Acqua Alta All Over Again

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Yogi Berra, the pithy catcher for the New York Yankees, once quipped “It’s like Déjà vu all over again.”  Today’s story might rightly seem like “Déjà blog all over again.”

In two earlier entries

http://www.openhazards.com/blogs/steve/acqua-alta-%E2%80%93-part-i

http://www.openhazards.com/blogs/steve/acqua-alta-%E2%80%93-part-ii

I explained how resonance can amplify ocean tides and I grounded the subject in Venice’s ‘Acqua Alta’.  Both Venice and tidal resonance merit a revisit. 

As for Venice, several factors contribute to increasingly severe floods apart from tides: global sea level rise, local subsidence and compaction, and seasonal ‘storm surge’ effects of wind and low pressure.

As for resonance, other places make Venice’s 1.5 -2 meter tidal range seem timid. The prequel blog already mentioned the Gulf of Gabes on the North African coast with its 3-4 meter tides. Far more spectacular examples exist.  England’s Bristol Bay hosts a 13 meter range. Nova Scotia’s Bay of Fundy however, wears the champion’s belt seeing high and low waters that differ by 17 meters. Considering that the tidal range in the absence of resonance would be about 70 cm, it is hard to imagine making a bell that responds so perfectly to its clapper as does Fundy’s.

A last mention goes to China’s Hangzhou Bay. As tidal flows squeeze into the Bay, a tidal bore forms on the Qiantang River effectively creating a twice daily tsunami several meters high that runs for miles up river. The bore has been a tourist attraction for one thousand years. in all that time, like at many wild natural attractions, over confident tourists regularly meet fatal consequences. Let’s just say that the Qiantang Bore makes Venice’s Acqua Alta look like a spring shower.

In a final nod to Yogi and employing the blogger’s universal disclaim, “I really didn’t say everything I said.”

Steven N. Ward   Santa Cruz

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