UNSTEADY Flow Errors – HTAB and WSEL

[gm1233]

I am having trouble with unsteady flow analysis. When I run the unsteady analysis I receive the errors“extrapolated above rating curve” and “solution solver went unstable at iteration 13 used computed changes in flow and stage at minimum error.” When I look at my profiles the water surface elevation for certain cross sections are much higher than the previous cross or next cross sections. I have edited the HTAB parameters for the cross section to 100 interval, 0.5 increment, and copied the inverted elevation but it doesn’t fix the problem. Is this a problem with my Manning’s roughness coefficient? Any help is appreciated.

[Luis Partida]

HTab parameters are def something that can help (100 may not be enough) but not just this. This could be a product of your min/initial flow, timestep….there are so many parameters to play with to help resolve your solution. But the easiest way, without messing w RAS default options and tolerances is to play with flows and time steps. You could just be drying out your model causing the spike

[gm1233]

How do you figure out an appropriate time step? I have the initial flows as blank in order to use the boundary conditions. I also have the boundary conditions reading off of the DSS.

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