Post-Processing: Creating Inundation Maps with very Large Terrain

Written by Mikell Warms  |  WEST Consultants 
Copyright © The RAS Solution 2016.  All rights reserved.   

We often have to deal with very large models covering hundreds of river miles.  The terrain data associated with these models can be equally massive, which can cause memory/processor issues when trying to delineate inundation maps.

This post is not concerned with pre-processing our model geometry. Instead, we have a completed model with results we are happy with, and now we want to make inundation shapefiles. This is often necessary when completing dam breach analyses/flood analyses/etc. 

There are typically two ways in which flood inundation shapefiles are created: (1) using HEC-GeoRAS and (2) RAS Mapper. Both methods can face uphill battles when terrain data begin to exceed 20-30 gigabytes, depending on performance characteristics of your computer. Below are some typical errors you may see when trying to create inundation shapefiles with RAS Mapper on very large terrain.

Figure 1: RAS Mapper Memory Error Messages

There IS a way to get around this using RAS Mapper. The first thing you’ll need to do is

Is it normal with a 2D full momentum model to run without any WSEL errors? Or should I expect a few?

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants 
Copyright © The RAS Solution 2016.  All rights reserved.   

You can have a 2D full momentum model without errors if:

• Your cell size and computation interval is appropriate
• Everything is moving very slowly
• Stages are high
• Slopes are shallow
• N values are high
• Discharge doesn’t change too suddenly
• It’s a 2D only model (i.e. no connections to 1D)
• You are a skilled HEC-RAS modeler
• A combination of any or all of these (and some other factors I’m not thinking of), sure, you can have no errors. 

Is it normal?  I certainly wouldn’t call it abnormal.  Actually, you’d be surprised at how well the 2D solution works compared with the 1D solution.  I’ve found I get a lot less errors and instabilities in 2D areas than I do with 1D reaches.  Most of the time, my big errors happen at and around 1D/2D connections.

If you are lucky to get a simulation with no errors, double check your solution-make sure it looks reasonable.  Your instincts are correct and you should question the results.  But if everything looks good and reasonable, chalk it up to your skill and claim your bragging rights!