CHAPTER 2 

 MECHANICS OF WAVE MOTION 



2.1 INTRODUCTION 



The effects of water waves are of paramount importance in the field 

 of coastal engineering. Waves are the major factor in determining the 

 geometry and composition of beaches, and significantly influence planning 

 and design of harbors, waterways, shore protection measures, coastal 

 structures, and other coastal works. Surface waves generally derive their 

 energy from the winds. A significant amount of this wave energy is finally 

 dissipated in the nearshore region and on the beaches. 



Waves provide an important energy source for forming beaches; assort- 

 ing bottom sediments on the shoreface; transporting bottom materials on- 

 shore, offshore, and alongshore; and for causing many of the forces to 

 which coastal structures are subjected. An adequate understanding of the 

 fundamental physical processes in surface wave generation and propagation 

 must precede any attempt to understand complex water motion in the near- 

 shore areas of large bodies of water. Consequently, an understanding of 

 the mechanics of wave motion is essential in the planning and design of 

 coastal works. 



This chapter presents an introduction to surface wave theories. 

 Surface and water particle motion, wave energy, and theories used in 

 describing wave transformation due to interaction with the bottom and 

 with structures are described. The purpose is to provide an elementary 

 physical and mathematical understanding of wave motion, and to indicate 

 limitations of selected theories. A number of wave theories have been 

 omitted. References are cited to provide information on theories not 

 discussed and to supplement the theories presented. 



The reader is cautioned that man's ability to describe wave phenomena 

 is limited, especially when the region under consideration is the coastal 

 zone. Thus, the results obtained from the wave theories presented should 

 be carefully interpreted for application to actual design of coastal struc- 

 tures or description of the coastal environment. 



2.2 WAVE MECHANICS 



2.21 GENERAL 



Waves in the ocean often appear as a confused and constantly changing 

 sea of crests and troughs on the water surface because of the irregularity 

 of wave shape and the variability in the direction of propagation. This 

 is particularly true while the waves are under the influence of the wind. 

 The direction of wave propagation can be assessed as an average of the 

 directions of individual waves. A description of the sea surface is 

 difficult because of the interaction between individual waves. Faster 

 waves overtake and pass through slower ones from various directions. 

 Waves sometimes reinforce or cancel each other by this interaction, and 

 often collide with each other and are transformed into turbulence, and 



2-1 



