1. Home
  2. Knowledge Base
  3. GeoHECRAS
  4. HEC-RAS Bridge Modeling

HEC-RAS Bridge Modeling

The GeoHECRAS software can model roadway crossing bridges and culverts by using the Bridge & Culvert Data command. The Bridge & Culvert Data command defines the bridge & culvert data, roadway deck geometry, bridge piers, sloping abutments, ineffective flow areas, bridge methodology & parameters, multiple opening stations, internal geometry, and more.

The user can define bridge data for 1D flow areas in the same way as defining bridge data for a 2D 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.

The following sections describe how to perform bridge modeling in GeoHECRAS.

Defining 1D Roadway Crossing Centerline

The roadway crossing centerline must be first defined to represent a bridge or culvert in a 1D model. The roadway crossing centerline must be drawn from left to right looking in a downstream direction.

To define the roadway crossing centerline, the user can use the Draw Roadway Crossings or Assign Roadway Crossings commands to either manually draw the roadway crossing centerline or assign an already existing polyline as the roadway centerline. Refer to this article in our knowledge base to learn more about the Draw Roadway Crossings or Assign Roadway Crossings commands.

1D Roadway Crossing Interior and Exterior Cross Sections

The software requires a minimum of four cross sections to be defined for modeling a roadway crossing. Two cross sections are defined at the downstream end of the roadway crossing structure. By contrast, the other two cross sections are defined at the upstream end of the roadway crossing structure. However, additional cross sections can also be defined upstream of the roadway crossing to account for additional backwater created upstream of the roadway crossing.

The two cross sections adjacent to the roadway crossing are placed in the full valley area adjacent to the roadway embankment. They represent the geometry of the river valley without the roadway embankment, which is the roadway fill area. These cross sections are called the Upstream Face Cross Section and the Downstream Face Cross Section since they are located very near the face of the roadway crossing structure.

The cross section drawn at the upstream end of the Upstream Face Cross Section is called the Approach Cross Section. This cross section is drawn upstream of the roadway crossing at a distance far enough where the impact of the flow contracting inward through the bridge or culvert opening is no longer felt. The cross section drawn downstream of the Downstream Face Cross Section is called the Exit Cross Section and is placed at a distance far enough in the downstream direction where the impact of the flow expanding outward is no longer considered significant.
1D Roadway Crossing Interior and Exterior Cross Sections

Entering Bridge Data

Once the 1D roadway crossing is defined, the user can enter the data for the 1D bridge in the Bridge & Culvert Data dialog box.

Follow the steps below to enter the bridge data:

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

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

Selecting Roadway Crossing

This section of the Bridge & Culvert Data dialog box is used to switch between roadway crossings. Note that if the user opens the Bridge & Culvert Data dialog box by double-clicking the roadway crossing on Map View, that roadway crossing will be selected by default.

This section contains the following entries:

  • River: This dropdown combo box shows all the currently defined rivers. When the user changes a river, then the Reach dropdown combo box also updates to show a valid corresponding river reach.
  • Reach: This dropdown combo box shows all the currently defined reaches that correspond to the selected river.
  • River station: This is an editable dropdown combo box that shows the currently defined roadway crossing river stations that correspond to the selected reach. The user can also change the river station of the current roadway crossing by clicking on the edit tool option adjacent to the River station dropdown entry or by clicking the […] pick button to graphically select the roadway crossing from the Map View.
  • Node name: This entry field allows the user to assign an optional text label to the current roadway crossing.

Roadway Crossing Specifications

This section of the Bridge & Culvert Data dialog box allows the user to select the various roadway specifications from the Roadway Crossing Specifications dropdown combo box. The Roadway Crossing Specifications dropdown combo box lists the following roadway specifications:

  • Deck Roadway
  • Culverts
  • Bridge Piers
  • Sloping Abutments
  • Ineffective Flow Areas
  • Bridge Methodology
  • Bridge Parameters
  • WSPRO Parameters
  • Multiple Openings
  • Internal Geometry
  • Build Bridge Opening
  • Opening Comparison Analysis
  • Geometry Adjustment
  • Geometry Point Reduction
  • Profile Results
    Roadway Crossing Specifications

Defining Deck Roadway Geometry

The Deck Roadway data panel is displayed by default when the Bridge & Culvert Data dialog box is displayed. This data panel provides a table for entering and editing the roadway high chord and bridge opening low chord geometry. The data in the table is used to describe the area that is blocked due to the roadway bridge deck, road embankment, and bridge opening vertical abutments.
Deck Roadway Geometry

To learn more about the Deck Roadway data panel, refer to this article in our knowledge base.

Defining Culverts

The Culverts data panel is used to define the culverts at a roadway crossing. Select the Culverts option from the Roadway Crossing Specifications dropdown combo box.
Select the Culverts option

The Culverts data panel will be displayed.
Culverts data panel will be displayed

To learn more about the Culverts data panel, refer to this article in our knowledge base.

Defining Piers and Sloping Abutments

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

Bridge Piers

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

The Bridge Piers data panel will be displayed.
Bridge Piers data panel

In the above panel, the user can enter the pier data in the same exact manner as required for a 2D bridge. To learn more about the Bridge Piers panel, refer to this article in our knowledge base.

Sloping Abutments

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

The Sloping Abutments data panel will be displayed.
Sloping Abutments data panel

The Sloping Abutments data panel is the same as for 2D bridges and works the same way. To learn more about the Sloping Abutments data panel, refer to this article in our knowledge base.

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 in order to compute accurate headwater and tailwater elevations for the bridge curves.

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

The Ineffective Flow Areas data panel will be displayed.
The Ineffective Flow Areas data panel

This data panel contains the following two types of ineffective flow areas:

Normal Ineffective Flow Areas

This section allows the user to define a left station and elevation and a right station and elevation for the ineffective flow area. Refer to this article in our knowledge base to learn more about Normal Ineffective Flow Areas.

Multiple Blocks Ineffective Flow Areas

This section allows the user to define up to 10 individual blocks for each cross section. Refer to this article in our knowledge base to learn more about Multiple Blocks Ineffective Flow Areas.

Defining Bridge Modeling Approach

The Bridge Methodology data panel allows the user to define which computational methods HEC-RAS will use at a bridge opening. The user can also select the bridge opening types and bridge pier shapes using this data panel.

Select the Bridge Methodology option from the Roadway Crossing Specifications dropdown combo box.
Select the Bridge Methodology option

The Bridge Methodology data panel will be displayed.
Bridge Methodology data panel

To learn more about the Bridge Methodology data panel, refer to this article in our knowledge base.

Defining Bridge Parameters

The Bridge Parameters data panel allows the user to define the bridge parameters to be used for all bridges (if multiple bridge openings) at a roadway crossing.

Select the Bridge Parameters option from the Roadway Crossing Specifications dropdown combo box.
Select the Bridge Parameters option

The Bridge Parameters data panel will be displayed.
Bridge Parameters data panel

To learn more about Bridge Parameters data panel, refer to this article in our knowledge base.

Defining WSPRO Parameters

The WSPRO Parameters data panel computes the water surface profile through a bridge by solving the energy equation. Select the WSPRO Parameters option from the Roadway Crossing Specifications dropdown combo box.
Select the WSPRO Parameters option

The WSPRO Parameters data panel will be displayed.
WSPRO Parameters data panel

To learn more about the WSPRO Parameters data panel, refer to this article in our knowledge base.

Defining Multiple Openings

The Multiple Openings data panel defines stagnation points for multiple opening roadway crossings. Select the Multiple Openings option from the Roadway Crossing Specifications dropdown combo box.
Select the Multiple Openings option

The Multiple Openings data panel will be displayed.
Multiple Openings data panel

To learn more about the Multiple Openings data panel, refer to this article in our knowledge base.

Defining Internal Geometry

The Internal Geometry data panel allows the user to edit the two-ground geometry cross sections inside of the bridge opening. This panel allows the user to change the station and elevation data, roughness coefficients, and main channel bank stations for each of the two internal bridge cross sections.

Select the Internal Geometry option from the Roadway Crossing Specifications dropdown combo box.
Select the Internal Geometry option

The Internal Geometry data panel will be displayed.
Internal Geometry data panel

To learn more about the Internal Geometry data panel, refer to this article in our knowledge base.

Defining Build Bridge Opening

The Build Bridge Opening data panel allows the user to define the geometry of the bridge opening by using span or abutment data. Select the Build Bridge Opening option from the Roadway Crossing Specifications dropdown combo box.
Select the Build Bridge Opening option

The Build Bridge Opening data panel will be displayed.
Build Bridge Opening data panel

To learn more about the Build Bridge Opening data panel, refer to this article in our knowledge base.

Bridge Opening Comparison Analysis

This data panel analyzes differences between bridge opening data for any two selected scenarios, and then exports the Opening Comparison Analysis results table.

Select the Opening Comparison Analysis option from the Roadway Crossing Specifications dropdown combo box.
Select the Opening Comparison Analysis option

The Opening Comparison Analysis dialog box will be displayed.
Opening Comparison Analysis dialog box

This data panel contains three sections: Select Existing Roadway Crossing for Comparison, Perform Opening Comparison Analysis, and Opening Comparison Results. To know more about this data panel, refer to this article in our knowledge base.

Adjusting Roadway Geometry

The user can adjust the roadway geometry at any time during the modeling process. Select the Geometry Adjustment option from the Roadway Crossing Specifications section.
Select the Geometry Adjustment option

The Geometry Adjustment data panel will be displayed.
Geometry Adjustment data panel

This data panel is similar to the Geometry Adjustment data panel of the 2D Bridge & Culvert Data dialog box. Refer to this article in our knowledge base to learn how to do the adjustment of the roadway geometry.

Geometry Point Reduction in Roadway Crossing

Select the Geometry Point Reduction option from the Roadway Crossing Specifications section.
Select the Geometry Point Reduction option

The Geometry Point Reduction data panel will be displayed.
Geometry Point Reduction data panel

Refer to this article in our knowledge base to learn more about the Geometry Point Reduction data panel in roadway crossing.

Defining Profile Results

The Profile Results data panel allows the user to select the analysis results. Select the Profile Results option from the Roadway Crossing Specifications section.
Select the Profile Results option

The Profile Results data panel will be displayed.
Profile Results data panel

To learn more about the Profile Results data panel, refer to this article in our knowledge base.

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). Note that this data is only used for unsteady flow simulations and is ignored for steady flow simulations. To change the flow data from steady to unsteady flow data, go to the Input ribbon menu, click the Scenario Manager dropdown menu, and then select the Scenario Manager command. The Scenario Manager dialog box will be displayed.
Scenario Manager dialog box

In the Scenario Manager dialog box, enable the Unsteady Flow Data radio button and add new unsteady flow data from the Unsteady flow dropdown combo box. Close the Scenario Manager dialog box and the current scenario flow data is now changed to unsteady flow.
Close the Scenario Manager dialog box

The user can now select the Hydraulic Parameters – Bridges & Culverts command from the Rating Curves – Hydraulic Parameters dropdown menu of the Analysis ribbon menu to define the parameters.
Hydraulic Parameters – Bridges & Culverts command from the Rating Curves – Hydraulic Parameters dropdown

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

Selecting Roadway Crossing

The Select Roadway Crossing section allows the user to select the roadway crossing for purposes of defining the hydraulic parameters. This section contains three entries: River name, Reach name, and Roadway crossing river station.

Family of Rating Curve Parameters

The Family of Rating Curve Parameters section is used to define the limits of the family of rating curves that are developed for the roadway crossing. This section covers the following parameters:

  • Number of points on free flow curve
  • Number of submerged curves
  • Number of points on each submerged curve
  • Use above parameters on all roadway crossings

Roadway Crossing Parameters

This section defines the parameters necessary to compute the rating curves for the selected roadway crossing. This section covers the following parameters:

  • Maximum headwater elevation
  • Maximum tailwater elevation, optional
  • Maximum flow, recommended

The Maximum headwater elevation defines the maximum elevation that the water surface can reach on the upstream side of the roadway crossing. Enter the maximum elevation that you expect the water surface to reach on the upstream side of the bridge in the Maximum headwater elevation entry field. Click on the […] button next to the Maximum headwater elevation entry field to pick the elevation from the roadway crossing plot. To learn more about the maximum headwater parameter settings, refer to this article in our knowledge base.

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 hydraulic property table at the roadway crossing.

To learn more about the various parameters provided in this dialog box, refer to this article in our knowledge base.

When the data are 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, the user must run the analysis by selecting the Compute Unsteady command from the Analysis ribbon menu.
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 1D bridge openings and then perform the 1D flow analysis.

Viewing Output Results

Once the model has finished running, the user can begin to view the output related to the 1D bridge hydraulics.

The software provides several types of output results for 1D 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. Stage and flow hydrographs
    Refer to this article in our knowledge base to learn how to view stage and flow hydrographs.
  3. Cross section plots
    Refer to this article in our knowledge base to learn how to view cross section plots.

About the Author cxscvlgeo

  • Was this helpful?
  • YesNo

Was this article helpful?

Related Articles