Below is a simple GIS ...
Published by Andrew Raal, Natural Scientific Professional
Below is a simple GIS methodology, the skill is to determine the height to which the water will rise during the event (e.g. 1:100y) one intends to model.
1).You need good surface elevation data (e.g. LiDAR) to create topographic surface (raster)
2).Then create water surface profile using hydraulic engineering methodology/software. i.e. expected height to which the water will rise during the specified flood event.
3).Get the difference between the water surface and land surface to determine the extents of the flood plain for the given event.
Methodology using ESRI ArcView tools.
http://resources.arcgis.com/en/help/main/10.1/index.html#//015w00000062000000