Clear-water Scour Research Articles

Clear-Water Scour Mechanism at Vibrating Monopile Foundations

Clear-Water Scour Mechanism at Vibrating Monopile Foundations

Extrapolating time development curves of clear-water scour around piles: From empirical fitting to physics-based approach

The currently available prediction formulas for the scour development around a pile predominantly rely on empirical fittings, lacking clear physical mechanisms. This study integrates the principle of sediment mass conservation, the flat-bed sediment transport model, and the phenomenological theory of turbulence to propose a semi-analytical model for predicting clear-water scour development around circular piles. This model overcomes scale-related issues and is governed by three fundamental parameters: the equilibrium scour depth, the characteristic time, and the ratio of the scour depth coefficient to the primary horseshoe vortex coefficient. The results demonstrate that the proposed model is consistent with experimental data. The proposed model offers significant improvements by addressing two critical issues observed in existing models: (i) empirical formulas expressed purely in mathematical terms exhibit low prediction accuracy for long-term scour, resulting in underestimation of the extrapolated equilibrium scour depth and premature attainment of equilibrium conditions; (ii) other models exhibit a divergence phenomenon, where the extrapolated equilibrium scour depth greatly exceeds the pile diameter, contradicting prevailing understanding. Applying the proposed model allows for a substantial reduction in test duration while accurately capturing the subsequent scour development process.

Numerical Investigation of Local Scour Protection around the Foundation of an Offshore Wind Turbine

The pile foundations of offshore wind turbines face serious problems from scour damage. This study takes offshore wind turbine monopile foundations as the research object and proposes an innovative anti-scour device for the protection net. A numerical simulation research method based on CFD-DEM was used to model the local scour of the pile foundation and protection net. The validity of the numerical model was verified by comparing the simulation results of the local scour of the pile foundation under the condition of clear water scour and the results of the flume test. The permeability rate was defined to characterize the overwatering of the protection net, and numerical simulations were performed for protection nets with permeability in the range of 0.681 to 0.802. The flow field perturbations, changes in washout pit morphology, and changes in washout depth development due to the protective netting were also analyzed. It was found that the protection net can effectively reduce the flow velocity around the pile, cut down the intensity of the submerged water in front of the pile, and provide scouring protection. Finally, the analysis and summary of the protection efficiency of the different protection nets revealed that the protection efficiency within the nets was consistently the highest. On the outside of the net, the protection efficiency is poor at a small permeability rate but increases with an increasing permeability rate.

Stage–discharge relationship in an erodible compound channel with overbank floods

Due to environmental changes and anthropogenic activities, a dramatically diminished upstream sediment supply can promote riverbed erosion and alter the stage–discharge relationship in a channel with overbank flows, which can impact flood control. It is important to understand how the stage–discharge relationship in an erodible compound channel is affected and how to predict this relationship. In this study, we performed laboratory experiments in a straight compound channel with a mobile main channel under clear water scour. Stage–discharge relationships were measured before the destruction of the armor layer and after the formation of a new armor layer at the riverbed surface. The results indicated that the impact of riverbed erosion on the stage–discharge relationship cannot be ignored. Under the same discharge, the flow depth in the main channel increased, while the flow depth in the floodplains decreased after the new armor layer was formed relative to the case before armor layer destruction. Considering the impacts of the erosion depth and the variation in the bed resistance, we proposed a new method for predicting the stage–discharge relationship in an erodible compound channel after riverbed erosion. The predicted stage–discharge relationships agreed with the measurements, indicating that the proposed method could be used to precisely predict the stage–discharge relationship after riverbed erosion occurrence. The comparison of the results of the proposed method with and without considering the erosion depth supported the idea that the impact of riverbed erosion on the stage–discharge relationship must be included. Finally, the proposed method was further used to examine the influence of floodplain encroachment on flood control and riverbed erosion. Floodplain encroachment produced a larger flood flow depth on floodplains and a higher mean velocity in the main channel, suggesting that floodplain encroachment should be avoided to eliminate possible increases in flood disasters and riverbed erosion risks.

Clear Water Scour Depth Prediction using Gradient Boosting Machine and Deep Learning

The scouring process in adjacent to spur dikes has the potential for compromising the stability of riverbanks. Hence, it is necessary for river engineering to conduct precise measurement of maximum scour depth in the vicinity of spur dikes. Nevertheless, the determination of the maximum scour depth has proven to be a challenging task, primarily due to the complex nature of the scour phenomena associated with these structures. In this study, two data-driven models, namely the Gradient Boost Machine (GBM) and Deep Learning (DL), were developed to predict the clear water scour depth near to a spur dike. A total of 154 distinct observations have been collected from previous literatures. A total of 103 observations were utilized for training the model, while 53 observation were allocated for validation purposes. Several performance assessment measures were employed to evaluate the performance of the models, including the correlation coefficient (CC), root-coefficient of determination (R2), scattered plot, variation plot, and box plot. GBM outperformed the DL on the basis of above-mentioned assessment measures. Sensitivity analysis suggests that l/d50 is the most influences input parameter. Thus, the conclusion suggested that both the data-driven model can be used in the prediction of the clear water scour depth around spur dikes but GBM have highest accuracy.

The universal two-fifths law of pier scour

Understanding scour at bridge piers is crucial for safeguarding public safety, ensuring infrastructure resilience, and planning effective maintenance. Despite over six decades of extensive studies aiming to develop predictive formulas for the equilibrium scour depth at bridge piers, more than 20 000 highway bridges in the United States have been spotted “scour critical.” The traditional reliance on existing empirical formulas has posed a severe challenge for researchers, hindering to achieve a unified relation for the equilibrium scour depth from a fundamental scientific tenet. This perspective article presents a breakthrough—a universal law governing the equilibrium scour depth at a circular pier embedded in a sediment bed, specifically in clear-water scour condition. Derived from a phenomenological model, the universal law reveals that the equilibrium scour depth to pier diameter ratio obeys a consistent two-fifths scaling law with the introduction of a newly coined pier-scour number. This number accounts for all the key parameters involved in a local scour phenomenon, including the approach mean flow velocity, threshold shear velocity for sediment grain motion, approach flow depth, pier diameter, and sediment grain size. Importantly, the scaling law contains an additional term involving the drag coefficient raised to the power of 2/5, addressing the impact of the pier shape on the equilibrium scour depth. The derived universal law undergoes the validation through an extensive dataset of experimental measurements on circular pier scour.

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.

Scouring around bridge pier: A comprehensive analysis of scour depth predictive equations for clear-water and live-bed scouring conditions

Abstract The failure of bridges, attributed to bridge pier scouring, poses a significant challenge in ensuring safe and cost-effective design. Numerous laboratory and field experiments have been conducted to comprehend the mechanisms and predict the maximum equilibrium scour depth around bridge piers. Over the last eight decades, various empirical methods have been developed, with different authors incorporating diverse influencing parameters that significantly impact the estimation of equilibrium scour depth around bridge piers. This paper aims to consolidate: (1) available experimental and field data sets on different types of bridge pier scouring, (2) the influence of flow and roughness parameters on both clear water scouring (CWS) and live bed scouring (LBS), and (3) existing empirical equations suitable for computing equilibrium scour depth around a bridge pier under CWS and LBS conditions. The presented research encompasses over 80 experimental/field data sets and more than 60 scour-predicting equations developed for CWS and LBS conditions in the past eight decades. Based on the performance of different empirical models in predicting scour depth ratio, suitable models are recommended for CWS and LBS conditions.

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.

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).

Clear Water Scour at Varied Pile-Cap Elevation and Skewed Bridge Piers

Clear Water Scour at Varied Pile-Cap Elevation and Skewed Bridge Piers

Numerical Study of the Flow and Blockage Ratio of Cylindrical Pier Local Scour

A three-dimensional large eddy simulation model is used to simulate the turbulent flow dynamics around a circular pier in live-bed and clear-water scour conditions. The Navier–Stokes equations are transformed into a σ-coordinate system and solved using a second-order unstructured triangular finite-volume method. We simulate the bed evolution by solving the Exner-Polya equation assisted by a sand-slide model as a correction method. The bedload transport rate is based on the model of Engelund and Fredsœ. The model was validated for live-bed conditions in a wide channel and clear-water conditions in a narrow channel against the experimental data found in the literature. The in-house model NSMP3D can successfully produce both the live-bed and clear-water scouring throughout a stable long-term simulation. The flow model was used to study the effects of the blockage ratio in the flow near the pier in clear-water conditions, particularly the contraction effect at the zone where the scour hole starts to form. The scour depth in the clear water simulations is generally deeper than the live-bed simulations. In clear-water, the results show that the present model is able to qualitatively and quantitatively capture the hydrodynamic and morphodynamic processes near the bed. In comparison to the wide channel situation, the simulations indicate that the scour rate is faster in the narrow channel case.

Influence of collar's shape on scour hole geometry at circular pier

Bridge collapses, which are frequently caused by scouring or other hydraulic complexities, have disastrous socioeconomically consequences. Accordingly, this paper offers clear water scour experimental investigation that assesses the impacts of circular and novel hexagonal collar shapes on scouring around circular piers which embedded in a uniform sand bed. Scour geometry was analyzed for both protected and unprotected piers, along with evaluating the percentage reductions of the scour hole dimensions and volume. The results demonstrate that the suggested collars have a considerable effect on separation, stagnation, and dissipation of the impinging downflow and vortices, resulting in a substantial impact on the scour geometry and significantly mitigates local scour. In conjunction with the laboratory data, empirical predicting formulas for scour dimensions and novel coefficients for the effect of collar shape are proposed. To incorporate a novel consideration of estimating scour depth at collared piers, the common prediction equations were updated by adding the newly developed collar shape coefficient. The proposed and upgraded formulas are compared and validated, and the findings demonstrate robust consistency.

Clear Water Scour at Rectangular Bridge Pier with Collar

Clear Water Scour at Rectangular Bridge Pier with Collar

Experimental Investigation and Field Evaluation of Anti-Scouring and Protection Material of Earth-rock Dam

Earth-rock dams have important flood control, silt reduction, water supply, and ecological benefits, but extreme rainstorms seriously threatens the safe operation of earth-rock dams. Therefore, based on the self-developed soil stabilizer, the laboratory tests and field evaluation tests for the anti-scouring performance for solidified soil were carried out to achieve the requirements of non-collapse or slow-collapse for the earth-rock dams. Firstly, the basic performance indexes of solidified soil using soil stabilizer and 42.5# P.O Portland cement were compared. Then, clear water scouring and muddy water scouring tests were carried out based on the self-developed anti-scouring test system. Furthermore, an earth-rock dam test dam was built by using the solidified soil as the material, and a field scouring test was carried out to analyze the corresponding hydraulic elements. Finally, a safety inspection of the dam structure was carried out. The results demonstrate that when compared to the results of solidified soil using 42.5# P.O Portland cement under the same contents, the strength and durability of soil stabilizer solidified soil rose by more than 5% and 10%, respectively. The performance of soil stabilizer is better than that of 42.5# P.O Portland cement. After 10 hours of approximately 15 m/s water flow scouring through the experiment test, the performance of the solidified soil under the higher content of soil stabilizer can meet the design requirements of anti-scouring performance. The field test results show that the maximum scouring depth is less than 2 cm, and the average scouring depth is less than 0.8 cm. The monitoring results show that the displacements are less than 5 mm, and the dam structure is safe and stable. It is recommended that the content of soil stabilizer should be higher than 20% to meet the overflow safety requirements of small and medium earth-rock dams.

Scour Development Around an Oblong Bridge Pier: A Numerical and Experimental Study

The complex flow structure around bridge piers is challenging for both experimental and numerical studies. Therefore, investigating the capabilities of Computational Fluid Dynamics (CFD) tools in resolving the flow structure and the mechanism of sediment entrainment into and out of the scour hole remains a challenging task. In this study, the scour depth around an oblong bridge pier and the bed shear stress distributions in time and space were numerically investigated using the Computational Fluid Dynamics (CFD) tool Sediment Simulation In Intakes with Multiblock option (SSIIM). Clear water scour conditions and sand of known granulometric composition were considered in accordance with the experimental study carried out. Laboratory data and the results of a scour characterization around a 0.11 m wide oblong bridge pier were considered to calibrate and validate the numerical model. The averaged form of the Navier–Stokes equations was considered to simulate the turbulent flow fields in anticipation of long time scales. The results show that calibrated numerical models can reproduce measured scour depths in the laboratory environment with considerable accuracy, with an average relative error of less than 3%, especially around oblong bridge piers.

Modelling of clear water scour depth around bridge piers using M5 tree and ANN-PSO

Abstract Scouring refers to the process by which bed sediment in a river is eroded around the periphery of a bridge abutment or pier. Many empirical models are available to estimate the scour depth for different flow, geometry, and bed roughness condition. However, none of them provide a better estimation of scour depth for a wide range of input parameters. Thus, in this paper, the scour depth around bridge piers has been modelled using M5 tree and hybrid artificial neural network (ANN)-particle swarm optimisation (PSO) techniques by considering the wide range of datasets. The clear-water scouring (CWS) datasets are collected from the literature and five different non-dimensional influencing parameters are selected as input parameters to model the scour depth. A Gamma test (GT) was performed to choose the best input parameter combinations. Based on the lowest gamma value and V-ratio, 4 out of 26 distinct input combinations for CWS depth modelling were chosen in the GT. According to statistical measures, the proposed M5 tree model predicts scour depth better than empirical approaches. Additionally, the developed ANN-PSO model is suitable for determining scour depth in both rectangular and circular shapes of piers. The results of both developed models are compared with other existing models and found to be satisfactory.

Simulation Study on Local Scour Characteristics of Tandem Bridge Piers in a Straight River under a Changing Environment

Hydrodynamics is a common manifestation that causes natural scouring of riverbeds, and it is one of the factors that exacerbate the natural disasters of local scouring of bridge piers, causing sustainability of environmental changes in the water. The evolution pattern and scour characteristics of the bed surface around the submerged structures under different scouring conditions vary greatly. In order to investigate the scour mechanism, the reformed group (RNG) turbulence model in computational fluid dynamics (CFD) simulation software (v11.2) was used to simulate the scour under the clear-water scour and live-bed scour environments, and different scour morphology characteristics around the tandem piers under the clear-water scour and live-bed scour environments were obtained in the final simulation. By capturing the cross-sectional vortex and bed shear stress during the scouring process, the characteristic pattern of scouring topography around the pier and the relationship between the scour hole structure scale were analyzed, and the relationship equation between the development of scour depth and time scale was established. The study shows that: under the clear-water scouring environment, the sediment transport rate lags behind, but the contribution time is superior; under the live-bed scouring environment, by the shading and reinforcement influence of the upstream piers, the extent and development of the downstream pier surrounding the scour hole is small; the development trend of the maximum sediment transport rate of the scour hole and the great value of the shear stress is more synergistic, and the peri-pier eddy is positively correlated with the bed shear stress; through the regression equation to compare the relevant test and simulation results, the two are in good agreement, indicating that the simulated local scour evolution law is consistent with the actual law.

Numerical investigation of the local impact of hydrokinetic turbine on sediment transport—Comparison between two actuator models

Hydrokinetic turbines interact with the dynamics of the sedimentary bottom at small and large scale. Despite the interest that the study of these interactions deserves, little research has been published in the field. In this paper, we investigate by numerical simulation the interaction between modeled hydrokinetic turbines and bed load under clear-water scour conditions. The mixture of water and sediment is accounted for by an Eulerian multiphase model developed in the open source platform OpenFOAM. The turbine blades are parameterized by two models: the Blade Element Method (BEM) and the Actuator Disc Theory (AD). A good agreement with measurements is obtained in the near wake. The effects of the two different turbine models on the bedload sediment transport and on the wake characteristics are then examined. The local impact of the turbine modeled by BEM is more pronounced on the bed than the one modeled by the AD. So, the BEM modeling is preferable to the AD one for local studies of the impact of a turbine on the bed morphology. However, combining BEM with the two-phase fluid-sediment is time consuming and make difficult the use of this method to study a farm of several stream turbines. In such a case the use of AD is still remains recommended for now. Moreover, more studies must be done at real scale as the turbine rotational speed is relatively low and could induced less impact of the swirl on the bottom shear stress.

A model for predicting the grain size distribution of an armor layer under clear water scouring

Clear water scouring often occurs in the channel downstream of a dam and changes the grain size distribution of the channel bed, which impacts the bed resistance and flood conveyance capability. Based on the proposed spatial-mean rolling probability of the non-cohesive sediment and the effects of the constantly decreasing sand content during riverbed armoring, a model was established to predict the grain size distribution of an armor layer under clear water scouring. Based on iterative calculation and the new spatial-mean rolling probability, the predicted grain size distribution of an armored bed was found to be in good agreement with measurements from laboratory experiments and field studies. This indicates that the new model is capable of accurately predicting the grain size distribution of an armor layer. Finally, the model was applied to predict the grain size distribution of the bed load and the depth of bed erosion in combination with an existing estimator of the thickness of the active layer in a channel bed. A high friction velocity transported more coarse sediment downstream and resulted in more significant scouring.