Wave height prediction for island regions

Abstract

During the period from January 1996 until December 1996, I worked on two different projects at the Department of Ocean Engineering at the University of Hawaii at Manoa. The State Department of Civil Defense initiated these projects as a result of the 1992 hurricane Iniki, which devastated the Island of Kauai and damaged the other Hawaiian Islands as well. The objective of these projects is to develop a wave measurement database and field verified wave models for use by the public and private sectors. The first project, which took up about 90 % of my time, deals with the development of computational capabilities to predict the refraction and diffraction of nonlinear, directional random waves in the nearshore environment. The Green-Naghdi (G-N) equations are used to determine the nonlinear diffraction of regular waves by topography in a numerical wave tank. In the first case, the Level II G-N equations are used to simulate monochromatic waves propagating over a steep shelf (slope 1 :2) that reaches a height of 70 % of the fluid depth. These results are compared with experimental data of Ohyama et al. (1995) and the Level I G-N simulations of Ertekin and Becker (1996). In the second case, the accuracy of the Level II G-N equations is assessed for the nonlinear diffraction of monochromatic waves over a gentle shelf (slope 1 :20) by comparing the numerical simulations with the measurements of Ohyama et al. ( 1994 ). The results show that nonlinear diffraction is simulated more accurately over a larger range of wave frequencies by the Level II G-N equations than the Level I equations. The frequency range over which the Level II G-N equations accurately simulate wave propagation over both types of topography is consistent with the frequency range over which the Level II G-N equations accurately model linear dispersion (Shields and Webster, 1988). In addition, it is the frequency of the highest harmonic generated by the nonlinear interactions that limits the accuracy of the Level II G-N simulations. The present work also shows that the G-N equations can be used in accurately determining the nonlinear diffraction of waves by submerged bodies, such as oil storage tanks and breakwaters. Thus the G-N equations may be used as an alternative to the solution of the fully nonlinear 3-D problem in modeling nonlinear diffraction. The second project deals with the development of a methodology for estimating wave conditions at unmonitored coastal sites around the Hawaiian Islands specifically, but for reef-fringed islands in general. field data is collected to test the accuracy of various wave transformation models. Model results are evaluated in the vicinity of reefs and at sites influenced by island sheltering. Computer simulations are used to determine the wave heights at selected sites at the South and West side of the Island of Oahu. At locations where the wave height increases considerably (hot spot), wave gages are deployed to measure the wave heights. These measurements will be compared with the computer simulations in order to verify the used computer models. A couple of the computer simulations have already been made and the wave gages are deployed as well. The comparison between the experimental data and the computer simulations is scheduled for the spring of 1997.