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The paper presents the results of an experimental study of mean fluid flow and turbulence over bed forms in a unidirectional flow with superimposed surface waves. Experiments were performed with only current and with combined wave‐current flows over a series of bed forms under different surface wave frequencies for two different Reynolds numbers. Three‐dimensional velocity was measured using a 3‐D micro acoustic Doppler velocimeter. The superposition of surface waves with increasing frequency leads to an increase in the apparent bottom roughness due to a vortex in the lee, which causes the resistance to the flow. The effect of surface waves is to increase the flow stability, consequently reducing flow separation and enhanced mixing in the lee side of the bed form. A stronger circulation pattern in the lee side of the bed forms is observed at a higher Reynolds number.

This article presents a review of the state of research on bridge pier scour under combined wave–current flow. The hydrodynamics and scour around the bridge pier under combined wave–current flow have been explained in detail based on the information available in the literature. The impact of relative flow velocity ( U cw ), Keulegan–Carpenter number ( KC ), absolute Reynolds number ( R ea ), and sediment characteristics on bridge pier scour under combined wave–current flow is presented. This study includes physical modelling of scour with various formulations to predict scour depth and calculation procedures related to scour under combined wave–current flow in the coastal environment. In addition, this study also provides the development of numerical models to investigate bridge pier scour in detail. In the end, future prospects of hydrodynamics and scour around the bridge pier under combined wave–current flow are delineated.

The development of pier scour in coastal environments severely affects the bridge’s stability. Therefore, estimating pier scour around the vertical cylinder is important for the safety of the bridge structure. The estimation of pier scour depth in combined wave-current conditions has become a challenging task for researchers in recent times. The existing empirical formulations that calculate scour in the combined action of current and wave are scarce and may not always provide accurate results. Machine-learning (ML) techniques have become increasingly popular for their prediction capabilities in the fields of hydraulics and coastal engineering in recent years. Therefore, the present study aims to develop Boosting ML techniques (i.e., AdaBoost, XGBoost, CatBoost, and LightGBM) of ML to estimate pier scour in combined wave-current conditions. The non-dimensional parameters, such as Keulegan–Carpenter (KC) number, Relative flow velocity ( U cw ), and Absolute Froude number (Fr a ), are used as input parameters, whereas scour depth (S/D) is the output parameter in Boosting ML models. The sensitivity analysis has been performed to demonstrate the relative importance of the input parameter on S/D . The performance metrics show that the XGBoost model with the input combination of F ra , KC, and U cw provides the highest accuracy of 92.47% and outperforms SVM, CatBoost, AdaBoost, and LightGBM models. The XGBoost model also outperforms the existing empirical formulations. Therefore, it can be concluded that the XGBoost techniques can be used as a reliable, accurate, and alternative tool to estimate pier scour depth in the combined action of current and wave.

The action of wave dominated flow on river bank leads to retreatment of the bankline thereby causing intense erosion issues. The understanding of the bank erosion process mechanisms is of great importance in the context of protecting or controlling the progressive growth of bankline which imposes a direct threat on near bank fertile agricultural land and habitats. The present study emphasizes on acquiring improved understanding on the bank erosion processes related to wave action that severely impact the bank erosion rate. Turbulent fluctuations of the near bank flow were observed to be modulated due to the interplay between eroded bank wall and stream flow under the influence of wave following and against the current. The fluctuating turbulent velocity field was measured using micro acoustic Doppler velocimeter (ADV) at regions close to the bank wall during the different stages of the erosion progress. Streamwise turbulence intensity was found to be relatively large upto a particular undercut depth during the erosion progression. The integral time and length scales and Taylor microscales were determined for different temporal stages using autocorrelation function. Results depict that wave current combined flow in conjunction with rough wall surface formed by the erosion process amplifies the turbulent kinetic energy and turbulent dissipation rate at vicinity of the wall. The velocity fluctuations show large intermittency as evaluated from Gaussian pdf for wave current combined flows. This may affect the near wall turbulence structures which is a causative factor for enhancement of erosion rate as compared to current only flow. Article highlights Modulation of turbulence scales under wave current combined flow. Interaction between wave current combined flow and roughness formed in bank wall due to erosion. Turbulent structures and its effects on progressive bank erosion process.

Sediment transport modeling for flows with cylinders is very challenging owing to the complicated flow–cylinder–sediment interactions, especially under the combined wave-current flows. In this paper, an improved formulation for incipient sediment suspension considering the effect of cylinder density (i.e., solid volume fraction) is employed to simulate the bottom sediment flux in the flow with cylinders. The proposed model is calibrated and validated using laboratory measurements under unidirectional and combined wave-current flows in previous studies. It is proved that the effects of cylinders on sediment suspension can be accounted for through a modified critical Shields number, and the proposed model is capable of simulating sediment suspension under both unidirectional and combined wave–current flows reasonably well with the average the coefficients of determination and model skills greater than 0.8 and 0.64.

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 U w /U c < 2.5, the time-averaged velocity peaked above the meadow, which was similar to pure current conditions. When U w /U c > 2.5, the presence of waves caused the time-averaged velocity to peak near the top of the meadow. For U w /U c > 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 U w /U c > 1 and dominated by currents when U w /U c < 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.

A 2D numerical model for viscous flow is established in OpenFOAM version 10 to analyze the hydrodynamic response of submarine power cables for offshore wind turbines under combined wave–current conditions. It focuses on analyzing the effect of the cable suspension ratio e/D and the current-to-wave velocity ratio Uc/Um on the Morison coefficient of the suspended cable. The results indicate that for the cable suspension ratio e/D of less than 0.5, the strength of the dependence of both the drag coefficient Cd and inertia coefficient CM on the cable suspension ratio e/D is significantly influenced by the current-wave-ratio Uc/Um, while this dependence becomes less pronounced for e/D greater than 0.5. And the inertia force coefficient CM decreases monotonically with the current-to-wave velocity ratio Uc/Um, while the drag force coefficient Cd demonstrates a more complex, non-monotonic relationship with it. Based on the simulation results in this paper, a quantitative relationship between Cd, CM, and the key governing parameters is established using a two-layer feedforward neural network model, providing a method for predicting wave–current forces on subsea suspended cables.

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.