Floodway Encroachments and Split Flow Optimizations

[Anonymous]

Hello,

I’ve got my model working the way I want it to, and now I’m trying to come up with my encroachment stations for my floodway. I’ve got several lateral weirs modeled to simulate the split flow between the main channel and an adjacent lake.

(see http://hec-ras-bloggery-forum.1091112.n5.nabble.com/Offline-Storage-Area-td123.html#a134 for more fun!)

Using lateral weirs, I have the option for Flow Optimization. I know I want this for my 100yr and 500yr floodplains, as I want the flow in the channel to drop (and therefore the WSEL) as you move downstream past the lake. Not selecting this option would result in the d/s flows being the same as the u/s flows.

My question is… do I want this for my Floodway run also? My gut says “yes” because the split flow is still possible, but only up to a certain point. To keep it simple, let’s say all of my lateral weirs are located at my left bank station. If my LEFT Encroachment Station reaches the left bank station, doesn’t that mean that I’ve effectively blocked off that entire lateral weir, and therefore the split flow? Meaning I would expect the d/s flows to be the same as the u/s flows for the two cross sections that the lateral weir is between, and that I’ve brought my encroachment station all the way into.

Thank you for your help!

[Anonymous]

If you know how much flow you want to divert to the lake, I think you would be better off just reducing the flows in the flow editor as you move downstream and not have to deal with flow optimization.

I’m not sure what happens if you try to combine an encroachment run with a flow optimizaztion run, but as far as the lateral structure is concerned I think it will still divert flow even if that side of the cross section is fully encroached/blocked.

[Author]

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