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robross

Irrigation diversions

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Hi all,

I am trying to figure out how to treat four irrigation ditches over the mainstem of a HEC-RAS in WMS model. Each one diverts and returns to either the mainstem or a tributary right before the tributary joins the mainstem. Any input on how to place diverted flow? Add a outlet point?

Thanks for any thoughts or ideas,

Rob

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And, as an afterthought, has anyone had problems with WMS not populating the downstream overbank and channel distances when exported to HEC-RAS?

Thanks!

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You will have problems with WMS not populating the overbank and channel distances if your bank arcs do not intersect all your cross sections or your bank arcs were created after the cross sections were created. If your bank arcs were created after your cross sections, you can try executing the Recompute All Stations menu item in the River Tools in the Map module in both the Cross Section and Centerline coverages. WMS should be able to handle branching flow in your HEC-RAS model. Just build your arcs from upstream to downstream in the 1D centerline coverage and branch your arcs downstream at the locations where you have diversions. There are other ways of handling your scenario that are less complicated, this is just one of them. You would not create an outlet point, outlet points are only for drainage coverages and hydrologic modeling, not for HEC-RAS modeling.

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Thanks for the reply, Chris! If I have modified cross section saved in an .idx, I can just merge those into extracted cross sections, right? It sounds like I need to widen my cross sections beyond the channel surveys that I did in order to generate the downstream length measurements. For the downstream branches of the diversions, I would just put change in flow at those branches where they divert and return?

Thanks,

Rob

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Rob,

Yes, you should be able to merge cross sections from another IDX file. A tutorial on cross sections that comes with WMS shows how to do this. Yes, you can just place cross sections with flow change locations in your HEC-RAS model before and after your diversions and return flows.

Chris

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Yep, got the cross sections merged last night, and am about to put the ditches and tributaries in. 17 river miles, four ditches, three tributaries, and 396 cross sections. I'll let you know how it goes.

Thanks,

Rob

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Is there another way to handle the diversions? I have four irrigation ditches ranging from 3 to 15 miles in length, and the creation of the bank arcs is causing WMS to do a Houdini-like crash, where the active window goes away without even a crash warning. Can I just do a small partial branch downstream, and a small partial branch returning, since I have discharge data both at diversion and return? Thank you!

Rob

Edited by robross

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Rob,

I'm not sure that you need to even model your diversions if you are having problems modeling them. You could just do a cross section before and after your diversion and place a flow change location at these cross sections in your HEC-RAS model. You might be able to do what you mention above in HEC-RAS, but I don't know how WMS would handle it if you have multiple downstream terminus points in your HEC-RAS model.

Chris

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Ok, I have a steady state model for the reach with no diversions, all diversions, and one diversion, and it seems to be behaving itself nicely. I am now having issues with creating a transient state simulation with the same framework. I am using a one week period doing a one day time step, with basically the same data that went into the steady-state, but for the whole week insetad of one day. It is not going so well; I am getting error messages about the matrix solution failing and error exceeding maximum tolerances. I am using flow hydrographs for the top and bottom boundary conditions, and uniform lateral inflow for the flow changes inside the system due to the ditches. Any thoughts on changing boundary condition types, or any other things that might be going on here?

Thanks,

Rob

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Rob,

I'm not sure what could be going on here. Right away, I would suggest lowering the timestep and see if that makes a difference. I would try hourly timesteps and then even a 1 minute timestep if you're still getting errors. If this doesn't solve the problem, I would suggest looking through the unsteady flow troubleshooting chapter in the HEC-RAS user's manual (p. 8-57 "Model Stability"). This has a number of suggestions for things to check to fix the problem (cross-section spacing, computation time step, etc).

Sorry I can't be of more help. If these suggestions don't help, you could also send the model to us and have us look at it for you, but that doesn't fall under normal tech support. If it comes to that, let me know and we'll work something out with you.

Thanks,

Clark

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Thanks, Clark. I have been playing with the time step, to no real avail - it seems like the least unlikely results come from a 12 hour time step, but I am still getting 50 year flood stage from seasonal low-flow input. I am wondering if I need to do initial flow change locations that correspond with all of the flow hydrograph/lateral inflow hydrograph/normal depth boundary conditions?

Thanks,

Rob

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Rob,

Yes, you do need to make sure your flow changes correspond to the event you're trying to run. If you're getting 50-yr flood stages with seasonal low-flow boundary conditions, then it could be that your flow changes are adding way too much flow. You could also fool around with the boundary conditions, e.g., change the ds boundary condition to normal depth and see if that drastically changes your results.

Clark

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As always, i appreciate the help. You guys go above and beyond. I had already changed the ds boundary conditions to normal depth, as I read on a blog that flow hydrographs are really tricky for those. Here is a question you might know the answer to offhand: I have flow changes for the one week I'm trying to simulate arranged both as actual flow change (i.e., -10.45 for a diversion, and 6.6 for a return), as well as tributary data in the same setup. I also have a "running total" of what flow in the channel should be (i.e., 88 cfs - 10.45 at the diversion, so 77.55). What should I be inputting for flow change and boundary condition?

Thanks a bunch,

Rob

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Rob,

I haven't used flow change locations much in unsteady flow. It seems though that if the tributary enters the main stem after your flow change locations, then the flow change should be entered simply as the total flow at that point. If the flow change locations occur downstream of the confluence, then you would also have to account for the tributary. I have a good feel for how this all works in steady flow, but again, not really for unsteady. I don't know that this affects your boundary conditions. It seems that if you use a flow time series upstream (on both the main stem and the trib), and normal depth for the ds boundary conditions (on both the main stem and the trib) that this should work fine. I guess I don't see how your boundary conditions would be affected by the flow changes.

Sorry if this response is completely useless to you. Feel free to clarify if there's something I missed.

Thanks,

Clark

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