Scour

What It Means

Scour is the removal of streambed material from around bridge piers and abutments by the erosive action of flowing water. It occurs in three forms: long-term degradation (gradual channel lowering over years), contraction scour (increased velocity through the bridge opening during floods), and local scour (vortex-induced erosion immediately adjacent to piers and abutments). Scour is the single largest cause of bridge failures in the United States, responsible for an estimated 60% of all bridge collapses with known causes, according to FHWA research. The Schoharie Creek Bridge collapse on the New York Thruway in 1987 killed 10 people after pier scour during a flood, and the collapse precipitated the creation of FHWA's formal scour evaluation program under Hydraulic Engineering Circular No. 18 (HEC-18). FHWA now requires every bridge over water (roughly 480,000 of the 617,000+ bridges in the NBI) to have a scour evaluation, with the scour vulnerability recorded as NBI Item 113 on a 0-9 scale: 9 (bridge foundations are on dry land), 8 (stable, above flood scour), 5-7 (stable for assessed scour condition), 4 (action required, Plan of Action in place), 3 (bridge is scour critical), 2 (extensive scour has occurred), 1 (failure imminent), and 0 (bridge has failed). Scour is most dangerous during floods when flow velocities can increase five- to tenfold over normal conditions. Countermeasures include riprap, grouted mats, pier extensions, and underpinning. Climate change is expected to increase scour risk as extreme precipitation events become more frequent, particularly in the Gulf Coast, Midwest, and Appalachian regions.

Frequently Asked Questions