On 1 April 1946, a tsunami struck the Hawaiian Islands with runup in 

 places as high as 17 meters (55 feet) above sea level (Shepard, MacDonald, and 

 Cox, 1950). The beach changes were similar to those inflicted by storm waves, 

 although "in only a few places were the changes greater than those produced 

 during normal storm seasons or even by single severe storms." Because a 

 tsunami is of short duration, extensive beach changes do not occur, although 

 property damage can be quite high. 



Several conclusions can be drawn from the above examples. If a beach has 

 a sufficient sand supply and fairly high dunes that are not breached, little 

 permanent modification will result from storms, except for a brief accelera- 

 tion of the normal littoral processes. This acceleration will be more 

 pronounced on a shore with low-energy wave conditions. 



IV. NEARSHORE CURRENTS 



Nearshore currents in the littoral zone are predominantly wind and wave- 

 induced motions superimposed on the wave-induced oscillatory motion of the 

 water. The net motions generally have low velocities, but because they 

 transport whatever sand is moved by the wave-induced water motions, they are 

 important in determining littoral transport. 



There is only slight exchange of fluid between the offshore and the surf 

 zone. Onshore-offshore flows take place in a number of ways that are not 

 fully understood at present. 



1. Wave-Induced Water Motion . 



In idealized deepwater waves, water particles have a circular motion in a 

 vertical plane perpendicular to the wave crest (Ch. 2, Fig. 2-4), but this 

 motion does not reach deep enough to affect sediment on the bottom. In depths 

 where waves are affected by the bottom, the circular motion becomes 

 elliptical, and the water at the bottom begins to move. In shallow water, the 

 ellipses elongate into nearly straight lines. At breaking, particle motion 

 becomes more complicated; but even in the surf zone, the water moves forward 

 and backward in paths that are mostly horizontal, with brief, but intense, 

 vertical motions during the passage of the breaker crest. Since it is this 

 wave-induced water particle motion that causes the sediment to move, it is 

 useful to know the length of the elliptical path travelled by the water 

 particles and the maximum velocity and acceleration attained during this 

 orbit. 



The basic equations for water-wave motion before breaking are discussed in 

 Chapter 2. Quantitative estimates of water motion are from small-amplitude 

 wave theory (Ch. 2, Sec. 11,3), even near breaking where assumptions of the 

 theory are not completely valid (Dean, 1970; Eagleson, 1956). Equations 2-13 

 and 2-14 give the fluid-particle velocity components u , w in a wave where 

 small-amplitude theory is applicable (see Fig. 2-3 for relation to wave phase 

 and water particle acceleration). 



For sediment transport, the conditions of most interest are those when the 

 wave is in shallow water. For this condition, and making the small-amplitude 



4-46 



