Bridge scour
[2] Stream channel instability resulting in river erosion and changing angles-of-attack can contribute to bridge scour.
Debris might also shift the entire channel around the bridge causing increased water flow and scour in another location.
[3] The most frequently encountered bridge scour problems usually involve loose alluvial material that can be easily eroded.
Similarly, relatively high velocities can be experienced when the flow is just contained within the banks, rather than spread over the floodplains at the peak discharge.
Urbanization has the effect of increasing flood magnitudes and causing hydrographs to peak earlier, resulting in higher stream velocities and degradation.
Channel improvements or the extraction of gravel (above or below the site in question) can alter water levels, flow velocities, bed slopes and sediment transport characteristics and consequently affect scour.
Contraction scour occurs over a whole cross-section as a result of the increased velocities and bed shear stresses arising from a narrowing of the channel by a construction such as a bridge.
Local scour arises from the increased velocities and associated vortices as water accelerates around the corners of abutments, piers and spur dykes.
The resulting stagnation pressure is highest near the water surface where the approach velocity is greatest, and smaller lower down.
Local pier scour begins when the downflow velocity near the stagnation point is strong enough to overcome the resistance to motion of the bed particles.
The examination process is normally conducted by hydrologists and hydrologic technicians, and involves a review of historical engineering information about the bridge, followed by a visual inspection.
The plan may include installation of countermeasures, monitoring, inspections after flood events, and procedures for closing bridges if necessary.
Each system can help to detect scour damage in an effort to avoid bridge failure, thus increasing public safety.
The numbering in the following table indicates the HEC-23 design guideline section:[6] Bend way weirs, spurs and guide banks can help to align the upstream flow while riprap, gabions, articulated concrete blocks and grout-filled mattresses can mechanically stabilize the pier and abutment slopes.
A number of physical additions to the abutments of bridges can help prevent scour, such as the installation of gabions and stone pitching upstream from the foundation.
Spur dykes, barbs, groynes, and vanes are river training structures that change stream hydraulics to mitigate undesirable erosion or deposits.
[8] However, research has shown that the standard equations in HEC-18 over-predict scour depth for a number of hydraulic and geologic conditions.
Most of the HEC-18 relationships are based on laboratory flume studies conducted with sand-sized sediments increased with factors of safety that are not easily recognizable or adjustable.
The consequences of using design methods based on a single soil type are especially significant for many major physiographic provinces with distinctly different geologic conditions and foundation materials.
This can lead to overly conservative design values for scour in low risk or non-critical hydrologic conditions.
