V. TSUNAMIS APPROACHING THE SHORELINE 



As a tsunami approaches a coastline, the waves are modified by the 

 various offshore and coastal features. Submerged ridges and reefs, 

 continental shelves , headlands , various shaped bays , and the steepness 

 of the beach slope may modify the wave period and wave height, cause 

 wave resonance, reflect wave energy, and cause the waves to form bores 

 which surge onto the shoreline. 



Ocean ridges provide very little protection to a coastline. While 

 some amount of the energy in a tsunami might reflect from the ridge, 

 the major part of the energy will be transmitted across the ridge and 

 into the coastline. The 1960 tsunami which originated along the coast 

 of Chile is an example of this. That tsunami had high wave heights along 

 the coast of Japan, including Shikoku and Kyushu which lie behind the 

 South Honshu Ridge (Hirono, 1961). 



1. Abrupt Depth Transitions . 



An ocean shelf along a coastline may cause greater modification to 

 a tsunami than an ocean ridge. Waves may become higher and shorter, and 

 dispersion may occur. Lamb (1932) gave the equations for a single wave 

 passing over an abrupt change in water depth as shown in Figure 16. He 

 considered only the case of a wave at a zero angle of incidence, i.e., 

 & 1 = 6 2 = 0. The equations he derived are 



H d 1 / 1 - d 1 / 2 

 r 1 2 



H. 6 W2 , Al s 2 (156) 



+ d J 

 2 



H t 2d l /2 



H. jl/2 a ,,1/2 



t 



d 1/z + d J 



1 2 



(157) 



and 



H H 



H, = H. + H 



t v r 



where 



H- = the incident wave height 



Hj, = the reflected wave height 



H. = the transmitted wave height 



d. = the initial water depth 



d 2 = the water depth under the transmitted wave 



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