Shoal margin collapses in the Western Scheldt estuary (2016-2019)

Shoal margin collapses of several million cubic meters have occurred in the Western Scheldt estuary, the Netherlands. Channel bank failure and collapses of shoal margins (flow slides) have been recorded systematically in Dutch estuaries for the past 200 years. Between 1800 and 1978 more than 1000 large failures with sediment volumes up to a million cubic meters were documented in soundings of the Eastern and Western Scheldt estuaries (Wilderom, 1979). In the last decade on average five collapse occurred per year. In many locations collapses reoccur at intervals of several years to decades. Flow slides are subdivided theoretically into two different types of underwater failure processes: rapid flow slides due to liquefaction and slow due to retrogressive breaching. While these collapses involve significant volumes of material, their effect on the channel-shoal morphology is unknown. The objective of this study is to investigate where shoal margin collapses occur, what shoal margin collapses volume are, predict shoal margin collapse locations in the Western Scheldt, and quantify its effect on the channel-shoal morphodynamics.

This study focuses on shoal margin collapses in the Western Scheldt. The Western Scheldt is located in the southwestern part of the Netherlands (51 deg 41'51"N, 5 deg 40'35"E) and is the seaward section (60 km) of the 200 km tide-dominated Scheldt estuary. The Scheldt is a well-studied and monitored estuary that provides access to various harbors, of which the port of Antwerp (Belgium) is the largest. The Western Scheldt is characterized by a convergent geometry and has a well-developed system of channels and shoals. In this study, we used two Delft3D schematizations that are both based on the NeVla-Delft3D model of the Scheldt estuary, which includes the upstream Flemish branches of the estuary, the Western Scheldt and part of the North Sea.

This project is close collaboration with D.R. Mastbergen, G.A. Van den Ham, J.J. van der Werf and M. Taal (Deltares), M. Schrijver, G-J. Liek (Rijkswaterstaat), J. Cleveringa (Arcadis), T. Bouma (NIOZ) and P. Dankers (RoyalHaskoningDHV).

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Dredging and Disposal in the Western Scheldt estuary

Estuaries worldwide are important centres of transportation and international commerce and most are continuously dredged since the early 20$^{th}$ century with an acceleration of activity in recent decades. This use also poses considerable issues. Continuous dredging is needed to maintain a minimum depth of the shipping fairways so that the large commercial and trade vessels can access major ports, e.g., Yangtze Estuary (Shanghai), Western Scheldt (Antwerp), Elbe Estuary (Hamburg). Dredging activities affect the hydrodynamics of estuaries. For example, tidal amplification increases circulation and increases flood-dominance of tidal asymmetry. It is site-specific which hydrodynamic processes dominate and how these affect sediment transport and morphodynamics of the system. For example, dredging activities can lead to a shift from a multi-channel system to a single-channel or loss of ecologically valuable intertidal flats\cite. Yet, it remains undiscovered what long-term effects current dredging and disposal strategy have on the sustainability of the multi-channel system, tidal asymmetry and tidal bifurcations and what changes occur to individual morphological features (shoals and channels) over time.

We (M.R. Hiatt and M.G. Kleinhans) developed a new network extraction approach (with Willem Sonke, Kevin Verbeek and Bettina Speckmann of TU Eindhoven), which we used to determine the development of

  • the multi-channel network
  • the shoal (bar) size over time
  • the flow field network during a tidal cylce
  • the tidal asymmetry through the channel network
  • the bifurcation asymmetry