The presence and dynamics of sawtooth bars along the ebb-tidal deltas of the wadden islands

Abstract

Sawtooth bars are located at the downstream side of the ebb-tidal deltas in the Dutch and German Wadden Sea. According to Brakenhoff et al. (2019), they are bar patterns with a down contour orientation of ±25° and with a typical wavelength of 300 to 900 meters. Their crest length is 800 to 2200 meters and they occur at a depth between 3 and 12 meters. Sawtooth bars are 0.5 to 2 meters high and migrate between 2 and 112 meters per year. However, the generation mechanism and the morphological evolution of sawtooth bars have not been studied. Therefore, the objective of this study is to clarify the generation mechanism and the morphological drivers of sawtooth bars.

A Delft3D model was set-up in which the modules FLOW and WAVE (SWAN) were used to model the hydrodynamics and morphodynamics in the study area with the forcing of tides, waves, and wind. Two types of simulations were accomplished, simulations with prescribed sawtooth bars and simulations with a random small amplitude perturbation between ±0.25m in which bars could naturally develop. The simulations with prescribed sawtooth bars were accomplished to study the sensitivity of the bar characteristics and forcing mechanisms on the bar dynamics. The simulations with a random small amplitude perturbation, were accomplished to research the feedback mechanisms that result in the natural development of sawtooth bars. These runs were forced by wind, waves, and tides of which only the significant wave height was a variable.

The main results show that the alongshore flow, driven by tides, waves and wind, is altered in its direction and magnitude due to the presence of sawtooth bars. As a result of the flow acceleration on the bar crests and the deceleration in the bar troughs, in combination with the down contour orientation of the bars, the resulting tidally averaged flow pattern attempts to follow the depth contours. This flow pattern results in a sediment transport pattern that causes erosion near the updrift side of the bar crest and deposition near the downdrift side of the bar crest. Sediment transport is mainly caused by the forcing of waves in the sawtooth bar area, which is overall suspended load dominated.

Overall, this study shows that sawtooth bars can be generated by stormy weather with high significant wave heights in combination with local high-angle waves, and that bar decay can be a result of fair-weather conditions. More research is necessary on the generation mechanism to find out what the physical key is for bar growth and decay, but this study gives valuable insights to accomplish this.