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Scour of seabed sediments can occur around offshore foundations. For monopile-supported offshore wind turbine structures, the reduction in foundation stiffness due to scour presents certain operational challenges. In cases where scour causes the natural frequency of the structure to become, for example, close to the range of rotor loading frequencies then—due to the increased risk of fatigue damage—turbine support structures are at risk of reduced operation, or even premature decommissioning. In practice, scour protection and/or remediation systems are typically used to mitigate the development of scour. As well as preventing further erosion, scour remediation systems may have a restorative effect on the stiffness of the foundation. This paper describes a one-dimensional (1D) finite-element model for the analysis of natural frequencies for monopile-supported turbine support structures with active scour process around the foundation. The model also incorporates procedures to model the influence of a rock fill scour remediation system on the foundation stiffness. The numerical model is calibrated and validated by comparison with a set of previously described reduced-scale model tests conducted in a flume. The calibrated 1D model is applied to a field case study at a UK offshore wind farm site. [ABSTRACT FROM AUTHOR]

Tsunamis continue to pose an existential threat to life and infrastructure in many coastal areas around the world. One of the risks associated with tsunamis is the formation of deep scour holes around critical infrastructure and other coastal buildings, compromising their structural integrity and stability. Despite its importance, tsunami-induced scour is still given limited and simplified consideration in design guidelines for coastal structures. To further improve the understanding of tsunami-induced scour processes, and thus provide the basis for safer design of coastal structures, novel large-scale laboratory experiments have been conducted. The experiments featured a unique combination of boundary conditions, including a square coastal structure on a sloping and dry sandy beach. Single broken solitary waves were used to simulate tsunami bores. The spatiotemporal scour development directly at the square column was monitored by a high-resolution camera system, allowing a detailed description of the highly dynamic flow and scour process. Differences in the scour process between the wave runup and drawdown phases are described, and maximum and final scour depths are given as a function of inundation depth, wave height, and distance of the column from the shoreline. The scour process is characterized by several distinct phases of varying intensity and scour rate, the sequence of which varies depending on the location on the sides of the column. It is shown that the drawdown phase has a large influence on the overall scour development, adding up to 58% to the scour depth obtained during the wave runup phase. As a result of significant sediment infilling during the drawdown phase, the maximum scour depths achieved during the drawdown phase are up to twice the final scour depths at the end of a test. This discrepancy between final and maximum scour depths is greater than in previous studies using a flat sediment bed. The results of this study therefore help to interpret scour depths measured during field investigations after a tsunami event and provide a basis for extending design guidelines for coastal structures. [ABSTRACT FROM AUTHOR]

This paper studies the effect of the wave front-current angle on the scour around a circular pile. An experimental study was carried out in the Coastal, Ocean and Sediment Transport (COAST) laboratory at the University of Plymouth (UK) using a single monopile of 0.125 m in diameter and an 8 m long by 1.5 m wide by 0.2 m high sand pit. The results obtained during the test campaign show the influence of the angle between waves and currents on both the maximum scour depth and the time scale of the process. Wave fronts partially aligned with current (65°) produce deeper scour holes than perpendicular forcing conditions (90°). Wave fronts partially against the current (115°) produce less scour than any of the two previous scenarios. The addition of waves reduced the maximum scour depth, compared with the current-only case. The development of the scour hole was found to be more rapid when waves are added to the current, with 50% of the final scour achieved in half the time. The results show that wave direction relative to the current is an important component in scour prediction. [ABSTRACT FROM AUTHOR]

A fully coupled hydrodynamic and morphologic numerical model was utilized for the simulation of wave-plus-current scour beneath submarine pipelines. The model was based on incompressible Reynolds-averaged Navier-Stokes equations, coupled with k-ω turbulence closure, with additional bed and suspended load descriptions forming the basis for seabed morphology. The model was successfully validated against experimental measurements involving scour development and eventual equilibrium in pure-current flows over a range of Shields parameters characteristic of both clear-water and live-bed regimes. This validation complements previously demonstrated accuracy for the same model in simulating pipeline scour processes in pure-wave environments. The model was subsequently utilized to simulate combined wave-plus-current scour over a wide range of combined Keulegan--Carpenter numbers and relative current strengths. The resulting equilibrium scour depths and trends were shown to be in accordance with existing experimentally based expressions from the literature. The variety of scour profile types emerging under various flow conditions is detailed and reconciled with experimental observations. The resulting matrix of scour depth time series was systematically analyzed, resulting in a new generalized expression for the scour time scale in combined waveplus-current flow environments. This expression is fully consistent with existing experimentally based relations at both pure-current and purewave limits and is appropriate for engineering use. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental investigation on edge scour adjacent to a stone cover laid on a sandy bed. The three-dimensional flow over the edge of the stone layer has been investigated by the use of particle image velocimetry. The flow measurements show a significant amount of turbulence in the primary flow near the junction between the stone layer and the sand bed and the formation of complex secondary-flow structures. The results show that the flow and the edge scour process in a steady current are governed by the size of the roughness elements and to some extent the side slope of the berm. The edge scour is caused by the combined action of the primary flow and the secondary flow. The primary flow stirs up the sediment and puts it into suspension, and the secondary flow carries it away from the junction between the stone layer and the sand bed, resulting in a scour hole forming adjacent to the toe of the stone layer. The measured scour depth attained a constant level of approximately one times the stone size in the live-bed regime; further, the scour depth showed a slight decrease when the side slope of the berm was increased. Design diagrams are presented for the scour depth and the time scale of the scour process. [ABSTRACT FROM AUTHOR]

Simple and effective technologies for promoting deposition and preventing scour have become the focus of intense research in recent years. This study investigated the hydrodynamic characteristics of an improved single suspended curtain (SC) structure for sediment trapping. The SC consisted of a thin, durable floating sheet, strings, and holding anchors. The floating sheet was fixed to the bed by the strings and anchors. The curtain itself was rigid, but could be free to orientate with the approaching flow. The majority of the flow passed over the curtain, and a large transverse recirculation zone developed on the lee side of the curtain. The inclination of the SC, recirculation zone, and forces on the SC under uniform flow condition were investigated. In addition, corresponding empirical formulas were proposed for calculating the inclination angle, recirculation length, drag coefficient, and anchoring force. [ABSTRACT FROM AUTHOR]

The evolution of a toe-protected nourished beach, located at Belvedere Marittimo (Italy), was investigated by means of both a physical model and a numerical morphological approach. Several surveys, including bottom profiles, sedimentology, and wave climate, were available after the nourishment for this site. Results showed that the physical model is in a fairly good agreement with the field surveys regarding localized phenomena such as the scour around the barrier, whereas it lacks in predicting the slope of the beach profile. The results of the morphological model, carried out on the basis of the field data, well reproduce the beach slope. On the other hand, when it is applied with the same parameters of the physical model, it lacks accuracy in predicting the experimental beach profile. [ABSTRACT FROM AUTHOR]

Laboratory experiments have been carried out to investigate the scour around single piles and twin-pile groups subject to oscillatory flows. The Keulegan-Carpenter number, which is proportional to the ratio between the maximum excursion of water particles and the pile diameter, ranges from 3 to 110 in the experiments. To verify the experimental setup, the scour around a tall monopile was first studied, and the results were found to agree well with the previous research. The effect of the pile height and the influence of the twin-pile layout on the scour have been subsequently investigated. It was found that the scour became independent of the pile height once the pile was four times higher than the diameter. An empirical expression was constructed to describe the exponential variation of the scour depth with the pile height. The individual piles in a twin-pile group were found to behave independently when the separation was three times greater than the pile diameter. The relationships between the scour depths around the twin-pile groups and the group configuration were established graphically. [ABSTRACT FROM AUTHOR]

In this paper, wave scattering by a vertical cylinder with a scour pit governed by the modified mild-slope equation (MMSE) is studied analytically. The scour pit around the cylinder is assumed to be axi-symmetric and idealized with its radial profile being a power function. This assumption permits transformation of the two-dimensional MMSE into an ordinary differential equation (ODE) in the radial direction through the technique of variable separation. By employing a newly derived explicit form of the resultant ODE of the MMSE in the scour pit region, an exact solution to the MMSE is constructed in terms of a Fourier-cosine series and Taylor series. To validate this new analytic solution to the MMSE, a comparison among the present solution, analytic solution to the long wave equation, and analytic solution to the Helmholtz equation is made and a good agreement is obtained. It is found that the present MMSE model is valid for a maximum bottom slope of approximately 0.927. Based on the present solution to the MMSE, the effect of dimensions of the scour pit, including both depth and width, on wave run-up around the cylinder is investigated. Finally, the influence of the wavelength of incident waves from shallow to deep water on wave run-up around the cylinder is also investigated. [ABSTRACT FROM AUTHOR]

This paper presents the findings from an experimental investigation into the nature and extent of the scouring that occurs when a ship berths in front of a perpendicular quay wall within the confines of a harbor. It examines the interaction between the relative position of the quay wall to the central axis of the ship and the influence of the angle of the vessel's rudder on the magnitude of the scouring produced. The experimental program covered a range of sediment sizes, propeller diameters and speeds of rotation, and rudder angles. Also, methods for calculating the depth of expected scour are presented in terms of initial semiempirical equations for the data range under investigation. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental investigation of the backfilling of scour holes around circular piles. Scour holes around a pile are generated either by a current or a wave. Subsequently, the flow climate is changed from current to wave, combined waves and current, or wave to a smaller wave, leading to the backfilling of the scour hole. The investigation has shed light onto the mechanism behind the backfilling process. The results show that the scour depth corresponding to the equilibrium state of backfilling is the same as that corresponding to the equilibrium state of scour around the pile for the same wave (or combined waves and current) climate. The time scale of backfilling has been determined as a function of three parameters, namely, (1) the Keulegan-Carpenter number of the initial wave or current (which generates the initial scour hole); (2) that of the subsequent wave, which backfills the scour hole; and (3) the Shields parameter associated with the latter wave, for live-bed conditions. In the case of the combined waves and current, the current-to-wave-velocity ratio is also involved. The time scale of the backfilling process is completely different from that of scour. The time scale of backfilling is much larger than that of scour when the Keulegan-Carpenter number associated with the backfilling is (typical wind farm application), while the time scale of backfilling can be smaller than that of scour when . [ABSTRACT FROM AUTHOR]

The scour process around monopiles caused by breaking waves is studied experimentally using regular waves. The use of regular waves is conservative, which made it possible to avoid scour phenomena caused by nonbreaking waves such as scour generation and backfilling. The waves were breaking on a flat sand section after shoaling on a mildly sloping ramp. Various monopiles were exposed to plunging breakers that were breaking at various distances from the pile. It was found that the scour was caused by turbulence generated by the breaking and was diverted toward the bottom by the pile. The maximum scour depth found was approximately 0.60. This was smaller than the scour observed around piles exposed to current; however, in some cases it was an order of magnitude larger than the scour caused by nonbreaking waves. This is apparently especially true for larger piles. [ABSTRACT FROM AUTHOR]

This paper presents results from laboratory experimental model studies investigating scour by waves around a pipeline attached to a flexible rubber plate or rigid spoiler. The rubber plate was placed between the submarine pipeline and bed, while the rigid spoiler is attached to the top of the pipe. The scour around pipelines with and without a rubber plate or a rigid spoiler under regular and irregular waves was observed and measured for a range of pipe sizes, wave amplitudes and frequencies, and the length of the plate/height of the spoiler. The experiments reveal that although the rigid spoilers can enhance the scour depth and extent (thus accelerating the self-burial of the pipe), they also have significant influence on both the upstream and downstream bed topography as sand ripples and dunes are formed. On the other hand, the rubber plates cannot only significantly increase the scour depth but also have little effect on the upstream and downstream bed. The experiments show that when the length of the plate is about 1.5 times the pipe size, it provides the optimum performance in terms of the largest scour depth while restricting the impact on nearby beds for the parameters investigated in this study. [ABSTRACT FROM AUTHOR]

This case study on the Mississippi River Gulf Outlet earthen berm/spoil bank (EBSB) illustrates a method to quantitatively analyze wave-induced lateral erosion and breaching through the flood side of coastal flood protection levees with grass armor prior to overtopping. Lateral wave-induced erosion was evaluated based on a three-step process: (1) velocity profiles were generated using a general purpose transient dynamic finite element program; (2) resistance to wave-induced erosion as a result of grass cover (turf) on the flood face of the EBSB was estimated and no wave-induced erosion was applied until the grass cover protection failed; and (3) estimated soil erodibility characteristics were used, in combination with the estimated wave-induced flood face velocities, to estimate the magnitude of lateral erosion. This case study identified that soil erodibility characteristics directly impact coastal flood protection levee performance, grass/turf armoring provides erosion protection for a limited duration, and identification and evaluation of pre-overtopping wave-induced erosion is an important failure mode to be considered in order to ensure reliable coastal flood protection. [ABSTRACT FROM AUTHOR]

A numerical model for local sediment scour with free surface and automatic mesh deformation is constructed and numerical results are compared with experimental results. For the turbulence closure, the two equation k–[variant_greek_epsilon] model is used. Two interfaces (water and air, water and sediment) in the domain are captured with different approaches. The free surface of the flow is simulated with the volume of fluid (VOF) scheme, which is an Eulerian approach. A new method for the VOF scheme is proposed to reduce the computational time while retaining relative accuracy. The behavior of the water-sediment interface (bed) is captured with a moving-mesh method, which is a Lagrangian approach. The flow field is coupled with sediment transport (both bed load and suspended load) using a quasi-steady approach. Good results have been obtained using the current model. The flow field is similar to the one observed in experiments. Scour patterns are similar to the experimental data as well. Large computational times are needed for morphological simulation and parallel computation is used to accelerate this process. The results obtained from the model are promising. [ABSTRACT FROM AUTHOR]

Self-burial of finite-length cylinders under pure oscillatory flow was studied with a large oscillating water-sediment tunnel (LOWST). Cylindrical models were placed on a 60 cm deep sandy bed and measurements of cylinder burial were made with the aid of video cameras. Thirty experiments were completed for a wide range of flow conditions to investigate burial due to scour of cylinders having length-to-diameter ratios between two and four. Experimental results showed a strong correlation between the Shields parameter and the equilibrium burial depth. On the other hand, the correlation between the equilibrium burial depth and the Keulegan-Carpenter number, commonly used to determine equilibrium scour depth of pipelines, was found to be less strong for the current case of short cylinders. Length-to-diameter ratio, density, and initial burial of the models were not found to influence the equilibrium burial depth. A time interval, necessary for burial initiation, was defined. The time scale for the burial of a cylindrical object was also identified and characterized by a semiempirical equation. Based on the observations, a new equation to estimate the burial of finite-length cylinders is advanced and a procedure to estimate the burial process is proposed. [ABSTRACT FROM AUTHOR]

This paper summarizes the results of an experimental investigation on wave scour around a circular pile in three kinds of soil, namely in dense silt (with a relative density of Dr=0.74), in medium dense silt (with Dr=0.38), and in sand (with Dr=0.23). The dense silt was achieved by wave liquefaction. Upon the completion of the liquefaction/compaction process, the pile was subjected to wave induced scour. The range of the Keulegan-Carpenter number (KC), tested in the experiments is from practically 0 to approximately 20. The Shields parameter in the experiments was such that the bed was live. The scour depth was increased by a factor of 1.6–2 when the bed soil was changed from medium dense silt or sand to dense silt. This is partly due to an increase in the angle of friction. The time scale of scour also was influenced by the density of the soil. The time scale was largest for the dense silt case, and smallest for the sand case in the experiments. [ABSTRACT FROM AUTHOR]

This paper presents the results of field experiments carried out around a bunker from the Second World War situated on the beach. The structure was surrounded by water at high tide and subject to intense wave forcing. By positioning pressure sensors at different levels inside the soil, simultaneous time series of pore pressure measurements were acquired. They demonstrated that the sediment layer on top of the bed does not remain continuously in static equilibrium. Momentary liquefaction was observed as the occurrence of an upward pressure gradient, which could overcome the effective weight of the sediment layer. Using a Fourier analysis, a comparison was made with the theory put forward by Sakai et al. in 1992 whereby the damping of pore pressure variations inside the soil can be quantified. This indicated the existence of a significant amount of gas inside the soil, which was confirmed in the field by geoendoscopic videos. [ABSTRACT FROM AUTHOR]

Results of an experimental study of the scour and burial of cylinders placed on a sandy bed under the combined flow of regular waves and currents are presented. Experiments were conducted for cylinder Reynolds wave number and Keulegan-Carpenter number within the ranges 1.5×104≤Re≤1.2×105 and 2≤KC≤48, respectively. Analysis of the observations indicates that existing semiempirical formulas for prediction of equilibrium-burial-depth and time scales for pipelines are not suitable for the case of a cylinder having a finite length. Thus the need for developing new expressions for both final-burial-depth and time scales based on new observations. In these experiments burial depth shows stronger dependency on the Shields parameter θ than on the KC, whereas burial-time scales seem to be influenced by both KC and θ. A new empirical predictor for estimating the equilibrium burial of short cylinders under combined flows is presented. The equation is based on the combination of θ, KC and the ratio of current velocity to the effective wave plus current velocity. [ABSTRACT FROM AUTHOR]

A vertical two-dimensional numerical model for local scour beneath an offshore pipeline exposed to waves is developed in this study. Waves are modeled as sinusoidally oscillatory flows, and the phase-resolved velocity field around the pipeline is simulated by solving the Reynolds-averaged Navier-Stokes equations with a k-ω turbulence closure. Both suspended and bed loads of sediment transport are considered in the model. A special technique for coupling the solution of the flow field to the scour calculation is developed to accommodate different timescales of the flow and morphological changes. The seabed morphology calculation is based on the period-averaged sediment transport rate with a morphological time step that is chosen by designating a maximum allowable bed deformation in each update. Numerical tests indicate that this technique significantly reduces the computational costs and is of acceptable accuracy. A sand-slide method is employed after each bed update to smooth out small bed irregularities. The model is applied to simulate three cases of wave-induced scour below a pipeline where laboratory measurements were published in the literature. The predicted scour-hole profiles compare favorably with the laboratory observations. [ABSTRACT FROM AUTHOR]

Presents the results of an experimental study of the scour created by a siphon flow through a vertical tube in a cohesionless sand bed. Factors governing the removal of cohesionless materials by siphon flows by a vertical straight tube; Technique for removing contaminated sediments; Mean grain size of the sand used on the study.

This paper summarizes the results of an experimental study on scour around spherical bodies and self-burial in steady current and in waves. The equilibrium scour depth below a fixed sphere in steady current for live-bed conditions was found to be S/D=O(0.3), D being the sphere diameter. The effect of the Shields parameter, θ, and that of the position of the sphere with respect to the bed, e, were investigated. It was found that, under live-bed conditions, the effect of θ is quite weak. It was also found that the effect of e is significant. No scour occurs when e/D>0.8. It was demonstrated that, in the case of waves, the Keulegan-Carpenter number, KC, is the main parameter that governs the equilibrium scour depth. S/D is always smaller in waves than in steady currents. Regarding the self-burial, the equilibrium self-burial depth in steady current was found to be e/D=O(0.5). The self-burial depth in waves is governed by KC, similar to the scour depth for a fixed sphere. e/D is always smaller in waves than in steady currents. The experiments indicated that the time scale of both the scour process and the self-burial process is a function of θ in the case of steady current, and θ and KC in the case of waves. [ABSTRACT FROM AUTHOR]

The hydrodynamics around a static horizontal cylinder and five static scoured bed profiles subjected to steady flow were simulated using a CFD model, FLOW-3D. Both a two-equation k-ε model and a Smagorinsky LES turbulence closure scheme were evaluated. The mean horizontal and vertical velocity estimates of both turbulence models show reasonable agreement with the data. Wake characteristics such as the reattachment point and vortex shedding were examined. The reattachment point for the k-ε closure scheme was consistent with the observations. However, the vortex shedding induced by the LES closure scheme resulted in a considerably larger and unsteady reattachment point. In all profiles, the size and strength of the eddies formed by the LES closure scheme were found to be grid dependent. The k-ε model was found to be grid dependent for the vortex shedding process. [ABSTRACT FROM AUTHOR]

Contraction scour in a firm-clay estuarine riverbed is estimated at an oil-unloading terminal at the Port of Haldia in India, where a scour hole attained a maximum depth greater than 5 m relative to the original bottom. A linear equation for the erosion flux as a function of the excess bed shear stress was semicalibrated in a rotating-cylinder device called SERF (Simulator of Erosion Rate Function) and coupled to a hydrodynamic code to simulate the hole as a clear-water scour process. SERF, whose essential design is based on previous such devices, additionally included a load cell for in situ and rapid measurement of the eroded sediment mass. Based on SERF’s performance and the degree of comparison between measured and simulated hole geometry, it appears that this device holds promise as a simple tool for prediction of scour in firm-clay beds. [ABSTRACT FROM AUTHOR]

This paper sheds light on the mechanism of post equilibrium sea bed scour around dynamically active marine structures such as wind turbines. Exposure of a fully developed scour hole (at equilibrium state) around a wind turbine mono-pile to the cyclic movement of the structure leads to the backfilling and deformation of the scour hole. The existing approaches to scour prediction for foundation design of offshore wind turbines generally consider wind turbines as static structures and ignore the physical impact of the cyclic movement of the pile on the supporting soil and, hence, on the scour process. Through an experimental program, this paper explains the influence of the cyclic movement of the pile on the local scour in noncohesive sediments. A series of flume tests at two scales were conducted. Simple hydrodynamic conditions and bed sediment configurations were adopted to highlight the effect of pile movement. The results obtained indicate that a mechanism exists by which the scour hole can be significantly deeper and wider in extent than that predicted by conventional methods. This arises through a multistage process consisting of periodically alternating cyclically loaded and unloaded stages simulating a sequence of storms. [ABSTRACT FROM AUTHOR]