Abstract
Saw-tooth bars are shore-oblique sand bars that are found along most ebb-tidal deltas of the Frisian Wadden Islands. Although they might significantly affect sediment transport pathways and volumes on the deltas, their general characteristics and dynamics are largely unknown. The main aim of this paper is to determine the typical saw-tooth bar heights, wave lengths, widths, orientations, migration rates and depths of occurrence. To this end, we analysed bathymetries from the Dutch and German Frisian Wadden Islands between 1970 and 2015. Bar heights range between 0.5 and 2 m, and wave lengths range between 300 and 900 m, with an average of 670 m, and the bar crests have a down contour orientation of approximately 25°. The bars are between 800 and 2200 m wide. Saw-tooth bars are located at depths from 3 to 12 m, depending on the slope of the area. Migration speeds of up to 112 m/year were found, using a spatial correlation method. Bar height and migration speed are positively linearly correlated, as well as bar wave length and bar width, bar height and the orientation of the depth contours and migration speed and the orientation of the area. The derived characteristics are compared to those of other bar types to evaluate hypotheses regarding the formation mechanism of the bars.
Highlights
Barrier coasts make up some 10% of the world’s coast line (Stutz and Pilkey 2011)
Barrier coasts consist of a series of islands, intersected by tidal inlets, with flood-tidal deltas on their landward side and ebb-tidal deltas on their seaward side (Hayes 1979, 1980)
The largest bar height is 2.27 m, which was found for 2006 on the ebb-tidal delta of the Ameland Inlet. Both median and maximum bar heights decrease from Ameland to the east
Summary
Barrier coasts make up some 10% of the world’s coast line (Stutz and Pilkey 2011). They are morphologically and ecologically highly dynamic, and understanding their dynamics is vital for managing purposes (Wang et al 2012). Ebb-tidal deltas are relatively shallow and can protect the adjacent islands and their back-barrier basin against high storm wave energy. Their high sediment volumes, between 1 and 100 × 106 m3 (Gaudiano and Kana 2001) can serve as a sediment source for adjacent islands and the back-barrier basin, as suggested for the Dutch Wadden Sea barrier coast by Elias et al (2012).
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