see in many estuaries and deltas worldwide". For 2050, Jana Cox found a negative sediment budget under all climate scenarios caused by extensive dredging (Storyline: The future of the Rhine-Meuse delta). A negative sediment budget can cause damage to flood infrastructure, bank instability, loss of intertidal ecosystems and bed erosion.
Composition is crucial
The composition of sediment in the Rhine-Meuse delta is important, not only for the type and cost of dredging, but also to determine where dredging material can be used, relocated or disposed. Since the 1990s, the overall trend has been an increase in the delivery of mud and silt to the system and a decrease in sand. "This is problematic”, explains Jana Cox, “because mud and silt tend to flocculate more easily with chemical waste and runoff and often cannot be reused, whereas sand tends to act as a filter”. The dredged mud is disposed of offshore or, if it is contaminated, it must be stored. Another problem with the decrease in sand as a stable building material is that it negatively impacts the capacity of the Rhine-Meuse delta to deal with relative sea-level rise.
Highly dynamic system
The coast in the Rhine-Meuse delta is characterized by a continuous row of dunes, without tidal inlets. Like deltas, sandy coasts are highly dynamic. They continually change in response to waves, currents and wind. Storm events can rapidly erode the dunes as energetic waves collide with the dunes and currents transport the eroded sand into the sea. The recovery of beaches and dunes is a much more gradual process, taking weeks, months or even years.
Model simulations show that sea-level rise and changes in storm, wave and surge characteristics enhance dune erosion. Under sea-level rise projections of one meter, erosion at Egmond aan Zee may be as high as 80 m-3 of sand per linear meter of beach, whereas at Noordwijk, erosion may be up to 52 m-3 of sand per linear meter of beach. Considering this trend, it is necessary to increase the size of sand nourishments to preserve flood safety levels of the Holland coast (Relocating sediment).
While it is possible to predict dune-erosion processes, this is not the case for dune growth
"While it is possible to predict dune-erosion processes to a certain extent, this is not the case for dune growth – especially on longer time scales", says Gerben Ruessink, professor of wave-dominated coastal morphodynamics. To address this, Gerben Ruessink and colleagues study wind-driven (aeolian) sand transport and the many factors that influence this transport, such as wind characteristics, the wetness of the sand and the shape of the beach profile. They compared field data collected at Egmond aan Zee with the aeolian sand transport model Aeolus and found that the model predicts the growth of the most seaward dune accurately during a four-year period. The applicability of Aeolus to other sites with different dune and beach characteristics is currently being assessed.
Muddy Wadden Sea
The Dutch Wadden Sea in the north of the Netherlands is a dynamic ecosystem with tidal flats, channels and salt marshes that naturally protects the mainland against flooding by reducing wave-heights. Maarten van der Vegt, associate professor of coastal morphodynamics, explains, “With every ebb and flood, large amounts of sediment are exchanged between the North Sea and the Wadden Sea, but the net values are only small".
Drowning tidal basins
Over the period 1935-2005 the sediment volume in the Wadden Sea increased by ~600 million m-3, which until now was more than sufficient to compensate for sea-level rise. However, projections for the Wadden Sea show a potential drowning of the tidal basins due to rapid sea-level rise, local sea-floor subsidence (both natural and anthropogenic) and insufficient sediment import.
Modelling morphodynamic development
Research at the Utrecht University on the Wadden Sea focusses on modelling the morphodynamic development of various parts of the Wadden Sea such as tidal inlets, barrier islands, sand bars and ebb-tidal deltas and how this impacts hydrodynamics and sand transport. PhD researcher Klaas Lenstra studied the long-term cyclic behavior of ebb-tidal deltas which are shallow features seaward of tidal inlets. He found that the cyclic dynamics of sand bars and tidal channels cause variations in the wave energy reaching the coast and the sediment exchange between the North Sea and the Wadden Sea. This directly affects the coastal safety functions of the ebb-tidal deltas.