Using 2D HEC-RAS Results for Bridge Scour Analysis

Bug in 7.0

The 2D Bridge Scour export workflow is available for testing in HEC-RAS 6.7 Beta 5. A last-minute issue prevented the feature from being active in the initial 7.0 release. Until the 7.0.1 fix release is available, use 6.7 Beta 5 to explore and test these capabilities. The workflow described below documents the intended 7.0.1 workflow which is very similar to the current 6.7 Beta 5 behavior.

This guide aims to help users fulfill the requirements of the Federal Highway Administration's (FHWA) 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 offer some operational tips on how to do these studies, but nothing in this guide supersedes anything in the Federal Highway guidance.

Automated Export Workflow

HEC-RAS 6.7 Beta 5/7.0.1 includes a tool that automatically exports the required 2D hydraulics to the FHWA toolbox.  This tutorial guides you through the use of that tool.  However, we have also documented how to manually transfer results from HEC-RAS reference lines to the toolbox and have annotated the *.hyd file with its associated HEC-RAS inputs.  We kept this legacy documentation because it may be useful to those annotating customized spreadsheets or other analyses.  Follow the above links to access these legacy tutorials.

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 these 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, current versions of HEC-RAS include new 2D 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 and a tool to help users extract the necessary data in the format required by the FHWA toolbox.  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.

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 accumulates them across the cross section.

QuickStart Guide

Detailed instructions are included below, but this quick start guide provides an overview of how to use HEC-RAS to export 2D hydraulic results to the FHWA Hydraulic Toolbox to compute bridge scour. 

Detailed Modeling Steps

While exporting data from HEC-RAS and importing it into the Hydraulic toolbox is relatively simple, you will have to make some modeling decisions that will affect your results.

The detailed modeling steps below describe the engineering context and best practices involved in using these tools to perform a bridge scour analysis.