Washover inundation and barrier island accretion

Abstract

The overall aim of this PhD-thesis was to improve the understanding of the role of the washover openings on the Wadden Islands on the storm-induced and cross-shore directed sediment transport on the short- (event scale) as well as medium-term (annual to decadal time scale). The Wadden Islands form a chain of barrier islands, located in the North Sea along the coast of the Netherlands, Germany and Denmark. Wadden Islands in a natural state often are characterized by the presence of several washover openings. In the past though many of these openings were closed off by artificial sand-drift dikes. This measure prevents the area behind the (artificial) dunes from flooding, but has as side effect that potential onshore-directed and storm-induced sediment transport is blocked. Coastal zone management organizations in the Netherlands consider the re-opening of the washovers by removing parts of the sand-drift dikes and it is hypothesized that this might lead to vertical accretion behind the dunes. The influence of wave, tide and storm surge conditions on sediment transport across the beach of a typical mesotidal Wadden Island during inundation was explored with a morphostatic 1D XBeach model study (Chapter 2).The medium-term sediment transport across the barrier island was studied. Six representative inundation classes were distinguished, ranging from frequently occurring, low-energy events to infrequent, high-energy events, and the hydrodynamics and sediment transport during these events were simulated. An analysis of the model simulations showed that larger storm events cause larger cross-shore sediment transport due to stronger currents and larger waves. However, the net onshore directed sediment transport during a storm levels off or even becomes smaller for the largest inundation classes because it is counteracted by larger mean water levels in the Wadden Sea (back-barrier). This opposes or even reverses sediment transport during inundation. The effects of the geometry of the washover openings on hydrodynamics and sediment transport during inundation were investigated with a morphostatic 2D XBeach model study (Chapter 3). Based on the XBeach simulations, two important effects of washover width were identified: firstly, for narrow openings flow contraction is important, causing relatively larger sediment exchange rates per unit width. Secondly, in a wider opening sediment is transported over a larger width, resulting in larger sediment mass exchange rates. These effects combined lead to a larger sediment load that is transported through a washover opening when the opening is wider. Morphological developments (25 years) were explored in a morphodynamic 2D XBeach model study (Chapter 4). Results showed that inundation events erode the washover openings with values of more than 0.5 m in total and deposit this sediment onshore. When a high rate of sea-level rise is taken into account (10 mm/y), deposition values increase with approximately 60%, and likewise vertical accretion rates increase as well. However, the accommodation space created by sea-level rise is not completely compensated by washover deposits. The natural evolution of a barrier island in absence of coastal management, and specifically the evolution of washover openings and the landward supply of sediment was investigated in a case study at the island of Rottumeroog (Chapter 5). The island is characterized by a natural washover, interrupting the dune system. The first years since 2005 were characterized by strong erosion of the beach and foreshore, followed by the development of a 600 m wide breach in the dunes. This breach developed into a washover and has been active since then. Bed level elevation in the washover increased with values up to 0.8 m, reflecting cross-shore supplies of sediment in a single storm season of about 62 m^3/m (volume per meter coastline).