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|Publication Number: Date: Nov/Dec 1999|
Issue No: Vol. 63 No. 3
Date: Nov/Dec 1999
State-of-the-Art Hydraulic Modeling Environment
Two-dimensional computer modeling of surface-water flows is an emerging technology within the hydraulic engineering field of civil engineering. The Federal Highway Administration (FHWA) has been a leader in the development of this type of computer model, which provides a level of detail and accuracy not previously available to highway hydraulic engineers. However, the complexity of defining the necessary finite element network (and the time associated with doing so) has somewhat limited the use of this type of engineering application until recently.
In 1994, FHWA collaborated with the Engineering Computer Graphics Laboratory at Brigham Young University. Recently, the lab has been reorganized to form the Environmental Modeling Research Laboratory (EMRL). Work previously completed by EMRL and the U.S. Army Corps of Engineers' Waterways Experiment Station was the basis of the FHWA-EMRL effort to create an interface to the FHWA's two-dimensional, depth averaged, Finite Element Surface-Water Modeling System, FESWMS-2DH. That interface and corresponding tools were made available as part of the Surface-Water Modeling System (SMS) in Version 4.0, which was released in 1995.
Tools created for use with two-dimensional finite element surface-water models generated an interest in the development of tools for creating one-dimensional data and for visualizing one-dimensional results in two and three dimensions. Therefore, an interface for graphical interaction with one-dimensional cross sections was developed to support FHWA's WSPRO (Water-Surface Profile) computer model. The WSPRO computer model is used by hydraulic units in many state highway agencies to accomplish their routine water-surface profile computations. This interface was developed and made available in SMS Version 5.0, which was released in 1997.
This article describes the methods and tools to perform numerical hydrodynamic analysis using SMS with FESWMS-2DH and WSPRO. The purpose of this article is to provide a greater awareness of state-of-the-art methods for modeling complex surface-water flows. Hopefully, this increased awareness will lead to more frequent and widespread application of these models.
The interface to SMS is divided into modules to simplify its use. Sets of random data points are grouped into scattered data sets" and are used in the Scattered Data Module. Finite element networks that consist of nodes, elements, element properties, and boundary conditions are created and manipulated through use of the Mesh Module. Input data files are also created through use of the Mesh Module. One-dimensional river cross sections, such as those that are used in conventional water-surface profile computation models such as FHWA's WSPRO, are created and manipulated though use of the River Module. Each module includes tools for operating with the data type used in that module.
Tools to convert data from a type compatible with one model to a type compatible with another model are also available in the system. SMS also includes a large set of tools and utilities for post-processing the results into an easily understandable graphical form for each of the system interfaces.
The following sections describe the four modules that apply directly to the work performed through the collaboration between FHWA and EMRL. These components include the Mesh Module, which contains the FESWMS-2DH tools and interface; the River Module, which contains the WSPRO tools and interface; and the Map and Scattered Data modules, which provide tools for automatic model generation.
Finite Element Network Creation and Editing
The Mesh Module is used to create finite element networks that consist of many interrelated nodes and elements. The Scattered Data Module is typically used to provide topographic information to the nodes in the finite element network. The system also allows the user to create and edit networks that have multidimensional attributes.
For example, the branches of a simple stream may be composed of a string of one-dimensional elements. As the stream merges with other streams or becomes larger, the elements can become a two-dimensional region of the finite element network. If the flow becomes more complicated, the network can also contain regions of three-dimensional elements.
The numerical model being used must support all the element types defined in the mesh. To prevent confusion and minimize errors, SMS includes a model checker that can compare aspects of a finite element network and data file with the capabilities of a selected model. If any errors are identified, they are reported to the users as potential errors.
The quality" of the mesh can also be evaluated through the use of a mesh quality tool. If poorly constructed elements are found within the network, they are graphically identified. The user may or may not correct the problem by using one of the numerous interactive editing tools within SMS to manually edit and modify the finite element network. Once a network is constructed, model-dependent material properties and boundary conditions are easily assigned.
SMS also includes a number of algorithms that can automate the creation of finite element networks. These algorithms include triangulation, patch-based network creation, adaptive tessellation (which forms a finite element network with a mosaic pattern), and finite element network and model creation using Geographical Information System (GIS) features and concepts. The GIS capabilities and concepts use arcs, polygons, and coverages that can easily be used to rapidly create and modify finite element networks. Because of space limitations, only finite element network and model creation using GIS features and capabilities will be discussed in this article. These techniques will be discussed more completely and will be illustrated by the figures.
|Figure 2 - Scanned background image with annotations and feature arcs.|
One-Dimensional Model Tools
The River Module supports one-dimensional, water-surface profile computation models. Tools are provided to create river stations and assign geometric data to those stations. Sectional data includes geometric valley cross sections and flow-control structure definitions. Flow-control structure definitions supported by SMS include culverts, bridges, roadways, and guide banks. Each section can be created and edited graphically, read from field data, or extracted from terrain data contained in a digital elevation map (DEM) or triangulated network.
The output from one-dimensional analysis includes data such as water-surface elevation at each cross section. The River Module includes visualization tools that can plot the computed data at any of the cross sections. The data can also be plotted at cross sections along the length of the river. Figure 1 is an image of the SMS screen showing a river cross section with a superimposed bridge. It also shows plotted profile results.
Automatic Model Generation
Automatic mesh or finite element network generation is most efficiently accomplished though use of the Map Module. The Map Module supports four types of objects: DXF objects, image objects, drawing objects, and feature objects.
DXF object files contain graphic images generated by external computer-aided drafting and design (CADD) packages, such as AutoCADÂ® or MicrostationÂ®.
Background information can be supplied to the model through either images contained in DXF files or image objects.
Image objects are graphic files that can be obtained from a scanned image of the section of the river being modeled. Image objects are also available through a variety of other sources. The image object files must be in tagged image file format (TIFF). When used as background information, both DXF and image objects are valuable in helping the user better understand and visualize the region being modeled.
Drawing objects include arrows, rectangles, and text. This feature allows the user to annotate regions of interest with important remarks and point out significant results without having to rely on additional software. This feature of SMS is valuable when preparing reports or presentations.
Feature objects include points, arcs, and polygons, which can be used to generate GIS-like coverage. Such coverage is used to generate detailed representations of a problem study area. The feature arcs define the boundaries of a conceptual model, and the annotated text provides information to the modeler. Regions enclosed by the feature arcs are converted to polygons, which can be assigned attributes such as boundary roughness characteristics and eddy viscosity. Feature arcs that bound the upstream and downstream boundaries of the section of the river being modeled can be grouped" to form a single feature arc that can be assigned attributes such as boundary conditions. The bathymetry (measurement of the depth of the water) of the model is defined by interpolating from a set of points that were digitized from a bathymetry map. Bathymetry data can also be provided through use of external data obtained from other sources, such as a survey.
The Scattered Data Module is used to manipulate the data from which the elevation data is interpolated. Figure 2 shows a background image created from a scanned U.S. Geological Survey (USGS) quadrangle map. The boundaries, and other important features in the study area, are defined by feature arcs and explained with text object annotations. The finite element network generated automatically from SMS's mesh generation from feature objects tools shown in figure 3. The finite element network is forced into the polygons formed by the feature arcs. Elements in each polygon are assigned a material type.
SMS supports numerous post-processing utilities such as vector plots, contour plots, animation, and particle-tracking. The post-processing tools are generalized in their application, which allows a consistent approach in visualizing output or preparing report information regardless of the engineering application applied.
|Figure 3 - Study reach with finite elements and boundary conditions.|
Data visualization usually begins with the input data and/or finite element network that has been prepared for use with an external engineering application such as FESWMS-2DH or WSPRO. Once the engineering application has been successfully applied, the output data can be read by SMS and processed into many useful formats. In the case of a FESWMS-2DH model, SMS reads an output flow file. From the output file, items such as ground elevation, water-surface elevation, water depth, and velocity can easily be contoured and displayed. Vectors showing the magnitude and direction of flow at each node point can also be displayed.
Each visualization can be manipulated and customized to suit the preference of the modeler. Figure 4 illustrates a study reach showing a finite element network with boundary conditions and the elevation contours in color. Figure 5 shows the same study reach with color-filled, water-surface elevation contours and velocity vectors. Figure 6 shows color-filled velocity contours and velocity vectors. If WSPRO is being applied, the water-surface profile can be plotted and evaluated.
Additional data sets can be created using the data-set calculator provided in SMS. This tool allows the user to compare data sets from one simulation to data sets generated from another. Differences in solutions can be computed and displayed. Another feature of the data-set calculator is the ability to perform mathematical operations on the model solution variables. For example, the data-set calculator can be used to compute and display other hydraulic variables that may be of interest to hydraulic engineers.
Data can also be visualized through the use of an animation solution. If time-dependant results are being considered (such as are commonly encountered in tidal modeling situations), the ability to view the successive time steps can be invaluable in understanding the progression of the data sets. SMS provides an animation tool that has the capability to create animated sequences of images. These images can be for each time step, or intermediate images can be generated by interpolating time steps. The images can then be played back on the computer monitor to quickly view the dynamics of the problem. The user is also able to look at individual frames and to step forward and backward through the frames of the animation. The animation sequence can then be saved for later presentation or evaluation.
Another type of visualization provided by SMS is referred to as flow tracing. A flow trace is a simulated release of dye droplets into the flow field. Subsequent steps show the paths taken by each of the droplets placed into the flow field. A droplet in a slow-moving portion of the flow field results in a small streak; the same particle in a fast-moving portion of the field results in a long streak. Each successive frame of the flow trace allows the droplet to migrate farther through the flow field until it has completely dissipated. The user can scale the velocity of the field to magnify the flow, as well as control the rate of dissipation of the droplet. This process can be used for both steady-state and dynamic cases.
|Figure 4 - Study reach showing finite element network, boundary conditions, and color-filled elevation contours.|
Animated sequences are generated by taking snapshots of the simulated particles decaying in the flow field. After a user-defined number of these images have been generated, they are animated, thus representing the movement of flow in the modeled region. Flow direction and areas of recirculating flow are easily identified, providing modelers with an excellent ability to understand and interpret the results of their analysis. An image representing one snapshot" in time of a flow trace is presented in figure 7.
SMS has been used with FESWMS-2DH by several state highway agencies.
The Alabama Department of Transportation has applied the models several times to model complex flow situations. One application evaluated flow patterns and bridge scour for several relief bridges and a main-channel bridge. The other application answered questions about sediment transport and flow into and out of a tidal inlet.
The Minnesota Department of Transportation has also used the models to help answer questions about a controversial bridge replacement over the St. Croix River.
Both the Florida and Georgia departments of transportation have completed two-dimensional flow analysis to evaluate complex flow situations at tidal bridges.
All of the state highway agencies that have used SMS and FESWMS-2DH to model complex hydraulic features report that they are confident the results of their studies are significantly more accurate than those that would have been obtained using a less sophisticated modeling process.
|Figure 5 - Study reach showing color-filled water-surface elevation contours and velocity vectors.|
SMS is a very powerful computer program used by engineers and scientists who need to model complex surface-water flows. However, each time the capability of the program has increased, ideas for new developments are planned. Future enhancements to SMS include new tools that will further facilitate the process of building finite element networks, creating data files, and visualizing results. Other improvements will include methods to more easily model hydraulic features such as bridge abutments, guidebanks, and piers. Future versions of SMS will also be more compatible with many of the readily available data sources. Methods for providing topography and bathymetry to finite element networks will continue to improve.
The next data conversion feature to be integrated into SMS is the integration of one- and two-dimensional data. The first step of this process is to use two-dimensional terrain data such as a digital elevation map or a triangulated irregular network to create the cross-sectional data required for a one-dimensional model such as WSPRO. The user would be required to graphically define the location of the centerline of flow in the terrain model and to select the location of the desired cross sections. SMS would then extract the elevation information for the cross sections from the terrain.
With these cross sections and user-specified flow quantities, one-dimensional models can compute water-surface elevations at each section. These elevations can then be mapped back to the terrain map and interpolated between cross sections in a manner similar to tracing out a contour on a terrain map. This defines the domain of a two-dimensional mesh or grid, and automatic mesh-generation tools can be applied to generate an initial finite element network.
Other information from the one-dimensional model, such as specified material zones on the cross sections, could similarly be interpolated to the two-dimensional data to create feature polygons if desired.
The Surface-Water Modeling System developed by EMRL in conjunction with FHWA and the Waterways Experiment Station takes a large, first step toward a comprehensive system for creating data files for use with state-of-the-art hydraulic models. SMS allows an unprecedented ability to view and analyze the results of these models.
|Figure 6 - Study reach showing color-filled velocity contours and vectors.|
The tools contained within SMS make it possible to apply data-intensive, state-of-the-art models such as FESWMS-2DH to numerically simulate complex surface-water flows. The simulations are cost-effective and provide realistic solutions to complex problems commonly encountered by hydraulic engineers. SMS contains user-friendly tools that can help hydraulic engineers apply a variety of types of surface-water models. In fact, the system makes it possible to solve a problem using multiple approaches or models and thereby gain a fuller understanding of the conditions.
The system is not designed to take the place of the engineer; instead, it is designed to perform the menial tasks of data management and compilation that, in the past, prevented the engineer from completely modeling complex flow conditions.
SMS, and one of its companion programs, called the Watershed Modeling System (WMS), is available for use by any state highway agency. FHWA has developed a licensing agreement that makes it possible for state highway agencies to obtain and use copies of the programs. In addition, training is available through the National Highway Institute. The available courses teach participants how to use SMS to build data files and create finite element networks for use with FESWMS-2DH and WSPRO.
|Figure 7 - Animated flow trace.|
For more information, please contact Larry Arneson at (303) 969-5772 ext. 349 or send e-mail to email@example.com.
1. W.A. Thomas and W.H. McAnally Jr. User's Manual for the Generalized Computer Program System: Open-Channel Flow and Sedimentation, TABS-2, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss., 1990.
2. J.K. Lee and D.C. Froehlich. Two-Dimensional Finite-Element Hydraulic Modeling of Bridge Crossings: Research Report, Federal Highway Administration, Washington, D.C., 1989.
3. D.C. Froehlich. Finite Element Surface Water Modeling System (FESWMS): Two .Dimensional Depth .Averaged Flow and Sediment Transport Module, Version 3, Reference Manual and Users Guide, Publication No. FHWA .RD .99 .160, Federal Highway Administration, Washington, D.C., 1999.
4. Surface-Water Modeling System, Version 4.0, Reference Manual , Engineering Computer Graphics Laboratory, Brigham Young University, Provo, Utah, 1995.
5. L.A. Arneson and J.O. Shearman. User's Manual For WSPRO - A Computer Model for Water-Surface Profile Computations, Publication No. FHWA-SA-98-080, Federal Highway Administration, Washington, D.C., June 1998.
6. Surface-Water Modeling System, Version 5.0, Reference Manual , Engineering Computer Graphics Laboratory, Brigham Young University, Provo, Utah, 1997.
Dr. Larry Arneson is a hydraulics engineer in the Denver, Colo., office of FHWA's Western Resource Center, and he is responsible for FHWA's development of advanced hydrologic and hydraulic, computer models. He has served in FHWA for more than 17 years. His professional interests include scour and sediment transport, scour countermeasures, hydraulic structures, and engineering-related computer applications. He performed laboratory research on scour in bridge openings with water flowing under pressure conditions. Dr. Arneson has developed numerous training courses and has taught more than 150 courses. He earned his bachelor's and master's degrees in civil engineering from Montana State University in Bozeman, Mont., and he received his doctorate in civil engineering from Colorado State University in Fort Collins, Colo.