Historical tsunami inundation data for the Kahuku Point area on Oahu, Hawaii, are compiled or deduced from comparison with Hilo, Hawaii. The data are considered in two parts; North Pacific tsunamis and South American tsunamis. The South American tsunamis are about 1/3 as large at Kahuku Point as they are at Hilo: North Pacific tsunamis are about the same at both locations. The maximum North Pacific inundation is 27 feet; the maximum South American inundation is 9 feet.
The distribution of inundation along the shoreline area of the point from Kawela Bay to Kahuku is rather uniform--being within 20 percent of the average.
The historic tsunami data are ranked by size. Assuming a Poisson distribution and independence between the observations, a graphical relationship for the frequency of occurrences is found. The extreme tsunami height encountered for a given duration and at a given level of accepted risk is graphically determined. The graphical presentation is modified to permit easier use. For example, accepting a risk of one in twenty for kO years, results in a tsunami design height of 43 feet. By accepting greater risk, a one in ten chance, the tsunami design level is reduced to 35 feet. The tsunami design level determined by the risk level actually accepted could define the height of the first floor. In the previous example of a risk of one in twenty, this height would be 17 feet (43 minus 26, the ground elevation of the structure); for a risk of one in ten, only 9 feet.
The probable sequence of events at Kahuku Point for an Aleutian Tsunami is:
Knowledge of the generating earthquake and the possible tsunami is reported to Kahuku Point at least two hours before the tsunami arrives.
The Kahuku Point area receives tsunami inundation highs relative to most parts of Oahu.
The inundation is of the form of a tidal bore.
Several inundations occur.
The surf breaks at or inland of the present beach line during periods of inundation.
The currents and waves at Kahuku Point are unusual for several days after the tsunami. The currents are much stronger than normal. No one should enter the water.
There is the possibility that protective structures against the potential tsunami damage at Kahuku Point can be built. These include armor, breakwaters, sloping walls, etc. A similar problem and situation occurs at Hilo, Hawaii. Recent studies of the Hilo situation indicate that the esthetic benefits of a natural environment are more important than the physical protection provided by the designed breakwater. Every breakwater design proposed is very expensive and severely mars the vistas.
By analogy, the findings at Hilo, Hawaii, apply to Kahuku Point on Oahu. Any protective structure would have to be so large as to blot out the seascape. The expense of any such protective structure is better invested in improving the design and construction of the buildings.
Figures & Tables
Prepared by the Case Histories Committee for the Engineering Geology Division of the Geological Society of America, these histories are intended as reference material for the practicing geologist and for the college student. This volume, the eighth in the Case History series, presents the seismological aspects of the works of man—the civil engineer or engineering geologist interacting with the environment. Topics are in two categories—changes at a point (nuclear or chemical explosions and well injection or withdrawal) and changes on a line (damming a river or construction along a coastline).