4 results found
Karmpadakis I, Swan C, 2020, On the average shape of the largest waves in finite water depths, Journal of Physical Oceanography, Vol: 50, Pages: 1023-1043, ISSN: 0022-3670
This paper investigates the average shape of the largest waves arising in finite water depths. Specifically, the largest waves recorded in time-histories of the water surface elevation at a single point have been examined. These are compared to commonly applied theories in engineering and oceanographic practice. To achieve this both field observations and a new set of laboratory measurements are considered. The latter concern long random simulations of directionally spread sea-states generated using realistic JONSWAP frequency spectra. It is shown that approximations related to the linear theory of Quasi-Determinism (QD) cannot describe some key characteristics of the largest waves. While second-order corrections to the QD predictions provide an improvement, key effects arising in very steep or shallow water sea-states are not captured. While studies involving idealised wave groups have demonstrated significant changes arising as a result of higher-order nonlinear wave-wave interactions, these have not been observed in random sea-states.The present paper addresses this discrepancy by decomposing random wave measurements into separate populations of breaking and non-breaking waves. The characteristics of average wave shapes in the two populations are examined and their key differences discussed. These explain the mismatch between findings in earlier random and deterministic wave studies.
Karmpadakis I, Swan C, Christou M, 2020, Assessment of wave height distributions using an extensive field database, Coastal Engineering, Vol: 157, Pages: 1-15, ISSN: 0378-3839
The present paper investigates the short-term statistical distribution of wave heights. Specifically, some of the most commonly applied wave height distributions are assessed using field measurements. The latter comprise of wave radar observations from 10 different locations in the North Sea and cover water depths between 7.7 m and 45 m. In total, the field database includes more than 200 million waves, making it one of the largest of its kind in this water depth regime. In using these data, the accuracy of existing wave height distributions has been examined and guidance is provided concerning the best performing models and their domain of applicability. Additionally, insights concerning the influence of key met-ocean parameters are also provided. Taken together, the results in this paper present an overview of the statistical behaviour of wave heights in finite water depths, as observed in the field.
Karmpadakis I, Swan C, Christou M, 2019, Laboratory investigation of crest height statistics in intermediate water depths, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 475, ISSN: 1364-5021
This paper concerns the statistical distribution of the crest heights associated with surface waves in intermediate water depths. The results of a new laboratory study are presented in which data generated in different experimental facilities are used to establish departures from commonly applied statistical distributions. Specifically, the effects of varying sea-state steepness, effective water depth and directional spread are investigated. Following an extensive validation of the experimental data, including direct comparisons to available field data, it is shown that the nonlinear amplification of crest heights above second-order theory observed in steep deep water sea states is equally appropriate to intermediate water depths. These nonlinear amplifications increase with the sea-state steepness and reduce with the directional spread. While the latter effect is undoubtedly important, the present data confirm that significant amplifications above second order (5–10%) are observed for realistic directional spreads. This is consistent with available field data. With further increases in the sea-state steepness, the dissipative effects of wave breaking act to reduce these nonlinear amplifications. While the competing mechanisms of nonlinear amplification and wave breaking are relevant to a full range of water depths, the relative importance of wave breaking increases as the effective water depth reduces.
Karmpadakis I, Swan C, Christou M, Wave height and crest height distributions in shallow water — Analysis of field data, Coastal Structures
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