-r; = Asincrt (D-21) 



and 



Jl = A a- cos o- t (D-22) 



or 



so that from Eqs. (D-20) and (D-22) horizontal velocity per unit width 

 becomes 



u = ^^ cos a t (D-23) 



s 



The flow is therefore periodic and for any given amplitude A and ground 

 slope s is inversely proportional to the wave period. This means, of 

 course, that a 30-minute period wave would have four tiixies as severe a 

 current as a 2-hour period wave. 



In most cases of inundation investigated for the Alaskan tsunami, the 

 flooding was brought about by a superposition of long waves of large ainpli- 

 tude. The nature of the superposition sometimes results in extremely 

 rapid time rates of change of water level — v-p- . If tide gage or water 

 level records are available, — v-p is directly measurable from the records 

 and water velocities are then calculable from Eq. (D-20). Obviously if the 

 ground slope s is very slight, water velocities can attain high values. The 

 The dictates of the flow are then governed by hydraulic considerations, as 

 for flow in rivers and canals. 



The point of overlap or criterion for which a flow of the type of 

 Eqs. (D-20) and (D-23) can become a tsunami bore or breaking wave is at 

 present not known to the writer and needs further research. 



Eq. (D-16) for the force of the flowing water on an obstacle is 

 obviously relevant only if the object is completely enveloped in the flow. 



D-9 



