224 Long Waves in Canals and Standing Waves in Closed Basins 



If V is positive, i.e. when the atmospheric pressure disturbance moves 

 along the canal away from the closed end, the term at the right is always 

 smaller than 1 ; the amplitude of the forced wave is smaller than the static 

 pressure disturbance. However, if V is negative, i.e. if the pressure disturbance 

 travels towards the closed end of the canal, the ratio can become great and 

 even very great, when the velocity of propagation of the pressure wave nears 

 the velocity of the free waves in the canal. The values of c for depths less 

 than 100 m lie below 30 m/sec, and the velocity of propagation of atmos- 

 pheric pressure disturbances is also of the order of magnitude of 30 m/sec. 

 Therefore, one can expect a strong impulse to wave disturbances in lakes 

 and bays, when these pressure disturbances travel towards the interior of 

 the bay, and the depth is not too great. 



In many cases, the tide gauges have registered this remarkable resonance 

 which is responsible for important variations of the level in lakes and in 

 shallow bays. These theoretical results explain the fact that in oblong lakes 

 extending in the west-east direction there are large variations in the level 

 at the eastern end. The same applies for bays which are shallow and have 

 an opening in the west. In lakes and bays open to the east, there are no such 

 variations in level. These variations occur seldom in lakes and bays extending 

 in a north-south direction. In most cases the pressure disturbances travel 

 approximately in a west-east direction, that is towards the closed end of the 

 canal, which is required to raise the water to a considerable amplitude for 

 the forced oscillations. 



Caloi (1938) has been able to prove, for the Bay of Trieste, the correctness of the resonance 

 condition (VI. 135), on the strength of gauge registrations made in the northern section of the Adria. 

 Figure 95 can be taken as an example. It shows the deviations from the computed tidal curves and 



to 



cm 



50 



1 tide— registration Trieste 



2 tide— registration Venice 



Observed— Computed 



Atm. pressure 1 : Trieste 

 Atm. pressure 2: Venice 



Fig. 95. Registration of atmospheric pressure and water level at Trieste and Venice on 



12,13 and 14 October, 1933. 



the curve for the atmospheric pressure for Trieste and Venice for the period 12—14 October 1933 

 (see p. 189). The low-pressure area which caused the strong fall in pressure on the 12th came from 

 the Po valley and crossed the northern Adria in about 2-2 h. Its velocity of propagation was ap- 

 proximately 50 km/h = 14 m/sec. As the average depth of water between Venice and Trieste is about 

 20 m, the velocity of the free waves will be 50 km/h. The condition for resonance is fulfilled. The 



