'hus for dg = 6 feet, Ug = 2^.5 fee+ per second, we find 



K = -S — = 1.76 (53) 



>^s 



In Appendix D, it has teen shown from a discussion of theoretical 

 ind experimental information bearing on tsunami surge velocities and 

 "orces , that a tsunami surge, with the characteristics of the experi- 

 lental waves illustrated in Figure 99, tends to have a frontal velocity 

 ,n accord with Equation (53) for K-values between 1.5 and 2.0. The 

 [-value 1.76 of Equation (53) supports the conclusions, despite the 

 'oughness of the analysis of Figure 152. 



Further support for the conclusion is discussed in Section IV-2, in 

 lonnection with the tsunami surge in the Near Island channel at Kodiak, 

 'here an eyewitness' estimate of water speed is found to be in approxi- 

 late accord with Equation (^2) or Equation (53) for a K-value of 2.0. 



The case of the light box at Crescent City (Figure 222) suggests that 

 I water velocity of 11.2 feet per second over a minimum depth of 6 feet 

 rould have caused failure of this light structure. We should here find a 

 [-value less than half that of Equation (53), but the probability is that 

 ,he nature of the flow was not like a surge but more like a fast-rising 

 ,ide, in which case Equation (D-20) of Appendix D would apply. 



It is convenient to summarize the findings of this study regarding 

 ,he water velocity of the front of a surge moving over dry or wetted 

 ;round. Thus the main conclusions of Appendix D and such other cases 

 is apply from the body of this report are drawn together in Table X, 

 rhich suggests that a suitable design value of K in Equation (53) or 

 D-17) would lie between 1.5 and 2.0. 



It is pointed out in Appendix D that, although calculations of force 

 )ased on the drag formula. Equation (D-I6), are probably valid for rela- 

 ,ively small objects or structures which become enveloped in the flow of 

 L tsunami surge, calculations for larger objects with broad or continuous 

 'rentage are better performed with a momentum formula such as Equation 



The behavior of a tsunami surge striking a wall or breakwater is 

 :haracteristic . The action is jetlike, and the momentiom is deflected 

 ipwards and possibly sideways if the approach is oblique to the structure. 

 Igainst a vertical wall of sufficient height the surge rises vertically 

 IS a wall of water to a height roughly equivalent to the velocity head 

 )f the stream. The collapse of this wall of water provides the physical 

 30dy for a reflected bore which moves out on the still incoming surge. 

 i'eak pressure on the structure occurs when this mass of water descends 

 3n the incident surge. When the structure walls are sloping inland, as 

 nth a seawall or breakwater, the pressure force on the structure is 

 relieved to the extent that moment^um is carried forward in the over- 

 topping volume of fluid. 



365 



