http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html
I just made a similar comment on the boundary conditions post but it’s not showing yet (not approved yet). So, please excuse the (almost) double post.
The gist of the post was:
1. The downstream end is almost at the ocean, so I used mean high-high tide as the boundary condition – is this appropriate? There is adverse slope at this point, so is normal depth an option?
2. The bridge overtops in both the existing and proposed (replacement bridge) condition. HEC-RAS defaults to critical depth a couple xsections downstream. Is this reasonable, since the bridge overtops? Channel velocity also increases from 6.2 ft/sec (just downstream of bridge) to 13.7 fps (where HEC-RAS defaults to critical depth).
3. HEC-RAS also defaults to critical depth upstream of the bridge, where velocities are also high, going from 12.4 fps (where HEC-RAS defaults to critical depth) to 6.1 fps (just upstream of the bridge).