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Design wave specification
Published in Dominic Reeve, Andrew Chadwick, Christopher Fleming, Coastal Engineering, 2018
Dominic Reeve, Andrew Chadwick, Christopher Fleming
A straightforward way of understanding swell is to return to Figure 2.1. Swell is a generic term applied to waves experienced at a point far from wave generation. Swell waves are characterised by being long-crested (focused in direction) and having long periods and relatively small wave heights. What makes these waves different from waves generated nearby is: (1) spreading – waves generated from afar will tend to be long-crested as waves propagating in different directions will not reach the observation point; (2) dispersion – in deep water, wave speed depends on wave period not water depth (see Section 2.2.4) and are ‘dispersive’. Thus far from the generation region, waves of different periods will be separated according to their speed, with the longer period waves travelling faster than shorter period ones. Due to the spreading and dispersion swell represents a fraction of the total energy that is imparted to the sea by storms, and the wave heights are correspondingly small. However, the energy within a single swell wave can exceed that in a single storm wave by virtue of its larger wave length. For example, a 1 m high swell wave of period 15 s has approximately the same energy as a 2 m high 7 s wind wave.
The offshore environment
Published in White David, Cassidy Mark, Offshore Geotechnical Engineering, 2017
Waves on the surface of oceans are primarily wind generated and propagate along the water–air interface. Waves form as the wind causes pressure and friction forces that perturb the equilibrium of the water surface and transfer energy from the wind into wave energy. Wind blowing over a calm sea forms small ripples. The undulations of the surface water provide a better surface for the wind to ‘grip’ and ripples grow into wavelets. When the wavelets become high enough to interact with the airflow, the wind becomes turbulent just above the surface of the water, and the wind transfers energy to the waves. As the sea becomes choppier, more energy is transferred to the waves, and they get bigger. The cycle continues and waves get bigger and steeper. The formation of ocean waves is affected by wind speed, wind duration and fetch – the distance over which the wind blows in a single direction, and these factors work together to determine the size of waves. A fully developed sea state occurs if, for a given constant wind speed, the depth, fetch and duration are sufficient that the wave travels at the same speed as the wind. In a fully developed state, the wave height and wavelength have reached their full potential: further energy cannot be transferred from the wind to the wave and waves will not continue to grow even if the depth, fetch or duration increases further. Waves that travel beyond the wind-affected zone (by distance or time) are referred to as ‘swell’. Swells propagate across the ocean away from their area of generation and can propagate in directions that differ from the direction of the wind. Swells can travel for hundreds and sometimes thousands of kilometres – swells generated from storms off the Antarctic often reach the equator.
Evaluating the potential of offshore wind energy in the Gulf of Oman using the MENA-CORDEX wind speed data simulations
Published in Engineering Applications of Computational Fluid Mechanics, 2021
Shahab S. Band, Sayed M. Bateni, Mansour Almazroui, Shahin Sajjadi, Kwok-wing Chau, Amir Mosavi
Sea surface roughness length () is an important parameter that affects the sea-atmosphere interaction. The momentum transfer between the sea surface and overlying air is strongly dependent on . Swells and wave shoaling can highly decrease and increase sea surface roughness length, respectively (Taylor & Yelland, 2001). The sea surface roughness length can be related to the significant wave height and wavelength via (Taylor & Yelland, 2001), where and are the significant wave height and peak wavelength for combined sea and swell spectrum, respectively. A and B are empirical coefficients, which are set to 1200 and 45, respectively (Taylor & Yelland, 2001).