TSUNAMI OF APRIL 1, 194G — MACDONALD, ET AL. 267 



height reached by the water and in violence of wave attack along 

 Hawaiian shores must be attributed to local influences modifying the 

 size and behavior of the waves. 



The factors found to have affected the height and intensity of tlie 

 waves during the tsunami of April 1, 1946, are : 



1. Orientation of the coast line with respect to the point of origin of the 

 tsunami. 



2. Shape of the island. 



3. Exposure to storm waves. 



4. Submarine topography. 



5. Presence or absence of reefs. 



6. Configuration of tlae coast line. 



7. Merging of waves from different directions, or of different types. 



Orientation of the coast line toith respect to the point of origin of 

 the tsunami. — In general, the heights reached by the water were great- 

 est on the sides of the islarsds facing the origin of the waves, and lowest 

 on the sides away from the wave origin. This is evident from even 

 a cursory inspection of the maps (figs. 3 to 7). Heights average con- 

 sistently greater on the northern than on the southern sides of the 

 islands. All the extreme heights were measured on the northern or 

 nortlieastern sides. Conversely, most of the lowest figures were found 

 on the southern and southwestern sides. It appears ahiiost self-evident 

 that this should be so. Waves striking northern shores retain their full 

 force, whereas the refracted waves striking southern shores suffer 

 a diminution in force and height. This effect is discussed for wind 

 waves in Breakers and Surf (U. S. Navy Hydrographic Office, 1944, 

 pp. 12-13) . No wave can be refracted or reflected without losing some 

 of its force. 



Shape of the island. — Waves were refracted around circular or 

 nearly circular islands much more effectively than around angular 

 or elongate islands. This fact had a marked effect on the height and 

 violence of waves on the southern shores. Thus the water reached 

 considerably greater heights along the southern coast of the nearly 

 round island of Kauai (fig. 3) than along the southern coast of the 

 angular and elongate island of Molokai (fig. 5), even though the 

 heights along the northern coast of Molokai were on the average per- 

 haps a little greater than those on the northern coast of Kauai. The 

 contrast between the very high average height on the northern coast 

 of Molokai and the very low average height on the southern coast is 

 greater than that between the two sides of any other island, although 

 the difference between the extreme highs and lows is almost exactly 

 the same as on the island of Hawaii (fig. 7) . 



Exposure to storm waves. — At the time of the tsunami, large storm 

 waves had been running for several days. As already pointed out, 

 these storm waves riding: in on the backs of the broad swells of the 



