Salauddin, Md (2020) Wave overtopping and toe scouring at sea defences with permeable shingle foreshores : a physical model study. PhD thesis, University of Warwick.

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Abstract

Wave overtopping and toe scouring are considered two primary coastal processes at the wave/structure interface of sea defence structures. It is therefore important to have reliable prediction methods for these coastal processes at coastal infrastructures. The existing prediction formulae currently available are principally based on fitting to experimental measurements, (e.g. empirical formulae) reported in the overtopping manual. However, to date, most parametric studies regarding these aspects (e.g. wave overtopping cited in the scientific literature) have tended to focus on structures having impermeable foreshores, with investigations on permeable beaches (e.g. shingle beaches) being less well-studied. Accordingly, there is a distinct knowledge gap on these coastal processes at sea defences (e.g. vertical seawall and sloping structure with permeable gravel slopes) due to limited field and laboratory research on these types of sea defences.

The purpose of this study was to investigate wave overtopping and toe scouring characteristics at sea defences (e.g. a vertical seawall and a 1 in 2 sloping structure) on permeable shingle foreshores. The small-scale laboratory study was conducted in two phases. Phase I focused on the overtopping and scouring processes at the plain vertical seawall, and Phase II investigated the overtopping and scouring characteristics at a smooth 1 in 2 sloping structure. Each Phase consisted of undertaking wave flume experimental investigations based on three configurations, namely a plain vertical wall or a sloping structure with permeable shingle foreshores of two different particle sizes (d50 of 13 mm and 24 mm) together with an impermeable foreshore representing the control condition.

The physical model experiments were undertaken in a two-dimensional wave channel at the University of Warwick, UK by adapting the well-established guidelines of EurOtop (2018), Powell (1990) and Wolters et al. (2009) for typical two-dimensional experimental investigations. An impermeable sloping foreshore with a uniform slope of 1 in 20 was constructed in front of the model structure. Two constant deep-water wave steepnesses (sm-1,0 = 0.02 v and 0.06) were tested with six different toe water depths. Each test comprised of 1,000 pseudo-random wave sequences. Permeable (gravel) foreshores, with slopes of 1 in 20 then simulated using crushed anthracite (specific gravity of 1.40 T/m3 ) following Powel (1990). For the 1 in 50 geometrical scaling, model anthracite d50 values of 2.10 mm and 4.20 mm represented prototype gravels with d50 values of 13 mm and 24 mm, respectively.

Detailed measurements were taken to parameterize the mean overtopping rate, mean sediment rate, individual overtopping volume, probability of overtopping and scour depths on a plain vertical seawall and a 1 in 2 sloping structure, for both impermeable and permeable shingle beach configurations. The resulting overtopping and scouring characteristics at the structures were then compared with existing empirical formulations from the literature to identify differences in the permeable and impermeable foreshore characteristics.

For both structural configurations (e.g. vertical wall and sloping structure) the measured baseline overtopping characteristics corresponding to the impermeable foreshore (control condition) showed an overall good agreement with the existing empirical prediction formulae for the tested wave conditions covered within this study. Within the experimental limitations of this study, it was found that the measured mean overtopping rate was reduced significantly for the case of permeable foreshores when compared to the predictions reported for the impermeable slope. However, when comparing the wave by wave overtopping volumes, no significant differences were observed.

The relationship of the scour depth with toe water depth, Iribarren number, and wall slope were investigated from the test results of this work and through a comparison with available datasets in the literature. For both plain vertical wall as well as sloping structure, the results of this study showed that the relative toe scour depth at the structure on a shingle beach, was influenced by the relative toe water depth and Iribarren number. Within the experimental limitations, the maximum toe scour depths were observed for the experiments under spilling and plunging wave attack. vi Before conducting this study, limited prediction guidance was available to predict the mean overtopping discharges and mean sediment rates at sea defences on permeable shingle foreshores. Therefore, for the prediction of overtopping characteristics at sea defences (e.g. vertical walls and sloping structures) on permeable gravel foreshores, revised predictions tools are suggested. The outcomes of this study are intended for practitioners and researchers in predicting wave overtopping characteristics at sea defence structures with permeable gravel foreshores.

Item Type:	Thesis (PhD)
Subjects:	G Geography. Anthropology. Recreation > GC Oceanography T Technology > TC Hydraulic engineering. Ocean engineering
Library of Congress Subject Headings (LCSH):	Sea-walls -- Design and construction, Ocean waves -- Mathematical models, Scour (Hydraulic engineering)
Official Date:	January 2020
Dates:	Date Event January 2020 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Pearson, Jonathan M.
Sponsors:	University of Warwick. Doctoral College
Format of File:	pdf
Extent:	xxvi, 182 leaves : illustrations (some colour)
Language:	eng

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