Hello, I made quite some ...

Hello, I made quite some flood risk mapping for Swizerland already. State of the art for open source would be QGIS (http://www.qgis.org/) which is really great to work with. It includes "grass" and is great to produce maps. In addition there are hydrology modules which you can implement there.

If you need to do more hydraulic-hydrological modeling I would recommend Octave (if you need to do some programming).

Not sure on how familiar you are with flood risk mapping but usually it is a good idea to create flood risk maps where you can see the intensity of an event but also the probability of occurence. So you might have the risk for a very severe event which is very unlikely or you have a rather low intense event which occurs almost every year. Or something in between.

Not sure about your socio-economic data. What kind of data do you mean?