There are approximately 575,000 highway bridges in the National Bridge Inventory. Of these, 85 percent are over streams or rivers. This level of exposure to the scouring action of floods dictates the need for expeditious development and implementation of reliable scour evaluation and related bridge inspection practices. Nationally, the annual cost for scour related bridge failures has been determined to be over $30 million.

In 1988, the FHWA issued a Technical Advisory, "Scour at Bridges." that provides guidance to the states for developing and implementing a scour evaluation program. If a bridge is determined to be scour critical, a countermeasure is recommended. One alternative to conventional countermeasures is to monitor actual scour activity to determine if significant scour affecting bridge safety has occurred.

Research was needed to develop low-cost, reliable, scour monitoring devices that could be mounted to a bridge to indicate the maximum scour depth that occurs at bridge piers and abutments during floods. Two practical devices were developed during this project. Details on the work can be found in NCHRP Report 396, "Instrumentation for Measuring Scour at Bridge Piers and Abutments". Specific details on the monitoring devices is contained in NCHRP Report 397A, "Sonar Scour Monitor: Installation, Operation and Fabrication Manual," and NCHRP Report 397B, "Magnetic Sliding Collar Scour Monitor: Installation, Operation and Fabrication Manual," respectively.