Help With HEC RAS ERROR

[Joseph Bartell]

Greetings. I have successfully ran a dam breach model in hec ras 2d. However, I now want to add a culvert going through a roadway and have the breach flow go through the culvert (as much as possible of course).

When I run the model with the new culvert I get the following error:

The SA 2D connection has a culvert lower than the cell it is connected to.

I measured the culverrt inverts off the GIS contours and they are correct. The only way to solve the problem is to have the culvert be ‘floating in air’ and flush with the top of roadway. Please help. Also, how do I post screen shots in here?

Joe

[Wick]

Look at the error codes, do they tell you how far off your elevation is? If it is close, you can just adjust the culvert inverts to match. A fraction of a foot here and there in your model is not going to drastically change your results for a dam breach.

The elevation is taken from the cell faces at the location, not necessarily the lowest point in the terrain. You can try adjusting your 2D flow area grid size smaller in that area to see if you can pick up the lower elevations you checked off the GIS contours.

Another option is to use RASMapper to make very small terrain modifications in the areas of the culverts to match the elevations you want (you would not have to adjust your culvert geometry data). By nature, terrain files tend to miss high and low spots depending on their resolution.

[Joseph Bartell]

George:
Thank you for the reply. The error is as follows:

the SA 2D connection has a culvert lower than the cell it is connected to. Culvert! elev 488.9, Cell 3955 cell elev 452.64 between face points 4216 4217 for 2D area.

How do I only make grid size in that location smaller? Can you provide direction or a video?

Can you provide further direction on using ras mapper to make small terrain mods? Can you provide a video? THANK YOU.

[alister.hood]

> How do I only make grid size in that location smaller?

Add a mesh refinement region. Page 3-10 in the 2D Modeling User’s Manual.
But be aware that introducing smaller cells may mean you need to user a smaller computation interval to keep the model stable. And there’s a good chance this won’t help – as Georgette mentioned, there’s a good chance your terrain isn’t actually low enough there anyway.

I would instead recommend using a simple circular or rectangular terrain modification at the end of the culvert, as Georgette suggested. Start on page 97 of the RAS Mapper User’s Manual.

[alister.hood]

> The elevation is taken from the cell faces at the location, not necessarily the lowest point in the terrain.

So when you add a terrain modification at the end of a culvert you need to make sure it isn’t fully contained within a cell, but intersects a cell boundary.
I think I’ve picked up on this before, and then forgotten, causing myself problems…

[Joseph Bartell]

This worked!!!!

THANK YOU!!!!!

[alister.hood]

> > The elevation is taken from the cell faces at the location, not necessarily the lowest point in the terrain.
>
> So when you add a terrain modification at the end of a culvert you need to make sure it isn’t fully contained within a cell, but intersects a cell boundary.
> I think I’ve picked up on this before, and then forgotten, causing myself problems…

Strange, I thought I’d replied again to this.

I’m sure this is incorrect, or at least the way I read it – I have verified that you can use a terrain modification located entirely within the cell to lower the terrain to the level you want to make a culvert.

I think in the past when I had problems setting culvert levels that I actually concluded they were related to this: https://therassolution.kleinschmidtgroup.com/the-ras-solution-forum/topic/why-ras-mapper-modifies-the-terrain/#post-15461

• This reply was modified 2 years, 8 months ago by alister.hood.

[alister.hood]

For the record, something else I’ve found is that I need to manually “compute property tables” after adding a terrain modification. I’m guessing this is only if I haven’t also modified the geometry.

• This reply was modified 2 years, 6 months ago by alister.hood.

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