Plans, Computations, and Results

Objective - This tutorial will walk the user through evaluating hydraulic results from multiple plans.

Data Files

The data for this tutorial are provided below.

Simple_Plans.zip

This is also available in the in-UI downloader here:

These data are for demonstration purposes only and should not be used for engineering analysis.

Steps

Step 1: Open the HEC-RAS Project

1. Open the HEC-RAS project named "Simple_Plans.ras"
2. Using the Project Browser, inspect the project files.
   How many n Value layers are there?

   There are two different n value layers.  They are both geospatially distributed, however, one of the layers has channel information.  This was done be defining a channel polygon on the land cover layer.

   

   What is the difference between "1a. 400ft" and "1b. 400ft nCh"?

   The geometry with the "nCh" attached to the name uses an n value layer with Channel information.

3. Use the Map to look at the data some more.
   
   What are the primary n values used for the model?

   The primary land cover types are:

   • Evergreen Forest, n = 0.12
   • Emergent Wetlands, n = 0.07
   • Channel (open water), n = 0.04
   • Scrub, n = 0.11

Step 2: Computation Information

1. Click on the "1a. 400ft" Plan.
   
   What equation set was used for the simulation?

   From the Components pane, on the General Options tab, you can see the simulation was run with the Diffusion Wave Equation set.

   

2. Click on the "1a. 400ft" Result.
   
   How long was the simulation?

   From the Compute Log, you can see the Diffusion Wave Equation was used and the simulation took approximately 18 seconds.

   

Step 3: Hydrograph Plot

You are able to plot time series of data a individual locations.  Use this capability to evaluate several model results.

1. Plot the Time Series  
   1. Turn on the Results for "1a. 400ft" and "1b. 400ft nCh" 
   2. Right-click on the map and choose Query Result Time Series
   3. Swap the plot to Water Surface
      
      What is the difference in the results?

      The hydrograph plot shows that the plan with the channel n values is has a lower water surface.

      

      What happens when you click on a legend item?

      The time series that is clicked stays "lit up" and the other time series "fade".

      

2. Animate the inundation map.
   
   
   What happens when you animate the map?

   The mapping changes for each output time step and bobber on the timeseries plot moves with the animation of the time series.

3. Plot the Time Series in other locations
   
   Are your findings consistent?

   Yes, the water surface for the geometry using the channel n values is consistently lower.  This is because the n values is lower than the other n values used in the other geometry.  This demonstrates that specifying the channel n values for this event (and geometry cell size is important).

4. We see the importance of defining channel n values.  Turn on the "1c. 400ft nCh wCh" Result.
   
   How does this plan differ from 1a. and 1b.?

   The 1c. plan has the channel defined by the mesh.

   How do the results differ?

   The result with the channel defined in the mesh results in much lower water surface.  This significant difference highlights the importance of defining the channel in river modeling.

   

5. Turn on the "1c. 400ft nCh wCh" and "2a. 200ft wCh1" Results.
   What is the difference in the Plans?

   • 1c. 400ft nCh wCh - 400ft cell size with channel defined
   •  2a. 200ft wCh1 - 200ft cell size with channel defined as single cell
   What does the differences in Results indicate?

   The 200ft cell size has a higher water surface.  This indicates that the 400ft cell size was not properly capturing the ground surface elevations and more detail is probably warranted to have an accurate water surface.

   

Step 4: Profile Lines

The hydrograph plot allows you to evaluate results and point locations.  To evaluate results spatially, use the profile lines option either along the river course or transverse to flow (cross sections).

1. Turn on the "1c. 400ft nCh wCh" and "2a. 200ft wCh1" Results.
   1. Select the Profile Lines layer
   2. Right-click on the river profile line (travels down the channel portion of the model) and choose Plot | Results Profile
   3. Select the Water Surface plot option
      
   4. Evaluate the water surface
      1. Use the mouse wheel to zoom in/out of the plot to evaluate the water surface
      2. Use the left mouse button to pan
      3. Zoom in use the "extent" along each axis
         
         What can you learn from the Profile Plot?

         The water surface elevations are similar.  However, if you zoom in you will see "significant" differences in the water surface.  The differences are most notable where the terrain is steeper and more varied.

   5. Use the Animation toolbar to evaluate multiple water surface profiles
      Note how the Profile Plot automatically updates
   6. Move the bobber along the profile.
      Note in the Map the red dot indicating the location of the bobber
      
2. Create Profile Lines to evaluate the water surface across the floodplain
   1. Select the Profile Lines layer
   2. Create a transverse profile line
   3. Right-click on the transverse profile line and choose Plot | Results Profile
      
   4. Right-click on the transverse profile line and choose Plot | Flow Time Series
      
      
      
3. Turn on the "2a. 200ft wCh1" and"2b. 200ft wCh3" Results
   
   What is the difference in Plans?

   The "wCh1" has one cell in the channel while the "wCh3" has three cells across the channel.

4. Use the Plotting tools to evaluate the results
   
   What are the differences in the Results?

   In general, the results are very similar, indicating that for water surface profile accuracy, one cell may be appropriate given the meshing and these hydraulic conditions for evaluating the water surface profile. 

5. Swap to the Velocity
   1. Click on the Results group
   2. Click on the Layer tab
   3. Select Velocity for the Map Type
      
      
      

      This will sync all mapped results to the Profile identified by the Keyframe Result.

   4. Plot the Velocity profile
      What do you see with the velocities?

      The model with 3 cells across the channel has higher velocities - the single cell "averages" out the velocity.
      
      
      
6. Turn on the "2a. 200ft wCh1" and "2b. 200ft wCh3" Results and the "3a. 100ft wCh1" and"3b. 100ft wCh3" Results
   
   What can you learn from these Results?

   The water surface for the smaller cell size results in a slightly higher water surface.  The differences are small, however, this indicates that final model calibration should be performed once with the resolved mesh.
   

   There is a larger discussion here - Manning's n values are used to compute friction losses, but account for other losses.  When developed for 1D modeling, other losses were lumped into this single value, however, with 2D modeling we are able to account for some of the additional losses through terms in the shallow water equation.

Step 5: Export Raster

1. Turn on the "2b. 200ft wCh3" Result (turn off all others)
2. Use the Animation bar to pick a water surface profile
3. Right-click on the "2b. 200ft wCh3" Result and choose Export Raster
4. Provide an output Name
   
5. Click the Export button
   When finished Window Explorer will open with the generated TIF.
6. Change the Symbology to use the Depths color ramp
7. Drag/Drop the TIF into the HEC-RAS map window
   What is the difference between the output Raster and Depth Map?

   The output floodmap shows discrete pixels where wet. The Depth Map in RAS shows a smoothed boundary due to the rendering algorithm.
   

Step 6: Export Contours

1. Turn on the "2b. 200ft wCh3" Result (turn off all others)
2. Use the Animation bar to pick a water surface profile
3. Right-click on the "2b. 200ft wCh3" Result and choose Export Contour
   1. Enter a Cell Size
   2. Enter a Simply Distance
      -- Change parameters and preview the result --
   3. Provide an output shapefile Name
      
      
4. Press the Export button to generate the flood polygon boundary.
5. Drag/drop the newly created shapefile into the HEC-RAS map
6. Investigate the polygon to see how it matches the Depth map.
7. Use the Edit tools to make a few modifications to the polygon(s)