Wave Hindcasting

Excerpt

Wave hindcasts refer to the predictions of wind waves on the water surface for a past event. Wave nowcasts and forecasts similarly refer to the predictions in real time and in the future, respectively. But the relations or models used for predictions for a past, present, or future event are the same. The wave parameters of interest are wave height and period, and the required wind parameters for predictions are wind speed (U) and U duration (t). Wind speed should represent an average, typically over a timescale of 1–15 min. Since wind waves represent an irregular undulated water surface comprising a multitude of superimposed wave frequencies, rather than a monochromatic wave, its parameters are best described in statistical terms. These terms are significant wave height (H _s) and significant wave period (T _s), which represent the average of the highest one–third of the parameters. However, in spectral-based computations, H _mo is used to represent wave height related to the total energy density as given by the zero-th moment of the wave spectrum. H _s is slightly larger than H _mo on most occasions, but they are equivalent for deepwater waves (Goda, 1974; Thompson and Vincent, 1985; Sorensen, 1993). Depth (d) and fetch (F) are the two water body variables required to compute wave parameters. Fetch is defined as a region of the water surface over which wind blows with speeds and directions that vary within a specified limit. The accepted limit for wind speed variation is 2.5 m/s (or 5 knots), and the same for wind direction is 45° (U.S. Army Coastal Engineering Research Center, 1984). In lakes and coastal water bodies, the fetch is often limited by land boundaries. The limiting lengths of fetch and duration give rise to three wave-generating conditions for a particular wind speed. A fetch-limited condition applies when the wind duration exceeds the wave travel time over the fetch. When the opposite happens, a duration-limited condition applies. If both the fetch and the wind duration are sufficiently large, a fully arisen sea develops for a particular wind speed. Depth of the water body is only important for shallow water wave hindcasts. …