Combined Wave-current Conditions Research Articles

How Artificial Salt Marsh Vegetation Reduces The Threshold For Sediment Resuspension In Wave-Current Flows

Suspended sediment transport and retention within salt marshes is a key factor in their resilience against erosion associated with threats such as sea level rise and coastal squeeze. Salt marshes are vegetated intertidal wetlands found in temperate climate zones. Their presence contributes to flood protection, coastal flora and fauna, and carbon sequestration (Temmerman et al., 2013). Vegetation-induced resuspension of fine sediment helps to transport fine particles deeper into salt marshes. The interaction between waves, currents, and vegetation may resuspend sediment sooner than it would without vegetation. It has been shown in conditions with only currents (Liu et al., 2021; Tinoco & Coco, 2014) or only waves (Tinoco & Coco, 2018) that the threshold for resuspension is lower within vegetated meadows than without vegetation. However, the threshold has not yet been studied for combined wave-current conditions, which often exist in the marsh environment. Identifying this threshold will enable us to predict when in a tidal cycle sediment resuspension occurs and can be used to improve sediment transport model. We use flume experiments to methodically identify the threshold of sediment resuspension in artificial salt marsh meadows of three different area densities under combined wave-current flows.

The dynamic response of floating offshore wind turbine platform in wave–current condition

In this paper, the fluid–structure interaction of floating offshore wind turbine (FOWT) platforms under complex ocean conditions is investigated using OpenFOAM and in-house developed models. Two types of FOWT platform, i.e., a semi-submersible platform and a barge platform, are studied for their dynamic responses to either wave or current. The results reveal that a semi-submersible platform exhibits larger cross-flow motion and lock-in phenomenon, while a barge platform experiences smaller motion with no significant lock-in within the velocity range examined. The combined wave–current conditions are further studied for the semi-submersible platform, with different angles between wave and current, the current speeds, and wave parameters. Unlike other investigations focusing on colinear wave–current interaction, in which the waves usually mitigate vortex-induced motion (VIM); here, we find that waves might lead to an enhanced VIM with a large angle between current and wave. The evaluation on the interaction effect factor shows that the largest wave height in the lock-in region does not lead to the most dangerous scenario, herein, the largest platform motion. Instead, a smaller wave height with a large wave period can induce even larger motion.

Experimental investigation of the horizontal bearing capacity of offshore wind-turbine monopiles with grouting protection against scouring

Seabed scouring can uncover the buried component of wind-turbine monopiles and this reduces their horizontal bearing capacity. Because rigid piles with large diameters and low slenderness ratios have been widely used in offshore wind farms, there is an urgent need to address this issue. In this study, grouting protection, as a new countermeasure against scouring, was examined, with scouring tests under constant-current, regular-wave, and combined wave–current conditions being conducted, along with static and cyclic load tests on the monopile. The scouring characteristics and horizontal bearing behaviours of a pile with grouting protection (Pile a) and a pile without grouting protection (Pile b) were investigated. We found that the ultimate scouring depths of piles with grouting were much lower than those without under constant-current scouring, regular-wave scouring, and combined wave–current scouring. The HL2Dγ′−y0D and p-y curves of the piles were obtained, which showed that grouting protection can significantly strengthen the ultimate horizontal bearing capacity under a static loading. The pile with grouting was more stable under cyclic load compared to the pile without grouting. There are two explanations for the strengthening of the horizontal bearing behaviours of piles with grouting protection. First, grouting lessens the loss of pile burial depth brought about by scouring. Second, grouted seabed has a much higher sediment stiffness than ungrouted seabed, thus the grout dramatically increases the strength of the pile– sediment structure.

A numerical flume for waves on variable sheared currents using smoothed particle hydrodynamics (SPH) with open boundaries

Combinations of waves and currents exist in a wide range of marine environments. The resulting, often complex, combined wave-current conditions largely determine the loading, response and survivability of vessels, offshore platforms and systems. Smoothed Particle Hydrodynamics (SPH) has becoming increasingly popular for free-surface flow problems which do not require special treatment to detect the free surface. The present work implements open boundaries for wave-current conditions within the SPH-based DualSPHysics solver. Open boundaries are applied for the generation of wave-alone, current-alone and combined wave-current conditions. A modified damping zone acting on the vertical velocity component is used for wave absorption and combined with open boundaries to allow particles to leave or enter the fluid domain when a current exists. Results of wave-alone (regular, irregular and focused) and current-alone (uniform, linearly sheared and arbitrary sheared) test cases demonstrate a general numerical flume is achieved. Tests of focused waves interacting with arbitrary sheared currents are validated with analytical linear solutions for surface elevation and velocities, demonstrating excellent agreement. The numerical flume may be extended to steep waves well suited to SPH and thus enable modelling complex and extreme wave-current conditions interacting with offshore platforms and systems.

Experimental study on dynamic characteristics of a monopile foundation based on local scour in combined waves and current

Scour is one of the key factors causing the collapse of buildings (structures) in water. Pile foundations are often subject to the combined actions of waves and current, resulting in weakened soil overturning resistance and changes in natural vibration frequency, which may lead to vortex-induced vibration under the action of fluid flow and cause fatigue damage. A series of experiments under current-only, wave-only, and combined wave-current conditions were carried out in a wave-current flume to investigate the scour phenomenon around a monopile and the change in natural vibration frequency of the monopile foundation along the flow and vertical directions caused by local scour. The scour depth under combined wave-current conditions is greater than that under current-only conditions, and increases accordingly with the increase of the flow velocity and the decrease of the water depth. Furthermore, the scour phenomenon under different flow patterns was analyzed according to the variation of pulsation pressure. The empirical formula for the natural vibration frequency of the monopile foundation considering the local scour under current-only and combined wave-current conditions was established respectively. Compared with the experimental results, the maximum error is 10%, indicating that the empirical formula has a high degree of confidence. The above empirical formula obtained from the indoor scale model is applied to the field test, and the maximum relative error is 14.37%, which indicates that the empirical formula is also suitable for the prototype scale.

Flow Structure in an Artificial Seagrass Meadow in Combined Wave-Current Conditions

Experiments were conducted in a laboratory flume using an artificial seagrass meadow, modeled after Zostera marina, to examine the impact of waves on the vertical structure of time-averaged current, Reynolds stress, and turbulent kinetic energy (TKE) under combined wave-current conditions. With the addition of smaller waves, defined by a ratio of wave velocity to current velocity Uw/Uc < 2.5, the time-averaged velocity peaked above the meadow, which was similar to pure current conditions. When Uw/Uc > 2.5, the presence of waves caused the time-averaged velocity to peak near the top of the meadow. For Uw/Uc > 1 the presence of waves reduced the magnitude of peak Reynolds stress. For all conditions considered, the wake production of turbulence dominated the shear production of turbulence in the meadow. However, the wave velocity was less efficient than the current velocity in generating TKE in the meadow because the movement of the blades forced by the oscillatory fluid motion reduced the relative velocity between the blades and the wave. A modified hybrid model for wake production of TKE in a flexible canopy under combined wave-current conditions was proposed to account for the relative contributions of waves and currents. Wake production of TKE was dominated by waves when Uw/Uc > 1 and dominated by currents when Uw/Uc < 1. The models and observations proposed in this study contribute to an enhanced understanding of the relative influences of waves and currents on seagrass meadow flow structure in realistic combined wave-current conditions.

SPH simulation of hydrodynamic responses for two novel types of silt curtain under combined wave-current conditions

Traditional silt curtain is a kind of commonly-used impermeable device for short-term control of suspended solids or turbidity in the water column generated during dredging operations. It is attempted to innovatively apply the silt curtain to more violent hydrodynamic conditions, such as functioning as an isolation structure for preventing sediment transportation into channels and harbor basins in a combined wave and current environment. The Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) code of Shi and Li et al. (2018), after incorporating an overlapped particle method (OPM), was applied to simulate the responses of a flexible impermeable curtain system as a thin-walled structure fixed to the sea bottom and moving violently under the action of static water pressure heads along and combined with waves. It is found that the modeled results of tensions in the curtain of a Silt Curtain System (SCS) working in emerged mode are in good consistency with the theoretical and experimental results and the SCS in submerged mode would be subject to much smaller tensions than in emerged mode. A large influx rate of q = 0.053 m3/s/m would enhance the wave-induced component of curtain tensions in the submerged mode. When subject to waves, the SCS in emerged mode imposes much more significant influence on wave transmission than in submerged mode, while the SCS in emerged mode produces lower horizontal oscillation ratio than in submerged mode.

Adaptive criterion curves describing incipient motion of sediment under wave and current conditions

Sediment incipient motion is a fundamental issue in sediment transport theory and engineering practice. Whilst Shields curve often is used to determine the threshold of sediment movement under unidirectional current conditions, it is unclear whether it can be directly applied for the wave or combined wave-current conditions. The study developed adaptive criterion curves describing incipient motion of sediment under wave and current conditions based on the flow pattern around the sediment particles. Firstly, the flow pattern law for fixed particles was recognized based on the friction law under various dynamic conditions (wave, current, and their combinations), and the flow pattern demarcations for incipient sediment motion were obtained with the threshold conditions for sediment movement under various dynamic conditions combined. Secondly, the exact shape of the Shields curve in each flow regime was derived under the current condition. By combining the flow pattern demarcations for incipient sediment motion under the wave condition, the criterion curve under the wave condition was derived. By combining the flow pattern demarcations for incipient sediment motion under the combined current-wave condition, the criterion curve for sediment incipient motion under the combined current-wave condition was derived. The results indicated that the flow pattern around incipient particles includes laminar, laminar-rough turbulent transition, and rough turbulent regimes. The criterion curves for sediment incipient motion under various dynamic conditions stayed the same in the laminar and rough turbulent regimes, but different in the transition regime. Depending on the relative strengths of the currents and waves, the shape of the criterion curve under the combined current-wave condition transitions adaptively between the criterion curve under the current condition and the criterion curve under the wave conditions.

Influence of Structural Elements on the Spatial Sediment Displacement around a Jacket-Type Offshore Foundation

This research advances the understanding of jacket-type platform induced local and global erosion and deposition processes for combined wave–current conditions. To this end, a laboratory study was carried out comparing the equilibrium scour depth for two structural designs that are differentiated in the geometrical distance of the structure’s lowest node to the seabed. Measurements of local scour depths over time have been conducted with echo sounding transducers. An empirical approach is proposed to predict the final scour depths as a function of the node distance to the seabed. Additionally, 3D laser scans have been performed to obtain the digital elevation model of the surrounding sediment bed. Novel methodologies were developed to describe and easily compare the relative volume change of the sediment bed per surface area due to structure–seabed interaction, enabling spatial analyses of highly complex erosion and deposition patterns. The seabed sediment mobility around the structure is found to be highly sensitive to a change in node distance. The decrease of the node distance results in a higher erosion depth of sediment underneath the structure of up to 26%, especially for current-dominated conditions, as well as an increased deposition of sediment downstream of the structure over a distance of up to 6.5 times the footprint length. The results of this study highlight the requirement to consider the interaction of the structure with the surrounding seabed within the design process of offshore structures, to mitigate potential impacts on the marine environment stemming from the extensive sediment displacement and increased sediment mobility.

Numerical Study of a Single-Point Mooring Gravity Fish Cage With Different Deformation-Suppression Methods

Abstract Compared with the multipoint mooring fish cage, the single-point mooring (SPM) fish cage can spread out the fish wastes and uneaten feeds in a larger area, and it can also prevent the local environment from being overwhelmed. Thus, it has attracted much attention recently. In this research, different deformation-suppression methods are applied to the SPM system with a typical Norwegian fish cage aiming to increase the cultivation volume under the action of current and/or wave loads. A well-validated software, fhsim, is used to conduct the full-scale numerical study. The effects of the three deformation-suppression methods, i.e., (i) adding the lower bridles, (ii) adding the frontal rigid frame and (iii) adding the trawl doors, are analyzed under pure current and combined wave–current conditions. The results indicate that all the three deformation-suppression methods can improve the cultivation volume at least by 32% compared to the original SPM fish cage when the current velocity is larger than 0.5 m/s. In addition, moving the conjunction point close to the bottom ring can bring a positive effect on the cultivation volume maintaining with an only small increment in the tension force. This study can provide practical advice and useful guides for the SPM fish cage design.

Local Scour for Vertical Piles in Steady Currents: Review of Mechanisms, Influencing Factors and Empirical Equations

Scour induced by currents is one of the main causes of the bridge failure in rivers. Fundamental knowledge and mechanisms on scour processes due to currents are often taken as a basis for scour studies, which are the focus of this review. Scour development induced by waves and in combined wave–current conditions are also briefly discussed. For the design of structure foundations, the maximum scour depths need to be estimated. The mechanisms of local scour and predictions of maximum local scour depths have been studied extensively for many years. Despite the complexity of the scour process, a lot of satisfying results and progresses have been achieved by many investigators. In order to get a comprehensive review of local scour for vertical piles, major progresses made by researchers are summarized in this review. In particular, maximum scour depth influencing factors including flow intensity, sediments, pile parameters and time are analyzed with experimental data. A few empirical equations referring to temporary scour depth and maximum scour depth were classified with their expressing forms. Finally, conclusions and future research directions are addressed.

Experimental study on the formation and migration of submarine sandwaves under wave-only and combined wave-current conditions

Based on the assumption that the crest lines of sandwaves are perpendicular to the main flow direction, the formation and migration of submarine sandwaves are simulated in a 2 D flume under wave-only and combined wave-current conditions. The effects of various flow parameters, including the wave height and current velocity, on the characteristic dimensions and migration rates of sandwaves are discussed. Measured morphological data indicate that the seabed is composed of both large-scale sandwaves and small-scale sand ripples, and sandwaves are dominant in shaping the seabed form. Starting from a flat seabed, the height of sandwaves increases gradually and tends to become dynamically stable, while the wave length of sandwaves reaches a constant value at the initial stage. Under wave-only conditions, the characteristic wave heights and wave lengths of sandwaves increase with increasing incident wave height. The relative locations of the sandwave peak and trough are almost constant, indicating that sandwaves scarcely migrate under wave-only conditions. Under combined wave-current conditions, the characteristic heights and lengths of sandwaves also increase with increasing current velocity. The migration direction of sandwaves coincides with the direction of the residual current, and the migration rate increases with increasing current velocity. Finally, the characteristic dimensions of sand ripples are also analyzed, revealing that the Reynolds number (based on the particle size) is dominant in determining the size of a sand ripple. Both the characteristic wave lengths and the characteristic wave heights of sand ripples decrease with increasing Reynolds numbers.

Wave-induced set-up over barrier reefs under the effect of tidal current

In this study, a series of new laboratory experiments were performed in a wave-current flume in the presence/absence of tidal current. Experimental results are reported for a variety of combined wave-current conditions based on an idealized barrier reef model. Result analysis shows that the mean water level on the reef flat was consistently lower during the shoreward current (flood tide) than that during the seaward current (ebb tide). Maximum wave set-up on the reef flat decreased significantly as the current flowrate changed from its seaward maximum to its shoreward maximum. A theoretical model, which was derived based on the cross-shore balances of momentum and energy, was proposed to reproduce the wave set-up on the reef flat in the presence of current. The model was then validated by the laboratory data. The key model parameter was found to decrease rapidly with increasing current flowrate.

Characteristics and simulation of sediment concentration in the northwestern head of Bohai Bay

The evolution and utilization of estuarine and coastal regions are largely restricted by sediment issues. Waves and currents are the main dynamic driving forces of sediment transport in estuary and coastal areas. The study of wave-current movement and sediment transport is of great significance in both academic research and engineering practice, which has received much attention from many scholars and engineers. The northwestern head of Bohai Bay belongs to a transitional area from silty coast to muddy coast, where the sediment movement is complex. Firstly, based on the field observation data measured with a tripod system and the hydrological data in the northwestern head of Bohai Bay, the characteristics of tidal currents, waves, and suspended sediment concentration (SSC) changes under different hydrological conditions were analyzed. Sediment density distribution was detected by a γ-ray densitometer. Results show that the SSC is mainly influenced by wave-induced sediment suspension: Under light wind conditions the SSC was very low, with the peak value generally less than 0.1 kg/m3; the SSC increased continuously under the gales over 6−7 in Beaufort scale, with sustained wind action. The measured peak SSC at 0.4 m above the seabed was 0.15−0.32 kg/m3, with the average value of 0.08−0.18 kg/m3, which is about 3−6 times the value under light wind conditions. The density at the bed bottom was about 1.4− 1.6 g/cm3, and decreased to 1.02−1.03 g/cm3 at 0.1−0.2 m above the bed; there was rarely little fluid mud in the study area. Secondly, the mechanism of wave-induced SSC was studied from the view of flow structure, shear stress in the bottom boundary layer. The friction velocity was calculated by fitting the datasets of the measured logarithmic velocity profile. Comparison shows that the wave shear stresses (calculated from the bottom velocity profile) is higher than the current shear stress (calculated from the upper velocity profile). Following the Reynolds’ decomposition method, expressions of wave-induced and current-induced shear stress were derived by splitting the variables into a fluctuating component, an averaged component and an oscillatory component. Compared with uniform flow, an additional item of wave streaming leads to a higher wave-induced shear stress. Finally, by analytically solving the sediment diffusion equation, a numerical procedure was proposed for sediment concentration under combined action of waves and currents. Some key approaches for fine sediment simulation were presented, including stratification effects, hindered settling, moving bed roughness, reference concentration and critical shear stress. Under combined wave-current conditions, the combined sediment diffusivity is given by the square sum of the wave-related and current-related diffusivities, and a toe-type distribution of wave-related eddy viscosity was employed. The stratification effects are considered by introducing the turbulence damping coefficient. The approaches of hindered settling for silt were employed. The enhanced roughness due to mobile bed effects was included. Comparison shows that, these approaches can properly simulate the measured wave-induced sediment concentration. This study provides basic data for coastal protection and utilization as well as for scientific study of silty-muddy coast, which is of great significance in practice and theory.

Scour development around a jacket structure in combined waves and current conditions compared to monopile foundations

This paper presents the results of an experimental study on the scour development of a hydraulic-transparent offshore foundation exposed to combined waves and current. Irregular waves propagating perpendicular to a current were simulated in a wave-current basin. The physical model tests were conducted in a length scale of 1:30 while measurements of the scour development over time were achieved by echo sounding devices placed at several locations at the upstream and downstream side of the jacket structure. Insights were gained on the scour development and time scale of the scouring process around a complex jacket structure for different wave-current conditions. The results were presented with respect to the Keulegan-Carpenter KC number and the relative wave-current velocity. Wave conditions were adjusted so that KC numbers between 6.7 and 23.4 could be tested in a systematic wave-current test program with tests reaching from wave dominated conditions up to current dominated conditions. Measured scour depths were critically assessed by an extrapolation to expected equilibrium scour depths. With respect to the current flow direction, the experiments showed generally larger scour depths at the upstream side and lower scour depths on the downstream side for each pile of the jacket structure. The development of global scour around the structure intensified with increasing relative wave-current velocity. As a result, a practical formulation is proposed for the reliable prediction of local scour depths around a jacket foundation in combined wave-current conditions. Finally, dimensionless time scales and observed as well as predicted scour depths are compared to values for the scour development around monopiles.

Experimental assessment of tidal turbine loading from irregular waves over a tidal cycle

Waves interact with currents in tidal channels with the resulting wave–current environment largely determining the loads experienced by tidal stream turbines. Over a tidal cycle, the magnitude and direction of the current velocity changes and hence so does the combined wave–current conditions the turbines must operate within. Here we demonstrate this effect experimentally, generating a realistic irregular wave case in both following (in the same direction as the waves) and opposing currents prior to assessing the resulting loads on a fully instrumented 1:15 scale tidal turbine model aligned with the current direction. Large changes in the environmental conditions, along with the turbine performance and loads, are demonstrated through the presentation of temporal, spectral and statistical outputs. The experimental results demonstrate that the full-scale equivalent significant wave height changes from 2.25 m in zero current to 6.11 m in 3.2 m/s opposing current and 1.56hbox { m} in 3.2hbox {m/s} following current. The corresponding standard deviations of measured turbine parameters for the opposing condition range between 215 and 260% of the following case, and between 340 and 565% of the current-only measurements. Hence, when waves are present, significantly greater fatigue damage will be accumulated during one-half of the tidal cycle. The mean values, however, appear to be unaffected by the presence of waves suggesting that the overall turbine performance is unaltered. These results demonstrate the requirement to understand the combined wave–current environment and to test and de-risk tidal stream turbines for operation in both following and opposing wave–current conditions. Significant additional insight is gained into the nature of loads experienced by tidal turbines in irregular wave conditions, a scarcely documented phenomenon.

Temporal Variation of Live-Bed Pier Scour under Combined Wave-Current Flow in a Large-Scale Flume

Chen, B. and Li, S.W., 2019. Temporal variation of live-bed pier scour under combined wave-current flow in a large-scale flume. Journal of Coastal Research, 35(2), 348–356. Coconut Creek (Florida), ISSN 0749-0208.Studies on the temporal variation of live-bed scour under waves alone and combined wave-current flow are not as extensive as those under current-alone conditions. Motivated by this, series of experimental tests were carried out in a large-scale flume (5 m wide, 8 m deep, and 450 m long). The scour processes in different locations of the scour hole were monitored simultaneously during the tests. Measured data indicated that the scour depth versus time can be expressed by an exponential equation for both waves-only and combined wave-current conditions. A piecewise equation for predicting the characteristic timescale under combined wave-current flow is proposed based on the data from the literature and the present experiment. In live-bed scour, the effect of wave-generated ripples on scour depth fluctuation could be ignored under waves alone. For the combined wave-current flow, the fluctuation amplitude of scour depth was approximately equal to the height of bed forms under current-dominated conditions and nearly half of the observed height of the bed forms in wave-dominated conditions.

Laboratory investigation of the breaking wave characteristics over a barrier reef under the effect of current

ABSTRACTRecent field studies have increasingly recognized the importance of tides in dominating the coral reef hydrodynamics. Aside from the tidal level, it is also important to evaluate the effect of tidal current on wave breaking over coral reefs. In this study, a series of laboratory experiments were performed in a wave-current flume in the presence of both flooding and ebbing tides. Experimental results are reported for a variety of combined wave-current conditions based on an idealized barrier reef model. Results analyses show that the relative reef-flat submergence (the ratio of reef-flat submergence to deep-water wave height) is a key parameter to characterize such breaking-wave characteristics as the breaker type, the breaking location, the surfzone width, and the breaker depth index. The influences of the current flowrate on the breaking location, the surfzone width, and the breaker height index appear to be significant within our tested ranges. Empirical formulas to predict these characteristics in the existing literature are also improved to account for the current effect.

Experimental Study of Local Scour around a Vertical Cylinder under Wave-Only and Combined Wave-Current Conditions in a Large-Scale Flume

AbstractA series of physical experiments under wave-only and combined wave-current conditions are conducted in a large-scale wave flume. It is found that waves with strong nonlinearity may lead to ...

Effective Roughness Height in High-Concentrated Flows

ABSTRACT Fang, J.Y.; Chen, Y.P.; Yao, P., and Su, M., 2016. Effective Roughness Height in High-Concentrated Flows. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 33–37. Coconut Creek (Florida), ISSN 0749-0208. The effective roughness height is an important parameter in coastal sediment transport models. It has been extensively investigated in the past but few research results are related to the high-concentrated flows which often occur in a silty coast. A series of experiments has been carried out in a wave-current flume with silt-sized sediment bed. The mean velocity profiles were measured under different combined wave-current conditions. The effective roughness heights were calculated based on the curve fitting of measured velocity profiles by following the velocity profile model of You (1994). The accuracy of three empirical models, namely, Gr...