Modeling Lateral Flows

In HEC-RAS, lateral structures are used to model flow being transferred between a river and adjacent elements, such as another river, storage area, or 2D flow area. The lateral structure acts as an internal boundary element between the above model elements, and can represent a levee or flood wall, a flow diversion structure, or the natural terrain.

Levee Overtopping

Lateral structures primarily describe the weir geometry, representing how water overflows into the adjacent flow element. (More information on these flow elements is contained in this knowledge base article.) These structures can also include other outlet elements, such as culverts, flood gates, and overflow spillways. Additionally, breaching failure of a lateral structure either from overtopping erosion or underground piping erosion can be modeled.

River Reach, Storage Area, 2D Flow Area and Zoomed In View

The lateral structure is generally placed along a physical element in the terrain, such as a roadway, as shown in the above figure. A lateral structure models the exchange of flow between a main river reach and an adjacent river reach, a main river reach and an adjacent storage area, or a main river reach and an adjacent 2D flow area. To model the exchange of flow between a storage area and a 2D flow area, a SA/2D connection internal boundary element is used.

Creating a Lateral Structure

To create a lateral structure, either the Assign Lateral Structures or Draw Lateral Structures command can be used. Both commands operate similarly.

Lateral Structure Ribbon Menu

Assign Lateral Structures Command

To use the Assign Lateral Structures command, a polyline that can be selected for the purpose of assigning the lateral structure must already exist on the Map View. The software will extract the lateral structure geometry from the underlying terrain model.

Assign Lateral Structures - Elevation Data Option

Draw Lateral Structures Command

The Draw Lateral Structures command allows the user to interactively draw the centerline of the lateral structure on the Map View. Again, the software will extract the lateral structure geometry from the underlying terrain model.

Draw Lateral Structures - Elevation Data Option

When creating a lateral structure, it is assigned to the main river reach to which it is adjacent.

River Reach Lateral Structure Definition

At a minimum, there must be at least one cross section upstream and downstream of the lateral structure. The upstream cross section can either be at the beginning of the lateral structure or a short distance upstream. The downstream cross section can be at the downstream end of the lateral structure or a short distance downstream. Additional cross sections should be placed between these two bounding cross sections to model the change in the conveyance area of the river reach and to model the flow as it exits or enters along the lateral structure.

Plan View of Example Lateral Weir

Lateral structures can be on either side or both sides of the river. The lateral structures can be adjacent to the channel bank or located along the far overbank. A lateral structure can extend along a river reach for up to 100 cross sections. For lateral structures that exceed this length, an additional lateral structure can start at the point where the other lateral structure ends. Lateral structures cannot exist beyond the downstream most or upstream most cross section in a river reach, nor can they exist where a river reach junction is defined.

Adjacent River Reaches Lateral Structure

Flow over the lateral weir can be computed using either the energy grade line elevation or water surface elevation. Water surface elevation is the most appropriate when the lateral structure weir is located close to the main channel. In this situation, the energy from the flow is in a downstream direction, and the velocity head is in the same direction and should not be considered in determining the flow over the lateral weir structure. Therefore, the computation of flow over the lateral weir is best computed using the water surface elevation option, which is the default computation option.

Trimming & Extending Cross Sections

When defining a lateral structure, it is important that the river reach cross sections extend out to the lateral structure, but do not extend beyond it. The cross sections should end at the inside top of the lateral structure. That is why the lateral structure alignment should coincide with something in the terrain model that corresponds to a bifurcation of the flow, such as a roadway or levee.

GeoHECRAS will automatically trim and extend the adjacent river reach cross sections, while defining the lateral structure, so that the cross sections extend to the inside top of the lateral structure.

Trimming Extending Cross Sections

If the lateral structure is between two adjacent river reaches, the software will automatically trim or extend the cross sections for both reaches.

Lateral Structure Overflow Weir

When defining a lateral structure, it is necessary for the modeler to define where the lateral overflow weir discharges. Lateral flow can discharge into an adjacent river, storage area, or 2D flow area.

Lateral Weir Geometry Example

The geometry of the lateral weir structure is defined, representing the overflow terrain ground geometry or levee geometry along the main river reach.

Lateral Weir Geometry

When defining the weir geometry, the weir type must be specified. The weir type dictates the amount of head loss that the flow encounters as it flows over the structure. The weir type is specified in the Weir crest shape dropdown combo box entry contained in the Overflow Weir panel of the Lateral Structure Data dialog box

Weir Crest Shape

Available weir shapes include:

  • Broad Crested
  • Sharp Crested
  • Ogee
  • Zero Height

Broad Crested Weir

Broad crested weirs are very common and are typically represented by overflow over a roadway, levee or other flat crested structure. The broad crested weir structure represents a rectangular obstruction across the flow, causing head loss as the flow passes over the obstruction.

Roadway Overflow

Sharp Crested Weir

Sharp crested weirs are generally used to measure flow rates, and the weir is constructed from steel plate or other metal. The crest of the weir is very sharp so that the water will spring clear of the crest.

Sharp Crested Weir


An ogee shaped (S-shaped) weir is commonly used for dams and flow diversion structures. This weir shape is typically used in spillway design because its shape naturally follows the lower surface of a horizontal jet emerging from a sharp crested weir, thereby limiting the amount of spalling damage that can occur due to the rapid flow of water over the concrete spillway.

Ogee Spillway

Zero Height

When using lateral structures to model discharge from a main river reach into an adjacent overflow area (i.e., river reach, storage area, 2D flow area) without an elevated structure (e.g., roadway or levee) separating the two elements, then a zero-height weir can be defined to represent the non-elevated overbank terrain. This corresponds to flow traveling overland with no weir at all.

Lateral Flow Between Reaches

Weir Flow Computations

Weir flow over a lateral structure can be quite different than over a normal spillway. If the water flow near the lateral structure is predominantly in the direction of the main river reach, then there is not much of a momentum component over the lateral structure weir and a lateral weir coefficient should be utilized. This can be looked up from the Weir coefficient entry contained in the Overflow Weir panel of the Lateral Structure Data dialog box, as shown above.

If the weir flow is perpendicular to the flow direction of the river, and the lateral structure is located near to the main flow area of the river, then a lateral weir coefficient should be used. If the weir flow is in an area where the water is fairly stagnant and not located near to the main flow are of the river, then an inline weir coefficient should be used. Both of these coefficients are provided in the lookup table.

There are two different equations available for computing weir flow for lateral structures:

  • Standard weir equation
  • Hager’s lateral weir equation

The Hager’s lateral weir equation is the same as the standard weir equation, except the weir discharge coefficient is computed automatically based on the physical and hydraulic properties of the lateral structure.

When Hager’s lateral weir equation is selected, the user can enter an optional average weir height, average bed slope, and a positive weir angle (in degrees) relative to the stream if the lateral structure is not parallel to the stream.

Hagers Equation Parameters

Lateral Structure Culverts

In addition to the weir geometry, lateral structures can include culverts. These are commonly used in levees to allow ponded stormwater contained in the overbank areas to flow back into the river. These culverts typically have flap gates to prevent flow reversal so that the flood in the main river channel does not flow out into the overbank areas.

Culvert Flap Gate

Within the Lateral Structure Data dialog box, the Connection Data panel is used to define culvert flap gates.

Culvert Direction - Culvert Flap Gates.

The following flap gate options are available to describe culvert flow:

  • No Flap Gates – Flow is allowed to flow in either direction through the culvert
  • No Negative Flow – Allows water to flow from the river into the overflow area
  • No Positive Flow – Allows water to flow into the river from the overflow area

Lateral Structure Gates

Manually controlled gates can be added to the lateral structure. These can be used to represent flow diversions or locations where a mobile flood gate is rolled into place across a roadway or railway to maintain the levee during a flood.

Flood Gate - Lateral Structure Gates

Closed flood gate during May 2017 flood in St. Louis, MO

Within the Lateral Structure Data dialog box, the Gates panel is used to define gates. A number of different gate types are supported.

Gate Panel

Diversion Rating Curve

A diversion rating curve can be defined to represent how flow is removed from the main river. The diversion rating curve is defined as water surface elevation in the river versus amount of diverted flow from the river.

Diversion Rating Curve Table

Linear Routing

The software provides a linear routing option, which is a simplified storage accounting method in which the user enters a linear routing coefficient. This coefficient ranges between 0.0 and 1.0, with 1.0 representing routing the maximum flow over the lateral structure and 0.0 representing no flow routed over the lateral structure. Typical values range from 0.05 to 0.2, although this coefficient needs to be calibrated to be accurate. This calibration is typically done using historical flooding data.

The linear routing option is useful when there is a lot of lateral structures connected to storage areas, and a detailed flow calculation over each lateral structure is not necessary. In addition, the linear routing method is computationally faster and more stable.

Split Flow Optimization

When performing a steady flow analysis or computing unsteady flow initial conditions with a model containing a lateral structure, the flow optimization option must be enabled (i.e., turned on) for the HEC-RAS software to compute how much river reach flow is lost or gained from the adjacent overflow area through the lateral structure. To access the flow optimization option, select Flow Optimizations from the Analysis ribbon menu.

Ribbon Menu - Split Flow Optimization

The Flow Optimizations dialog box will be displayed.

Split Flow Optimizations dialog box

When the Optimize option is enabled (i.e., turned on), the software calculates the flow out of the lateral structure or back into the river, depending upon the adjacent river reach, storage area or 2D flow area water surface elevation. The result of this calculation either reduces or increases the flow in the main river. The software then recalculates the water surface profile in the main river and the whole operation repeats itself. This iteration continues until there is a balance between the calculated and assumed flows in the main river.

If the Optimize option is not enabled (i.e., turned off), the HEC-RAS software will assume all the water in the main river is still going downstream, although it will calculate what could have gone out of the lateral structure based upon the computed water surface elevation in the main river.

About the Author Chris Maeder

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