Acqua Alta – Part II

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Earlier we learned half of the Acqua Alta story in terms of favorable tides. The second half of the lesson involves resonance.

Thumbnail Tutorial Continued:

(2) Resonance: I discussed resonance previously here under the title “Seismic Slosh”...

http://www.openhazards.com/blogs/steve/seismic-slosh-swimming-pools-and-...

In that example, the resonance was in reservoirs and the cause was seismic shaking. 

Tidal resonance is similar but on a much larger scale. The gravitational pull of the sun and the moon act to raise and drop water piles over intervals of about 12 and 24 hours. Due to the shape and depth of the sea in certain places, water may also like to drain or fill in 12 or 24 hour intervals. Such reinforcing resonance can amplify the height of the tide many times.  Guess what? Located at the head of the closed Adriatic Sea, Venice is one of those places. When you mix just the right locations of the sun and moon with a favorable resonance  ---- BINGO---- Acqua Alta.

This movie shows computed tidal motions of the Central Mediterranean Sea over seven days in mid Summer including a new moon.  It’s easy to see that most of the Mediterraean has minimal tidal motions, whereas the Adriatic tends to “breath in and out” over a 24 hour period. On this account, the tides near Venice are several times larger than those on the western Italian coast near Genoa. (Red versus Blue lines upper right.) You might spot another resonance on the coast of Africa.  That place is the Gulf of Gabes. It hosts the highest tides in all the Mediterranean.

OK, now that you’ve competed your tutorial on Acqua Alta, feel free to enjoy Venice from perspectives other than geophysical.  Class dismissed.

Steven N. Ward   Santa Cruz

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