Calibration of a HEC-RAS modell

[László Vas]

Hello!

I have some problem with the calibration of a HEC-RAS modell.
My problem is the following:

I have to set up a HEC-RAS modell for a lesser reach of the river Danube, and within this reach there is a side-branch.
So we measured the bed of these reaches, we measured discharges at the junctions and measured the elevation of the water surface.
When I’m tying to calibrate the modell with Manning roughness, the calibration is succesful at the Danube, but I can’t calibrate the side-branch of it.
During the lowering the roughness of the side-branch, the water levels become lesser, but it won’t reach the correct elevation. Now I’m using irationally low roughness values, and the water level is still above the correct elevation at about 60-80 centimeters.

Could somebody help me?

[Anonymous]

Manning n is a coefficient that is measured in the lab when the river is flowing at “normal depth”, meaning your bed slope, water surface slope, and energy slope are all identical.

If there is a big backwater effect, which you might expect to see at a junction with a large river, this assumption breaks down. I think would then expect you manning n to be fudged lower to account for the back water effect.

Maybe first try and calibrate the reach upstream of the backwater in some areas that are approaching normal depth condition. This would be the reach’s “correct” manning value. Then in the area effected by backwater you know it should be somewhere between this and zero.

Hope that helps.

[Anonymous]

Also, the main channel is your DS boundary, so any calibration error from this is going to propogate itself upstream to the lesser reach. I might also start by rechecking all your geometries if you havent done that yet.

[Author]

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