Equilibrium Scour Depth Research Articles

Experimental Study on Aspect Ratio and Velocity Intensity of Scour Around Submerged Pile Groups

Submerged pile groups are important components of complex piers in hydraulic engineering and are analogs for a range of subsea structures. These may sustain severe damages from local scour. The velocity intensity (U / Uc, the ratio of critical velocity to mean velocity) and aspect ratio (H / Dp, the ratio of pile height to pile diameter) are critical variables in this scour process. However, previous studies on scour around submerged pile groups were conducted in clear-water conditions (U / Uc < 1). In addition, many research studies are being conducted in shallow water flow conditions, which cannot eliminate the effect of the water depth on the scour process. Thus, these research studies cannot directly be applied to live-bed scour around submerged structures. To expand the velocity intensity and aspect ratio of scour around submerged pile groups, flume experiments were conducted with uniform quartz sand in both clear-water and live-bed conditions. Pile groups with different heights are adopted as experimental models. An improved time factor for both clear-water and live-bed scour around submerged structures is determined with the present experiments to extrapolate the scour depth of the present work to the equilibrium scour depth. This new time factor is then tested with experimental data from the literature. The tests demonstrate that this new time factor can reliably predict the scour process and equilibrium scour depth for submerged structures. Empirical relationships to demonstrate the effects of the studied variables including the equilibrium scour depth, scour area, and scour volume are presented. Different methods to predict the effects of the velocity intensity and aspect ratio on the scour depth are compared based on the present work. Furthermore, a correction coefficient is proposed to illustrate the effect of aspect ratio on scour depth. Then, equations of scour area and volume are derived according to the present experiments.

Local Scour at Triangular Labyrinth Side Weirs Located on an Alluvial Channel

The studies on labyrinth side weirs have received considerable attention in existing literature. A mobile bed, which causes significant morphological bed changes, can be induced by the side overflow weir. In this study, the local scour depths that occur near a triangular labyrinth side weir at a 90° angle in a straight channel with an alluvial bed were experimentally examined under steady flow conditions with free overflow of the weir. Based on a dimensional analysis applied according to the parameters of the study, it was determined that dimensionless equilibrium scour depth is dependent on the approach flow intensity, dimensionless side weir crest height, dimensionless median grain size, dimensionless side weir opening, and dimensionless volumetric amount of upstream sediment feed under live-bed scour conditions. For different combinations of weir opening length and weir height, data for a set of 81 runs were obtained. The results show that an increase in weir crest height results in a decrease in dimensionless maximum scour depth. In the clear-water scour experiments, sediment deposits were observed downstream of the weir. In the live-bed scour experiments, the dimensions of the scours and sediment deposits that occurred upstream and downstream of the weir exhibited periodic changes. Good consistency was attained in the developed empirical equations between the calculated and measured values of the equilibrium scour depth.

Relative importance of parameters controlling scour at bridge piers using the new toolbox ScourAPP

The new toolbox ScourApp is presented and applied for analysis of the complex interactions between local scour and sediment deposition at bridges during floods and to determine the relative importance of the controlling parameters on scour depth. ScourApp represents the time-dependent sediment transport processes through modern empirical formulations to compute the evolution of scour depth at a bridge pier. Results show that the mathematical model reproduces observed scour depths with high accuracy and confirm that ScourApp is a suitable tool for scour analysis. The model parameters controlling the time dependent scour depth were the equilibrium scour depth, and two constants representing the effects of pier geometry, flow and sediment. Sensitivity analysis showed that all three scour model parameters exhibited a nearly linear relation with the final scour depth. Moreover, results considering sediment deposition obtained for the Rapel Bridge showed that scour depths are highly sensitive to the sediment supply rate. ScourApp is recommended in scour assessment for bridge planning, design, management, and forensic analysis, as well as for engineering education.

Equilibrium scour hole size at setback abutments with varied aspect ratios in floodplains

The key features of the equilibrium scour depth dse, width Wse, length Lse and the volume Vse with large abutment aspect ratios (i.e., abutment width Lc divided by its length L) in floodplain are different compared to narrow abutments. Seven models with Lc/L ranging from 0.125 (narrow abutment) to 4 (very wide abutment) were tested. The results show that the combined effect in terms of the abutment aspect ratio Lc/L is a key parameter with wide abutments. Furthermore, the average equilibrium scour width observed was much larger than previous studies and extends up to 3.5 times the floodplain water depth. This implies the current guideline by FHWA (Federal Highway Administration), (2009) to provide a riprap countermeasure apron width for 2 times the floodplain water depth may be insufficient. The much wider scour formation is caused by the migration of the maximum scour location around wide abutments. The results show generally the scour hole dimensions for setback abutments in compound channel are less than that abutments in rectangular channel under the same flow conditions. To this end, empirical equations, which agree well with the data from the present and previous studies are proposed to predict these characteristics at the equilibrium state.

Numerical Study of Local Scour around Tripod Foundation in Random Waves

In this study, the local scour around tripods in random waves is numerically investigated. The seabed-tripod-fluid numerical model with an RNG k−ε turbulence model is built and validated. Following that, the scour characteristics and flow velocity distribution are analyzed using the present numerical model. Finally, a revised stochastic model is proposed to predict the equilibrium scour depth, Seq, around tripods in random waves. The results indicate that the present seabed-tripod-fluid numerical model is capable of depicting the scour process and of capturing the flow field around tripods with high accuracy. Due to the blockage effects of the main column and structural elements, there is enhanced flow acceleration underneath the main column and the lower diagonal braces, which increases the turbulence intensity and seabed shear stress, causing more particles to be mobilized and transported, resulting in more severe scour at the site. The revised stochastic model shows the best agreement with the numerical and experimental results when n = 20, but more experimental data and numerical results are still needed to verify the adaptation of the revised stochastic model for larger Keulegan–Carpenter (KC) number conditions (KCrms,a > 4).

Sticky Stuff: Biological Cohesion for Scour and Erosion Prevention

This study examines the potential for biological cohesion to arrest scour erosion at marine infrastructure. Biological cohesion occurs naturally in sedimentary environments, and is caused by extracellular polymeric substances (EPS) which result from the life cycles of microorganisms. EPS is known to dramatically increase the resistance of natural biomediated sediment to erosive hydrodynamic forces. In this study we test, for the first time, whether EPS can be deliberately added to a sediment to mitigate against scour erosion – a process we term ‘biostabilisation’. A systematic laboratory experiment is used to investigate the effects of an EPS additive on scour erosion around a monopile in a sand substrate. Results show that increasing EPS content causes a progressive reduction in equilibrium scour depth, the volume of excavated material and the timescale required to reach equilibrium scour morphology. These parameters are linearly related to EPS content, showing that the effects of EPS on the physical processes required for erosion to occur are concentration dependent. It can be concluded that biostabilisation offers a potential new ecologically engineered, nature-based solution to a range of scour and erosion scenarios. The economic and environmental advantages are discussed, and a methodology for biostabilisation use in individual erosion mitigation scenarios is proposed.

Scour Characteristics and Equilibrium Scour Depth Prediction around a Submarine Piggyback Pipeline

Local scour around a submarine piggyback pipeline in combined waves and current is investigated experimentally. Based on the experimental results, the scour evolution and scour morphology are firstly analyzed. Then, a comparison with the equilibrium scour depth Seq between the present experimental data and predicted results is conducted. After that, the correlation between the dimensionless scour timescale T* and the maximum Shields parameter θcw is investigated, and a formula is obtained to describe the variation trend between T* and θcw for different gap ratios G/D. Furthermore, the parametric study is carried out to study the effects of Reynolds number Red and θcw on Seq, respectively. The results indicate that the Seq below the piggyback pipeline increases when the gap ratio G/D increases from 0 to 0.1, and it gradually decreases when G/D > 0.1. For a given KC, the Seq increases with the increase of the ratio of velocities Ucw. In addition, when Ucw is fixed, a higher KC results in a greater Seq. The T* is closely related to θcw and G/D. The higher Red and θcw both tend to result in the greater scour depth below a piggyback pipeline in combined waves and current.

Three-Dimensional Simulations of Scour around Pipelines of Finite Lengths

In the past few decades, there have been many numerical studies on the scour around offshore pipelines, most of which concern two-dimensional setups, with the pipeline infinitely long and the flow perpendicular to the pipeline. Based on the Ansys FLUENT flow solver, this study establishes a numerical tool to study the three-dimensional scour around pipelines of finite lengths. The user-defined functions are written to calculate the sediment transport rate, update the bed elevation, and adapt the computational mesh to the new boundary. The correctness of the model has been verified against the measurements of the conventional two-dimensional scour around a long pipe and the three-dimensional scour around a sphere. A series of computations are subsequently carried out to discover how the scour hole is dependent on the pipeline length. It is found that the equilibrium scour depth increases with the pipeline length until the pipeline length exceeds four times the pipe diameter.

Local scour at offshore windfarm monopile foundations: A review

In this article, current research findings of local scour at offshore windfarm monopile foundations are presented. The scour mechanisms and scour depth prediction formulas under different hydrodynamic conditions are summarized, including the current-only condition, wave-only condition, combined wave-current condition, and complex dynamic condition. Furthermore, this article analyzes the influencing factors on the basis of classical equations for predicting the equilibrium scour depth under specific conditions. The weakness of existing researches and future prospects are also discussed. It is suggested that future research shall focus on physical experiments under unsteady tidal currents or other complex loadings. The computational fluid dynamics-discrete element method and artificial intelligence technique are suggested being adopted to study the scour at offshore windfarm foundations.

Large-scale experimental study on scour around offshore wind monopiles under irregular waves

The Keulegan-Carpenter (KC) number is the main dimensionless parameter that affects the local scour of offshore wind power monopile foundations. This study conducted large-scale (1:13) physical model tests to study the local scour shape, equilibrium scour depth, and local scour volume of offshore wind power monopiles under the action of irregular waves with different KC numbers. Systematic experiments were carried out with the KC number ranging from 1.0 to 13.0. With a small KC number (KC < 6), and especially when the KC number was less than 4, the scour mainly occurred on both cross-flow sides of the monopile with a low scour depth. When the KC number exceeded 4, the shape of the scour hole changed from a fan to an ellipse, and the maximum scour depth increased significantly with KC. With a large KC number (KC > 6), the proposed method better predicted the equilibrium scour depth when the wave broke. In addition, according to the results of three-dimensional terrain scanning, the relationship between the local equilibrium scour volume of a single offshore wind power monopile and the KC number was derived. This provided a rational method for estimation of the riprap redundancy for monopile protection against scour.

Experimental Study on Local Scour Depth around Monopile Foundation in Combined Waves and Current

Local scour is one of the key factors that cause the collapse of structures. To avoid structure failures and economic losses in water, it is usually essential to predict the equilibrium scour depth of the foundation. In this study, several design models which were presented to predict the equilibrium scour depth either under steady clear water conditions and combined waves and current conditions were recommended. These models from China, the United States and Norway were analyzed and compared through experiments. Moreover, flume tests for monopile foundation embedded in sand under different flow conditions were carried out to observe the process and gauge the maximum depth around the pile. Based on this study, for predicting the equilibrium scour depth around bridge piers, the computational results of three design methods are all conservative, as expected. For the foundation of offshore structures in marine environment, most of the predicted scour depths by design methods are different from field data; in particular, the mean relative error with these design methods proposed may reach up to 966.5%, which may lead to underestimation of the problem, overdesign and consequently high construction cost. To further improve the ability of the scour prediction in a marine environment, data from flume tests and some field data from a previous study were used to derive the major factors of scour. Based on the dimensional analysis method, a new model to estimate the equilibrium scour depth induced by either current or waves is proposed. The mean relative error of the new formula is 49.1%, and it gives more accurate scour depth predictions than the existing methods.

Experimental Study of Local Scour around Tripod Foundation in Combined Collinear Waves-Current Conditions

A series of laboratory experiments were conducted in a wave-current flume to investigate the scour evolution and scour morphology around tripod in combined waves and current. The tripod model was made using the 3D printing technology, and it was installed in seabed with three installation angles α = 0°, 90°and 180° respectively. In the present study, the scour evolution and scour characteristic were first analyzed. Then, the equilibrium scour depth Seq was investigated. Furthermore, a parametric study was carried out to study the effects of Froude number Fr and Euler number Eu on equilibrium scour depth Seq respectively. Finally, the effects of tripod’s structural elements on Seq were discussed. The results indicate that the maximum scour hole appeared underneath the main column for installation angle α = 0°, 90° and 180°. The Seq for α = 90° was greater than the case of α = 0° and α = 180°, implying the tripod suffered from more severe scour for α = 90°. When KC was fixed, the dimensionless time scale T* for α = 90° was slightly larger than the case of α = 0° and α = 180° and the T* was linearly correlated with Ucw in the range of 0.347 &lt; Ucw &lt; 0.739. The higher Fr and Eu both resulted in the greater scour depth for tripod in combined waves and current. The logarithmic formula can depict the general trend of Seq and Fr (Eu) for tripod in combined waves and current.

Development of scour hole depth around setback abutment in a compound channel

AbstractThis study focuses on scour depth development around 45° wing‐wall setback abutments in a compound channel. The results show that the scour development curves almost have similar shape and they can be described using exponential function. It is generally observed that the scour hole achieved 80% of the equilibrium scour depth at a scour time equals to 25% of the equilibrium scour time. Additionally, an increase in the relative abutment length and the contraction ratio causes an increase in the equilibrium time. However, this time decrease with increasing flow depth ratio. Finally, new empirical equations are proposed to predict the development of scour depth and the time to reach the equilibrium state. These equations will be useful to predict these characteristics of scour hole especially for setback abutments in compound channel and predict the temporal development scour depth for previous studies that have available data related to maximum scour depth.

Experimental study on local scour around submerged monopile under combined waves and current

Submerged pile foundation have been widely used in ocean. Plenty of promising research concentrate on local scour around mono-pile or pile groups. Yet submerged situation remains unawareness. Based on a series of laboratory flume experiments, local scour in the vicinity of the submerged monopile foundation under the action of combined wave and current are studied in this research. During the experiments progress, simultaneously the scour depth and pore pressure in the sand are measured under various hydrodynamic factors. Then 3D laser scanner is applied to capture the scour topography. The experimental results indicate that with decreasing submergence ratio (hc/D), or declining monopile height, the horseshoe vortex and vortex shedding weaken accordingly. Subsequently, the scour rate, radius and depth perform a correspondingly reducing tendency. Considering dimensionless parameters Keulegan-Carpenter number and Froude number (KC and Fr), the data analysis centered on the relative equilibrium scour depth (s/D), demonstrate a satisfactory consistency comparing to previous research results. Under the action of combined wave and current, the pore pressure around the monopile presents periodic sinusoidal variation. The pore pressure exerts an upward seepage force inside the sand which bearing the cyclic loading. Hence, the sand-bed express extra susceptibility and vulnerability to scour induced by combined wave and current.

Propeller jet flow scour around pile group supported piers

In this study, scouring formation around pile groups with different configurations and exposed to a ship's main propeller jet flow was investigated experimentally. The experiments were conducted in a flume by installing cylindrical piles on a horizontal sand bed. The selected pile configuration was a 3 × 3 square arrangement. Different tests were performed by changing the space between the piles. The effect of the distance between the piles with respect to the densimetric Froude number (Frd) on scour formation was investigated by considering three different propeller rotation speeds (590, 670 and 745 rpm) with 5 and 7 cm pile diameters (d0), using propellers with diameters (Dp) of 6.5 and 10 cm located 10 cm above the sand bed (G) and 10 cm away from the front piles (X). In addition, the 3D global scour-hole geometry was obtained using photogrammetric data collection and analysis techniques. The equilibrium scour hole morphology was described in detail, and the characteristic dimensions of the resulting scour hole were thoroughly analyzed. The equilibrium scour depth of the pile groups was found to depend on the space between piles (Xt), densimetric Froude number and also position of the propeller. The equilibrium (maximum) scour depths around piles with respect to their configuration were larger than the scour depth around a single pile under same flow conditions.

Three-dimensional Computational Fluid Dynamics modelling of scour around a single pile due to combined action of the waves and current using Level-Set method

This paper presents three dimensional (3D) numerical modeling of local scour around a single pile under the combined action of waves and currents. A semi-coupled hydrodynamic and morphological model (REEF3D) is used in this study which solves the Reynolds-averaged Navier–Stokes (RANS) equation with k-ω turbulence model closure. The Exner equation is used to calculate the changes in bed elevation. The use of the Level-Set method is done to capture the changes in the free surface. Realistic scour and deposition calculations are aided with the use of modified critical bed shear stress reduction formulation on a sloping bed supplemented with the use of Sand Slide algorithm in the morphological model. The scouring process is modeled in this study using a reduced-length numerical wave tank with wave generation and wave absorption implemented using active wave absorption method. The model is validated for the numerical modeling of local scour around a single pile in steady current, waves alone, and combined wave–current environment. The validated model is used to analyze the effect of Keulegan–Carpenter (KC) number and combined wave–current parameter (Ucw) on normalized equilibrium scour depth. It is found that the normalized equilibrium scour depth for scour under waves alone, waves combined with weak currents, and waves combined with mild current do not significantly differ for flow developed in a low KC number environment. However for waves combined with strong currents the normalized equilibrium scour depth increases significantly. For large KC numbers, the normalized equilibrium scour depth is found to have increased with an increase in KC number as well as combined wave–current parameter (Ucw).

Investigation of the flow behaviour and local scour around single square-shaped cylinders at different positions in live-bed

Scour around non-circular cylinders has seldom been studied before. This paper aims to numerically investigate the flow behaviour and evolution of local scour around two single square-shaped piers with 90° (square pier) and 45° (diamond pier) orientation angles to the flow under steady-current and live-bed conditions. For this purpose, a coupled approach between a hydrodynamic and a morphodynamic model is developed and validated against flow and scour experimental data sets. Hydrodynamic model simulations reveal the formation of strong horseshoe vortices (HSVs) upstream of the square pier compared to the diamond pier. The scour simulation results show a faster rate of increase in the scour depth around the square pier, which leads to a deeper equilibrium scour hole. The model gives values of normalized quasi-equilibrium scour depths of Seq/wp = 1.94 and 1.00 for the square and diamond piers, respectively, where Seq is the quasi-equilibrium scour depth, and wp is the pier's projected width under steady current flow and live-bed conditions. It is discovered that the impact of other factors, such as bed material gradation, Froude number, Shields Parameter, as well as flow depth, can be as important as the shape factor in the study of equilibrium scour depth in live-bed conditions.

Scour Characteristics and Equilibrium Scour Depth Prediction around Umbrella Suction Anchor Foundation under Random Waves

A series of numerical simulation were conducted to study the local scour around umbrella suction anchor foundation (USAF) under random waves. In this study, the validation was carried out firstly to verify the accuracy of the present model. Furthermore, the scour evolution and scour mechanism were analyzed respectively. In addition, two revised models were proposed to predict the equilibrium scour depth Seq around USAF. At last, a parametric study was carried out to study the effects of the Froude number Fr and Euler number Eu for the Seq. The results indicate that the present numerical model is accurate and reasonable for depicting the scour morphology under random waves. The revised Raaijmakers’s model shows good agreement with the simulating results of the present study when KCs,p &lt; 8. The predicting results of the revised stochastic model are the most favorable for n = 10 when KCrms,a &lt; 4. The higher Fr and Eu both lead to the more intensive horseshoe vortex and larger Seq.

The equilibrium depth of wave scour around both slender and large piles

The equilibrium stage of the scour caused by the wave effect around the marine piles was investigated experimentally for both slender and large piles. Unlike the experimental studies in the literature, some of the experiments were carried out under clear water scour condition, since the coarse bed material was used in the present study. While the effective parameter on scour for slender piles is only the Keulegan-Carpenter number, it is understood that the diffraction parameter is also effective for large piles. It has been determined that when the steady-streaming structure around a large pile is effective, the scour increases with the increase in the Keulegan-Carpenter number and the diffraction parameter. For the estimation of the relative equilibrium scour depth around the slender piles, the results were evaluated together with the previous experimental study results and a new empirical equations based on the Keulegan-Carpenter number (KC > 4) was proposed. A new empirical correlation is derived, which includes the Keulegan-Carpenter number and the diffraction parameter (10−3<D/L < 3·10−1) for both slender and large piles. In addition, alternatively, a new equation is proposed using the modified Ursell parameter (Urp<105). The nominal limit of the relative scour depth according to the diffraction parameter was determined as 0.0082·(D/L)−1.

Modeling Clear-Water Scour around the High-Rise Structure Foundations (HRSF) of Offshore Wind Farms

Xiao, Y.; Jia, H.; Guan, D.; Liang, D.; Yuan, S., and Tang, H., 2021. Modeling clear-water scour around the high-rise structure foundations (HRSF) of offshore wind farms. Journal of Coastal Research, 37(4), 749–760. Coconut Creek (Florida), ISSN 0749-0208.The high-rise structure foundation (HRSF) has been used in building the Donghai offshore wind farm in Shanghai, China. So far, few published studies can be found on the local scour around an HRSF. This study presents the results of a physical modeling investigation of the clear-water scour around two typical HRSF models consisting of eight piles and 10 piles, respectively. In the study, the flume bed was covered with uniform noncohesive sediment and the incoming flow was steady. The effects of the alignment angle of the HRSF, water depth, and relative sediment size on the equilibrium scour depth were analyzed. Results showed that the maximum scour depth always occurred around the middle piles on the shoulders of the HRSF. The alignment angle of the HRSF had little influence on the final equilibrium scour depth. For permeable multipile foundations, the scour depth increased when the water depth increased within the range of the testing conditions, which indicates a similar trend between a multipile foundation and a monopile. An empirical formula is proposed for estimating the equilibrium scour depth.