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Scour is the primary cause of bridge failures in the United States. Figure 1 shows statistics compiled by the Structures Division of the New York State Department of Transportation (DOT) and calculated using the National Bridge Failure Database. From 1966 to 2005, there have been at least 1,502 documented bridge failures. Of those bridge failures, 58% were the result of hydraulic conditions. Second on the list, but substantially behind, were collisions by ships, trucks, or trains, and overload. Earthquakes were a distant eighth on the list. According to the FHWA, the number of bridges declared âscour criticalâ total more than 20,904. During and following the successful completion of NCHRP Project 21-03, Instrumen- tation for Measuring Scour at Bridge Piers and Abutments, more than 120 of these bridges were instrumented for scour measurements. Often these bridges are instrumented because the scour estimates appear overly conservative and it is prudent to observe scour activity during flood events before spending resources on other types of countermeasures. Other bridges are scheduled to be replaced, and monitoring is an alternative measure to help ensure the safety of the traveling public until the new bridge is completed. This synthesis is a report of the state of knowledge and practice for fixed scour monitor- ing of scour critical bridges. It includes a review of the existing knowledge and research and an examination of current practice. The project included a survey of transportation agencies and other bridge owners to obtain their experiences with fixed scour monitoring systems. For those agencies that have not employed scour monitoring systems, their opinions were requested regarding problems and suggestions. Thirty-seven state DOTs responded to the survey. Information on scour monitoring for non-responding states was obtained from the literature review. Many of these instrumented bridges have been monitored for more than ten years and some valuable field data have been accumulated. Exploring what data and associated evalu- ations are available will be useful for improving the technologies of predicting bridge scour as well as monitoring scour. Thirty-two of the 50 states use, or have employed, fixed scour monitoring instrumentation on their highway bridges. A total of 120 bridge sites were identified that are using or have employed fixed monitors. The respondents to the survey provided information on their expe- riences with fixed scour monitoring installations and detailed data on at least one representa- tive bridge site. Not surprisingly, the states that had the largest number of scour monitoring installations were also locations with extreme weather conditions, Alaska and California. The monitoring systems used by the states, with the exception of time domain reflectometry, are described in the current FHWA guidelines on scour countermeasures and monitoring, Hydraulic Engineering Circular 23. The third edition of these guidelines, expected to be published in 2009, includes an expanded chapter on scour monitoring, with information on time domain reflectometers. The problems reported by the states were very similar. The difficulties with maintenance and repairs to the scour monitoring systems were the most common theme throughout the survey responses. The leading cause of damage to the systems was debris flows and accumulation. Other common problems were vandalism and corrosion. SUMMARY MONITORING SCOUR CRITICAL BRIDGES
The advancements that bridge owners would like to see for future fixed scour monitoring technology included the development of durable instrumentation, with increased reliability and longevity, decreased costs, and minimum or no maintenance. This equipment would include instrumentation that measures streambed scour and other hydraulic variables includ- ing water elevations and velocities. These would provide information for hydraulic design and analysis, and for the improvement of scour prediction methodologies. 2 FIGURE 1 Causes of bridge failures in the United States (Courtesy: New York State DOT and Texas A&M University).
