Morphological coupling in a double sandbar system

Nearshore sandbars are ubiquitous morphological features along many sandy, wave-dominated, coasts. They are approximately shore-parallel ridges of sand, located in water depths of less than 10 m. Depending on the wave conditions, sandbars regularly develop remarkable alongshore periodic undulations in their height and cross-shore position, so-called crescentic sandbars. Sandbars safeguard beaches by dissipating wave energy before it impacts the shore, and, in addition, they constitute a sediment buffer within the nearshore zone. The protection of coastal areas against erosion and flooding mostly involves engineering measures consisting of the upkeep of this natural sediment buffer within the nearshore zone through sand nourishments. Accordingly, the ability to better understand and predict nearshore sandbar behaviour will facilitate more efficient nourishment programs, both in terms of time and money. Most of the current research on alongshore sandbar variability has focussed on the initial formation of crescentic patterns, and to a lesser extent on the behaviour of existing crescentic sandbars, their finite-amplitude behaviour. Moreover, in a double sandbar system, with a more landward inner bar and a more seaward outer bar, the crescentic outer-bar morphology may lead to similar rhythmic perturbations in the inner-bar morphology and the shoreline. This can lead to localised, severe beach and dune erosion, and subsequent property losses during storms. The main objective of this thesis is to increase our understanding of existing crescentic patterns in double sandbar systems, with a focus on morphological coupling. The analyses in this thesis are based on 9 years of wave data and daily video observations of the alongshore sandbar patterns from a natural, double-barred beach on the Gold Coast, Queensland, Australia. From the video images, the inner- and outer-sandbar patterns are derived and the occurrence of morphological coupling between the sandbars is quantified. Coupling is an inherent property of the double sandbar system studied here, as the alongshore variability in the inner bar is coupled to that in the outer bar for some 40% in time. Coupling is predominant when the outer bar is alongshore variable, both in position and depth, except for excessively large offshore angles of incidence (larger than 30 degrees) or wave heights (larger than 2 m) leading to outer-bar straightening and sandbar de-coupling. In addition to wave height and depth variation along the outer bar, the offshore angle of wave incidence is crucial to the type of coupling that emerges. Using a 2DH non-linear morphodynamic model, it is demonstrated that the angle of wave incidence strongly controls the type of flow pattern over the inner bar, with a change from cell-circulation patterns for approximately shore-normal waves to an alongshore meandering current as the angle increases. The latter type of currents lead to the development of the coupling type dominating the present data set – cross-shore undulations of the inner bar that are coupled to the outer bar morphology. These results show that the individual sandbars in a double-barred system should not be studied as independent features, but, instead, the interaction within the composite sandbar system should be taken into account