Gerald Herrling

My research project deals with the short- and medium-term morphodynamic response of barrier island systems in the Wadden Sea to extreme events such as storm surges in comparison to normal hydrodynamic conditions.

Storm induced surges temporally increase the tidal prism, resulting in high energy flows in the tidal inlet and adjacent channels. Waves on depth-limiting ebb tidal shoals, bars or beaches together with enhanced alongshore and transversal currents can be considered as the main drivers for short-term morphological changes. Nowadays, the driving forces of these extreme events can be measured and modeled reasonably well, but the underlying morphological development is still poorly understood due to a lack of suitable field observations.

Process-based numerical models offer an alternative means of improving our understanding of the underlying physical processes. Coupled hydro- and morphodynamic models including the interaction of waves are applied to hindcast extreme events and be used to predict sediment pathways and bed elevation changes under complex interacting processes. However, the modelling approach is very different whether an extreme event of the duration of hours to a few days is aimed to reproduce or the long-term morphological response under fair-weather conditions is simulated in time scales of several months to years.

Fig.1: In the case of storm-induced surges the model boundary conditions such as astronomical tides, waves and meteorological data, i.e. non-stationary wind and pressure fields, are based on meteorological (German Weather Service) and hydrodynamic models of a spatially larger extent.
For the underlying study, a cascade of hydrodynamic models with decreasing spatial sizes and increasing numerical resolutions was set-up starting far outside the area of interest in order to transfer the forcing conditions from the open sea to the East Frisian coast.

Fig. 3: Near-shore morphological features as the shore-oblique sand bars connecting the sediment bypass from the ebb delta shoal to the beach as well as the shoreface-connected ridges in water depths of about 15-20m will be studied with respect to their functioning in sediment transport processes under extreme events and fair-weather conditions.

The investigation is part of the joint research project WIMO (www.wimo-nordsee.de) which aims to develop innovative scientific monitoring concepts for the model region German Bight. Two ministries in Lower Saxony, the Ministry for Environment and Climate Protection and the Ministry for Science and Culture, are financing the project in equal parts.