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How can I enter the lenght between the junction and the downstream cross section?įor now, RAS assume the same lenght between the primary river cross section and the juntion.įor computations in RAS, the legnth from the junction to the downstream cross section (and the length from the upstream cross section to the junction are irrelavant. Just make sure they don't intersect flowlines and they only intersect any given cross section once. Just be aware that the downstream cross section in the upper reach of the primary river will have 0 (zero) for its reach lengths-which is okay-they're not used anyway.īank stations can be continuous through the junction. The primary river can have flow lines that project through the junction and are continuous from the upper reach to the lower reach. Each junction needs to have 3 or more discrete polylines representing reaches that come together at a single point.įlow paths should terminate after intersection the downstream most cross section on the tributary reach. Your reaches should snap together at the junction. What do I need to watch out for when digitizing junctions in HECGeo-RAS? As far as, do my flow paths, banks, rivers need to intersect, and where? If you want to exchange volume from one reach into another, you have to connect them using a lateral structure.
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However, there is no automatic exchange of volume into the adjacent cross section when this happens. RAS will just extend vertical "walls of water" in a cross section where you are above the end points. If I set up my cross sections in HEC GeoRAS as you have them drawn above – will the model know to overtop on the left side of the right river and flow into the left river? I am having a lot of issues with the water level raising above my cross sections. I am currently modeling a dam breach that breaks in a river that has a large slope and then enters a junction where it flattens out. That’s okay as long as it is just a geometric schematic issue, and that cross sections were not really surveyed overlapped. Sometimes you’ll find that interpolated cross sections might overlap at times. You certainly do not want cross sections overlapping or extending into another reach’s flow path. The new cross sections will be missing the high ground on its “confluence” side, but that’s okay, because the other cross section (in the opposite reach) will capture that. This plane of confluence will generally follow high ground between the two reaches, but technically should define the boundary between flow lines of one reach and flow lines of the other reach. What I’ll usually do is draw an approximate “plane of confluence” (the yellow line), and then butt the new cross sections up to that line, without crossing over it. With this example, I would certainly add more cross sections upstream of the junction like so… With junctions, generally it is important to get cross sections as close to the junction as possible-even more so with unsteady flow modeling ( ).