Blocked Junctions – Discontinuous Edge Lines in RAS Mapper?

[LorenAmelang]

I finally have my QGIS / RiverGIS / SAGA project imported to HEC-RAS 5.0.4, and cleaned up enough to actually run an unsteady flow simulation. Testing with very tiny late summer California flows, it “goes unstable” in about 20 minutes of simulated time. Looking at all the resulting profiles, it looks like the water can’t escape from upstream reaches into the next downstream river.

This shows three reaches with the same river name, that seem continuous and connected. The water level jogs down to the level of the next river junction over the last station distance.

I happened onto this view in RAS Mapper:

Sure looks like a serious discontinuity at each junction! But that’s the same area shown in reaches 1-2 of the profile plot, and the edge lines are broken both between the connected reaches and at their end. I’ve never seen the term “edge lines” anywhere else in this project, and I can’t find any place to show or edit them in Geometric Data Editor.

When I enter Unsteady Flow Data -> Flow Hydrographs it seems to show that only the very last downstream station of the final reach needs a Downstream Boundary Condition. Does that mean it thinks the others are connected? Again, it handles the junctions that seem connected in the same way as the blocking ones.

I checked the elevations of the channels across the junctions and they match well.

But the water seems blocked. Are those edge lines a clue? How do I connect them? What else could be blocking junctions between different river names? I guess the stationing distances start over for different river names…

[Anonymous]

This kind of looks like the joining cross sections do not all have similar channel elevations. Junction hydraulics can be extremely complicated event when joining the 2 reaches into 1 2-dimensional area that outflow to a 1-dimensional reach. To succesfully model a junction in hec-ras 1D there are some rules that apply:

-The elevations of the combining reaches must be very very similar in geometry

^^if not the drop or change in elevation can be manipulated just upstream of the junction as a direct drop to trick the software as a work around

-The cross sections joining the reaches should be as close to the junction as possible without overlapping while still following the rules of cutting cross sections

-It should always be experimented with both methods of solving hydraulics at the junctions

[LorenAmelang]

Thank you Luis for the reply! A lot has happened since I wrote my post… I wanted to check the example projects, and noticed HEC-RAS 5.0.5 was there, so I installed it. Can’t find any examples of junctions.

But I did discover RAS Mapper’s editing functions! The Release Notes document announced them like a finished addition, but I notice the Menu choices all still say “BETA”. And rightly so – I saw at least ten “exceptions” yesterday. They randomly re-order a few Cross Sections, or move a couple of them way out in the mid-Pacific on “River (Missing), Reach (Missing)” – and then redraw all your bank lines zigzagging up and down the reach, or flipping around to the other end of it. If you can get through fixing those without generating any new ones, it will automatically fix all your bank lines… If they ever make editing reliable it will be wonderful!

So I was able to edit those junctions with the Terrain visible under them, and cleaned up several places where my original river lines didn’t match the new 1-meter LIDAR DEM.

There is still a pileup of water ahead of junctions, but it is now ~5 feet instead of ~32 feet. But the “Max WS” levels still wildly overreact to every little bump in the streambed! A 2′ rise to a downstream cross section can make a 100′ spike in the Max WS. I guess that’s what they mean by “unstable”?

All my junctions have less than 2′ drop across the adjacent cross sections, similar to differences between other cross sections. Does that require a “direct drop”?

The cross sections joining the reaches are as close to the junction as they can be without being a star of arrow points. Maybe that’s what I need, a really short perpendicular line with splayed tails barely avoiding each other?

But I’m now seeing worse pile-ups midstream than at the junctions, so I think my scenario is just way too sensitive all over. Maybe little ~1 cfs and ~2 cfs wandering mountain streams are just tricky to model?

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