HEC-RAS Model with 2D Mesh Only.

Written by Christopher Goodell, P.E., D.WRE
Copyright © The RAS Solution 2015.  All rights reserved.

There have been many questions lately about whether HEC-RAS 5.0 can perform a simulation with ONLY a 2D mesh (i.e. no cross sections).  The answer is yes.  In fact, in many ways it’s a lot easier.  In this example, the standard Muncie project offered by the Hydrologic Engineering Center has been converted first to SI units, then to a single 2D mesh.  To keep things simple, the levee breach was removed and flow moving into the overbank areas is purely due to overtopping of the levee.  The following figure presents the geometry file for Muncie when modeled with a single 2D area.   




Once you’ve drawn your 2D area boundary and have assigned a mesh cell center spacing (DX and DY) and Manning’s roughness value(s), really all that is left is to
assign boundary conditions at the upstream and downstream ends using 2D Area BC Lines.  These were placed at the same location as the upstream and downstream cross sections from the original model. 
The easiest way to set initial conditions is to just apply a single starting water surface elevation for the entire domain, [or start the 2D area out dry].  This will not always work, especially if there is significant changes in water surface elevation throughout the 2D area.  But in this case it worked, with a little help from the Initial Conditions Ramp-Up option in the Unsteady Computation Options and Tolerances window.  I used a ramp-up time of 4 hours to set the model up with a more appropriate initial conditions solution.



In the 2D mesh, you’ll also notice I’ve added some refinement to the initial 40m x 40m cell center spacing to better capture the geometry along the levee and to minimize inundation fragmentation. 
Notice in this figure I’ve oriented some additional cell centers to try to align the cell faces with the high ground just to the southwest of the main channel.  This helps to prevent “leaking” through the high ground, which occurs when a cell is large enough to straddle the high ground feature. [orientation of cell faces along high ground features can now be done in the latest version of HEC-RAS quite easily with breaklines]



As shown in this figure, I added some resolution and strategic cell center orientation to minimize inundation fragmentation.  A lot more of this could be done with this data set to completely eliminate fragmentation. 



Over 2 days of simulation time with a 10 second computation interval and 40 m x 40 m initial cell size ran rather quickly at around 2.5 minutes on my computer. 



This sample data set is available from The RAS Solution Google Drive site at:
https://drive.google.com/file/d/0B0bpiyLiUeRXMHhZVU9ibm5faUk/view?usp=sharing

I didn't include the output files, so make sure to run the simulation first.

Please note that this data set is completely fictional and represents no real flood event-past, present, or future.  This data set was originally developed by the Hydrologic Engineering Center and modified by me.  It is meant strictly for demonstration purposes.  Enjoy!