270 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



wave than does the deeper water alongside it. Consequently the por- 

 tion of the wave over the ridge is retarded more than that away from 

 the ridge, and the wave front becomes bent, with its concavity di- 

 rected toward the ridge head. The result is a focusing of wave force 

 on the shore at the head of the ridge (U. S. Navy Hydrographic Of- 

 fice, 1944, p. 13). 



Similarly, in moving toward shore along the axis of a submarine 

 valley, the part of the wave in the deep water along the valley axis 

 moves faster than that in shallower water on the two sides. In con- 

 sequence the wave front becomes bent, with its convexity toward the 

 valley head. In the vicinity of the valley head the force lines (orthog- 

 onals) of the wave are diffused or spread apart, and over any unit 

 area the force of the waves striking shore is greatly decreased. 



An example of the effect of a submarine valley in lessening the 

 force of the waves at its head is found at Kahana Bay, on Oahu (fig. 

 4). There the waves dashed to heights of 11 to 17 feet on the coasts 

 north and south of the bay, but reached heights of only 4 to 7 feet 

 in the bay itself. A small submarine valley extends 2 miles north- 

 eastward from the bay, to a depth of 150 feet. An example on a 

 much larger scale is afforded by the zone of small heights along the 

 northwestern shore of Kauai (fig. 3), at the head of a broad swale 

 extending outward to oceanic depths. The broad valleylike depres- 

 sion off the eastern coast of Hawaii south of Hilo Bay probably also 

 was somewhat effective in reducing the heights reached by the water 

 along that coast. Although fairly great, ranging from 16 to 19 feet, 

 the heights there are not much greater than those reached by ordinary 

 storm waves. 



Presence or absence of reefs. — The presence of a well-developed 

 fringing reef appears to have had a decided effect in reducing the 

 intensity of wave onslaught. Along the reef -protected northern coast 

 of Oahu the heights reached on shore by the waves were on the average 

 decidedly less than on the unprotected northern coasts of Molokai 

 and Hawaii, or on the less-protected northern coast of Kauai. The 

 best-developed coral reef in the Hawaiian Islands fills Kaneohe Bay 

 on Oahu, where it has a width of about 3 miles. Despite the fact that 

 the broad mouth of Kaneohe Bay is open to the north and northeast, 

 the tsunami produced a rise in water level at the bay head which was 

 so small as to be hardly perceptible to observers, and, so far as could 

 be determined, nowhere exceeded 2 feet. Along the shore north of the 

 bay the heights ranged from 4 to 10 feet, and on the end of Makapuu 

 Peninsula southeast of the hvcy the heights reached more than 20 feet 



(% 4). 



The lesser heights along the southern shore of Molokai were prob- 

 ably partly due to the wide protecting reef. The effect of the reef in 

 reducing wave violence along that shore is well shown at places 



