234 Long Waves in Canals and Standing Waves in Closed Basins 



not satisfactory in all points, they nevertheless show that the main causes 



of the generation and development of a surge were considered as correctly 



as possible and that a successful numerical prediction of sea level can be 



achieved by further development of the theory and numerical computation 



methods. 



(d) Oscillations on the Shelf and Meteorological Tsunami 



We have already mentioned previously that tidal recordings on straight 

 coasts include also seiches-like motions of the water surface, which can only 

 be interpreted as being oscillations of the water-masses on the shelf. It is 

 probable that they are developed under certain conditions, which cause 

 a given section of the shelf to behave like an open bay. The period of the 

 oscillations depends in the first place upon the width of the shelf and, as 

 according to the records, varies from a few minutes to 2 h and over. Nomitsu 

 and Habu (1935) and Nomitsu and Nakamiya (1937) have continued 

 the work started by Honda and his collaborators. These oscillations are 

 caused by both atmospheric disturbances and submarine earthquakes. The 

 damping of the waves is considerably smaller when they originate from an 

 earthquake. Perhaps this is due to the fact that a storm disturbs the waters 

 to a great extent and, consequently, the coefficient of viscosity is then larger 

 while an earthquake is more distant and the weather and sea remain 

 usually calm. 



The agreement between the observed and computed periods for oscilla- 

 tions on the shelf is not bad, even when Merian's formula for bays using the 

 dimensions of the shelf is applied. In most cases, however, there will 

 be a more or less wide range within which to fix the outer limit of the 

 shelf against the open sea, whereas it will not be difficult to take into account 

 the various depths at the shelf, which so far has not been attempted in single 

 cases. A theoretical investigation of seiches which can be produced in the 

 open ocean by a submarine ridge has been made by Hidaka (1935). 



Assume for the depth a curve of the form h = h (l + x 2 /a 2 ) llz , then the 

 bottom configuration characterized by a submarine ridge rising up till the 

 water depth h at x = 0; at the distance a the water depth is //„) 2. The 

 possible oscillations are shown in the curves in Fig. 99, in which the longer 

 wave has a node over the ridge; its period is T\ = 6-95(a/} gh ). The shorter 

 wave has an anti-node over the ridge and the numerical factor in the period 

 is 324, instead of 6-95. 



Figure 99 contains in its lower part also the possible seiches on a shelf 

 obeying the same law of depth. Here both seiches represented have, of course, 

 anti-nodes at the land end of the shelf, where the depth is h ; the period 

 is T = 3 -21 (a/ \ / gh ); for the shorter wave the numerical factor is 1-925. 



The Japanese shores are frequently the scene of devastations caused by 

 strong, wave like disturbances of the sea surface. Their development is either 

 of a seismic or of a meteorologic origin. On the east coast of the island Hondo 



