🌉Bridge Engineering Unit 12 – Bridge Hydraulics and Scour Analysis

Key Concepts in Bridge Hydraulics

• Hydraulics plays a crucial role in bridge design, construction, and maintenance by analyzing the flow of water around bridge structures
• Bridges must be designed to withstand various hydraulic forces, including hydrostatic pressure, hydrodynamic forces, and uplift forces
• Hydraulic engineers use principles of fluid mechanics to calculate flow velocities, discharge rates, and water surface profiles near bridges
• Froude number ($Fr = \frac{V}{\sqrt{gD}}$) is a dimensionless parameter used to characterize the flow regime around bridges (subcritical, critical, or supercritical)
• Manning's equation ($Q = \frac{1}{n}AR^{2/3}S^{1/2}$) is commonly used to estimate the discharge capacity of channels and rivers near bridges
• Backwater effects occur when a bridge constricts the flow, causing an increase in water surface elevation upstream of the structure
• Local hydraulic effects, such as flow contraction and expansion, can lead to increased flow velocities and turbulence around bridge piers and abutments

Flow Dynamics Around Bridge Structures

• Bridge piers and abutments obstruct the flow, creating complex flow patterns characterized by flow separation, vortex shedding, and wake formation
• Horseshoe vortices form at the base of bridge piers due to the interaction between the approaching flow and the pier, leading to increased local velocities and shear stresses
• Wake vortices develop downstream of bridge piers, characterized by alternating low-pressure zones that can induce vibrations in the structure
• Flow contraction occurs when the flow area is reduced by the presence of a bridge, resulting in increased flow velocities and potential scour
• Pressure flow conditions can occur when the water surface elevation exceeds the low chord of the bridge deck, leading to increased hydraulic forces on the structure
• Debris accumulation around bridge piers and abutments can alter the flow dynamics, increasing the risk of scour and structural damage
• Hydraulic jumps may form downstream of bridges in supercritical flow conditions, dissipating energy and potentially causing erosion

Types of Scour and Their Mechanisms

• Scour is the erosion of streambed or bank material due to flowing water, which can undermine the stability of bridge foundations
• General scour occurs naturally in river channels due to the erosive action of flowing water, lowering the overall bed elevation
• Contraction scour results from the increased flow velocities and shear stresses caused by the constriction of the flow area at a bridge crossing
• Local scour occurs around bridge piers and abutments due to the formation of vortices and turbulent flow structures
  • Horseshoe vortex scour forms at the base of bridge piers, causing a deep scour hole to develop
  • Wake vortex scour occurs downstream of bridge piers, resulting in the formation of a shallow but wide scour hole
• Lateral stream migration can lead to scour at bridge abutments as the channel shifts its position over time
• Bed form propagation, such as dunes and ripples, can affect the local scour depths around bridge foundations
• Pressure flow scour can occur when the bridge deck is submerged, leading to increased flow velocities and erosive forces on the streambed

Hydraulic Analysis Methods

• One-dimensional (1D) hydraulic models, such as HEC-RAS, are widely used for analyzing flow conditions and water surface profiles at bridge crossings
• Two-dimensional (2D) hydraulic models provide a more detailed representation of the flow field around bridges, accounting for lateral flow variations and complex geometries
• Three-dimensional (3D) computational fluid dynamics (CFD) models offer the highest level of detail but require significant computational resources and expertise
• Physical hydraulic modeling involves constructing a scaled model of the bridge and river system to study flow patterns and scour processes under controlled laboratory conditions
• Hybrid modeling approaches combine physical and numerical models to leverage the strengths of both methods
• Field measurements, such as acoustic Doppler current profilers (ADCPs) and particle image velocimetry (PIV), can provide valuable data for calibrating and validating hydraulic models
• Probabilistic methods, such as Monte Carlo simulations, can be used to account for uncertainties in hydraulic parameters and boundary conditions

Scour Prediction Models

• Empirical scour prediction equations, such as the Colorado State University (CSU) equation and the Melville and Coleman equation, estimate scour depths based on hydraulic and geometric parameters
• The Hydraulic Engineering Circular No. 18 (HEC-18) provides a comprehensive methodology for evaluating scour at bridges, incorporating both live-bed and clear-water scour conditions
• Numerical scour models, such as SRICOS (Scour Rate In COhesive Soils), predict scour evolution over time by simulating the erosion process of cohesive soils
• Machine learning techniques, such as artificial neural networks (ANNs) and support vector machines (SVMs), have been applied to scour prediction using historical data and field measurements
• Probabilistic scour models account for uncertainties in input parameters and provide a range of possible scour depths with associated probabilities
• Scour prediction models should be calibrated and validated using field data to ensure their accuracy and reliability
• The limitations and assumptions of each scour prediction model should be carefully considered when interpreting the results

Design Considerations for Scour Mitigation

• Scour countermeasures are designed to prevent or mitigate the adverse effects of scour on bridge foundations
• Riprap is a common countermeasure that involves placing large rocks or concrete blocks around bridge piers and abutments to armor the streambed and prevent erosion
• Guide banks, also known as spur dikes, are structures placed upstream of bridge abutments to redirect the flow and minimize scour potential
• Gabion mattresses are flexible, permeable mats filled with rocks that can be used to protect the streambed and banks from erosion
• Articulating concrete blocks (ACBs) are interconnected concrete units that provide a flexible, erosion-resistant surface for scour protection
• Pile foundations can be designed to extend below the anticipated scour depth, providing additional stability to the bridge structure
• Continuous monitoring systems, such as scour sensors and tiltmeters, can be installed to detect scour development and provide early warning for maintenance and repair
• Regular inspection and maintenance of scour countermeasures are essential to ensure their long-term effectiveness

Field Assessment Techniques

• Visual inspections are the most common method for assessing scour at bridges, involving the examination of the streambed and foundations for signs of erosion
• Probing involves the use of rods or poles to measure the depth of scour holes around bridge piers and abutments
• Sounding techniques, such as lead lines or weighted tapes, can be used to measure the water depth and detect changes in the streambed elevation
• Subsurface geophysical methods, such as ground-penetrating radar (GPR) and seismic reflection, can provide information on the extent and depth of scour without the need for excavation
• Sonar systems, including single-beam and multi-beam echosounders, can create detailed bathymetric maps of the streambed around bridges
• Scour monitoring devices, such as magnetic sliding collars and tilt sensors, can be installed on bridge piers to continuously track scour development over time
• Underwater inspections using divers or remotely operated vehicles (ROVs) may be necessary for a more detailed assessment of scour conditions and foundation integrity
• The frequency and timing of field assessments should be based on the bridge's scour criticality and the observed rate of scour progression

Case Studies and Practical Applications

• The Schoharie Creek Bridge collapse (New York, 1987) highlighted the importance of scour monitoring and timely maintenance, as undetected scour led to the failure of a pier foundation
• The Hintze Ribeiro Bridge collapse (Portugal, 2001) demonstrated the need for considering the effects of debris accumulation on scour, as a combination of scour and debris buildup led to the failure of a pier
• The I-5 Skagit River Bridge collapse (Washington, 2013) emphasized the significance of regular inspections and load capacity evaluations, as an oversized load struck and damaged the bridge, causing it to collapse
• The Malahide Viaduct collapse (Ireland, 2009) underscored the importance of hydraulic analysis and scour protection, as the failure of a pier foundation due to scour resulted in the collapse of the railway bridge
• The Wuyishan Bridge scour mitigation project (China, 2012) showcased the successful application of riprap and gabion mattresses to protect the bridge foundations from scour in a highly erosive environment
• The real-time scour monitoring system installed at the Choshi Bridge (Japan, 2008) exemplified the use of advanced sensor technologies to continuously assess scour conditions and provide early warning for maintenance
• The Hercílio Luz Bridge rehabilitation project (Brazil, 2019) demonstrated the use of numerical modeling and physical hydraulic modeling to design effective scour countermeasures for a historic bridge
• The Shuang Yuan Bridge field assessment (Taiwan, 2015) highlighted the application of sonar and geophysical techniques to evaluate scour depths and foundation conditions in a rapidly eroding river channel