Laboratory investigation of crest height statistics in intermediate water depths.

I Karmpadakis,C Swan,M Christou

doi:10.1098/rspa.2019.0183

Abstract

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.

Highlights

The statistical distribution of crest heights in severe storms is key to the characterization of the surface wave conditions in both coastal and offshore locations, and represents a critical input for the design of all royalsocietypublishing.org/journal/rspa Proc
Evidence of the importance of crest heights in intermediate depths was provided by hurricanes Rita and Katrina in the Gulf of Mexico. These were both category five hurricanes which together destroyed 113 offshore platforms, 87 of which were located in water depths of less than 60 m [3]. This paper addresses this challenge, providing a laboratory investigation of crest height statistics in intermediate water depths
Several realistic sea states based upon a representative spectral shape (JONSWAP) are addressed. These cover a broad range of sea-state steepness, effective water depth and directional spreading; the primary purpose being to establish the success of existing crest height models and to explore the competing influences of nonlinear amplifications and wave breaking

Summary

Introduction

The statistical distribution of crest heights in severe storms is key to the characterization of the surface wave conditions in both coastal and offshore locations, and represents a critical input for the design of all. The majority of this work has been undertaken in deep water This is despite the fact that the statistical representation of water waves becomes progressively more complicated as the water depth reduces. These were both category five hurricanes which together destroyed 113 offshore platforms, 87 of which were located in water depths of less than 60 m [3]. Several realistic sea states based upon a representative spectral shape (JONSWAP) are addressed These cover a broad range of sea-state steepness, effective water depth and directional spreading; the primary purpose being to establish the success of existing crest height models and to explore the competing influences of nonlinear amplifications and wave breaking

Background

Experimental set-up

Experimental results

Concluding remarks