Long Waves in Canals and Standing Waves in Closed Basins 243 



phenomenon. Their interference with the tsunami waves permits only an ap- 

 proximate determination of the time of arrival and of the amplitude of the 

 individual waves. Matuzawa and his collaborators (1933) have given an 

 accurate analysis of the water motion in the Bay of Sasu (in the innermost 

 part of the Great Toni Bay), for the tsunami of 3, March 1933. 



Figure 102 shows the section of the Sasu Bay flooded by the tsunami. 

 This section is almost completely enclosed by the contour line of 10 m depth 

 and has a pronounced trough form. The sudden end of the flood in the 

 innermost part is caused by small hills which produce a partly inward barrier. 

 Figure 103 gives a longitudinal section of the profile close to its right bank 

 and above it the heights of the flood along this profile. The velocity of the 

 "tidal" wave can be determined by considering the inner part of the bay 



150 



350 



Fig. 103. Longitudinal section of Bay of Sasu (see Fig. 102). Lower line is the bottom 

 profile. Dotted line: observed height of the flood. Full drawn line: computed height of 



the flood. 



with the inundated area as one single oscillating system, co-oscillating with 

 the wave coming from the outside. The assumption that the cross-section 

 is rectangular and that the bottom slopes gently, first less strongly and then 

 stronger in the innermost part, corresponds rather well to the reality. The 

 theoretical computation gives an oscillation time of 224 sec and a dis- 

 tribution of the vertical amplitude as represented in Fig. 103 by the solid 

 line. As the maximum velocity of the tidal wave at the beach, we get around 

 8 m/sec, all of which agrees very well with the observations. Essentially 

 this tsunami caused a simple co-oscillation of the water-masses of the bay 

 with the strong dislocation wave coming from the open ocean. 



Since tsunami are shallow water waves, they are propagated at a speed 

 c = \ gh. A typical tsunami, one having a period of 20 min for example, 

 has a wave length of 90 miles in water of 5000 ft depth. At the continental 

 slope and walls of deep oceanic trenches, an appreciable depth change takes 

 place over a distance which is small compared with this typical wave length. 

 Therefore a reflection is to be expected. 



16' 



