Using 2D HEC-RAS Results for Bridge Scour Analysis

This page is under active construction and should be ready for the final v6.7 release.

Nothing in this draft document should be considered official guidance.

Introduction

For years, the 1D bridge scour tools in HEC-RAS were widely used by US Department of Transportations.

But over the last few years, The US Federal Highway Administration (FHWA) released new bridge scour guidance (Hydraulic Engineering Circular No. 18 or HEC 18*) making the equations in the HEC-RAS version incomplete (no pressure flow or cohesive options) and - in some cases - obsolete.  Additionally, the FHWA started recommending 2D analysis for many bridges, making the 1D interface in HEC-RAS less useful.  Because HEC-RAS did not have pressure flow options in 2D bridges and did not have good methods to one-dimensionalize 2D model results until recently, it was difficult to use HEC-RAS for 2D Bridge Scour Analysis.

However, HEC-RAS version 6.7 includes new bridge algorithms that compute pressure flow and a robust "reference line" output system that averages hydraulic parameters over cross sections.  These features provide all the data users need to perform HEC-18 bridge scour analyses with 2D HEC-RAS output.  This guide demonstrates how you can set up reference lines to get the required data from a 2D hydraulic model in HEC-RAS and input those data into the FHWA Hydraulic Toolbox to compute scour.

*It is often confusing that tools from the Hydrologic Engineering Center (HEC) like HEC2, HEC-RAS, etc. have the same abbreviation as the FHWA's Hydraulic Engineering Circulars (HEC) like HEC 18.

The Bridge Scour Equations are Empirical, 1D Equations

While FHWA does recommend 2D hydraulic analysis for many bridge scour computations, it is important to remember that these are still, fundamentally, 1D equations.  The guide below will help you one-dimensionalize your 2D results to feed these post-processed 1D results into the HEC-18 algorithm.  However, in many cases, 1D results from HEC-RAS will also work well.  Carefully consider if the bridge hydraulics require 2D modeling to capture the scour-sensitive hydraulic parameters.

Conceptual Approach

Scour at Bridges has four components.  These components are additive and include:

1. Long term, reach scale, degradation (estimated outside of the local scour calculations and added)  
2. Contraction Scour 
3. Peir Scour
4. Abutment Scour

FHWA Hydraulic Toolbox

This tutorial will describe how to extract data from HEC-RAS to conduct an HEC-18 analysis using the FHWA's Hydraulic toolbox.  

You can download the toolbox here:
https://www.fhwa.dot.gov/engineering/hydraulics/software/toolbox404.cfm
(USACE users can download the toolbox on your ACE-IT machines with the AppPortal.
Some practitioners have developed their own tools and spreadsheets for this analysis.  The tutorial below should provide guidence on how to extract the correct data from HEC-RAS for any of these approaches, but we will focus on parameterizing the official Hydraulic Toolbox.

This toolbox performs many transportation related calculations, but we will only use the Bridge Scour tool in this tutorial.  Press the bridge scour button or go to Calcuators→ New Bridge Scour Analysis.   Then click on the Bridge Scour Analysis region node in the Project Explorer

Using Reference Lines to Access Transect Hydraulics in HEC-RAS

Reference Lines are a relatively new feature in HEC-RAS that allow users to query and summary hydraulic parameters (averages and maximums) from a specified transect.  They are different than "Profile Lines" in a several important ways. 

Most importantly, reference lines are part of the geometry and HEC-RAS writes summary results to them during the simulation.  The 2D results HEC-RAS writes to these reference lines intentionally includes the one-dimensionalized summary results that HEC-18 requires for bridge scour analysis (e.g. hydraulic depth, average velocity, max velocity, etc...).  Profile lines have some utility in Bridge Scour analyses, but most of the 2D HEC-RAS results you will use will come from Reference Lines.

Reference lines use more sophisticated logic than profile lines to project the fluxes (e.g. flow) and vectors (e.g. velocity) from cell faces to a summary transect.  But it is still worth recognizing that HEC-RAS only knows flux and vector properties at cell faces.  Therefore, the more that reference lines align with cell faces, the more accurate the summary data in the reference line will be.  Enforcing a break line (or an arc in 2025) is good practice to make align cell faces with a break line.

Laying out Reference Lines for 2D Bridge Scour Analyses

The FHWA HEC-18 analysis requires three cross sections, and each of these cross sections are divided into three sub-sections (channel, left overbank, right overbank). 
You will define:

• An Approach Cross Section: This cross section is upstream of the bridge where the river is at its widest in flood flows - occupying the maximum floodplain extent. 
• A Contraction Cross Section: This cross section follows the center line of the bridge and represents the narrowest flow constriction.
• An Upstream Face Cross Section:  Most of the scour calculations just use the approach and contraction cross sections, but a few components of the scur algorithms require summary data from a cross section just upstream of the bridge.  This is sometimes confused with the Approach cross section (and the same cross section was often used for both in 1D analyses) but is distinct.  The upstream face cross section is usually about one pier lengh (or bridge width) upstream of the upstream face of the bridge and is used to compute a maximum channel velocity and depth entering the bridge.  

To define a reference line, start editing the geometry.

Select the Reference Line node of the tree and digitize each cross section component independently (e.g. make the Approach Channel and Approach ROB adjacent but separate cross sections).  Digitize these reference lines from left to right looking downstream.  (Cross sections digitized in the opposite direction will work, but will report negative signed results).

Tip: It is useful to simulate the design flow before you define your reference lines to harvest results.  The inundation extents and trace lines can help you select the approach cross section location and lay out the cross sections perpendicular to flow.

Channels are seperated from "banks" by different criteria for bridge scour

Contraction Scour

A bridge induces contraction scour when the bridge reduces the cross-sectional area of the river ("contracting" the flow).  The river must push the same flow through a smaller area (Q=VA) which causes the water to accelerate through the bridge.  This contraction increases the river's potential to pick up and transport sediment through the bridge.  A bridge contraction is often lateral as the width of the river decreases through the bridge.  But if the flow increases to engage the low chord of the bridge (i.e. the bottom of the bridge deck) the bridge can also contract flow vertically, inducing pressure flow.  HEC-18 treats contraction scour differently with and without pressure flow, so we will also separate them below.

Contraction Without Pressure Flow

The Example Data Set Does Not Compute Much Scour

It is worth noting that while the example data set used in this model does include substantial contraction, the contraction is geologic.  The bridge itself does not introduce much additional contraction.  Therefore, the river already scoured substantially through the bridge opening - presumably before the bridge was built.  We selected this bridge because we had bathymetry and bridge deck info and it does demonstrate the data transfer process, but it does not compute much contraction scour.