HEC-RAS

HEC-RAS is a software program developed by the US Army Corps of Engineers for steady and unsteady flow hydraulic calculations. It performs one-dimensional and two-dimensional simulations across natural and constructed channel networks, including floodplains, sediment transport, and water quality analysis.

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• Steady flow water surface profiles. This function computes water surface profiles for gradually varied flow in subcritical, supercritical, or mixed regimes. Solves the one-dimensional energy equation with friction losses (Manning's equation) and contraction/expansion losses. Uses the momentum equation for rapidly varied flow situations such as hydraulic jumps, bridges, and stream junctions.
• One-dimensional and two-dimensional unsteady flow simulation. Capability to model combined 1D and 2D unsteady flow through open channels, floodplains, and alluvial fans. Handles mixed flow regimes, dam break analysis, levee breaching and overtopping, pumping stations, navigation dam operations, pressurized pipe systems, user-defined rules, and automated calibration features.
• Sediment transport and movable boundary computations. Simulates one-dimensional scour and deposition over moderate time periods (years or single flood events). Computes transport potential by grain size fraction, allowing hydraulic sorting and armoring. Models full stream networks, channel dredging, levee alternatives, and multiple sediment transport equations. Evaluates reservoir deposition, channel contractions for navigation depths, dredging impacts, and maximum scour during large floods.
• Water quality analysis. Advection-dispersion module solving the one-dimensional equation using the QUICKEST-ULTIMATE explicit numerical scheme with a fully implemented heat energy budget. Simulates transport and fate of dissolved nitrogen (NO3-N, NO2-N, NH4-N, Org-N), dissolved phosphorus (PO4-P, Org-P), algae, dissolved oxygen (DO), and carbonaceous biological oxygen demand (CBOD).
• Common geometric data representation. All four analysis components (steady flow, unsteady flow, sediment transport, and water quality) share identical geometric data and hydraulic computation routines. Enables switching between analysis types without redefining cross-sections, junctions, bridges, culverts, or control structures.
• RAS Mapper geospatial capabilities. Generates inundation maps directly from computed water surface profiles. Creates depth grids in binary raster floating-point format (.flt) and floodplain boundary datasets in ESRI Shapefile format. Produces additional layers for velocity, shear stress, stream power, ice thickness, and floodway encroachment data. Requires a terrain model in .flt format.
• Dam and levee breach modeling. Simulates progressive or instantaneous failure of dams and levees including overtopping, piping, and slope failure mechanisms. Calculates outflow hydrograph, flood wave propagation, and inundated areas downstream in 1D or 2D modes. Allows specification of breach parameters (final width, formation time, side slopes).
• Hydraulic structures. Models bridges (energy, momentum, and WSPRO methods), culverts (circular, box, arch, and ellipse shapes), lateral weirs, radial and lift gates, pumping stations, and navigation dams. Evaluates rating curves, submergence, localized losses, and user-defined operational rules.
• Data management with HEC-DSS and HDF5. Stores input data in flat ASCII and binary files organized by project, plan, geometry, steady flow, unsteady flow, sediment, and water quality categories. Output data stored in separate binary files (HEC and HDF5). Data transfer between HEC-RAS and other programs uses the HEC Data Storage System. Interface automatically creates and names all associated files from a single project filename.
• Graphics and customizable reporting. Produces X-Y plots of river system schematics, cross-sections, profiles, rating curves, hydrographs, and inundation maps. Provides three-dimensional plots of multiple cross-sections. Reports for input and output data can be customized by content and format. Output can be displayed on screen, sent to printer, or passed through Windows Clipboard to word processors or spreadsheets.
• Coupled 1D/2D modeling. Connects one-dimensional reaches with two-dimensional flow areas within a single simulation. Exchanges mass and momentum at lateral interfaces or through linear structures (levees, roads). Suitable for modeling extensive floodplains together with entrenched main channels without losing detail in either domain.
• Automated parameter calibration. Adjusts roughness coefficients (Manning), contraction/expansion loss coefficients, and infiltration parameters by comparing computed stages with observed measurements. Uses optimization algorithms to minimize error between simulated and measured hydrographs at gaging stations.

HEC-RAS was created by the Hydrologic Engineering Center (HEC) of the US Army Corps of Engineers. Development began in 1995 as the successor to HEC-2 (steady flow) and HEC-6 (sediment transport). The first operational version (1.0) was released in 1998. The primary programming language is C++ (from version 3.0 onward), using the Qt graphical interface libraries. Versions 5.0 and later incorporate the two-dimensional module written in C++ with OpenMP parallelization. The executable is distributed at no cost to government agencies, academic institutions, and consultants.

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