Tsunami of April l, 1946 — Macdonald et al. 
effect on the heights at their heads; but it 
should be noted that they extend across the 
general direction of wave advance, not 
toward it. 
The greater heights reached by the water 
at the heads of submarine ridges are not 
difficult to explain. The ridge has a greater 
effect in limiting the movement of water 
particles in the advancing wave than does 
the deeper water alongside it. Consequently 
the portion of the wave over the ridge is 
retarded more than that away from the ridge, 
and the wave front becomes bent, with its 
concavity directed 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 Office, 1944: 13). 
Similarly, in moving toward shore along 
the axis of a submarine valley, the part of 
31 
the wave in the deep water along the valley 
axis moves faster than that in shallower 
water on the two sides. In consequence the 
wave front becomes bent, with its convexity 
toward the valley head. In the vicinity of 
the valley head the force lines (orthogonals ) 
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 northeast¬ 
ward from the bay, to a depth of 150 feet. 
An example on a much larger scale is af- 
Fig. 6. Map of the island of Maui, showing heights (in feet above lower low water) reached by 
the water during the tsunami of April 1, 1946. 
