What areas in Austin Texas are prone to flooding and are slab foundation dependable?

Onion creek area floods quite a bit.

Marble Falls
Downtown Austin
Anywhere right next to Mopac Freeway

If you go to your local city or county office they will have maps to show you where the flood zones are located. You can also check out News8Austin.com to find out what areas were flooded in the past.

I have a slab foundation and it is very dependable for this type of area. I live on a rocky area. However, you need to make sure the land they are placing it on is sturdy enough to handle it, because if it sinks, the foundation and the house will start to shift and crack.

The city has a GIS viewer on their website that you can view floodplains on. Austin has a large network of lakes, rivers, streams, floodwater easements, etc, so it's best to look at the floodplain map to see where the waterways closest to you are. West Austin is more prone to flash flooding/runoff because the top soil is thin, and lies over limestone.
http://www.ci.austin.tx.us/development/default.htm (Under Geographic Information, there's a link to their flood plain viewer).