and the wind speed so high that a relatively long duration would be required 

 for the waves to reach the limiting heighto Such durations are rather infre- 

 quent., In lakes and land-locked bays the fetch is limited by the size of the 

 body of water, and large waves , therefore , cannot be formed „ Within the 

 generating area, steep s short-crested waves of all possible heights, lengths, 

 and period are present, and many of them break as whitecaps The waves 

 which leave a generating area and move into relatively calm water (commonly 

 called the decay area) are known as swell „ The length, period, and velocity 

 of the swell gradually increase with distance from the generating area| 

 whereas, the height gradually decreases , In the Pacific Ocean where the 

 generating areas may be several thousand miles from the coast, the decay 

 distances are relatively large „ The waves which travel such long distances 

 become very regular, and when they reach the coast they become a series of 

 long, low, and fairly regular undulations which usually are called ground 

 swells o In deep water the presence of this swell may be almost totally ob- 

 scured by small waves which are newly formed by local winds „ It should be 

 mentioned that in relatively small bodies of waters, as lakes and bays, 

 which are subjected to wind action the entire water surface usually is a 

 generating area. There is no decay area in such instances and the waves 

 lose their energy during the storm by breaking on an adjacent shore 



Waves usually are classed as either deep-water or shallow-water waves. 

 When waves are moving in water depths less than half the wave length, their 

 motion is influenced by the bottom,, Thus, waves moving in depths greater 

 than half a wave length are termed deep-water waves, and when moving in 

 lesser depths they are considered as shallow-water waves „ Shallow-water 

 waves moving into shoaling water undergo important transformations „ The 

 velocity and length of the waves decrease, while the period remains the 

 same, and the total energy is slightly reduced by bottom friction „ Except 

 for a small initial decrease in height when the wave first "feels the 

 bottom" , and height usually increases up to the point of breaking,, If 

 waves approach a shore line at an angle, the inshore portion of the wave 

 travels at a lower velocity than the portion in deeper water,, The result 

 of refraction is a change in the wave heights and the direction of travel, 

 with the wave fronts becoming oriented nearly parallel to tiie bottom con- 

 tours „ These factors may vary considerably at closely adjacent points on 

 a coast, due solely to the underwater topography,, For example, refraction 

 concentrates wave energy over submarine ridges, causing the waves to in- 

 crease considerably in heighto Over a submarine canyon, on the other hand, 

 low wave heights occur due to the spreading of wave energy „ Methods of 

 estimating the changes in wave heights and direction of travel are dis- 

 cussed subsequently,, 



For waves with a relatively large ratio of height to length in deep 

 water (usually short-period waves) the breaker height is about the same 

 as the wave height in deep water, but for waves with a small height-length 

 ratio in deep water (usually long -period waves) the breakers become much 

 higher than the deep-water heigh t„ A long low swell which hardly may be 

 observed in deep water can thus cause higher breakers than short period 

 waves of much greater deep-water height (Figure 1)„ It is the swell which 

 usually is responsible for the predominant breakers in the surf zone and 

 for damaging coastal structures „ 



