Underwater Bridge Scour Is the #1 Cause of Bridge Failure and Hardest to Detect

Scour, the erosion of riverbed material around bridge foundations by flowing water, is the leading cause of bridge failure in the United States. FHWA estimates that 60% of all bridge failures are scour-related. Unlike cracking or corrosion, which are visible, scour happens underwater and underground, invisibly undermining the foundation that holds the entire bridge up. A bridge can look perfectly fine from the surface while its foundations are critically compromised. This matters because scour failures tend to be sudden and total. When a bridge foundation loses enough soil support, the pier or abutment can shift or collapse with little warning, often during flood events when the bridge is most needed and rescue is most difficult. The Schoharie Creek Bridge collapse in New York in 1987, which killed 10 people, was caused by scour that had gone undetected despite inspections. The problem is compounded by climate change. More intense rainfall events and changing flood patterns are increasing scour risk on bridges that were designed for historical hydrology. A bridge that was safe under the 100-year flood event calculated in 1970 may not be safe under the 100-year flood event recalculated with current climate data. Yet most bridges have not been re-evaluated for updated flood risks. This problem persists because underwater inspection is expensive, technically difficult, and dangerous. Inspecting a bridge foundation for scour requires divers or sonar equipment, and conditions are often poor: murky water, strong currents, debris. Many agencies only perform underwater inspections on a 5-year cycle, and some bridges classified as scour-critical have never had a full underwater inspection of their foundations. The structural root cause is that bridge inspection standards were originally designed around what inspectors could see from the surface. The National Bridge Inspection Standards were created after the 1967 Silver Bridge collapse, which was caused by a visible defect (a corroded eyebar). The inspection framework is inherently biased toward detecting visible, above-water defects and poorly suited to detecting invisible, below-water ones. Fixed scour monitoring instrumentation exists but is installed on fewer than 5% of scour-vulnerable bridges due to cost.