water surface elevation issues – channeled river

[Anonymous]

Hello,

I am using a full 2D model to study the impacts of channeling a river (flood study). The idea is to add high walls on both banks to avoid all spillage.

Logically, I expected the computed water surface elevation after the walls were added would be higher that before. However, this is not the case, WSE is lower. Would you have any idea why ?
All entries to the model are the same (TN, hydrogramm, etc). The only changing parameter is the geometry, as I added lateral structures.

Thanks for you help!

[cameron]

did the velocities go up in the channel? You are basically forcing water to stay in the channel which probably has a lower roughness value.

[Anonymous]

Yes they do!
Does this parameter weights a lot if my river section is 40 meters large ?
Would there be any way to compensate this friction issue?

[Scott Miller]

What roughness values are in channel and overbanks of the model?

[Anonymous]

For the roughness values, I have :
n = 0.067 overbanks and 0.045 in the river bed/channel .
My weir coefficients for the lateral structures are 1.1

[Scott Miller]

Those values look reasonable. Changing them has a significant influence on the accuracy of the model, regardless of channel width. The roughness can be adjusted so that the modeled water surface elevations matches observed elevations.

What would happen to the water surface elevation if you brought the levees a bit closer together?

[Anonymous]

They would stand in the river bed..

[Anonymous]

This could make sense as the contraction may cause the flow to become supercritical flow. I would review a specific energy chapter in an open channel flow/hydraulics textbook to see if this would help you to better understand this phenomena. Often a contraction/expansion in open channel flow will not always preform quite as one would intuitively expect.

[Author]

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