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Classification of embayed beach morphology and quantifying the influence of headlands on beach and surf zone morphodynamics
thesisposted on 2022-03-28, 10:39 authored by Thomas Edmund Twysden Fellowes
Embayed beaches are common on mountain and cliff dominated coastlines, represent a large proportion of beaches globally, and have a range of embayment and headland geometries. The headlands around embayed beaches can shadow (protect) areas in their lee through wave refraction processes, creating an alongshore gradient in wave energy and hence beach and sand bar morphology. The varied headland morphology and wave conditions that exist on embayed beaches means that their responses to both storm and fair-weather conditions can be highly variable and hard to predict. Thus, embayed beaches do not typically fit the standard assumptions of beach morphodynamic classifications that assume some alongshore uniformity. Due to this, this thesis aims to classify embayed beaches globally and understand how headlands influence embayed beach morphodynamics across multiple scales. This thesis presents a multi-scale analysis of embayed beaches, including a global analysis that included a total of 168 beaches from Australia, New Zealand, Brazil, United Kingdom, Spain and Portugal. This global analysis classifies the morphological settings of embayed beaches, combined with detailed site-specific studies over days to years that focuses on quantifying how embayment settings and headlands influence the beach morphodynamics. Chapter 2 proposes a new embayed beach classification that categorises embayed beaches by their embayment geometry with and embayment morphometric parameter, which is an empirical relationship between embayment area and coastal indentation; and ranges from Class 1 (least embayed) to Class 4 (most embayed). Chapter 3 uses monthly surveys in southeast Australia (2015-2019) at 9 beaches to demonstrate that the four classes responded differently to storms, whereas recovery (this study and literature; n = 22) was consistent at 0.22 m to the power of 3/m/day across low-moderate embayed beaches (Class 1-Class 3), but for high embayed beaches (Class 4) was considerably lower at 0.08 m to the power of three/m/day, suggesting Class 4 beaches are the most vulnerable to storm impacts. Furthermore, Chapter 3 proposes a new trigonometry-based method quantified the headland shadow edge length alongshore, which highlighted alongshore responses and showed headland protected (shadowed) zones experienced 7 times less volume losses than exposed zones to 8 powerful storms. Chapter 4 demonstrates a new method developed to quantify sand bar migration rates and vorticity from daily video images (2012-2018) at Bondi Beach (Australia). Results in this chapter show 4 key examples of sand bar response patterns to storms including, opened sand bar-rip channel, meandering sand bar/trough, concentric sand bar, and sand bar curvature switching, and these provide insights into storm-scale shifts in sand bar morphology. Alongshore gradients showed that protected zones experience 25% less sand bar dynamics than exposed zones. Overall, this thesis quantifies fundamental relations between the degree of embaymentisation, headland shadowing and controls, alongshore morphodynamics and shifts in beach and sand bar morphology under storm and fair-weather conditions, under the framework of a new morphodynamic classification for embayed beaches -- abstract.