Longshore Current Velocity Research Articles

The Environmental Impact of a Wave Dragon Array Operating in the Black Sea

The present work describes a study related to the influence on the shoreline dynamics of a wave farm consisting of Wave Dragon devices operating in the western side of the Black Sea. Based on historical data analysis of the wave climate, the most relevant environmental conditions that could occur were defined, and for these cases, simulations with SWAN spectral phase averaged wave model were performed. Two situations were considered for the most representative patterns: model simulations without any wave energy converter and simulations considering a wave farm consisting of six Wave Dragon devices. Comparisons of the wave model outputs have been carried out in both geographical and spectral spaces. The results show that although a significant influence appears near the wave farm, this gradually decreases to the coast line level. In order to evaluate the influence of the wave farm on the longshore currents, a nearshore circulation modeling system was used. In relative terms, the longshore current velocities appear to be more sensitive to the presence of the wave farm than the significant wave height. Finally, the possible impact on the marine flora and fauna specific to the target area was also considered and discussed.

PREDICTIONS OF CIRCULATING CURRENT FIELD AROUND A SUBMERGED BREAKWATER INDUCED BY BREAKING WAVES AND SURFACE ROLLERS

This paper develops a quasi-three-dimensional nearshore current model which accounts for excess shoreward volume fluxes due to waves and surface rollers. The model splits the water column at the wave trough level and solves two different layers of 2DH momentum equations. The first layer covers above the trough level and determines the shear stress at the trough level while the the second layer covers over the entire depth and determines the volume flux due to mean current components. Linearly approximated vertical profile of the shear stress and corresponding turbulent eddy viscosity model enables the present model to yield analytical explicit expression of the horizontal velocity profiles, which enhance computational efficiency of the model. The extended model was applied to predictions of longshore current velocity on the long straight beach and circulation current patterns behind the detached submerged breakwater on the plane beach. Excellent agreement between model predictions and measurements supports the validity of the model and importance of quasi-three-dimensional features around the submerged break water.

A One-Dimensional Parametric Model for Undertow and Longshore Current Velocities on Barred Beaches

A one-dimensional parametric model for undertow and longshore current velocities assuming a triangular velocity distribution in a surface roller was developed. This model as well as a parametric model with the assumption of a uniform velocity distribution in a roller was compared with field data obtained on barred beaches at Hasaki in Japan and at Duck in the USA. The comparisons showed that the present model predicted the velocity fields at the two sites reasonably well, and the prediction accuracy of the present model is slightly better than that of the other model. However, the present model underpredicted the undertow velocities on the trough regions, and overestimated the longshore current velocities near the shorelines.

Effects of Low Water Temperature on Longshore Sediment Transport on a Subarctic Beach, Hudson Bay, Canada

Measurements of longshore sand transport, wave breaker height, longshore current velocity, and water temperature and salinity were carred out on a subarctic beach under varying water temperature conditions during summer and fall. Water temperature during summer varied from 9°C to 17°C, while temperature ranged from −1.75°C to 1.25°C during the fall experiments. During periods with low water temperatures, considerably higher rates in both bedload and suspended sediment transport were observed compared with the results obtained during summer with similar wave heights and current velocities. The increase in suspended sediment transport associated with low water temperatures is likely due to lower sediment fall velocities resulting from significant increases in fluid kinematic viscosity, which ranged from 1.1 to 1.37 × 10−6 m2 s−1 in summer and from 1.74 to 1.92 × 10−6 m2 s−1 in the fall. Less but still significant increase in bedload sediment transport in cold water is interpreted to be the result of temperature-induced variations in fluid viscosity that affect turbulence and boundary layer dynamics as well as bed configuration. Frazil ice occurred on some occasions during the fall experiments, but with the small number of samples collected when frazil ice crystals were present in the water column, it was not possible to establish a relationship between sediment transport rates and the presence or absence of ice. Our results nevertheless indicate that in cold climate regions extremely low water temperatures may cause a substantial increase of coastal sediment transport.

Numerical Model for Cross-shore Distribution of Longshore Sediment Transport Rate Including Suspended and Bed Loads in and outside the Surf Zone

A one-dimensional numerical model for predicting the cross-shore distribution of the longshore sediment transport rate was developed. The model predicts the suspended load due to wave breaking and bed load due to longshore current velocity and velocity skewness and atiltness. The model validity was examined for steady flows and for waves and wave-induced longshore currents. The sediment transport rates for steady flows estimated by the present model agreed with those by Ribberink's formula. The total longshore sediment transport rates estimated by the present model were smaller than those estimated by CERC's formula. However, because CERC's formula was found to overestimate the total longshore sediment transport rate, the present model likely predicts the longshore sediment transport in the field properly.

Numerical Simulation of Long-Shore Currents Induced by Regular Breaking Wave

The hydrodynamics of coastal zones are extremely complicated, being influenced greatly by shallow water waves and currents induced by wave breaking. This paper presents numerical simulations of long-shore currents induced by the breaking of oblique incident waves in shallow coastal zones. The wave numerical model is based on parabolic mild slope equation, and so the wave radiation stress required for the generation of wave-induced currents are calculated based on the variables in the parabolic mild slope equation, and the long-shore currents have been numerically simulated based on these. The numerical models are validated against experimental data, and the results suggest that the long-shore current velocity and wave set-up increase with the increasing incident wave amplitude and offshore slope steepness, as well as the wave set-up increase with the increasing incident wave period.

Cross-shore variation of long-term average longshore current velocity in the nearshore zone

The cross-shore variation of long-term average longshore current velocity was investigated on the basis of a 15-year data set of longshore current, wave and wind. The longshore current velocities were measured once a day along a 427-m-long pier. The results show that the direction of the long-term average longshore current velocity away (>200 m) from the shore was the opposite to that near the shore. The southward current was dominant offshore, whereas the northward current was dominant near the shore. The cross-shore variation of the long-term average longshore current velocity was formed owing to a difference between the wave and wind conditions when the northward and southward currents developed. When the northward current developed, the offshore wave height was relatively small and the frequency of the southward wind velocity was almost equal to that of the northward one. As a result, the northward current developed only near the shore and decayed outside the narrow surf zone. On the other hand, when the southward current developed, the offshore wave height was relatively large and the southward wind velocity was predominant, which resulted in the southward current developing not only in the wide surf zone but also outside the surf zone. The superposition of the two cross-shore variations produced a cross-shore variation of the long-term average longshore current velocity with a northward velocity near the shore and a southward velocity away from the shore.

A hybrid framework for predicting waves and longshore currents

An easily operable computational system has been designed to simulate waves and longshore currents. The system is composed of a MATLAB GUI in the foreground, which directs the integration of the SWAN shallow water wave model with a 1D surf model in the background. This newly developed tool first performs a rapid implementation of SWAN in a specific site followed by a comprehensive evaluation of the wave model output. Cross-shore profiles and other surf model input parameters are defined graphically directly from the SWAN output results. Longshore currents are then evaluated immediately in various locations. Besides testing the application of the described system in different environments, a comparison to measurements from a field experiment is also performed. Comparisons were made for longshore current velocities from both ‘in situ’ measurements as well as currents resulting from quasi-3D nearshore circulation model (SHORECIRC) simulations. The results indicate that the system developed provides reasonable predictions and that the hybrid modeling approach used can be also applied in some areas where the 1D beach paradigm is not valid.

Cross-shore Distribution of Long-Term Average Longshore Sediment Transport Rate

Cross-shore distributions of longshore sediment transport rate at the Hasaki Coast of Japan for 15 years from 1987 to 2001 were estimated with a one-dimensional numerical model and the cross-shore distribution of the long-term average longshore sediment transport rate was examined. The model was calibrated with longshore current velocities measured with a spherical float once a day at about 50-m intervals along a 427-meter-long pier at the study site. Numerical simulations showed that the average longshore sediment transport rate was northward near the shore, but southward away from the shore.

Influence of Coastal Processes on High Fecal Coliform Counts in the Mississippi Sound

Microbial source tracking efforts have historically focused on the input of fecal bacteria from sources such as storm drains, sewers, and runoff. Fecal coliform levels in the Mississippi Sound have been analyzed and compared with physical factors in an attempt to characterize possible nonpoint sources of pollution. Results from this study show that a primary factor in elevated levels of fecal coliform is a change in wind direction. The passage of warm and cold fronts through the northern Gulf of Mexico causes numerous 90°–180° shifts in wind directions over a period of 6–8 days. Commonly, a rise in fecal coliform counts is observed at coastal monitoring stations after an abrupt shift in wind direction and wind speed. When these trends of increased fecal coliform levels occur before rainfall, it is inferred that the sediment could be a source of the fecal coliform observed in the water column. The changes in wind direction and velocity might induce more energetic conditions at the shoreline (e.g., increased wave heights and increased longshore current velocities). Fecal coliform counts collected from five monitoring stations along the Harrison County, Mississippi, coast during 2002–2003 have been compared with wind, wave, and current records from within the Mississippi Sound. The occurrence of high fecal coliform counts at multiple stations can be correlated with higher energy events in the Sound. Statistically, wind direction and high bacterial counts are correlated, and higher counts are most likely to occur when the winds are out of the west or southwest at most stations.

Morphodynamic evolution of double‐barred beaches

The linear and nonlinear morphological behavior of double‐barred coastal systems under the forcing of obliquely incident waves is studied using a nonlinear numerical model. The linearly most unstable bed forms consist of crescentic patterns (rip channels), whose spacing depends on the magnitude of the longshore current velocity. Using the nonlinear model, six morphodynamic experiments have been performed with various initial bed perturbations in order to assess, among others, the influence of the initial bed perturbation on the morphodynamic evolution. The nonlinear experiments have been pursued well into the nonlinear regime, showing that after a phase of initial exponential growth, a highly dynamic behavior is observed and no equilibrium state is reached. The spacings predicted with the linear stability analysis are observed during the exponential phase of the nonlinear experiments. In the dynamic phase, however, four to seven modes significantly contribute to the resulting bed features. In this final stage, the apparent wavelength of 1000 m of the resulting bed forms on the inner bar is quite insensitive to the initial bed perturbation. On the outer bar it seems that the longer the wavelength of the initial bed perturbation, the longer the wavelength of the final bed forms in the dynamic phase and the larger the migration celerity. In general, the bed forms can be characterized as crescentic or undulating bed patterns. Good correspondence between simulated and observed spacings, shapes and migration celerities are found.

Hydrodynamics and Sediment Fluxes across an Onshore Migrating Intertidal Bar

Abstract Detailed hydrodynamic and morphological data are presented from a field deployment spanning 2 days (four tide cycles). The data include bed-elevation changes measured at each low tide and continuous records of water-surface elevation, cross-shore and long-shore current velocities, and suspended sediment concentrations all measured within 20 cm of the bed. During the deployment, an intertidal bar migrated onshore and infilled a runnel on its landward side. The depth of this runnel was initially 0.6 m. During the migration of the bar, the significant wave height in deep water was ca. 2 m and wave period was 7 seconds. The significant wave height over the intertidal bar crest was about 0.25 m. Suspended sediment fluxes were estimated (product of current velocity and suspended sediment concentration profile) and partitioned between mean and oscillatory components with the latter further partitioned between short and long wave contributions. When the bar was migrating shoreward and infilling the runne...

Examination of the influence of several factors on longshore current computation with random waves

Influence of various factors affecting the longshore currents induced by obliquely incident random waves is examined through numerical calculation. Seven numerical models for random wave breaking process are found to yield large differences in the wave heights in the surf zone and longshore current velocities. The turbulent eddy viscosity formulation by Larson and Kraus [Larson, M. and Kraus, N.C. (1991): Numerical model of longshore current for bar and trough beaches, J. Waterway, Port, Coastal, and Ocean Eng., ASCE, 117 (4), pp. 326-347.] functions almost equal to that by Battjes [Battjes, J.A. (1975): Modeling of turbulence in the surf zone, Proc. Symp. Modeling Techniques, pp. 1050–1061.], but the formulation by Longuet-Higgins [Longuet-Higgins, M.S. (1970): Longshore current generated by obliquely incident sea waves, 1 and 2, J. Geophys. Res., 75 (33), pp. 6779–6801.] produces excessive diffusion of longshore currents into the offshore zone. The generation and decay process of the surface roller is indispensable in the longshore current analysis. The random wave transformation model called PEGBIS (Parabolic Equation with Gradational Breaker Index for Spectral waves) by Goda [Goda, Y. (2004): A 2-D random wave transformation model with gradational breaker index, Coastal Engineering Journal, JSCE and World Scientific, 46 (1), pp. 1–38.] produced good agreement with several laboratory and field data of longshore currents.

Longshore Sediment Transport by Nonlinear Waves and Currents

A time-dependent model for obliquely incident nonlinear waves is developed and applied to predict longshore current and sediment transport. The wave model is based on the Boussinesq equations for breaking and nonbreaking waves. Wave breaking is introduced by adopting the surface roller concept. Longshore current velocity is calculated using the time-averaged alongshore momentum equation, including the effects of the cross-shore circulation on the dispersion of momentum. The wave module provides the longshore current and the sediment transport modules with all required hydrodynamic information such as radiation stress, bottom velocity, undertow velocity, and eddy viscosity coefficient. The Dibajnia and Watanabe formula is adopted to predict sheet-flow transport; whereas, for the suspended load, an energetics approach is used. Model results are compared with experimental data as well as with the Kamphuis and Costal Engineering Research Center formulas for the total alongshore sediment transport rate.

Vertical flow structure during Sandy Duck: observations and modeling

Observations of the vertical distribution of time-averaged cross-shore and alongshore flows during the Sandy Duck field experiment are compared with model predictions to assess the parameters governing the flow behaviour. The measurements were obtained with a vertical stack of eight two-component current meters, with the lowest and highest sensor at, respectively O(0.1) and O(2.7) m, above the bed. Observations under breaking wave conditions within the surfzone show that the maximum return flow velocities occur in the lower part of the water column, consistent with laboratory observations. Under non-breaking conditions outside the surfzone the maximum return flow velocities are observed closer to the water surface, again in line with laboratory results. Analogous to previous observations the measured longshore current velocity profiles are logarithmic under non-breaking conditions and become more depth-uniform under breaking conditions. The model description of the vertical structure of the flow includes the presence of wind stresses, wave stresses, pressure gradients, turbulent eddy viscosity and a wave boundary layer. The model utilizes parabolic shape functions to describe the vertical distribution of the turbulent eddy viscosity in the middle layer and within the bottom boundary layer. Eddy viscosity is enhanced in regions where turbulence is produced, i.e. near the surface in the case of breaking waves and within the bottom boundary layer. Estimates of the wave-breaking-induced turbulent eddy viscosity and bottom friction are obtained by minimizing the model–measurement discrepancies. Predictions utilizing calibrated expressions for both the turbulent eddy viscosity and bottom friction are in general agreement with the observations, provided the wave transformation and associated mass flux are modeled correctly and a parabolic eddy viscosity distribution is used. Using a piecewise constant eddy viscosity distribution generally results in a degrading of the agreement between measurements and model results.

Cross-shore distribution of longshore sediment transport rates on a barred non-tidal beach

This study was conducted to relate the cross-shore distribution of longshore sediment transport and grain size characteristics to cross-shore and longshore current velocities on a sandy low-energy beach in a non-tidal embayment of the Baltic Sea. Simultaneous measurements of current velocities and amount of sand caught in streamer traps were made on 31 sampling runs on 6 d in April 1999 at three fixed sites including the swash zone on the upper foreshore, the lower foreshore, and the crest of the most landward of four bars. Spilling waves broke frequently on the bar but rarely on the lower foreshore, even during onshore wind speeds up to 11.0 m s−1. Waves always broke as plunging waves at the step at the base of the upper foreshore and were converted directly into swash. The greatest longshore current velocities in the swash occurred when wind speeds and water levels were greatest, but wind direction was nearly directly onshore. Longshore velocities were greater in the swash zone than at other sites except when relatively strong winds blew nearly parallel to the shoreline, causing a pronounced wind-induced current at the other two sites. Calculated longshore shear stress and rate of sediment trapped were highly correlated on the bar (r=0.90), less highly correlated in the swash zone, and least highly correlated (r=0.66) on the lower foreshore. Mean trapping rates in the swash were 14.6 times greater than on the lower foreshore and 7.2 times greater than on the bar. Greater trapping rates in the swash are attributed to the greater turbulence mobilizing sediments in the uprush and backwash. Little of the finer-grained sediment on the offshore sites was reworked under low energy conditions. The study reveals the dominance of swash transport on steep, reflective, low-energy beaches where wave energy dissipation takes place over small distances on the upper foreshore.

The predictability of cross-shore bed evolution of sandy beaches at the time scale of storms and seasons using process-based Profile models

Deterministic and probabilistic Profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons. The large-scale laboratory experiment is a pure 2D case and offers and ideal test case for cross-shore Profile models, as disturbing alongshore non-uniformities are absent. The field experiments are performed at the Egmond site (The Netherlands) during the EU-COAST3D project and represent storm time scale (Oct.–Nov. 1998) as well as seasonal time scale conditions (May 1998–Sep. 1999). The objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities Coastal Process-based Profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons. Profile models can quite accurately (errors smaller than 10%) represent the cross-shore significant wave height distribution in the surf zone, if the wave breaking model is properly calibrated. The wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results. Profile models can reasonably represent the cross-shore and longshore currents (undertow) in a pure 2D case and in 3D field conditions. Profile models including cross-shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross-shore current velocities. Profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale; the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale. Profile models can reasonably simulate the post-storm onshore bar migration, provided that the near-bed orbital velocities and wave asymmetry-related sand transport are represented in a sufficiently accurate way (using non-linear wave theories). Profile models cannot simulate the beach recovery processes on the post-storm time scale, because these essentially 3D processes are not sufficiently known to be included in the models. Profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale; the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles. The simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning.

STUDY ON ABILITY OF PERMEABLE BREAKWATER TO EXCHANGE WATER IN A SMALL HARBOR

The authors have already shown that breakwaters with slope channel and step channel effectively reduce transmitted wave height when compared with a straight channel, and the flow rate through the breakwater with channel can be estimated from the non-linear Darcy's law.In this study, the authors examined the ability of the breakwater with channel to exchange water in the model harbor in a three-dimensional wave basin. The authors also proposed numerical model for predicting the rate of water exchange in the harbor and the applicability of the proposed model was also examined using experimental results. The time required to change whole water in the harbor can be estimated from the water volume in the harbor, the representative longshore current velocity, and the opening ratio of the channel in the breakwater.

Experimental investigation of vanes as a means of beach protection

Vanes are essentially flat vertical flow guiding structures. In the beach protection application vanes are placed, inshore of the breaking zone, in the upper part of the vertical water column and with a predefined angle of attack to the design wave orthogonal. They are thus subjected to a generally inshore-directed flow, which induces a lift, and drag force on the vane. These forces, in turn, induce a flow that counteracts the wave-induced longshore current, thus decreasing the longshore current velocity, which facilitates sediment deposition. Two experiments were conducted in a small-scale wave basin with the purpose of investigating the vane function. Both experiments were conducted with a movable bed that formed a crenulate-shaped bay between two headland structures. The first experiment showed that the vanes created a salient upstream of the vanes. This result is also supported by initial prototype results. The second experiment showed that the longshore velocity was reduced upstream of the vane and that the bulk of the velocity reduction took place in the nearshore part of the breaking zone. The main flow feature is identified as a large horizontal vortex inshore of the vane.

A one-dimensional model for undertow and longshore current on a barred beach

A one-dimensional model was developed for undertow and longshore current. The model predicts time- and depth-averaged undertow and longshore current velocities with longshore uniformity in depth, waves and current. It was calibrated with field data obtained over longshore bars at Hazaki Oceanographical Research Station (HORS). Verification with the other field measurements and large-scale experiments showed that the present model predicted well the undertow velocities over the longshore bars, while a previous model, calibrated with small-scale experiment data, underestimated the velocities. The verification also showed that although the cross-shore distributions of the longshore current velocity over the longshore bars predicted with the previous model had peak velocities seaward of the bar crests, the distributions predicted with the present model had peaks shoreward of the crests and fitted those measured in the field.