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HEC-RAS is designed to perform one-dimensional hydraulic calculations for a full network of natural and constructed channels.
The following is a description of the major capabilities of HEC-RAS.
User Interface
The user interacts with HEC-RAS through a graphical user interface (GUI). The main focus in the design of the interface was to make it easy to use the software, while
still maintaining a high level of efficiency for the user. The interface provides for the following functions:
- File Management
- Data Entry and Editing
- Hydraulic Analyses
- Tabulation and Graphical Displays of Input and Output Data
- Reporting Facilities
- Context Sensitive Help
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Hydraulic Analysis Components
The HEC-RAS system contains four one-dimensional river analysis components for: (1) steady flow water surface profile computations; (2) unsteady flow simulation; (3)
movable boundary sediment transport computations; and (4) water quality analysis. A key element is that all four components use a common geometric data
representation and common geometric and hydraulic computation routines. In addition to the four river analysis components, the system contains several
hydraulic design features that can be invoked once the basic water surface profiles are computed.
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Steady Flow Water Surface Profiles
This component of the modeling system is intended for calculating water surface profiles for steady gradually varied flow. The system can handle a full
network of channels, a dendritic system, or a single river reach. The steady flow component is capable of modeling subcritical, supercritical, and mixed flow
regimes water surface profiles.
The basic computational procedure is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction (Manning's equation)
and contraction/expansion (coefficient multiplied by the change in velocity head). The momentum equation may be used in situations where the water
surface profile is rapidly varied. These situations include mixed flow regime calculations (i.e., hydraulic jumps), hydraulics of bridges, and
evaluating profiles at river confluences (stream junctions).
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Unsteady Flow Simulation
This component of the HEC-RAS modeling system is capable of simulating one-dimensional unsteady flow through a full network of open channels. The unsteady flow
equation solver was adapted from Dr. Robert L. Barkau's UNET model (Barkau, 1992 and HEC, 1997). The unsteady flow component was developed primarily for subcritical
flow regime calculations. However, with the release of Version 3.1, the model can now perform mixed flow regime (subcritical, supercritical, hydraulic jumps, and
draw downs) calculations in the unsteady flow computations module.
The hydraulic calculations for cross-sections, bridges, culverts, and other hydraulic structures that were developed for the steady flow component were incorporated
into the unsteady flow module. Special features of the unsteady flow component include: Dam break analysis; levee breaching and overtopping; pumping stations; navigation
dam operations; and pressurized pipe systems.
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Steady Transport/Movable Boundary Computations
This component of the modeling system is intended for the simulation of one-dimensional sediment transport/movable boundary calculations resulting from scour and
deposition over moderate time periods (typically years, although applications to single flood events are possible).
The sediment transport potential is computed by grain size fraction, thereby allowing the simulation of hydraulic sorting and armoring. Major features include
the ability to model a full network of streams, channel dredging, various levee and encroachment alternatives, and the use of several different equations for the
computation of sediment transport.
The model is designed to simulate long-term trends of scour and deposition in a stream channel that might result from modifying the frequency and duration of the
water discharge and stage, or modifying the channel geometry. This system can be used to evaluate deposition in reservoirs, design channel contractions required to
maintain navigation depths, predict the influence of dredging on the rate of deposition, estimate maximum possible scour during large flood events, and evaluate
sedimentation in fixed channels.
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Water Quality Analysis
This component of the modeling system is intended to allow the user to perform riverine water quality analyses. An advection-dispersion module is included with this
version of HEC–RAS, adding the capability to model water temperature. This new module uses the QUICKEST-ULTIMATE explicit numerical scheme to solve the one-dimensional
advection-dispersion equation using a control volume approach with a fully implemented heat energy budget. Transport and Fate of a limited set of water quality
constituents is now also available in HEC-RAS. The currently available water quality constituents are: Dissolved Nitrogen (NO3-N, NO2-N, NH4-N, and Org-N); Dissolved
Phosphorus (PO4-P and Org-P); Algae; Dissolved Oxygen (DO); and Carbonaceous Biological Oxygen Demand (CBOD).
For details on how to use the water quality capabilities in HEC-RAS, please review Chapter 19 of the User's Manual.
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Data Storage and Management
Data storage is accomplished through the use of "flat" files (ASCII and binary), as well as the HEC-DSS. User input data are stored in flow files under separate
categories of project, plan, geometry, steady flow, unsteady flow, and sediment data. Output data is predominantly stored in separate binary files. Data can be
transferred between HEC-RAS and other programs by utilizing the HEC-DSS.
Data management is accomplished through the user interface. The modeler is requested to enter a single filename for the project being developed. Once the project
filename is entered, all other files are automatically created and named by the interface as needed. The interface provides for renaming and deletion of
files on a project-by-project basis.
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Graphics and Reporting
Graphics include X-Y plots of the river system schematic, cross-sections, profiles, rating curves, hydrographs, and many other hydraulic variables. A three-dimensional
plot of multiple cross-sections is also provided. Tabular output is available. Users can select from pre-defined tables or develop their own customized tables. All
graphical and tabular output can be displayed on the screen, sent directly to a printer (or plotter), or passed through the Windows Clipboard to other software,
such as a word-processor or spreadsheet.
Reporting facilities allow for printed output of input data as well as output data. Reports can be customized as to the amount and type of information desired.
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RAS Mapper
HEC-RAS has the capability to perform inundation mapping of water surface profile results directly from HEC-RAS. Using the HEC-RAS geometry and computed water surface
profiles, inundation depth and floodplain boundary datasets are created through the RAS Mapper. Additional geospatial data can be generated for analysis of velocity,
shear stress, stream power, ice thickness, and floodway encroachment data. In order to use the RAS Mapper for analysis, you must have a terrain model in the binary
raster floating-point format (.flt). The resultant depth grid is stored in the .flt format while the boundary dataset is store in ESRI's Shapefile format for use with
geospatial software.
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