Tuesday, January 19, 2010

Another Reason for Interpolated Cross Sections

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.

Here’s a classic case for interpolation of cross sections in a RAS model.

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Notice the drop in water surface to near critical depth at the grade break. Then, at the next cross section upstream of the break, RAS seems to overestimate the head loss. Sometimes this phenomenon can be much more dramatic, occasionally initiating oscillations upstream of the break that can lead to instabilities.

When scanning the results of your model run, or when trying to diagnose sources of instability, keep an eye out for this phenomenon. Interpolation of cross sections around this point will provide a much more accurate and stable answer. Notice in the figure below, that further interpolation upstream of the newly interpolated reach is probably warranted.

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