Contrary to what some people think, a bridge designed to current standards and with a maintenance of the structure following current criteria can, in fact, collapse. If the potential influence of external factors and phenomena on the bridge is not controlled, these can compromise their integrity and stability.
As will be explained below, being able to identify, monitor and determine the relative importance of phenomena such as scour on a certain bridge plays a fundamental role in order to guarantee its safety.
Importance of scour in the failure of bridges
About 60% of all bridge failures are estimated to have a hydraulics-related cause. Among them, scour is the main reason, constituting in global terms one of the three main causes of bridge failure.
Some recent examples of bridge failures in Europe due to scour are the Sava Bridge in Zagreb (Croatia), the Malahide Viaduct in Dublin (Ireland) or the collapse of the Hintze Ribeiro Bridge in Entre-os-Ríos (Portugal) where the number of victims amounted to 59 people.
Scour has been identified as the most common cause of road bridge failure in the United States. In the framework of a study that analyzed the causes of collapse of more than 500 bridges in the country between 1989 and 2000, it was determined that 53% of the failures were due to scour.
In the context of future climate change, some regions will face extreme and changing rainfall regimes, together with the alteration of snow melting patterns. This will result in higher frequencies and intensities of flooding, which directly influences scour.
In fact, the increased vulnerability of bridges to climate change is already the subject of numerous studies. Simulations considering climate change scenarios carried out by the European Commission determined that around 20% of bridges in Europe will present high risks of scour in the next 20 years. This percentage varies from country to country and it is estimated that the highest risks will occur in Austria (60%), Portugal (50%), Spain (42%) and Italy (39%).
What exactly is scour and why does it increase the vulnerability of bridges so much?
Scour can be defined as the excavation and removal of material from the bed and the banks of streams as a result of the erosive action of the water flow itself. Although scour seems like a simple concept, it takes several different forms and can range from a natural phenomenon to being caused by man-made changes in a river.
Although when it comes to scour many tend to think only of the phenomenon that produces the scour of bridge piers, total scour is generally the result of three different components, the combination of which is the existing scenario: long-term degradation of the river bed (what many experts call natural scour), contraction scour of the bridge and local scour on the piers or abutments. Although some experts define other types of scours that occur in more specific situations, we could affirm that these three components are the main ones:
- Natural scour are changes in the elevation of the stream that take place in the long term, generally due to natural causes, which can affect the section of the river where the bridge is located. The processes that take place are called aggradation and degradation. While aggradation involves the deposition of eroded material from the channel or the basin, degradation involves the descent or dragging of the stream in relatively long sections due to a deficit in the supply of upstream sediments, contributing to the total scour of the river bed.Natural scour generally occurs when changes are made to the hydraulic parameters that govern the shape of the channel, such as changes in flow velocity or in the amount of sediment. It is related to the evolution of the channel and is associated with the progression of scour and filling, in the absence of obstacles.
- Contraction scour is the descent of the bed in the vicinity of the bridge. These descents can be uniform or non-uniform; therefore the scour can be deeper in some parts of the cross section. Contraction scour occurs as a result of the narrowing of the channel cross-sectional area due to the construction of structures such as piers and abutments of bridges. Reducing the cross-sectional area of the canal at the location of a bridge produces an increase in the flow velocity due to the contraction (or constriction) to which it is subjected, causing the appearance of shear stresses on the bed. Increased shear stresses can exceed the threshold shear stress of the canal bed and mobilize sediment across the entire (or nearly all) width of the canal.Contraction scour differs from degradation associated with natural scour in that it occurs in the vicinity of the constriction or the bridge, it can be cyclical, and / or related to flooding.
- Local scour is a complex three-dimensional phenomenon that occurs as a result of the encounter of the water flow with the piers and abutments of the bridge. In the case of the piers, an acceleration of a downward flow occurs in the front face, resulting in a pressure gradient. This gradient generates a vertical current towards the bottom of the channel, which impacts the bed, forming a very localized erosion (hole) around the structure that can cause their subsidence and / or rotation.When said hole is produced, the downward flow changes direction, then going upwards and consequently acquiring a rotary movement that creates horseshoe vortices, which drag the bed material towards the areas adjacent to the pier and downstream. Therefore, the horseshoe-shaped vortices are the result of the beginning of the scour and not the main cause of it. These vortices grow, both in size and intensity, as the depth of erosion increases (although they grow at a decreasing rate), so the vertical current towards the bed of the channel, and therefore the erosion, also increases. Thus, the scour hole grows in this area continuously, until reaching a maximum or equilibrium depth.In addition, the separation of the flow to the sides of the pier creates wake vortices downstream of it, which cause all these raised and transported sediments to accumulate downstream of the pier.The magnitude of the local scour will depend on the shape of the channel and the flow conditions, and of course, it will be more severe during floods, that is, under strong hydraulic conditions (significant speeds and drafts).
Diagram of types of scour (from Melville et al, 2000)
Additionally, these three types of scour can be subdivided into two groups:
- Clear-water scour. It usually occurs on piers with relatively low flow rates and consists of the simple removal of material from the bed by the flow of water. Upstream of a bridge, water does not carry significant amounts of bed material (hence the term clear water). On the bridge, the bed material is removed and transported, but no material from the top is deposited at the same time. Therefore, the scour holes that form remain present when flows decrease and can be seen during underwater inspection.
- Live-bed scour is the continuous erosion and deposition of material from the bed during flood periods. In the worst case, the bed under the foundation of a pier will become fluid as the material is constantly removed and replaced. This type of scour can be very difficult to detect by inspection. When a diver is sent to inspect the pier, the flow will have been reduced and the bed will have stabilized at a level much higher than the maximum level of scour during the flood.
What could avoid this kind of collapse
Where it occurs, scour poses serious problems to the stability of bridge structures as it reduces the rigidity of foundation systems and can cause bridge abutments to fail.
Current practice for the most critical design dictates that the depth of the scour is determined by the sum of the individual scour depths caused by the mentioned mechanisms (natural, contraction and local). Due to the large number and variety of factors involved and the different nature of the phenomena that take place, being able to mitigate or eliminate the scour, or even predict its appearance or evolution, is a really complex process for engineers.
Currently, scour is combated in several ways, as the added cost of making a bridge less vulnerable to scour is small compared to the costs incurred by its collapse, which can easily be 2 to 10 times the cost of the bridge itself if not only the material cost or the interruption of traffic is taken into account, but also the fact that the administrator is exposed to the potential loss of human life.
Thus, at the bridge design stage, it is possible to propose both hydraulic and structural solutions to scour.
Hydraulic solutions involve the prevention of rapid expansion or contraction of flow caused by sudden induced changes in the direction of flow that can lead to the occurrence of scour. Maintaining larger openings in the bridge and streamlining the geometries of the abutments are steps that can be taken at the design stage. During bridge operation, keeping openings clear by removing debris such as downed trees and other objects that can often lodge in bridge openings, obstructing flow, are some of the steps that can be taken.
However, it should be noted that maintaining large bridge openings and aerodynamic pier fronts can often be a futile exercise, as natural changes in channel deposition and erosion upstream of a bridge can change often the flow angle relative to the alignment of a bridge, causing these hydraulic problems.
Structural measures can be applied at the design stage by ensuring that the continuous footings are below the maximum design scour depths. Maintenance or repair measures related to scour include adding rip-rap or rock-type reinforcement to the base of the piers and abutments.
Similarly, the application of these measures is limited by uncertainties in determining the predicted scour depth, especially since it may change over the lifespan of the bridge.
For example, in scenarios with seismic events, if the scour is at a very advanced stage, the suspended mass of the bridge increases and, faced with severe horizontal actions, the structure can be subjected to high displacements, causing damage to resistant elements and even loss of stability if adequate means are not available (seismic stops or ties, for example).
Our opinion as experts
We, like many experts, think that the most effective and economically viable method to combat scour is to monitor its evolution over time and carry out the necessary repair or reinforcement works based on a predictive approach through simulation of critical scenarios.
Currently, it is most common to carry out scour control through visual inspection. This involves the use of divers to inspect the condition of the foundations and measure the depth of the scour using basic instruments. Two particular disadvantages associated with this inspection method are the fact that inspections cannot be carried out in flood season, when the risk of scour is greatest, and the maximum depth of scour may not be recorded correctly since scour holes can be refilled, as explained previously in the case of living-bed scour. The fact that scour holes tend to refill can be dangerous and misleading, as the true magnitude of the scour problem can be overlooked on inspection.
To overcome the limitations of visual inspections, instrumentation has been developed to measure the depth of scour continuously or discretely. Floating devices, devices based on TRD technology (time domain reflectometry), fiber optic sensors (Bragg grating) or sound wave equipment are just some examples of the technologies used. When analyzing the advantages and limitations of the available technologies, we can affirm that those based on fixed underwater instruments can be damaged by suspended sediments during moments of turbulent flow, when the risk of scour is higher, and those based on discrete measurements do not offer immediate or continuous information, which limits its usefulness.
To determine what repair work is necessary, while having an understanding of the extent of the scour is important, it is even more important to know how that scour is affecting the overall stability of the bridge, and how it will do so in the future. This means that the solutions must be designed by engineers considering the reality of the bridge.
In our methodology we use sensors strategically located on the bridge, ensuring the integrity of each node, to validate and calibrate numerical models of the structure, which gives us in real time the response of each bridge to the scour process it is facing. By simulating scenarios, customers can make decisions about what repair measures to take and when is the optimal time to do so.
References
Maddison, B. (2012). Scour failure of bridges. Proceedings of the Institution of Civil Engineers-Forensic Engineering, 165(1), 39-52.
Melville, B. W., & Coleman, S. E. (2000). Bridge scour. Water Resources Publication.
Nemry, F., & Demirel, H. (2012). Impacts of Climate Change on Transport: A focus on road and rail transport infrastructures. European Commission, Joint Research Centre (JRC), Institute for Prospective Technological Studies (IPTS).
Prendergast, L. J., & Gavin, K. (2014). A review of bridge scour monitoring techniques. Journal of Rock Mechanics and Geotechnical Engineering, 6(2), 138-149.
Wardhana, K., & Hadipriono, F. C. (2003). Analysis of recent bridge failures in the United States. Journal of performance of constructed facilities, 17(3), 144-150.
Contrary to what some people think, a bridge designed to current standards and with a maintenance of the structure following current criteria can, in fact, collapse. If the potential influence of external factors and phenomena on the bridge is not controlled, these can compromise their integrity and stability.
As will be explained below, being able to identify, monitor and determine the relative importance of phenomena such as scour on a certain bridge plays a fundamental role in order to guarantee its safety.