264 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



2 days, but with occasional waves which were larger than those just 

 before and after them. Such temporary increases in wave height 

 probably resulted from mutual reinforcement by the essentially simul- 

 taneous arrival, in phase, of waves which had traveled different paths, 

 or from the coincidence of tsunami waves with storm waves or seiche 

 oscillations. 



Measures of the height of the waves approaching shore in shallow 

 water, but before they dashed up on shore, are poor. At Kawela Bay, 

 Oahu, Shepard estimated the height of the waves advancing across 

 the reef to have been as much as 18 feet, and observers estimated the 

 height of the waves crossing the reef off Lanikai, on Oahu, to have been 

 about 7 feet. Photographs taken at Hilo show the top of the breakers 

 to have been 25 feet above the normal bay surface where they struck 

 Cocoanut Island, but the waves may have increased considerably in 

 height in crossing the breakwater, and the effect of dashing up on the 

 shore was probably already present, further exaggerating the height. 

 Photographs of some of the late waves at the mouth of the Wailuku 

 Kiver, in Hilo, show them to have been 6 to 8 feet high (pi. 6) , and 

 early waves undoubtedly were higher. In general, these heights cor- 

 respond fairly closely with the measured heights to which the water 

 dashed on the shore at those localities. At any rate it appears clear 

 that the waves not only slowed down, but increased in height on 

 entering shallow water. George Green (1838) states that the wave 

 height varies inversely as the fourth root of the depth of the water. 



Most observers reported the first movement on Hawaiian shores to 

 have been a withdrawal of the water. However, the only available 

 instrumental records, at Honolulu and Waimea, both indicate the first 

 movement to have been a rise. The instrumental records are prob- 

 ably more reliable than the reports of untrained observers. The initial 

 rise at Honolulu was small (fig. 2), and a similar small rise at other 

 localities may easily have been overlooked. Certainly it would have 

 been less impressive than the large withdrawal of the water from shore 

 as the succeeding trough approached. It is interesting to note that 

 the records of tide gages along the coasts of North and South America 

 obtained by C. K. Green (1946, p. 497) all show the initial movement to 

 have been a rise, with amplitude of about one-third that of the ensuing 

 trough. 



HEIGHTS REACHED BY WAVES ON HAWAIIAN SHORES 



Measurements of high-water marks have been made around the 

 shores of all five major islands of the Hawaiian group. The measured 

 heights are shown on figures 3 to 7. All heights are stated in feet 

 above lower low water. At each point sea level was estimated, the 

 height of the high- water mark above that level was measured by 

 means of hand level or steel tape, and the measurement reduced by 



