There are many examples of the application of HEC-RAS to dam break flood simulation. Included in the distribution of RAS Ver. 4.1 are several example data sets for unsteady flow analysis. One of those is for a dam break application; it is titled “Bald Eagle Creek Example Dam Break Study.” Also, a recent (Aug. 2014) publication is available from HEC that describes all aspects of applying HEC-RAS to a dam break study. That publication is TD-39, “Using HEC-RAS for Dam Break Studies.”
Hope this helps,
Michael