How was Venice flooded?
Venice can't really be flooded by rivers, since there are none worth mentioning throughout the city.
High tides are predictable, yes.
Take a look at how Venice is situated. The historic city is sheltered behind low sandbanks/islands, which form the foremost coastline. Still the laguna is - obviously - connected to the open sea. Meaning: Huge waves, like in the open sea, wont rush through the city's channels, since they are dispersed by these islands. Still, in some images the water is quite choppy - a sign for rather windy conditions. Furthermore, all the houses of the city act like additional "islands", calming the surface of the water by dispersing larger waves into smaller and smaller ones. However, wind does not only create waves, it pushes the water also into the direction it is blowing to. In case of the Scirocco, which blows from northern Africa across the Mediterranean right into the Adriatic Sea, at which northern end Venice is situated. Thus, this wasn't simply a high tide, it was a spring tide, a combination of favourable planetary alignment and wind conditions. And, as already commented, winds/weather are quite hard to predict.


The whole process actually has a local name: "Acqua alta". It is nothing new, simply the intensity was noteworthy.