How to delineate flood limits in Civil 3D using HEC-RAS 5

With the release of HEC-RAS 5, we now have between Civil 3D and HEC-RAS all the tools we need to send our terrain and stream geometry from Civil 3D to HEC-RAS, delineate flood limits in HEC-RAS, and send the inundation limits back to Civil 3D.

Watch the 10-minute video

The quick outline of the process for a simple stream is like this:

1. Prepare terrain, stream centerline, and cross sections in Civil 3D.
2. Send to HEC-RAS
3. Add roughness coefficients, banks, flow, and boundary conditions (elevations).
4. Compute water surface profile in HEC-RAS
5. Map inundation limits in the RAS Mapper feature of HEC-RAS
6. Import to Civil 3D

Don't hesitate to ask for additional info if you are stuck.  We can do this!

Stream Geometry

See this post to get stream geometry from Civil 3D to HEC-RAS.

Terrain

In Civil 3D, create a Geotiff Civil 3D Terrain Export file of the cross section surface.

1. Right-click the surface in Toolspace, Prospector
2. Choose Export to DEM.... 
3. Specify the file.  Be sure to choose GEOTIFF instead of DEM file type.
4. Make sure a geographic projection is set right.  Talk to a GIS person or comment below if you need help with this.  (If nothing else, do this: Under Toolspace, Settings, right-click your drawing name and select Edit Drawing Settings, Units and Zone, Available coordinate systems, and pick the system that seems most appropriate to you.  Then use the GEOMAP command (you will have to sign in to Autodesk A360) to bring in a map at world coordinates and move your project into approximate place on that map.)

Export to DEM from Civil 3D.  Make sure a projection is set right.

In HEC-RAS, go to GIS Tools, RAS Mapper.  Start with a blank new Mapper.
Do not set a projection.
Tools, New Terrain...

Answer No to skip setting a projection.  Civil 3D terrain will not add with projection already set.

Click the + to add the Civil 3D Terrain Export file.  Accept to use the projection in the file.  Click Create. Bug in HEC-RAS 6.2: When you get a creation error in proj_get_ellipsoid, use Create New Terrain again, but add the tif file HEC-RAS created on your first attempt (probably in the Terrain sub-folder).

Calculate Water Surface

Use HEC-RAS Geometric Data, Tables menu to set Manning Roughness for all cross sections.
Use HEC-RAS Steady Flow Data to add flow and reach boundary elevation conditions.  Use critical depth for a quick and dirty start.  But normal depth is usually better if you don't have known water surface elevations at the ends of your stream.
Use HEC-RAS Steady Flow Analysis, Compute button to calculate the water surface.

HEC-RAS basic analysis buttons

Inundation Limits

Show inundation limits in RAS Mapper

Make sure you have saved your Plan under the Steady Flow Analysis File menu.
In HEC-RAS RAS Mapper, go to Tools, Manage Results Maps.
Click Add New Map button.
Choose Inundation Boundary under Map Type.  Click Add Map button.
Highlight Inundation Boundary line in Manage Results Maps. Click Compute/Update Stored Maps.  This creates the inundation boundary including the shape files you need in Civil 3D, located somewhere under your HEC-RAS project folder in a group named Inundation*.*

Import Inundation limits into Civil 3D

MAPIMPORT command.
Check the box to change regions to polylines.
Import the inundation shp file. 

Smooth inundation limits

Use PEDIT Spline to smooth the plines or...
Use WeedFeatures multiple (around 10) times with only Close Point Removal, 3D distance checked and set to 5 to 10 times the original export DEM grid spacing.

And finally a conscientious plea: Go hug and thank a family member and then move your money from a bank to a credit union.