HEC-RAS Lateral Structure to cross sections

[BenjaminHartman]

So I obviously can't get HEC-RAS to operate as a distributary properly, so I was going to trick the program. There are several passes in the lower Mississippi river that I am trying to model. Normally I just set the downstream tailwater to each and have it run. But based on the stage downstream and the stage in the river, the passes don't have the right flow. The main channel has been calibrated well. I have curves that say what the flow in the passes should be based on the flow in the main channel, so I applied them in HEC-RAS and set the lateral structure to "between two cross sections of a reach" and assigned the reach. The lateral structure shows the right flow leaving, the main channel shows the right flow up and downstream of the structure, but in the pass itself there is a difference in magnitude and some negative flows. Any know what is going on or what I should do? Thanks!

 

[ChuckDavis]

It sounds like the issue might possibly be that the "optimization" feature of the program has not been turned on. If you go into the "steady flow analysis" window for your plan, click on "options" menu, then click on the "flow optimizations" feature, have you checked the button that says "optimize" for your lateral structure? I have found that the lateral structure calculations can computer correctly, but those computations don't always match the flows in the cross sectional output of the model in the upstream and downstream reach of the main channel for the lateral structure or in the connected reach on the other side of the lateral structure if this box is not checked. As a side note, you can optimize your flow across a distributary junction using this "flow optimizations" feature as well.

If you have already checked these boxes for your lateral structure and/or your junction and the flows still don't seem to be computing correctly, then the issue might be in the way you've defined the connectivity of the lateral structure. I have some other possible ideas if this didn't resolve your issue, so feel free to post a response if that doesn't work!

 

[BenjaminHartman]

Hey Chuck,

Good feedback. I haven't tried this yet because I got hung up on particulars of Bohemia Spillway these last few days.

If I had to guess I would say that this won't work because my model is an unsteady one that operated off observed gage data and flow optimization is a steady flow only feature. Nonetheless, I believe I have seen the feature available during browsing and will pursue the details.

Thanks, I'll get back with a response soon.

 

[BenjaminHartman]

Looking through the unsteady options and it looks like the tool is used for determining the initial flow conditions and can't distribute water continuously based on flows in channel.