Loss of velocity

[Anonymous]

Hey all

My little model isn’t working properly. At cross section 1 it looks all good but then down the river the steady flow analysis results in a velocity loss and so the height of my water line is going way up…
What could be wrong?

Thanks in advance

[Anonymous]

A few things to consider,

Do your cross sections really overlap? You don’t want to double account this area.

What are your boundary conditions?….potential problems there.

Is your channel concrete or excavated? Is your channel and floodplain manning’s n defensible?

Your first cross section is at critical….this doesn’t seem right….

I would put the bank points on the banks, and consider adding levee points.

cheers!

[Anonymous]

You may have your channel in the wrong direction. River stationing should go to zero at the downstream end.

just a thought….

[Anonymous]

Correct! This was my fault. It’s kinda weird to do the wright steps in civil 3d to produce working HEC RAS exports.

In between I tried unsteady flow with 2D mesh only ( http://hecrasmodel.blogspot.ch/2015/01/hec-ras-model-with-2d-mesh-only.html )

And I get similar results like the official hazard map of the country hazard management (for a 100yearly flood) but my flow is only like 10% (1 m3/s) of the 100yearly flood (10.4 m3/s)!?!?
I put manning n to 0.04 for the whole area. My lower BC is normal depth (0.02) and my upper BC is a flow hydrograph (in 30 minutes from 0.05 m3/s to 1 m3/s and then back to 0.05 m3/s in 90 minutes). Delivering Slope is again 0.02.
I put the lower and the upper BC only in the mesh cell exactly where the deepest point of my flume is.

What could be wrong?

[sidargul]

I think it might be related to your upstream BC, delivering slope being 0.02, you should increase that value gradually, maybe try 1 and see the difference. I didnt experience myself but I remember seeing it in one of the posts in this blog and this adjustment solved the problem.
Good luck!

[Author]

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