Tuolumne Meadows floodplain modeling using HEC-GeoRAS (Part 2)

After a long break, today we will see how to import the preprocessing data generated in part 1 of this tutorial into the HEC-RAS hydraulic model.

HEC-RAS is a program that models the hydraulics of water flow through natural rivers and other channels. It is one-dimensional, meaning that there is no direct modeling of the hydraulic effect of cross section shape changes, bends, and other two- and three-dimensional aspects of flow.

Launch the HEC-RAS program (Start > Programs > HEC > HEC-RAS > HEC-RAS 4.1.0). The latest version of HEC-RAS can be downloaded for free from the Hydraulic Engineering Center of the U.S. Army Corps of Engineers website: http://www.hec.usace.army.mil/software/hec-ras/hecras-download.html

Save the new project. Go to File > Save Project As… Save it as “TuolumneMeadows.prj”

To import the GIS data into HEC-RAS, first go to the geometric data editor (Edit > Geometric Data…)

Next, click on File > Import Geometry Data > GIS Format. Browse to “GIS2RAS.RASImport.sdf” file previously created in HEC-GeoRAS, and click OK. An import wizard will appear. In the first window (Intro), confirm SI (metric) units.

In the second window (River Reach Stream Lines), make sure all import stream lines boxes are checked, and click Next. Finally, in the last window (Cross Sections and IB Nodes), make sure all Import Data boxes are checked for cross-sections and click OK.

Click on Finished-Import Data. Data will be imported into the HEC-RAS geometry editor.

Save the geometry file as “TuolumneGeometry” (File > Save Geometry Data). Note that the geometry data has a .g01 extension.

To check the quality of the imported data, we can use the graphical cross-section editor (Tools > Graphical Cross-section Edit) in the geometric data editor.

We can use the editor to move bank stations, change the distribution of Manning’s n, add or delete ground points, etc.

The next step is to edit the structures data (bridges). Click on “Bridge/Culvert” editing button. In our case, we have two bridges at river stations RS=5360.882 and RS=4123.291.

Pacific Crest trail bridge (RS=4123.291)

We see that, in both cases, the deck elevation is not accurate. So we need to edit this information. Click on Deck/Roadway editor, and delete all information. Enter the data for the deck hight and low chords height as shown in the next picture:

Click on the “Copy US to DS” button to copy the information from upstream to downstream, and click OK. The result should look something like this:

Now, we will enter the pier information. The first bridge (RS=5360.882) has three piers and the second (RS=4123.291) has only one. Click on the “Bridge Design” button and enter the information as shown below:

Close the bridge/culvert editor. Save the geometry data and close the geometric editor.

The next step is entering Flow Data and Boundary Conditions. In HEC RAS every flow to be simulated is called a profile. In our case, we are going to define one profile that represents the peak discharge at the most upstream location of the Tuolumne River for a 100 year return period.

100-year flows were assessed from a frequency analysis of the nearest gauging station data (1127479 Tuolumne River at Grand Canyon of Tuolumne), with the maximum discharge being 200 cubic meters per second (7060 cubic feet per second). Enter 1 for number of profiles, and click on “Apply Data”.

We now define flow conditions upstream for steady flow (Edit > Steady Flow Data). To define downstream conditions, click on “Reach Boundary Conditions”, select “Downstream” for Toulumne River, click on Normal Depth, and enter 0.001.

The basic computational procedure of HEC-RAS for steady flow is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction and contraction – expansion coeficients.

To learn more about boundary conditions in HEC-RAS, check chapter 3 of the Hydraulic Reference Manual.

Click OK. Save the flow data as “TuolumneMeadows.f01”. Now we are ready to run the model.

In the HEC-RAS main window, click on Run > Steady Flow Analysis… Click on File > New Plan. Name it “TuolumneMeadows.p01” and “100year” for the short ID. Select the Subcritical Flow Regime, and click on “Compute”.

We have to take into account that the maximum number of station-elevation points is 500. The number of points can be reduced with the XS Points Filter Tool (geometric data window > Tools > Cross section point filter…).

Please note that this is a very simple example for educational purposes only. An actual hydraulic simulation requires the verification of results, identification of possible errors in the data and, if necessary, repetition of calculations. Assessment of the quality of the simulation comes with the knowledge of the study area and experience.

After the computation is finished, we will export the results to ArcGIS to delineate the inundation extent. To export the data to ArcGIS click on File > Export GIS Data… in the HEC-RAS main window. Leave the rest of the parameters as default.

Click on “Export Data” button. A “TuolumneMeadows.RASexport.sdf” file will be created in our working directory. Save the HEC-RAS project and exit. Now we will now return to HEC-GeoRAS to delineate the extent of the flooding.

Open ArcMap and load the “Tuolumne.mxd” file previously created. In the HEC-GeoRAS toolbar, click on “Import RAS SDF file” button to convert the SDF file into an XML file. Browse to “TuolumneMeadows.RASexport.sdf” and click OK.

Next, click on RAS Mapping > Layer Setup. In the Layer Setup window, select “New Analysis”  and name it as “Tuolumne Steady”. Browse to TuolumneMeadows.RASexport.xml for RAS GIS Export File. Select “Single” for Terrain Type, and browse to “tm_dem_flood” GRID. HEC-GeoRAS will create a geodatabase with the analysis name (Tuolumne Steady) in the output directory.

A new data frame named “Tuolumne Steady” will be added to ArcMap.

Click on RAS Mapping > Import RAS Data. This will create a bounding polygon by connecting the endpoints of the XS Cut Lines.

Click on RAS Mapping > Inundation Mapping > Water Surface Generation. Select PF1 as a profile and click OK. A TIN named “t PF 1” is created. This TIN define a zone that will connect the outer points of the bounding polygon.

And now the final step. Click on RAS Mapping > Inundation Mapping > Floodplain Delineation Using Rasters. Select PF1 (100-year flow), and click OK. At this stage the water surface TIN (t PF 1) is first converted to a GRID, and then the terrain surface (tm_dem_flood) is subtracted from the water surface grid. The area with positive results (meaning water surface is higher than the terrain) is flood area, and the area with negative results is dry.

The final result is a polygon (b PF1) which is the final flood inundation polygon, and a GRID (d PF1) that represents the final depth water map. We can see how the maximum water depth is 3.68 meters along the Tuolumne River main channel.

Ok, we are now done with delineating the 100-year flow floodplain in the Tuolumne Meadows area of Yosemite National Park.

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About Sergio Perez
An environmental scientist passionate for the world of Geographical Information Systems and its application to the field of hydrological management.

One Response to Tuolumne Meadows floodplain modeling using HEC-GeoRAS (Part 2)

  1. thanks. i understand all the process. but i have 1 question to ask; let say, i apply hidraulic data from “department of drainage and irrigation”, then they provide me the data, i will open the data using Hec-Ras to convert into arcgis format (shapefile/.shp), but what about the projection of the data? how to make the hidraulic data will be the same projection as mine in in arcgis? can you show me the tutorial?-please.thank you..

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