Erosion dynamics of a sediment depleted coastal embayment and dunefield

Author: Samuel G. Davidson

Davidson, Samuel G., 2022 Erosion dynamics of a sediment depleted coastal embayment and dunefield, Flinders University, College of Science and Engineering

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Climate change induced sea level rise and increased storm magnitude and frequency will result in more frequent coastal erosion through scarping (Psuty and Silveira, 2010; Maximiliano-Cordova et al., 2019). Scarping occurs when foredunes and other dunes situated foremost at the coast are partially eroded by waves, generally during periods of high-water level (Carter et al., 1990; Sallenger, 2000; Hesp, 2002; Pye and Blott, 2008; Splinter and Palmsten, 2012; Houser et al., 2018). This thesis specifically investigates the effect an increase in scarping will have on complex coastal embayment’s with limited to no sediment supply. The study uses Salmon Hole, located near Beachport, South Australia, a rapidly eroding headland bay beach system, backed by a heavily eroded dunefield and partially protected by an aeolianite reef as a proxy for how similar systems will be affected into the future. This PhD addresses four key components of scarping and the erosion at Salmon Hole to gain insight into this question.

Firstly, the controls that dictate the degree of spatio-temporal change to foredunes following scarping are reviewed. A new conceptual model summarising the key controls and their relationship/significance to the magnitude and extent of scarping is presented. Water level height and duration is found to be the most significant control. The effects scarping has after the initial erosion event are discussed and include moderate changes such as the foredune translating landwards to large change such as the transition of an entire dune system into a new transgressive dunefield phase.

Secondly, the progress, dynamics and cause of the erosion at Salmon Hole were analysed in detail by examining historical shoreline change and the processes driving it. This was primarily achieved using historical aerial imagery and shoreline change statistics. Findings show that a combination of the formation of the ‘lagoon’ between the mainland/dune system and the offshore reef and the resultant breakthrough of the tombolo that have led to the acceleration of the erosion processes seen at Salmon Hole. The formation of the lagoon initiated a divergent evolution that continues in the form of a significant geologically controlled longshore current and terminal rip that enhances removal of sediment during and following erosion of the beach and dunes.

Thirdly, flow dynamics over the steep scarp at Salmon Hole with a larger, higher and longer stoss slope than previously studied were observed through a wind flow experiment. The scarp slope is comprised of segments of varying slope angles that have a significant impact on flow dynamics over the dune. Wind flow dynamics observed include percentage speed up, fluctuations in turbulence, topographic steering, flow expansion and a leeward reversing vortex.

Finally, unique subaqueous transverse dunes found lined perpendicular to the shore in the Salmon Hole lagoon were studied for the first time using bathymetric surveys, orthomoasics, oblique aerial imagery and current meter data. The morphology of the dunes changes seasonally with fluctuations in wave energy and current speed transitioning the dunes between being narrow, asymmetric, and transverse to wide and symmetric.

The four components of this study demonstrate that a lack of sediment delivery back to the beach and thence to the dune between storm events results in the inability for dune recovery or much translation (small blowouts do occasionally form in the dune crest) and that this will result in similar sandy coastlines and dunefields being removed with increased erosion due to climate change.

Keywords: Coastal, geomorphology, dunes, erosion, foredunes, scarps, aeolian

Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2022
School: College of Science and Engineering
Supervisor: Professor Patrick Hesp