Effects of storms on short and medium-term morphodynamics of a tide-dominated coastal region

Etri, Talal

A storm event over shallow nearshore coastal waters or shallow inland water bodies can generate large water fluctuations if the storm is sufficiently strong. It can cause both a rise (set-up) and fall (set-down) of the water level. Generally, as a result of the water level fluctuations and the associated changes in the wave conditions the morphology will change at temporal and spatial scales. The main concern of this work is to study the storm events and their effects on the short- and the medium-term morphological developments in the central Dithmarschen Bight, German North Sea. In this study, the morphological changes on the short- and the medium-term will be a result of a singular event from a few hours to days and from a few weeks to months, respectively. In previous works, the individual process models (flow, wave and sediment transport) have been extensively calibrated and validated against measurements. In this work a morphodynamic model for the Dithmarschen Bight region has been setup. The morphodynamic model is based on Delft3D model developed by WL-Delft Hydraulics in which the on-line approach was applied. The Dithmarschen Bight morphodynamic model implies a morphological acceleration factor. Moreover, the sediment transport model concerns the sediment distribution for cohesive and non-cohesive sediments for the Dithmarschen Bight. In the short-term morphodynamic model the simulations were obtained by nesting the Dithmarschen Bight Model (DBM) in the German Bight Model (GBM), which in turn is nested in the northwest European Continental Shelf Model (CSM). For the Dithmarschen Bight model the bathymetry update takes place every computing time step of the flow simulation and the analysis was focused on two severe events that took place in January 1994 and December 1999. In the medium-term morphodynamic model an input filtering approach has been applied in which a limited number of representative conditions by means of tides, wave climates and storm conditions were defined. The interactions between the flow, sediment and wave models were obtained every time step of the flow model, however, the bathymetric update will take place each morphological acceleration factor value. The performance of the short-term morphodynamic model showed that the maximum morphological changes take place during the storm event (the peak of the storm) with about 1m depth of erosion in the Tertiussand sandbank at most. It has been also noted that the morphological changes due to the tide alone were relatively small within the period considered. The morphological changes due to the two storms events indicate that more significant changes took place during the storm of December 1999 and lower morphological activity was observed from the storm of January 1994. The medium-term morphodynamic model showed that the tide is the main driving force for the initiation of the tidal channels and the storms are responsible for the erosion of the tidal flats. The model also showed no indication that the storm sequences and the chronologies of the wave climates have major effects on the morphodynamics for the simulation period. The evaluation of the differences between the cases simulated including only the tides and those including tides and storms together showed that there are increases in erosion and sedimentation of about 0.4m in the Tertiussand sandbank and about 1m in the Piep tidal channel after including the storms.

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Etri, Talal: Effects of storms on short and medium-term morphodynamics of a tide-dominated coastal region. 2007.

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