What is Bridge Scour?
Bridge scour is the removal of streambed material around bridge abutments or piers, such as sand and gravel. It can potentially occur from fast-flowing water or pressure flow, debris, and water flowing over the structure or overtopping. If scour continues to occur, it can compromise the integrity of a structure, causing holes or other defects, including loss of foundation material beneath the structure footings. In the United States, bridge scour is one of the three main causes of bridge failure.
Evaluating Scour Critical Bridges
As part of routine bridge inspections, McCormick Taylor uses field observations and data gathered by our certified bridge safety inspectors to determine if a bridge is scour critical. In Pennsylvania, scour critical bridges are categorized by the structure’s vulnerability to scour. Category A is the most susceptible to scour while Categories B, C, & D have decreasing scour vulnerability. As such, Category A bridges are given higher priority and monitored more frequently.
Once a bridge is identified as scour critical, the development of a Plan of Action (POA) is required. The POA establishes a systematic process for the monitoring of the bridge and subsequent closing during a significant rain event. This ensures public safety as there are specific criteria that must be met through an inspection prior to the bridge re-opening after a flood event. The POA also assists bridge owners with maintenance programming and prioritizing scour countermeasures designed to protect scour critical bridges from flood damage. McCormick Taylor utilizes a bridge-specific monitoring log during each visit to a scour critical bridge to provide our clients with quick access to the latest information on a scour critical bridge status.
Action Items and Preventative Measures
McCormick Taylor tracks rainfall totals to identify the potential for flooding. When flooding occurs on a Category A bridge, bridge monitoring begins immediately to evaluate potential signs of distress, and observations are recorded in the bridge’s monitoring log. If the approach roadway is submerged or pressure flow occurs, the bridge should be closed. If the bridge must remain open, then continuous monitoring is performed. For Category B and C bridges, monitoring will occur as needed and at a lesser frequency than Category A or as directed by PennDOT. Category D bridges are considered stable and do not require monitoring during an event, but do require countermeasures and an inspection after each serious flood. An example of a countermeasure is the placement of riprap (large rock) in close proximity to the bridge foundations to protect the underlying streambed against erosion and scour caused by fast-flowing water. Below are the Monitoring Actions for each Category bridge.
