# 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.

Lateral structures primarily describe the weir geometry, representing how water overflows into the adjacent flow element. 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.

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.

The **Assign Lateral Structures** command allows the user to assign the already imported GIS shapefiles as a lateral structure polyline. To learn more about the **Assign Lateral Structures** command, refer to this article in our knowledge base.

The **Draw Lateral Structures** command allows the user to interactively draw the lateral structures on the Map View. To learn more about the **Draw Lateral Structures** command, refer to this article in our knowledge base.

## 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.

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.

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.

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.

The geometry of the lateral weir structure representing the overflow terrain ground geometry or levee geometry along the main river reach is defined within the **Lateral Structure Plot** section of the **Lateral Structure** **Data** dialog box.

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.

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.

### 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.

### Ogee

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.

### 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.

## Weir Flow Computations

Weir flow over a lateral structure can be quite different from flow 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 value can be looked up using 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 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 the main flow area of the river, then an inline weir coefficient should be used. Both of these coefficients are provided in the Overflow Weir panel described above.

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 the following parameters:

**Weir coefficient (Cd)**

This field defines the weir coefficient that will be used for the first iteration of trying the Hager’s lateral weir equation. The default weir coefficient is 2.6. The Hager’s weir equation is iterative and requires hydraulic results in order to make a weir coefficient calculation. The defined weir coefficient is only used for the first guess at the hydraulic computations. Clicking on the**[…]**button will display the**Weir Discharge Coefficient**reference dialog box as shown below.

**Average weir height**

This field defines the average height (not elevation) of the weir above the ground.**Average bed slope (optional)**

This field defines the average slope of the stream bed in the river reach containing the lateral structure. If this entry is left blank, then software will compute the slope by estimating an average bed elevation for each cross section and then compute the slope from the average bed elevations. The average bed elevation of the cross sections is obtained by subtracting the hydraulic depth from the water surface elevation.**Weir angle (optional)**

This field defines the angle (in degrees) for the lateral structure overflow weir. If the overflow weir is parallel to the stream, then the weir angle is assumed to be zero. If the weir is angled inwards towards the center of the river flow, then a weir angle is required. This is used for channels that have a contraction where the weir flow is allowed to go over the contracted section. A diagram showing the weir angle is shown below.

**Average radius**

This field defines the average radius of the ogee weir for Hager’s equation. This entry is disabled (grayed out) when an**Ogee**weir crest shape is not specified.

## 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.

Within the **Lateral** **Structure** **Data** dialog box, the **Connection** **Data** panel is used to define the 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.

*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.

## 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 the amount of diverted flow from the river.

## 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 are 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.

The **Flow** **Optimizations** dialog box will be displayed.

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 channel.