HEC-RAS has been developed for the U.S. Army Corps of Engineers. It is also available for download and use by individuals outside of the Corps of Engineers without charge. HEC will not provide user assistance or support for this software to non-Corps users.
HEC-RAS ia a free Hydraulic Modeling and Analysis Software Developed and Distributed by US Army Corps of Engineers. This is a really useful tool for Hydraulic Analysis and Flood Plain Mapping. Link: http://www.hec.usace.army.mil/