Morphological coupling in multiple sandbar systems – a review

T D Price,B G Ruessink,B Castelle

doi:10.5194/esurf-2-309-2014

Abstract

Abstract. Subtidal sandbars often exhibit alongshore variable patterns, such as crescentic plan shapes and rip channels. While the initial formation of these patterns is reasonably well understood, the morphodynamic mechanisms underlying their subsequent finite-amplitude behaviour have been examined far less extensively. This behaviour concerns, among other aspects, the coupling of alongshore variable patterns in an inner bar to similar patterns in a more seaward bar, and the destruction of crescentic patterns. This review aims to present the current state of knowledge on the finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar systems. In this context we include results from our recent study, based on a combination of remote-sensing observations, numerical modelling and data–model integration. Morphological coupling is an inherent property of double sandbar systems, where the inner bar may attain a type of morphology not found in single bar systems. Coupling is governed by water depth variability along the outer-bar crest and by various wave characteristics, including the offshore wave height and angle of incidence. In recent research, the role of the angle of wave incidence for sandbar morphodynamics has received more attention. Numerical modelling results have demonstrated that the angle of wave incidence is crucial to the flow pattern, sediment transport, and thus the emerging morphology of the coupled inner bar. Moreover, crescentic patterns predominantly vanish under high-angle wave conditions, highlighting the role of alongshore currents in straightening sandbars and challenging the traditional conception that crescentic patterns vanish under high-energy, erosive wave conditions only.

Highlights

This review aims to present the current state of knowledge on the finite-amplitude behaviour of crescentic sandbars, with a focus on morphological coupling in double sandbar
Considerable research has been devoted to the state dynamics of a double-barred system, observations were mostly based on data which were either temporally limited to a single accretionary/erosional sequence (e.g. Van Enckevort et al, 2004; Ruessink et al, 2007a), spatially limited to the inner bar (e.g. Lippmann and Holman, 1990; Shand et al, 2003; Sénéchal et al, 2009) or based on data acquired at different locations or at irregular intervals (Short and Aagaard, 1993; Castelle et al, 2007)
Morphological coupling is an inherent property of double sandbar systems

Summary

Introduction

Subtidal sandbars are shore-parallel ridges of sand in less than 10 m water depth fringing wave-dominated coasts along great lakes, semi-enclosed seas and open oceans (e.g. Evans, 1940; Saylor and Hands, 1970; Greenwood and DavidsonArnott, 1975; Lippmann et al, 1993; Ruessink and Kroon, 1994; Shand et al, 1999; Almar et al, 2010; Kuriyama, 2002; Ruessink et al, 2003; Wijnberg and Terwindt, 1995, and references therein). For single-barred beaches, Wright and Short (1984) developed the most widely accepted and applied beach state classification model, based on observations of beaches with contrasting environmental conditions over a period of 3 years. Such an aggregation facilitates answers as to when certain behaviour, such as morphological coupling, happens. In a double sandbar system, with a more landward inner bar and a more seaward outer bar, the distinction between a forcing template and self-organisation becomes blurred (Castelle et al, 2010a, b) In this case, the crescentic outer-bar morphology acts as a morphological template for the inshore flow patterns through the breaking and focussing of waves across the outer bar.

Alongshore sandbar variability

Observations

Findings

Conclusions and perspectives