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 Tan1. INTRODUCTIONOcean surface waves are the major factor in determining the geometry and composition ofbeaches and significantly influence the planning and design of harbours, waterways, shoreprotection measures, coastal structures and other coastal works. Surface waves derive theirenergy from wind, and a significant amount of this energy is dissipated in the nearshoreregion. The wave energy shapes the coastline, transports and sorts bottom sediments, andexerts forces on coastal structures; hence the information obtained from wave climate data isimportant for planning and design in coastal and offshore engineering. The local wave climateis also important in other fields, such as shipping, commercial fishing, coastal research andrecreation (Lemm 1996).The conventional method to analyse wave records is through the relatively easymonochromatic wave approach, however this simple method does not consider the directionof wave propagation. The analysis of directional wave is accomplished via the spectralmethod. The main drawback of utilising spectral analysis is the relatively complex and timeintensivecomputations involved, which is the main reason why engineers are reluctant toabandon the monochromatic wave approach. However, the uses of computer models andmultidirectional wave basins have indicated that there are significant differences between thetransformations of monochromatic and directional waves (Goda 1995), and that directionalwave data offer a more accurate representation of natural ocean waves.Directional data is often needed for the design of harbour and coastal structures, and for theanalysis of sediment transport and pollution control. The loading and performance of manymarine structures are directly affected by wave directions, and the consideration of directionaldistribution of wave energy can assist in obtaining an efficient structural design. Takingdirectional wave data into consideration usually results in reduced design wave loads andpossibly a lighter structure, which would reduce construction costs for the structure (Wiegel1981). According to Hashimoto (1995), it is apparent that “with the ever-increasing demandfor the utilisation of coastal areas… there is a need for directional wave data to ensureadequate though cost-effective safety margins be implemented during the design process ofcoastal structures.” (p. 140). The directional wave spectrum is now a standard tool ofIntroduction 1
Directional waves in the nearshore coastal region of Perth, Western AustraliaHuey Jean Tanengineering practice in the planning and designing of many harbour and coastal projects(Goda 1995).The aim of the present study is to carry out an analysis of the nearshore directional waves inthe Perth coastal region. Perth is the capital city of Western Australia, and is located on thesouth-west coast of the Australian continent. Beaches along the coastline of Perth exhibit anapparent seasonality in beach morphology, however not all beaches are characterised by thesame seasonal cycle (Masselink & Pattiaratchi 2001). Some of the Perth beaches becomewider during summer (i.e. accretion) and narrower during winter (i.e. erosion) whereas othersexperience summer erosion and winter accretion (Masselink & Pattiaratchi 2001). It isgenerally accepted that seasonal beach cycles are due to seasonal variations in the incidentwave energy conditions; however the study by Masselink & Pattiaratchi (2001) demonstratedthat the seasonal changes on most beaches along the Perth coasts are better explained by theseasonal reversal in the littoral drift direction. The direction of littoral drift depends largelyupon the direction of the waves incident on the beaches (i.e. nearshore waves), which is, inturn, affected by the direction of wind. During summer, northward sediment transport prevailsdue to sea breeze activity blowing in a predominantly south-southwest direction; howeverduring winter, the longshore sediment transport is towards the south due to southwardflowingcurrents generated by northwesterly storms (Masselink & Pattiaratchi 2001). Due to alack of nearshore directional wave data at the time of the study by Masselink & Pattiaratchi(2001), conclusions could only be drawn on the basis of the wind speed and direction. Themain objective of the analysis is to quantify the directional wave response to changes inmeteorological conditions (specifically storm and sea breeze events). The information fromthis study can then be applied to the findings by Masselink & Pattiaratchi (2001). This is thefirst time a study of nearshore directional waves is done for the Perth coastal region..2 Introduction
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Directional Waves in the Nearshore Coastal Region of Perth ...

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