Lateral Structures in HEC RAS

[Dr. Khan]

Could someone please clarify how the lateral structures in HEC ras actually work.
When the water is transferred between 1D/2D through a lateral structure in HEC RAS, is it transferred at Left or right bank level of cross-section or is it transferred at the lateral structural weir level.

What is difference in setting HW position to “next to left bank station” instead of “left overbank” under lateral structure editor?
Thanks

[Anonymous]

That is incorrect.

When mass transfers from a 2-dimensional cell to a 1-dimensional cross section, mass is immediately transferred to channel of the 1D cross section. This is irregardless if the 2D area is connected to a 500-ft ROB, mass is transfered to the channel and will skip the overbank. This requires the user to manipulate the cross sections to terminate or begin exactly at the bank.

When a weir is connected to the ROB it will connect to where you have defined the overbank. (try it) Even if the overbank is far into the 1D cross section, you will see it visually move. The connection to the cross section connects to the end of the cross section

[Lonnie A]

The bank vs overbank connection is a hold over from before you could geospatially define the lateral for the most part. Flow from 2D to 1D doesn’t differentiate if it is going into overbank or channel in 1D. It is only assigning the flow to the 1D XS as a whole. Flow going from 1D to 2D is assigned to the edge cells the lateral is connected to. Where the differentiation of connection location comes into play is reach length differences and how the lateral stationing fits between XS locations. You should be able to visually see how the connection changes XS reference in the lateral editor. Switching between overbank and bank should move the XS location (if reach lengths vary from channel reach length).

With geospatially defined XS and laterals you should always connect to the overbank in my opinion. This is because you don’t want much if any overlap between the 1D and 2D portions of the model to avoid double counting volume. If you don’t have overlap then your connection is truly in the overbank.

Keep in mind how the reach lengths in 1D come into play on figuring out the corresponding cells that the lateral is tied to. If your XS is basically only covering the channel portion your overbank and channel reach lengths will likely be the same or very nearly the same. But if you are modeling some of the floodplain in 1d then the overbank reach may be longer or shorter and effect the stationing the lateral uses for its cell connection reference. One of the more common errors I see is that someone will take a 1D model, trim the XS to near banks and attach lateral to overbank but not correct the reach length.

[Anonymous]

Your picture is very out of date

[Lonnie A]

I know

[Dr. Khan]

Thanks Lonnie and Luis for helping on this forum. Your help is much appreciated.

[Zoran]

I’m trying to understand how the connections between river XS and the LS weir are defined and what influence it has on the results to choose a “right overbank” or “next to right bank station” type connection.

I observe that the weir stationnings change in the HW lateral structure connection table when I change the connection from “right overbank” to “next to right bank station”. However, I struggle to understand how these distances are calculated by HEC-RAS?

From what I understand, if “right overbank” is selected, the LS is positioned at the end of the XS, while if “next to right bank station” is selected the LS is positioned at the right bank stations. When I compare the distance between the right bank stations I get the same result as indicated in the corresponding table of HW lateral structure connections (231.6 m). On the other hand, I do not obtain the same result when I compare the distance between the ends of the XS with the corresponding table of HW lateral structure connections (249.4 m instead of 230.9 m).

I don’t understand how the distances are calculated in the HW lateral structure connections table?

For the positioning of the LS, it seems to me that the most correct solution is to connect the LS to what is really closest: right overbank or next to right bank station. It seems to me that once the water level in the XS has reached the level of the weir, water starts to overflow in the 2D flow area / SA. So even if there is an overlap, the volume shouldn’t be counted double (cf. Lonnie’s post)?

The choice of a HW position can have an influence on the simulation results as shown in the following figures.

[Author]

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