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4. HEC-RAS 2D Bridge Modeling
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4. HEC-RAS 2D Bridge Modeling
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4. HEC-RAS 2D Bridge Modeling

# HEC-RAS 2D Bridge Modeling

GeoHECRAS can model roadway crossing bridges and culverts inside of 2D flow areas. Bridges inside 2D flow areas can handle the full range of flow regimes, from low flow to pressure flow, combined pressure flow, and flow going over the top of the bridge deck or roadway.

The user can define bridge data for 2D flow areas in the same way that bridge data is defined for a 1D model. HEC-RAS provides the same low flow (energy, momentum, and Yarnell) and high flow (energy and pressure/weir) bridge modeling approaches for both 1D and 2D bridge modeling.

For 2D bridge modeling, the software takes the user input bridge data and modeling approaches to develop a family of rating curves for the bridge, similar to what it does for 1D bridge modeling. However, for 2D bridge modeling, the bridge’s curves are used to obtain a water surface difference through the bridge for each set of cells being used to model the bridge. This difference in water surface is then equated to a force, which is distributed and put into a special version of the momentum equation for each set of cells spanning the bridge centerline. So instead of calculating friction forces, pressure forces, and spatial acceleration forces, these forces are obtained from the bridge curves. Then the 2D equations are solved as they are normally solved at any cell/face in the model. This approach used for 2D bridge modeling allows for equivalent forces to be computed for low flow, pressure flow, combined pressure flow/weir flow, or even low flow/weir flow.

Note that the amount of force given to each cell is based on the percentage of the total flow passing through that particular set of cells. This 2D bridge modeling approach allows for varying flow, water surface, and velocity at each of the cells around the centerline of the bridge opening. Therefore, the flow is still computed as a two-dimensional flow through and over the top of the bridge. Flow can pass at any angle through the bridge opening based on the hydraulics of the flow and the number of cells being used to represent the bridge opening.

Note that the WSPRO low flow method is not available for 2D bridge modeling.

## Defining 2D Roadway Crossing Centerline

The roadway crossing centerline must be first defined to represent a bridge or culvert in a 2D model. The roadway crossing centerline must be drawn from left to right looking in a downstream direction.

## 2D Roadway Crossing Interior and Exterior Cross Sections

The software will construct two parallel interior roadway crossing cross sections that represent the upstream and downstream interior geometry of the roadway deck using the roadway width value. The software assumes that the upstream and downstream interior cross sections are parallel to the roadway centerline, dividing the defined roadway width in half to determine their placement.

The software will construct two parallel exterior roadway cross sections that represent the upstream and downstream full valley geometry using the distance from face to full valley value. This value represents the distance that the upstream and downstream exterior roadway cross sections are placed relative to the roadway crossing exterior faces.

## Enforcing Roadway Crossing as a Breakline

After defining the centerline of the roadway crossing, the user needs to enforce the structure as a breakline into the 2D mesh. This is done using the Enforce as Breakline command available from the 2D roadway crossing right-click context menu.

The Enforce as Breakline command aligns the 2D mesh faces with the roadway crossing centerline, guaranteeing that the flow computations are perpendicular to the roadway crossing.

## Entering Bridge Data

Once the 2D roadway crossing is defined, the user can enter the data for a 2D bridge in the 2D Bridge & Culvert Data dialog box.

Follow the steps below to enter the bridge data:

1. From the Input ribbon menu, select the 2D Bridge & Culvert Data command.
Alternatively, the user can either double click the roadway structure on the Map View or choose the 2D Bridge & Culvert Data command from the Roadway Crossings dropdown menu of the Input ribbon menu as shown below.
2. The 2D Bridge & Culvert Data dialog box will be displayed.

The following sections describe how to enter the bridge data and interact with the above dialog box.

The Select 2D Roadway Crossing section allows the user to select the 2D roadway crossing to edit. The user can select the 2D roadway crossing from the 2D roadway crossing ID dropdown combo box or click the […] button to graphically select the roadway crossing from the Map View.

Note that if the user opens the 2D Bridge and Culvert Data dialog box by double-clicking the roadway crossing on Map View, that roadway crossing will be selected by default.

### Defining General Specifications

The General Specifications data panel is displayed by default when the 2D Bridge and Culvert Data dialog box is displayed.

In the above data panel, the user must enter the following data:

• Structure Type
This dropdown combo box allows the user to specify the roadway structure type. The following options are provided to the user:

1. 2D Bridge
2. Culvert

Based on the option selected by the user, the other data panels in the 2D Roadway Crossing Specifications dropdown combo box get enabled or disabled.

1. If the user selects the 2D Bridge option, then all data panels are enabled except the Culverts data panel.
2. If the user selects the Culvert option, then all the data panels are disabled except the Deck Roadway and Culverts data panels.
• Roadway width (parallel to the flow)
This field is used to define the roadway width in the direction of flow.
• Distance from face to full valley
This field is used to define the distance from the upstream (and downstream) side of the roadway deck to the upstream (and downstream) full valley cross section.
• Upstream embankment side slope (V:H)
This field is used to define the roadway crossing upstream embankment side slope. If this field is left blank, the software assumes a vertical embankment.
• Downstream embankment side slope (V:H)
This field is used to define the roadway crossing downstream embankment side slope. If this field is left blank, the software assumes a vertical embankment.
• [Extract Geometry] button
Clicking the [Extract Geometry] button allows the user to extract cross section geometry from the selected cross sections defined in the Cross Section Geometry panel (explained below). This allows the user to quickly update the geometry at the roadway crossing when changing either the Roadway width or the Distance from face to full valley values.
• Minimum weir elevation (optional)
This field is used to define the minimum elevation at which weir overflow over the roadway begins. If this field is left blank, software will scan through the specified roadway geometry to determine the minimum roadway elevation. Generally, this field is used to artificially raise or lower the minimum elevation at which weir flow is considered. Click the [Pick] button to interactively select the minimum elevation from the roadway crossing plot.
• Maximum submergence ratio
This spin control entry is used to specify the maximum allowable submergence ratio that can occur during weir overflow over the roadway. If this ratio is exceeded, the software will automatically switch to an energy-based (standard step method) flow calculation rather than standard pressure and weir flow calculation.
• Weir Crest Shape
This dropdown combo box allows the user to select the weir crest shape. By default, the weir crest shape is set to “Broad Crested,” which is appropriate for a roadway crossing.
• Weir coefficient (Cd)
This field is used to specify the discharge coefficient to be used in the weir flow equation for roadway overflow computations. Typical values range from 2.50 for a broad-crested rectangular shaped weir to 3.00 for a trapezoidal shaped weir. However, a value between 2.50 and 2.60 should be used for typical roadway crossings. By default, the software uses a value of 2.60. Click the […] browse button to display a weir coefficient reference dialog box.
• From SA/2D area ID, To SA/2D area ID
These fields are used to specify the upstream to downstream connection details of the roadway crossing. These fields are used if there are two different storage areas or 2D meshes that the roadway connection is connected to.
• Cell spacing along roadway centerline
This entry represents cell spacing along the roadway centerline. If this entry is left blank, then the cell spacing used in the vicinity of the roadway centerline will be used. Click the […] button to measure the cell spacing from the Map View.
• Relaxed cell spacing for roadway centerline
This entry represents the cell spacing further away from the roadway centerline. If this entry is left blank, then the cell spacing used in the vicinity of the roadway centerline will be used. This value should not be the same as the Cell spacing along roadway centerline value, or there may be many cell errors introduced along the roadway centerline. Click on the […] button to measure the cell spacing from the Map View.

Notes:

While importing a model from HEC-RAS, the 2D bridge structure will be imported to the 2D Bridge & Culvert Data dialog box. If the user has defined a culvert, then by default, the structure will get imported as the 2D bridge and culvert unless the user has specified some parameters that are specific to the SA/2D connection. In that case, the culvert will be imported as the SA/2D connection. For example, if the user has defined the following items, then the culvert will be imported as the SA/2D connection:

• Culvert Flow Direction (Flap Gates)
• Overflow Computation Method
• Gates
• Outlet Rating Curves
• Outlet Time Series

To specify the deck roadway geometry, select the Deck Roadway option from the 2D Roadway Crossing Specifications dropdown combo box.

The Deck Roadway data panel will be displayed.

The above panel provides a table for entering and editing the roadway high chord and bridge opening low chord geometry. The data in the table are used to describe the area that is blocked due to the roadway bridge deck, road embankment, and bridge opening vertical abutments.

Note that there are two tabs at the top of the geometry table that correspond to the upstream and downstream faces of the roadway crossing. The user can copy the current bridge deck and roadway geometry from the upstream cross section to the downstream cross section (or vice versa) by clicking on the [Copy to Downstream Cross Section] button.

### Defining Piers and Sloping Abutments

After entering the deck roadway data, the user must define bridge piers and sloping abutments that are inside of the bridge opening.

#### Bridge Piers

Select the Bridge Piers option from 2D Roadway Crossing Specifications dropdown combo box.

The Bridge Piers data panel will be displayed.

In the above panel, the user can enter the pier data in the same exact manner as required for a 1D bridge. A centerline station is required for both the upstream and downstream side of the bridge pier. The pier is formed by entering pairs of elevations versus pier widths, starting below ground, and continuing up past the low chord of the bridge deck. To learn more about 1D bridge modeling, refer to this article in our knowledge base.

The paired elevation and pier width values must be completed for both the upstream side and downstream side of the bridge. However, if the upstream and downstream sides are the same, then fill in the upstream side and use the [Copy to Downstream] button to copy the data to the downstream side (or vice versa).

The user can add abutments inside of the bridge opening that are different than the natural ground. For example, “Spill through Abutments” are abutments that have a slope and often a rounded or angled approach to guide the flow through the opening.

#### Sloping Abutments

Select the Sloping Abutments option from the 2D Roadway Crossing Specifications dropdown combo box.

The Sloping Abutments data panel will be displayed.

The Sloping Abutments data panel is the same as for 1D bridges and works the same way. The user can enter station and elevation data going from left to right and proceed accordingly for each consecutive abutment in order to modify the terrain through the bridge opening.

After entering the bridge deck/roadway and piers/abutments data, the 2D Roadway Crossing Plot section will display the bridge information graphically.

### Enter Manning’s Roughness for 1D Cross Section

The user can enter Manning’s n values for all of the 1D cross sections that are automatically formed as a result of the user entered bridge data.

To enter Manning’s n values for the 1D cross sections, select the Cross Section Geometry options from the 2D Roadway Crossing Specifications dropdown combo box.

The Cross Section Geometry data panel will be displayed.

In the above panel, the user must enter Manning’s n values for each of the four bridge cross sections (External upstream and downstream cross sections, internal face upstream and downstream cross sections). Manning’s n values are entered as horizontally varying values, starting with the very first station within the cross section. At least one n value must be entered for each cross section.

The user can modify the station/elevation data, Manning’s n values, and the main channel bank station locations. However, the length of the cross section must stay the same as what is spatially laid out from the bridge centerline data and other bridge information. The left and right main channel bank stations can also be changed. By default, they are set to the first and last point of each cross section.

### Geometry Bank Stations

This section defines the additional cross section geometry data.

The Bank Stations field defines the left and right bank stations. The defined bank station must match an existing ground station. The user can click the […] pick buttons under the Left and Right entry fields to select the left and right bank stations from either Map View or the cross section plot.

The Manning’s checkbox entry defines Manning’s n roughness values for left overbank, channel, and right overbank. Clicking the […] lookup button displays a Manning’s roughness lookup table. Unchecking this checkbox entry disables the underlying fields and enables the Horizontal Roughness column under the Cross Section Ground Geometry table.

### Extracting Cross Section Geometry

From the Extract Elevation Data section, the user can extract the cross section geometry at any time to account for changes in Roadway width and Distance from face to full valley values.

The Primary Elevation Data and Secondary Elevation Data panels are used to define the primary and secondary (if available in the project) elevation data sources for extracting the cross section geometry. Depending on the selected elevation data source type, the content of these panels changes to include the additional elevation data information.

When a secondary elevation data source is available, the software forms a concave hull around the primary elevation data source to identify its bounds. For locations where elevation data from the primary data source is unavailable, the software will use elevation data from the secondary data source.

Note that the user cannot utilize the same data source to define both the primary and secondary elevation data.

The user can click the [Swap Sources] button to swap the selected elevation source from primary elevation data to secondary elevation data and vice versa.

The software allows the user to extract the cross section geometry from each of the following cross sections associated with the bridge opening:

• Upstream full valley cross section
• Upstream internal face cross section
• Downstream internal face cross section
• Downstream full valley cross section

The user can check the respective checkboxes. Additionally, for upstream and downstream internal face cross sections, the user can elect to either extract the cross section geometry using the roadway edges or roadway centerline. By default, the Extract From Roadway Edge option is selected.

Once all the options have been defined, the user can click the [Extract Geometry] button to extract the cross section geometry.

### Defining Bridge Modeling Approach

To define which computational methods HEC-RAS will use at a bridge opening, select the Bridge Methodology option from the 2D Roadway Crossing Specifications dropdown combo box.

The Bridge Methodology data panel will be displayed.

The following sections are available in this data panel:

#### Support Pier Shape

This section allows the user to select the bridge pier shape. By default, the Select pier shape entry will be set to None for new bridges. However, the following options will be available:

• None (Default)
• Dual-Column Circular Piers
• Dual-Column Square Nose Piers
• Dual-Column Triangular Nose Piers
• Single-Column Circular Pier
• Wall-Type Round Nose Pier
• Wall-Type Square Nose Pier
• Wall-Type Triangular Nose Pier

When importing a HEC-RAS model, the software will automatically set the pier shape by reviewing the defined pier drag coefficient value. Based upon the defined pier shape, the software will render the bridge pier to match. Circular piers, square piers, and triangular nose piers are roughly the same—just the leading (upstream) edge looks different.

#### Low Flow Computational Methods

This section allows the user to instruct HEC-RAS to use any or all low flow computational methods by selecting the checkboxes under the Compute label. If the Momentum and/or Yarnell methods are selected, the user must enter pier loss coefficients corresponding to each method.

The following table lists the drag coefficient for different pier shapes:

Pier ShapePier Drag Coefficient CD
Circular pier1.20
Elongated piers with semi-circular ends1.33
Elliptical piers with 2:1 length to width 0.60
Elliptical piers with 4:1 length to width0.32
Elliptical piers with 8:1 length to width0.29
Square nose piers2.00
Triangular nose with 30° angle1.00
Triangular nose with 60° angle1.39
Triangular nose with 90° angle1.60
Triangular nose with 120° angle1.72

The following table lists the Yarnell K coefficient for different pier shapes:

Pier ShapeYarnell K Coefficient
Semi-circular nose and tail0.90
Twin-cylinder piers with connecting diaphragm0.95
Twin-cylinder piers without diaphragm1.05
90° angle triangular nose and tail1.05
Square nose and tail1.25
Ten pile trestle bent2.50

Once the user has selected the low flow bridge methods to be computed, a specific method muse be selected that will be used as the final answer to continue the computations upstream. Only one of the methods can be chosen as the answer, which is accomplished by selecting the corresponding radio button under the Use label to continue the computations upstream.

An alternative to selecting a single method is to instruct HEC-RAS to use the answer with the highest computed upstream energy elevation. This is accomplished by selecting the Highest energy answer radio button option under the Use label. By default, the software selects the Highest energy answer radio button option.

#### High Flow Computational Methods

This section allows the user to instruct HEC-RAS on how to compute high flows (flow at or above the maximum low chord elevation). For high flows, the user can choose between Energy only (Standard step) or Pressure and/or weir flow calculations.

If Pressure and/or weir flow is selected as the high flow method, the user must enter coefficients for the pressure flow equations. The first coefficient (Submerged inlet discharge coefficient) applies to the equation that is used when only the upstream side (inlet) of the bridge is submerged. If this coefficient is left blank, HEC-RAS selects a coefficient based on the amount of submergence. If the user enters a coefficient, that value is used for all degrees of submergence. The second coefficient (Submerged inlet & outlet discharge coefficient) applies to the equation that is used when both the upstream and downstream end of the bridge is submerged. By default, this coefficient is defined as 0.8.

The Pressure flow trigger elevation (optional) field is used to set the maximum elevation of the deck low chord and defines the elevation at which pressure flow calculations will begin. If this field is left blank, then the elevation that triggers pressure flow calculations is based on the highest low chord elevation on the upstream side of the bridge deck. If the user enters a value in this field, the entered value will be the trigger for pressure flow calculations to begin.

Pressure flow is triggered when the energy elevation exceeds the maximum low chord. When pressure flow is calculated, the answer is compared to the low flow answer, and the highest energy elevation of the two is selected. Alternatively, the user can tell the program to use the water surface elevation instead of the energy elevation to trigger pressure flow calculations.

### Defining Ineffective Flow Areas

The user can define ineffective flow areas for the upstream and downstream cross sections outside of the bridge. If the user has included the left and right roadway approaches as part of the bridge, then it may be necessary to define ineffective flow areas for the outside cross sections to compute accurate headwater and tailwater elevations for the bridge curves.

Select the Ineffective Flow Areas option from the 2D Roadway Crossing Specifications dropdown combo box.

The Ineffective Flow Areas data panel will be displayed.

This data panel is similar to the Ineffective Flow Areas data panel of the 1D Bridge & Culvert Data dialog box. Multiple Blocks Ineffective Flow Areas are not supported in 2D bridge modeling. Refer to this article in our knowledge base to learn how to define ineffective flow areas.

Note that this panel is disabled when the user selects Culvert as the structure type.

The user can adjust the roadway geometry at any time during the modeling.

Select the Geometry Adjustment option from the 2D Roadway Crossing Specifications section.

The Geometry Adjustment panel will be displayed.

Typically, this panel is used to revise the roadway geometry where there is insufficient terrain data available to adequately define the roadway geometry. The user can adjust the elevations or stations of roadway geometry as well as shift the horizontal stationing at a bridge. Additionally, the user can also define the extent of adjustments in the Adjustment Extent section.

## Defining HTAB Parameters

After the user has defined the necessary data for the roadway crossing structure, the user needs to define the parameters necessary to create the HTAB (hydraulic table of rating curves). To define these parameters, the user can select the Hydraulic Parameters – 2D Bridges & Culverts command from the Rating Curves – Hydraulic Parameters dropdown menu of the Analysis ribbon menu as shown below.

The Hydraulic Parameters – 2D Bridges & Culverts dialog box will be displayed.

In the above dialog box, the following parameters must be entered by the user:

• Number of points on free flow curve
• Number of submerged curves
• Number of points on each submerged curve

The Maximum tailwater elevation and Maximum flow parameters are optional. However, entering a maximum flow value is recommended as it will help control the limits of the connection hydraulic property table.

When the data is defined, the user can click the [OK] button to save the entered data and close the dialog box.

## Performing the Analysis

After the user has entered the roadway crossing data and has ensured that the 2D mesh and cell faces are well-formed around the bridge, the user must run the analysis by selecting the Compute Unsteady command from the Analysis ribbon menu.

When the analysis run is complete, the software will then generate a family of rating curves for any 2D bridge openings and then perform the 2D flow analysis.

## Viewing Output Results

Once the model has finished running, the user can begin to view the output related to the 2D bridge hydraulics.

The software provides several types of output results for 2D bridges, as described below:

1. Inundation maps including water surface, velocity, etc.
Refer to this article in our knowledge base to learn how to view inundation maps.
2. Profile line plots
Refer to this article in our knowledge base to learn how to view profile line plots.
3. Stage and flow hydrographs
Refer to this article in our knowledge base to learn how to view stage and flow hydrographs.
4. Cross section plots
Refer to this article in our knowledge base to learn how to view cross section plots.