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.

This guide aims to help users fulfill the requirements of the Federal Highway's HEC 18 (5th edition) analysis:
Evaluating Scour at Bridges, Fifth Edition

Practitioners evaluating scour must be proficient with that guidance and those methods.
This guide will help users extract hydraulic data from HEC-RAS for these analyses and some operational tips on how to do these studies, but nothing in this guide supplants or supersedes anything in the Federal Highway guidance.

Automated Tool Coming in version 6.7

This guide walks users through the process of manually extracting hydraulic variables from HEC-RAS and using them in the FHWA Hydraulic Toolbox.
HEC-RAS 6.7 will have an automated tool that reads the hydraulic variables from the reference lines and writes them to a Toolbox Input File
We will integrate the use of that tool into this guide but will keep the framework of this guide with tips and best practices to include both manual input and the automated tool because it is important to understand processes that software automates.

Introduction

For years, the 1D bridge scour tools in HEC-RAS were widely used by US Department of Transportations.  But in 2012 the US Federal Highway Administration (FHWA) released new bridge scour guidance (Hydraulic Engineering Circular No. 18, Fifth Edition or HEC 18*), making the equations in the HEC-RAS version incomplete (no pressure flow or cohesive options) and - in some cases - obsolete.  Federal Highway no longer recommends using the Hydraulic Design tool in HEC-RAS for analyses.  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 cross-sectionally average 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.

*Note: 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 equations do use average hydraulic properties for the channel and the overbanks.  The guide below will help you average your 2D results over these three conveyance regions, to feed these post-processed into the HEC-18 algorithm.  However, the 1D results from HEC-RAS also report average results in these three conveyance regions and may still be appropriate in some cases.  Carefully consider if the bridge hydraulics require 2D modeling to capture the scour-sensitive hydraulic parameters.

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. Pier Scour
4. Abutment Scour

The HEC18 Bridge Scour approach computes each of these in turn and then adds them across the cross section.

Steps

Follow the steps below to use HEC-RAS 2D with the FHWA Hydraulic Toolbox: