30 
PACIFIC SCIENCE, January, 1947 
ward coasts which were sheltered from the 
storm waves also experienced less violent 
waves. Thus at Kalaupapa, on the sheltered 
side of the peninsula on the windward side 
of Molokai, both photographs and the testi¬ 
mony of observers indicate that the rise of 2 5 
feet caused by the tsunami was not violent. 
On the windward coasts, much of the rapid 
variation in intensity of wave attack may 
have resulted from the caprice of storm 
waves. 
Submarine topography. — Owing to their 
great wave length, the waves were somewhat 
affected by the ocean bottom throughout 
their course. However, the effect of the bot¬ 
tom increased greatly as the waves moved 
into shallow water, and caused a slowing of 
the wave, an increase in its height, and a 
steepening of its front. A direct evidence of 
the increase in height of the waves in shal¬ 
low water was afforded by the lesser heights 
reached by the water at the ends of certain 
peninsulas projecting into deep water and 
not prolonged seaward by pronounced 
ridges, as compared with the heights on ad¬ 
jacent shores rising from shoal water. Thus 
at the end of Kalaupapa Peninsula, on the 
northern coast of Molokai (Fig. 5), the 
water dashed only 7 feet above normal sea 
level, distinctly less than do the waves of 
ordinary storms; whereas on the coasts ris¬ 
ing from shoal water both east and west of 
the peninsula, the water swept up to heights 
of 30 to 54 feet. At the end of Keanae Pen¬ 
insula, on the northern coast of Maui (Fig. 
6), the tsunami reached heights only a little 
greater than large trade-wind waves. 
Submarine ridges and valleys, particularly 
those pointing toward the wave source, were 
of great importance in their effect on the 
strength of the waves. The best examples of 
the effect of ridges are found on the north¬ 
ern coast of Kauai. A long ridge extends 
in a direction slightly west of north from 
Haena, to a depth of about 8,000 feet 
(Plate 2). Another extends northeastward 
from Kilauea Point, to even greater depths. 
The greatest heights (45 feet) reached by 
the water on the shores of Kauai were at 
the heads of these two ridges (Fig. 3). An¬ 
other ridge extending northwestward from 
the western coast of Kauai is probably re¬ 
sponsible for heights of 35 to 38 feet at its 
head. Long ridges projecting from Kaena 
and Kahuku Points on Oahu similarly caused 
an increase in wave heights there as com¬ 
pared to the heights on both sides (Fig. 4). 
The ridges projecting eastward north of Hilo 
Bay and at Cape Kumukahi on Hawaii had, 
on the other hand, no such pronounced 
Fig. 5. Map of the island of Molokai, showing heights (in feet above lower low water) reached 
by the water during the tsunami of April 1, 1946. 
