[Ian Sharp]

HEC-RAS has no awareness of the n values in-between the cross sections. The n value at a cross section should be representative of the conditions half-way between it and the upstream section to half-way between it and the downstream section. If there are significant n value changes then it would be advisable to add cross sections.

[Ian Sharp]

I don’t fully understand the problem but I wonder if there is a leaky cell issue going on. Double check you break lines and cell faces to make sure they are along the high ground.

[Ian Sharp]

I have had this too. 1D/2D connections are notorious for this. There is a problem somewhere in your model but it is hard to find with these vague errors.

[Ian Sharp]

Your solution is probably a good one, I’ll have to give it a try. If I understand it correctly, you are essentially moving the cell faces to avoid the high ground. I think for most situations this will work fine, but as you observed you may find some with Courant issues, etc. You will still get some strange results when Mapper plots the depths, but I think that is acceptable.

I think your solution deserves a new post. Good work.

[Ian Sharp]

You may be having the same issue I identified in another thread. If the terrain does not include the channel through the bridge, then RAS will treat it as a dam.

http://hec-ras-help.1091112.n5.nabble.com/Ras-6-0-Bridges-through-embankments-td9519.html

[Ian Sharp]

I’m glad someone else is seeing this as I do, thanks for the responses Luis. Terrain modifications seem to be the solution, and thankfully they are not that hard in Mapper, but it seems this is a place for HEC to improve.

[Ian Sharp]

I would have a hard time believing a jump would occur as the flow exited the upper pond and entered a steeper reach. It seems the jump would occur as it entered the upper pond.

[Ian Sharp]

We have run into this on several models. I’ve played with all the things you played with and also switching between Full Momentum and Diffusion Wave. I have found that the DW runs have much less of these types of issues while FM runs tend to have some strange velocity and WSE issues along any interface. I see in your cross section you have shallow flow on a terrace which is above the flow in the channel. I have seen this phenomena occur with the use of the FM equations in channelized flow conditions.

We have had several models with fully channelized/contained flow that the 2D FM equations over-estimate water surface elevations and/or place water where a 1D model would not. That is to say, if you model the reach with a 1D model you will get a certain answer. Then if you model it with a 2D model with the FM equation set, the water surface will some times be 2-3 feet higher. So what may be happening in your situation is the 2D model is simply giving an answer that disagrees with the 1D model at this interface.

I have not yet found a great answer for this issue. You can try to move the connection to a more favorable location and adjust some roughness values. I have not found much benefit in changing the grid size at the interface. Sometime the warmup will help, but not always. I suppose that as long as the model is not having a volume conservation issue that maybe the answer is to live with the problem and address it in the mapping.

[Ian Sharp]

Thanks Lonnie. That was my conclusion too.

[Ian Sharp]

I have had the exact same problem in a number of models. There appears to be a mismatch in the velocity at the 1d/2d boundary. If you plot the velocity along the flow centerline you might see an abrupt spike and the velocity will reach zero at the connection.

It occurs in both 2D to 1D and 1D to 2D. And it occurs if I use Diffusive Wave or Full Momentum, although DW does better in handling this. I have tested this further to analyze grid size, orientation, stream slope, and other issues. Ultimately it seems to me that this is an insurmountable boundary constraint type issue at this time. Maybe it will be resolved in later releases?

[Ian Sharp]

Manning’s n too high?

[Ian Sharp]

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From: Ian Sharp
To: “guana [via HEC-RAS Help]”
Subject: Re: Bridges in 2D
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Will do. Thanks!

—
Ian Sharp, P.E., CFM
Project Manager/Engineer | JE Fuller/Hydrology & Geomorphology

Direct Line: 520-207-0008| [email protected]

Tucson Office: 520-623-3112 |www.jefuller.com<http://|www.jefuller.com>

________________________________
From: guana [via HEC-RAS Help]
Sent: Wednesday, October 5, 2016 4:05:02 AM
To: Ian Sharp
Subject: Re: Bridges in 2D

Maybe look at this:

http://hecrasmodel.blogspot.de/2016/05/bridges-in-2d-important-update.html

Combining 1D and 2D for modeling bridges in 2D areas was not considered. If=
you find out a good way, please share with us.

________________________________
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Will do.  Thanks!

—
Ian Sharp, P.E., CFM
Project Manager/Engineer | JE Fuller/Hydrology & Geomorphology

Direct Line: 520-207-0008| [email protected]

Tucson Office: 520-623-3112 |www.jeful=
ler.com

---

From: guana [via HEC-RAS He=
lp] <ml-node+[email protected]>
Sent: Wednesday, October 5, 2016 4:05:02 AM
To: Ian Sharp
Subject: Re: Bridges in 2D

 

Maybe look at this:

http://hecrasmodel.blogspot.de=
/2016/05/bridges-in-2d-important-update.html

Combining 1D and 2D for modeling bridges in 2D areas was not considered. If=
you find out a good way, please share with us.

---

If you reply to this email, your message wi=
ll be added to the discussion below:

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16.html

To unsubscribe from Bridges in 2D,
click here.

in reply to: Bridges in 2D #10094

Ian Sharp

Participant

Thanks Toby. I have tried to add more cells and have also adjusted the roughness values to better match, but the problem remains. I am wondering if there might be a problem that is occurring because I am only modeling the channel in 1D and not the entire floodplain (I am modeling 2D over the bridge approaches).

in reply to: Bridges in 2D #10092

Ian Sharp

Participant

Thanks for the response Jarvus. I have used the 1d/2d iterations and have found no improvement. The image shows a typical example of what I am finding where the velocity at the downstream interface is zero in the 2D section.

[Ian Sharp]

Thanks Toby. I have tried to add more cells and have also adjusted the roughness values to better match, but the problem remains. I am wondering if there might be a problem that is occurring because I am only modeling the channel in 1D and not the entire floodplain (I am modeling 2D over the bridge approaches).

[Ian Sharp]

Thanks for the response Jarvus. I have used the 1d/2d iterations and have found no improvement. The image shows a typical example of what I am finding where the velocity at the downstream interface is zero in the 2D section.

[Author]

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