How are water scenes (with flooding) achieved without injuring actors?

There are a hand full of ways that flood and water scenes can be done in movies.

The first instance, and one that's been used more commonly the last few years, is cgi. They film the actors in front of a green screen and replace everything with computer graphics.

Another example is building a set specific to the scene/s where flooding occurs. The environments are constructed such that the crew have control of the situation at all times and everyone's safety is kept in check.

In some scenes where there is a lot of open water, a special set is constructed in water tanks with backdrops to make it seem that you're not looking at the actors in a tank.

The Impossible's' tsunami scene is one of the well praised example and here are the insiders on it :-

"We started off with a test with like six submergible pumps to try to get the current," said Costa. "We ended up with 33 submergible pumps, and each pump weighed like 1,322 pounds." Each pumped about 80 gallons per second. Four large generators supplied power to the pumps, which had to be adapted for the tank because they couldn't be visible on camera.

The actors would sit in carts that moved on two rails inside the channel, and they were pulled by steel cables at the same speed as the current, said Costa. "They were very protected. They were sitting in the baskets with their arms and legs sticking out, and we would pull the camera next to them and behind them." - latimes.com

In behind the scene of Bollywood film Satyam Shivam Sundaram during TV telecast, it is told that they used real flood scene to catch up the intensity for film. They said , on the similar time real flood got happened.

Otherwise CGI is the solution for the rest but it doesn't look as realistic as above methods.