Scour at bridge abutments with erodible embankments

This study presents the extensive data and observations from a laboratory study of bridge-abutment scour. Abutment scour is a common and major cause of bridge failure, especially during floods. The data and observations lead to a practical new approach for estimating realistic, maximum scour depths at bridge abutments. Prior design approaches have usually lead to unrealistically deep estimates of abutment-scour depth, largely because these studies have not accurately simulated the actual construction features of bridge abutments. Additionally, the present study includes field information on actual bridge failures as reported by various agencies responsible for bridges. This information indicated that abutment scour was poorly understood, and that several scour conditions could occur. The study, accordingly, focused on elucidating the various erosion processes whereby bridge abutments fail owing to scour.; An important feature of actual bridge abutments is that they comprise a pile-supported structure set amidst an erodible earthfill embankment. This feature has a pronounced set of influences on scour processes at bridge abutments. This study is the first time in which these processes were recorded and described in a comprehensive manner.; The study required extensive experimentation conducted with a laboratory flume, and abutments of realistic design that were subject to the scour for a range of abutment and flow conditions. The experiments were conducted with abutments with approach embankments configured in a range of erodibility conditions: fixed embankment on fixed floodplain; riprap-protected erodible embankment on readily erodible floodplain; and, unprotected readily erodible embankment on readily erodible floodplain.; The design approach developed during the study discards the old notion of linearly combining bridge-waterway constriction scour and local scour at the abutment structure, a notion that the study's findings do not support. The new approach entails estimating an abutment-induced local amplification of constriction scour at the bridge opening, and separately estimating a maximum local scour depth at the abutment when exposed by embankment failure. Verification with field data confirmed the appropriateness of the new approach.