Directional Waves in the Nearshore Coastal Region of Perth ...

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Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tanfaster than shorter period waves, and will reach distant destinations first. The shorter periodcomponents may arrive at the same destination several days later (USACE 2002).In the area of wave generation, energy is transferred from the shorter period waves to thelonger period waves (USACE 2002). Waves can travel large distances without losing muchenergy, but energy can be dissipated internally within the fluid, by interaction with the air,and by friction with the bottom sediment. Due to the fact that short-period wave componentslose energy more easily than long-period components, the periods of swell waves tend to belonger than those of sea waves.2.1.2 Factors Influencing Nearshore WavesOcean surface waves undergo certain transformations as they approach the coastline. Thespeed of wave propagation decreases as water depth decreases, with those of longerwavelengths (such as swell waves) slowing down first. The water depth may vary at differentpoints along the length of the wave, hence different sections of the wave may travel atdifferent speeds (BOM 1993). Waves travelling through shallow water are strongly affectedby seabed bathymetry and currents, and a sloping or undulating sea bed can cause largechanges in wave height and direction of propagation. For instance, shoals can focus wavesand sometimes waves can double in height behind a shoal (USACE 2002). The magnitude ofwave changes depends on the wave period and direction and the distribution of wave energyin frequency and direction.The following points outline the factors affecting wave propagation from deep into shallowwater (USACE 2002):• Propagation effectsThese result from the convergence or divergence of waves caused by shape of bottomtopography, causing wave energy to focus or spread out. Propagation effects includerefraction, shoaling and diffraction. Diffraction is also caused by presence of coastalstructures that interrupt wave travel.• Sink mechanismWave energy is dissipated due to friction, percolation and breaking• Source mechanismThe addition of wave energy due to windLiterature Review 5
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tan• Wave-current and wave-wave interactionsWave transformation analyses for nearshore waves are discussed in detail in USACE (2002)2.2 THEORIES AND METHODS OF WAVE ANALYSIS2.2.1 Regular WavesIn nature, ocean surface waves are three-dimensional, irregular and random in amplitude,period and direction (Horikawa 1988). The ocean surface typically varies in time and isunsteady, and therefore the ocean surface cannot be adequately described in its fullcomplexity. The simplest method to deal with irregular waves is to reduce them to arepresentative two-dimensional monochromatic wave of infinite wave crest, to enable theapplication of the vast knowledge available on periodic waves (Horikawa 1988). Wavetheories have been developed only as approximations to real waves based on variousassumptions. The most elementary wave theory is the linear first-order wave theory (alsocalled small-amplitude or Airy wave theory), and this theory is widely used in coastalengineering and design because it can be applied with ease whilst giving a reasonableapproximation of wave characteristics for a wide range of wave parameters (USACE 2002).The nonlinear, finite amplitude periodic wave theories include Stokes, Cnoidal and solitarywave theory. The details of these theories can be found in several texts such as Horikawa(1988), Kinsman (1965) and USACE (2002).A simple, sinusoidal progressive wave passing a fixed point in the ocean (as depicted inFigure 2.2) can be represented by the horizontal spatial coordinates x and time t. Thefollowing wave parameters are used to describe a simple sinusoidal oscillatory wave:x – horizontal spatial coordinatest – timeL – wavelength: horizontal distance between corresponding points on two successive wavesT – wave period: time interval between two successive crests at a given pointC – wave celerityω – angular or radian frequency," = 2!Tk – wave number,k = 2!Lθ – phase," = kx # ! tH – wave height: vertical distance to its crest from the preceding trough6 Literature Review
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