Connecting 2d area and 1d Elements

[amgeda]

Hello!
I am trying to connect the 2d flow area with 1D Cross sections and some questions came up:

– What is Hec-Ras doing if cross-sections and 2d-area is overlapping?
– What if i do not add any 1d lateral structure for the Connection?
– What is the best Position for the lateral structure and how can i add it?
– Do I allways have to connect the area via a lateral structure?
– What ift there is a gap between 2darea/lateral structure and the cross sections?

Thanks a los for your help!

[Anonymous]

Whether or not XS’s and 2D flow areas overlap, RAS aims to connect XS end points with 2D mesh cell faces. These can be interrogated and revised, as appropriate. Ideally, enough mesh boundary definition is provided to avoid a 2D mesh boundary overlapping with XS’s.

A lateral structure is needed as a connection of XS edges in order to allow flow to transfer from a XS geometry (a vertical wall is assumed at a XS extent if no connection is specified).

The best position for a lateral structure is the real/actual position of the transfer of flow from a XS edge and the connected 2D mesh.

Whether or not there is a gap between a lateral structure and XS’s, the terrain in between will define the mesh cell properties. Generally, the further away a modeled connection is from the actual location of flow transfer from a XS edge to a mesh cell, the less accurate the modeled transition of flow. Connections should be representative of the actual conditions being modeled. The weir coefficient for flow over a lateral structure across ground should be much smaller than that of a lateral structure behaving like a broad-crested weir (e.g. say about 0.2-0.5, instead of a typical 2.6).

[Anonymous]

Thanks for your reply I still have some questions:

1. I have to define a RS where to put the LS. This RS is not at directly at the position of a XS. The HW distance to Upstream XS will be the difference between those two. Am I right?

2. Is it better to define the position of LS within the 2D area or right at the border to the 2D area or within a distance? Does it matter? I do not know the actual position of the transfer from a XS edge and 2D. I will be always there where I define it?

3. What if I want to define a continous LS, but the number of points should not be over 500. There will be a gap between those two LS. Does it matter?

4. I wonder about the difference of between “Normal 2D Equation Domain” and “Use weir equation”. In both cases the LS should be higher than the (Minimum?) Elevation of the Cell face? Do you just copy the terrain profile xyz data for the weir profile xyz?

5. How does it come that there is a difference between weir length and Centerline length. What do I change if I move the points of the centerline? The centerline length?

Thanks a lot for your help!

[Anonymous]

I have to disagree with you here markwood. Or maybe im uncertain as to what set up you are talking about.

To clarify:

From a 1D reach (upstream) connection to a 2D storage area (downstream) or vis versa…the only thing that is needed is to move the reach end point onto the face of the 2D mesh. At this point RAS will ask you to verify to make this 2D area the DS condition for the 1D reach. What this does, is immediately connect all faces of the 2D area along the entire length of the cross section. No lateral structure connection is needed.

When connecting two 2D areas a lateral structure is absolutely needed though.

[Anonymous]

I was talking about the 1D/2D Connection with a 1D river reach and a 2D floodplains on the right and left side of the reach. The connection is sideways then via Lateral Structure.

[markwood]

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From: “Markwood, David”
To: “amgeda [via HEC-RAS Help]”
Subject: RE: Connecting 2d area and 1d Elements
Thread-Topic: Connecting 2d area and 1d Elements
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amgeda,
These are very good questions, and I expect others on this blog can provide=
more than provided here. But, hopefully this is helpful:

1) Generally, yes, the HW distance to Upstream XS will be the RS of the=
XS less the RS of the lateral structure. The RS of a lateral structure can=
be a thousandth of a foot less than a XS RS. It’s worth noting that what i=
s important here is a reasonable model of the actual flow overtopping the X=
S and into the adjacent 2D flow area, therefore, the RAS-assumed connection=
s to the cell faces of the 2D flow area are what counts, and these should b=
e scrutinized and revised, as appropriate.

2) The actual position of the transfer of flow leaving a XS edge and en=
tering a 2D grid is more accurately modeled when the literal boundary or th=
reshold between these two is placed accurately, based on topography. (i.e. =
no gap, adjacent, boundaries are more ideal than having RAS align XS edges =
and 2D mesh cell faces based on a rough/questionable geometry configuration=
)

3) For setting up model geometry, it would be awesome to have lateral s=
tructures with more than 500 points. This is a (reasonable) limitation of t=
he software.

4) The equation selected is separate from the need to have 2D mesh cell=
face elevations at or above lateral structure station-elevation data. Esse=
ntially, yes. Copying the terrain profile into the lateral structure geomet=
ry is a viable way to define that geometry, though you’ll inevitably be fac=
ed with mesh cell elvations below connected lateral structure elevations, a=
nd will have to resolve those errors (not a big deal, if you’re satisfied w=
ith mesh and 1D configurations, but something you’ll have to reconcile).

5) There is an option for georeferncing a known weir/lateral structure.=
I believe a typical lateral structure is defined by the XS edges of a 1D r=
each, which produces a ‘straight-line’ depiction of the lateral structure (=
straight-line from a XS edge to the next). Clicking on this feature within =
the Geometry Editor window provides the option for georeferencing, which is=
an awesome feature not previously available.

From: amgeda [via HEC-RAS Help] [mailto:[email protected].=
com]
Sent: Thursday, September 08, 2016 4:46 AM
To: Markwood, David
Subject: Re: Connecting 2d area and 1d Elements

Thanks for your reply I still have some questions:

1. I have to define a RS where to put the LS. This RS is not at directly at=
the position of a XS. The HW distance to Upstream XS will be the differenc=
e between those two. Am I right?

2. Is it better to define the position of LS within the 2D area or right at=
the border to the 2D area or within a distance? Does it matter? I do not k=
now the actual position of the transfer from a XS edge and 2D. I will be al=
ways there where I define it?

3. What if I want to define a continous LS, but the number of points should=
not be over 500. There will be a gap between those two LS. Does it matter?

4. I wonder about the difference of between “Normal 2D Equation Domain” and=
“Use weir equation”. In both cases the LS should be higher than the (Minim=
um?) Elevation of the Cell face? Do you just copy the terrain profile xyz d=
ata for the weir profile xyz?

5. How does it come that there is a difference between weir length and Cent=
erline length. What do I change if I move the points of the centerline? The=
centerline length?

Thanks a lot for your help!

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amgeda,

These are very good questions,=
and I expect others on this blog can provide more than provided here. But,=
hopefully this is helpful:

 

1) &nbs=
p;  
Generally, yes, the HW=
distance to Upstream XS will be the RS of the XS less the RS of the latera=
l structure. The RS of a lateral structure can be a thousandth
of a foot less than a XS RS. It’s worth noting that what is importan=
t here is a reasonable model of the actual flow overtopping the XS and into=
the adjacent 2D flow area, therefore, the RAS-assumed connections to the c=
ell faces of the 2D flow area are what
counts, and these should be scrutinized and revised, as appropriate.<= /o:p>

2) &nbs=
p;  
The actual position of=
the transfer of flow leaving a XS edge and entering a 2D grid is more accu=
rately modeled when the literal boundary or threshold
between these two is placed accurately, based on topography. (i.e. no gap,=
adjacent, boundaries are more ideal than having RAS align XS edges and 2D =
mesh cell faces based on a rough/questionable geometry configuration)<= /o:p>

3) &nbs=
p;  
For setting up model g=
eometry, it would be awesome to have lateral structures with more than 500 =
points. This is a (reasonable) limitation of the software.

4) &nbs=
p;  
The equation selected =
is separate from the need to have 2D mesh cell face elevations at or above =
lateral structure station-elevation data. Essentially,
yes. Copying the terrain profile into the lateral structure geometry is a =
viable way to define that geometry, though you’ll inevitably be faced=
with mesh cell elvations below connected lateral structure elevations, and=
will have to resolve those errors (not
a big deal, if you’re satisfied with mesh and 1D configurations, but=
something you’ll have to reconcile).

5) &nbs=
p;  
There is an option for=
georeferncing a known weir/lateral structure. I believe a typical lateral =
structure is defined by the XS edges of a 1D reach, which
produces a ‘straight-line’ depiction of the lateral structure =
(straight-line from a XS edge to the next). Clicking on this feature within=
the Geometry Editor window provides the option for georeferencing, which i=
s an awesome feature not previously available.

 

 

From: amgeda [=
via HEC-RAS Help] [mailto:ml-node+[email protected]]

Sent: Thursday, September 08, 2016 4:46 AM
To: Markwood, David
Subject: Re: Connecting 2d area and 1d Elements

 

Thanks for your reply=
I still have some questions:

1. I have to define a RS where to put the LS. This RS is not at directly at=
the position of a XS. The HW distance to Upstream XS will be the differenc=
e between those two. Am I right?

2. Is it better to define the position of LS within the 2D area or right at=
the border to the 2D area or within a distance? Does it matter? I do not k=
now the actual position of the transfer from a XS edge and 2D. I will be al=
ways there where I define it?

3. What if I want to define a continous LS, but the number of points should=
not be over 500. There will be a gap between those two LS. Does it matter?

4. I wonder about the difference of between "Normal 2D Equation Domain=
" and "Use weir equation". In both cases the LS should be hi=
gher than the (Minimum?) Elevation of the Cell face? Do you just copy the t=
errain profile xyz data for the weir profile xyz?

5. How does it come that there is a difference between weir length and Cent=
erline length. What do I change if I move the points of the centerline? The=
centerline length?

Thanks a lot for your help!

---

If you reply to this ema=
il, your message will be added to the discussion below:

http://hec-ras-help.1091112.n5.nabble.com/Connecting-2d-area-and-1d-Ele=
ments-tp3010p3047.html

ml-node+s109=
[email protected]

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[ce18m092]

I am trying to connect 2D flow area to 1D river element but at the DS side there are 2 rivers , How to connect them??

[Author]

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