Scour Program Management

Risk-Based Approach for Bridge Scour Prediction: Applications for Design

  • Paul Clopper (Ayres Associates)
  • P. F. Lagasse
  • L. W. Zevenbergen


NCHRP Project 24-34 was completed in September 2013 with the publication of NCHRP Report 761, "Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction." The project accomplished its objectives of developing risk/reliability-based methodologies that can be used in calculating bridge pier, abutment, contraction, and total scour at waterway crossings so that scour estimates can be linked to a probability. The developed probabilistic procedures are consistent with LRFD approaches for bridge design used by structural and geotechnical engineers.

As a necessary first step in developing the statistical parameters for risk and reliability analyses, the key bridge scour equations (HEC-18 pier scour, Florida DOT pier scour, HEC-18 contraction scour, and NCHRP 24-20 abutment scour) were tested against available laboratory and field data sets. The results which are summarized in this presentation include:

  • · The pier scour equations (HEC-18 and FDOT) are design equations which do not under predict observed scour very often. Consequently, the probabilistic reliability indexes for pier scour compare favorably with those used by structural and geotechnical engineers in LRFD applications for bridges.
  • · In contrast with the pier scour equations, the HEC-18 contraction scour equations are essentially predictive, given that they are derived from sediment transport principles and theory. Therefore, under predictions of observed scour are much more common, and the resulting reliability is very low compared to typical target values used in LRFD applications.
  • · The NCHRP 24-20 equations for live-bed and clear-water abutment scour both use a calculation for contraction scour and then apply an amplification factor to account for the additional scour caused by local effects at the tip of the abutment. The scour predicted by this method is the total scour at the abutment. The reliability of the abutment scour equations was found to be intermediate between those of the pier scour and contraction scour equations.

This presentation will be followed by Part II which will provide an example application of the basic risk-based methodology in a typical design situation.

How to Cite:

Clopper, P. & Lagasse, P. F. & Zevenbergen, L. W., (2014) “Risk-Based Approach for Bridge Scour Prediction: Applications for Design”, National Hydraulic Engineering Conference 2014 1(2014).

Rights: Copyright © 2014, P.E. Clopper, P.F. Lagasse, and L.W. Zevenbergen

Publisher Notes

  • Panel moderated by Dave Claman, IOWADOT.
  • About the Presenters: Paul Clopper is the Director of Applied Technology in the Fort Collins, Colorado office of Ayres Associates. His training and experience are in the fields of surface water and ground water hydrology and hydraulics, river mechanics, bridge scour, and erosion/sediment control. He has 34 years of experience in consulting engineering, joining Ayres Associates in 1992. Paul is responsible for planning, coordination, technical conduct, and management of a variety of projects involving hydrologic and hydraulic data collection and analysis, water resource assessment and characterization, and engineering design. Typical projects have included: • Bridge scour and stream stability countermeasure performance, selection, and design • Applied research in erosion control product performance • Development of adult learning materials, including course curricula and web-based training. • Surface water and groundwater monitoring networks • Levee and roadway overtopping protection – applied research • Levee foundation seepage analysis Since 2007, Paul has been an active member of Transportation Research Board (TRB) Committee AFB60, “Hydrology, Hydraulics, and Water Quality.” He currently serves as the Chairman of the Hydraulics Subcommittee of AFB60. Paul is currently working on two on-going NCHRP research projects: • Co-Principal Investigator for NCHRP Research Project 24-39: “Evaluation and Assessment of Environmentally Sensitive Stream Bank Protection Measures,” and • Principal Investigator for NCHRP Research Project 24-42: “Underwater Installation of Filter Systems for Scour and Other Erosion Control Measures.”

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Published on
22 Aug 2014
Peer Reviewed