Overview

Federal levee systems are part of the Annual Flood Damages Reduced report prepared by USACE every year. A without-levee HEC-RAS geometry (and HEC-RAS simulation) is often needed to calculate the damages reduced by the levee system as part of the CWMS FDR forecast. HEC-RAS needs to be set up to run a simulation with levees in place and another without levees in place. This tutorial will walk through how to set up the without levee geometry for use in the without levee Flood Damages Reduced simulations. More information on incorporating the without levee HEC-RAS alternative into HEC-FIA and CWMS is available here.

Step 1. Determine how many without levee geometries are needed for the watershed.

The first step is to determine how many without levee geometries are needed for the watershed. Review the Guide to Computing Annual Flood Damages Reduced using CWMS to determine how many geometries will be needed for the without levee scenarios.  The projects in the watershed will dictate how many geometries will be needed to compute FDR without levees.  Watersheds with headwater reservoirs only and levees will need 2 geometries and watersheds that have reservoirs in tandem in the same HEC-RAS model and levees could need up to 4 geometries depending on the setup in the model.

Step 2. Gather data on the levee systems in the watershed. 

Information about the levees in the watershed primarily come from, design documents or as-builts, if available, and the National Levee Database (NLD).  The NLD website can be found at the location below.  (Note you may need to contact the help desk to request an account if accessing for the first time.)

https://levees.sec.usace.army.mil/#/ 

To estimate the without levee scenario, the levee needs to mimic the pre-construction condition.  This can be done by revising the levee or storage area connection to the elevation of that pre-construction condition. Depending on data available for the levee, levee elevation data can be adjusted for the lateral structures or storage area connections primarily in two ways:

  1. Using known pre-construction natural ground elevations from design or as-built documents.
  2. Removing the levees from the terrain and using that as the natural ground.

It is recommended to use the second option, removing levees from the terrain, for flood damages reduced computations.  The next section describes one way to remove levees from the terrain file.   

Step 3. Downloading Levee Data for the Watershed

Depending on the quality of the terrain data the levees will have varying levels of definition, ie a 10m DEM might not capture the levee crest while other levees may have very detailed 1 ft Lidar. 

Levee data can be brought into HEC-RAS two ways.

  1. There is an import tool in HEC-RAS Mapper for version 6.5 that will bring in shapefiles with all the levee information as a shapefile (second figure below).
  2. Download the shapefile with all the data directly for the NLD website (link provided above)

The HEC-RAS Mapper User’s Manual has thorough instructions on how to download the NLD data using HEC-RAS Mapper.  This can be found at Using the NLD for Terrain Modification.

If using HEC-RAS Mapper, make sure to have the projection set prior to download as shown in the figure below.

Recommendation:  USGS terrain data can also be downloaded using RAS Mapper.  If the CWMS model was completed years ago with 10m DEMs it may be worth it to download newer Lidar data if available.  There is much more Lidar available now than when many CWMS models were being completed.  For more information on how to import USGS Terrain or Grid data see the HEC-RAS Mapper User’s Manual currently  at Terrain Layer.  Upgrading the terrain may cause some issues as cross sections are updated and may impact the model calibration. Another option is to have it on hand for mapping only and incorporate into the HEC-RAS geometry when time allows.

Step 4. Removing Levees from the Terrain Data used for the Without Levee Geometry

Start by creating a clone of the terrain with levees in HEC-RAS Mapper as shown in the figure below.

In the Maps Layer add a reference layer for the levees to help define the footprint for the modification (seen in the figure below). Right click on the Maps Layer, hover over the Reference Layers, and select Add Reference Layer. Use the levee centerline either downloaded from the NLD or with HEC-RAS 6.5.

If adding the levee centerline from HEC-RAS Mapper download (NLD), add the RASMergedAlignments.shp file (seen in the figure below).  This has all the relevant information about the levee itself.  This file can also be used to add levees directly to the terrain for the levee scenarios, or to update the terrain file with the NLD elevations. 

Next, add a new multipoint polygon modification layer to the cloned terrain (as seen in the figure below).  This will be used to outline the base (toe to toe) and remove the levee in the terrain.  Right click on the Cloned Terrain file that was created, hover over Add New Modification Layer, then Polygons, and select Multipoint.  Name the new modification layer something that describes what is being done.  “NoLevee” or “WithoutLevee” are good options.

To get a better idea of the base of the levee, change the properties of the terrain file being modified. 

Right click on the terrain and select Image Display Properties (as seen in the figure below).  Check the box that says Update Legend with View. This will change the shading based on the extents on the screen, making the levees much more visible in refined terrain files.

Start editing the NoLevee modification layer that was created in the previous steps (seen in the figure below).  Right click on the layer and select Edit Layer.

Create a polygon that encompasses the entire levee, this is basically a toe-to-toe estimate of the base of the levee.  Name the polygon, the levee name would be a good option.  Select Lower (Terrain/User) Value for the Modification Method and Use Terrain Elevations or Control Points Override for the Perimeter Profile Mode (seen in the figure below).  This is interpolating the lower elevation for the levee based on the terrain and the bounding polygon that was created.  A comparison of the original terrain file and the modified terrain is shown below.


It is a good practice to look at the modified area to make sure the lowered elevations make sense and will be good to use as the new natural ground elevations for the HEC-RAS geometry.  This can be done by either drawing polyline features at locations of interest along the levee or using the measure tool to check various points on the fly.  The image below shows the comparison of the terrain with the levee and the cloned terrain where the levee was removed.  The example shown is done using a polyline feature.

Step 5. Creating the Without Levee Geometry

Take the HEC-RAS geometry that is used for the regulated simulations and save as a new geometry.  Include somewhere in the new name “NoLevee” or some other sort of identifier that indicates this is the without levee geometry (seen in the below figure).  There should now be at least two geometries (4 if you have a watershed that has reservoirs in tandem) for the Flood Damages Reduced scenarios. 

Step 6. Assigning the Revised Terrain File to the New Without Levee Geometry

Open HEC-RAS Mapper and assign the no levee terrain to the without levee geometry.  This is done by right clicking on the Geometries layer and selecting Manage Geometry Associations (as shown in the figure below).  Select the clone (Without Levee) terrain from the terrain dropdown for the no levee geometry that was created in the last step.

Step 7. Adjust Levee Elevations to Represent Pre-construction Estimates in the Without Levee Geometry

If using the without levee terrain elevations for the pre-construction levee, simply extract the elevations from the terrain file to the lateral structures or storage area connections in the no levee geometry (Option 1). Since the levees have been removed from the terrain all that needs to be done is to cut the new structure elevation data from terrain. 

If the decision from the district is to use natural ground elevations from documents with pre-construction ground profiles to revise the lateral structure or storage area connection, this should be done in the structure editor in the geometry (Option 2).

Option 1. To use the without levee terrain file as the pre-construction natural ground, open HEC RAS Mapper and right click on Lateral Structures for the without levee geometry, and select Edit Geometry (as shown in the figure below). Update the Elevation Profiles from Terrain for each of the levees. This can be done individually or select a group of structures to update at once.

Option 2 - To update the lateral structure or storage area connection using known pre-construction natural ground, simply edit the lateral structure and replace the Weir/Embankment profile with the known station and elevation points for that structure. Right click on the Lateral Structure or Storage Area Connection, and select Edit Lateral Structure or Edit Connection. The editor for the structures is very similar so the same steps will be taken for either the Lateral Structure or Storage Area Connection. The figures below are for a lateral structure example.

Caution, when using known data with the estimated without levee terrain with a 2D area there may be additional edits that either need to be done to the no levee terrain, 2D area or the pre-construction elevations to get the model running since they might have differences, as seen in the figure below.


Step 8. Reference AFDR SOP

Reference the Guide to Computing Annual Flood Damages Reduced using CWMS to complete the rest of the setup in RAS and FIA for the no levee computations.