Bridge Scour

The erosion of sediment near bridge foundations by water, a phenomenon known as bridge scour, is a critical concern in civil engineering and a significant contributor to structural failures. This article delves into the intricacies of bridge scour, its causes, the methods for its evaluation, and the countermeasures employed to prevent it. While the provided examples and perspective primarily focus on the United States, the underlying principles are globally relevant.

Areas Affected by Scour

Bridge scour refers to the removal of sediment – material like sand and gravel – from the immediate vicinity of bridge abutments or piers. This process, driven by hydrodynamic forces from fast-flowing water, can excavate scour holes that dangerously undermine the structural integrity of the bridge.

In the United States, bridge scour stands as one of the three primary culprits behind bridge failure, alongside collision and overloading. It's estimated that a staggering 60% of all bridge failures are attributable to scour and other hydraulic-related issues. This makes it the most prevalent cause of highway bridge failure in the US, with historical data indicating that 46 out of 86 major bridge failures between 1961 and 1976 were linked to scour occurring near piers. The Mississippi Highway 33 bridge over the Homochitto River, for instance, serves as a stark reminder of flood-induced erosion leading to catastrophic failure.

Causes of Bridge Scour

The inherent nature of water flow around bridge components makes them particularly vulnerable. Water typically accelerates as it encounters piers and abutments, leading to localized scour. Furthermore, when a bridge opening is narrower than the upstream channel, the water is forced to accelerate through the constricted passage, a phenomenon known as contraction scour. Beyond these localized effects, degradation scour can affect broader areas both upstream and downstream of the bridge, gradually lowering the stream bed over extended periods.

Several factors can exacerbate bridge scour. Instability within the stream channel itself, leading to river erosion and shifting angles-of-attack relative to the bridge, can significantly contribute. The accumulation of debris, such as logs and branches, also plays a substantial role. Debris can reduce the effective waterway opening, intensifying contraction scour. It can also increase the obstruction area around abutments, thereby intensifying local scour. Moreover, debris can deflect water flow, altering the angle of attack and further promoting local scour. In some instances, debris can even shift the entire channel course around the bridge, concentrating flow and scour in an unexpected location.

While loose alluvial materials like sand and gravel are easily eroded and frequently implicated in scour problems, it would be a grave error to assume that cohesive or cemented soils offer complete immunity. Scour can and does occur in these materials, though its development may simply take longer.

The equations used to predict scour depths often stem from laboratory studies. The challenge lies in accurately extrapolating these findings to real-world conditions, as the range of applicability can be difficult to ascertain. Many studies have concentrated on piers and pile formations, yet the majority of bridge scour issues are associated with the more complex geometry of bridge abutments. While some laboratory findings have been validated with limited field data, scaling physical models to accurately represent field conditions is fraught with difficulties. During field measurements, for example, a scour hole that forms during the rising limb of a flood, or even at its peak, might partially refill as the flood recedes. This makes it challenging to accurately model the maximum scour depth after the event.

Scour also complicates the hydraulic analysis of a bridge. Significant scour can deepen the channel through the bridge opening, effectively reducing or even eliminating the backwater effect. However, relying on this reduction is ill-advised due to the inherent unpredictability of scour processes.

When discussing scour, a crucial distinction is made between non-cohesive (or cohesionless) sediments, often found in alluvial environments and the focus of many laboratory studies, and cohesive materials. Cohesive materials necessitate specialized techniques and remain comparatively less researched.

A fundamental consideration in scour analysis is the difference between clear-water scour and live-bed scour. This distinction hinges on whether the mean bed shear stress of the flow upstream of the bridge is below or above the threshold required to move the bed material.

In the case of clear-water scour, the upstream shear stress is insufficient to dislodge the bed material, meaning the approach flow is relatively clear of sediment. Any bed material removed from a developing scour hole is not replenished by incoming sediment. The scour hole grows until the altered flow dynamics within it reduce the local shear stress to a critical value, at which point the flow can no longer excavate further.

Live-bed scour, conversely, occurs when the upstream shear stress exceeds the threshold, and the bed material upstream of the bridge is actively moving. This implies that the approach flow continuously transports sediment into the scour hole. In a uniform channel, a live bed alone does not create a scour hole; an additional increase in shear stress is required. This increase can be caused by a constriction, whether natural or artificial (like a bridge), or by a local obstruction, such as a bridge pier. Equilibrium scour depth is reached when the rate at which sediment is transported into the scour hole matches the rate at which it is transported out.

Generally, the maximum equilibrium clear-water scour depth can be approximately 10% greater than that of live-bed scour. Conditions conducive to clear-water scour include situations where the bed material is too coarse to be transported by the flow, where vegetated or artificially reinforced channels create localized high velocities only due to scour, or during low flows with gentle bed slopes.

It is also possible for both clear-water and live-bed scour to occur concurrently or sequentially. During a flood event, the bed shear stress can fluctuate as the flood intensity changes. The process might begin under clear-water conditions, transition to a live bed as the flood peaks, and then revert to clear-water conditions as the water levels recede. Crucially, the maximum scour depth may actually occur during the initial clear-water phase, not necessarily at the flood peak when live-bed scour is prevalent. Similarly, high velocities can be experienced when the flow is confined within the channel banks, rather than spread across floodplains at peak discharge.

Urbanization often leads to increased flood magnitudes and earlier, sharper hydrograph peaks, resulting in higher stream velocities and accelerated degradation. Alterations to the channel, such as improvements or gravel extraction upstream or downstream of a bridge site, can modify water levels, flow velocities, bed slopes, and sediment transport characteristics, thereby influencing scour. For example, if an alluvial channel is straightened, widened, or otherwise modified to increase flow energy, it will naturally seek a lower energy state through upstream degradation and downstream aggradation. The implications of degradation scour for bridge design are significant; engineers must ascertain whether the existing channel elevation is likely to remain stable throughout the bridge's lifespan or if changes are probable. If changes are anticipated, they must be accounted for in the design of the waterway opening and foundations.

The lateral stability of a river channel can also impact scour depths. Channel migration can lead to a bridge being misaligned or positioned incorrectly relative to the approaching flow. While this is a concern under many circumstances, it poses a particularly serious risk in arid or semi-arid regions and with ephemeral streams. The rates of lateral migration are notoriously difficult to predict. A channel that has remained stable for years may suddenly begin to shift, influenced by factors such as floods, bank material composition, vegetation on banks and floodplains, and land-use practices.

Scour at bridge sites is typically categorized into two main types: contraction (or constriction) scour and local scour. Contraction scour affects the entire cross-section of the channel due to increased velocities and bed shear stresses resulting from the narrowing of the channel by a structure like a bridge. Generally, a smaller opening ratio leads to higher waterway velocities and a greater potential for scour. When flow contracts from wide floodplains, significant scour and bank failure can occur. Very constrictive openings may necessitate ongoing maintenance for decades to combat erosion. Consequently, widening the opening is a direct method to mitigate contraction scour.

Local scour, on the other hand, is a result of increased velocities and the associated vortices generated as water accelerates around the corners of abutments, piers, and spur dykes. The flow pattern around a cylindrical pier, for instance, involves the approaching flow decelerating as it nears the cylinder, coming to rest at the pier's center. The resulting stagnation pressure is greatest near the water surface, where the approach velocity is highest, and diminishes with depth. A downward pressure gradient on the pier face directs the flow downwards. Local pier scour commences when this downflow velocity near the stagnation point becomes strong enough to overcome the resistance of the bed particles to motion.

During flood events, even if the bridge foundations themselves remain intact, the fill material behind abutments can be susceptible to scour. This type of damage is particularly common with single-span bridges featuring vertical wall abutments.

Bridge Examination and Scour Evaluation

The process of examining bridges for scour is typically undertaken by hydrologists and hydrologic technicians. It commences with a thorough review of historical engineering documentation pertaining to the bridge, followed by a detailed visual inspection. Information gathered includes the type of rock or sediment present in the river and the angle at which the river flows towards and away from the bridge. The area beneath the bridge is meticulously inspected for the presence of holes or other tell-tale signs of scour.

The initial phase of bridge examination involves an office investigation, where the bridge's history and any prior scour-related issues are documented. Once a bridge is identified as a potential "scour bridge," it proceeds to further evaluation, which includes field reviews, a scour vulnerability analysis, and prioritization. Bridges are categorized and ranked based on their scour risk. If a bridge is assessed as "scour critical," the owner is expected to develop a scour action plan to address existing and potential deficiencies. This plan might encompass the installation of countermeasures, ongoing monitoring, post-flood inspections, and established procedures for bridge closure if deemed necessary.

Advancements in sensing technologies are also being integrated for scour assessment. These technologies can be broadly categorized into three levels: general bridge inspection, collection of limited data, and collection of detailed data. Various scour-monitoring systems exist, including fixed, portable, and geophysical positioning systems, all aimed at detecting scour damage to prevent bridge failure and enhance public safety.

Countermeasures and Prevention

The Hydraulic Engineering Circular Manual No. 23 (HEC-23) provides comprehensive design guidelines for scour countermeasures applicable to both piers and abutments. The numbering in the accompanying table refers to specific sections within these guidelines.

Description of Countermeasure	HEC-23 Design Guideline No.
Bend way weirs and stream barbs	1
Soil cement	2
Wire-enclosed riprap mattress	3
Articulated concrete block system	4
Grout-filled mattresses	5
Concrete armour units	6
Grout- or cement-filled bags	7
Rock riprap at piers and abutments	8
Spurs	9
Guide banks	10
Check dams and drop structures	11
Revetments	12

Bend way weirs, spurs, and guide banks are employed to help align the upstream flow, while riprap, gabions, articulated concrete blocks, and grout-filled mattresses serve to mechanically stabilize pier and abutment slopes. Rock riprap remains the most commonly utilized countermeasure for preventing scour at bridge abutments. Physical additions to bridge abutments, such as upstream gabions and stone pitching, can also provide protection. The installation of sheet piles or interlocking prefabricated concrete blocks offers further protective measures. It's important to note that these countermeasures do not alter the fundamental scouring flow patterns and can be temporary, as their components may shift or be washed away during floods. The Federal Highway Administration (FHWA) advocates for design criteria, outlined in HEC-18 and HEC-23, which prioritize avoiding unfavorable flow patterns, streamlining abutments, and designing pier foundations to be inherently resistant to scour, thereby reducing reliance on riprap or other external countermeasures.

Trapezoidal-shaped channels through a bridge opening can significantly reduce local scour depths compared to vertical wall abutments. These trapezoidal shapes provide a smoother transition, eliminating abrupt corners that generate turbulent flow zones. Spur dykes, barbs, groynes, and vanes are examples of river training structures designed to modify stream hydraulics and mitigate undesirable erosion or deposition. They are typically employed in unstable stream channels to redirect flow towards more favorable paths through the bridge. Reinforcing bridge foundations by driving piles or installing deeper footings is another common practice.

Estimating Scour Depth

The Federal Highway Administration (FHWA) published Hydraulic Engineering Circular Manual No. 18 (HEC-18), which details several methodologies for estimating scour depth. Chapter 5 of this manual presents empirical scour equations for live-bed scour, clear-water scour, and local scour at piers and abutments, all within the section on General Scour. The total scour depth is calculated by summing three components: long-term aggradation and degradation of the river bed, general scour occurring at the bridge, and local scour at the piers or abutment. [8] However, research has indicated that the standard equations within HEC-18 may overestimate scour depth under certain hydraulic and geological conditions. Many of the relationships presented in HEC-18 are derived from laboratory flume studies using sand-sized sediments and incorporate safety factors that are not easily discernible or adjustable. [9] While sand and fine gravel are the most easily eroded materials, streams often contain more scour-resistant materials such as compacted till, stiff clay, and shale. The consequences of employing design methods based on a single soil type become particularly pronounced in physiographic provinces with distinct geological conditions and foundation materials. This can lead to excessively conservative design values for scour in low-risk or non-critical hydrologic scenarios. Consequently, ongoing efforts are focused on refining these equations to minimize both the underestimation and overestimation of scour.

Bridge Disasters Caused by Scour

History is punctuated by tragic bridge failures directly attributable to scour. Notable examples include: