28 
PACIFIC SCIENCE, January, 1947 
on Hawaii, 54 feet at Waikolu Valley on 
Molokai, and 45 feet at Haena and Kilauea 
Point on Kauai, to 2 feet at Kaunakakai on 
Molokai, 2 feet at Milolii and Hoopuloa on 
Hawaii, and less than 2 feet at the head of 
Kaneohe Bay on Oahu. Causes of the varia¬ 
tions in height will be discussed in a later 
section. 
Most of the heights measured are, of 
course, not the heights of the actual waves, 
but rather the heights to which the water 
was driven on shore. On a vertical cliff di¬ 
rectly across the path of the wave, this height 
may theoretically amount to twice the height 
of the actual wave. On slopes less than ver¬ 
tical, or on cliffs at an angle to the direction 
of wave advance, it should be somewhat less 
than twice the wave height. This measure 
represents the height of dash of solid water, 
but very abundant spray may be thrown 
much higher. Moreover, storm waves riding 
on the crest of the broader swells of the 
tsunami undoubtedly added in places to the 
height to which water dashed on shore. 
There are places where normal trade-wind 
waves are flung to a height nearly as great 
as that reached by the tsunami, and many 
places, particularly on shores facing away 
from the origin of the tsunami, where waves 
of heavy storms reached appreciably higher 
than did the waves of the tsunami. 
It is not possible to make reliable esti¬ 
mates of the magnitudes of these complicat¬ 
ing factors, as there are too many unknown 
elements involved. However, it is probable 
that most of the water heights recorded for 
the tsunami on the northern and eastern 
sides of the islands were appreciably in¬ 
creased by these factors. 
FACTORS INFLUENCING THE HEIGHTS 
AND INTENSITIES OF THE WAVES 
It may be assumed that the size and speed 
of the waves approaching the islands from 
the open ocean to the north were essentially 
the same throughout the length of the Ha¬ 
waiian Archipelago. Differences in height 
reached by the water and in violence of wave 
attack along Hawaiian shores must be at¬ 
tributed to local influences modifying the 
size and behavior of the waves. 
The factors found to have affected the 
height and intensity of the 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 the coast line. 
7. Merging of waves from different direc¬ 
tions, or of different types. 
Orientation of the coast line with respect to 
the point of origin of the tsunami. —In gen¬ 
eral, the heights reached by the water were 
greatest on the sides of the islands 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 (Fig. 3 to 7). Heights average 
consistently greater on the northern than on 
the southern sides of the islands. All the 
extreme heights were measured on the north¬ 
ern or northeastern sides. Conversely, most 
of the lowest figures were found on the 
southern and southwestern sides. It appears 
almost self-evident that this should be so. 
Waves striking northern shores retain their 
full force, whereas the refracted waves strik¬ 
ing 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: 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 
