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Re: Boundary condition not respected in steady flow analysis
For steady flow, if the downstream boundary is below critical depth, the program will compute a subcritical profile based on the critical depth. If mixed flow is turned on, it will then try and compute a supercritical profile, if applicable. Look at section 4.6 ("Mixed Flow Regime") of the Hydraulic Reference manual. Available under Help, or off of HEC's web site.
Unsteady flow will use ("respect") the downstream water surface. However, this is not always hydraulically correct.
Imagine you have a storage area (a lake) at the bottom of a very high cliff. If the stream goes over this cliff (makes a waterfall), it would not be correct to force the water surface of the stream (at the top of the cliff) to the water surface in the storage area down below. But unsteady flow will do this. In reality, the downstream boundary (in this scenerio) should, somehow, be set to critical depth.
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