HEC-RAS allows users to model a variety of structures including bridges, culverts, inline structures, and lateral structures. The lateral structure element is typically used to model flow coming out of a river into a storage area. However, lateral structures can be used to model a variety of hydraulic structures such as morning glory spillways, berms, levees, or flow diversion structures.
In HEC-RAS, lateral structures require the user to input a weir coefficient. Typically lateral broad-crested weir coefficients should be around 2.0 (US Units) or 1.0 (SI Units) which is much lower than weir coefficients used for inline structures.
This blog post will be a detailed guide on how to choose the appropriate lateral weir coefficient and weir equation for your hydraulic model.
What is a Lateral Structure in HEC-RAS?
A lateral structure is typically used in HEC-RAS to represent flow leaving a river laterally. For example, you can use a lateral structure to model a levee being overtopped or breached. You can also add culverts, gates, and spillways, which are common elements of a flow diversion structure, to your lateral structure. In addition, lateral structures can also be used to connect HEC-RAS 1D elements and 2D flow areas.
You can now define a lateral structure geospatially in HEC-RAS. First, use the measure tool to trace the lateral structure. Then copy and paste the coordinates into the Lateral Structure Centerline GIS Coordinates table.
Lateral Weir Coefficients vs. Inline Weir Coefficients
It is important to note that lateral weir coefficients are different than inline weir coefficients. Generally, lateral weir coefficients are much lower. This is due to the energy/momentum loss from turning flow lines to a lateral direction.
Many textbooks and other resources provide guidance on selecting weir coefficients for inline structures. However, there are far fewer resources for lateral weir coefficients. Fortunately, the United States Army Corps of Engineers provides some guidance.
Because lateral structures can represent a variety of hydraulic elements, it is impossible to recommend a single lateral weir coefficient that would be appropriate for all cases.
Recommended Lateral Weir Coefficients
Keep in mind that lateral weir coefficients are not dimensionless. Therefore, weir coefficients used for determining flow in SI Units are different from weir coefficients used for determining flow in US units by a factor of 0.55. This value is the square root of the ratio of the SI gravitational constant to the US gravitational constant.
The following lateral weir coefficients are specified in the Hydrologic Engineering Center’s “Combine 1D and 2D Modeling with HEC-RAS” document which is dated August 2013.
| Hydraulic Structure Being Modeled | SI Units | English Units |
| Levee/Roadway – 3 ft (1 m) or higher above natural ground | 0.83 to 1.2 (1.1 default) | 1.5 to 2.2 (2.0 default) |
| Levee/Roadway – 1 to 3 ft (0.3 to 1.0 meter) elevated above ground | 0.55 to 1.1 | 1.0 to 2.0 |
| Natural high ground barrier – 1 to 3 ft (0.3 to 1.0 meter) high. | 0.28 to 0.55 | 0.5 to 1.0 |
| Non-elevated overbank terrain. Overland flow escapes the main river. | 0.06 to 0.28 | 0.1 to 0.5 |
In addition, HEC-RAS will calculate a weir coefficient for you if you select the Hagar Equation option.
Other Journal Articles
In addition to the guidance provided by the Hydrologic Engineering Center, there are some journal articles that might be worth checking out. You should note that many of these articles do not specify values. Rather, they describe how to calculate a lateral weir coefficient.
- Hager, Willi (1987). Lateral Outflow Over Side Weirs. Journal of Hydraulic Engineering, ASCE, 113(4).
- Mohammed, Ahmed (2011). Theoretical Analysis of Flow over the Side Weir Using Runge Kutta Method. Internal Journal of Engineering, 9(2).
- Delkash and Bakhshayesh (2014). An Examination of Rectangular Side Weir Discharge Coefficient Equations under Subcritical Condition. 3(1): 24-34.
Weir Modeling in HEC-RAS
HEC-RAS can model the following three cases: ungated inline weirs, ungated lateral weirs, and gated weirs.
Difference Between the Weir Coefficient and the Discharge Coefficient
Some people think that the weir coefficient and discharge coefficient are equivalent. They are not! So before I talk about the specific weir equations available in HEC-RAS, I would like to talk about the difference between the weir coefficient and the discharge coefficient.
The weir coefficient is a parameter that accounts for the discharge coefficient as well as the gravitational constant.
Standard Weir Equation
Selecting the Standard Weir Equation allows you to choose from four options for weir crest shape: Broad Crested, Ogee, Sharp Crested, and Zero-Height. However, you should NEVER select Zero-Height because it should not be used with the standard weir equation. This is a bug in the program. If you do select Zero-Height as your weir shape, HEC-RAS will assume a Broad Crested shape.
Hager’s Equation
When you select Hager’s Equation, HEC-RAS will require you to enter parameters. Then the program will calculate the weir coefficient automatically. Each of the four weir types has its own method for computing the weir coefficient based on hydraulic parameters entered.
Unlike the Standard Weir Equation, the Hager’s Equation allows you to model a Zero-Height weir shape. You should use a Zero-height weir in situations where flow leaves a channel laterally, but there is no defined obstruction or hydraulic control separating the two. This is common when simulating flow from the main channel to a storage area or 2D flow area via a lateral structure.
Because the weir coefficient is calculated based on hydraulic parameters, the computations are iterative in nature. The default weir coefficient (Cd) value is only used for the first iteration of the computations discussed on page 8-18 of the HEC-RAS Hydraulic Reference Manual.
Gated Weirs
When modeling gated spillways at lateral structures (or inline structures), you must enter a weir coefficient that represents flow over the spillway when the gate is completely opened. This is different from the discharge coefficient used to represent flow over the top of an inline structure.
For gated spillways, you can choose from three options: Broad Crested, Ogee, and Sharp Crested. Broad Crested and Ogee weir shapes are treated in the same manner as previously discussed. However, the user can choose from three options for calculating the weir coefficient for a sharp-crested weir.
The Sharp Crested Weir option allows you to choose from three weir coefficient calculation methods: user-entered, the Rehbock equation, and the Kindsvater-Carter equation. The weir coefficient will be calculated at each time step for the Rehbock and Kinsvater-Carter methods.
The Rehbock equation for the discharge coefficient was developed for rectangular weirs.
Although the Kindsvater-Carter equation was developed for English units, HEC-RAS will automatically convert units if you are building your model in SI units.