Unsteady floodways

[Guillermo]

Has anyone found an efficient way to develop a floodway model using an unsteady HEC-RAS model (for FEMA applications)? As far as I know, the preferred option still requires the user to create a steady flow model, although this modeling is seldom done at all (why bother with an unsteady analysis if we have to map steady results anyway?).

Thanks!

[Anonymous]

what do you mean by mapping steady flow. We can get outputs for dicrete time step ahead of the input data, so isn’t that useful?

[Vince Moody]

Chapter 6 of the new RAS 2D Modeling User’s Manual presents an entire discussion on steady vs unsteady modeling.

I will tell you from my perspective that I would go with a steady model over an unsteady model every time, if the steady model will meet project requirements. Steady models are often much more straightforward to develop and refine compared to unsteady. Besides, if FEMA calls for a steady model that’s what you should use.

[Anonymous]

I actually found the unsteady floodway capability within 4.1 later on, months after I posted my question. But as you suggest, steady modeling is the way to go. FEMA didn’t really call for steady, it was just a possibility given some complex situations I resolved without having to go the unsteady way. As you said, I agree that an unsteady model would cause future users more headaches than benefits.

So, I am glad you mentioned this…because I am almost finished with it!

Thank you much!

[Author]

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