On the physical interaction between ocean waves and coastal cliffs

Abstract

Wave impacts have long been posited as the primary forcing mechanism of coastal cliff recession. Recent developments in the study of hydrodynamics at coastal structures such as seawalls and breakwaters have shown that wave pressures are stochastic in nature and have a broad range of first- and second-order controls. This understanding has yet to be translated to coastal cliffs, where it is still largely assumed that wave impact characteristics can be predicted by simple deterministic formulae. Hydraulic components in coastal models are limited by the lack of in-situ measurements of waves at the cliff toe due to the difficulties in deploying instrumentation in such energetic and inaccessible environments.
To address this, I have approached the problem threefold. Monthly high-resolution terrestrial laser scanning (TLS) was undertaken over a year at multiple sites at Staithes, North Yorkshire, to evaluate the recession rate and detachment characteristics of the lower cliff section. Concurrently, wave gauges were deployed at the cliff toe of each site to monitor wave conditions. A novel method of measuring wave impacts was undertaken at one of the sites for nine low-to-low tidal cycles. New and established methods for processing this data were used.
Analysis of the erosion dataset revealed distinct temporal patterns of erosion, with accelerated erosion rates during winter. Vertical variations in detachment volumes below 0.1 m3 related to the tidal elevation were also observed, suggesting a key marine influence. Detachment frequency and volume were found to be influenced by lithology type and joint density. Wave conditions over the study period were found to be depth-limited, yet some waves at the toe were found to be larger than those offshore due to shoaling. Wave breaking conditions were strongly influenced by platform morphology and tidal stage. Up to 9% of all waves were breaking on impact. Measurements of wave impacts revealed approximately 14% of wave exhibited high-magnitude impulsive pressures generated by breaking and broken waves. These were analysed probabilistically and found to be controlled primarily by the ratio between wave height and water depth.
These data were used to develop a conceptual model of forcing at the cliff toe, including an evaluation of the ability of waves to remove material via enhanced pressure inside discontinuities and fragmentation of weathered material. These results have broad implications concerning the process geomorphology of rock coasts and the evaluation of wave forcing in coastal models.