High Tides in Venice

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DaveWalsh
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Joined: Sat Oct 26, 2013 9:37 am

High Tides in Venice

Post by DaveWalsh » Sun Nov 17, 2019 6:55 pm

Apparently Venice is experiencing unprecedented high tides this last few days causing immense damage in a spot where high water is a regular occurrence, so there must be something very unusual going down.
First research stop was the tide tables for Galway, which maxed at 5.0 a few days ago, on say Wednesday and Thursday, the usual 36 hours after the full moon which was last Tuesday. Folks, 5.0 is nothing in Galway. You need 5.6 or 5.7 before the dreaded "sandbag" word is heard. Even then you need other factors complicateding things.
But Venice is still getting hammered and this is Sunday, they are back to 4.6 in Galway (which actually is OHW, ordinary high water, the level that legally determines the foreshore from contiguous privately owned land) so what is going on?
  • Is it low pressure ? Remember the rule "an inch is a foot", i.e. an inch of mercury is a foot of water, meaning 2.5cm less of mercurial pressure equates to 30cm of tide height.
  • Is it wind? I can't see wind was the major factor.
Knotman, where are you when you are needed most in our hour of peril?
Advise us, advise us now.
DWalsh

knotman
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Joined: Sat Oct 26, 2013 9:37 am

Re: High Tides in Venice

Post by knotman » Tue Nov 19, 2019 9:38 pm

Believe it or not, Mr Walsh, I was in Venice this weekend observing first hand.
I have a picture which will answer all your questions as to the real cause of all this flooding.
.Will post ASAP.

PS: the locals were most worried about the wind direction.

Knotman .

knotman
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Joined: Sat Oct 26, 2013 9:37 am

Re: High Tides in Venice

Post by knotman » Tue Nov 19, 2019 9:48 pm

Look up SEICHE. Standing waves {rocking motion} set up in elongated bays, closed at one end eg, Fundy , Adriatic,


knotman
Posts: 127
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Re: High Tides in Venice

Post by knotman » Sun Feb 09, 2020 1:27 pm


DaveWalsh
Posts: 127
Joined: Sat Oct 26, 2013 9:37 am

Re: High Tides in Venice

Post by DaveWalsh » Sun Feb 09, 2020 5:32 pm

Knotman,
There are big tides the next few days which we tend to associate with flooding (a HW 5.6m + sustained west winds as well, Galway beware), but the low waters are also very low including one LW 0.1m beloved of razor fishermen. Is that what we are seeing here, big tides going a long way out at LW or is there more to it, that resonance thing you go on about?
DWalsh

PS More simply put, six hours after these photos were taken, would the canals be overflowing? Again?

knotman
Posts: 127
Joined: Sat Oct 26, 2013 9:37 am

Re: High Tides in Venice

Post by knotman » Tue Feb 11, 2020 8:02 pm

Is such an acqua bassa common? When was the latest important 'acqua bassa'?

Really low tides are not a common phenomenon: before the “acqua bassa” of December 2016, the latest recorded “acqua bassa” happened in 2008, 8 years before.

Knotman

DaveWalsh
Posts: 127
Joined: Sat Oct 26, 2013 9:37 am

Re: High Tides in Venice

Post by DaveWalsh » Thu Feb 13, 2020 9:48 am

Knotman,

I still struggle.

When the LWs get really low, should not the HWs also get really high?

In Galway I have always operated off a simple enough formula that (ignoring extreme outliers) high HWs get to 5.6, low HWs only to 3.6 and the average is 4.6, while low LWs get to 0.1, high LWs only to 2.1 and the average is 1.1.

In that way the "range" is 5.5 for really big tides, 1.5 for really small tides, and the average is 3.5.

Massive differences. The change takes place in one week. On a week's holidays, you might see both.

But, and this is the big "but" of this enquiry - the next time we have a 0.1 we have a 5.6 immediately before and after, yet you seem to be saying that the entire operation just gets lower - the LWs get lower and so do the HWs ???????

I can't get my head around that, sorry.

Is that what you are saying?

DWalsh

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