Iowa's Proactive Approach to Bridge Scour Monitoring

Scour, which is the result of the erosive action of flowing water excavating and carrying away material from the bed and banks of streams and from around the piers and abutments of bridges, is the most common cause of bridge failure. The structural instability and undermining caused by scouring are affected by factors such as channel and bridge geometry, floodplain characteristics, flow hydraulics, bed material, channel protection and stability, riprap placement, and ice formation and debris. Directing its attention to the scour problem, the Federal Highway Administration issued a Technical Advisory in 1988 and again in 1991, revisiting the National Bridge Inspection Standards to require evaluation of all bridges for susceptibility to damage resulting from scour. Of special concern were scour-critical bridges, or those bridges that could experience catastrophic failure or become structurally unstable as a result of excessive scour caused by a destructive flood event. Over the past 4 years, the Iowa Department of Transportation has devoted significant time to evaluating the bridges under its jurisdiction and assigning scour-classification codes to them. This article discusses the scour evaluation of Iowa's 2,100 waterway bridges, 180 of which have been classified as scour-critical, and the scheduled hydraulic and structural construction countermeasures being taken to prevent their catastrophic failure.