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The seiche

In their passage, the atmospheric disturbances act on the tide as a impulsive stimulation, the "storm surge", following which the Adriatic, because of the shape of the basin, behaves as a resonant cavityat the end of the pulse a series of longitudinal and transverse oscillations appears, the "seiches", whose amplitude progressively fades. The fundamental oscillation, in the longitudinal direction, has a period of 22 hours.
The effect must be taken into account when predicting tide. In the days following the storm surge, other surges will appear, and due to the phase shift of almost two hours between the frequency of the astronomical tide (about 24 hours) and the seiche (about 22 hours), peaks could eventually be on phase and trigger a flood in Venice.
A clear example can be seen in the graph below which refers to the period going from February 12th  to 15th, 1972: the maximum meteorological contribution occurred on the 12th, while the observed sea level has reached its maximum on February 14th, despite improved weather conditions, when astronomical tide and meteorological contribution became on phase.

 
Graphic reporting the seiche
Observed level tide in Venice, astronomical tide and meteorological contribution from 12 to 15 February 1972
 
 
 
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