A hurricane (unnamed) coincided with spring tide on the New England 

 coast on 21 September 1938. Property damage and loss of life were both 

 high. A storm of this magnitude was estimated to occur about once every 

 150 years. A study of the beach changes along a 12-mile section of the 

 Rhode Island coast (Nichol and Marsten, 1939) showed that most of the 

 changes in the beach profile were temporary. The net result was some 

 cliff erosion and a slight retrogression of the beaches. 



Beach changes from Hurricane Donna which hit Florida in September 

 1960 were more severe and permanent. In a study of the southwestern 

 coast of Florida before and after the storm. Tanner (1961), concluded 

 that "Hurricane Donna appears to have done 100 year's work, considering 

 the typical energy level thought to prevail in the area." 



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

 places as high as 55 feet above sea level. (Shepard, et al., 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 nor- 

 mal 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, and if the dunes are 

 not breached, little permanent modification will result from storms, except 

 for a brief acceleration of the normal littoral processes. This accelera- 

 tion will be more pronounced on a shore with low-energy wave conditions. 



4.4 NEARSHORE CURRENTS 



Nearshore currents in the littoral zone are predominantly 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 set in motion 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, which 

 at present are not fully understood. 



4.41 WAVE- INDUCED WATER MOTION 



In idealized deepwater waves, water particles have a circular motion 

 in a vertical plane perpendicular to the wave crest (Fig. 2-4, Section 

 2.235), but this motion does not reach deep enough to affect sediment on 

 the bottom. In depths where waves are affected by the bottom, the circu- 

 lar 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 hori- 

 zontal, with brief, but intense, vertical motions produced by the passage 



4-39 



