Sunday, April 16, 2017

How Roughness and Loss Coefficients are Applied to Reaches in HEC-RAS

It's really hard to find a clear explanation of how roughness and loss coefficients are applied to reaches in HEC-RAS.  I ran extensive scenario testing today and here are my results.

Roughness

Roughness at a cross section is applied in both directions, upstream and downstream, as the conveyance of two cross sections are averaged to determine the energy slope between the sections.  So if you want to model precisely an abrupt change in roughness, you need to have two cross sections at the abrupt change.

Loss Coefficients

Expansion and contraction loss coefficients are applied to the reach downstream of the section were they are declared.  If an expansion coefficient is 1 at a section, the water surface will never increase going downstream from that section, because 100% of energy is lost when flow slows down rather than being recovered as a pile-up (increase) of water surface.

Documentation

If you can find any documentation of the above, I would love to reference it.

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