Hurricane Eloise Directional Wave Energy Spectra

ABSTRACT Directiona1 wave energy spectra, calculated from data recorded during Hurricane Eloise (Gulf of Mexico, 1975), are presented. The spectra, based on an energy flux concept, allows one to identify the fundamental properties of the wave climate, the total wave energy content and the magnitude and direction of wave energy propagation. The spectra show the energy and directional characteristics of the sea state at three stations during the storm's passage. Based on these spectra, conclusions are drawn regarding the dominate wave directions, wind wave generation characteristics, wave refraction, and wave energy transfer across frequency bands due to non-linear wave interactions. INTRODUCTION This paper presents directional wave energy spectra developed from wave data measured in the Gulf of Mexico during Hurricane Eloise, September 21-23, 1975. The purpose of the paper is to illustrate an energy flux principle for calculating directional ocean wave spectra. The Hurricane Eloise spectra provides information on the magnitude and direction of wave energy propagation during the storm and provides a basis for evaluating wave refraction, long period wave generation, and non-linear wave energy transfer. The development of the energy flux concept is presented elsewhere. (Black 1974). Engineers who design and construct marine structures have long recognized the need for wave climate models that account for wave directionality. Structural dynamic and fatigue analyses of offshore platforms and vessel motion calculations are examples of engineering tasks that need directional wave information. While there is a definite need for directional data, such data are limited. Often the data that are obtained are presented in a form that tends to obscure the wave physics, making it difficult for engineers to reach a concensus on how directionality should be treated. Directional spectra based on the energy flux concept provides a concise description of the wave climate. It identifies, as a function of frequency, the total wave energy content and the magnitude and direction of wave energy propagation. Non-propagating energy, the difference between total and propagating energy, indicates the presence of crossing wave trains. The field data required for calculating the energy flux directional spectrum are time records of wave profile and water particle velocity, horizontal components. Shell Development Company conducted a field measurement program recording wave and current data in the Gulf of Mexico for several years. The results presented in this paper are based on data recorded in the Shell program in 1975. Recent publications by Forristall, et al., (1977, 1978) utilizes measurement program data for ocean current and directional wave spectra analyses. DISCUSSION Wind and wave climate data were recorded during the passage of Hurricane Eloise at the three Gulf of Mexico sites shown in Figure 1. Station 1, offshore Galveston, is in 66 feet of water. Station 2, in Offshore Continental Shelf location Eugene Island Block 331, is in 246 feet of water, and Station 3, in South Pass Block 62, is in 325 feet of water.