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Title: Directional characteristics of waves in shallow water
Authors: Oregon State University. College of Oceanography.
Long, Charles E. (Charles Edward)
Oltman-Shay, Joan M.
Keywords: Directional spectra
Nearshore processes
Wave climate
Wave measurements
Wind waves
Ocean waves
Mathematical models
Publisher: Coastal Engineering Research Center (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Technical report (U.S. Army Engineer Waterways Experiment Station) ; CERC-91-1.
Description: Technical Report
Abstract: Proper coastal engineering design relies ultimately on a complete description of sea states to which natural or man-made structures are exposed. The character of directionally distributed wind wave energy is an intuitively and demonstrably important aspect of sea state definition, yet no long-term high-resolution coastal observations exist upon which to base engineering guidance. To remedy this, a multi-year series of directional wind sea and swell measurements has been undertaken at the Field Research Facility (FRF) of the US Army Engineer Waterways Experiment Station, Coastal Engineering Research Center (CERC). This report contains a preliminary analysis of 1,046 directional observations obtained during the first year of data collection. The climate during this year was typical of this site based on longer time series observations of winds, waves, and currents at the FRF. Bulk directional characteristics are deduced from a spread parameter, based on the angular arc which subtends the central half of the area under a wave energy directional distribution, and an asymmetry parameter, which indicates how evenly energy is distributed about the peak of a directional distribution. The spread parameter for energy integrated across all frequencies ranges from 20 to 60 deg with a distinct maximum occurrence near 40 deg. Unidirectional sea states do not exist in this data set. The same general behavior is found when data are evaluated frequency by frequency. The tendency for spread to be near 40 deg appears to occur under all conditions, being nearly independent of conventional sea state descriptors such as characteristic wave height, peak period, peak direction, and bulk steepness. Multiple modes (distinct peaks in a directional distribution at a given frequency) occur in about one-third of the observations. Where multimodal distributions exist, energy tends to be evenly distributed among modes. Causes and effects of this feaure of wave climate are subjects for future research. An indication of ordered structure in directional distributions at individual frequencies is apparent when select distributions are classed by ranges of spread and asymmetry parameter and then averaged to find mean shapes. Select distributions are those which are unimodal and contain enough wave energy to avoid noise contamination. The shapes thus found can be used as structural elements with which to constitute a complete sea state in numerical models or in signal generation algorithms for directional physical models. Only about one-third of all distributions are found to be symmetric. Symmetry is a common assumption in specifying analytic functions with which to model directionally distributed wave energy. The remaining two-thirds of observations are strongly asymmetric, a feature which is sure to have important consequences in wave dynamics inshore of the observation site. NOTE: This file is large. Allow your browser several minutes to download the file.
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