Steep initial reach problems

[JAKE SAYELL]

Hello everybody,

Please would anyone be able to advise on a model I am creating. I have attached an image containing the profile plot view of the first section of my reach and the output hydrograph after running an unsteady flow analysis . The output hydrograph has an extremely steep decline initially I believe to be due to the steepness of the reach. Is there any way to solve this problem to get more realistic results? The hydrograph also has minor fluctuations throughout would anyone have suggestions how to generate results which fluctuate less?

Thank you in advance,

Regards

[Chris G.]

The rapid initial drop in stage in your hydrograph is because you’ve set initial conditions that are not what RAS wants to compute. Try adjusting your initial conditions to match the first time step conditions.

At the upstream end of your reach, the instability can be fixed by one or more of the following:

Higher n values: http://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.html

Tighter cross sections: http://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.html

Refinement of HTAB parameters: http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.html

Turn on mixed flow: http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html

This other post may help as well: http://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.html

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