Boundary Condition – Normal Depth or known Water Surface Elevation

[m@nation]

Hello,

I am completing a 1D Hec-RAS model and I am modelling a smaller tributary that outlets into a larger river.

Is it better to use normal depth or a known water surface elevation? The only water surface elevation data I have is from a model developed 25 years ago. At the most downstream cross sections the difference in depth between normal depth and the known water surface elevation are around 1 foot. Is it better to just use the most conservative value if I have no data to calibrate or validate?

Thank you.

[Anonymous]

First validate the use of normal depth, as this is the most misunderstood boundary condition in hydraulics. For a great read and example of when Normal Depth is acceptable click on the blog tab on the top right where Chris Goodell has explained a normal depth scenario and the output

[Anonymous]

It depends. try to extend your model further downstream so that the B.C. will not have a huge impact on your results. Or you need to consider your study areas, study purpose and goal to choose the best and mos reasonable B.C. for example, If the channel you are modeling has a much smaller drainage area than the downstream receiving river (B.C.) and you are doing 100-yr modeling, you do not want to use 100-yr WSE of the receiving river as your B.C.

[Author]

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