Wave height

About: Wave height is a research topic. Over the lifetime, 5920 publications have been published within this topic receiving 100257 citations.

Energy dissipation of wind-generated waves and whitecap coverage

Abstract: [1] The energy dissipation per unit area of the ocean surface attributed to fetch- or duration-limited wind-generated waves can be expressed in terms of wind speed, significant wave height and peak wave frequency. Such a parameterization equation can be exploited for obtaining a first order estimation of the rate of energy input through the air-sea interface in the world's oceans using satellite output of wind speed, wave height and wave period. For general wind wave events in the ocean with event duration longer than one hour, the energy dissipation (in W/m 2 ) is equal to the product of the density of air, wind speed cubed and a proportionality coefficient between 0.00037 and 0.00057. Using the equation to calculate the wave energy dissipation, the whitecap coverage is proportional linearly to the energy dissipation. The threshold energy dissipation for whitecap inception is between 0.013 and 0.038 W/m 2 , which corresponds to a threshold wind speed of between 2.5 and 3.6 m/s. The proportionality coefficient is relatively constant for a wide range of wave growth conditions in comparison to the data scatter in the whitecap measurements. This may explain why it is so difficult to establish an unequivocal dependence on the explicit surface wave parameters in the whitecap data. The weak explicit wave signal can be detected after the cubic wind speed dependence is removed.

Field data support of three-seconds power law andgu *σ−4-spectral form for growing wind waves

Abstract: Observational data on air-sea boundary processes at the Shirahama Oceanographic Tower Station, Kyoto University, obtained in November, 1969, was analyzed and presented as an example representing the structure of growing wind-wave field. The condition was an ideal onshore wind, and the data contained continuous records of the wind speed at four heights, the wind direction, the air and water temperatures, the tides, and the growing wind waves, for more than six hours. The main results are as follows. Firstly, in both of the wind speed and the sea surface wind stress, rather conspicuous variations of about six-minute period were appreciable. Secondly, the three-seconds power law and its lemma expressed byH*=BT*3/2 andδ=2πBT*−1/2, respectively, are very well supported by the data, whereH*(≡gH/u*2) andT*(≡gT/u*) are the dimensionless significant wave height and period, respectively,δ the wave steepness,u* the friction velocity of air,g the acceleration of gravity, andB=0.062 is a universal constant. Thirdly, the spectral form for the high-frequency side of the spectral maximum is well expressed by the form ofΦ(σ)=αsgu*σ−4, whereσ is the angular frequency andΦ(σ) the spectral density. The value ofαs is determined as 0.062±0.010 from the observational data. There is a conspicuous discrepancy between the spectral shape of wind waves obtained in wind-wave tunnels and those in the sea, the former containing well-defined higher harmonics of the spectral peak, and consequently there is an apparent difference in the values ofαs also. However, it is shown that the discrepancy ofσs may be eliminated by evaluating properly the energy level of the spectral form containing higher harmonics.

Effect of wave frequency and directional spread on shoreline runup

Abstract: [1] Wave breaking across the surf zone elevates the mean water level at the shoreline (setup), and drives fluctuations about the mean (runup). Runup often is divided into sea-swell (0.04–0.3 Hz) and lower frequency infragravity (0.00–0.04 Hz) components. With energetic incident waves, runup is dominated by infragravity frequencies, and total water levels (combined setup and runup) can exceed 3 m, significantly contributing to coastal flooding and erosion. Setup and runup observations on sandy beaches are scattered about empirical parameterizations based on near-shoreline beach slope and deep water wave height and wavelength. Accurate parameterizations are needed to determine flooding and erosion risk to coastal ecosystems and communities. Here, numerical simulations with the Boussinesq wave model funwaveC are shown to statistically reproduce typical empirical setup and runup parameterizations. Furthermore, the model infragravity runupRs(ig) strongly depends on the incident wave directional and frequency spread (about the mean direction and peak frequency). Realistic directional spread variations change Rs(ig) equivalent to a factor of two variation in incident wave height. The modeled Rs(ig)is shown to vary systematically with a new, non-dimensional spreading parameter that involves peak frequency, frequency spread, and directional spread. This suggests a new parameterization forRs(ig) potentially useful to predict coastal flooding and erosion.

Wave loads on offshore structures

Abstract: The author discusses how knowledge of wave-induced loads is essential in both the design and operation of offshore structures. In hostile areas like the North Sea, the significant wave height (mean of the highest one third of the waves present in a sea) can be larger than 2 m 60% of the time. The most probable largest wave height in 100 years can be more than 30 m. The mean wave period can be from 15 to 20 s in extreme weather situations, and it is seldom below 4 s. Environmental load due to current and wind are also important, and in some cases the interactive effect between waves and current is significant. Current velocities of 1-2 m s{sup {minus} 1} and extreme wind velocities of 40-45 m s{sup {minus}1} must be used in designing offshore structures in the North Sea.