Floodplain Visualization Using HEC-GeoRAS
Prepared by Daniel Snead and David R. Maidment
Terrain Model and Hydrologic Data provided by Esteban Azagra-Camino
Center for Research in Water Resources
October 2000
Table of Contents
Goals of the Exercise
Software and Data Requirements
Starting ArcView
Terrain Model Processing
GeoRAS Pre-processing
Preparation of ArcView GIS Themes
Generation of Additional Attributes and 3D Spatial Data
Generation of the HEC-RAS Import File
Developing Steady-State Simulations in HEC-RAS
Geometry Data
Flow Data
Export HEC-RAS Data into GIS
GeoRAS Post-processing
Theme Setup
Read GIS Export File
Water Surface TIN Generation
Floodplain Delineation
Editing the Floodplain Profile
3-D Floodplain Development
Conclusion
This exercise is an introduction to floodplain analysis and representation through computer modeling. There are various methods and programs for such modeling. This exercise will use the HEC-River Analysis System(HEC-RAS) and the ArcView GIS extension HEC-GeoRAS. The exercise covers the following concepts:
The study area selected for the exercise is the section of the Waller Creek Basin that flows through the University of Texas at Austin. If you have problems in doing this exercise, please contact Dr Maidment or Dan Snead at dsnead@mail.utexas.edu.
Software and Data Requirements
The HEC-GeoRAS extension (Version 3.0) was developed through a Cooperative Research and Development Agreement between the Hydrologic Engineering Center (HEC) and the Environmental Systems Research Institute, Inc. (ESRI) and is provided for this exercise. It is the result of continued development of the AVRAS 2.2 extension written previously by ESRI in cooperation with HEC. You need to have ArcView version 3.0 or later running with the 3D Analyst and Spatial Analyst extensions to support 2-D and 3-D modeling using grid, vector, and TIN files.
This exercise also requires that you have HEC-RAS version 2.2 or greater. The HEC-GeoRAS extension was developed for use with HEC-RAS 3.0, which is currently being tested as a Beta release. For the purpose of this exercise, HEC-RAS 2.2 will be used. Most of the computers at the LRC have HEC-RAS 2.2 on their hard drives. The HEC-RAS and the HEC-GeoRAS software can be obtained from the Hydrologic Engineering Center's home page at http://www.wrc-hec.usace.army.mil/. A user's manual is also available from the same internet site.
The data files used in the exercise consist of ArcView shape files, the HEC-GeoRAS extension file, the digital terrain model of the Waller Creek study (in the TIN format), a geometry file for use in HEC-RAS, and a MS Excel Spreadsheet with flow data from HEC-HMS for inputting into HEC-RAS. The following files are required for this exercise, and are stored in georas.zip:
Select a working directory on your computer and download these files into it (except the HECGeoRAS extension).
When you start Arcview, first set the working directory. Open a new view from the Views window. From the File menu, choose Set Working Directory. Type the name of your working directory (where you downloaded/copied the files) and click OK.
Save the Untitled Project as wallerfld.apr in the working directory.
Select the File/Extensions...; menu option and check the HECGeoRAS, 3D Analyst, and Spatial Analyst extensions from the Extensions window. Click OK and the extensions will be loaded. If the HECGeoRAS extension is not available in the list, the extension was not properly loaded in the /ext32/ subdirectory. Go back and ensure the extension is in the correct directory.
GeoRAS extracts terrain information stored in TINs and generates a HEC-RAS import file. TINs are created from points, polygons, and lines stored in different formats. Since the focus of this exercise is on the development of a river model and its spatial import/export features within Arcview GIS, the TIN is provided for you. Esteban Azagra-Camino previously developed the terrain model in the TIN format.
Terrain model development is the most critical step in computer modeling of flood events. There are limits to using TINs developed only from a 30-m Digital Elevation Model, the obvious one being accuracy. If additional data exists (i.e., surveyed cross-section data, building coverages, 1-2 ft. contour files, etc.), it should be incorporated into the TIN for use with GeoRAS. A number of students that have worked at the Center for Research in Water Resources (CRWR) have done extensive work on terrain model development. For further information, check out the web pages of Esteban Azagra-Camino, Eric Tate, and David Anderson, found on the Graduated Students Pages on the CRWR web site, http://www.crwr.utexas.edu/.
Start the exercise by adding the TIN theme to View 1, first change the name of the view to "Preprocess" using the Properties command from the View window. Also set the Map Units and Distance Units to feet.
Click on the Add Theme button 
,
select “TIN Data Source” from Data Source Types and double click on “walterrain”.
Double click on the TIN’s legend bar to open the TIN
Legend Editor. Click off the
check box next to Lines and then click the Edit button in the Faces
part of the window to open the regular Legend Editor. Change the Color Ramp to
“Terrain Elevation #2”. Switch the
color scheme by clicking on the 
button. Click on the Apply button and close both the Legend
Editor and TIN Legend Editor. Your TIN should look similar to the following figure. You can change the ranges of each color
as desired.
To see the TIN triangles, turn off the Faces display and turn on Lines, choosing All Feature Types. Zoom in to see the triangles in some detail.
Use Theme/Properties to examine the number of TIN triangles and the extent of the coverage. The map units are in feet.
Save the project. You are now ready to pre-process geometric data for HEC-RAS!
To be turned in: What is the area covered by the TIN? How many triangles are there? What is the average area covered by a triangle? What is the range of elevation covered by the TIN?
The goal of this section is to develop the spatial data required to generate a HEC-RAS import file with a 3-D stream network and 3-D cross sections defined. The process is divided in three steps:
Results of this section will be provided as results to your homework assignment. The georas.zip file contains all the shape files created previously using the GeoRAS pre-processor. These files will be unzipped into a folder called Results in your working directory. If you get frustrated with the pre-processing (it can get tedious and time consuming) work, you can pick up the completed result from the Results folder and move on to the next section. If you want to skip the preprocessing of the GIS files, add the themes banks.shp, streamcl.shp, xscutlines.shp and flowpath.shp to your view from the Results folder and go to Generation of Additional Attributes and 3D Spatial Data.
The pre-processing section of this exercise is NOT required for the additional sections of this exercise. All the work done for pre-processing is to obtain an understanding of the process required to input geometric data into HEC-RAS, but will not be used in the next section. An import file for HEC-RAS has been provided for you in the georas.zip file.
Preparation of ArcView GIS Themes
The development of a stream centerline, cross-sections, stream banks, and flow
path lines as shape files are required for preprocessing. Digitizing polylines with the 
button creates the themes while in the edit mode. It is helpful to have a
visual data source, like a Digital Orthophoto Quadrangle (DOQ) to identify the
centerline, banks, and flow paths. For this exercise, you will use the TIN as the visual source. Follow the rules that are provided
below when digitizing. Focus more on the process necessary to complete this section of the exercise than on the
overall accuracy of the digitization. Do the best you can. A
HEC-RAS geometry file will be provided to you for the next section.
Stream Centerline
For developing the stream centerline, use the stream.shp theme provided for you in your working directory. Click on the add theme button, change the Data Source Types to “Feature Data Source”, add the stream.shp theme to the “Preprocess” View.
Select the Create Stream Centerline command from the preRAS
menu. Name the new shape file as streamcl.shp. The file will automatically be in the
edit mode. Under the Theme menu, click on Stop Editing and save the edits to streamcl.shp.
We are going to copy the features of the stream.shp theme and paste them into the streamcl.shp
theme. Highlighting the stream.shp theme in the legend, click on the Start Editing command under the Theme
menu. Using the highlight tool, 
, highlight
the entire extent of the stream.shp theme. Under the Edit menu, click on the Copy Features
command:
Stop editing the stream.shp theme and start editing the streamcl.shp theme. Click on the Paste command under the Edit menu, stop editing, and save the edits. Delete the stream.shp theme from the “Preprocess” View. The theme should now have the stream centerline as well:
Check the attribute table of streamcl.shp. There should be three polylines depicting the three separate
reaches in the View. Once edits are completed on the stream centerline theme, river and reach identifiers must
be added using the 
(River ID) tool. After clicking on the River ID
tool, select one of the three reaches in the streamcl.shp theme. The following menu will appear:
Identify the three reaches as the following:
|
Location in View |
River ID |
Reach ID |
|
Northwest Reach |
Wallercrk |
WallerUS |
|
Northeast Reach |
Trib |
Trib |
| Southern Reach |
Wallercrk |
WallerDS |
The requirements for developing the stream centerline when using GeoRAS is to ensure all reaches are properly connected at junctions. For this exercise, this has been done for you. If you had digitized the reaches, the Snap tool in Arcview would be used to “snap” the three reaches together at the same location. In addition, stream centerline arcs must point downstream in the direction of flow. This can be verified by changing the line to an “arrow” line in the Legend Editor for the streamcl.shp theme. Check the attribute table to ensure reach IDs properly designated for the streamcl.shp. You are now ready to develop the main channel banks theme.
Stream Banks Theme
The stream banks separate the main channel from the overbank areas when flooding occurs. It differentiates the resistance of the main channel and the overbanks. This is important for a steady-state simulation. To create the stream banks theme, click on the Create Banks command under the preRAS menu. Arcview will ask you for the name of the new theme, call it banks.shp. The theme will appear in the legend in edit mode. Before we begin to create the banks theme, remember the following rules when digitizing:
To start editing, first ensure the
button is pressed. Let’s zoom into the upper left corner
of the View and start digitizing the theme. As you digitize, you can click on the right button on your
mouse, click on Pan, to continuously digitize your line. Remember to keep the TIN coverage on to
use as a reference. Look for differences in the shading of the TIN’s contours to identify the stream
channel. Once you have completed a line in the theme, double click at the final point.
There are some areas of the TIN that have incorporated the elevation of a bridge, so you will notice a drastic change in the elevation as you digitize the stream banks. Disregard the elevation change and continue digitizing the stream banks as if the bridge did not exist. We will discuss how we will overcome these discrepancies before developing the HEC-RAS Import File for the next section of this exercise. Once the theme has been completed, Save Edits under the Edit menu. Your view should now look something like this:
Creating the stream banks theme has been completed.
Flow Path Centerlines Theme
The Flow Path Centerlines theme is used to identify the hydraulic flow path in the left overbank, main channel, and right overbank. If the stream centerline already exists (as in our case), you have the option to copy the stream centerline for the flow path of the main channel. To accomplish this, click on the Create Flowpaths command under the preRAS menu. Specify the name of the theme as flowpath.shp. The following window will then appear in Arcview:
Click on Yes, and the features of the streamcl.shp will be copied to the flowpath theme. To complete the theme, digitize the flowpaths for the left and right overbanks while the flowpath.shp theme is in the edit mode. Make sure to adhere to the following rules regarding flowpath development:
Do not be concerned with the buildings in the TIN. Digitize the flowpaths as if the buildings did not exist in the terrain model. Once the polyline theme is completed, Save Edits under the Edit menu. Notice that you should have three flowpaths for each reach, flowing from upstream to downstream.
Once you have completed digitizing the flow paths, each flow path must mow be identified as a left, right, or channel flow path. Obviously the channel is the flow path along the center of the stream channel. Determining the left and right flow path is accomplished by an upstream to downstream perspective. For example, imagine yourself at the most upstream point on Waller Creek. Look downstream the stream’s channel. The left flow path would be to your left, the right flow path to your right.
To identify each flow path, first click on the 
button.
The following window will appear:
Choose Flowpath.shp as the flow path theme, and click OK. Your cursor will now have a “Tag”, which allows you to identify each polyline in the Flowpath.shp theme as left, right, or channel. Make sure you set the correct identification tag to all the polylines in the theme. To check this, pull up the attributes table of Flowpath.shp. The attributes table should look like the following, with a Linetype field identifying each polyline’s flow path:
Once you have correctly tagged each polyline in the Flowpath.shp theme, you have completed the process of creating flow path centerlines.
Cross Section Cut Lines Theme
The location, position, and expanse of cross sections are represented by the Cross Section Cut Line theme. This theme will identify the planar location of the cross sections and the station-elevation data being extracted from the TIN along each cut line for use in HEC-RAS. The rules for developing this theme are as follows:
To begin digitizing the Cross Section Cut Line theme, choose the Create XS Cut Lines command from the preRAS menu. Specify the name of the theme as xscutlines.shp and begin digitizing. Choose approximately 10-20 cross section locations for each reach. When accomplishing this in the Edit mode, click on the starting location and double-click where each line ends, then move to the next location.
There are Four Key Points to Remember:
1) For buildings, act as if they do not exist in the TIN,
2) Elude locations where possible bridges and/or overpasses exist in the TIN,
3) Ensure to place cross sections at upstream and downstream boundaries, and
4) Start and end the cut lines well beyond the extent of the flowpaths, since this theme will subsequently become the extent of the floodplain’s bounding polygon.
Use the arrow line in the Legend Editor to ensure the cut lines are directed from right to left, as you look downstream:
This completes the development of themes for use in the GeoRAS preprocessor.
Here is the point to continue the exercise if you just add the banks.shp, flowpath.shp, xscutlines.shp and streamcl.shp to your view from the Results folder.
Generation of Additional Attributes and 3D Spatial Data
For this part of the exercise, the polyline themes you created will be used to extract the 3-D attributes of the TIN through the themes’ spatial relationship with the terrain. Let’s first set up the theme attributes. Select the Theme Setup… command from the preRas menu and fill in each window with the identification of the created themes as shown in the graphic below.
The Land Use input data is Null since we are not using it for this exercise. It can be used to extract spatial data to determine Manning’s roughness value, h, for the HEC-RAS model. GeoRAS will process the intermediate data (the Stream Centerline (3D) and XS Surface Line (3D)) in the subsequent steps of this exercise. Name the RAS GIS Import File rasinput.geo, as shown above. Click OK to confirm the data. It is possible you may get an error message once you press OK. Disregard the message and continue with the exercise.
GeoRAS will generate three additional data sets prior to using HEC-RAS. They are the 3D Centerline theme, the 3D Cross Section theme, and the RAS GIS Import File. If you looks at the preRAS menu, you will notice a number of commands indented under those three data sets. The processing for each data set can be done step-by-step, for those cases where data is limited. For example, we will have to use the step-by-step process for developing the 3D Cross Section theme since we are not including a Land Use theme for the processing.
Centerline Completion
To complete the 3D centerline theme, click on the Centerline Completion command under the preRAS menu. Specify the name of the theme as streamcl3D.shp and press OK. Check the attribute table to ensure the three reaches in the theme have been connected properly. Each field should now have a “PolylineZ” shape and distances “to_ST” and “from_ST” for each reach. This ensures the connectivity of the three reaches. If the attribute table is missing the data, the development of the centerline theme will need to be repeated again before moving to the next step.
XS Attributing
Since we are not using a Land Use theme to develop Manning’s h values, we must accomplish the final attributing of the cross section theme step-by-step. First click on the Stream/Reach Names command under the preRAS menu. The following window should appear:
Press OK and continue down the list in the preRAS menu. Continue with the Stationing, Bank Stations, and Reach Lengths commands.
NOTE: Before moving to the next step, make sure the xscutlines.shp theme is active. If all the cross section cutlines are not highlighted already, use the Highlight tool to highlight all the cross section cutlines in the view. Click on the XS Elevations command and specify the name of the 3-D cross section file as xscutlines3D.shp. Click OK, and look at the theme in the “Preprocess” View. Make sure all of the cross sections are in the 3-D cross section theme. If not, then return to the Centerline Completion section, ensuring you highlight all the cross sections in xscutlines.shp theme prior to beginning the XS attributing.
Generation of the HEC-RAS Import File
The Generation of the HEC-RAS Import File is the last step of the GeoRAS pre-processing. The idea is to create a HEC-RAS input file in RAS Import format which includes the terrain elevation extracted from the TIN, the 3-D stream centerline, and the 3-D cross sections themes as z values (z value is the elevation above mean sea level and, for our case, is in units of feet).
To be turned in: Look at the attributes of Streamcl3d.shp. How many routing reaches are there? What is the river stationing on each reach (From_st, To_st). What is the total length of river which will be modeled over the three reaches? Look at the attributes of Xscutlines.shp. Choose a particular Cross-section. What Reach_Id and Station is it located at? At what percent distance across the cross-section are the left and right banks located? How far downstream is the next downstream cross-section located?
Developing Steady-State Simulations in HEC-RAS
The goal of this section is to develop the HEC-RAS model for our study area, using the geometric data extracted from the TIN using GeoRAS. The model’s geometry data and steady flow data is provided for you in the georas.zip file. The geometry file was developed from GeoRAS and imported as a HEC RAS Import File. The steady flow file was extracted from a HEC HMS hydrologic model and is shown in HMSResults.xls. The process is divided in two steps:
Geometry Data
To open HEC-RAS, look for a folder called “HEC” from the Start button on your computer, there should be a HEC-RAS 2.1, HEC-RAS 2.2, or HEC-RAS icon. Double click the icon to open the main project window, which looks like this:
When you extracted the files from georas.zip, the HEC RAS files (waller1.prj, waller1.g02, waller1.f01) should currently be in your working directory. Click on the Open Project… command from the File menu. Scroll down to your working directory and highlight the project titled “Use of GeoRAS Input”, file waller1.prj. Click OK. The Project, Geometry, and Steady Flow information lines should now be filled with the titles of those respective files as shown:
As shown in the main menu, four files are required to run a HEC-RAS project. First, the Project File acts as a file management tool and identifies which files are used in the model. The Plan File sets the model conditions as subcritical, supercritical, or mixed flow and runs the simulation. The Geometry File contains all the geometric attributes for the model (which, for our case, are imported from GIS using GeoRAS). Lastly, the Steady Flow File establishes the steady-state flow and boundary conditions at numerous points in time for the model. (If you are using HEC-RAS 3.0, you will have a row in the main menu for Unsteady Flow. Disregard the Unsteady Flow information line and continue with the exercise.)
Geometry Data
Let’s first examine the geometry data that was developed using the same methodology we used in the previous section. Select Edit/Geometric Data… from the main project window. You should have the Geometric Data editor appear:
Additional editing of Geometry Data is accomplished from this window. Click on the Cross Section button to see the tabular data for each cross section. Control structures along a stream can be manually entered using the corresponding buttons from this editor. We will not be using any control structures for this model. From the Cross-section menu, choose Plot to plot the Cross-section Use File/Copy to Clipboard to save your cross-section plot.
Select File/Save Geometry Data and then File/Exit Geometry Data Editor. You will be back to the main project window. We are now ready to input the flow data for the model.
To be turned in: Take
the Cross-section whose results you described previously and make a plot of the
cross-section. How wide is the cross-section in feet? What Manning’s “n” values have been used at this
cross-section? At what
distance (ft) from the left end of the cross-section are the left and right
banks?
The flow data has been extracted from a HEC-HMS hydrologic model of the Waller Creek Basin. The flow data is in cubic feet per second (cfs) and is derived from a 100-yr design storm for the Austin area. For reference only, the data is also provided for you in the MS spreadsheet, HMSResults.xls.
First, let’s open the Steady Flow Editor. Select the Steady Flow Data… command from the Edit menu. The Steady Flow Editor should look like the following:
There are three sections of Steady Flow Data that has been inputted using this window: Enter/Edit Number of Profiles, Flow Change Location/Profile Names and Flow Rates, and Reach Boundary Conditions.
Enter/Edit Number of Profiles
A “Profile” is a list of flow conditions at specified locations along the extent of the model’s reaches at a specified point in time. Each entered Profile will calculate a steady-state water surface elevation along the length of each reach. Open completion of running the model (which we will do in the next section) results can be observed from the View Profiles, View Cross Sections, or View 3D Multiple Cross Section Plot buttons provided in the HEC-RAS Main Menu window.
For this exercise, a total of ten profiles were entered. The profiles correspond with flow data extracted from the HMSResults.xls file at one-hour time intervals, at 0100 hours, 0200 hours, 0300 hours, and so on, for Decemeber 1, 1999.
Open Flow Data
The editor initially showed the upstream cross sections of each reach in the model: cross sections 3543.919 (upstream boundary of the Tributary), 7745.596 (upstream boundary of WallerUS), and 4378.570 (upstream boundary of WallerDS). In addition, the cross sections acting as outlets of watersheds for rainfall runoff were added as well. If you examine the spreadsheet, HMSResults.xls, you will find flow data for upstream boundaries as well as runoff data for specified cross sections along both Waller Creek and the Tributary. This data was used for this model. When adding the flow data to HEC-RAS, the values entered for a specified cross section is an accumulated flow. All flows upstream of a flow change location in the model must be added to the flow at that point. In other words, the model is obtaining a “snapshot” in time of the flow at locations along each reach to develop the water surface profiles.
Reach Boundary Conditions
The final step in the Steady Flow Data development is establishing the Reach Boundary Conditions. Click on the Reach Boundary Conditions button. The Steady Flow Boundary Conditions Editor is used to set the water surface elevation boundary conditions. Click in the “Known WS” cell in the Downstream column. Notice that the water surface elevation for cross section 14.035 on “WallerDS” is inputted for each profile, establishing the boundary conditions for each steady-state profile.
HEC-RAS Steady Flow Simulation
We are now ready to run a Steady-State Flow simulation. From the Simulate or Run menu, select Steady Flow Analysis. Under the File menu, select New Plan. Enter “Hypothetical flow conditions” as the Plan’s title and put into the next window which appears a 12-character short identifier, “Hypoflow”. Click OK.
Ensure the Flow Regime is set to Subcritical and press the COMPUTE button. This starts a FORTRAN program called SNET, which carries out all the calculations for the simulation. A DOS window with a message indicating the end of the process will appear when the computations are completed. Close the DOS window by clicking the Close button in the upper right corner.
You can see the results of the model using the 3D Multiple Cross Section
plot button. Click on the 
button. Under the Options menu, click on Reaches… Click on the Select All
button and then click OK.
Adjust the Rotation and Azimuth Angles to see the attributes of the
buildings in the cross sections.
Choose different profiles by clicking on Profiles.. under the Options
menu. This is what Profile #3
should look like:
Select File/Exit to return to the HEC-RAS main window.
To be turned in: A 3-D profile plot of the computed water surface elevation for Profile 3. (You can use File/ Clipboard to copy the plot in the X-Y-Z perspective plot window)
To export the simulation data into Arcview, select File/Export GIS Data… from the HEC-RAS main window. Select the export file name as "Waller1.gis" in the Export File box. Under the Export Options, check the Export Water Surfaces box. Click on the Select Profiles to Export button and select all 10 profiles. The ten profiles should automatically be listed as “Profiles to Export”:
Click the Export Data button on the new window. The data has been properly extracted
into a readable export file for use with GeoRAS. You may want to check your working directory to ensure it
has been saved properly. Saving
the project, you can now exit from HEC-RAS.
With the development of a GIS Import File from HEC-RAS, we can now begin the last portion of the exercise. Post-processing using GeoRAS incorporates the water surface profiles derived from the HEC-RAS model into the spatial environment of GIS. The water surface profile data is used to develop a water surface TIN, and the intersection of the water surface TIN with the terrain model TIN provides flood visualization. The results can be shown in 2-D or 3-D views.
Start ArcView and open the wallerfld.apr project you had saved during pre-processing. Select Theme Setup… command from the postRAS menu and fill in the necessary data for the window as shown below:
Select the GIS export file, waller1.gis, and the terrain model TIN, walterrain, from your working directory. Set the output directory for the post-processing files as “Postprocess”. This will also be the name of the post-processing View in Arcview. Set the Rasterization cell size to 5 (which, since the “Preprocess” View is in feet, the subsequent grid cell sizes for rater-based files will be 5 ft by 5 ft). Click OK to confirm your selections. The “Postprocess” View should automatically appear in Arcview.
The next step is to click on the Read RAS GIS Export File command under the postRAS menu. A window will inform you that the themes have been extracted successfully. Arcview will add the terrain TIN theme; banks, cross sections, and stream centerline polyline themes; and water surface profile polygon themes for the one profile created in HEC-RAS. The water surface profile polygon acts as a bounding polygon for the floodplain. Double-click on the walterrain TIN theme legend. From the TIN Legend Editor, turn off the Lines application. Click on the Edit button under the Faces section and change the Color Ramp to “Terrain Elevation #2”. Switch the color scheme as we did in the first section of the exercise. Click the Apply button and close both the Legend Editor and TIN Legend Editor windows. You may have to readjust the elevation values if you want to depict the same legend as used during pre-processing.
Take a moment and look at the attributes table for the Postprocess_XS.shp theme. Notice there are profile fields from “PF_1” to “PF_10” with numeric values in them. These values are the water surface elevations at each cross-section location. Each cross section polyline is assigned the corresponding water surface elevation value for each profile computed in HEC-RAS. The water surface elevation data, along with the water surface polygon themes for each profile, are used to develop a water surface elevation TIN. This is accomplished in the next step.
Click on the WS TIN Generation command under the postRAS menu. The following window will appear:
We are going to select the “PF_3” profile. Using the left mouse button, highlight the “PF_3” profile and click OK. Arcview will inform you, the “Water Surface TIN completed successfully”. Click OK. You may want to hide the legends for the water surface and terrain TIN by highlighting the TIN themes Wstinpf_3 and Walterrain, then clicking on the Hide/Show Legend command under the Theme menu.
Click on the water surface profile TIN theme, Wstinpf_3. Notice how the elevation shows a slope of the water surface from upstream to downstream. The TIN theme’s extent was developed from the water surface profile polygons, irrespective of the terrain TIN. Elevation level was interpolated from each cross section along the TIN. Highlight the Wstinpf_3 and use the Info tool to click on the TIN and find elevation values
To be turned in: What is the
range of water surface elevation values represented on the TIN?
This is where we will develop the flood plain for Profile #3. The process will take the water surface TIN (which we just created) and compare, or intersect it, with the terrain TIN. The floodplain is where the elevation values for the water surface TIN are either 1) equal to the terrain TIN’s, or 2) limited by the water surface TIN’s bounds. The output will be in vector and raster form. The raster form will have a grid cell size of 5-ft by 5-ft (which we assigned during the Theme Setup step).
Under the postRAS menu, click on the Floodplain delineation command. The created shape and grid themes will appear in the legend as Fppf_3.shp and Gdpf_3, respectively. Double click on the Gdpf_3 theme in the legend, set the Legend Type to “Graduated Color” and change the Color Ramps to “Blue Monochromatic”. Change the “No Data” value color to transparent by double-clicking on the “No Data” existing color, and changing the color in the Color Palette to the “X” value:
Check-mark only the Gdpf_3 grid theme and the Walterrain TIN in the legend. The “Postprocess” View should show the grid representation of Waller Creek’s water surface elevation for Profile #3. Zoom into the view to see a more detailed look at the flooding. The more you zoom into the view, the higher the resolution becomes, maybe even to the point that you can visually differentiate each 5-ft by 5-ft grid cell from the Gdpf_3 theme. You should be able to see how the terrain model’s intersection with the water surface creates flood inundation in the model. The advantage of the grid theme is to portray differences in water depth along the extent of the floodplain.
To be turned in: What is the
maximum water depth in the flood plain?
Where does this occur?
Editing the Floodplain Profile
Some faults and inaccuracies in the data can be seen at higher resolutions, once you Zoom into the view. In some cases, “pits”, or depressions, in terrain models that fall within the extent of the water surface profile’s bounding polygon may show inundation from the storm event. In other cases, some may realize that the cross sections (used to define the floodplain bounding polygon) developed during pre-processing may be limiting the overall floodplain. Whether these are cases or not, the modeler must continuously investigate and scrutinize. HEC GeoRAS uses a process that requires numerous iterations of pre- and post-processing to reach an optimum visual model.
To improve upon the immediate floodplain representation, without returning to pre-processing or to HEC-RAS, we can edit the floodplain shape theme, Fppf_3.shp. Unfortunately, we cannot do the same for the grid theme, Gdpf_3. This would have to be accomplished by modifying the terrain model to reduce the number of “pits” created.
For this exercise, there are a numerous “pits” of water, which actually
wouldn’t exist. We can make the
necessary corrections immediately to the shape theme, Fppf_3.shp. Make the Fppf_3.shp active, and
click on the Start Editing command under the Theme menu. We will use the 
(Select) button to select
unnecessary polygons, and delete them from the theme.
The first “pit” lies directly west of Texas Memorial Stadium. Select the polygon with the Select tool, and then press the Delete button. The polygon should be removed from the view. The remaining four polygons that need to be removed are near the confluence of the Tributary with Waller Creek. Pan up, then Zoom into that area:
Holding down the Shift key, select the four polygons shown above, then hit the Delete key. Click on the Stop Editing command under the Theme menu. Save the edits you made. Using the Color Palette, change the color of the Fppf_3.shp theme to a water-like blue color. Also change the Outline in the Color Palette to “None” as well. Click on the Apply button under the Legend Editor window. With only the Walterrain TIN theme and Fppf_3.shp check-marked in the legend of the “Postprocess” View, the floodplain visualization should look like the following:
The post-processing step of this exercise has now been completed. What you should to take from this section is the iterative process that is necessary to optimally use HEC GeoRAS. The post-processing provides floodplain visualization that can assist at identifying additional revisions necessary in the modeling. The modeler must weigh the results versus time availability, accuracy tolerance, and computer memory limitations to find the optimum solution.
To be turned in: A map of
the completed floodplain delineation
The final step is to analyze the flood data in three-dimensional space using the 3D Analyst extension. Under the View menu, click on the 3D Scene… command. Choose “Themes” in the 3D Scene window and click OK. In the 3D Scene Legend check mark the Walterrain and Wstinpf_3 TIN themes only.
Double click on the Wstinpf_3 theme. Remove the check mark from the Lines box in the TIN Legend Editor. Next, change the legend from “Elevation Range” to “Single Symbol” under the Faces section of the TIN Legend Editor. Click the Edit… button under the Faces section. Be patient, processing these changes in a 3D Scene may take longer than in previous views.
Double-click on the color in the Legend Editor. Change the color to blue in the Color Palette. Click the Apply button in the Legend Editor. Close both the Legend Editor and TIN Legend Editor windows. Lastly, choose the Properties command under the 3D Scene menu. Change the Vertical Exaggeration Factor from “None” to “3” and click OK.
Using the “Ship” button in the upper left corner of the 3D Scene View, you can zoom in and out, pan, and change the perspective of the 3D scene. By holding the left mouse button down and moving the “Ship” around in the 3D Scene, you can change the perspective. The zoom commands are accomplished by holding down the right mouse button and moving the “Ship” vertically up (zoom in) or down (zoom out) within the 3D Scene. To pan, hold both left and right mouse buttons down and move the “Ship” to the direction you want to pan in the 3D Scene.
You have just finished this exercise!
To be turned in: A 3-D map
of the floodplain created in ArcView
What you have just completed is a really complex study that normally takes months of effort. The exercise illustrates a fundamental goal of GIS in Water Resources, namely the combination of mapping tools to describe the environment and flow simulations models to describe how water moves through the environment.
Summary of Items
to be Turned In
What is the area covered by the TIN? How many triangles are there? What is the average area covered by a triangle? What is the range of elevation covered by the TIN?
Look at the attributes of Streamcl3d.shp. How many routing reaches are
there? What is the river
stationing on each reach (From_st, To_st). What is the total length of river which will be
modeled over the three reaches?
Look at the attributes of Xscutlines.shp. Choose a particular Cross-section. What Reach_Id and Station is it located
at? At what percent distance
across the cross-section are the left and right banks located? How far downstream is the next
downstream cross-section located?
Take the Cross-section whose results you described previously and make a
plot of the cross-section. How wide is the cross-section in feet? What Manning’s “n” values have been
used at this cross-section?
At what distance (ft) from the left end of the cross-section are the
left and right banks?
A 3-D profile plot of the computed water surface
elevation for Profile 3.
What is the range of water surface elevation values
represented on the TIN?
What is the maximum water depth in the flood plain? Where does this occur?
A map of the completed floodplain delineation
A 3-D map of the floodplain created in ArcView
These materials may be used for research and educational purposes
only.
Please credit the authors and the Center for Research in Water
Resources of The University of Texas at Austin.
All commercial rights
reserved. Copyright 2000 Center for Research in Water
Resources.