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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.

 

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Comments

Hey There,
Thanks for the post. I was wondering why in RAS the flow through the bridge opening is reduced when a bridge is over topped. I am running steady state RI flows and at low design flows (i.e. 122 cfs) the bridge opening passes the flow, seen in the "bridge Output Table". But at higher flows (i.e. 462 cfs) the flow through the "Q bridge" flow is less than the 122 cfs which it was originally passing. Does this have to do with RAS switching equations from an orifice flow equation to a weir + orifice flow? Is it correct to say at a higher RI the bridge is conveying less flow through the opening?
Thank you.
Tim Bedford

Hi Tim,

It’s tough to say exactly what’s happening without knowing the specifics of your case here, but what follows are some general ideas for you to think about. When you have flow overtopping the bridge, HEC-RAS is iteratively balancing the equations for the energy required in each equation to convey that total flow.

To determine if you are actually seeing lower flow through the bridge opening in the higher return interval events, you’ll want to make sure that the ineffective flow areas upstream and downstream of the bridge are appropriately placed. For example, the ineffective flow areas should be set just below the lowest overtopping return interval flow in your simulation. Additionally, the bridge modeling approach makes a difference in the calculations. There is also a difference in the equation being used for pressure flow depending on whether your bridge is flowing full or not.

There is some great material regarding Bridge Modeling in Chapter 5 of the HEC-RAS Hydraulic Reference Manual – www.freese.com/sites/default/files/files/HEC-RAS_4.1_Reference_Manual.pdf. A discussion of situations where particular methods are applicable begins on page 5-26 of that document.

Feel free to leave another comment or reach out to us at water@freese.com if you have other questions.

Jeremy

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