By: Gibson, Ph.D.
Flood risk is not just about floods escaping banks and levees. Some of the most dangerous flood damage happens where the flood flows accelerate through bridges. When high flows encounter fixed in-channel infrastructure such as bridge constrictions, piers, abutments, and embankments, the flow accelerates and redirects, scouring the channel bed and banks. These local scour processes can undermine the very piers and abutments that create them, sometimes leading to bridge failure.
The Federal Highway Administration’s (FHWA) Hydraulic Engineering Circular No. 18 (HEC-18) has been the industry standard for bridge scour analysis for decades, and for much of that time the one-dimensional (1D) HEC-RAS bridge scour calculator was the primary tool engineers used to apply those equations. But two developments in recent years made that workflow outdated: FHWA released a new set of equations and began recommending two-dimensional (2D) hydraulics for bridge scour analysis.
HEC has been working with FHWA to incorporate a 2D bridge scour workflow, using the latest equations, into HEC-RAS 7.0, the final feature release of this series.
Figure 1: The 2D Bridge Scour tool in RASMapper
HEC-RAS 7.0.1 will include a 2D bridge scour tool that is compliant with the latest FHWA guidance. The tool allows users to define nine reference lines that extract and average the 2D hydraulic variables required by HEC-18. RAS Mapper then writes an input file for the FHWA Bridge Scour Toolbox directly from the HEC-RAS results. This workflow is intended to help keep HEC-RAS analyses aligned with current FHWA methods, even if a new HEC-18 manual is released in the next 18 months. It also should make 2D bridge scour analysis simpler and more repeatable within the hydraulic model many engineers already use for the rest of their work.
Figure 2: Bridge scour results from the FHWA hydraulic toolbox base on 2D HEC-RAS inputs and bridge deck geometry.