How do I prevent flood waters from entering my house?
DA's comment is correct.  If water has gotten to your house all you can hope to do is minimize the damage...water WILL get in if it is up against the door.

The best bet (a picture of the area in question would be useful) would be to build up the ground around the house to add a barrier to keep water off the house in the first place.  That may or may not be feasible in your situation, this is where the picture would be handy.