Hi Gareth,
As Wu suggested this is highly specialized stuff, on which many scholars and scientists break their heads.
At SIGGIS in Brussels, we recently integrated waterinfo.be. This is a flooding (and drought) portal for Flanders, the northern part of Belgium. The site displays a.o. predicted flooding contours, but these are made by highly specialized calculations models (a.o. Floodworks). These models take into account many variables, like measured precipitation, predicted precipitation, DTM, soils, soil saturation, surface resistance, subsurface flow, capillary rise, evapotranspiration, … and probably several other parameters. Per basin it takes up to 2 hours to run a calculation.
It is also key to note, that climate and geography will have a very profound impact on the structure and details of prediction models. It is hard to just copy a flood prediction model from one site to another. However the documents you’ll find on Google will explain you the basics.
Best you extend your research to local universities with a geography/hydrography department. Alternatively you can try to contact your local government agency that is dealing with floods. Both should have already some ideas on how local floods are predicted, if not, than you might have found a gap in the market (and call me ;-)).
Good luck!