HEC-RAS : Bridge Models

  HEC-RAS : Bridge Modeling

Written by Andy Carter,PE | October 13, 2020

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Know the Lingo

When it comes to bridge modeling in HEC-RAS, there are a few items that you need to take into consideration:

• Station Control:  All bridges inside of HEC-RAS are a based upon a alignment layout.  Think of it like a cross section.  You have a horizontal alignment where a station (X value) has a corresponding elevation (Z value). 

• Bounding Cross Sections:  Cross sections both upstream and downstream of the structure to be modeled are necessary.  To make modeling easier, it is recommended that both of these cross sections be the same length and therefore have the same station control.

• High Chord:  The 'high chord' is the top of the bridge to be modeled.  For an existing bridge, often LiDAR is available to help model these station-elevation pairs, which is typically the roadway elevation, rail road or other similar item.  Sometimes the high chord should be modeled to include a concrete barrier, hand rail or metal beam guardrail.

• Low Chord:  The 'low chord' is the bottom of the bridge.  These are station-elevation pairs modeled together with the high-chord.  Often the low chord represents the bottom of the girder section or the bottom of the I-beams.

• Piers:  Between each bridge span a pier can be modeled.  These are entered along the station control at width-elevation pairs.  I typically model the piers at an elevation of zero and ending at an elevation between the high and low chord.  Often the bents capping the piers are wider than the piers and are modeled as part of the pier.

• Abutments:  At the end of bridge span, bridges have abutments.  If this abutment is vertical it can be modeled with geometric points of the low chord.  It the abutment is sloping, it can be modeled in HEC-RAS with station elevation-pairs along the control alignment.  This is done to make sure that the conveyance (flow) area under the bridge is correctly modeled.

• Ineffective Flow Areas  As water flows though a bridge, it accelerates and funnels into to area under the bridge.  As water flows out from under a bridge, the water spreads out and expands back into the full conveyance area of the floodplain.  While water moves quickly through the bridge opening, areas of the upstream and downstream bounding cross sections that are not adjacent to the bridge opening may be wet but have little to no velocity or conveyance.  Around a bridge, it is necessary to model ineffective flow areas so that HEC-RAS knows areas that are likely to get wet but have no velocity or flood conveyance.

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A video tutorial

Wow! It seems like there are several items to manage when modeling a bridge.  How to keep all of them straight?  I have added a video that walks through the whole bridge modeling process.  Enjoy.

Video Walkthrough (27 minutes)

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