Thursday, June 25, 2009

New Features 4.0.1 Part 2...Junctions

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

Up until now, unsteady flow HEC-RAS uses a very simplistic method for handling juctions. It simply projected the water surface elevation of the downstream cross section to the two cross sections upstream of the junction. For shallow, deep systems, this worked fine-no problems. However, if you tried to model a steep reach with shallow depths you run into a host of problems at junctions. I've blogged about this problem before and provided examples of ways around to work around it. However, in the new version soon to be released (4.0.1) HEC has taken care of this problem and it works quite nicely.

In the Junction Editor, you'll notice that there is a new option for unsteady flow.

You can either keep the original, simplistic computation mode, or require RAS to perform an energy balance around the junction. I'm assuming HEC left the original "Force Equal WS Elevations" method in as the default so that your results will remain the same when you upgrade to 4.0.1. I don't believe that the new method is computationally burdensome, so I would expect that for new models, you'll want to check the "Energy Balance Method", particularly if you'll be working with steep reaches and/or shallow depths.

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