The leave velocity is the rate of travel of the 

 wave form through the water. Here again, as in 

 the case of height, there is considerable varia- 

 tion in these characteristics in any given wave 

 train. For obsen'ation purposes, one should 

 determine the average values for the signifi- 

 cant waves. Wave direction is the direction, in 

 degrees true, from which the waves come. 



If wave systems cross each other at a con- 

 siderable angle, the result is a vei-y irregular sea 

 surface comprised of apparently unrelated 

 peaks and hollows and is tenned a cross sea. 

 Waves are said to be shoi't-erested when the 

 crests are short compared to the wave length and 

 long -crested when crests are long compared to 

 the wave length. Waves are commonly short 

 crested in cross seas and in the early stages of 

 generation, while swell is generally long crested. 

 In deep water, where the orbital motion of water 

 particles is uninhibited by the bottom while the 

 waxes proceed through the water, the wave 

 period is related mathematically to the wave 

 length and wave velocity in such a manner that 

 they travel at different speeds, and are con- 

 stantly overtaking or pullmg away from their 

 neighbors. If the crests of two waves happen to 

 be at the same point at the same time, their com- 

 bination results in a crest that is higher than 

 either of the component crests. This phenom- 

 enon, know as loave interference, accentuates 

 the variability in wave height. Conversely, inter- 

 ference also can cause flat zones when the trough 

 of one wave meets the crest of another. The 

 hydrodynamics of surface wave motion is such 

 that as the period increases, the speed and wave 

 length increases as well. The following approx- 

 imate formulas show this relation, where the 

 units are knots, feet, and seconds. 

 Wave speed = 3.0 (Period) 

 Wavelength=5.12 ( Period) = 

 Wave Period=0.3 (Wave Speed) 

 5. Whitecaps. — In deep water, the wind may 

 blow strong enough to raise steep and choppy 

 wind waves. When the ratio of height to length 

 becomes too large, the water at the crest moves 

 faster than the crest itself, causing the water to 

 topple forward and form whitecaps. The term 

 whitecap is confined to deep water waves while 

 the term hrenker is used to describe waves 

 breaking in shoal water or in strong-tidal cur- 

 rents which oppose wave motion. "\^^^itecaI}s 

 owe their instability to a too rapid addition of 

 energy from the wind to the wave form and 

 breakers to the restrictive eft'ect of the sea bot- 

 tom or opposing currents upon the water move- 

 ment in the wave form. 



(>. Breakers. — A breaker is an ocean wave, 

 either wind wave or swell, which has traveled 

 over a gradually shoaling bottom and reached 

 the point in its transformation where it is no 

 longer stable and plunges over or breaks. As a 

 rule, when swell is definitely predominant, the 



breakers are regular with smooth profiles. When 

 wind waves are predominant, the breakers are 

 choppy and confused. Swell coming into a beach 

 increases in height up to the point of breaking. 

 Wind waves, on the other hand, are already so 

 steep that there is little if any incre-ase in height 

 just before breaking. Thus, swell often defines 

 the period of the breakei-s even though the wind 

 waves appear to predominate in deep water. A 

 long, low swell in deep water may be obscured 

 by choppy wind waves and be detectable only 

 on the beaches. 



7. Surf. — The zone of breakers, termed surf, 

 includes the region of white water between the 

 outermost breaker and the waterline on the 

 beach. During a storm, it may be difficult to dif- 

 ferentiate between surf inshore and whitecaps 

 in deep water just beyond. 



8. Wave Refraction and Longshore Cur- 

 rents. — When waves approach a coastline at any 

 angle, they tend to swing around and break 

 parallel to the beach. The waves are slowed down 

 as they come into shallow water by the inhibit- 

 ing effect of the bottom, and owing to the change 

 in velocity and contour of the bottom, are de- 

 flected, or refracted from their original parts. 

 Consequently, refraction is only noticeable on 

 beaches with gradual profiles since the bottom 

 must influence the waves over several wave- 

 lengths. Wlien waves do not swing all the way 

 aromid before breaking and break at an angle 

 with the shoreline, a current in the direction of 

 the open angle is generated. The strength of 

 this current depends chiefly on the height, pe- 

 riod, angle of approach of the waves, and beach 

 configuration. If a longshore current develops 

 during unloading operations, it may swing ves- 

 sels sideways and broach them. 



B-11 Effect of Tidal Currents.— If tidal cur- 

 rents acquire velocities of two or tliree knots or 

 greater, they affect the waves which travel into 

 the area of their influence. Waves opposing cur- 

 rents tend to steepen and increase in height; 

 those moving with currents flatten and decrease 

 in height. Unless the current is effective over a 

 considerable area with a moderate to high ve- 

 locity, wave changes are not usually noticeable. 

 Near headlands and in tidal races, however, 

 they may be appreciable, and zones of white- 

 caps where the waves are breaking because of 

 this effect occur at many places. 



B-12 Effect of Shoals. — Waves (either sea or 

 swell) in passing over shoals or bars tend to 

 steepen and increase slightly in height. Long 

 swells feel tlie effect of a deeper shoal more than 

 wind waves and may steepen more noticeably, 

 but will not break as quickly because their 

 height to length ratio is initially very low. 

 Wind waves, on the other hand, may suddenly 

 steepen and break. 



B-12 



