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HEC-RAS is a computer program 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 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 developed the River Analysis System (RAS) to aid hydraulic engineers in channel flow analysis and floodplain determination. It includes numerous data entry capabilities, hydraulic analysis components, data storage and management capabilities, and graphing and reporting capabilities.
How it works 
The basic computational procedure of HEC-RAS for steady flow is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction and contraction / expansion. The momentum equation may be used in situations where the water surface profile is rapidly varied. These situations include hydraulic jumps, hydraulics of bridges, and evaluating profiles at river confluences.
For unsteady flow, HEC-RAS solves the full, dynamic, 1-D Saint Venant Equation using an implicit, finite difference method. The unsteady flow equation solver was adapted from Dr. Robert L. Barkau’s UNET package.
HEC-RAS is equipped to model a network of channels, a dendritic system or a single river reach. Certain simplifications must be made in order to model some complex flow situations using the HEC-RAS one-dimensional approach. It is capable of modeling subcritical, supercritical, and mixed flow regime flow along with the effects of bridges, culverts, weirs, and structures.
HEC-RAS is a computer program for modeling water flowing through systems of open channels and computing water surface profiles. HEC-RAS finds particular commercial application in floodplain management and flood insurance studies to evaluate floodway encroachments. Some of the additional uses are: bridge and culvert design and analysis, levee studies, and channel modification studies. It can be used for dam breach analysis, though other modeling methods are presently more widely accepted for this purpose.
HEC-RAS has merits, notably its support by the US Army Corps of Engineers, the future enhancements in progress, and its acceptance by many government agencies and private firms. It is in the public domain and peer-reviewed, and available to download free of charge from HEC's web site. Various private companies are registered as official "vendors" and offer consulting support and add on software. Some also distribute the software in countries that are not permitted to access US Army web sites. However, the direct download from HEC includes extensive documentation, and scientists and engineers versed in hydraulic analysis should have little difficulty utilizing the software.
Users may find numerical instability problems during unsteady analyses, especially in steep and/or highly dynamic rivers and streams. It is often possible to use HEC-RAS to overcome instability issues on river problems. HEC-RAS is a 1-dimensional hydrodynamic model and will therefore not work well in environments that require multi-dimensional modeling. However, there are built-in features that can be used to approximate multi-dimensional hydraulics.
HEC-RAS Programs 
Aquaterra is a professional software for canals and river engineering works design. Seamless integration in CAD environment makes it really easy to learn and use. Engineers use Aquaterra to create detailed designs as well as for conceptual studies and designs. HEC-RAS interface is intended for communication between CAD environment and HEC-RAS. This module prepare the HEC-RAS geometric input file based on cross sections, banks an Manning coefficient data and transfer the resulting water levels back to Aquaterra.
HEC-RAS can also be used in the GIS program, ArcView (With 3-D analyst and spatial analyst extensions). ArcView can read-in AutoCAD drawings for backgrounds and can create a DTM. In a similar way to river CAD, GEO-RAS can be used to create cross sections and other geometrical data for use in HEC-RAS and can be used to export water surface data from HEC-RAS back into ArcView to create flood maps with depths and extents of flooding.
HEC-RAS has a dll library that allows you to create an application for use without a user interface. This dll proved to be useful for advanced analysis such as Monte Carlo uncertainty. The freeware application ARAS takes advantage of such capability, and allows to automate several tasks like running several plans of updating roughness.
HEC-RAS can be used along with AutoCAD. A digital terrain model (DTM) can be constructed in AutoCAD and sections can be cut from the DTM and imported into HEC-RAS.
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WMS (Watershed Modeling System) 
WMS (Watershed Modeling System) provides pre and post-processing tools for use with HEC-RAS. The development of WMS was funded primarily by The United States Army Corps of Engineers. Commercial licenses to WMS are available from the developer, Aquaveo LLC. Features include:
- Use feature objects (centerline, cross section lines) and a TIN to develop the geometry of a HEC-RAS model.
- Edit, merge, and create cross sections in a database for use with HEC-RAS and other hydraulic models.
- Delineate flood plains from water surface elevation data. Water surface elevations can be computed by HEC-RAS, defined interactively, or imported from a file.
- Link multiple simulations of HEC-1 to HEC-RAS to determine the uncertainty in modeling parameters on a delineated flood plain. Curve Number and Precipitation can be stochastically varied among HEC-1 parameters and Manning's n value for HEC-RAS.
- HEC-Archives by Dodson & Associates, Inc.
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See also 
- Aquaterra River Design Software
- WMS River Modeling Software
- ISIS River Modelling Software
- Hydrologic Engineering Center - Institute for Water Resources - U.S. Army Corps of Engineers
- HEC-RAS home page at the US Army Corps of Engineers, Hydrologic Engineering Center
- HEC-RAS Bloggery - HEC-RAS help site
- Hands-On HEC-RAS Seminar on CD by Dodson
- An out put video of an flood analysis that done with HEC-RAS and visualizaten in ArcGIS
- Freeware applications that automates HEC-RAS features, such as creating geometry files, updating roughness and running several plans
- HEC-RAS Users Group on LinkedIn