When a very high tide combines with strong winds, there is the risk of flooding. This is fun if you are prepared with wellies - but sometimes the flood is so high you need thigh-high waders or else you must be prepared to stay indoors until the tide lowers. Fortunately the flooding in Venice is tidal and WILL go down within 12 hours.
They do sell disposable wellies in the local markets, which you can put over your normal shoes and which are cheap enough to leave behind when you go home. Some hotels have wellies you can borrow but not usually enough pairs for everyone and it depends on your size. Most Venetians have waders; at times of high water they can be seen going to work in their suits, with wellies over their designer trousers.
When there is going to be an exceptionally high tide, a siren is sounded over the whole city in plenty of time to warn you to get to where you need to be, or to batten down the hatches against the floods. Most shops and hotels have effective flood defences by way of flood-proof half doors that can be stepped over by pedestrians but keep the water out. It is quite an adventure if you are prepared.
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