Objective

If you have never created a basic clear-water model in HEC-RAS it is useful to build one before you jump into mud and debris flow.  In this workshop, you will learn how to create a simple 2D model with HEC-RAS of the main channel and floodplain area.  This workshop will require you to use multiple terrain datasets to form one terrain model, develop a 2D mesh, provide inflow hydrograph information, downstream boundary conditions, and perform model runs.

1     Background

You will be working on Bald Eagle Creek near Lock Haven, PA.  Flow hydrographs will be used to simulate flows from Joseph Sayer’s Dam downstream to the town of Lock Haven and the confluence with the West Brach Susquehanna River.  Inflows will also be specified for local tributaries on Marsh Creek, Beech Creek, and Fishing Creek.  A Normal Depth Boundary Condition will be used.


                                               




                 

                                                                               

2     Terrain Model Preparation

This part of the workshop will guide you through the process of importing terrain data.  The terrain data will be used as the basis for the mesh used for 2D hydraulic computations.

  1. Start HEC-RAS.
  2. Start a NEW project using File | New Project… Go to the workshop directory for this workshop (“Creating a Simple 2D Model”) and then providing a Title and File Name.  Press OK to save it.
  3. Launch RAS Mapper .
  4. Select Project | Set Projection. Select the Folder button at the top left and navigate to the “prj” provided in the “GIS_Data” folder.  Select and Open this file.  This sets the coordinate system for all the data you will view in RAS.  The press the OK button to close the window.
  5. Import terrain data for use in RAS by selecting the Project | Create New RAS Terrain menu item.
  6. Click the Add Files button and navigate to the “Terrain” folder.  Select the “tif” and “DEM.tif” files, then press the Open button at the bottom.  The “BEC_10ft.tif” file contains gridded data at 10ft postings for the channel created from a TIN and the “DEM.tif” is a DEM (lower resolution) downloaded from the USGS for the remainder of the floodplain. 
  7. Use the New Terrain Layer dialog to order the files for import. Note that the “most important” layer should be on the top of the list and will get the highest priority when creating the Terrain Layer.
  8. Press the Create
    As the Terrain is created, a computation window will inform you of progress.


  9. When the terrain process is finished, select Close on the processing window, this will close both windows. Turn on the Terrain Layer.
  10. Right-click on the Terrain Layer and choose Zoom to Layer (if necessary to see the terrain).

Below is what the terrain should look like.  Also shown are all the boundary condition locations for this workshop.


 

Note the location of Sayer’s Dam (labeled Inflow) and tributaries, shown in the background image above.  You will be creating a 2D Model below the Dam with tributary inflows.


To get a sense of the region we are modeling, Right click on Map Layers then choose Reference LayersàAdd Reference Layer.  Then navigate to the GIS folder and select bec_boundary.shp.



3     Create the 2D Flow Area

The 2D Flow Area will go from just downstream of the Dam to the confluence (labeled Downstream Boundary) with West Branch Susquehanna River and will include a portion of Marsh Creek, Beech Creek, and Fishing Creek tributaries.

  1. Right-click on the Geometries group and choose Add New Geometry.
  2. Enter “Simple Geometry” for the name.  Press OK to create the geometry layer.
  1. A dialogue will open asking you to associate the geometry with a terrain, but since you only have one terrain, it will automatically choose the terrain you just made.  You can close out of that dialogue.
  2. Right-click the new Geometry Layer and select Edit Geometry.
  1. Select the 2D Flow Areas Perimeter layer. 
  1. Draw the perimeter using the Add New Feature  tool.


Question:  What things should you consider when creating/defining the 2D Flow Area polygon?


  1. Enter a name, when finished.
  2. In the Edit 2D Area Properties that come up, Enter 500 for the DX and DY Point Spacing.
  1. Change the Default Manning’s n Value to 04.
  1. Click on the Generate Computation Points button.
  2. Close the Editor.



  1. Turn on the Computation Points.
  1. Inspect the edge of the 2D Flow Area Mesh for any mistakes in the boundary.  Make sure each cell has only one point in it. Turn off all background layers to more easily inspect the mesh. (Mesh problems should show up with cell points colored red.)
    To fix errors in the mesh, use the Editing Tools to Add, Delete, or Move points.

  2. Stop Editing to Save the Geometry!  Right Click on the Geometry and select Stop Editing or press the Stop button at the top of the map.

4     Flow Data Connections

After creating the 2D Flow Area computation mesh, add all the locations for flow boundary conditions.

4.1          RAS Mapper

  1. Start Editing in RAS Mapper.  (right click or press the pencil )
  2. Select the BC Lines layer.
  1. Create boundary conditions, left to right when looking downstream at each of the boundary locations shown below.  Double click when you are done with each.  Enter a name for each (Inflow, Beech Creek, Marsh Creek, Fishing Creek and Downstream) using the map below.

***WARNING:  Boundary Conditions must be drawn outside of the perimeter of the mesh!***

  1. Stop Editing and Save (Right click on geometry or press stop sign ).

4.2          Geometric Editor

  1. Close RAS Mapper.
  2. Return to the Main HEC-RAS Editor and open the Geometry Editor.  Select the Geometry button  or the EditàGeometric Data menu.

  

  1. Open the Geometric Schematic and Open the new Geometry (“Simple Geometry”).  Select the FileàOpen Geometry Data menu and choose the geometry file you just made in RASMapper.


  1. Verify the Geometry came in correctly.
  1. Close the Geometric Editor.

4.3          Unsteady Flow Editor

Open the Unsteady Flow Data editor and enter boundary condition information.

  1. Select the Unsteady Flow Editor from the main HEC-RAS dialogue by pressing the unsteady flow button  or selecting EditàUnsteady Flow Data. 

 

  1. Start with the Downstream Boundary Condition.  Press Select the empty field next to the Downstream boundary condition.  Select the Normal Depth option.



  1. Use Normal Depth with Slope = 0.001 for the downstream boundary condition.
  1. The other boundary conditions are all flow boundary conditions.  We have flows stored in a DSS file (e.g. this is how HEC-HMS results are stored).  You will use the “Simple2DModel_Flows.dss” file to define the Flow Hydrograph for the upstream boundary and tributaries.  Start with the Inflow Boundary.
  2. Click on the Inflow Boundary and press the Flow Hydrograph button.
  1. To import flow from DSS you will ignore most of the flow editor, and just focus on the top portion.
  2. Select the Read from DSS before simulation option and press the Select DSS File and Path button.
  1. This will launch the DSS editor. You will need to click the Add DSS File  button to add the DSS file (Simple2DModel_Flows.dss in the Flow Data folder) prior to picking a path.

Note: The hydrograph for the inflow is named “Sayers Dam Outflow”.  The other DSS records are named the same as the boundary condition.


  1. 2D boundary conditions need a slope to distribute flow.  Specify the “EG Slope for distributing flow along BC line” as 0.001. (This is located at the bottom right on the Flow Hydrograph window.)  This must be done for each flow hydrographs attached to the 2D Flow Area.
  2. Repeat previous 2 steps for all inflow locations.
  3. Save the Unsteady Flow data when complete (call it “Flows”).


 

5     Plan and Simulation

  1. Open the Unsteady Flow Analysis window, by pressing the Unsteady Plan button  or selecting the RunàUnsteady Flow Analysis.
  1. Save as Plan, by selecting FileàSave As and enter all of the necessary information to make a run.
  2. Note: You have to give HEC-RAS plans two names.  A full name, and a shortID that shows up on the plots. 
  1.  Set up the time window:
    Start Date: 02JAN1999      Start Time: 0000
    End Date: 06JAN1999        End Time: 0000
  2. Turn on the Geometry Preprocessor and Unsteady Flow Simulation programs.
  3. Specify a Time step.

    Question:  What is an appropriate time step to use for the simulation?

How would you figure it out?



  1. Save a plan by the name of the grid cell size and time step selected (“Initial Run”).
  2.  Run the simulation (press the Compute button).


Question:  What else SHOULD we have done before simulating?


  1. Go to the Options | Calculation Options and Tolerances in Plan editor and set the 2D Flow Option for the Initial Conditions Time (and Ramp Up Fraction).



Question:  What is an appropriate Initial Conditions Time to use?  What happens if you don’t use an Initial Conditions Time?  How else could you set the initial conditions?

  1. What hydrodynamic equation are we using? Do you think that is appropriate? Change the Equation Set to SWE-ELM (original/faster).
  2. Press Compute and simulate again.

 

 

Visualize Results

  1. Open RASMapper Again and Expand the Results portion of the tree. Turn on the Depth and Velocity Results in turn and experiment with animating them and right clicking on results to get time series.