Quasi-3D modelling of nearshore currents

Existing concepts of wave-induced nearshore current models, in the cross-shore vertical plane (2DV) and depth-integrated (2DH), are combined to a quasi-3D mathematical model. This combination is tested for reproducing correct results in 2DV and 2DH situations. The importance of the various contributions to the wave-induced secondary circulation in the vertical plane is investigated for realistic parameter ranges, which leads to the conclusion that both the non-breaking and the breaking fraction of a random wave field in the surf zone generate important secondary currents.Additional computations show the relevance of a 3D-approach of nearshore currents, even in seemingly simple situations like a plane sloping beach with obliquely incident waves.     

Nonlinear and 3D effects in leaky infragravity waves

In this paper, infragravity (IG) waves, forced by normally and obliquely incident wave groups, are studied using the quasi-3D (Q3D) nearshore circulation model SHORECIRC [Van Dongeren, A.R., I.A. Svendsen, 1997b. Quasi 3-D modeling of nearshore hydrodynamics. Research report CACR-97-04. Center for Applied Coastal Research, University of Delaware, Newark, 243 pp.], which includes the Q3D effects. The governing equations that form the basis of the model, as well as the numerical model and the boundary conditions, are described. The model is applied to the case of leaky IG waves. It is shown that the Q3D terms have a significant effect on the cross-shore variation of the surface elevation envelope, especially around the breakpoint and in the inner surf zone. The effect of wave groupiness on the temporal and spatial variation of all Q3D terms is shown after which their contribution to the momentum equations is analyzed. This reveals that only those Q3D coefficients, which appear in combination with the largest horizontal velocity shears make a significant contribution to the momentum equations. As a result of the calculation of the Q3D coefficients, the IG wave velocity profiles can be determined. This shows that in the surf zone, the velocity profiles exhibit a large curvature and time variation in the cross-shore direction, and a small — but essential — depth variation in the longshore direction.     

Numerical simulation of nearshore circulation on field topography under random wave environment

In order to investigate surf zone hydrodynamics through two-dimensional numerical simulations of nearshore circulation under random wave environment, a nearshore circulation model, SHORECIRC, and a random wave model, SWAN, were combined and utilized. Using this combined model, a numerical simulation of the October 2, 1997 SandyDuck field experiment was performed. For this simulation, field topography and an input offshore spectrum were constructed using observed data sets synchronized with the experiment. The wave-breaking model in SWAN was modified by using breaker parameters varied according to bottom slope. The simulation results were compared with the experimental data, which revealed a well-developed longshore current, as well as with results using other combinations which were SHORECIRC and its original monochromatic wave-driver, and SHORECIRC and the default of SWAN. The results from the novel combined model agreed well with the experimental data. The results of the present simulation also indicate that alongshore field topography influences shear fluctuation of longshore currents.     

Sedimentology and morphodynamics of a macrotidal beach, Pendine Sands, SW Wales

The foreshore of Pendine Sands forms the seaward part of an extensive, sandy coastal barrier in a shallow Carmarthen Bay, SW Wales. The sedimentological features of the macrotidal foreshore reflect a tide-induced modification of nearshore wave characteristics. As the tide ebbs, the breaker height may decrease, the surf zone widens and becomes increasingly dissipative, and swash/backwash velocities diminish. A concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient.

A zero net transport model demonstrates that the beach profile is self-stabilising in the short-term, and periodic levelling has shown that the beach is in long-term equilibrium with prevailing conditions, though this does not preclude a significant dynamic response to changing tides and waves.

The flow regimes of wave-generated currents decline as the tide ebbs, and normal beach processes do not usually affect the lower foreshore. Accordingly, there is an overall seaward-fining of the primary framework component of the sands. In more detail, this framework component displays a slight seaward-coarsening across an upper foreshore dominated by high water swash and surf; a rapid seaward-fining across the mid-foreshore in response to the ebb-attenuating swash zone flow velocities; and a slight seaward-fining across the lower foreshore under the action of nearshore shoaling waves. Bedforms vary from a swash/backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore.

The surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions, partly to storms. The upper foreshore is characterised by swash/backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities. The mid-foreshore is composed of coarser-grained shelly traction clogs arranged as landward- and seaward-dipping large-scale cross bedding and/or plane bedding; these are probably storm breaker/surf deposits. The lower foreshore, though partially and sometimes totally bioturbated, shows landward-dipping small-scale cross bedding in very fine sands sorted by nearshore shoaling waves.

Tide- and storm-induced modification of the nearshore flow regimes therefore produces a distinctive shore-normal array of sedimentary facies. Each facies is characterised by diagnostic textural and structural signatures. A prograding sequence of such macrotidal deposits would be similar to, but more extensive than, a comparable microtidal sequence.     

Implementation and modification of a three-dimensional radiation stress formulation for surf zone and rip-current applications

Regional Ocean Modeling System (ROMS v 3.0), a three-dimensional numerical ocean model, was previously enhanced for shallow water applications by including wave-induced radiation stress forcing provided through coupling to wave propagation models (SWAN, REF/DIF). This enhancement made it suitable for surf zone applications as demonstrated using examples of obliquely incident waves on a planar beach and rip current formation in longshore bar trough morphology (Haas and Warner, 2009). In this contribution, we present an update to the coupled model which implements a wave roller model and also a modified method of the radiation stress term based on Mellor (2008, 2011a,b,in press) that includes a vertical distribution which better simulates non-conservative (i.e., wave breaking) processes and appears to be more appropriate for sigma coordinates in very shallow waters where wave breaking conditions dominate. The improvements of the modified model are shown through simulations of several cases that include: (a) obliquely incident spectral waves on a planar beach; (b) obliquely incident spectral waves on a natural barred beach (DUCK'94 experiment); (c) alongshore variable offshore wave forcing on a planar beach; (d) alongshore varying bathymetry with constant offshore wave forcing; and (e) nearshore barred morphology with rip-channels. Quantitative and qualitative comparisons to previous analytical, numerical, laboratory studies and field measurements show that the modified model replicates surf zone recirculation patterns (onshore drift at the surface and undertow at the bottom) more accurately than previous formulations based on radiation stress (Haas and Warner, 2009). The results of the model and test cases are further explored for identifying the forces operating in rip current development and the potential implication for sediment transport and rip channel development. Also, model analysis showed that rip current strength is higher when waves approach at angles of 5° to 10° in comparison to normally incident waves.     

An Analytical Solution for Nearshore Circulation Driven by Focused/Defocused Waves

An offshore shoal or bar refracts ocean surface waves and causes wave focusing/defocusing on the adjacent beach.Wave focal patterns characterized by alongshore variations in wave height, wave angle, and breaking location induce alongshore non-uniformities of wave setup and nearshore circulation, e.g., rip currents and alongshore currents, in the surfzone. A simplified analytic model for nearshore circulation generated by focused/defocused waves on a planar beach is developed and theoretical solutions are obtained using transport stream function and perturbations in alongshore distributions of wave height and wave angle at the breaker line. The analytic model suggests that alongshore currents are strongly affected by a pair of counter-rotating vortices generated shoreward of the wave focal zone. The vortices are persistent, and their strengths depend on the amplitudes of alongshore variations in wave height and wave angle. The alongshore gradient in wave height tends to intensify the vortices while the convergence of wave angle tends to weaken the vortices. Divergent flows associated with the vortices in the surfzone are intense,strengthening alongshore currents in the downstream of the wave focal zone and weakening alongshore currents or causing flows reversal in the upstream. Alongshore currents are modulated by rip currents associated with the wave focusing/defocusing patterns.     

Two-dimensional wave-current interaction with a locally enriched quadtree grid system

Wave-induced currents may drive nearshore mixing and transport processes, including coastal pollutant dispersion, littoral drift, and long-term morphological changes through beach erosion and accretion. In this study, a numerical model is newly developed to simulate wave climate and localized currents in complicated coastal environments. The model developed is based on a quadtree grid system. The two-dimensional hydrodynamic governing equations are solved by using an explicit Adams-Bashforth finite difference scheme. Effects of wave breaking, shoaling, refraction, diffraction, wave-current interaction, set-up and set-down, turbulent mixing, bed friction, and shoreline movement are incorporated in the model. Results are presented for set-up at a beach in a flume due to normally incident waves, and longshore currents generated by oblique waves on a plane beach.     

NearCoM-TVD — A quasi-3D nearshore circulation and sediment transport model

The newly developed nearshore circulation model, SHORECIRC, using a hybrid finite-difference finite-volume TVD-type scheme, is coupled with the wave model SWAN in the Nearshore Community Model (NearCoM) system. The new modeling system is named NearCoM-TVD and the purpose of this study is to report the capability and limitation of NearCoM-TVD for several coastal applications. For tidal inlet applications, the model is verified with the semi-analytical solution of Keulegan (1967) for an idealized inlet-bay system. To further evaluate the model performance in predicting nearshore circulation under intense wave–current interaction over complex bathymetry, modeled circulation patterns are validated with measured data during RCEX field experiment (MacMahan et al., 2010). For sediment transport applications, two sediment transport models are applied to predict three sandbar migration events at Duck, NC, during August to October 1994 (Gallagher et al., 1998). The model of Kobayashi et al. (2008) incorporates wave-induced onshore sediment transport rate as a function of the standard deviation of wave-induced horizontal velocities. The modeled beach profile evolution for two offshore events and one onshore event agrees well with the measured data. The second model investigated here combines two published sediment transport models, namely, the total load model driven by currents under the effect of wave stirring (Soulsby, 1997) and the wave-driven sediment transport model due to wave asymmetry/skewness (van Rijn et al., 2011). The model study with limited field data suggests that the parameterization of wave stirring is appropriate during energetic wave conditions. However, during low energy wave conditions, the effect of wave stirring needs to be re-calibrated.     

Investigation and Discussion on the Beach Morphodynamic Response Under Storm Events Based on A Three-Dimensional Numerical Model

A well-established 3D phase-averaged beach morphodynamic model was applied to investigate the morphodynamics of a typical artificial beach,and a series of discussions were made on the surfzone hydro-sedimentological processes under calm and storm events.Model results revealed that the nearshore wave-induced current presents a significant 3D structure under stormy waves,where the undertow and longshore currents exist simultaneously,forming a spirallike circulation system in the surfzone.Continuous longshore sediment transport would shorten the sediment supply in the cross-shore direction,subsequently suppress the formation of sandbars,showing that a typical recovery profile under calm waves does not necessarily develop,but with a competing process of onshore drift,undertow and longshore currents.Sediment transport rate during storms reaches several hundreds of times as those under calm waves,and two storm events contribute approximately 60%to the beach erosion.Sediment transport pattern under calm waves is mainly bed load,but as the fine sands underneath begin to expose,the contribution of suspended load becomes significant.     

Numerical modelling of sand beach evolution around coastal structures

I~crIOWIn the coastal area, especially at the sandy seashore, wave and nearshore current are the major factors which affect sediment transPOrt and the motyhChdynamics.The numerical models of predicting the beach evolution can be classified intO the medi~term and long-term models according to their space and time scales (De Briend et al., 1993;Watanabe, 1990; Watanabe et al., 1986; Tao, 1996). In the medium-term model the effects ofwave, nearshore current and sediment transport are conside…     

A numerical model of beach morphological evolution due to waves and currents in the vicinity of coastal structures

A numerical model was developed of beach morphological evolution in the vicinity of coastal structures. The model includes five sub-models for random wave transformation, surface roller development, nearshore wave-induced currents, sediment transport, and morphological evolution. The model was validated using high-quality data sets obtained during experiments with a T-head groin and a detached breakwater in the basin of the Large-scale Sediment Transport Facility at the Coastal and Hydraulics Laboratory in Vicksburg, Miss, USA. The simulations showed that the model reproduced well the wave conditions, wave-induced currents, and beach morphological evolution in the vicinity of coastal structures. Both salient and tombolo formation behind a T-head groin and a detached breakwater were simulated with good agreement compared to the measurements.     

Effects of cross-shore boundary condition errors in nearshore circulation modeling

The paper analyzes the effect which prescribed errors in the cross-shore boundary conditions for a computational domain along a beach have on the flow field predicted inside the domain. This problem is relevant because errors in boundary conditions are unavoidable when modeling limited domains of a nearshore region. For simplicity, we consider a longshore uniform plane beach with monochromatic, obliquely incident waves, and assume depth uniform currents. It is then studied analytically and numerically how small perturbations of the boundary conditions along both upstream and downstream cross-shore boundaries spread inside the computational domain. It is found that the errors at the upstream cross-shore boundary tend to spread over a long distance downstream of the boundary, while the influence of the errors in the downstream boundary condition is limited to the adjacent upstream area of the computational domain. Both the numerical and analytical solutions show that the errors introduced at the upstream boundary decay exponentially in the surf zone at a rate proportional to the bottom friction. A simple formula is developed to estimate the influence distance of the upstream errors. If we consider the mismatch in the volume flux at the upstream boundary, the error merely redistributes in the cross-shore direction to conserve volume. In the case of excessive flux or velocity specified at the cross-shore boundaries, a circulation cell tends to appear in the offshore region where the errors caused by the boundary mismatch increase with the cross-shore width of the model domain.     

Numerical Simulation of Spatial Lag Between Wave Breaking Point and Location of Maximum Wave-Induced Current

A quasi three-dimensional numerical model of wave-driven coastal currents with the effects of surface rollers is developed for the study of the spatial lag between the location of the maximum wave-induced current and the wave breaking point.The governing equations are derived from Navier-Stokes equations and solved by the hybrid method combining the fractional step finite different method in the horizontal plane with a Galerkin finite element method in the vertical direction.The surface rollers effects are considered through incorporating the creation and evolution of the roller area into the free surface shear stress.An energy equation facilitates the computation process which transfers the wave breaking energy dissipation to the surface roller energy.The wave driver model is a phase-averaged wave model based on the wave action balance equation.Two sets of laboratory experiments producing breaking waves that generated longshore currents on a planar beach are used to evaluate the model's performance.The present wave-driven coastal current model with the roller effect in the surface shear stress term can produce satisfactory results by increasing the wave-induced nearshore current velocity inside the surf zone and shifting the location of the maximum longshore current velocity landward.     

琼州海峡南岸海岸动力地貌研究

岬湾相间的琼州海峡南岸在海岸动力条件作用下,岸滩发生侵蚀或堆积,特别是南岸中部的南渡江三角洲沿岸岸滩演变剧烈。该文从海岸动力地貌的角度,对琼州海峡南岸的海岸动力特征、泥沙运动以及岸滩演变进行分析。根据海峡南部三维潮流场数值模拟结果,结合经验公式初步分析潮流引起的泥沙运移速率和方向,得到岸外水域总的泥沙运移趋势为从西向东。根据波浪动力计算分析沿岸泥沙运移,探讨沙质岸滩的动态与地貌演变之间的关系,得出海峡南岸海岸地貌演变与盛行的NE和NNE向风浪有密切关系,岸滩的演变过程主要受制于这两个方向的风浪及其引起的泥沙沿岸运移。     

Large-scale experiments on beach profile evolution and surf and swash zone sediment transport induced by long waves,wave groups and random waves

New large-scale laboratory data are presented on the influence of long waves, bichromatic wave groups and random waves on sediment transport in the surf and swash zones. Physical model testing was performed in the large-scale CIEM wave flume at UPC, Barcelona, as part of the SUSCO (swash zone response under grouping storm conditions) experiment in the Hydralab III program (Vicinanza et al., 2010). Fourteen different wave conditions were used, encompassing monochromatic waves, bichromatic wave groups and random waves. The experiments were designed specifically to compare variations in beach profile evolution between monochromatic waves and unsteady waves with the same mean energy flux. Each test commenced with approximately the same initial profile. The monochromatic conditions were perturbed with free long waves, and then subsequently substituted with bichromatic wave groups with different bandwidth and with random waves with varying groupiness. Beach profile measurements were made at half-hourly and hourly intervals, from which net cross-shore transport rates were calculated for the different wave conditions. Pairs of experiments with slightly different bandwidth or wave grouping show very similar net cross-shore sediment transport patterns, giving high confidence to the data set. Consistent with recent small-scale experiments, the data clearly show that in comparison to monochromatic conditions the bichromatic wave groups reduce onshore transport during accretive conditions and increase offshore transport during erosive conditions. The random waves have a similar influence to the bichromatic wave groups, promoting offshore transport, in comparison to the monochromatic conditions. The data also indicate that the free long waves promote onshore transport, but the conclusions are more tentative as a result of a few errors in the test schedule and modifications to the setup which reduced testing time. The experiments suggest that the inclusion of long wave and wave group sediment transport is important for improved near-shore morphological modeling of cross-shore beach profile evolution, and they provide a very comprehensive and controlled series of tests for evaluating numerical models. It is suggested that the large change in the beach response between monochromatic conditions and wave group conditions is a result of the increased significant and maximum wave heights in the wave groups, as much as the presence of the forced and free long waves induced by the groupiness. The equilibrium state model concept can provide a heuristic explanation of the influence of the wave groups on the bulk beach profile response if their effective relative fall velocity is larger than that of monochromatic waves with the same incident energy flux.     

Down-scaled regional ocean modeling system (ROMS) for high-resolution coastal hydrodynamics in Korea

A down-scaled operational oceanographic system is developed for the coastal waters of Korea using a regional ocean modeling system(ROMS).The operational oceanographic modeling system consists of atmospheric and hydrodynamic models.The hydrodynamic model,ROMS,is coupled with wave,sediment transport,and water quality modules.The system forecasts the predicted results twice a day on a 72 h basis,including sea surface elevation,currents,temperature,salinity,storm surge height,and wave information for the coastal waters of Korea.The predicted results are exported to the web-GIS-based coastal information system for real-time dissemination to the public and validation with real-time monitoring data using visualization technologies.The ROMS is two-way coupled with a simulating waves nearshore model,SWAN,for the hydrodynamics and waves,nested with the meteorological model,WRF,for the atmospheric surface forcing,and externally nested with the eutrophication model,CE-QUAL-ICM,for the water quality.The operational model,ROMS,was calibrated with the tidal surface observed with a tide-gage and verified with current data observed by bottom-mounted ADCP or AWAC near the coastal waters of Korea.To validate the predicted results,we used real-time monitoring data derived from remote buoy system,HF-radar,and geostationary ocean color imager(GOCI).This down-scaled operational coastal forecasting system will be used as a part of the Korea operational oceanographic system(KOOS) with other operational oceanographic systems.     

Quasi-three-dimensional modelling of the morphology of longshore bars

A morphological quasi-three-dimensional (Q3D) area model for barred coasts has been developed. The model combines a two-dimensional depth integrated model for wave-driven currents with a model for undertow circulation currents. The combined model makes a simultaneous simulation of the bar-forming processes associated with the undertow and the horizontal wave-driven circulation currents, which may cause instabilities of the bar and the formation of rip channels. Situations with normal and oblique wave incidence are considered. Compared to the depth integrated approach the Q3D model produces less pronounced alongshore irregularities for obliquely incident waves. For normal incident waves the Q3D model produces a crescentic bar while the depth integrated model predicts almost straight sections of the bar interrupted by rip channels. The sensitivity to variation of wave angle and beach slope is further investigated.     

Nearshore tidal currents and sand transport in a macrotidal environment

Benthic nearshore currents were measured continuously for a week over the subtidal zone fronting an open coast, macrotidal beach (spring tide range 9.5 meters) in Northwestern Australia. The shore-parallel currents were dominated by the semidiurnal tide; however pronounced asymmetries expressed the contributions of higher harmonics. Northerly flows at high tide were considerably stronger and of longer duration than southerly flows at low tide. Considering the combined effects of sediment agitation by waves and net transport by the tidal currents, it is shown that a mechanism may exist which could produce net northerly transport of “bed load” and southerly transport of suspended load.     

Quadtree grid numerical model of nearshore wave–current interaction

This paper describes an adaptive quadtree-based 2DH wave–current interaction model for evaluating nearly horizontal wave-induced currents in the surf-zone. The model accounts for wave breaking, shoaling, refraction, diffraction, wave–current interaction, set-up and set-down, mixing processes (turbulent diffusion), bottom frictional effects, and movement of the land–water interface at the shoreline. The wave period- and depth-averaged governing equations, which conserve mass, momentum, energy and wave action, are discretised explicitly by means of an Adams–Bashforth second-order finite difference technique on adaptive hierarchical staggered quadtree grids. Grid adaptation is achieved through seeding points distributed according to flow criteria (e.g. local current gradients). The model is verified for nearshore circulation at a sinusoidal beach and nearshore currents at a multi-cusped beach. Reasonable agreement is obtained with experimental data from da Silva Lima [da Silva Lima, S.S.L., 1981. Wave-induced Nearshore Currents. PhD Thesis, Department of Civil Engineering, University of Liverpool] and Borthwick et al. [Borthwick, A.G.L., Foote, Y.L.M., Ridehalgh, A., 1997. Nearshore measurements at a cusped beach in the UK Coastal Research Facility, Coastal Dynamics '97, Plymouth, 953–962]. The modelling approach presented herein should be useful in simulating nearshore processes in complicated natural coastal domains. Of particular value is the local grid enrichment capability, which permits refined modelling of important localised flow behaviour such as rip currents and surf-zone circulation systems.     

Rip current charactersitics and their role in the exchange of water and surf diatoms between the surf zone and nearshore

Photogrammetric measurements of dye dispersion in a high-energy surf zone provided semi-quantitative measurements of the pattern and flux of water exchange between the surf zone and nearshore. The intensity of current flow, the rip morphology and its position within the surf zone have important ecological implications for the surf zone and nearshore biota. A variety of water-movement patterns was found, ranging from currents which remained confined to the beach terrace to those which involved substantial exchange across the breaker line. In considering rips as exchange mechanisms, two rip types are recognized. Depending on the exchange of water across the breaker line, a rip may be classified as ‘exchange’ or ‘non-exchange’. The ecological significance of these current types is discussed and a classification scheme for rip currents is proposed. Offshore fluxes across the breaker line by rip currents ranged from negligible to 80 m³s⁻¹ rip⁻¹. The estimated maximum flux per running meter of the Sundays River Beach was calculated as 0·0.32m ³ s⁻¹. The half-residence period of surf-zone water ranged from as little as 22 min t0 5 days, averaging 3·6 h.A dichotomy of current patterns found between the water column and surface layers is used to explain the build up of a concentration gradient in cell numbers of the surf diatom Anaulus birostratus within the surf zone despite extensive flushing by surf circulation.