What Causes these Floods? What has been done?

Usually, the rise and fall of tides will result in the floodings. In Venice, the Adriatic rises and falls twice daily.

Here is a flash about why the tides rise and fall twice daily:

http://www.pbs.org/wgbh/nova/venice/tide_flash.html

How did Venetians cope with this problem?

"High water, or acqua alta, is a persistent phenomenon throughout the Venetian lagoon. Over the centuries, the city's leaders, merchants, and residents have dealt with rising sea levels either by demolishing old buildings and erecting new ones on higher, impermeable-stone foundations, or by raising the entrances to buildings that line the dozens of canals criss-crossing the historic city. Or, on the then-rare occasions when the city was flooded by storm-driven tides, Venetians simply waited it out. The water went up, a few hours later it went down, and life, scarcely disturbed, went on."

Now how much will the tide rise in Venice?

"By 1989, however, such inundation occurred no fewer than 40 times a year. In 1996, water nearly as high as the average tourist's knee lapped 99 times at piles of sandbags placed to guard the doorways of the Doges' Palace and St. Mark's Basilica. More and more frequently, visitors must walk on elevated wooden walkways, or passarelle, as peak tides flow over the city's sidewalks. Boats cannot navigate the canals during the highest tides because they cannot pass under bridges.

The high tides are not just annoying but damaging. Instead of merely washing against the impermeable marble that makes up the city's foundations, high waters are splashing with increasing frequency against the soft, permeable bricks that sit above the foundations. Saltwater from the Adriatic soaks into this brick, inching ever higher into the walls and creeping into interiors, destroying frescos and other irreplaceable relics. Unless they have been restored with new, waterproof brick, many of these buildings crumble imperceptibly."

more was found on this link here:http://www.pbs.org/wgbh/nova/venice/solutions.html