Island at about 07 hours GMT of March 28, 196U. The crest of the high 

 tide reached the same area two hours later at 09 hours GMT. Since the 

 tsunami period may be considered nominally 2 hours, the first tsunami 

 crest would occupy the frontal position (OT hours) at about 08 GMT, or 

 very near the peak of the high astronomical tide, and the first few high 

 waves would effectively ride the tide wave and cause the high runup in 

 the area. 



Farther south below the Canada-U.S.A. border the tsunami, as also 

 the tide wave, rolled in practically normal to the coastline (Figure 79). 

 Here the concurrence of high seismic sea waves with high spring tide 

 again caused runup of serious proportions. 



The tide wave and the tsunami ran up the Columbia River with the 

 tsunami front preceding the tide crest by about 1.5 hours. The vast 

 size of the tide wave ensured that the leading waves of the tsunami rode 

 on the top of the tide wave, and propagated upriver in this virtually 

 interlocked fashion. 



The effects, recorded at tide-gage stations spaced along the 90 

 nautical miles of estuary, make possible the derivation of important 

 conclusions regarding the tsunami. Three samples of the estuary mari- 

 grams (Figure 80) reveal the development of the tide wave as an un- 

 symmetrical wave effectively made up of harmonics of the tide riding 

 the fundamental wave as already noted (cf. Section 11-7) . Beaver Tide 

 Gage in particular, shows that, other than the tsunami waves riding the 

 tide crest, the intermediate waves have lost their identity and hardly 

 register at the low tide, though later waves are again found on the 

 succeeding high tide. This absence of tsunami waves (other than the 

 first three) on the rear slope of the tide wave would have been pro- 

 moted by the tsunami beat interference effect evident in the fourth, 

 fifth and sixth waves of Figure U8d for Victoria, Canada, even if 

 frictional damping of the waves at low tide were not the major cause. 



The data from the Columbia River tide gages permit us to derive 

 some quantitative information about the tsunami and the tide propagation 

 up river. Figure 8la shows a space-time plot of the progression of the 

 first, second and third tsunami crests on the first tide crest and of 

 two other tsunami crests riding the subsequent tide crest. At the mouth 

 the leading tsunami waves show a period T - 1.75 hours, in good agreement 

 with Table III, but the period changes and increases with distance be- 

 cause the wave velocities differ, in accordance with Equation (^), with 

 the depth of water provided by the tide wave and the river water. By 

 the time the third wave loses identity at about 70 nautical miles from 

 the mouth, its effective period is 2.2 hours. 



In the first 50 nautical miles the tide crests advance more rapidly 

 than the tsunami, but then slow rapidly, presumably as a result of 

 shallow water and increasing tidal friction. The tsunami crests, less 

 susceptible to friction, then outrun and advance through the tide crest. 

 The reason for the initially greater speed of the tide may be surmised 



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