Morphodynamics of Barrier Island Systems

Barrier island systems, also referred to as multiple-inlet systems, are coastal environments with shallow, interconnected tidal basins that are fringed by a chain of elongated islands. Their geomorphology and tidal habitats encompass numerous transitional zones between the land and the sea that are rich in species specially adapted to the varying hydrodynamic conditions. Morphodynamics at mixed-energy barrier island coasts have been studied for several decades on the basis of aerial photographs, field observations and numerical or analytical models. A process-based understanding of the morphological response to the driving hydrodynamic forces, however, has still not been achieved. The aim of this study is to assess the system morphodynamics in response to the interaction of tidally- and wave-induced currents, wind stress and the availability of mobile sediments. The study area is the East Frisian Wadden Sea (Germany), a lagoon-type environment with intertidal flats that are sheltered by seven inhabited barrier islands; it belongs to the Wadden Sea extending along the southern North Sea coast. A state-of-the-art process-based model is applied as a hindcasting and experimental tool for the evaluation of relevant processes at short term (tidal cycle) to medium term (annual) time scales. The spatial scales encompass sand shoals (meso-scale) as typical morphological features at ebb-tidal deltas, to the entire system covering the upper shoreface, the barrier islands and the back-barrier basins (large-scale)

Morphodynamik von Barriereinselsystemen

Barrier island systems, also referred to as multiple-inlet systems, are coastal environments with shallow, interconnected tidal basins that are fringed by a chain of elongated islands. Their geomorphology and tidal habitats encompass numerous transitional zones between the land and the sea that are rich in species specially adapted to the varying hydrodynamic conditions. Morphodynamics at mixed-energy barrier island coasts have been studied for several decades on the basis of aerial photographs, field observations and numerical or analytical models. A process-based understanding of the morphological response to the driving hydrodynamic forces, however, has still not been achieved. The aim of this study is to assess the system morphodynamics in response to the interaction of tidally- and wave-induced currents, wind stress and the availability of mobile sediments. The study area is the East Frisian Wadden Sea (Germany), a lagoon-type environment with intertidal flats that are sheltered by seven inhabited barrier islands; it belongs to the Wadden Sea extending along the southern North Sea coast. A state-of-the-art process-based model is applied as a hindcasting and experimental tool for the evaluation of relevant processes at short term (tidal cycle) to medium term (annual) time scales. The spatial scales encompass sand shoals (meso-scale) as typical morphological features at ebb-tidal deltas, to the entire system covering the upper shoreface, the barrier islands and the back-barrier basins (large-scale)

Water level time series and sediment grain size model

The environment of ebb-tidal deltas between barrier island systems is characterized by a complex morphology with ebb- and flood-dominated channels, shoals and swash bars connecting the ebb-tidal delta platform to the adjacent island. These morphological features reveal characteristic surface sediment grain-size distributions and are subject to a continuous adaptation to the prevailing hydrodynamic forces. The mixed-energy tidal inlet Otzumer Balje between the East Frisian barrier islands of Langeoog and Spiekeroog in the southern North Sea has been chosen here as a model study area for the identification of relevant hydrodynamic drivers of morphology and sedimentology. We compare the effect of high-energy, wave-dominated storm conditions to mid-term, tide-dominated fair-weather conditions on tidal inlet morphology and sedimentology with a process-based numerical model. A multi-fractional approach with five grain-size fractions between 150 and 450 µm allows for the simulation of corresponding surface sediment grain-size distributions. Net sediment fluxes for distinct conditions are identified: during storm conditions, bed load sediment transport is generally onshore directed on the shallower ebb-tidal delta shoals, whereas fine-grained suspended sediment bypasses the tidal inlet by wave-driven currents. During fair weather the sediment transport mainly focuses on the inlet throat and the marginal flood channels. We show how the observed sediment grain-size distribution and the morphological response at mixed-energy tidal inlets are the result of both wave-dominated less frequent storm conditions and mid-term, tide-dominant fair-weather conditions