I would appreciate some help/advice on modeling a complex diversion structure. There is a small diversion dam to create head on a canal headgate. The canal has a very shallow longitudinal slope (1/4%) so backwater is a concern (i.e. it affects water levels in the river). The river is also pretty flat, but not that bad.
I have a schematic I can post if someone tells me how.
I have split my model into to “rivers”: RIVER and CANAL (very imaginatively named).
The diversion dam is modeled as an inline structure on RIVER. The IS goes from the left (looking downstream) flood plain to the right until it reaches the intersection with the headgate. The IS is modeled as a weir/embankment with a varying top elevation and with a gate at the sluiceway (it will always be open but that’s beside the point).
The headgate is modeled as a lateral structure of RIVER with a TW location of the first (most upstream) section of CANAL. The LS is modeled as a weir/embankment with a top elevation at the top of canal elevation and with three gates. Immediately downstream of the first CANAL section I have put in an inline structure to model the culverts. The IS is modeled as a weir/embankment with a top elevation at the top of canal and three culverts.
What are some of the pitfalls I should look for and try to avoid? Do you have any other advice on setting up the model? I am quite new at HEC-RAS and so you could tell me that my model is completely wrong and I would accept that as long as you told me the logic used to come to that determination and how to fix it.
At first, I had problems with not getting any flow into CANAL. I found the flow optimization option to fix that. Currently, I have problems with not having any flow through the CANAL IS even though there is flow in CANAL downstream of the IS. (It makes a difference because no flow through IS means no head loss from the culverts, which will be submerged most of the time.)
Any assistance would be greatly appreciated!!
Is this an interview question for a job kind of like how they tricked will smith and then boom hes hired and killing aliens?
...anyways post some screen shots, plan view. If you are modeling this, based on what you have explained it does not seem that a steady state model is the way to go but ill hold off on that until you post some images.
The 1D approach looks reasonable. Take a closer look at how the headgate culverts are set up. See if there is anything there, or at the lateral structure, that would be limiting flow. Viewing the canal profile may reveal what's going on. This is steady flow, isn't it?
When 1D is working like you expect it, give 2D a try. The results ought to be comparable. A detailed terrain model including the channel bottom is needed for 2D. The 2D can capture effects of that sharp bend in the river. The 1D is still a good way to go for the culverts and canal. What velocities are actually going through the culverts?
This is in middle-of-nowhere Wyoming (which is a little bit redundant). We don't have alot of the extra resources other places have.
I'm mostly concerned with the interaction with the inline and lateral structures in the model. Currently I just have the headgate as uncontrolled culverts in the lateral structure.
Any advice with adding in gates?
What things should I look for regarding flow limitations? This is for steady flow.
As for doing a 2D model, we didn't have the budget to do the surveying and modeling at this stage. We are doing preliminary design - will be recommending doing 2D for final (new budget - doing prelim design so they know how much to apply for next phase funding).
I think I will post screen shots of the inline and lateral structure model info so others can take a look.
I would expect the pipes to show, like in the screenshot below. (I changed the lateral structure hatch.) The culverts in your model appear to be at the right elevation. Have you got both upstream and downstream elevations in the geometry? Do the culverts run the entire length of the in-line structure? Those are the first things that come to mind.
I've done this model two ways: First I did the River to Canal lateral structure without any gates - just culverts. Below are screen shots of geometry and results tables.
The second is with gates on the lateral structure and an inline structure with culvert. Below are screen shots of geometry and results. It is confusing that the Canal inline structure doesn't show any flow in the culverts but there is flow in the Canal downstream of the inline structure.
I'm really beginning to hate HEC-RAS! I *REALLY* appreciate your looking at this!
How about backing up a bit? It could help to simplify. What about modeling the canal and gate with a constant head? And make sure the whole thing works part by part. The model goes from 1D reach to 1D reach after all. Any complex hydraulics due to the river bend and the intake structure configuration are lost. It's steady. How uniform are the upstream and downstream river channels, not considering meanders? What, ultimately, is the task?
Pardon my ignorance, what do you mean by modeling the canal and gate with a constant head?
One of the main goals of this project (besides rehabbing the deteriorating dam structure) is to reduce the amount of effort that goes into opening and closing the headgate slide gates. For almost the entire irrigation season, our client is limited to what they can take from the river. The river flows vary greatly hour to hour, day to day, week to week, and so they have to adjust the opening of the gates. This requires a significant amount of time. They are considering a SCADA system to take care of it, but costs may be prohibitive and even then, we would want to try to minimize the amount of operation (= electricity to run the gates). At the very least, I need to be able to show how often they will need to adjust the gates given average river flows and the constant flow they are allowed (every drop of which they very much need).
The canal and gate structure could be isolated from the river. I suggested putting 'constant' head on this, but that would be an unsteady model. A stage hydrograph would be the upstream boundary condition.
The upstream boundary condition for the gate and canal could be a steady flow, to verify that this part of the model works. If your clients have a way to know the river stage at this location, this part of the model may be all that is needed.