Water surface elevations not corresponding to breach flows

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Water surface elevations not corresponding to breach flows

ardenb
Hello,

I currently have an approximately 10 mile long 2D Hec Ras model that I am using to evaluate the flood risk to dwellings downstream of a dam. I am comparing two dam breach scenarios, one using Froehlich 1995 parameters and the other using Xu Zhang. The breach flows just downstream of the dam show the Froehlich breach as being higher than Xu Zhang, but when I look at a profile line at the dwellings approximately 7 miles downstream, the Xu Zhang WSE and breach flow are higher. My expectation is that the WSE and flow for the Froehlich parameters would remain higher throughout the length of the model.

There are no bridges, ineffective areas, significant channel slope changes, or flow regimes changes as the channel is uniform throughout the length I am modeling. I have ruled out attenuation differences as causing the problem as well. The cell spacing and run time are also appropriate for the velocities and channel width through the model.

Has anyone experienced this scenario before or have suggestions as to how I might be able to remedy the situation?

I appreciate any help!
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Re: Water surface elevations not corresponding to breach flows

jarvus
It is hard to say for sure.  Are you using full momentum?  Diffusion wave will not get good results for a dam break.  Are you using a small enough time step?  Any signs of instability?
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Re: Water surface elevations not corresponding to breach flows

ardenb
I am using Full Momentum yes. The time step I have set using the courant condition being less than 1 based on the highest velocities and the smallest cell size. No signs of instability either.
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Re: Water surface elevations not corresponding to breach flows

cameron
What do the breach hydrographs look like?