The Effect of Grain Size Distribution and Bimodal Sea States on Coarse Beach Sediment Dynamics

Polidoro, Andrea (2019). The Effect of Grain Size Distribution and Bimodal Sea States on Coarse Beach Sediment Dynamics. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f53e

Abstract

In this work we investigated the effect of gravel beach profile response under wave spectra characterised by swell and wind wave periods in various combinations. This was done by running an extensive series of 2D physical model tests. It was found that even a small percentage of wave energy within the low frequency range triggers a significant landward displacement of the beach crest. Based on this 2D physical model study, a new parametric model, Shingle B, was derived and an online tool was developed and made available.

This research also presents a study on the effect of the grain size distribution, i.e. permeability, on the beach profile response. It was investigated, using a permeameter, how a stationary porous flow is influenced by the grain size distribution. The results clearly show that the D15 parameter dominates the flow/resistance behaviour for all the tested samples and the other parameters (D₅₀ or D₈₅) have only second order effects on the flow/resistance relationship.

Additionally, in order to investigate how the grain size distribution influences both the wave induced pore pressure and beach profile evolution, a 2D physical model study was carried out using ten different gravel beaches. Observations made during this study, in which more complex phenomena were involved, proved that the pore pressure attenuation was mainly influenced by the grading width parameter D₈₅/D₁₅. Measurements on the internal wave set up recorded during these experiments showed that the internal wave set-up was strongly influenced by both the incident wave conditions and the sediment characteristic D₅₀.

Post-storm beach profiles, with similar grading parameters (e.g., D₅₀, D₁₅ and D₈₅/ D₁₅), were then compared. These comparisons demonstrated that the crest moves upwards and shoreward as D₅₀ and D₁₅ decrease. In particular the crest elevation increases with increasing internal wave set-up.

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