To enter the pier scour data in the Hydraulic Toolbox press the Define button in the Define Pier Scour Parameters row of each pier.
The HEC-RAS Bridge Scour tool will automatically export the pier length, width, and angle of attack.

The three pier geometry factors in the pier scour editor come from three different locations in the 

For information on how to import the rest of the pier geometry see the section on Copying Your Bridge Data into the FHWA Toolbox in this tutorial.

  1. Selecting Hydraulic Variables for Pier Scour in the Channel 

The HEC-18 Pier Scour analysis requires two hydraulic parameters Depth (y1) and Velocity (V1) "Upstream" of the pier.
This can be confusing because there are two "approach" cross sections (reference lines) upstream of the pier.
The depth and velocity in the pier analysis are from the Pier Approach XS placed one pier length upstream of the Contraction (Bridge) cross section, NOT the Approach Cross Section in the fully expanded flow field.

The Hydraulic toolbox includes two approaches to pier scour "Thalweg" and "Local."
HEC-18 recommends "Thalweg" (Max) for channel pier design.
The thalweg method assumes that the thalweg can migrate across the channel, so it uses the hydraulics from the 2D cell with the maximum scour potential (unit discharge) for every pier in the channel.

The thalweg of the channel is usually the critical pier scour condition (Dmax*Vmax).
But the cross section will not necessarily maintain its shape during a scour event.
The thalweg or the critical scour condition can migrate laterally, especially during bridge scour.
FHWA recommends using the maximum, cross section scour condition for each pier based on the assumption that the thalweg or critical condition could migrate through any or all of the channel piers.
See discussion below for how this applies to floodplain piers.


Therefore, the Depth (y1) and Velocity (V1) upstream of the Pier should represent the maximum scour condition across the channel reference line.
The maximum scour condition is estimated with the unit discharge (V*D) to identify the critical V and D at a shared cell, not the highest V and D independently.
(Note: The beta version of the toolbox does not do this unit discharge analysis but returns the local maxes but has been fixed for the 7.0.1 release). 


Why use V and D associated with the max unit flow


The HEC-RAS bridge scour export tool automatically writes the local, channel Dmax and Vmax for all piers.

2. Pier Scour Parameters for Floodplain 

In the same way that HEC-18 considers thalweg migration to compute channel pier scour, the analysis also considers channel migration for floodplain piers. If the channel could migrate into the floodplain piers (e.g. Figure below), then use the max channel hydraulics for those floodplain piers as well.  But if the channel is not likely to migrate, then the maximum hydraulics from the overbank pier approach XS are appropriate.

In the example below, there are three piers (red) that are clearly associated with the channel.
They used the channel hydraulics from the pier approach XS.
Then, a geomorphic assessment found it unlikely that the channel would migrate into the three piers on the right (blue).
They used the hydraulics from the right overbank section of the pier approach cross section.
But the river channel could shift into the footprint of the other two piers (orange - dotted).
Therefore, these piers use the channel hydraulics even though they are officially in the overbanks.

HEC-RAS Export Tool (Beta) assigns channel hydraulics to ALL Piers

The current beta version (6.7 beta 5) of the tool associates the channel variables to all piers.
Use this display to identify the overbank hydraulics for the pier approach XS and associate them with the appropriate piers.

The final release will:

  1. Compute the depth and velocity from the max conveyance cell (instead of the independent max velocity and max depth) and 
  2. Export the parameters for the approach reference line for each selected pier, not just export the channel for all of them.


Pier scour results will depend on your mesh

Because the pier scour equations use maximum parameters, they are mesh dependent.  Finer meshes will usually identify a larger maximum unit flow (because they are averaging over a smaller area) and, therefore, more pier scour.

Meshing around piers or handling different pier shapes for these detailed hydraulics required for bridge scour analysis can be difficult in 6.7.

This page describes different approaches to developing a pier-sensitive mesh in 6.7/7.0.1, or preferably with improved tools in RAS2025, which can be imported back into 6.7/7.0.1, as well as how to handle non-standard pier shapes.

The main approaches are pictured below: