What It Means
A scour critical bridge is one whose scour evaluation, performed under FHWA Hydraulic Engineering Circular No. 18 (HEC-18), has determined that the foundations are unstable for observed scour or for scour predicted during the design flood or check flood. Scour critical designation is recorded as code 3 in NBI Item 113 (Scour Critical Bridges). A scour critical bridge must have a Plan of Action (POA) on file with the state DOT, describing monitoring protocols (typically enhanced inspection before, during, and after flood events), closure criteria (flow levels or rainfall totals that trigger bridge closure to traffic), and countermeasure installation plans. Common scour countermeasures include riprap aprons (large stone placed around piers and abutments), grouted riprap, articulating concrete mats, partially grouted riprap, piled retaining structures around the pier perimeter, and pier extensions founded below the predicted scour depth. FHWA data shows approximately 17,000 U.S. bridges are classified as scour critical, concentrated in states with high-energy river systems such as California, Kansas, Missouri, and Tennessee. Scour has caused numerous catastrophic failures including the Schoharie Creek Bridge (New York, 1987, 10 deaths), the Hatchie River Bridge (Tennessee, 1989, 8 deaths), and the Arroyo Pasajero Bridge (California, 1995, 7 deaths). Climate change is expected to significantly increase scour risk as extreme precipitation events become more frequent and intense, particularly in the Gulf Coast, Midwest, and mountain West. The 2022 NBIS strengthened scour monitoring requirements and introduced risk-based inspection intervals that shorten the inspection cycle for scour critical bridges to as little as 12 months.
Frequently Asked Questions
What does "Scour Critical" mean?
A bridge where scour analysis has determined the foundations are unstable for observed or predicted flood conditions (NBI Item 113 code 3).
Why does Scour Critical matter for bridge safety?
A scour critical bridge is one whose scour evaluation, performed under FHWA Hydraulic Engineering Circular No. 18 (HEC-18), has determined that the foundations are unstable for observed scour or for scour predicted during the design flood or check flood. Scour critical designation is recorded as cod...