12.2 Scour mechanisms and analysis
3 min read•july 30, 2024
Bridge scour can seriously threaten structural integrity. It's caused by flowing water eroding sediment around foundations. Different types include affecting the whole riverbed, and localized scour around specific bridge elements.
Estimating is crucial for bridge safety. Engineers use standard equations and advanced modeling techniques to predict how deep scour might go. This helps assess risks and plan appropriate measures to protect bridges from potential failure due to scour.
Scour types for bridge foundations
Erosion mechanisms and general scour
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Analysis on River Bank Erosion-Accretion and Bar Dynamics Using Multi-Temporal Satellite Images View original
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ESurf - Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid ... View original
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ESurf - Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid ... View original
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Analysis on River Bank Erosion-Accretion and Bar Dynamics Using Multi-Temporal Satellite Images View original
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Top images from around the web for Erosion mechanisms and general scour
Analysis on River Bank Erosion-Accretion and Bar Dynamics Using Multi-Temporal Satellite Images View original
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ESurf - Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid ... View original
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ESurf - Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid ... View original
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Analysis on River Bank Erosion-Accretion and Bar Dynamics Using Multi-Temporal Satellite Images View original
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- Scour erodes sediment around bridge foundations due to flowing water compromises structural integrity
- General scour lowers entire riverbed over time affects whole channel cross-section
- Degradation progressively lowers channel bed due to natural or human-induced causes over longer river reach
- Lateral stream migration causes bank erosion and scour potentially exposes bridge foundations originally buried in floodplain
Localized scour phenomena
- occurs when flow area reduces typically by bridge or piers causes increased flow velocities and bed erosion
- removes sediment from around or abutments due to complex flow patterns and vortices
- happens when upstream flow does not transport bed material
- occurs when there continuous (sand-bed rivers)
Estimating scour depth
Standard methodologies and equations
- Hydraulic Engineering Circular No. 18 () provides standard methodologies for scour depth estimation in United States
- Colorado State University (CSU) equation commonly estimates local scour depth around bridge piers
- Considers factors flow depth, pier width, Froude number, correction factors for pier shape, angle of attack, bed condition
- serves as alternative method for estimating pier scour depth particularly useful for wide piers
- applies for estimating local scour at bridge abutments when ratio of projected abutment length to flow depth exceeds 25
- Contraction scour depth typically calculated using either clear-water or live-bed scour equations depending on sediment transport conditions
Advanced modeling techniques
- Time-dependent scour equations () estimate scour depth evolution over time
- 2D and 3D numerical models (, ) provide more detailed scour predictions by simulating complex flow patterns around bridge structures
- (CFD) models simulate turbulent flow structures and vortex shedding around piers for improved scour predictions
Analyzing scour risks
Scour depth assessment and classification
- Compare scour depth predictions to depth of bridge foundation elements determines risk of structural instability
- Total scour depth concept sums effects of general scour, contraction scour, and local scour for comprehensive assessment
- Scour critical bridges have predicted scour depth exceeding foundation depth require immediate attention and potential countermeasures
- FHWA's HEC-18 provides guidelines for assessing scour vulnerability and recommending appropriate actions
Risk evaluation and management
- Risk assessment considers uncertainty in scour predictions, importance of bridge, potential consequences of failure
- Temporal variations in scour depth (during flood events) factor into interpreting results and assessing long-term risks
- Interpretation of scour analysis results informs design of and development of bridge inspection and monitoring programs
- Probabilistic scour analysis incorporates uncertainties in hydraulic and geotechnical parameters for more robust risk assessment
Limitations of scour models
Empirical and theoretical constraints
- Empirical scour equations based on laboratory experiments may not fully represent complexities of natural river systems
- Scour equations often assume steady-state conditions may not accurately represent dynamic nature of flood events and sediment transport processes
- Spatial and temporal variability of riverbed materials introduces significant uncertainties in scour depth predictions
- Complex flow patterns around irregular bridge geometries or in compound channels may not be adequately captured by simplified scour prediction methods
Data and modeling uncertainties
- Accuracy of scour predictions highly depends on quality and reliability of input data (hydrologic, hydraulic, geotechnical information)
- Climate change and anthropogenic alterations to watersheds affect long-term validity of scour predictions based on historical data
- Limitations of 1D hydraulic models in representing 3D flow structures around bridge piers and abutments lead to inaccuracies in scour depth estimates
- Uncertainty in future flood magnitudes and frequencies impacts reliability of long-term scour predictions and risk assessments
Key Terms to Review (29)
Abutments: Abutments are structural elements that support the ends of a bridge or an arch, transferring loads from the bridge to the ground. They play a crucial role in maintaining the stability and alignment of the bridge, especially in arch designs where they resist lateral forces and provide a solid foundation. The design and analysis of abutments are critical for ensuring the integrity of a bridge under various conditions, including seismic activity, scour effects, and load distribution.
ASCE Standards: ASCE Standards refer to a set of guidelines and criteria developed by the American Society of Civil Engineers, aimed at ensuring safety, reliability, and efficiency in civil engineering practices. These standards cover various aspects of engineering design and analysis, including hydrology, structural integrity, and materials usage, which are crucial for the successful construction and maintenance of infrastructure. Compliance with these standards is essential for engineers to meet regulatory requirements and industry best practices.
Bedload: Bedload refers to the sediment particles that are transported along the bottom of a water body, such as a river or stream, through rolling, sliding, or hopping. This type of sediment transport is crucial for understanding erosion processes, especially around structures like bridges, where the movement of bedload can lead to scour. Recognizing how bedload behaves helps engineers predict potential damage to infrastructure and design effective protection measures.
Bernoulli's Equation: Bernoulli's equation is a principle in fluid dynamics that describes the relationship between pressure, velocity, and elevation in a flowing fluid. It states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or potential energy. This equation is crucial for understanding how fluid flow interacts with structures, particularly in analyzing scour mechanisms around bridge foundations.
Bridge piers: Bridge piers are vertical structural elements that support the superstructure of a bridge, transferring loads from the deck and superstructure to the foundation. They are essential for maintaining the stability and integrity of the bridge while also providing resistance against various forces, such as wind and water. Their design and placement are critical, especially in areas prone to scour, which can undermine their stability.
Clear-water scour: Clear-water scour refers to the erosion of sediment around a submerged object due to the flow of clear water, which is free of sediment. This phenomenon occurs when the flow velocity exceeds the critical threshold for sediment movement, leading to localized erosion and potential undermining of structures such as bridge foundations. Understanding clear-water scour is crucial for predicting potential risks and designing effective countermeasures to protect infrastructure.
Computational fluid dynamics: Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and algorithms to solve and analyze problems involving fluid flows. It connects theoretical principles of fluid dynamics with computational techniques, enabling engineers to simulate and visualize complex fluid behaviors around structures such as bridges. CFD helps in understanding how fluids interact with surfaces, which is essential for predicting phenomena like scour around bridge foundations.
Contraction scour: Contraction scour refers to the erosion of sediment caused by the increased flow velocity and turbulence that occurs when a river or stream narrows at a bridge or similar structure. This phenomenon is particularly critical for bridge engineering as it can lead to significant undermining of foundations, affecting stability and safety. Understanding contraction scour is essential for assessing potential risks associated with hydraulic conditions and ensuring proper design measures are implemented.
CSU Equation: The CSU (Coefficient of Scour Under the influence of Flow) equation is a formula used to estimate the depth of scour around bridge piers and abutments caused by flowing water. This equation helps engineers predict how much sediment will be eroded from the riverbed, which can compromise the stability and safety of bridge structures. Understanding this equation is crucial for designing effective countermeasures to prevent excessive scour and ensure the longevity of bridges.
FHWA Guidelines: FHWA guidelines refer to the policies and standards established by the Federal Highway Administration for various aspects of highway and bridge design, construction, and maintenance. These guidelines help ensure safety, efficiency, and environmental stewardship in transportation infrastructure, influencing everything from geotechnical assessments to scour evaluations and protection measures.
Flow velocity: Flow velocity is the speed at which water moves in a specific direction, usually measured in meters per second. This term is crucial in understanding how water interacts with structures, particularly in the context of scour, where fast-moving water can erode the bed and banks around bridges and other infrastructures, leading to potential instability.
Flow-3d: Flow-3D is a computational fluid dynamics (CFD) software designed for simulating complex fluid flows and interactions in three dimensions. It utilizes advanced algorithms to accurately model various flow phenomena, such as turbulence, free surface flows, and sediment transport, making it especially useful in engineering applications like scour analysis around bridge structures.
Froehlich's Equation: Froehlich's Equation is a mathematical relationship used to estimate the scour depth at bridge foundations due to flowing water. It takes into account factors like the velocity of the approaching flow and the size of the foundation, allowing engineers to predict how much sediment will be eroded away around a bridge pier or abutment. This equation is crucial for ensuring the stability and safety of bridge structures by analyzing potential erosion caused by hydraulic forces.
General scour: General scour refers to the removal of sediment from the riverbed or streambed around bridge piers or abutments due to the flow of water, leading to a lowering of the bed elevation. This phenomenon typically occurs when the flow velocity exceeds a critical threshold, causing the sediment particles to be dislodged and transported downstream. Understanding general scour is crucial for assessing the stability of bridge foundations and ensuring their long-term safety.
Hec-18: HEC-18 is a guideline developed by the U.S. Army Corps of Engineers that provides procedures for the analysis of bridge scour, which is the removal of sediment from around bridge foundations due to flowing water. This guideline is crucial in understanding the different mechanisms that cause scour and in assessing the vulnerability of bridge foundations to hydraulic forces.
HEC-RAS: HEC-RAS, or Hydrologic Engineering Center's River Analysis System, is a software program used for modeling the hydraulics of water flow through rivers and channels. It provides engineers and researchers with tools to analyze flow characteristics, water surface profiles, and sediment transport, making it essential for understanding river dynamics and assessing flood risks.
Hire Equation: The Hire Equation is a mathematical relationship used to calculate the potential for sediment transport and the onset of scour around bridge foundations. This equation helps engineers understand how the flow of water can lead to erosion and removal of sediment, which is critical in evaluating bridge stability and safety. By applying this equation, engineers can better predict scour depths and design appropriate countermeasures to protect bridge structures from failure due to hydraulic forces.
Hydraulic modeling: Hydraulic modeling is a method used to simulate the flow of water and sediment in rivers, streams, and other bodies of water to predict how changes in the environment will affect hydraulic conditions. This process is crucial for understanding the interaction between water flow and structures like bridges, helping engineers assess potential issues such as scour, which can undermine the stability of foundations. Through hydraulic modeling, engineers can also design effective protection measures to enhance structure resilience and ensure safety.
Live-bed scour: Live-bed scour is the process of sediment removal around a bridge pier or abutment caused by the movement of sediment-laden water, which can lead to significant erosion and changes in riverbed topography. This phenomenon occurs when flow conditions allow sediment to be picked up and transported downstream, resulting in an ongoing interaction between sediment and hydraulic forces that can jeopardize structural stability.
Local scour: Local scour refers to the process by which sediment is eroded and removed from the vicinity of a bridge foundation due to the forces of water flow, leading to a localized depression around the structure. This phenomenon can compromise the stability and integrity of bridge foundations, making it essential to understand the mechanisms behind it and how to effectively design bridges that can withstand these hydraulic forces.
Manning's Equation: Manning's Equation is a widely used formula that estimates the velocity of water flowing in an open channel based on the channel's slope, roughness, and hydraulic radius. It provides crucial insights for understanding flow conditions and is essential for designing hydraulic structures like bridges to ensure they can withstand the forces of flowing water and prevent issues such as scour.
Melville and Chiew Method: The Melville and Chiew Method is a widely recognized approach for estimating scour depth around bridge piers and abutments, particularly under various flow conditions. This method emphasizes the relationship between flow characteristics and sediment transport mechanisms, providing engineers with a reliable tool for predicting potential scour in riverine environments. Its significance lies in enhancing the safety and design of bridge structures by allowing for better assessment of erosion risks due to flowing water.
Numerical simulation: Numerical simulation is a computational technique used to model and analyze complex systems by solving mathematical equations that represent their behavior. This method allows engineers to predict how a system will respond under various conditions, making it especially valuable for studying phenomena such as fluid dynamics, structural responses, and, specifically, scour mechanisms around bridge foundations. By creating detailed simulations, engineers can visualize potential issues and optimize designs without the need for costly physical experiments.
Riprap: Riprap refers to a protective layer of large stones or broken concrete placed along shorelines, riverbanks, or other areas prone to erosion and scour. This material helps to stabilize the soil and prevent the loss of land due to water flow, making it an essential element in managing erosion and protecting structures from the forces of water.
Scour countermeasures: Scour countermeasures are methods or structures designed to prevent or reduce the erosion of soil around bridge foundations caused by flowing water. These countermeasures help to maintain the stability and integrity of bridges by protecting them from the damaging effects of scour, which can lead to foundation failure. The effectiveness of these measures is analyzed through various mechanisms that understand how water interacts with the surrounding soil and structures.
Scour depth: Scour depth refers to the vertical distance from the original riverbed or seabed elevation to the lowest point of erosion caused by flowing water around bridge foundations or other structures. Understanding scour depth is critical for analyzing potential erosion mechanisms and determining the necessary protection measures to ensure structural stability and safety in hydraulic conditions.
Sediment transport: Sediment transport refers to the movement of solid particles, typically soil, sand, and gravel, from one location to another by the action of water, wind, or ice. This process is critical in shaping riverbeds and shorelines and is closely related to phenomena such as erosion and deposition, which significantly influence the stability and integrity of structures like bridges.
Suspended load: Suspended load refers to the portion of sediment that is carried by a fluid, such as water, and remains in suspension due to turbulence and other forces. This type of load is crucial in understanding how rivers and streams transport materials, which directly affects processes like erosion and deposition, as well as the design of structures near water bodies. The dynamics of suspended load play a significant role in scour mechanisms, influencing how sediment is removed from around bridge foundations and other infrastructures, as well as informing effective protection measures to minimize potential damage.
Water Surface Elevation: Water surface elevation refers to the height of the water surface above a reference point, usually expressed in feet or meters. This measurement is crucial for understanding hydraulic conditions and plays a vital role in analyzing how water interacts with structures such as bridges. Variations in water surface elevation can indicate changes in flow conditions, which are essential for assessing scour mechanisms around bridge foundations and other critical structures.