The influence of currents on ocean wave climates: Fid-Bau Portal

The influence of currents on ocean wave climates

Burrows, R. / Hedges, T.S.

Abstract The ocean wave climate at a site is often described in terms of the bivariate histogram of significant wave height, H s, and mean zero-upcrossing wave period, T z. Such a representation is usually referred to as a ‘scatter diagram’. The parameters H s and T z are statistics of the random wave field which is more fully described by the surface elevation spectrum, S ηη(ω). The changes which currents produce in the surface elevation spectrum have been described previously by the authors. In this paper, the effects of currents on wave statistics are investigated. Use is made both of the Pierson-Moskowitz spectral form and of a modified version developed by Det norske Veritas in order to specify wave conditions on quiescent water. The principle of wave action conservation is then employed, together with Phillips' constraint on the energy densities associated with the equilibrium range of the spectrum, in order to predict changes as the waves encounter a current. The influence is shown of current-induced changes to a typical scatter diagram. In the absence of wave refraction, an adverse current (one opposing the direction of wave propagation) is seen to increase the range of H s in the scatter diagram whilst reducing the spread in T z values associated with each level of H s. A following current reduces the range of H s. The consequences of the changes in the scatter diagram on predictions of extreme wave heights are also investigated. It is shown, for the particular wave climate considered, that adverse currents of 1 m/s and 2 m/s increase wave heights by approximately 11% and 17%, respectively, for extreme conditions with return periods ranging from one year to one hundred years. In contrast, following currents of 1 m/s and 2 m/s decrease these extreme wave heights by about 14% and 18%, respectively. For simplicity, it is assumed in the study that the water is deep and that the waves propagate from quiescent water onto a current which is steady and has streamlines which are parallel with the wave orthogonals. Despite these limitations, the work provides a basis for engineers to make judgements when faced with the problem of translating a known wave climate from an essentially current-free site to a location where currents are significant. These preliminary calculations suggest that wave climate description may suffer only a small net change in situations where currents are of tidal origin with little bias in magnitudes between flood and ebb, and where there is correspondingly little correlation between tidal phase and sea-state intensity. However, in estuaries, where current strengths could show considerable directional bias, currents may exert a strong influence both on the scatter diagram and on the estimates of extreme wave conditions.