Introduction

Nearshore sandbars (alongshore ridges of sand, located in 2 – 10m water depth, typical of micro- to mesotidal coasts) often exhibit remarkable alongshore-periodic undulations in their height and cross-shore position (figure 1a). These so-called crescentic sandbars evolve from an initially linear and shore-parallel sandbar following a storm period with high waves. During the next storm period, the sandbars are almost immediately straightened again (figure 1b). Crescentic sandbars often occur as part of a double bar system, where the shape of the outer bar may govern the evolution of the inner bar, introducing coupled patterns in the double bar system.

Figure 1: Planview images from the Gold Coast, Australia, showing a crescentic outer sandbar with similar undulations in the inner bar (A) and linear shore-parallel sandbars (B).

Objectives

This project aims to determine the conditions under which the outer crescentic sandbar (a) controls the morphology of the inner bar and (b) is reshaped into a shore-parallel linear bar. Besides the analysis of field observations, numerical models and sophisticated data analysis techniques will be applied to examine and quantify the morphodynamic processes involved.

Figure 2: The Australian study site is located at Surfers Paradise in Queensland.

Methods

The data for this project consists of multi-year data sets of video images (figure 1) of the double-barred beaches at Surfers Paradise, Queensland, Australia (figure 2) and Duck, N. C., USA with simultaneous hourly time series of offshore wave parameters (height, period, direction). The images are used to determine the occurrence of crescentic bars and are processed into bathymetric maps, allowing quantification of the outer bar and, together with the wave data, serve as input for the morphodynamic models. The interaction between the inner and outer bar is modelled using a non-linear two-dimensional horizontal (2DH) model, whereas the open-source quasi-3D model XBeach is used to examine the bar straightening under high waves.

Results

Preliminary results have shown that initially uncoupled inner sandbar patterns appear to couple with the crescentic patterns in the outer bar. The inner bar patterns were found to develop in response to the increasingly crescentic, onshore propagating outer bar. Model simulations with the 2DH model have shown that specific shapes of the outer bar and the distance between both bars govern the wave height variability and associated circulation patterns at the inner bar (figure 3) and hence govern the shape of the inner bar. During periods of high waves the horizontal circulation patterns (with rip currents) change to a more vertical flow circulation (with undertow), resulting in the straightening of crescentic bars. Field observations indicate the simultaneous straightening and offshore migration of the entire crescentic bar. Furthermore, strong alongshore currents, associated with obliquely incident waves, may move large amounts of sand alongshore and enhance bar straightening.

Figure 3: Modelled morphological evolution. From (A) to (C) the initial shape of the outer bar is increasingly crescentic, causing the inner bar rhythmicities to become increasingly coupled to the outer bar shape.

Links

Gold Coast, Australia study site.
Duck, N.C., USA study site.
The open-source quasi-3D storm model XBeach.

Please contact Timothy Price for more info on this project.