It can be seen that very little reserve force remains to resist drag 

 forces from the surge. With a lower level of oil in the tank, the 

 buoyant force could overcome the mass of the tank and the oil plus 

 the strength of any structural anchorages. 



************************************* 



In Shepard, MacDonald, and Cox's (1950) discussion of the 1946 

 tsunami in Hawaii, a house at Kawela Bay on Oahu was floated off its 

 foundation and deposited in a canefield 61 meters inland, leaving break- 

 fast cooking on the stove and dishes intact on shelves. Many other 

 houses were also gently floated from their foundations, and some houses 

 could be moved back to their original foundations with very little repair 

 work required. Damage caused by buoyant forces was the result of build- 

 ings being deposited on uneven ground, the fact that some buildings had 

 weak structures and broke apart when lifted from their foundations, and 

 minor damage from the breaking of water pipes and electric lines. 



In many instances, where tsunamis act like rapidly rising tides, the 

 current velocity associated with the waves is very low, so that the major 

 damages are similar to those discussed above. Shepard, MacDonald, and 

 Cox (1950) mentioned instances of people wading through chest-high water 

 to escape from the tsunami. 



b. Surge Forces . Cross (1967) showed that the force per unit length 

 of vertical wall, from the leading edge of a surge impinging normally to 

 the wall could be given as 



F = — — + C„ pu 2 h (325) 



2 c 



where F is the force in newtons per meter of width, h the surge height 

 in meters, u the surge velocity in meters per second, and C^ a force 

 coefficient defined by 



C p = (tan O) 1 * 2 + 1 (326) 



where 6 is the inclination of the water surface of the surge shown in 

 Figure 66; tan 6 is given by the equation 



tan = — = V- + b (327) 



dz C 7 2 h 



h 



where C^ is the Chezy roughness coefficient, Z the distance from the 

 leading edge, and b given by 



b=±^-S (328) 



g dt 



174 



