Local Scour Process Research Articles

Laboratory Study of Local Scour Around an Array of Pile Groups in Clear-Water Scour Conditions

Current-induced local scour around pile groups weakens the capacity of structures. In this paper, experimental tests of local scour around an array of 5 × 5 pile groups were conducted in a steady current in a hydraulic flume. The pile-to-pile space was five times the diameter of a single pile. All the tests were in clear-water scour conditions. The effects of upstream piles on the local scour characteristics of downstream piles, as well as the outer-arranged side piles on the inner-arranged piles, were studied within flow intensities of 0.37–1.0. Both the three-dimensional topography of bed elevation changes and the maximum temporal scour depths are discussed. The results showed that the minimum threshold of flow intensity that can induce local scour around the pile groups was 0.40. The scour holes were independent of each other, though a global scouring phenomenon occurred between piles at a flow intensity of 1.0. The temporal scour depths of the downstream piles increased slowly throughout the local scour processes. During the initial scouring stage, they accelerated rapidly. At flow intensities of 0.60, 0.80, and 1.0, the scour development then progressed gradually, resembling the behavior of a single pile. The developing scouring stage can hardly be distinguished in the case of flow intensity of 0.80. The maximum scour depths in the flow intensity of 0.60 showed irregular variations with increasing row and column numbers. The equilibrium scour depths in the central-positioned piles tended to a constant value of 0.5 times the pile diameter. In larger flow intensities of 0.80 and 1.0, they decreased linearly with pile row number, with the maximum scour depths at the piles in the first row. The local scour depths of the inner-positioned piles in the parallel arrangement showed few differences at the front and rear piles.

Computational Simulation of Monopile Scour under Tidal Flow Considering Suspended Energy Dissipation

Local scour around bridge foundations significantly impacts the stability and safety of marine structures. The development of scour holes adjacent to the pile foundations of sea-crossing bridges, influenced by tidal currents, involves multidimensional physical fields, multiscale coupling, and complex variations in marine loads. However, experimental models alone are inadequate for investigating the underlying mechanisms. Numerical simulation, a critical tool for studying local scour processes, faces the challenge of accurately modeling sediment transport, particularly under tidal flow conditions near pile foundations. To solve this challenge, this research considers the effect of reciprocating flow on sediment shear as well as its characteristic dissipation based on the immersed boundary method, introduces a reciprocating flow dissipation mechanism, and adds a momentum exchange term between the fluid and the sediment to derive a new controlling equation; a new tidal flow localized scour solver is ultimately constructed, termed TidalflowFOAM. The solver effectively simulates complex flow conditions under tidal currents, extending the modeling capabilities to more realistic three-dimensional bridge scour scenarios under combined wave and current conditions. Validation through cases reported in the literature and a series of controlled experiments, encompassing varying depths, flow velocities, and pile diameters, demonstrates the solver’s proficiency in capturing post-vortex data and accurately reflecting the influence of key factors on scour depth. However, the fidelity of the simulated scour hole morphology under tidal flow conditions behind the piles requires enhancement. The proposed numerical model for tidal flow conditions has high solution accuracy and can guide practical engineering applications.

Incompressible Smoothed Particle Hydrodynamics Simulation of Sediment Erosion around Submarine Pipelines

Sediment erosion around submarine pipelines is a popular topic, widely investigated in both ocean and submarine-pipeline engineering. In this paper, the incompressible smoothed-particle hydrodynamics (ISPH) method is modified for simulation of local scouring process around the submarine pipeline under the action of unidirectional flow. The erosion model is based on the Clear Water Particle–Turbid Water Particle–Critical Shear Stress (CWP-TWP-CSS) concept, and a sand–water two-phase model is proposed to deal with the sediment-entrained flow. The results of the numerical simulation are compared with the experimental data to verify the accuracy and applicability of the numerical model. The scouring process around the pipeline is investigated under different conditions, i.e., pipeline diameters, gap ratios, and flow velocities. The ISPH model is further used to study the flow characteristics of the scour pits around the submarine pipeline and the influence of the vortices on the maximum scour depth, to provide a theoretical basis for the stability design of submarine pipelines.

Experimental investigation on local scour downstream of stepped chutes with a flip bucket

Stepped chutes and flip buckets are the most prevalent energy dissipators in emergency spillways at dams. Given the impact that scour depth has on the stability and safety of these structures, the effects of stepped and smooth chutes on the local scour process downstream of a flip bucket for various hydraulic and geometric conditions were experimentally investigated in the present study. Experiments were conducted for 1:1 and 1:2 chute slopes (V: H), bucket exit angles of 15° and 30°, and a ratio of critical flow depth to step height (yc/h) between 0.53 and 0.97. The comparison of results revealed that installing steps over the chute profile reduced the maximum scour depth downstream in range of 15–51 percent, compared to smooth chute. By changing chute slope from 1:1 to 1:2, the maximum scour depth was reduced by an average of 16 and 25 percent for smooth and stepped chutes, respectively.

Tidal currents-induced scour development around pile foundations: Effects of flow velocity hydrograph

There is a lack of research in the existing literature regarding the scour around foundations for offshore wind turbines under tidal currents, which primarily relies on laboratory experiments with simplified flow velocity hydrographs like square tidal currents. To improve the prediction accuracy of scour development under tidal currents, a series of experiments were conducted to investigate the local scour process around a pile foundation under two typical tidal velocity hydrographs (sinusoidal and square tidal currents) in a specially designed fluid-structure-soil coupling flume. The results demonstrate that reciprocating tidal currents lead to a continuously evolving process of sediment erosion and backfilling around the pile. Although the shapes of the scour holes are similar between sinusoidal and square tidal currents, there are significant differences in the evolving process of the scour depth, presenting a short-platform shape and serrated shape, respectively. A consistent relationship is found between the dimensionless scour depth and the dimensionless effective flow work (DFW) under both tidal and unidirectional currents. An equivalent velocity expression for sinusoidal and square tidal currents is proposed and verified using existing experimental data. Furthermore, an empirical expression for the scour depth reduction coefficient between square tidal currents and unidirectional currents is proposed. These outcomes not only establish a theoretical approach for simplifying tidal currents hydrographs in laboratory experiments, but also provide practical guidance for assessing the tidal currents-induced scour development around pile foundations for in-situ offshore wind turbines.

A study of the effect of local scour on the flow field near the spur dike

The flow field near a spur dike such as down flow and horseshoe vortex system (HVS) are susceptible to the topographic changes in the local scouring process, resulting in variation of the sediment transport with time. In this study, large eddy simulations with fixed-bed at different scouring stages were conducted to investigate the changes in flow field. The results imply that the bed deformation leads to an increase in flow rate per unit area, which represent the capability of sediment transportation by water, in the scour hole. Moreover, the intensity of turbulent kinetic energy and bimodal motion near the sand bed induced by the HVS were also varied. However, the peak moments between the two sediment transport mechanisms were different. Hence, understanding the complex feedback mechanism between topography and flow field is essential for the local scour problem.

An Experimental Investigation on Dike Stabilization against Floods

A flood protection dike blends seamlessly with natural surroundings. These dikes stand as vital shields, mitigating the catastrophic effects of floods and preserving both communities and ecosystems. Their design not only aids in controlling water flow but also ensures minimal disruption to the local environment and its biodiversity. The present study used a uniform cohesionless sand with d50 = 0.9 mm to investigate the local scour process near a single combined dike (permeable and impermeable), replicating a flooding scenario. The experiments revealed that the maximum scour depth is likely to occur at the upstream edge of the dike, resembling a local scour observed around a scaled-down emerged dike in an open channel. The scour hole downstream of the dike gets shallower as it gets smaller, as do the horseshoe vortices that surround it. Additionally, by combining different pile shapes, the flow surrounding the dike was changed to reduce horseshoe vortices, resulting in scour length and depth reductions of 48% at the nose and 45% and 65% at the upstream and downstream dike–wall junction, respectively. Contrarily, the deposition height downstream of the dike had a reciprocal effect on permeability, which can severely harm the riverbank defense system. The combined dike demonstrates their ability to mitigate scour by reducing the flow swirls formed around the dike. The suggested solutions can slow down the rapid deterioration and shield the dike and other river training infrastructure from scour-caused failures.

Numerical Modeling of Local Scour in the Vicinity of Bridge Abutments When Covered with Ice

The occurrence of bridge collapse is frequently attributed to the prevalent erosion in the vicinity of bridge abutments. Accurate determination of the maximum scour depth in the vicinity of bridge abutments is imperative to ensure a secure and reliable bridge design. The phenomenon of local scour at bridge abutments can exhibit significant variations when compared to open-flow conditions, primarily due to the additional obstacle posed by the presence of ice cover. Research has demonstrated that the erosion of bridge abutments is more pronounced in the presence of ice. The vertical velocity distribution has a direct impact on both the bed shear stress and the resulting scour geometry in ice-covered conditions. This study aims to analyze the effects of flow through open channels and covered flow conditions on the local scour process around semi-circular and square bridge abutments using FLOW-3D V 11.2 software. The utilization of the volume of fluid (VOF) approach is employed for monitoring the free surface, while the Reynolds averaged Navier-Stokes (RANS) equations and the RNG k turbulence model are employed for simulating the flow field in the vicinity of bridge abutments. The sediment transport equations formulated by Meyer-Peter and Müller were employed for the purpose of simulating the movement of sediment particles. The numerical simulation results are compared with the experimental results. The result shows that the presence of ice cover and its roughness can increase the maximum scour depth both in numerical and experimental studies. The results also indicate that the maximum scour depth is located at the upstream section of the bridge abutments. These findings demonstrate the ability of the numerical model to predict the occurrence of local scour in the vicinity of bridge abutments under conditions of ice presence.

Numerical investigation of scour around the monopile using CFD-DEM coupling method

The risks posed by local scour around offshore monopile foundations are substantial and threaten the structural safety of these installations. It is therefore essential to study the mechanisms of local scour to mitigate these risks. Numerical simulation is an effective tool to help us understand the mechanisms of local scour. However, the accurate modeling of the local scour process remains to be challenging due to the complexity of sediment transportation, in which the continuity threatment of sediment phase is typically used, the discrete behavior of the sediment is hard to be described. Therefore, in this paper, in order to conduct the deepin investigation into the mechanism of the local scour around monopile, a three-dimensional CFD-DEM model was established by coupling the computational fluid dynamic method (CFD) and discrete element method (DEM). To increase the efficiency and accuracy of the CFD-DEM model, the coarse grain method (CGM) was used to decrease the number of required DEM particles in the simulation, and the angle of repose was used to calibrate the contact parameters between DEM particles. During the validation process of the numerical model, it was found that the development of the scour depth and the morphology of scour pit obtained by the CFD-DEM model showed good agreement with the previously published results. The CFD-DEM simulation revealed that seepage-induced vertical drag force plays a crucial role in the initial stage of local scour. Two sediment transportation processes, "Pile toe erosion-Slope avalanche" and "Push-Accumulation-Wash", were observed and the relationship between bedload sediment flux and the Shields number was quantified. In the end of this paper, the critical evaluation of the CFD-DEM method was conducted.

Numerical Study of the Local Scouring Process and Influencing Factors of Semi-Exposed Submarine Cables

Local scouring might result in the spanning of submarine cables, endangering their mechanical and electrical properties. In this contribution, a three-dimensional computational fluid dynamics simulation model is developed using FLOW-3D, and the scouring process of semi-exposed submarine cables is investigated. The effects of the sediment critical Shields number, sediment density, and ocean current velocity on local scouring are discussed, and variation rules for the submarine cables’ spanning time are provided. The results indicate that three scouring holes are formed around the submarine cables. The location of the bottom of the holes corresponds to that of the maximum shear velocity. The continuous development of scouring holes at the wake position leads to the spanning of the submarine cables. The increase in the sediment’s critical Shields number and sediment density, as well as the decrease in the ocean current velocity, will extend the time for maintaining the stability of the upstream scouring hole and retard the development velocity of the wake position and downstream scouring holes. The spanning time has a cubic relationship with the sediment’s critical Shields number, a linear relationship with the sediment density, and an exponential relationship with the ocean current velocity. In this paper, the local scouring process of semi-exposed submarine cables is studied, which provides a theoretical basis for the operation and maintenance of submarine cables.

Scour around the single emergent cylinder due to subcritical and supercritical approach flow conditions

ABSTRACT Flume experiments with a uniform cohesionless sand with d 50 = 0.9 m m were utilized in the current study to examine the local scour process around a single emergent cylinder under various geometrical circumstances and with a various flow characteristic that simulates an inundating tsunami flow. This single emergent cylinder represents a piloti-type column located over a movable bed material because piloti-type columns are used to elevate the building structures from the ground level. In the experiments, it was noted that the maximum scour depth probably occurs at the cylinder’s upstream face, resembling local scour around a scaled-down bridge pier in an open channel. Additionally, the scour hole around the cylinder is shallower the smaller they are, as are the horseshoe vortices that surround them. Furthermore, the effects of the surface bow-wave, hydraulic jump detachment and the wall-jet over the front face of the emerging cylinder were used to identify the relationship concerning the scouring pattern and selected flow characteristics over a piloti-type emergent column under tsunami flow. Empirical formulae have been finally developed to determine the maximum scour depth in front of the cylinder, which is beneficial to characterize the stability of the piloti-type columns, considering the specified Froude number conditions.

Riverbed Morphologies Induced by Local Scour Processes at Single Spur Dike and Spur Dikes in Cascade

Spur dikes are elongated structures extending from banks into rivers that mitigate erosion by forcing the flow away from the bank. The research on grouped spur dikes is insufficient in comparison with those on isolated spur dikes. Most of the studies focus on the maximum scour depth, omitting the bed morphological changes induced by local scour processes. Moreover, as yet, there is no established procedure for predicting the scour depth around spur dikes. This study aims to provide insights into the temporal and spatial morphological patterns around a single spur dike and spur dikes in cascade (three, five, and seven, consecutively). Experiments (of up to 318 h) were performed on a rectangular straight channel with dimensions of 20 m (length) × 1.0 m (width) × 1.0 m (depth). Nearly uniform sand with median grain size of 1.7 mm and sediment gradation of 1.5 was used for the mobile bed. The spacing between the elements for the spur dikes in cascade was 3b, where b is the spur dike width. All runs were conducted under a clear-water regime and steady flow conditions. Some limitations of the formulas for the equilibrium scour depth at the first spur dike reported in the literature were emphasised, with underestimations up to 160% and overestimations up to 200% at the earlier scour stages. The temporal evolution of the scour depth at the first spur dike was satisfactorily predicted with a correlation coefficient of 0.91. The scour processes at the other spur dikes were delayed and started at a dimensionless time greater than approximately 103. However, the scour rates increased to a high degree, with the scour depths tending to match those at the first spur dike.

Numerical Simulation on the Local Scour Processing and Influencing Factors of Submarine Pipeline

To investigate the different influencing characteristics of local scour around submarine pipelines, hydrodynamic and sediment transport two-dimensional models based on Flow-3D are used to numerically simulate the local scour around the pipeline under steady currents. An RNG k-ε turbulence model is applied to simulate the turbulent flow field around the pipeline. The instantaneous shear stress of the bed surface is taken as the starting and transporting conditions of the sediment. The simulation results of the equilibrium scour depth and terrain around the pipeline are verified with the previous experimental results, which perform with good agreement. Then, the numerical simulation method is applied to investigate the local scour process around the pipeline. The results show that shear stress is the main driving force of scour around a pipeline. The velocity, sediment grain size, pipeline diameter, and the initial gap between the pipeline and the seabed, significantly affects submarine pipeline equilibrium scour depth and terrain in varying degrees.

Experimental Study of Local Scour Around Circularcrossing and Square-crossing Piles in Waves and Current

Experimental tests of local scour around a circular-crossing pile (CCP) and a square-crossing pile (SCP) were carried out to study the effects of superimposed waves upon current in the local scour process and the differences in characteristics in the two shaped piles. Non-uniform sediment with a gradation of 1.6 was used in the tests. Stokes waves of 0.11 m in height and 1.6 s in period were generated in a 50 cm water depth flume. The ratios of Shields number to critical Shields number in the current-only tests, the waves-only test, and the waves-current tests were individually 0.90, 1.48, and 1.83, respectively. The Keulegan-Carpenter number (KC) was kept at a constant value of 2.9, which was expected not to generate a horseshoe vortex or a wake vortex in the waves-only condition. The topography at various scour durations, temporal bed elevation profiles, and temporal scour depths were measured and discussed. The results showed that sediment backfilling into the scour hole was caused by moving sand ripples. The limit value under which the horseshoe vortex or wake vortex was absent in the waves-only condition was reduced in the waves-current condition. The temporal bed elevation profiles and scour depth development were very similar between the SCP and CCP, although the scour hole and sand dunes in the former were individually deeper and larger than the latter. The maximum scour depth in the SCP was nearly equal to that of the CCP in the waves-current condition, but it was much larger in the current-only condition.

Scouring of a granular bed by dam break: Experimental study and numerical simulation by a VOF-LPT coupling

The first part of the research reported here consists of an experimental campaign to study the scouring of a granular bed (glass beads, sand) induced by a dam break in an open channel. Two configurations are considered: with and without cylinders. In the second part of this study, the volume of fluid method coupled with the shear stress transport turbulent model and the lagrangian particle tracking method is used to simulate the local scour processes. The four-way coupling is realized by considering the drag force and collisions between particles. Using a vortex map and velocity profiles, the flow dynamics and scour pattern can be studied. The current research also focuses on the profiles of the erodible particle bed and the sediment transport rate to study the effects of a cylinder on local scour. The presence of the cylinder increases the experimental erosion depth by up to 42%. It was found that for relatively close particle diameters (2 and 3 mm), a granular bed with larger porosity (larger diameter) erodes the most. Experimentally and numerically it was found that the center of the granular bed experiences more erosion compared to the channel sides. In the last part of the research, comparisons between the numerical results and the experimental data allowed risk areas to be identified and the roles of porosity, sediment density, and flow dynamics on erosion to be identified.

Temporal Scour Variations at Permeable and Angled Spur Dikes under Steady and Unsteady Flows

Spur dikes are river protection structures typically used for flow diversion from erodible banks. However, scouring might be a severe problem that compromises their stability and, consequently, their hydraulic function. This paper aims to study the maximum scour depth at permeable and angled spur dikes under hydrographs of different duration. Experiments were carried out in a rectangular channel 10 m long, 0.76 m wide, and 0.6 m deep. The mobile bed was made of nearly uniform sand with a median grain size of 0.8 mm. A total of 36 new experiments were performed with a detailed data collection over the time (i.e., 216 datasets), which could provide a useful contribution to the topic. The impact of the spur dike orientation angle, θ, and the degree of permeability, φ, on the temporal scour evolution were explored. Results were found physically consistent and revealed that the spur dike permeability implies a significant attenuation of the scouring processes in comparison to the impermeable spur dikes and generally its effect is more beneficial than that from a favorable orientation angle. The differences in percentage between the maximum scour depth for impermeable spur dikes and the maximum scour depths for various degrees of spur dike permeability were found ranging from 44% (at φ = 33% and θ = 60°) up to 88% (at φ = 66% and θ = 120°). Other results include the effect of the hydrograph base-time on the scour depth and the comparison between scouring processes under steady and unsteady flow conditions. By quadrupling the hydrograph base-time, keeping constant the peak and base flood discharges, the maximum scour depths increased by about 29%, 42%, and 25% in case of impermeable spur dike, spur dike with 33% degree of permeability, and spur dike with 66% degree of permeability. Furthermore, starting from dimensional analysis a new empirical model (with coefficient of determination R2 equal to 0.94) is introduced to predict the time-dependent scour depth due to the passage of a flood wave. The model suggests that the main independent dimensionless variables which control local scour processes are: the densimetric Froude number, the time t normalized to the hydrograph base-time, the degree of permeability, and the orientation angle. These dimensionless variables would generalize the laboratory results to the real-world scenarios, although caution should always be taken because of possible scale effects.

Numerical study of scour below vibrating pipelines under waves and currents

Local scour of the seabed near a submarine pipeline is a major factor in pipeline damage and failure. Previous studies on local scour of submarine pipelines mainly focused on fluid flow and sediment transport, ignoring the influence of vibration on the local scour process. In this study, a two-dimensional scour model based on the time-averaged N–S equation was established to analyze the local scour and vortex-induced vibration of the pipeline. The clear-water and live-bed scours beneath submerged horizontal pipelines are presented under current-wave coupling conditions. The study shows that when the submarine pipeline is partially suspended, the vibration of the submarine pipeline is gradually intensified under the action of waves and currents and then affects the scour process near the pipeline, especially for KC numbers larger than 0.8. In addition, thresholds of embedment depth e/D = 0.4 and relative water depth h/D = 6.7 were found, that is, the influence of bed and free surface on the scouring process are greatly reduced once these thresholds are met.

Local Scour around Tandem Double Piers under an Ice Cover

Compared to the scour around a single pier, the local scour process around tandem double piers is much more complicated. Based on laboratory experiments in a flume, we conducted the scour process around tandem double piers under an ice-covered flow condition. The results showed that when the pier spacing ratio L/D = 2 (where L = the pier spacing distance, and D = the pier diameter), the rear pier (the downstream one) will intensify the horseshoe vortex process behind the front pier, and the scour depth around the front pier will increase by about 10%. As the pier spacing ratio L/D increases, the scour depth around the front pier will gradually decrease. When the pier spacing ratio L/D = 5, sediment scoured around the front pier begins to deposit between these two piers. To initiate a deposition dune between piers, the pier spacing distance under an ice-covered condition is about 20% more than that under an open flow condition. The results also showed that the existence of the rear pier will lead to an increase in the length of the scour hole but a decrease in the depth of the scour hole around the front pier. The local scour around the front pier interacts with the local scour of the rear pier. The maximum scour depth of the scour hole around the rear pier increases first, then decreases and increases again afterward. When the pier spacing ratio L/D = 9, the scour depth around the rear pier is the least. With an increase in the pier spacing ratio, the influence of the local scour around the front pier on the local scour around the rear pier gradually decreases. When the pier spacing ratio L/D is more than 17, the scour around the front pier has hardly any influence on that around the rear pier. The scour depth around the rear pier is about 90% of that around the front pier.

Investigation of Local Scour Hole Dimensions around Circular Bridge Piers under Steady State Conditions

The local scour around bridge piers is one of the main causes of bridge failures. In this study, scour hole dimensions around circular bridge piers were investigated under clear water scour conditions for various steady flow rates. The experiments were performed with four different bridge pier diameters and seven different flow rates by using uniform sediment with a median diameter of 1.63 mm and geometric standard deviation of 1.3. After each experiments the bathymetry of scour hole was determined. New empirical equations to estimate scour hole length, scour hole width and scour hole volume (V) are proposed by using experimental findings and experimental data available in the literature. The experimental results were also compared with those calculated using several empirical equations given by previous studies. Since there is a lack of data about scour hole dimensions, the experimental findings presented in this study are useful for the researchers investigating the local scour process, and have contributed to the few experimental data in the literature.

Influence of Cohesion on Scour at Piers Founded in Clay–Sand–Gravel Mixtures

An accurate prediction of scour depth around bridge piers is crucial for economical and safe design of bridge pier foundations. The main objective of the present study is to identify the influencing cohesive parameters and their effects on the local scour processes around bridge piers, depending on various proportions of clay–sand–gravel mixtures. Twenty experimental tests were performed in a channel 25 m long and 1.0 m wide for this purpose. Runs lasted from 16 to 40 h. It was noted from the experimental work that an increment of clay fraction significantly reduces the scour depth around bridge piers. It was also observed that the initiation of scour occurred at the sides of the pier where separation of flow occurred. Typically, the maximum scour depth at the equilibrium stage was still observed at the sides of the pier. A dimensional analysis was used to propose mathematical relationships assessing the temporal scour depth variation at the wake and sides of the pier. The developed relationships yielded reasonable results with maximum error of two folds for 95.22% of total data sets for scour depth at the wake and 92.57% of the total data sets for scour depth at the sides of the pier.