HEC-RAS is a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 that models the hydraulics of water flow through natural rivers and other channels. The program is one-dimensional, meaning that there is no direct modeling of the hydraulic effect of cross section shape changes, bends, and other two- and three-dimensional aspects of flow. The program was developed by the US Department of Defense, Army Corps of Engineers

United States Army Corps of Engineers

The United States Army Corps of Engineers is a federal agency and a major Army command made up of some 38,000 civilian and military personnel, making it the world's largest public engineering, design and construction management agency...

 in order to manage the rivers, harbors, and other public works under their jurisdiction; it has found wide acceptance by many others since its public release in 1995.
The Hydrologic Engineering Center (HEC) in Davis, California

Davis, California

Davis is a city in Yolo County, California, United States. It is part of the Sacramento–Arden-Arcade–Roseville Metropolitan Statistical Area...

 developed the River Analysis System (RAS) to aid hydraulic engineers

Hydraulic engineering

This article is about civil engineering. For the mechanical engineering discipline see Hydraulic machineryHydraulic engineering as a sub-discipline of civil engineering is concerned with the flow and conveyance of fluids, principally water and sewage. One feature of these systems is the extensive...

 in channel flow analysis and floodplain

Floodplain

A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...

 determination.