Observations on decadal sandbar behaviour along a large‐scale curved shoreline

J Visscher,B.G Ruessink,T Schlurmann,R Gijsman

doi:10.1002/esp.5041

J Visscher, B.G Ruessink + Show 2 more

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https://doi.org/10.1002/esp.5041

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Abstract

AbstractNearshore sandbars are characteristic features of sandy surf zones and have been observed with a variety of geometries in cross‐shore (e.g. location) and longshore direction (e.g. planform). Although the behaviour of sandbars has been studied extensively on spatial scales up to kilometres and timescales up to years, it remains challenging to observe and explain their behaviour on larger spatial and temporal scales, especially in locations where coastline curvature can be prominent. In this paper, we study a data set with 38 years of coastal profiles, collected with alongshore intervals of 50 m, along the 34 km‐long curved sandy shoreline of Sylt island, Germany. Sylt's shoreline has an orientation difference of ~20° between the northern and southern half of the island. We found that the decadal coastal profiles on the southern half show features of a low‐tide terrace and a sandbar located further from the shoreline (~441 m). On the nothern half, the sandbar was located closer to the shoreline (~267 m) and was less pronounced, while the profiles show transverse bar and rip features. The alongshore planform also differed systematically and significantly along the two island sides. The sandbar on the southern island half, with alongshore periodicity on a larger length scale (~2240 m), was coupled out‐of‐phase to the shoreline, while no phase coupling was observed for the sandbar with periodicity on a shorter length scale (~670 m) on the northern half. We related the observed geometric differences of the sandbars to the difference in the local wave climate along Sylt, imposed by the shoreline shape. Our observations imply that small alongshore variations in wave climate, due to the increasing shoreline curvature on larger spatial scales, can lead to significant alongshore differences in the decadal evolution of coastal profiles, sandbars and shorelines. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

Highlights

IntroductionNearshore sandbars, often frame as crescentic plan shapes with alongshore periodicity in cross-shore location and depth
Shoreline parallel sand ridges, or nearshore sandbars, often frame as crescentic plan shapes with alongshore periodicity in cross-shore location and depth
The large-scale and long-term evolution of nearshore sandbars is important for coastal management because sandbars induce depth-limited wave breaking, while sandbar–shoreline coupling can result in alongshore variability in dune erosion (Thornton et al, 2007; Galal and Takewaka, 2011; Castelle et al, 2015)

Summary

Introduction

Nearshore sandbars, often frame as crescentic plan shapes with alongshore periodicity in cross-shore location and depth These crescentic sandbars are characteristic of many surf zones of wave-dominated sandy beaches and their alongshore periodicity, three-dimensionality, or 3D length scale has been observed to vary from 100 to 3000 m along different individual stretches of shoreline worldwide (Van Enckevort and Ruessink, 2003; Van Enckevort et al, 2004; Ribas et al, 2017). These sandbars are often coupled to the shoreline. Most existing sandbar (Wright and Short, 1984; Lipmann and Holman, 1990; Van Enckevort and Ruessink, 2003; Splinter et al, 2011) and sandbar–shoreline coupling (Ruessink et al, 2007; Castelle et al, 2010a,b; Price and Ruessink, 2013; Van de Lageweg et al, 2013) studies were, limited to individual shorelines, with spatial scales up to kilometres and timescales up to years

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