scouring and the increased hydrostatic pressure from initial overtopping 

 may cause failure. The concrete seawall along a highway between Hadenya 

 and Mitobe on Shizukawa Bay (Fig. 54) collapsed seaward. Similar fail- 

 ures occurred along a highway on Onagawa Bay and along a quay wall at 

 Kamaishi, Japan. Magoon (1962) noted that approximately 2 meters of 

 sand was scoured at the seaward toe of a steel sheet -pile retaining wall 

 at Crescent City, California, in 1960 which contributed to its partial 

 failure. Also, a woodpile-mooring dolphin was destroyed as a result of 

 the loss of sand at its base. Matuo (1934) mentions a concrete retaining 

 wall which was overturned seaward by the 1933 Sanriku tsunami. 



The damage from the 1960 tsunami in Hawaii is evidence of the erosive 

 force of a tsunami. Concrete seawalls 0.9 meter high were washed out and 

 a gully about 3 meters deep and 27 meters wide was washed into a highway 

 along the shoreline at Hilo, extending inland about 18 meters. Large 

 stones from a seawall, weighing up to 20 metric tons (22 tons), were 

 carried inland (Eaton, Richter, and Ault, 1961). Shepard, MacDonald, 

 and Cox (1950) mention a case where water overtopping sand dunes cut a 

 channel about 30 meters wide and about 5 meters (15 feet) deep. 



Tsunamis will not always produce the maximum forces on a structure. 

 A concrete seawall protected the buildings at the Puu Maile Hospital at 

 Hilo during the 1946 tsunami. The seawall was undamaged by the tsunami, 

 but a few months later storm waves destroyed parts of the wall and damaged 

 the lower floor of the hospital (Shepard, MacDonald, and Cox, 1950). 



Matuo (1934) reports on a dynamometer located on a breakwater at 

 Hatinohe harbor, Japan, during the 1933 Sanriku tsunami. The dynamometer 

 was located 0.76 meter (2.5 feet) below the level of the water surface at 

 the time of arrival of the tsunami. The recorded maximum pressure was 

 38,300 newtons per square meter (800 pounds per square foot) for a wave 

 with a height of 3.2 meters (10.5 feet) and a period of 6 minutes. 



Nasu (1948) developed some empirical criteria for the stability of 

 breakwaters based on the geometric shape of the breakwater. For a break- 

 water with a seaward slope of 1:2.5 and a landward slope of 1:2, he gives 



h + 0.89b 



u 2 < _J> (320) 



0.0358 



for the condition of geometric stability, where u is the current veloc- 

 ity in meters per second, hy the height in meters of the vertical seg- 

 ment of the face of the breakwater against which the current acts, and 

 b the top width of the breakwater in meters. 



Kaplan (1955) gives an empirical equation for the volume of overtop- 

 ping of a seawall at the shoreline. This equation can be rewritten as 



21.65(Kh - h ) 3 

 V ^ — (321) 



YT h 



s 



159 



