(from the introduction in the Users' Manual)
The HEC-RAS modeling system was developed as a part of the Hydrologic Engineering Center's "Next Generation" (NexGen) of hydrologic engineering software. The NexGen project encompasses several aspects of hydrologic engineering, including: rainfall-runoff analysis; river hydraulics; reservoir system simulation; flood damage analysis; and real-time river forecasting for reservoir operations.
HEC-RAS is an integrated system of software, designed for interactive use in a multi-tasking, multi-user network environment. The system is comprised of a graphical user interface, separate hydraulic analysis components, data storage and management capabilities, graphics and reporting facilities.
The HEC-RAS system will ultimately contain three one-dimensional hydraulic analysis components for: (1) steady flow water surface profile computations; (2) unsteady flow simulation; and (3) movable boundary sediment transport computations. A key element is that all three components will use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the three hydraulic analysis components, the system contains several hydraulic design features that can be invoked once the basic water surface profiles are computed.
The current version of HEC-RAS only supports steady flow water surface profile calculations. New features and additional capabilities will be added in future releases.
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.
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 regime water surface profiles.
The basic compoutational 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 is utilized 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).
The effects of various obstructions such as bridges, culverts, weirs, and structures in the flood plain may be considered in the compoutations. The steady flow system is designed for application in flood plain management and flood insurance studies to evaluate floodway encroachments. Also, capabilities are available for assessing the change in water surface profiles due to channel improvements, and levees.
Special features of the steady flow component include: multiple plan analyses; multiple profile computations; and multiple bridge and/or culvert opening analyses.
Unsteady Flow Simulation. This component of the HEC-RAS modeling system will be capable of simulating one-dimensional unsteady flow through a full network of open channels. The unsteady flow equation solver will be adapted from Dr. Robert L. Barkau's UNET model. This unsteady flow component was developed primarily for subcritical flow regime calculations.
The hydraulic calculations for cross-sections, bridges, culverts, and other hydraulic structures that were developed for the steady flow component will be incorporated into the unsteady flow module. Additionally, the unsteady flow component will have the ability to model storage areas, navigation dams, tunnels, pumping stations, and levee failures.
Sediment Transport/Movable Boundary Computations. This component of the modeling system is intended for the simulation of one-dimensional sediment tranpsort/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 will 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 will be 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.
Data storage is accomplished through the use of "flat" files (ASCII and binary). User input data are stored in flat 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 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, moving, and deletion of files on a project by project basis.
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.
The Applications Guide contains written descriptions of 13 examples that demonstrate the main features of the HEC-RAS program. The project data files for the examples are contained on the HEC-RAS program distribution diskettes, and will be written to the HEC/RAS/DATA directory when the program is installed. The discussions in the manual contain detailed descriptions for the data input and analysis of the output for each example. The examples display and describe the input and output screens used to enter the data and view the output. The user can activate the projects within the HEC-RAS program when reviewing the descriptions for the examples in this manual. All of the projects have been computed, and the user can review the input and output screens that are discussed as they appear in this manual. The user can use the zoom features and options selections (plans, profiles, variables, reaches, etc.) to obtain clearer views of the graphics, as well as viewing additional data screens that may be referenced to in the discussions. The examples are intended as a guide for performing similar analyses.
(Abstract from Proceedings of the First Federal Interagency Hydrologic Modeling Conference, held in Las Vegas, NV, April 19-23, 1998, prepared by the Subcommittee on Hydrology of the Interagency Advisory Committee on Water Data)
The eruption of Mount St. Helens, spring of 1980, resulted in a deposition of vast quantities of sand-sized sediment which created a threat of flooding in Southwest Washington and navigation disruption on the lower Cowlitz and Columbia Rivers. Corps of Engineers levees on the Cowlitz River, a Sediment Retention Structure (SRS) on the North Fork Toutle River (a Cowlitz River tributary), and a dredging program were designed to reduce these threats.
In November 1995 and February 1996, large floods occurred on the Cowlitz River. There was considerable local concern that these events deposited sediment and decreased the flood protection afforded by the levees along the lower 17 miles of the Cowlitz River. The Federal Emergency Management Agency provided the Corps of Engineers, Portland District, funds to update the discharge-frequency curve, to determine risk of flooding, and to map flooded areas.
The frequency of events was estimated at a USGS gage at Cowlitz River at Castle Rock (RM 17.1). Since 1968, flows at this gage have been regulated by Mossyrock Dam at river mile 65.5. The previous frequency curve was developed using annual peak instantaneous flow data from 1928-1962. For this study, flow data provided by the USGS were used to estimate natural (unregulated) flood peaks at Castle Rock. A regulated discharge-frequency curve was revised from the updated natural discharge-frequency curve and a new natural versus regualted discharge relationship.
With the deposition of debris flow material during the eruption of Mount Saint Helens, the lower Cowlitz changed from a gravel bed to a sand bed stream. Following the closure of the SRS in 1987, the bed has gradually been returning to its pre-eruption gravel bed characteristics. For flood plain planning purposes, the existing risk of flooding (1997) reflecting existing channel bed characteristics and the long-term risk of flooding reflecting the expected change of the stream bed back to primarily gravels are required.
The Corps of Engineers backwater application HEC-RAS (River Analysis
System) was used to develop elevation-frequency data for existing and
long-term conditions. The Corps HEC-FDA (Flood Damage Reduction
Analysis) Monte Carlo simulation techniques were applied to the
elevation- and regulated discharge-frequency curves. These computations
resulted in the expected value of the events that would exceed the safe
levee elevations accounting for the uncertaintly in the derivation
curves. The existing and long-term safe protection provided by the
levees was estimated from these data.
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