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GSSHA - outlet slope is negative

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Keep encountering this error when trying to run with diffusive wave.

Model works fine with no routing.

In .prj - OUTSLOPE = 0.01 (have tried it with higher numbers as well)

I see no obvious errors in the .gst or .cif. Streams were smoothed and .gst indicates arc exiting basin is lower than those entering.

I'm fairly new to GSSHA...


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Make sure the elevation of the arc at your outlet point is not higher than the elevation just upstream. Also make sure your outlet point is located on a 2D grid cell and is not outside the 2D grid. One other thing you can check--make sure all your streams are set to either be trapezoidal or cross section channels. The error is probably because you have a negative slope at your cross section point (item #1), but it could be one of the other problems also. Let me know if these things fix your problem.



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I confirmed issues 1 and 3. Did not examine number 2, but I'm not sure how it would get placed out of the grid. Got tired of messing with it so I coarsened up the stream network and it works fine. Originally there were several arcs converging on the node just upstream of the outlet. Unsure if that would cause problems. Arcs were pointed in the correct direction.


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This error usually means that you have a stream segment that has no vertices. Vertices represent computational nodes in GSSHA, and each stream arc must have at least one vertex, or computational node, to run. I'm not sure why the model says you have a negative outlet slope because as far as I've seen, the outlet slope has nothing to do with it. Simply add another vertex to the arc (I usually redistribute the vertices along the arc and set the number of segments option to 2) and rerun and it should work.


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