Scour Hole Depth Research Articles

An investigation of scouring mechanism and equilibrium state by the submerged jet using synthesized oblique velocity

This paper uses a numerical simulation method based on synthesized oblique velocity to simulate the process of submerged oblique jet scouring of the sand bed. The concept of the initial scouring equilibrium state is introduced, and curves showing the variation of scour hole depth over time at varying angles are fitted based on experimental data. This enables predicting the scour hole depth at any point during the scouring process and the time required to reach the initial scouring equilibrium state. An ultrasonic displacement sensor was used to measure the depth and width of the scour holes accurately. The evolution of the scour holes formed by the submerged oblique jet on the sand bed was systematically studied within the range of oblique angles from θ = 30°–80°. The flow structure during the scouring process and its interaction with sediment movement were further revealed through numerical simulations. The synthesized oblique velocity method effectively addresses the complexity of simulating the oblique jet. This provides a new approach to investigating the submerged oblique jet scouring of the sand bed. The findings of this study offer valuable guidance for practical applications, such as underwater equipment cleaning and deep-sea dredging.

Three-dimensional numerical study on reservoir sediment flushing through an initially covered bottom tunnel

Sediment flushing through bottom tunnels has been widely applied in engineering practice to alleviate reservoir sedimentation. However, there is limited understanding of whether flushing can be accomplished once the tunnel intake is covered by deposited sediment. Here, a three-dimensional model is used to resolve the hydro-sediment-morphodynamic processes and reveal the physical mechanism for the occurrence of reservoir sediment flushing through an initially covered bottom tunnel. The results demonstrate that the reservoir water level and initial cover layer thickness have a significant impact on the pressure difference between tunnel inlet and outlet and the resistance of tunnel walls, determining the momentum of the water-sediment mixture. If water level is low or cover layer is thick, the pressure is not sufficient to overcome the resistance. The momentum tends to zero, and tunnel blockage occurs. If water level is high or cover layer thickness is small, the scour hole depth will exceed the cover layer thickness and the sediment concentration in tunnel decreases. The resistance is reduced significantly, and sediment flushing is accomplished. The present work facilitates enhanced understanding of sediment flushing, thereby informing reservoir engineers and managers of potential approaches to benefit maximization and sustainable sediment management.

Deformation of vegetated channel bed under ice-covered flow conditions

A common feature of cold regions is the presence of ice cover on water surface. In the presence of vegetation in the channel bed which is normally leafless in winter, interaction between river ice and vegetated channel bed becomes very complex. In the present study, the impacts of ice cover and submerged leafless vegetation on channel bed deformation and flow resistance have been investigated based on 144 experiments conducted in a large-scale outdoor flume. The independent variables associated with the maximum scour depth around vegetation elements have been assessed and equations have been developed. Results indicate that the most important variable affecting the maximum scour depth around vegetation under ice-covered flow conditions is the ratio of the ice cover roughness to the bed roughness (ni/nb). However, under open channel flow conditions, the flow Froude number is the most important variable influencing the maximum scour depth. The methods based on the law of the wall provide reliable Manning's coefficient for both smooth and rough ice covers in the presence of submerged vegetation in the channel bed. As the vegetation density increases, the size of scour holes becomes smaller. With the increase in vegetation density, the median grain size of the armour layer in scour holes decreases correspondingly. When vegetation elements are placed in a staggered configuration, the median grain size of the armour layer in scour holes is less than that of squared configuration of vegetation elements. Regardless of the roughness of ice cover on water surface and the grain size of sediment in the bed, the maximum depth of scour holes always occurs at the upstream front face of each vegetation element. Under the rough ice-covered flow condition, the maximum depth of scour holes is more than that under the smooth ice-covered and open flow conditions.

An investigation of the effect of the pulse width and amplitude on sand bed scouring by a vertical submerged pulsed jet

This paper investigates the effects of the pulse width and amplitude on the scouring of sand beds by vertical submerged pulsed jets using a combination of experimental and numerical calculations. The reliability of the numerical calculations is verified through a comparison between the numerical simulations with the sedimentation scour model and the experimental data at a low pulse width T2 of 0, with the result that the various errors are within 5%. The results show that the scour hole depth |hmin| grows with the relative low pulse width T3 throughout three intervals: a slowly increasing zone I, a rapidly increasing zone II, and a decreasing zone III, producing a unique extreme value of |hmin|. The optimal scouring effect equation was obtained by analytically fitting the relationship curve between the pulse amplitude V and the relatively low pulse width T3. Including the optimal T3 and optimal duty cycle ƞ. The difference in the scour hole depth |hmin| under different pulse amplitudes is reflected in the initial period F of the jet. With an increasing pulse amplitude, |hmin| goes through three intervals: an increasing zone M, decreasing zone N, and rebound zone R. It is found that the scouring effect in the pulse jet is not necessarily always stronger with a larger amplitude. The results of the research in this paper can provide guidance for optimizing low-frequency pulsed jets for related engineering practices, such as dredging and rock-breaking projects.

Precise forecasting of scour depth downstream of flip bucket spillway through data-driven models

Flip-bucket spillways are utilized in hydraulic engineering to diminish the kinetic energy of flowing water by redirecting the flow jet into the air. In the downstream stailing basin with low tail-water, sediment particles movement results in scour hole formation, posing a threat to spillway stability. The accurate prediction of scour hole depth is a crucial area of the present research work. This study endeavors to employ four data-driven models (DDMs), namely Support Vector Machine (SVM), Gene Expression Programming (GEP), Multilayer Perceptron (MLP), and Multivariate Adaptive Regression Splines (MARS), in combination with five selected empirical equations. The objective is to accurately predict scour depth utilizing field-collected data from site number 84. Relative scour depth, dsH1, was simulated based on the readily extracted parameter i.e. Froude number, Fr=qgH13. The evaluation of model performance was conducted using fundamental metrics, including root mean square error (RMSE), coefficient of determination (R2), mean average error (MAE), and the maximum value of the developed discrepancy ratio (DDRmax). Among the DDMs, the MARS model demonstrated superior performance in both the training and testing phases. In the training phase, it yielded metrics (RMSE = 0.08665, MAE = 0.05714, R2 = 0.99169, DDRmax = 4.519), and in the testing phase, it produced metrics (RMSE = 0.0252, MAE = 0.0170, R2 = 0.09933, DDRmax = 9.144). This exceptional performance of the MARS model surpassed the initially selected (Wu, 1973) [1] experimental model, which exhibited metrics (RMSE = 0.39667, MAE = 0.17463, R2 = 0.96172, DDR = 1.428). The evaluation indices conclusively establish the MARS method's absolute superiority over the experimental approach proposed by Wu (1973) [1].

Experimental study of propeller and ducted propeller’s wake inducted scour

This study investigates the scour profile, scour hole depth, deposition mound height, and the positions produced by the propeller and the ducted propeller that vary over time. The terrains produced by the Ka4-70 propeller and the Ka4-70 propeller + No. 19A duct are obtained using a linear laser three-dimensional terrain scanning system. Compared with the Acoustic Doppler velocimetry (ADV) and other approaches, the system presented here could efficiently and precisely capture the terrain. The principle and system layout are elaborated in this work. Notably, the system can perform underwater scanning. Moreover, compared with the ADV, our system is more efficient and has a smaller data interval. The scour profile, max scour depth and height, and their positions over time are analyzed. The development of the ducted propeller scour is faster than that of the propeller. The ducted propeller scour has a longer, narrower, and shallower influence range compared to the propeller scour. The scour hole range is bigger than that of the scour deposition mound. The general trends of the propeller and ducted propeller development are similar. In each development, the terrain is also morphologically similar at different phases. In addition, the relationship between the max deposition mound position and the max scour hole depth is analyzed, and a scour classic formula is used to fit the scour data on a time scale.

Characterizing the Importance of Porosity of Large Woody Debris Accumulations at Single Bridge Piers on Localized Scour

AbstractThe accumulation of large woody debris (LWD) at bridge piers is a serious hazard to the structural integrity of bridges across watercourses worldwide. The exacerbated scour that can directly result from LWD accumulations can lead to major structural damage or even catastrophic collapse. Recent research has led to empirical equations to estimate the scour depth for given LWD accumulation size; however these are mostly based on experimental tests with prismatic and impervious solid LWD accumulations, ignoring field observations that have shown that accumulations are neither impervious nor prismatic but are porous with inverted conical shapes. In this study, we therefore investigate the effects of porous LWD accumulations having shapes commonly observed in the field on scour holes. Results reveal that LWD size and shape, and flow characteristics are the primary factors influencing the erosion of sediments at the base of bridge piers. However, the porosity of accumulations is also observed to have a considerable effect on the size and maximum depth of scour holes. In particular, porous LWD reduce the maximum scour depth by up to 50% (and on average in the range of 5%–25%) relative to the respective solid impervious accumulation. The results shown in this study also provide a practical tool for arriving at more realistic and less conservative estimates of scour depths at bridge piers when affected by LWD accumulations.

Scour pattern around a pier in a 180° sharp bend: influence of pier shape under unsteady currents

ABSTRACT The effect of unsteady currents on scour at piers was investigated. The tests were run for nine pier shapes, three installation positions along a 180° bend, four flood durations, and four ratios of rising limb to flood duration for the 100-minute flood durations. Overall, including seven tests performed in the bend without piers, 196 tests were performed. The flow rapidity ranged from the movement threshold to live bed situations. The results indicated that the deepest scour-hole in front of the piers with different shapes was a function of flood duration and the ratio of ascending branch to descending branch had little effect on the final maximum scour-hole depth around the pier. The scour-hole volume is affected by the nose and the cross-sectional shape of the pier. For the rectangular pier, the scour-hole volume is greater than that of the other piers. The average shape coefficient was calculated.

Channel Bed Deformation around Double Piers in Tandem Arrangement in an S-Shaped Channel under Ice Cover

Flow structure and channel bed deformation caused by double piers in a tandem arrangement under ice-covered flow conditions in a bent channel is more complicated than those around a single pier in a straight channel. Based on experiments in an S-shaped flume, the scouring phenomenon at double piers in a tandem arrangement under an ice cover has been conducted by varying pier spacing distance, bend apex cross section (BACS), and hydraulic parameters. Results show that, under identical hydraulic conditions, the variation trend of the scour depth in the vicinity of double piers in a tandem arrangement in a bent channel is similar to that in a straight channel. The deepest depth of scour holes at the upstream BACS is more than that at piers at the downstream BACS. At each BACS, the effect resulting from the interaction of double piers gradually decreases with the pier spacing distance. Different from the characteristics of local scour at double piers in a tandem arrangement in the straight flume, when the ratio of pier spacing distance to pier diameter (L/D) is more than 15, the horseshoe vortex generated by the front pier has negligible impact on the rear pier, and the maximum depth of scour hole at the rear pier scour hole is about 90% that of the front pier. Also, when L/D is higher than 15, the influence of the rear pier on the front one is negligible, and the scour hole depth at the front pier remains the same. However, this phenomenon occurs when the straight flume’s L/D is greater than 17.

Accuracy evaluation of scour depth equations under the submerged vertical jet

Abstract Scouring of sediment materials under submerged vertical jets is shown at grade-control structures, downstream of weirs, and submerged vertical water jets have several uses, including seabed sediment removal and dredging in ocean engineering. Numerous studies have been conducted to demonstrate the scour process and the variation of scour profile utilizing various influencing parameters. Many researchers developed equations for predicting the scour hole characteristics. Previous studies identified two types of scour hole depth configurations: static and dynamic scour depth and the variation of scour hole variations differs for long- and short-impinging jet height conditions. In this study, extensive data on non-dimensional static and dynamic scour depth under short- and long-impinging jets acquired from prior literature, and the previously proposed equations of static and dynamic scour depth were analyzed using graphical and statistical analysis. The findings demonstrated that the relationships proposed by Aderibigbe and Rajaratnam (1996) for long-impinging jets and Amin et al. (2021) for short-impinging jets predict the static scour depth better than the other equations. The proposed equations for dynamic scour depth under long- and short-impinging jets are highly biased and inaccurate.

Scour depth downstream sharp-crested weir

Weirs are hydraulic obstructions constructed across an open-channel flow causing a rise in water which is useful to convert water to the channels upstream as well as using these structures as a measuring device. There are different types, and a sharp-crested weir is one of them. Local scour downstream hydraulic structures are one of the most critical problems affecting their safety and stability. Many studies conducted to prevent these facilities from collapsing, but the effect of adding a solid apron downstream weir still need deep studies. The current study included a survey of scour reduction downstream sharp-crested weir using a solid apron consisting of three different lengths, four different discharges, and sand bed material to calculate the maximum depth and length of scour hole when using an apron solid. The results proved that the depth and length of the scour hole are reduced to 19.3% and 27.7%, respectively, when an apron solid is used. Two empirical equations were found to determine the maximum depth and length of the hole with a coefficient of determination are 0.892 and 0.804, respectively.

Gridded cemented riprap for scour protection around monopile in the marine environment

A novel gridded cemented riprap (GCRP) composed of loose riprap stones and cemented stone clusters is proposed, which combines high stability of large stones and excellent flexibility to follow ground deformation. A series of physical experiments were performed in a large-scale flume, and two extreme wave-current conditions of an offshore wind farm with a return period of 50 and 100 years, respectively, were selected. The anti-scouring performance of riprap and GCRP is studied in detail. It is found that the magnitude of wave leads to downstream movement of the maximum scour point. The armor stones around monopile cannot maintain their stability under the extreme wave-current condition, and riprap layer gradually disintegrates during scour process. The stability of riprap layer is significantly improved by fully or gridded grouting. Fully cemented riprap (FCRP) and GCRP remain intact even under the combined wave-current condition of return period of 100 years. For FCRP, scour occurs at the downstream edge and the erosion of underlying sand bed takes place. The anti-scouring performance of GCRP is better than riprap and FCRP, the maximum depth of scour hole around GCRP is reduced 80% than unprotected monopile.

A laboratory study of wave-induced local scour at an emergent pile breakwater

A pile breakwater is a row of closely-spaced piles, placed in a side-by-side arrangement, to dissipate wave energy and prevent coastal erosion. It is important to understand the local scour at a pile breakwater for its foundation design. This paper reports a set of wave-flume tests on the local scour around the piles in regular waves. A laser scanner was used to measure the change in the three-dimensional bed elevation. Five values of Keulegan–Carpenter number were examined in this study. Scour holes near the pile breakwater were developed due to the oscillatory jet flow through the gaps between adjacent piles, and sand ripples were observed on both sides of the structure. The migration of the sand ripples was either onshore or offshore, depending on the wave conditions. The maximum scour-hole depth had an exponential growth with the duration of wave attack, and the equilibrium depth was found to be deeper than that for a standalone pile. Both the scour-hole depth and length were found to be scaled by the pile diameter. The ripple length and steepness were in general agreement with recent empirical formulas in the literature in spite of the presence of a pile breakwater.

Effect of scour erosion on mode shapes of a 5 MW monopile-supported offshore wind turbine

This paper investigates the influence of scour erosion on the mode shapes of the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine (OWT) in varying soil conditions, where modes are derived from wind and wave-induced accelerations. A numerical model, which considers aerodynamic and hydrodynamic loading with soil-structure interaction (SSI) and servo-dynamics, is developed. The superstructure is modelled using OpenFAST and the foundation is modelled with SESAM, where SSI is incorporated using Winkler (p-y) springs. The foundation stiffness is subsequently integrated in OpenFAST using mudline interface springs. The influence of soil density, scour hole depth, and loading on the derived mode shapes is studied. Accelerations are extracted at nine locations along the tower. The first two system natural frequencies and mode shapes are estimated using frequency domain decomposition (FDD) applied to the acceleration time-histories for each scour depth and soil condition. Modal Assurance Criterion (MAC) values are calculated to evaluate the changes in mode shapes that occur due to scour. It is shown that the mode shapes present consistent changes as scour depth increases, and the second mode shape exhibits more sensitivity to scour. The sensitivity of mode shape changes to scour under variations in wind speed and measurement noise are also investigated.

Comparative study of soft computing models for prediction of scour below two symmetric crossing jets

The scour estimation downstream of the dam’s hydraulic structures is a serious issue and has long been considered an important topic by hydraulic engineers. The literature shows that the computational methods are good alternatives for predicting the scour in hydraulic structures in which the conventional methods demonstrate shortcomings. In the present paper, various machine learning techniques have been used for the first time to predict the scour below the two symmetric crossing jets. Four types of artificial neural networks (ANNs), including feedforward back-propagation (FFBP), cascade-forward back-propagation (CFBP), radial basis function (RBF), generalized regression neural network (GRNN) along with the adaptive neuro-fuzzy inference system (ANFIS), and support vector regression (SVR) were employed and evaluated. Two important scour hole characteristics, namely the maximum scour hole depth and its location relative to the scour hole origin, were predicted at three crossing angles. The soft computing models were also compared with the traditional empirical methods. The results indicated that the applied computing techniques perform satisfactorily and improve the results remarkably. They can estimate the scour more precisely than the regression models and can be considered robust alternative tools. A detailed sensitivity analysis was also performed on the FFBP that shows the crossing angle has a powerful effect on the predictions. The SVR improved the results to 47% and 31.71% for the scour hole depth and its location, respectively, in terms of the correlation coefficient.

Local Scour of Highway Cross Drainage Structures

In this paper methods for determination of maximum local scour depth and eroded material volume downstream of highway drainage structures were developed. Severe local scour leads to undermining followed by collapsing of the drainage structure. The analysis method is based on integrating a new technique of improved drainage structure outlet geometry, jet hydraulic characteristics and soil properties. The jet velocity, induced shear stress, critical shear resistance to erosion and particle size are the main factors that control the erosion process downstream of a drainage structure. The study is of experimental and theoretical nature. The developed mathematical model was calibrated using collected field data in addition to laboratory tests. The produced scour holes were plotted for each laboratory run and the scour hole depth, width and length were measured. A semi- empirical relation has been developed for predicting the depth of local scour hole. It is based on Densimetric Froude number. It is found that the semicircular apron gives less scour depth and width when compared to the linear front apron. The comparison of the model outputs and field study is found of a good agreement. The model is recommended for engineering design of highway drainage structures located on streams of sandy bed.

Effect of sediment gradation on scour by symmetric crossing jets: an experimental investigation

Abstract A laboratory study with 162 experiments was designed and conducted to investigate the effect of three non-uniform sediment samples on the scour morphology by the symmetric crossing jets. The bed sediment materials with close median diameter values and different non-uniformities were utilized to investigate the scour at various crossing angles. The results were also compared with the equivalent single jet. The proper novel relationships were presented to predict the scour hole depth, and dimensions. The results showed that at a crossing angle of 30° with low and medium tailwater depth, the scour depth increases with increasing the non-uniformity of the bed materials. At the crossing angle of 30°, the armor layer was created perfectly that was thick on the bottom of the scour hole, but for the crossing angles of 70° and 110° it was thin and was a mixture of the coarse and fine particles. At low tailwater depth at the crossing angles of 70° and 110°, by increasing the non-uniformity of the sediments, the scour tended to be more asymmetric. At the high tailwater depth, for the crossing angles of 70° and 110° the scouring was less than that of the crossing angle of 30° and single jet.

Use of spur dikes with different permeability levels for protecting bridge abutment against local scour under unsteady flow conditions

Local scour around abutments is one of the most important reasons for a bridge collapse, making it a major concern for hydraulic and river engineers. Available studies on scour control around abutments have been limited to steady flow conditions. In this light, the present experimental study investigates using a single spur dike with different levels of permeability (0%, 35%, 50%, and 65%) to reduce scour around a short vertical-wall abutment (abutment length/flow depth ≤ 1) under unsteady flow conditions. The hydrographs with Gaussian distribution and different base flow times (i.e., duration of 15, 30, 60, and 90 min) were used to simulate the unsteady flow. Abutment scour depth variations with time showed that the final scour depth always appeared after the peak discharge of the hydrograph regardless of whether a spur dike was used. It was shown that the spur dike has successfully reduced local scour around the abutment. The maximum depth of scour hole around abutment was reduced by 47%, 32%, and 11%, when spur dikes with 35%, 50%, and 65% permeability were, respectively, used. Using an impermeable spur dike not only prevented the scour upstream nose of the abutment but also led to some deposition that raised the bed level by nearly 0.47 La (where La is the abutment's length) in that area. However, the maximum depth of scour hole around the impermeable spur dike was nearly identical to that occurred around the abutment for the experiments without a spur dike, making it necessary to arrange for scour protection around the spur dike.

Estimating the equilibrium scour depth around two side-by-side piers with different spacing ratios in live-bed conditions

The aim of this research was to numerically investigate the flow behaviour and evolution of the local scour around a group of two side-by-side piers in live-bed conditions. A coupled hydro-morphodynamic model was developed in OpenFOAM®. The hydrodynamic part of the model solved the unsteady Reynolds Averaged Navier-Stokes equations. The morphodynamic part of the model entailed solving both bedload and suspended load sediment transport. The model validation was performed for the available experimental data in the literature. The modelling for the pier groups was carried out with three different spacing ratios of G/D = 1, 2, and 3, where G represents the distance between the piers and D is the pier diameter. The results presented detailed information regarding the flow field and scour hole progression adjacent to two side-by-side piers. The dimensionless equilibrium scour hole depths of 1.74, 1.38 and 1.29 were predicted for the pier groups with G/D = 1, 2, and 3, respectively. The results showed an inverse correlation between the spacing ratio and the scour hole depth adjacent to the piers. The results were generalised, and an equation was obtained for the prediction of the equilibrium scour hole depth as a function of the spacing ratio.

Experimental Study of Scour Hole Depth around Bridge Pile Using Efficient Cross-Section

Extensive numerical and analytical studies exist that focus on the scours around bridge piers with different cross-sections. However, studies addressing the scour around bridge piles are rather limited and not conclusive. Therefore, the most efficient cross-section reducing the local scour around a pile group is experimentally examined herein. Accordingly, comparative scouring of three different shapes, namely, circular, rectangular and lenticular, is investigated by installing a cohesion-less bedding material around scaled bridge piles immersed in clear water. It is also shown that the available pier scour predictive models grossly underestimate the depth of pile scour holes. For example, the pier predictive models underestimate the pile scours hole by more than 50% in the case of square and circular cross-sections. It also transpires from the experimental investigation that the scour hole depth around the square shape pile is significant, around the circular pile is moderate and around the lenticular pile is minimal.