TY - JOUR

T1 - Observations on decadal sandbar behaviour along a large-scale curved shoreline

AU - Gijsman, R.

AU - Ruessink, B. G.

AU - Visscher, J.

AU - Schlurmann, T.

N1 - Funding Information: The authors gratefully acknowledge the Coastal Authority of Schleswig‐Holstein (LKN.SH), in particular Arfst Hinrichsen, for the provision of the wave buoy and coastal profile data, the discussion on the findings and the comments on the draft manuscript. We acknowledge Finn Mielck of the Alfred‐Wegener‐Institute (AWI) for sharing the grain size measurement. The study was funded through project STENCIL (Contract No. 03F0761) of the German Ministry of Education and Research (Bundesministerium für Bildung und Forschung or BMBF). We acknowledge the reviewers for their thoughtful comments on the manuscript.

PY - 2021/2/9

Y1 - 2021/2/9

N2 - Nearshore 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.

AB - Nearshore 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.

KW - alongshore variability

KW - coastal profiles

KW - curved coastlines

KW - nearshore morphology

KW - nearshore sandbars

KW - sandbar–shoreline coupling

KW - Landforms

KW - Earth surface process

KW - Geometric difference

KW - Low tide terraces

KW - Phase coupling

KW - Sandy shoreline

KW - Spatial and temporal scale

KW - Spatial scale

KW - Wave climates

KW - Coastal engineering

UR - http://www.scopus.com/inward/record.url?scp=85099367180&partnerID=8YFLogxK

U2 - 10.1002/esp.5041

DO - 10.1002/esp.5041

M3 - Article

AN - SCOPUS:85099367180

VL - 46

SP - 490

EP - 503

JO - Earth Surface Processes and Landforms

JF - Earth Surface Processes and Landforms

SN - 0197-9337

IS - 2

ER -