If evacuations have been ordered, then it is safe to assume the calulations are about right. After New Orleans, disaster management officials will get their way on flooding risks for a few years, without too much political interference.

Shutting the stable door after the horse has bolted may not stop the first one, but after that, it's useful.

RE GPS. Depends waht kind. Most units designed for hiking should give you an elevation. I doubt any designed for marine navigation, or in-car sat-nav types would. HOWEVER, without a good topographical map of your area, the GPS data alone is of very little use. If your house is higher than the current water level in the dam, it is definitly safe. Beyond that, you need to look at the gradient and profile of the channel, but it's not an easy job.

If you're seriously worried, look at the area close to your house- within 1/4 of a mile or so. If there are people being evacuated who live 3 feet below you, then you could rightly feel that maybe you're too close for comfort. If the nearest people being evacuated are 30 foot below you, then you can see you're safe.

But seriously, trust the professionals on this one. I've approched channel capacity calulations from both the engineering and geological angles, and I wouldn't go near anything but the simple, idealised cases. Working with (presumably) an irregular channel, and a non-constant flow is going to be a right pain, and the saftet margins thrown in as a result will be big.