Riprap Sizing for Bridge Pier and Abutment Countermeasures

Many equations are available for sizing riprap protection for bridge piers and abutments. These design equations can give significantly different results for protecting the same installation. This paper provides an overview of a review of riprap sizing methods performed as part of the National Cooperative Highway Research Program (NCHRP) Project 24-23, "Riprap Design Criteria, Specifications, and Quality Control." The performance of twelve (12) pier riprap sizing equations was evaluated by comparing the predicted size against three laboratory data sets. Adjustment factors for pier shape were also evaluated because the laboratory data includes tests of square and round nose piers. Flow velocity is the primary variable for determining riprap size. One-dimensional models often do not provide accurate estimates of flow velocity for bridge abutment areas located at the ends of embankments that encroach significantly into floodplains. Therefore, 2-dimensional hydraulic modeling was performed to test methods for estimating flow velocity at abutments and guide banks and additional guidance was developed.