illustrate the damage to the railroad bridge on the Wailuku River (Fig. 

 63) and the railroad trestle on Kilekole Stream (Fig. 64) caused by the 

 1946 tsunami in Hawaii. Iwasaki arid Horikawa (1960) show a case of 

 Mangoku, Japan, where a bridge support (Fig. 65) slumped almost 1 meter 

 due to the heavy scouring of the channel bottom. 



4. Tsunami Surge on the Shoreline, 



The determination of the runup height of a tsunami is discussed in 

 Section VII, 1. After the runup height of a tsunami has been established, 

 the effects of this runup on structures and other objects located near 

 the shoreline must be determined. When the tsunami acts as a rapidly 

 rising tide, the resulting incident current velocities are relatively 

 low, and most initial damage will result from buoyant and hydrostatic 

 forces and the effects of flooding. Shepard, MacDonald, and Cox (1950) 

 noted that in many instances the withdrawal of the water occurred much 

 more rapidly than the runup and flooding. In some instances, damage may 

 result from the higher current velocities associated with the withdrawal. 

 These velocities would be on the order of those normally associated with 

 an incident surge. More concern is therefore given to a tsunami which 

 approaches the shoreline as a bore. 



When the tsunami forms a borelike wave, the runup on the shoreline 

 has the form of a surge on dry ground. This surge should not be confused 

 with the bore approaching the shoreline 3 as different equations govern 

 the motion and profile of the surge. Miller (1968) noted from laboratory 

 observations that a bore approaching a shoreline exhibits a relative 

 steepening of the bore face just before reaching the shoreline, and that 

 this is followed by a flattening of the face of the surge on the dry 

 slope. The current velocities associated with the surge are proportional 

 to the square root of the surge height, and approximations of the current 

 velocities can be obtained from equations (318) and (319), with equation 

 (318) providing the more conservative result. For a surge height approach- 

 ing 5 meters, the estimated current velocity would be about 14 meters (46 

 feet) per second. When the tsunami runup acts as a high velocity surge 



169 



