If now we assume a mean depth d, over the quarter wave length, of 1Z8 ft. , 

 we find, for T = 1.8 hrs. 



-A- ~ 8. 6 n. ml. (D-19) 



4 



These figures are consistent with a sea bed slope of about 1/200 which 

 is fairly reasonable. 



With nnaximiumL amplitude of the wave over the coastline, we find 

 (for a sinusoidal wave) that the 1. 8 hr. wave will have 95 percent of this 

 amplitude at a distance from the crest of about 3. 5 n. mi. on either side. 

 Because inundation distances from the Alaskan tsunami never exceeded 

 1/Z n. mi. , it is clear that inundation water level immediately seaward 

 and landward of the coastline would be effectively horizontal. Further, 

 since the surface slope of so large a wave is extremely gradual, the rise 

 of the crest at the coast would be tantamount to the lifting of a virtually 

 horizontal sea level in the time of the quarter wave-period. This argument 

 n"iay be extended successfully to wave periods of much smaller periods — 

 at least to T :^ 30 nnins. 



Assuming then a rise of horizontal sea level over the land at the 

 vertical rate b t] / ^t where rj is the wave elevation of the wave above 

 still water level at any point, and t is variable time, the continuity con- 

 dition governing the speed of water over dry land of slope s requires a 

 velocity of horizontal flow per unit width 



u = - -^ (D-20) 



s d t 



For a sinusoidal wave at the coastline of amplitude A and angular 

 frequency cr 



D-8 



