In HEC-RAS, lateral structures are used to model flow being conveyed out of a river and into an adjacent area (e.g., another river/stream, a 2D area, or a storage area). In this way, lateral structures are useful because they allow the user to model situations where water is leaving a stream/river system. By default, HEC-RAS water surface profiles are generated by assuming water builds up like a wall rather than calculating overtopping when the water surface extends beyond the defined cross-section geometry. This means the user must manually add an element, like a lateral structure, to their 1D HEC-RAS model to account for water leaving the system.
A lateral structure can be added to an HEC-RAS model to represent a variety of physical features such as levees, diversion structures, spillways, or even natural berms. Although lateral structures are versatile and important elements for any HEC-RAS user to understand, they can be tricky because they do have the tendency to cause stability issues. The following article will provide an overview of how to add lateral structures in HEC-RAS and some strategies for addressing some of the stability problems associated with lateral structures.
What is a lateral structure?
As previously mentioned, lateral structures are model elements in HEC-RAS that define how water flows laterally to the stream centerline. Lateral structures do not necessarily need to represent an actual structure. However, lateral structures are commonly used to model levees, diversion structures, spillways, or even natural berms. They can also be used to model the amount of water that overtops stream banks and enters an offline 2D area, storage area, or another stream. Alternatively, the user can allow water to simply leave the stream system and disappear into nowhere. However, it is worth noting that HEC-RAS will not be able to account for any submergence effects or tailwater when you allow flow to simply flow into the abyss.
How does water pass over a lateral structure in HEC-RAS?
Water passes over a lateral structure from a 1D cross section into an adjacent area when the water surface elevation for a cross section exceeds the user-defined crest of the lateral structure. HEC-RAS does account for the fact that a lateral structure is a continuous surface with varying water surface elevations at each cross section by interpolating to determine whether the lateral structure is being overtopped between cross sections.
Adding a lateral structure in HEC-RAS
Users can add lateral structures in the Geometric Data Editor. Click the Lateral Structure button on the left side of the widow. Once the Lateral Structure Editor appears, select Options from the top menu and click Add Lateral Structure as shown in the image below.
Defining the location of a lateral structure
After clicking Add a Lateral Structure, HEC-RAS will ask for a river station. The river station should be any number between the two cross sections where a lateral structure will be located. HEC-RAS does not position the lateral structure based on the river station. The river station number is simply an indication of which cross sections the lateral structure is between.
The position of a lateral structure is based on the user-defined. To define the headwater distance, click on the Weir/Embankment button within the Lateral Structure Editor. Under the Weir Stationing Reference section, you will see a box for HW – Distance to Upstream XS. The headwater distance to the upstream cross section is the distance between the cross section upstream of the lateral structure and the upstream end of the lateral structure itself.
After drawing your lateral structure, you can use the measure tool (hold down the CTRL key and click where you want to measure) to determine the headwater distance. The following section will discuss how to draw a lateral structure in the Geometric Data Editor.
Drawing your lateral structure
To draw your lateral structure in HEC-RAS, navigate to the Geometric Data Editor. Then hold down the CTRL button while drawing the path of the lateral structure. Next, copy the coordinates by clicking the Copy coordinates to clipboard button as shown below.
Finally, paste the coordinates into the Lateral Structure Centerlines GIS Coordinates dialog box located within the Lateral Structure Editor as shown below.
It is important to draw your lateral structures along high ground because the flow on the “stream side” of that high ground is likely to be 1D and the flow on the “overbank side” of that high ground is likely going to be 2D.
In addition, it is good practice to overlap the cross sections and the 2D flow area a little bit so there are no gaps in mapping. However, this overlap will cause HEC-RAS to double-count the storage in the system a little bit. However, this error should not be an issue if the overlap is not too significant. It is also worth noting that, computationally, HEC-RAS can still run with a gap between the edge of your cross sections and the boundary of the 2D flow area.
Finally, it is important to ensure that your weir length and the centerline length generated by the coordinates are the same. Otherwise, you will get an error.
How to define the crest of a lateral structure
After drawing the centerline of the lateral structure, you must define the crest. If you have high-quality terrain data, you can define the crest of your lateral structure using your terrain data. In this case, HEC-RAS will account for the fact that water may spill over your stream banks in one location and not in another location by allowing water to move into the cells that are adjacent to the lower crest elevations and not the cells adjacent to areas where the lateral structure is at a higher elevation. You can define the crest elevation of your lateral structure by clicking the Terrain Profile button in the Lateral Structure Editor. Then click on the Table tab and copy the two columns. Finally, paste this information into the Embankment Station/Elevation Table, which is accessed by clicking the Weir/Embankment button.
If you are defining a proposed levee or other structure, you can simply enter your own station/elevation information into the Embankment Station/Elevation Table.
When defining a lateral structure that will convey flow from a stream to an offline 2D area, it is important to ensure that the crest of your lateral structure is higher than the minimum elevation of the adjacent cells. The line shown in red on the image below depicts the minimum elevation of the adjacent cells. If you are using the terrain to define the crest of your lateral structure, there are areas where the terrain is a bit too low. Luckily, in the newer versions of HEC-RAS, you can simply click the Clip Weir Profile to 2D Cells button to raise the crest elevation where necessary. This new feature is a time saver. In the past, users had to manually raise the crest elevation of the lateral structure based on (potentially) dozens of errors that would pop up when running the program.
Headwater Connections and Tailwater Connections
The weir stationing table, which can be accessed by clicking the Headwater Connections button in the Lateral Weir Embankment dialog box, defines where each HEC-RAS cross section is located along the lateral structure. This is because the reach associated with the lateral structure is the headwater side of the lateral structure. If your model is not georeferenced, this table will be populated based on either the left overbank, right overbank, or main channel reach distances (depending on which Headwater Position you select in the Lateral Structure Editor). For this reason, I recommend always starting with a georeferenced HEC-RAS model file.
A similar process is used to define the tailwater connections. Whatever you connect the lateral structure to (e.g., a 2D flow area, storage area, etc.) is the tailwater side of the lateral structure.
Overflow Computation Method
HEC-RAS allows the user to choose between two overflow computation methods for lateral structures: the weir equation and the normal 2D equation. The weir equation is the default option. However, the weir equation may not be the best choice when your lateral structure will be simply conveying overland flow from cross sections to a neighboring 2D flow area rather than directing flow over a raised surface/berm. HEC-RAS does have a “Zero Height” weir option for these types of situations. However, in these cases, the Normal 2D Equation Domain may be a better option. When you select this option, HEC-RAS uses the profile of the water surface along the lateral structure and uses the 2D equation to transfer water to the bounding cell faces.
If you determine that the weir equation is more appropriate for your HEC-RAS model, you must enter a weir coefficient in the Lateral Weir Embankment dialog box. There are many publications available that recommend lateral weir coefficients for “inline” situations, but there are not very many resources applicable to lateral structures. In general, lateral weir coefficients should be much lower than their inline counterparts. This is because an inline structure will be more hydraulically efficient than a lateral weir. The default weir coefficient in HEC-RAS is 2, but Table 3-1 of the HEC-RAS 2D User’s Manual lists the range of appropriate weir coefficients for various situations.
| What is being modeled with the lateral structure? | Description | Range of Weir Coefficients (English Units) |
| Levee/Roadway – 3 ft or higher above natural ground | Broad crested weir shape, flow over levee/road acts like weir flow. | 1.5 – 2.6 |
| Levee Roadway – 1 to 3 ft elevated above ground | Broad crested weir shape, flow over levee/road acts like weir flow, but becomes submerged easily. | 1.0 – 2.0 |
| Natural high ground barrier – 1 to 3 ft high | Does not really act like a weir, but water must flow over high ground to get into 2D flow area. Flow does not pass through critical depth. | 0.5 – 1.0 |
| Non-elevated overbank terrain. Lateral structure not elevated above ground. | Overland flow escaping the main river | 0.2 – 0.5 |
2D Boundary – Use Velocity
When the Use Velocity box is unchecked, HEC-RAS computes the volume of water that passes over the lateral structure. In other words, there is no momentum coming out of the 1D reach into the 2D flow area. In contrast, when the Use Velocity box is checked, HEC-RAS calculates a velocity over the cell faces adjacent to the lateral structure and will use that velocity to solve the momentum equations. This box, by default, is unchecked for stability purposes. However, I recommend running your final model with this box checked because it does increase the accuracy of your model results.