Scour has been linked to nearly 95% of all severely damaged and failed highway bridges constructed over waterways in the United States. There are two issues associated with such scour-induced damage to bridge pier footings. The first effect is the loss of foundation material, which exposes the footing and lowers its factor of safety with regard to sliding or lateral deformation. The greatest loss of sediment to scour occurs at high water velocities, such as during floods. Secondly, pier movement may occur because of material loss beside and beneath the base of the footing, which produces undesired stresses in the bridge structure and ultimately results in structural collapse.

The various scour mechanisms are illustrated in figure 46. Total scour is comprised of long-term channel aggradation and degradation, contraction scour, and local scour. Local scour involves the removal of material from around underwater structures such as bridge piers, abutments, spurs and embankments; thus, it has the greatest impact on bridge integrity. Local scour results from acceleration of flow and subsequent development of vortices around obstructions. The action of both horseshoe and wake vortices is to displace material from the base of the obstruction. In this case, it is a cylindrical pier, which can result in undermining the structure.

Figure 46. Scour mechanism.

The most critical factors contributing to local scour are the velocity and depth of flow, both of which are significantly increased during heavy storms and floods. As the velocity and/or depth increase, the amount of scour increases. Other factors affecting bridge scour include the dimensions and orientation of piers, bed configuration and material size/gradation, and accumulation of ice and debris along the piers.

Current methods for measuring scour include visual underwater bridge inspection and various sonic devices such as sonar, sounding rods, and buried or driven rods, along with other buried devices. In these methods, measurements can be performed manually or can be integrated with real-time systems to monitor flood stage scour. Some of these devices are non-geophysical (i.e., sounding rods) and will not be discussed in this web manual.

Methods used to detect scour include Time Domain Reflectometry (TDR), Parallel Seismic, Ground Penetrating Radar (GPR), Continuous Seismic Profiling (CSP), and Fathometer. These methods are described below.