Overview of HEC-RAS as a Hydraulic Modeling and Analysis Tool
The HEC-RAS application is a computer based software that is used for hydraulic modeling and analysis of water flow through rivers and other water channels. The software has undergone different upgrades to version 5.0. The program was one-dimensional before an upgrade to version 5.0. As such, there was no direct hydraulic effect modeling of the cross sections, bends, and other elements of flow that need to be represented in two to three dimensions. In the done example from the user manual, the version 5.0 has been used that allows a two dimensional modeling of flow, besides other aspects of flow such as sediment transfer modeling capabilities. HEC – RAS is software developed by the US Department of Defense, Army Corps of Engineers for purposes of river management, harbor management, and any other water related public works that fall under the jurisdiction of the department (US Army Corps of Engineers, 2016).
The software is integrated and its design is such that it is interactive and allows a multitasking user interface. The primary features of the software are a graphical user interface (GUI), separate analysis components, data storage components, management of data elements and, graphics and reporting facilities.
The river analysis components in HEC – RAS include: (1) water quality analysis. (2) unsteady flow simulations (both in one dimensional and two dimensional), (3) steady flow water surface profile computations and (4) Quasi unsteady or fully unsteady flow movable boundary sediment transport computations. All the primary elements of the software use the same geometric data representation and geometric and hydraulic computations routines that are similar.
In the example done from Chapter four of the user manual, the following are the process were done: staring a New project, entering geometric data, entering steady flow data, performing the hydraulic calculations, viewing results, and existing the program.
After starting a new project, various assumptions were made during project processing and execution. The example done in the application is an imaginary example, and that no data was obtained from any field for analysis.
The first assumption made is that the river to be analyzed exists and it is a two river hydraulic system, i.e. a three hydraulic reaches system as it is depicted in the project files. In development of this model, the next major assumption made is that the river profile has georeferences. However, these georeferences are not real, they do not exist. As such, an imaginary georeferenced background map was selected, or was brought into the application.
The Design and Features of HEC-RAS Software: A Graphical User Interface (GUI), Separate Analysis Components, Data Storage Components, Management of Date Elements and Graphics and Reporting Facilities
Another major assumption made during project execution is the cross sections for the river system actually exists and the parameters given for the river system are actual parameters. The cross sections were also assumed to be uniform throughout the river profiles, and because of that, they were copied, one to the next. However, different but almost similar parameters were utilized to create some arbitrary differences for the river profile.
The river system contained three reaches separated by a junction. The junction separated the upper reach and the lower reach for the Fall River, and also the reach for the Butter Cr. River (the tributary). The junction data is ideal – a major assumption for computation and analysis of the river in subject. Junction data for the river was assumed to accurately contain the precise figures for river description, and the lengths of the reaches across the junction. The geometry data for the project was saved and the analysis done step by step as from the example in the user manual.
Before the selection and generation of the various graphs from the software, different analyses and data entries were done to form the basis for graph generation. Steady flow data was entered into the software to compute the profiles. Three river profiles were calculated. For the three profiles, flow data was entered starting the upstream side to the downstream of the river. It this step of the analysis, another assumption was made – the flow is constant until another flow value is encountered within the reach. Other follows were added at the river sections within the reaches.
The graphs generated and selected for the example as shown in the next section of the report were also defined by the river profile under analysis, boundary conditions. The boundary conditions serve as the parameters for the establishment of the point where the river starts and the end of the river system under analysis. The beginning of a river is essential in the analysis because in helped the application to start making calculations. The selected regime for the river was a subcritical flow regime. In a subcritical flow regime, analysis boundary conditions are only required at the downstream end of the river system. For supercritical flow regimes, the boundary condtions are only set at the upper end of the river. There are cases where the flow is kof a mixed regime. In such cases, the calculations for the river are done with the coundary conditions entered both at the upstream end of the river and the downstream end of the river.
River Analysis Components in HEC-RAS software: Water Quality Analysis, Unsteady Flow Simulations, Steady Flow Water Surface Profile Computations and Quasi-Unsteady or Fully Unsteady Flow Movable Boundary Sediment Transport Computations
The example from chapter 4 of the HEC- RAS user manual sets a base of the various processes, or analyses that can be accomplished by the software. In the example, the different hydraulic analyses of a river profile were done and results saved in the HEC – RAS files, contained in a separate folder. The analysis done in the example is a steady flow analysis of a river. A steady flow analysis is the computation of a river or channel of water which has a continuous flow of water.
The steady flow analysis done for this example involved an assignment different numbers of flow profiles as different flow scenarios. The analysis helped to show the stages of the river at different points in the river and also helps to show the initial ideas of where the channel converges. In case of unsteady flows, temporal river changes in flow are captures at different locations and this analysis cannot be replaceable by steady analysis.
HEC- RAS provides an option for computation of different hydraulic calculations covered under steady flow analysis (Tate, 1999). After data has been entered into the steady flow analysis platform/window, the compute button is used to do all the necessary calculations and the results are displayed. The steady flow simulation window displays all the computation done in the application.
The graphs and tables for the various analyses are displayed in different windows. A successful computation of the results led to the results for the example project. Using the scenario output options, the Cross section plots, profile plots, general profile plots, rating curves, X-Y-Z perspective plots, detailed tabular outputs, and limited tabular outputs were generated.
The figure below shows the cross section plot for the example application:
The figure below is a plot generated for multiple water surfaces from the HEC – RAS;
A rating curve generated from the analysis based of the water surface profile is shown in the diagram below.
The X-Y-Z perspective plot for the river system was done as shown in the screenshot below. The profile summary table was used to generate the plot. The table displays a limited number of hydraulic variables for several cross sections.
X-Y-Z perspective of the river profile.
Cross sections tabular output
Profile formats.
There are other options provide by HEC- RAS for printing the graphs and output from various analyses. However, they are beyond the scope of this report.
Conclusion
After all the analyses were done, the project was saved and the program closed. The file folder Hydraulics_HEC – RAS contains all the project files and results for the example application described in Chapter 4 of the HEC RAS user’s Manual Version 5.0.
HEC – RAS is a hydraulics application which is free and available in the public domain, besides being peer reviewed software (Dyhouse, Hatchett, & Benn, 2003; Bonner, Brunner, & Jensen, 1994). It is not only used by different consulting organizations but also other public works departments, besides being an add on software. With its extensive documentation, scientists and engineers can use the application for various hydraulic analyses with little difficult. It is notable however, in its use, users may find some numerical instability problems especially is an unsteady analysis is being done e.g in steep and highly dynamic channels.
References
Bonner, V., Brunner, G., & Jensen, M. (1994). HEC river analysis system (HEC – RAS). ASCE.
Dyhouse, G., Hatchett, J., & Benn, J. (2003). Floodplain modeling using HEC – RAS. Haestad Press.
Tate, E. (1999). Introduction to HEC – RAS. Center for Research in Water Resources.
US Army Corps of Engineers. (2016). HEC – RAS River Analsys System. Hydrologic Engineering Center.
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