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Publication Number:  FHWA-HRT-17-054    Date:  October 2017
Publication Number: FHWA-HRT-17-054
Date: October 2017


Advanced Methodology to Assess Riprap Rock Stability At Bridge Piers and Abutments

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The following changes were made to the document after publication on the Federal Highway Administration website:

Location Corrected Values URL
Page 2

Editorial Correction
Added source information.
Source: Modified by FHWA from
NCHRP report 568. See
Acknowledgment page.
Page 10

Editorial Correction
In addition, rocks around the upstream
corner of the first abutment were
Page 88

Editorial Correction
Added reference 43. Lauchlan, C.S. (1999).
Pier Scour Countermeasures, Ph.D. Thesis,
University of Auckland, Auckland, NZ.
Page 89

Editorial Correction
Added Acknowledgment page. Figure 1 was
modified by FHWA from figure 2.5 of
NCHRP report 568, which in turn was
modified from an original graph in C.S.
Lauchlan's thesis, "Pier Scour



Riprap is one of the most common materials used to protect bridge abutment and pier foundations from scour. A key element of the design of riprap countermeasures is rock sizing, which is based on equations generally derived from simplified laboratory experiments. In this study, an advanced modeling approach is developed and applied to evaluate rock stability. The advantage of this approach is that it can incorporate site-specific conditions that complicate riprap design. This report describes this advanced, numerical modeling procedure for analyzing the stability of riprap at bridge abutments and piers. The report will be useful for designers and engineers responsible for protecting bridge foundations. The study described in this report was conducted at the Federal Highway Administration’s (FHWA) Turner-Fairbank Highway Research Center (TFHRC) J. Sterling Jones Hydraulics Laboratory.

Cheryl Allen Richter, P.E., Ph.D.
Director, Office of Infrastructure
Research and Development


This document is disseminated under the sponsorship of the U.S. Department of Transportation (USDOT) in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.


Technical Report Documentation Page

1. Report No.


2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Advanced Methodology to Assess Riprap Rock Stability at Bridge Piers and Abutments

5. Report Date

October 2017

6. Performing Organization Code
7. Author(s)

Oscar Suaznabar, Cezary Bojanowski, Steven A. Lottes, Jerry Shen, Kornel Kerenyi, and Roger Kilgore

8. Performing Organization Report No.


9. Performing Organization Name and Address

11848 Rock Landing Drive, Suite 303
Newport News, VA 23606

10. Work Unit No. (TRAIS)

11. Contract or Grant No.


12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

Pooled Fund Study Report
October 2011–September 2016

14. Sponsoring Agency Code


15. Supplementary Notes

The Contracting Officer's Representative (COR) was Kornel Kerenyi (HRDI-40).

16. Abstract

The objectives of this research study were to: (1) assess whether detailed fluid structure interaction (FSI) modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons; and (2) develop recommendations for the design, installation, and monitoring of riprap aprons at bridge piers and abutments, where feasible.


A new advanced computational methodology for assessing failure risk of geometrically complex riprap installations was developed for this study. The study demonstrated that detailed FSI modeling can inform evaluation of rock riprap movement for both the analysis of existing riprap aprons and for the design of new riprap aprons. The approach solved the FSI problem for the onset of rock riprap motion using weakly coupled computational fluid dynamics and computational structural mechanics software. The flow threshold for the onset of motion of riprap rocks was computed for a set of representative riprap rocks for both simplified laboratory and complex field conditions. Physical laboratory experiments were used to validate the numerical procedures. The FSI approach was also tested on a complex field case study of a riprap installation at a pier for a bridge over the Middle Fork of the Feather River. While the case study application was considered successful, the approach is limited by its high costs and limited availability. Therefore, good candidate applications for using FSI analysis to assess new or retrofit riprap installations would be those where the project cost is significant or the risks of failure are catastrophic.


The study also identified recommendations for improving the design, installation, and monitoring of riprap apron installations at bridge piers and abutments, where feasible. These included: (1) verifying as-built conditions for assuring that the intended level of protection has been achieved, (2) inspecting for changes in stream morphology that may significantly change conditions from those anticipated at design, (3) recording the date of rock riprap installations and monitoring the performance of the installations after major floods, (4) applying sonar technologies for riprap monitoring, and (5) avoiding rock riprap installations for new bridge piers as they are not recommended by FHWA policy.


The FSI numerical modeling approach has promise for supporting the design and evaluation of riprap installations for bridge abutments and piers. As computer capabilities increase and more detailed representations of rock riprap installations become more practical, the approach should continue to increase in its utility.

17. Key Words

Riprap, countermeasure, bridge pier, bridge abutment, fluid-structure interaction, computational fluid dynamics, computational structural mechanics, numerical modeling, flume modeling of riprap

18. Distribution Statement

No restrictions.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price
Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors













Figure 1. Graph. Comparison of riprap sizing curves at a rectangular pier
Figure 2. Illustration. FHWA tilting flume
Figure 3. Photo. Automated flume carriage in the TFHRC Hydraulics Laboratory
Figure 4. Photo. ADV probe
Figure 5. Sketch. Plan view of the test section
Figure 6. Sketch. Cross-section view of the test section
Figure 7. Photos. Riprap installation
Figure 8. Photo. Riprap apron installation for testing
Figure 9. Photos. Shear failure sequence for riprap apron installed flush with channel bed
Figure 10. Photos. Shear failure sequence for riprap installed on a slope against the abutment face
Figure 11. Graphic. Bathymetry of the riprap apron after failure in isometric view
Figure 12. Photo. After rock shear failure at the upstream corner of the abutment
Figure 13. Sketch. Plan view prototype domain
Figure 14. Sketch. Cross-section view of the prototype domain
Figure 15. Graphics. Rock layout in the CFD model
Figure 16. Graphics. Positions of movable rocks
Figure 17. Graphic. Velocity profile in a horizontal slice just above the riprap rocks with an inlet velocity of 4.27 ft/s (1.3 m/s)
Figure 18. Graphic. Location of the rocks with the highest forces in a CFD analysis
Figure 19. Graph. Initial stabilization of vertical forces on movable rocks Figure 20. Graphic. Definition of forces on a single rock
Figure 21. Graphics. FSI simulation for an inlet velocity of 4.27 ft/s (1.3 m/s)
Figure 22. Graphics. FSI simulation for an inlet velocity of 4.59 ft/s (1.4 m/s)
Figure 23. Graphics. FSI simulation for an inlet velocity of 4.92 ft/s (1.5 m/s)
Figure 24. Photo. Historic, realigned, and current channel alignment
Figure 25. Photo. River bathymetry before installation of the riprap
Figure 26. Photo. Rock layout design for protection of pier 3
Figure 27. Photo. Riprap installation near pier 3 (August 2012)
Figure 28. Photo. River bathymetry in 2013 after riprap installation
Figure 29. Schematic. CFD model surface characterization
Figure 30. Graphic. CFD model boundaries
Figure 31. Graphic. Surface mesh of the riverbed in CFD model
Figure 32. Graphic. Cross-section through the finite volume mesh used for the CFD model
Figure 33. Graphic. Cross-section through the subregion used in the FSI computations
Figure 34. Drawing. Design drawing (typical section) of riprap around pier 3
Figure 35. Drawing. Design drawing (section B-B) of riprap around pier 3
Figure 36. Graphic. Geometry of the riprap used in the CFD model
Figure 37. Image. Sonar image of pier 3 with installed riprap
Figure 38. Graphic. The extent of the riprap derived from the sonar bed scan
Figure 39. Graphic. Extent of riprap in the updated CFD model with movable rocks
Figure 40. Graphic. Placement of movable rocks around the pier
Figure 41. Graph. Depth-averaged velocity estimates under the bridge (looking upstream)
Figure 42. Graphic. Location of the rocks with the highest forces in CFD analysis
Figure 43. Graphic. Water surface (velocity overlain) for condition of 1.6 times the 100-year flood
Figure 44. Graphic. Velocity vectors on the interface between the FSI subregion and the CFD domain
Figure 45. Graphic. Velocity vectors on the plane just above the rocks
Figure 46. Graphics. FSI simulation for the 100-year discharge
Figure 47. Graphics. FSI simulation for 1.1 times the 100-year discharge
Figure 48. Graphics. FSI simulation for 1.2 times the 100-year discharge
Figure 49. Schematics. Stages of FSI mesh morphing
Figure 50. Sketch. Definition of the domains for FSI analysis of riprap stability
Figure 51. Flowchart. Weak FSI coupling scheme
Figure 52. Flowchart. Strong FSI coupling scheme
Figure 53. Flowchart. Implementation of coupling workflow between STAR-CCM+ and LS-DYNA
Figure 54. Schematics. Stages in the coupled simulation for water induced rock motion
Figure 55. Graphic. Rock representation with feature curves created on all edges of the mesh
Figure 56. Graphic. Surface triangulated representation of a rock
Figure 57. Graphics. Test CFD models
Figure 58. Equation. Laursen’s equation for critical velocity
Figure 59. Equation. Neill’s equation for critical velocity
Figure 60. Graphics. Time series for layout 1 at an inlet velocity of 9.8 ft/s (3.0 m/s)
Figure 61. Graphics. Time series for layout 2 at an inlet velocity of 9.8 ft/s (3.0 m/s)
Figure 62. Schematic. CFD domain and grid for analysis of flexible plate protruding into the flow setup for FSI analysis coupling with LS-DYNA
Figure 63. Schematics. Morphed mesh at maximum plate deflection
Figure 64. Graphs. Plate deflection



Table 1. Flow conditions for flume experiments
Table 2. Forces on ten movable rocks with varying inlet velocities
Table 3. Comparison of physical experiments and computational simulations
Table 4. Comparison of 3D and 2D modeling results for the 100-year discharge
Table 5. Forces on the critical rocks



AASHTOAmerican Association of State Highway and Transportation Officials
ADVAcoustic Doppler Velocimeter
ALEArbitrary Lagrangian-Eulerian
ASCEAmerican Society of Civil Engineers
CaltransCalifornia Department of Transportation
CD-AdapcoComputation Dynamics—Analysis and Design Application Company
CFDcomputational fluid dynamics
CSMcomputational structural mechanics
CURCenter for Civil Engineering Research and Codes (translated from Dutch)
DGDesign Guideline
DOTDepartment of Transportation
FHWAFederal Highway Administration
FSIfluid structure interaction
GPSGlobal Positioning System
HECHydraulic Engineering Circular
LSTCLivermore Software Technology Corporation
NCHRPNational Cooperative Highway Research Program
NHINational Highway Institute
NTSBNational Transportation Safety Board
NYSTANew York State Thruway Authority
RANSReynolds-Averaged Navier-Stokes
RSProck slope protection
RWSRijkswaterstaat, Dutch Public Works Department (translated from Dutch)
TFHRCTurner-Fairbank Highway Research Center
TRACCTransportation Research and Analysis Computing Center
TRBTransportation Research Board
USGSUnited States Geological Survey
VOFvolume of fluid
D50median particle (rock) size (ft (m))
FrFroude number (dimensionless)
imiteration index (dimensionless)
KUunit conversion constant (varies with each equation)
l1abutment length (ft (m))
l2abutment width (ft (m))
mnumber of structural solver time steps within a fluid solver time step (dimensionless)
ntime step index (dimensionless)
triprap apron thickness (ft (m))
V1approach velocity (ft/s (m/s))
V2contracted section velocity (ft/s (m/s))
VCLcritical velocity estimated from Laursen’s equation (ft/s (m/s))
VCNcritical velocity estimated from Neill’s equation (ft/s (m/s))
W1approach flow width (ft (m))
W2contracted (bridge opening) width (ft (m))
WRriprap extent (ft (m))
yflow depth (ft (m))
y1approach flow depth (ft (m))
y2contraction flow depth (ft (m))
δtffluid solver time step (s)
δtsstructural solver time step (s)
Γboundary condition (dimensionless)
Ωffluid occupying space (dimensionless)
Ωssolid body occupying space (dimensionless)




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Turner-Fairbank Highway Research Center | 6300 Georgetown Pike | McLean, VA | 22101