On Finite-Depth Wind-Wave Generation and Dissipation
The results from the finite-depth transient wave growth and dissipation event discussed in this paper demonstrate the sensitivity of the finite-depth parameter, kph, in the study of shallow-water wave generation and show that wind-wave spectra and their associated scaled parameters differ most from their deep-water counterparts when kph ⩽ 0.75. Even a small Doppler shift was shown to cause very rapid changes in the spectrum (and its associated spectral parameters). Energy overshoot was found to be important in the growth of the wave spectrum. There were, for three frequency components, two significant exponential growth periods each followed by energy overshoot which, though not as pronounced as the exponential growth, was consistent with all the transient-event spectral-parameter data and therefore assumed to be real. The second and predominant overshoot was associated with a significant shift in the forward face and peak of the spectrum to lower frequencies and a reduction in the peak spectral density in the manner suggested by Phillips (1977). This overshoot, furthermore, occurred when kph ≈ 0.67 [where the resonant response rate has been calculated by Hasselmann and Hasselmann (1980) to be greater by a factor of more than 3 than the deepwater rate], so was probably the result of resonant interactions. The spectral peak moved back toward higher frequencies during a period when wind speed was nearly constant, h<92 cm and kph ⩽ 0.75. Initially, this movement (accompanied by a general decrease in wave-height variance) was apparently the result of enhanced resonant interactions; as h dropped below 85 cm, however, the movement and variance decrease was more likely the result of greatly increased dissipation by percolation in the coarse sand near shore and bottom friction at midshelf. Finally, the repeated evidence of enhanced resonant interactions suggests that new criteria more applicable to finite-depth wave generation be established for determining the importance of these interactions.
KeywordsWind Speed Transient Event Resonant Interaction Wave Growth Nonlinear Energy Transfer
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