TSUNAMI OF APRIL 1, 194 6 — MACDONALD, ET AL. 



269 



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 northern coast of Kauai. A long ridge extends in a direction 

 slightly west of north from Haena, to a depth of about 8,000 feet 

 (fig. 9) . 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). 



Figure 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. 



Another ridge extending northwestward from the western coast of 

 Kauai is probably responsible for heights of 35 to 38 feet at its head. 

 Long ridges projecting from Kaena and Kahuku Points on Oahu simi- 

 larly caused an increase in wave heights there as compared 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 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 sub- 

 marine ridges are not difficult to explain. The ridge has a greater 

 effect in limiting the movement of water particles in the advancing 



