A contribution to understanding the recently enhanced coastal siltation in the German Wadden Sea
Creators: name: Lepper, Robert
Publisher: name: Institut für Wasserbau (TU Hamburg)
Year: 2023
Subjects: subject: North Sea
subject: Wadden Sea
subject: sea level rise
subject: siltation
subject: tidal flats
subject: tidal asymmetry
subject: sediment transport
subject: morphodynamics
classificationCode: 550
subject: Earth Sciences, Geology

description: The Wadden Sea on the Northwestern European Shelf is world's largest channel-shoal system that comprises broad coherent intertidal flats, islands, shoals, and several estuaries. Coastal management has shaped the Wadden Sea's coastline and its estuaries for centuries which is why navigational channel dredging and coastline maintenance have become routine coastal engineering tasks. Since the 2010s, German coastal stakeholders reported further enhanced coastal siltation. My research objectives were (1) to identify whether there was quantifiable evidence of enhanced coastal siltation and (2) to find the driving processes behind this development. An analysis of bathymetry data confirmed that intertidal flats accreted and that lateral expansion and subtidal deepening occurred. Balanced subtidal and intertidal sediment budgets suggested a redistribution of local subtidal sediments to intertidal flats. In addition, surface sediment data indicated a gradually increasing mudflat coverage over time. Based on this information, numerical modeling and observational data were used to quantify changes in tidal and non-tidal forcing: I found near-constant wind-wave and storm surge conditions which indicated that event-driven phenomena were an unlikely driver. Spatial tidal analysis and observational data revealed amplitude and phase shifts of principle and shallow water tidal constituents which reflected in increased tidal amplitude, and reduced flood dominance. Furthermore, diminishing flood flow velocity and increased slack duration were noted. These phenomena were related to increased relative intertidal storage and to subtidal deepening from bathymetry evolution. Hence, the reduction in channel volume and the intertidal accretion likely decreased tidal and wave energy on the tidal flats and enabled the deposition of more fines. Longer flood and ebb slack duration should add to this effect. The import of sand into the coastal zone was likely reduced as flood dominance diminished at the entire study site.
description: Die Einwirkung des Menschen auf das weltweit einzigartige Küstensystem „Wattenmeer“ hat in den letzten Jahrhunderten die Küstenlinie und Morphologie dieses Systems dauerhaft verändert. Stakeholder und Hafenbetreiber im Küstenbereich beklagen seit Jahrzehnten stetig zunehmende Verschlickung. Die Aufgabe dieser Dissertation war es (1) Nachweise für zunehmende Verschlickung im Zeitraum von 1996 bis 2016 zu finden und (2) mögliche Wirkungsketten zu untersuchen. Eine Topographie-Analyse bestätigte dies zunächst durch die Zunahme der mittleren Watthöhe und des Sedimentvolumens im Wattbereich. Diese morphologischen Veränderungen hatten Auswirkungen auf die Eigenschaften der Tide, insbesondere durch abnehmende Strömungsgeschwindigkeiten, längere Stauwasserdauer und abnehmende Flutdominanz. Eine Verringerung des Tiderinnenvolumens und höher liegendes, breiteres Watt verringerte voraussichtlich die Energie von Wellen und Tide, was Verschlickung begünstigt.