INLINE STRUCTURE

[Anonymous]

Hello, I’m quite new of Hec-Ras and I need your help, please, because I’m getting crazy!
I’m from Italy so I’m sorry about my English. I try to explain my problem.

I have a private soil dam at the end of a very small reach (actually this reach it’s a sort of overflow of a bigger one). “It’s a lateral small lake”, hope you understand.
I already calculated the flood coming from the main river than I prepared a separated model regarding the dam.

Now I should do an unsteady flow analysys for the flood to verify the lateral structure of the dam + the dam breach analysys.

I drawn the dam like an inline structure and the overflow of the dam like a lateral one. The difference in high between the bottom of the lateral overflow and the top of the weir is 1.50 m.
I fixed ineffective areas at the level I assume the water at the arrival of the flood (level of the overflow bottom).

My main problem is the following:
– for ANY flow, even very tyny, the water elevation goes over the dam top level, raising the level of all the sections.If I delete the dam and I keep only the lateral structure, it doesn’t happen and the ws is very lower.
– it seems to me that the program consider the inline weir as a normal section and so whatever flux I give, it raises over the top of the weir, but it’s surely wrong for very small flow!!!
I understand that I’m doing something wrong, but I cannot understand what!
Please help me
thank you very much
PS. if needed I can post picture of the model, but I don’t know how to do.

[Lonnie A]

Have you set a “pilot flow” value on the inline? It sounds like you don’t have a low flow outlet in the model. The pilot will let flow through the structure. RAS needs a flow in every XS meaning you can’t have a “dry” section which is why you have flow over the weir at the start.

If you use RAS V5.0 they have added more options for the inline weir so you can model more than just a weir and gate which will give you additional way to carry the low flow through the structure..

[Anonymous]

Hello Lonnie,
first of all thank you for yout time to reply to me.
After your suggestion I set to 1 in pilot flow box (is that a flow dimension?which is the correct number?) and re-run the unsteady analysys. Actually for the first time in these conditions the ws slightly decreased under the weir top level but not to much. If I put 10 it crashes. What could it be?
I noticed also something else that can maybe help: the WS is over or coincindent with the top level of the inline structure already at the beginning of the simulation, when the inflow idrograph still needs to arrive from upstream.
So is it possible that the problem can be that hec-ras, before starting the unsteady process, maybe calculate a sort of initial WS and it fix it at the top of the weir (that is also the bottom of the flooding sections, maybe equal to critical depth?).. Trying to understand.
I don’t know how to post pictures of this ridicoulos model to show you..
I think it’s a stupid thing, but I am not able to understand it.

If you menage to help me, sure you’ll be my guest once you come to Italy!!!
I’m from Tuscany :-))

[PaoloP]

Giacomo,

the pilot flow is in m3/s and you have to define it based on the discharge upstream the inline structure. If pilot flow is larger than US discharge you dry out the US reach.

Paolo

[Robert]

Hello Giacomo,

Do you solve your problem?
You can send me your model using Hec-RAS- File-Debug report. I will check your bugs during weekend.

have a nice day
Robert

[Anonymous]

Hello Everybody,
thank you very much for your help. Some info:
1 – I don’t know how to post or send my model here to let you see it, eventually please teel me.

2- After your suggestions, my attention moved to the initial conditions and I saw that hec-ras was actually automatically setting the initial WS over the inline weir major quote (318.50 m)
So I run a hostart model to try to artificially recreate the initial condition file. I followed the instructions from web, that was also helping me to find some critical parts and calibrate the model to let it be more stable (adding some futher flow to avoid dry flow).
Now the model doesn’t crash but at the end of simulations time I still don’t get the initial condition I would like (I need to fix the beginning WS of the lake to 317 m hight). I think I’ll try to extend the idrograph time.

3- Is there any alternative way to create the initial conditions? Is it possibile to manually write the initial condition file? I had no time to try…

Thank you very much
Giacomo

[Anonymous]

Hello,
this is my model debug file. If anyone could help me, also about general results.

https://www.dropbox.com/sh/2wu542dg1qx360j/AACAnTgXJa-6KSzKAggnzF6Oa?dl=0

Some brief info to resume:
– I’m modeling the flood in this “lateral lake” done with a soil dam (high 318.5 m see level) + lateral overflow (bottom high 317 m) and connected to the main river that of the system. This model represents only from the derivation from the main river to the dam, using inflow hidrograph separately calculated.

– First step: verify the overflow of the system in case of flood.
I did a hostart file to fix the initial condition in the lake at about 317.00 (actually the hotstart file was not very stable but at the end of simulation WS in the lake was about 317.00 m so I took it)
– Geometry: ineffective flow in lake fixed at 316.00 (higher gives instability); some interpolated XS; DS section drawn at same level of bottom of overflow;
– Boundary: idrograph Return Time 3000 years (it lasts 45 minutes with peack of 20m3/s flood) + normal depth 0.017
– time step: I fixed computation interval 1 minute

In general what is strange:
– I expect to see the level start from about 317, increase and then decrease. It doesn’t happen
– velocity very low (0.10 m/s) is that coerent with the situation?
– some errors and warning

Second step:
– I should do dam breach analysys, not yet started until not having a good model.

Thank you very much
Giacomo

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