Morphological and sedimentological response of a mixed-energy barrier island tidal inlet to storm and fair-weather conditions

Abstract

Abstract. 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 Langeoog and Spiekeroog in the southern North Sea has been chosen here as an exemplary 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 graduated grain-size fractions between 150 and 450 microns allows 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-dominant less frequent storm conditions and mid-term tide-dominant fair-weather conditions.