Unsteady model unstable

[Anonymous]

Hello,

Im running a 1d model under the unsteady flow and it goes completely unstable. It crashes in the first minutes of the simulation and says that extrapolate above several cross sections table.
My objective is to calibrate the mannings coefficient, based on two hydrographs, one wich was obtained by a monitoring station located at the last cross section (downstream) and one wich was obtaing by modelling by HEC-HMS and is used as a boundary condition to upstream.
My cross sections are spaced by a maximum of 5 meters.
I already tried to put more cross sections but it doesn’t help. I am using the mixed flow option and my simulation lasts for 2 days.
If anyone can help, i would apreciate it!

[Scott Miller]

Hello Pedro. Since the model will run at least for a few minutes, set the output interval to a minute. Run it, and take a look at the profile animation. Watch for whether any of the cross sections goes dry – empties out before the next time step. Can’t have that happen, or the model will blow up.

Have you copied the starting elevation in the HTAB parameters to invert? Check the forum, Chris’s blog, or the web for info on that and other unsteady stability issues.

[Anonymous]

Hi, thanks for responding.
I changed the output intervals and now the model runs from 16:00 wich is the starting time until 16:08 (the simulation end time is supose to be 2 days forward). It is producing a lot of negative flow values but none of the cross sections goes dry.

[Lonnie A]

try increasing your initial conditions flow and run a warm up period. I usually set a minimum flow on my top hydrograph ~5% of my 100yr flow. Other thing to try is setting htab to start at min channel elevation and have a fairly defined HTab.

[Anonymous]

Hi, thanks for the tips.
I changed the simulation time and inserted another hydrograph where the flow is more significant. I changed the htab parameter to 100 points at every cross section and put it to start at minimum channel elevation. I put my minimum flow as 5% of the peak flow. The model runs for 8 minutes (should last for a few hours) and crashes. The profiles show the increase of the flow but it reaches a certain time where the flow extrapolates.
I have no clue what it is going wrong now, if anyone has any idea on what it is.

[Scott Miller]

This document is a set of things to check: Common Model Stability Problems – Unsteady

A variety of things might be causing instability. Insert an image of the profile. That could help narrow possibilities down.

[Anonymous]

Here’s a link for a image of the profile plot, for the profile of the first simulation minute.
https://uploaddeimagens.com.br/imagens/hec-ras-jpg-cafbc4bd-0d30-4dc6-9480-ad1d23883385

And here’s a link for when it crashes
https://uploaddeimagens.com.br/imagens/hec-ras_crash-jpg

Aprecciating all the help!

[Scott Miller]

It looks like instabilities are generated at two locations.

The drop from about elevation 265 has a problem even before the crash. Put in some more cross sections where the energy grade line goes high. Maybe use a shorter time step. Take a look at the velocities and calculate the Courant number to see what distance and time step are reasonable. Set the Courant number to 2 or less.

Is the downstream boundary condition normal depth? It looks like it might be emptying the channel too quickly. Try using a less steep slope for the boundary condition. Alternatively, since the channel appears to be going steep again at the tail end, it may work to lock it in at critical depth.

[Anonymous]

Im interpolating more cross sections on every energy peak. In some cross sections the velocity goes negative and so the flow.

The downstream boundary condition that i am using is another flow hydrograph since i am trying to compare the observed hydrograph (downstream) with the simulated by the HEC-HMS (upstream) and then calibrate the mannings coefficient.

[Scott Miller]

I use normal depth, or possible critical depth, as the downstream condition. Let the channel flow, then compare the output hydrograph with observed flow. Minimize the absolute error by adjusting the n-value, so that the channel stores more or less volume in active storage. If the hydrograph volumes match, that approach should work well.

You might at least ease the downstream constraint until the rest of the model is stable.

The energy grade line should only increase if energy is added to the system, like with a pump or from a tributary.

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