2D Mesh “Leaking”

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

To follow up my post on fragmented inundation, I want to highlight another 2D mesh issue we should all be aware of.  Unlike fragmented inundation, which is an artifact of how HEC-RAS discretizes the 2D domain and the way it maps the results, Leaking is a result of terrain features not aligning with cell faces and/or cells that are too large, and can produce very wrong results. 
Take the following example of leaking.



Here we see a high ground feature that is straddled by rather large cell.  HEC-RAS will preserve the underlying terrain on the cell faces, but the cell itself is resolved to a volume-elevation curve.  Since the high ground feature runs diagonally through the cell, it is not picked up by the cell faces.  As a result, HEC-RAS does not know that there is a barrier that should keep water on one side of the high ground feature before it is overtopped.  The consequence is that water leaks through the high ground and is available to move further down the channel even before the high ground is overtopped.  This is incorrect.  To better capture the high ground feature, cell faces in this vicinity should be aligned to the high ground feature so that the terrain is picked up on the cell faces, which will prevent leakage.  The following figure shows the resulting flood map at the same time in the simulation as the figure above. 



Here we see the entire mesh has been overall refined to a smaller cell center spacing, but in addition, much more resolution was added by manually straddling cell centers around the high ground feature.  Notice around the crest of the high ground feature, the cell centers were placed to align the cell faces with the contours.  This ensures that the high ground is picked up by the cell faces.  The result is a higher resolution flood map, but also, and more importantly prevents leakage through the high ground before it is overtopped.  Also important is to provide much smaller cells on the downstream slope of the high ground feature, to prevent fragmented inundation.  In hindsight, I probably went a little overkill on adding cell centers, but it didn’t really add any noticeable time to the simulation, so I’m good with it. 
Manually adding cell centers is not particularly precise, and can take a bit of time.  Fortunately HEC will be including a new feature in the full release of 5.0 that allows the user to define a breakline along high ground terrain features like this.  The mesh is then generated, automatically aligning the cell faces to that breakline.