Scour Process Research Articles

Scour mechanism around a pipeline under different current-wave conditions using the CFD-DEM coupling model

Understanding the pipeline scour mechanism under different conditions is essential for protection measures. The computational fluid dynamics (CFD) and discrete element method (DEM) are coupled for the simulations. To reduce the computational effort, the coarse-grained method is applied, and the Darcy-Forchheimer porous model is used to replace the far field elements. Both of the porous medium and particles-fluid interaction model are validated by comparison between simulation results and theories. The scouring simulation results show a good agreement with the experiments conducted with single-diameter spherical particles under different current-wave conditions. Further, the flow around the pipeline, the seepage in the sediment bed and the force acting on the particles are analyzed. The vortex shedding exists downstream under current but the vortex moves around under waves. The seepage depends on the pressure, influenced by the pipeline and the water depth, and the variation of the seepage is discussed. The force acting on the particles is not uniform along bed and the distribution is analyzed.This paper provides an insight into scour mechanics from the aspects including seepage and flow-particle interaction. Different methods are used for modification, giving a reference for the usage of CFD-DEM model in the analysis of scour process.

Time development of clear-water scour around a pile foundation: Phenomenological theory of turbulence-based approach

Existing prediction formulas for clear-water scour development are typically empirical fittings of lab-scale results. However, it is unreasonable to evaluate the in-situ clear-water scour process around a pile based on small-scale flume tests due to inherent scale effect. This study proposes a time-dependent model of clear-water scour development around a pile foundation under steady currents. A scaling expression of shear stress acting on sediment particles at the front of a circular pile is established based on the phenomenological theory of turbulence. By applying the sediment transport model of flat-bed to local scour around a circular pile, a physics-based ordinary differential equation for predicting the scour depth development is derived. The analytical solution of scour depth development is generally more consistent with the experimental data compared with previous models. The probability density function distribution of the proposed model's error mainly concentrates within the range of ±10%, which is significantly superior to previous models. The proposed model integrates all pertinent parameters that govern the scour process using fundamental principles, rendering it free from scale issues and applicable to prototype conditions. The present model is applied to evaluating clear-water scour development around typical prototype piles with diameters ranging from 2.0 m to 10.0 m. The predicted variations of equilibrium scour time with pile diameter, flow velocity and sediment particle size aligns closely with previous experimental observations.

On the onset of pipeline scouring: Reconciling waves and currents forcing

The combined action of waves and currents can lead to the generation of freespans that have significant influence on both pipeline on-bottom stability and structural integrity. Several studies have been carried out since the 80’s to analyse the onset of the scour process and the related phenomena in the presence of either waves or currents, while only a few studies regard the combination of waves and currents. Moreover, the main empirical expressions coming from such studies cannot adequately represent the physics of the phenomena and lead to non-conservative results. For that purpose, a reanalysis of existing datasets on the onset of scour due to waves and/or currents has been carried out to obtain a coherent method for the evaluation of the critical embedment associated to such mixed flow conditions. In addition, an analytical study has been used to quantify the Keulegan–Carpenter parameter when waves are combined with currents, for a more correct application of such empirical formulas. These results will be integrated into a probabilistic model for the long-time evolution of freespans for the evaluation of the structural stability of pipelines in the marine environment, although the present approach can be exploited for a broader range of offshore applications that deal with the combined action of waves and currents.

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.

Experimental study on the optimal spur dike shape under downward seepage

ABSTRACT The successful stabilization of riverbanks and the construction of sustainable foundations, extensive knowledge of the scour process and accurate assessment of scour depth around spur dikes are required. This study focuses on optimizing the spur dike shape to manage flow turbulence effectively and minimize local scour while considering the impact of downward seepage. The investigation was conducted within an experimental flume featuring a mobile bed under a clear water regime and the provision for downward seepage. The investigation concentrated on three distinct spur dike shapes (T, L, and rectangular) under different seepage velocities (V S1 and V S2) and a no-seepage condition. The current study also illustrates changes in bed morphology, temporal evolution and longitudinal profile of scour depth with and without downward seepage. The results reveal that the downward seepage intensifies the motion of sediment particles, and more seepage leads to an escalated particle detachment, resulting in deeper scour depressions. The T-shaped spur dikes exhibited the lowest scour depths compared to the other shapes, both with and without seepage. The negative impact on velocity and RSS magnitude was observed at the near channels bed where maximum scour depth was achieved. This phenomenon is closely linked to horseshoe vortices’ formation and current generation. These factors play a pivotal role in the detachment of particles from the base of the structure.

Scour around twin-piles under combined wave–current flows

The paper presents three dimensional simulations of scour around a pair of piles arranged side-by-side configurations under combined wave–current flow conditions using a computational fluid dynamics model. The Reynolds-averaged Navier–Stokes (RANS) equation is solved using the k-ω turbulence model in the present work. The Exner equation is used to measure the variations in bed elevation. The level-Set approach is used to capture the free surface realistically. The numerical model couples the hydrodynamic module with the morphological module to simulate the scour process. For accurate erosion and deposition calculations in the sediment bed, the morphological model employs a modified bed shear stress formula on a sloping bed in combination with a sand slide algorithm. In the present study, the simulations have been done in a truncated numerical wave tank with the Dirichlet boundary condition and active wave absorption method. The numerical model is validated with the experimental results of combined wave–current hydrodynamics and scour around a pair of piles. The validated numerical model is utilized to study the effect of the gap ratio and the effect of KC number in the various combined wave–current environment. In low KC conditions, normalized scour depth increments owing to waves alone, weak currents, and moderate currents are less, whereas the scour depth increases dramatically for waves with high currents. It is also found that the gap flow between the piles increases the depth of scour in a significant manner. For a given pile gap ratio, the scour depth is dependent on the KC number, which implies the larger the KC number, the larger scour depth. For a given KC number, the scour hole stretch in the flow direction reduces as the gap ratio increases, whereas the perpendicular stretch increases. It is also observed that the normalized scour depth decreases as the gap ratio increases for a fixed KC number and fixed Ucw. It is evident from the present research that the normalized scour depth increases with increase in KC number for a fixed wave–current parameter (Ucw).

Local scour depth at piles group exposed to regular waves: On the assessment of expressions based on classification concepts and evolutionary algorithms

An accurate estimation of local scour depth around piles group is inevitably essential to provide stability of marine structures. Over the past decades, many investigations have been made to understand the scouring process at piles group exposed to waves for field and experimental scales. This study aims to predict the wave-induced local scour depth by various robust Data Driven Models (DDMs) and Machine Learning Models (MLMs) developed by classification and evolutionary concepts: Model Tree (MT), Evolutionary Polynomial Regression (EPR), Multivariate Adaptive Regression Spline (MARS), and Gene-Expression Programming (GEP). From relevant literature, 125 data series were employed to provide empirical relationships for scour depth predictions. The raw variables were related to bed sediment, pile configuration, pile geometry, approaching flow, and wave characteristics. Non-dimensional parameters have been obtained through the Buckingham theorem in order to control local scour depth. In this way, the ratio of spacing between piles to pile diameter (G/D), sediment number (Ns), pile arrangement number (ratio of the number of piles parallel to the flow; m; to the number of piles normal to the flow; n), Shields parameter (θ), and Keulegan-Carpenter (KC) were considered. From the training and testing stages of Artificial Intelligence Models (AIMs), it was found that regression equation given by MARS model provided better performance (e.g., Correlation Coefficient [R] = 0.9297, Root Mean Square Error [RMSE] = 0.3489, and Scatter Index [SI] = 0.2765) than other AI models’ efficiency. Additionally, the performance of AI models was assessed for various ranges of dimensionless parameters (i.e., G/D, Ns, and θ) versus KC variation. MARS model had the best performance for G/D = 0–1 and KC < 10 (R = 0.9917 and RMSE = 0.2198) and 0.4≤θ < 0.5 and 15 < KC < 25 whereas EPR model had promising efficiency in the its highest level for Ns = 1–3 and 10 < KC ≤ 15 (R = 0.9941 and RMSE = 0.0771) than other AI models. For the sensitivity analysis, the mutation rate and number of chromosomes are examined for the GEP model, the K-fold number and number of parameters for the MARS model, and the number of algebraic terms and number of chromosomes for the EPR model. Furthermore, ranking analysis of AI models indicated that MARS model had the best performance (Ranking Performance Index [RPI] = 0.7348) and followed by GEP (0.3412), M5MT (0.2999), and EPR (0.2848).

Experimental Investigation of the Evolution Process of Suspended Pipelines through River Bottoms under Unsteady Flow Conditions

One of the major geological hazards that can cause harm to long-distance oil and gas pipelines are water-induced disasters. These disasters are quite common and widespread. Pipelines that cross river channels are at a higher risk of facing damage due to flood-induced erosion. To shed light on the evolution pattern of riverbeds adjacent to pipelines under the influence of unsteady flow conditions, a flume model test was conducted, and the underlying mechanisms of local scour were elucidated. The experimental results demonstrate that pipelines are more susceptible to suspension during flood conditions. The suspension process of pipelines under flood conditions could be broadly divided into five stages. In comparison to constant flow conditions, the evolution process of local scour and the suspension of pipelines under unsteady flow lacked the erosion pit expansion stage, and the scour duration was shorter. Each stage exhibited distinct erosion characteristics, and both the peak flow rate and the number of flood peaks significantly impacted the maximum range and depth of the erosion pit. During pipeline-laying projects, selecting a covering layer with a larger particle size can enhance the erosion resistance of the riverbed around the pipeline. The study of the local erosion process of underwater crossing pipelines under unstable flow conditions can provide a reference for pipeline engineering design and riverbed pipeline protection strategies.

Sills and macro-roughness elements as countermeasures for bridge scour

Bridge are key infrastructures deserving attention because of their strategic role. In particular, bridges whose piers and foundations are located in water bodies are subjected to complex, localized erosion processes which may lead to undesired consequences, such as the failure of the structure. Therefore, in the last decades many researchers have analyzed the phenomenon, developing and implementing countermeasures that can mitigate the risk of a collapse. Among others, structural countermeasures like riprap, collars, and sacrificial piles have shown to be effective in reducing the maximum scour depth, under certain hydraulic conditions and geometric configurations. However, there are still some countermeasures that require further investigations. Namely, only recently, the effectiveness of transversal structures like sills and gabions have been tested. However, there are still many aspects that remain unclear, like the kinetics of the scour evolution depending on locations of the sills. In addition, macro-roughness elements may be also adopted downstream of bridge piers to protect the channel bed. This paper aims at comparing the effectiveness in controlling scour process of the last two countermeasures and provides interesting insights on the physics of the phenomenon that may be useful for practitioners. Future lines of research are also discussed.

Experimental Study on Influence of Different Patterns of an Emergent Vegetation Patch on the Flow Field and Scour/Deposition Processes in the Wake Region

AbstractFlume experiments were conducted to comprehend the impact of different patterns of an emergent vegetation patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different vegetation patch patterns, which were simulated inspired by the expansion process of a typical instream vegetation. The patch expansion process was idealized with an initially circular patch of rigid emergent stems becoming elongated due to positive and negative feedbacks. The expansion of the vegetation patch was considered to occur in three stages, in which the density of the patch from the previous stage was increased while the patch was also elongated by connecting at its downstream side with another sparser vegetation patch. These stages were replicated succesively by increasing the density and elongating the patch. In this way, two processes (i.e., elongation and decrease in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies that as the patch expands, in the wake region, the steady‐wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density.

Study on the Sand-Scouring Characteristics of Pulsed Submerged Jets Based on Experiments and Numerical Methods

Water-jet-scouring technology finds extensive applications in various fields, including marine engineering. In this study, the pulse characteristics are introduced on the basis of jet-scouring research, and the sand-scouring characteristics of a pulsed jet under different Reynolds numbers and the impact distances are deeply investigated using Flow-3D v11.2. The primary emphasis is on the comprehensive analysis of the unsteady flow structure within the scouring process, the impulse characteristics, and the geometric properties of the resulting scouring pit. The results show that both the radius and depth of the scour pit show a good linear correlation with the jet-flow rate. The concentration of suspended sediment showed an increasing and then decreasing trend with impinging distance. The study not only helps to enrich the traditional theory of jet scouring, but also provides useful guidance for engineering applications, which have certain theoretical and practical significance.

Clear-water scour at monopile foundations subjected to cyclic lateral loads in different directions

Monopile foundations have been widely used to support offshore wind turbines in shallow waters. Current-induced local scour at monopile foundations, as a universal problem threatening the structure safety and the operational efficiency of the turbines, has been extensively studied in recent decades by flume experiments and numerical models. Recent studies reported that the scour process around monopile foundations is affected by cyclic lateral loads induced by waves, currents, and winds. However, the scour mechanism around monopile foundations subjected to cyclic lateral loads in different directions remains unclear, necessitating further investigation. This paper presents an experimental study investigating the clear-water scour characteristics around monopile foundations under cyclic lateral loads applied parallel and perpendicular to the flow direction at a relatively low flow intensity. The experimental results reveal that when the amplitudes of the cyclic lateral loads exceed 4% of the monopile diameter at the monopile’s top, the equilibrium scour depths at monopile foundations subjected to cyclic lateral loads perpendicular to the flow direction are generally larger than those under cyclic lateral loads parallel to the flow direction. Sour holes extent more significantly in the direction of cyclic lateral loads rather than the unload direction, and the slope angles in the load directions are gentler than those in the unload directions. Furthermore, based on the experimental data, an empirical equation is proposed to predict the current-induced equilibrium scour depth at cyclically vibrating monopile foundations in different directions.

Effect of Chemical Pre-treatment on Screen Printed Fabric Performance Properties

Printing is the process of transferring a specific pattern or design onto the surface of textile fabric. In the printing process, screen printing is one of the fundamental printing methods on textiles and is widely used worldwide. However, screen-printed fabrics have specific properties like fastness, water absorbency, and whiteness index that increase the end-use properties of the printed fabrics. There are several factors that affect the technical characteristics of printed fabrics. This research focuses on an experimental approach to ascertain and confirm the pre-treatment process as one of the most important factors in pigment dye screen-printed fabrics. The main raw material used in this research was 100% cotton fabric with a plain weave structure. The experimental method used in this research involved the pre-treatment (singeing, scouring, and bleaching) process with different recipes and four samples. Screen-printed fabric after printing was tested for its technical properties using standard test methods. The results of a one-way ANOVA (Analysis of Variance) were analysed using MINITAB and Microsoft (MS) Excel software. The results indicate superior fabric technical properties like color fastness to rubbing and washing, coupled with improved water absorbency and excellent whiteness index values of samples printed after bleaching with Hydrogen Peroxide (H2O2). In this research, we conclude that printed fabric after bleaching with H2O2 has the best color fastness, and bleaching with NaOCl is the next one. However, printed fabric has a low fastness property after scouring and singeing.

Reducing Scour around Semi-Elliptical Bridge Abutments: Application of Roughness Elements

Bridge abutments in river channels induce local scour. Recent research indicates that introducing roughness elements on the surface of the bridge abutments can influence the flow pattern around the abutment, reducing the intensity of eddies and diverting the flow away from the abutment. The roughness elements protruding from the abutment surface, with specific thickness, protrusion, and spacing, influence the scour process by enhancing turbulence. This study investigates the impact of roughness elements and their spacing on clear water scour at bridge abutments. The results reveal a noteworthy reduction in scour depth as the spacing between roughness elements decreases and their thickness increases on the abutment surface. Furthermore, an increase in the roughness spacing to roughness protrusion ratio (s/p) leads to an amplified scour depth. Additionally, the presence of roughness on the abutment surface alters the slope characteristics of the scour hole in response to changes in flow depth. In particular, the absence of roughness exhibits an increased slope as flow depth increases, while the presence of roughness results in a reduced slope across all three flow depths examined. Notably, the maximum slope and depth of the scour hole under the influence of roughness elements occurs at angles of 50 to 70 degrees. Also, the slope and depth of the scour hole decrease to a minimum value at specific roughness dimensions (s = 0.17 L and p = 0.17 L).

Numerical investigate of the effects of surface roughness on local scour around pipeline under steady current condition

A two-dimensional numerical model was developed to investigate the effects of the surface roughness on local scour around pipeline based on Naiver-Stokes equations. Both suspended load and bedload transport are considered for the sand motion equation. Present numerical model is verified accurately for simulating local scour around pipeline. The numerical results show that the surface roughness has a significant effect on the process of local scour, i.e. forces, Shields parameter and scour depth. For rough pipeline, the average value of the drag force coefficient on the pipeline, the RMS value of the lift coefficient, the scour depth and the scour width all decrease rapidly with increasing roughness and then change slowly.

Experimental investigation of local scour around complex bridge pier of sea-crossing bridge under tidal currents

To study the process of local scour around the large-scale complex bridge pier under tidal currents, local scour experiments were carried out using the pile group-cap of the Hong Kong-Zhuhai-Macao Bridge (HZMB) as prototypes. The geological conditions at the bridge site were explored and the sampled soil was experimentally analyzed to determine the model sand. Based on the long-term observation data of tidal currents at the bridge site, a harmonic analysis of tidal currents was conducted to obtain the hydrodynamic characteristics of the tidal currents. Then a flume experiment of local scour around the large-scale pile groups under tidal currents was carried out. Research results indicate that the tidal currents velocity at the HZMB site exhibits the characteristic that the falling tide velocity is greater than the rising tide velocity. The frequency of the flow direction in the N and S directions is more significant. The tidal currents at the bridge site belong to the irregular semi-diurnal tide. The maximum depth of scour under tidal currents occurs on the side with highest velocity in the rising and falling tide process, and its value is slightly lower than the value under unidirectional flow. The duration of equilibrium scour under tidal currents is longer than under unidirectional flow. The shape of the equilibrium scour pit under tidal currents has changed from a circular arc shape on the upstream side to a saddle shape with deep ends and shallow middle, and has formed a longer sedimentation zone at both ends of the rising and falling tides compared to the situation under unidirectional flow. The size and layout of the pile foundation have a significant impact on the shape of the scour pits that achieve equilibrium scour.

Impacts of wave-induced seabed response on local scour around a pipeline: PORO–FSSI–SCOUR–FOAM

The problem of local scour around a submarine pipeline is vital for oil and gas projects. Conventional scour models around pipeline practically include the flow and sediment transport characteristics, but they ignore the interactions between flow and seabed soil in the scouring process. In this study, a new two-dimensional model for simulating the scouring process is established while considering the impact of the wave-induced soil response and the associated seepage. With the present model, the influence of seepage and soil response on the modified Shields number, repose angle and prediction of the local scour depth and profile are examined. The numerical results indicate that the influence of repose angle when modified by seepage in the whole scour process is small. Meanwhile, the wave-induced dynamic soil response promotes the development of the scour profile and eventually increases the maximum scour depth to some extent, regardless of the change in the soil characteristics. However, at smaller KC numbers, the variations in the maximum scour depth with seabed properties become more significant.

Sea level rise and the increasing frequency of inundation in Australia’s most exposed estuary

The large tidal lake systems along the Southeast Australian coast are amongst the most vulnerable estuaries in Australia to the effects of sea level rise. In these lakes, reduced tide ranges compared with the ocean, in combination with modest flood extremes, have allowed development to occur in close vertical proximity to the current mean sea level. In this study, we examine water levels within Lake Macquarie, Australia’s most exposed estuary to sea level rise. We analyse water level data from the entrance channel and the lake to investigate recent changes to the frequency and duration of inundation or flooding of low-lying streets and examine the potential impacts of future rises in sea level. Our analysis shows that the numbers of days each year when water levels exceed those of low-lying streets, while subject to some variability, have increased significantly over recent decades. The increasing frequency of inundation is attributed to both mean sea level rise and an increase in tide range over the period of available data, which is thought to be associated with scour processes related to ongoing morphological adjustment to entrance training works undertaken over a century ago. Comparison of the projected behaviour of lake and open coast water levels under sea level rise shows the lake has significantly greater sensitivity to sea level rise. Projected inundation frequency for a given amount of sea level rise within the lake is double that of open coast sites, exposing infrastructure in the estuary to increasing risk of damage.

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.

A Review of Literature on the Scour Process under Different Jets Conditions

A Review of Literature on the Scour Process under Different Jets Conditions