| Title: | THE EVOLUTION OF LARGE WAVES IN SHALLOW WATER |
| DOI No: | 10.1142/9789812701916_0004 |
| Source: | COASTAL ENGINEERING 2004 (pp 69-81)
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| Author(s): | VASILIKI KATSARDI
Department of Civil & Environmental Engineering, Imperial College, London, SW7 2AZ, UK
CHRIS SWAN
Department of Civil & Environmental Engineering, Imperial College, London, SW7 2AZ, UK
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| Abstract: | The evolution of large waves in realistic JONSWAP spectra is calculated using the fully nonlinear wave model proposed by Bateman, Swan & Taylor (2001 & 2003). Comparisons between wave fields of varying nonlinearity and between "equivalent" deep and shallow water wave cases confirm that the water depth has a profound influence on the characteristics of large waves in uni-directional seas. In deep water, the evolution of large waves is controlled by linear dispersion. However, as the water depth reduces frequency dispersion is weakened, and the competing process of wave modulation becomes dominant. As a result, the evolution of nonlinear waves in an intermediate water depth (d=15m) is shown to be characterised by a narrowing of the underlying frequency spectrum in which resonant or near-resonant wave interactions create a quasi-regular wave pattern, not previously present in the wave field, and the modulation between closely spaced frequency components, or side-band instabilities, leads to the generation of a large wave event always appearing at the front of an elongated wave group. Calculations involving limiting, or near-limiting, wave events suggest that the largest waves may be characterised by a wave height to water depth ratio of H/d=0.55. This is significantly lower than the established value of H/d=0.78, commonly adopted in engineering design, and appears to be consistent with recent laboratory observations reported by Kamphius (1991b) and Massel (1998). |
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