Three Abutment Scour Conditions at Bridge Waterways

This paper presents findings from an extensive, laboratory-based study of scour at bridge abutments located in compound channels. The study considered aspects of abutment construction, including abutment and floodplain erodibility, and examined how these aspects influence scour development. Its findings show that three basic scour conditions can develop at abutments in compound channels: scour of the main-channel bed, when the floodplain is far less erodible than the bed of the main channel (Scour Condition A); scour of the floodplain around an abutment, for abutments set back on a floodplain (Scour Condition B); and scour at an abutment column exposed once an abutment is breached (Scour Condition C). These conditions may combine, depending on abutment location and site circumstances. The authors present laboratory observations and data associated with Scour Conditions A and B. The data are analyzed in terms of a formulation concept that treats Scour Conditions A and B as essentially scour at a short contraction (the bridge opening), for which flow nonuniformity and turbulence amplify scour depth associated with a long contraction. The amplification is greatest for small contractions and diminishes for larger contractions that force greater magnitude and uniformity of flow velocity through a bridge opening.