Bridge Modeling in HEC-RAS

In riverine systems, bridges typically represent an obstruction to channel and floodplain flow. The obstruction to flow can be a source of substantial energy loss. In order to appropriately model the energy loss due to a bridge in HEC-RAS, there are some things you should know.

Bridge modeling in HEC-RAS has several components:

• Bounding cross sections
• Ineffective flow areas
• Bridge opening

This post provides the information necessary to model a simple bridge using HEC-RAS.

Bounding Cross Sections

Bounding cross sections are a crucial aspect of bridge modeling in HEC-RAS. The bounding cross sections provide a means of analyzing the impacts of the bridge structure on the flow.

• The bridge routines in HEC-RAS use 4 cross sections to compute energy losses due to the structure.
• There are 2 cross sections upstream and 2 cross sections downstream of the bridge.
• Bounding cross sections should represent the natural ground of the main channel and overbanks, and generally should not include embankments.

A plan view of a bridge and bounding cross sections from the HEC-RAS Hydraulic Reference Manual is shown in the figure below.

The Contraction Reach (from Cross Section 4 to Cross Section 3):

• Represents the area where flow is contracting from the channel (and overbanks) into the bridge opening.
• The additional losses associated with the Contraction Reach are represented with the contraction coefficient at Cross Section 3.
• Cross Section 4 represents the channel before the flows are contracted. The bridge opening does not affect this cross section.

The Expansion Reach (from Cross Section 2 to Cross Section 1):

• Represents the area where flow is expanding from the bridge opening back to the channel (and overbanks).
• The losses associated with the Expansion Reach are represented with the expansion coefficient at Cross Section 2.
• Cross Section 1 represents the channel where the flows are already expanded. The bridge opening does not affect this cross section.

Ineffective Flow Areas

Ineffective flow areas are used in HEC-RAS to represent areas where flow is not being conveyed. Modeling bridges generally requires ineffective flow areas because the profile of the bridge is typically an obstruction to flow in the overbanks, or possibly within the channel itself. In these cases, ineffective flow areas are defined to ignore (for conveyance calculations) the areas where water is being stored and not conveyed.

Typically, it is appropriate to use a ratio of 1:1 for the ineffective flow area contraction (CR) and a ratio from 2:1 to 4:1 for the ineffective flow area expansion (ER). The ratio is distance in the direction of flow compared to distance perpendicular to the direction of flow.

Defining the elevation of ineffective flow areas at bridges can potentially require a bit of work, but is straightforward.

• The ineffective flow areas at upstream sections should be no higher than the lowest road elevation.
• At downstream sections, ineffective flow areas should be defined lower than any overtopping profile, but higher than all other profiles.
  • Start by defining the downstream ineffective flow area halfway between the low chord and high chord on each side of the bridge.
  • Adjust the elevation of the ineffective flow area until the lowest overtopping profile is overtopped in the downstream cross section.
  • If the bridge is overtopping, flow in the overbanks will likely contribute to the total conveyance.

The Bridge Opening

The bridge opening is defined by the upstream and downstream cross sections intersected with:

•  The Deck/Roadway
• Piers
• Abutments

The deck or roadway of the bridge is defined similar to a cross section, with a station, high chord, and low chord. If no low chord is defined, the bridge will extend straight down to the ground surface. The Deck/Roadway menu is also where the bridge width and distance to the upstream cross section can be input. At this menu you can also add the type of weir crest that will define how flows are treated once the road is overtopped.

Piers can be added based on the centerline of the pier at the upstream and downstream sections. Piers are drawn from the ground to the first point with the specified pier width at a certain elevation.

In the example below, the pier is 1 foot wide from the ground level to elevation 50, and transitions to 3 feet wide at elevation 63

Abutments are defined with station and elevation data from left to right.

In the example below, the left abutment is Abutment #1.

Recap

• Bridges should have two cross sections upstream and two cross sections downstream with contraction and expansion coefficients of 0.3 and 0.5, respectively.
• The bounding cross sections should have ineffective flow areas that are below the top of the deck upstream of the bridge and below the lowest overtopping profile downstream of the bridge.
• Bridges should have deck/roadway data that describes the elevation of the crest of the bridge, the lowest chord, the width, and the distance to the upstream cross section.
• Bridges may or may not have piers and/or abutments.

Additional Resources

Some additional resources on bridge modeling are listed in the table below.