How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

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How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

Andrea
Hi everyone,

I am working on a HEC-RAS 5.0.3 project and I need to connect the 1D channel to some 2D areas. To do that, I am using the Lateral Structures, as suggested by the manual.

To assign the elevation profile to the Lateral Structure, I am copying the terrain profile underneath it and pasting it in the Enbankment Station/Elevation Table. However, in this way I obtain a Lateral Structure's profile that sometimes is higher and sometimes is lower than the TW Cell Min Elevation (to avoid the error saying that the elevation at my weir is lower than that of my cell faces, I increase the Lateral Structure elevation in all the interested points suggested by the error message). The result of this process, however, is a Lateral Structure that often is much higher than the cell face, so that the Lateral Structure behaves like a small dyke (sometimes it is 1m high). This of course doesn't correspond to the real situation, in which there is no dyke.

What I need to do is, therefore, assigning to the Lateral Structure the same elevation of the TW Cell Min Elevation, so that the artificial dyke is 0 meters high and the water passes from the 1D area to the 2D area without crossing any dyke (that is the real situation).

Is there any way to do that automatically (I could do that manually by changing the elevation of every single point of the lateral structure and replacing it with the elevation of the grid cell underneath it, but it takes too much time).

Any help would be appreciated!

Best Regards,
Andrea
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Re: How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

cameron
You should be using the elevation data directly below the lateral structure.  Does the lateral structure and 2D boundary snap together?  They should be right on top of each other.
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Re: How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

Andrea
I am using the terrain profile below the lateral structure to assign the elevation to the lateral structure, but the lateral structure and the 2D area's boundary don't snap perfectly together (if you have any suggestion in order to make them snap perfectly, it would be really helpful). However, they are not so far each other: they can be far up to 0.5 meters (the resolution of the DEM file is 10m).

But even if I perfectly overtop the two lines, I think I would have the same problem anyway. In Picture_1 it is shown the profile of the lateral structure and the TW Cell Min Elevation: I would like to find a way (not manually) to set the elevation of the lateral structure equal to the TW Cell Min Elevation.

Anyway, my aim is not to have a small dyke (created with the lateral structure) separating the main channel with the 2D flow areas. I want the water to flow from the 1D area to the 2D area without any levee or dyke. What can I do to accomplish this aim?

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Re: How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

cameron
I set up the lateral structures in GIS and import then into HEC-RAS so I can get them right on top of each other and snapped together.  

There is not a automated way to do what you ask that I am aware of, unless you are good at programming and can change the input files directly.  

Your cross-sections should be snapped to the lateral structure as well and the cross-section end elevation should match the lateral structure, but in your case it is off by 0.5 m at station 8484.475.  

To accomplish what you ask, I would go back to GeoRAS or HEC-RAS if that is all you can do, adjust the lateral structure so it snaps or is very close to the 2D boundary and the 1D cross-section.

The TW min elevation represents one value for the entire grid cell it is attached to and if you go and adjust the entire lateral structure to match it, that would cause more flow to leave or return to the channel than really would. It would also impact the mapping so that you might have a gap between the 1D floodplain and the 2D floodplain.
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Re: How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

Sarah
In reply to this post by Andrea
I would adjust lateral structure location so it always captures the high ground.
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Re: How to assign to the Lateral Structures the same elevation of the TW Cell Min Elevation

Igloo_79
In reply to this post by cameron
I followed Cameron's suggestion of snapping together the lateral weir and 2D flow area in GIS, then importing the GIS coordinates. Since I am modelling an entire reach in 1D-2D, my weirs are long (3-4km) which means that once I filtered the weir sta/elevation to 500 points, this created deviations between the weir elevation and the terrain - thus, I received errors that some 2D cells had higher elevation than the weir. Since this difference was small (~0.005m), I raised the weir elevation in Excel by 0.015/0.025 m, which resolved the error.

The model still won't run (1D/2D Flow Errors), but at least I don't have geometry errors (!)