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.     

Boussinesq modelling of transient rip currents

The flow on a plane beach with a random, directionally spread wave field was simulated with a Boussinesq model. The random wave spectra were directionally symmetric with their central direction perpendicular to the beach, so no constant longshore current was generated. Variable wave-averaged currents were generated because of the spatially variable wave field, and sometimes formed offshore directed rip currents that appear in variable longshore locations. The rip currents are associated with a vortex pair which is generated within the surfzone and subsequently propagates offshore. Analysis of the vorticity balance show that the main vorticity input occurs within the inner surfzone. Three different beach slopes and four different wave spectra are simulated. The frequency, duration, and intensity of the transient rips depend on both the beach slope and the incident wave spectra. The results have important engineering implications for the transport of material in the nearshore zone, in particular on longshore uniform beaches.     

A wave-resolving model for nearshore suspended sediment transport

This paper presents a wave-resolving sediment transport model, which is capable of simulating sediment suspension in the field-scale surf zone. The surf zone hydrodynamics is modeled by the non-hydrostatic model NHWAVE (Ma et al., 2012). The turbulent flow and suspended sediment are simulated in a coupled manner. Three effects of suspended sediment on turbulent flow field are considered: (1) baroclinic forcing effect; (2) turbulence damping effect and (3) bottom boundary layer effect. Through the validation with the laboratory measurements of suspended sediment under nonbreaking skewed waves and surfzone breaking waves, we demonstrate that the model can reasonably predict wave-averaged sediment profiles. The model is then utilized to simulate a rip current field experiment (RCEX) and nearshore suspended sediment transport. The offshore sediment transport by rip currents is captured by the model. The effects of suspended sediment on self-suspension are also investigated. The turbulence damping and bottom boundary layer effects are significant on sediment suspension. The suspended sediment creates a stably stratified water column, damping fluid turbulence and reducing turbulent diffusivity. The suspension of sediment also produces a stably stratified bottom boundary layer. Thus, the drag coefficient and bottom shear stress are reduced, causing less sediment pickup from the bottom. The cross-shore suspended sediment flux is analyzed as well. The mean Eulerian suspended sediment flux is shoreward outside the surf zone, while it is seaward in the surf zone.     

Spatial and temporal variability of titanomagnetite placer deposits on a predominantly black sand beach

Titanomagnetite forms rich placer deposits along the northwest coast of New Zealand. These deposits were sampled along 3 shore-normal transects spaced over the southern 2 km of a dissipative high-energy beach on the west coast in 5 field campaigns covering one year. The percentage of opaque minerals (mainly titanomagnetite) was extremely high in the upper 30 m of the beach face, extending seaward where these opaque minerals were gradually replaced with variable amounts of lighter augite, hornblende and plagioclase. The pattern appeared to be divided into two regions, a lower seaward and an upper landward region, separated by a point where either marine dominated over aeolian processes or where swash dominated over breaking processes. In the seaward region, the percentage of opaques increased and particle size fined landward as undertow removed the lighter larger particles seaward. In the landward region, the percentage of opaques and particle size were more constant, or even showed the reverse pattern as wind transported the lighter material shoreward, or swash asymmetry transported the heavier material seaward. The similarity of settling velocities over the whole beach face suggests that sorting by size rather than weight plays a dominant role in separating the mineral assemblages. Considerable variations existed between transects. This could be explained by the spatial changes in surfzone waves and currents that were associated with proximity to the southern headland and various rip current channels that characterised this dissipative site. Surprisingly, the percentage of opaques decreased when the wave conditions of the day of sampling were more energetic. In contrast to many other placer deposits, these deposits are abundant on the beach face, forming an armouring layer during lower wave energy conditions. During higher wave conditions, the surface layer erodes allowing lighter augite, plagioclase and hornblende to be released from the sediments below.     

基于XBeach模型的深圳金沙湾裂流的数值模拟

金沙湾是粤港澳大湾区重要的滨海旅游景点之一,深受世界各地游客的青睐.利用XBeach模型模拟金沙湾海滩的近岸环流,研究了不同模拟波况下裂流的发生情况.结果表明,金沙湾产生裂流很大程度上受到波高和地形的影响,在年平均有效波高波况下,金沙湾沿岸无明显裂流,当入射波高超过某个阈值后,沿岸裂流风险提高.裂流的强度和离岸距离与入...     

Establishment,validation and discussions of a three dimensional wave-induced current model

A three-dimensional numerical model was established to simulate the wave-induced currents. The depth-varying residual momentum, surface roller, wave horizontal and vertical turbulent mixing effects were incorporated as major driving forces. A surface roller evolution model considering the energy transfer, roller density and bottom slope dissipation was developed. The expression of the wave-induced horizontal turbulent mixing coefficient proposed by Larson and Kraus (1991) was extended to three-dimensional form. Plenty of experimental cases were used to validate the established model covering the wave setup, undertow, longshore currents and rip currents. Validation results showed the model could reasonably describe the main characteristics of different wave-induced current phenomena. The incorporation of surface roller for breaking waves should not be neglected in the modeling of surfzone hydrodynamics. The wave-induced turbulent mixing affects the structures of wave-induced current either in horizontal or in vertical directions. Sensitivity analysis of the major calibration parameters in the established model was made and their ranges were evaluated.     

Rip currents under obliquely incident wind waves and tidal longshore currents

A field experiment on the nature of rip currents was conducted on the Dutch coast, which differs from previous rip current study sites because it is a wind-sea dominated environment with mostly obliquely incident waves and tidally-driven longshore currents. During the experiment three distinct flow patterns, obtained with GPS tracked drifter instruments, were observed: (1) a locally governed circulation cell, (2) an offshore current that is deflected shore parallel outside the surf zone and (3) a meandering longshore current. The transition from rip currents (flow patterns 1 and 2) to meandering longshore currents (flow pattern 3) occurred gradually within the tidal cycle with longshore currents prevalent at mid to high tide. Rip currents at this site appeared at depressions in the surf zone bar and typically occurred when the water level fell below NAP (equivalent to MSL), even in the presence of obliquely incident waves and tidally driven longshore currents. Hindcast simulations of the drifter experiments were performed with the numerical model XBeach and showed good agreement with field observations. The model was subsequently used to investigate the influence of tidal water level fluctuations, longshore currents and obliquely incident waves on rip currents.Rip currents were initiated when the water level dropped below a specific threshold with the magnitude of the rip current associated with the water level. The strength of the tidal current and its orientation with respect to the incident waves governed the offshore extent and orientation of the rip current. In contrast to other studies that suggest that rip currents solely occur under shore normal (or slightly oblique waves), in this study both observations and numerical model simulations indicate that rip currents can exist under large angles of wave incidence, when the rip channel is sufficiently wide and the wave height is small.     

Surfzone currents

Investigations of surfzone currents including longshore currents, rip currents and nearshore circulations have been conducted worldwide for over sixty years. The motivation is understanding and prediction of sediment transport and coastal pollution processes. This article summarizes a recent state-of-the-art review of theory and experiments conducted since 1967 (Basco, 1982, with Volume II including over 350 annotated bibliographic entries). Emphasis is on the now generally accepted, time-averaged theory based upon radiation stress principles. The newly emerging Boussinesq-type theory, which follows the instantaneous water surface and current variations within each wave period, is also briefly considered. Both approaches assume uniform currents with depth, rely upon empirical wave breaking and surfzone energy empiricism, and suffer from a limited data base for verification. Emphasis throughout is on needed areas for further research.     

A morphological stability analysis for a long straight barred coast

A morphological stability analysis is carried out for a long straight coast with a longshore bar. The situation with oblique wave incidence and a wave-driven longshore current is considered. The flow and sediment transport are described by a numerical modelling system. The models comprise: (i) a wave model with depth refraction, shoaling and wave breaking, (ii) a depth integrated model for wave driven currents and (iii) a sediment transport model for the bed load transport and the suspended load transport in combined waves and current. The direction of the sediment transport is taken to be parallel to the depth integrated mean current velocity, neglecting the effects of a bed slope and secondary currents. An instability is found to develop around the bar crest. The instability is periodic in the alongshore direction, and tends to form rip channels and to steepen the offshore face of the bar between the rip channels. The alongshore wave length of the most unstable perturbation is determined for different combinations of the wave conditions and the geometry of the profile.     

Tidal-cycle changes in oscillation ripples on the inner part of an estuarine sand flat

Oscillation ripples form on subaqueous sand beds when wave-generated, near-bottom water motions are strong enough to move sand grains. The threshold of grain motion is the lower bound of the regime of oscillation ripples and the onset of sheet flow is the upper bound. Based on the relation between ripple spacing and orbital diameter, three types of symmetrical ripples occur within the ripple regime. In the lower part of the ripple regime (orbital ripples), spacing is proportional to orbital diameter; in the upper part (anorbital ripples) spacing is independent of orbital diameter. Between these regions occurs a transitional region (suborbital ripples).

Oscillation ripples develop on a sandy tidal flat in Willapa Bay, Washington, as a result of waves traversing the area when it is submerged. Because wave energy is usually low within the bay, the ripples are primarily orbital in type. This means that their spacing should respond in a systematic way to changes in wave conditions. During the high-water parts of some tidal cycles, ripples near the beach decrease in spacing during the latter stage of the ebb tide while ripples farther offshore do not change. Observations made over several tidal cycles show that the zone of active ripples shifts on- or offshore in response to different wave conditions.

Detailed bed profiles and current measurements taken during the high-water part of spring tides show the manner in which the oscillation ripples change with changes in orbital diameter. Changes in ripple spacing at the study site could be correlated with changes in orbital diameter in the manner suggested by the criterion for orbital ripples. However, there appeared to be a lag time between a decrease in orbital diameter and the corresponding decrease in ripple spacing. Absence of change during a tidal cycle could be attributed to orbital velocities below the threshold for grain motion that negated the effects of changes in orbital diameter.

Because changes in sand-flat ripples depend both upon changes in orbital diameter and upon the magnitude of the orbital velocity, exposed ripples were not necessarily produced during the preceding high tide. In fact, some ripples may have been just produced, while others, farther offshore, may have been produced an unknown number of tides earlier. Therefore, when interpreting past wave conditions over tidal flats from low-tide ripples, one must remember that wave periods have to be short enough to produce velocities greater than the threshold velocity for the orbital diameters calculated from the observed ripple spacings.     

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.     

威海国际海水浴场裂流调查与区划研究

裂流发生迅速,发生位置隐秘,发生范围相对较小,发生时海浪大、流速快,这些特点给裂流观测带来困难。本文以威海国际海水浴场为研究对象,通过水深地形测量、海浪海流观测及高空摄影等多种观测方式相结合,尝试采用可行且易于实现的方法对裂流易发区域进行定位,在此基础上对该海域裂流类型和产生原因进行分析,并进一步进行威海国际海水浴场裂流区划。研究表明:威海国际海水浴场水下地形较为复杂,离岸200 m范围内存在5个沙丘-沟槽结构,沟槽和东侧栈桥前沿是裂流危险区,分属海滩型裂流和边界型裂流,裂流流速在0.5~1 m/s之间,观测期间裂流平均风险等级为中等。本研究可为裂流观测和海水浴场管理提供一定的参考。     

Numerical Modeling of the Undertow Structure and Sandbar Migration in the Surfzone

A process-based 3D numerical model for surfzone hydrodynamics and beach evolution was established. Comparisons between the experimental data and model results proved that the model could effectively describe the hydrodynamics, sediment transport feature and sandbar migration process in the surfzone with satisfactory precision. A series of numerical simulations on the wave breaking and shoaling up to a barred beach were carried out based on the model system. Analyzed from the model results, the wave-induced current system in the surfzone consists of two major processes, which are the phase-averaged undertow caused by wave breaking and the net drift caused by both of the nonlinear wave motion and surface roller effect. When storm waves come to the barred beach, the strong offshore undertow along the beach suppresses the onshore net drift, making the initial sandbar migrate to the seaside. Under the condition of calm wave environment, both the undertow and net drift flow to the shoreline at the offshore side of the sandbar, and then push the initial sandbar to the shoreline. The consideration of surface roller has significant impact on the modeling results of the sandbar migration. As the roller transfer rate increases, the sandbar moves onshore especially under the storm wave condition.     

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.     

广东省14个海滩裂流类型及统计特征分析

裂流是沿海地区频发的一种高速离岸流。根据Castelle的裂流分类模型对广东省14个海滩的Google Earth影像进行了解译分类,对裂流分类模型在遥感影像中的应用进行了探讨。结果表明:在高风险月份,海滩裂流的组成较为单一,混合型裂流少,水深控制的裂流占据主导地位,裂流数量总体维持在较高水平;在中风险月份,水动力控制的裂流和混合型裂流的比例增加,水深控制的裂流仍然占有一定比例;在低风险月份,裂流很少出现,甚至无裂流发生。裂流的类型与海滩的状态密切相关。对于海湾中的海滩,海湾长度影响裂流的数量,海湾凹入度影响不同类型裂流的组成比例。考虑到实验条件的限制,这一规律需要后续结合实际地形地貌进行更细致和深入的研究和验证。按照本文解译标准得出的分类结果,与前人提出的裂流风险性评价模型吻合度较高,进一步说明了该解译标准的有效性,为今后对海滩裂流的研究和分类工作提供了一定的参考。     

离岸流灾害水动力学过程的数值模拟研究

离岸流是近岸流的重要组成部分,当波浪受到特殊海滩地形的影响,会形成一股沿着离岸方向运动的高速水流,能够迅速将人带离海岸,对海滨安全造成威胁。为了深入探究离岸流的形成机理及水动力学特性,本文基于二阶Stokes波浪理论,采用了更为光滑的变截面沙坝模型,通过流体体积法捕捉自由液面,对离岸流进行三维数值模拟探究。本文重点分析了离岸流产生时流场的瞬时速度、时均速度、压强等不同参量的分布规律,结果显示在沙坝和海岸线之间,有一对方向相反的水循环体系;对比不同流层离岸流的速度,了解到波浪与离岸流的耦合作用;并探究了入射波波高对离岸流强度及分布区域的影响,深化了对离岸流水动力学过程的认识。     

Western boundary current crossing a ridge

Investigated is a possibility of two-dimensional model in the study of the dynamics of the western boundary current by a numerical experiment. Emphasis is laid on the effect of bottom barrier corresponding to the Izu Ridge.The western boundary current in the model is formed by source and sink of the water prescribed at an artificial eastern wall (600 km offshore). The bottom topographyconsists of a continental slope parallel to the straight western coast, and a ridge protruding from the western coast to 500 km offshore (1,500 m deep and 400 km wide). The grid size of 12 km× 25 km (offshore and longshore directions, respectively) resolves both the western boundary current and the bottom topography.The assumption of homogeneity of the water density makes the western boundary current detour along the isobath of the ridge.A steady state solution is obtained under the assumptions that the horizontal velocity does not change direction vertically (equivalent barotropic), and that the geostrophic relationship holds at the bottom. Homogeneity of the water density is not assumed. The solution shows that most of the volume transport of the western boundary current cross the ridge and the current has cyclonic vorticity near the summit of the ridge. It seems to suggest that the investigation by three-dimensional models is neccesary in order to study the complete dynamics of the western boundary current crossing the ridge.     

Continental shelf waves off the Fukushima coast

Numerical experiments are performed on shelf waves forced by wind stress with a spectral peak around a period of 100 hr. Water depth in the numerical model is a function of offshore distance only and resembles a bathymetric profile off the Fukushima coast. A pair of vortices alined in the offshore direction and a large vortex are reproduced and they propagate southward outside the forced region. Judging from the propagation speed, the former corresponds to the second-mode and the latter to the first-mode shelf waves. In the forced region, the propagation speed of a trough and a ridge is slow, 3–5km hr^–1. These propagation characteristics reproduce those observed along the Fukushima coast and this propagation speed corresponds to that of second-and third-mode shelf waves. Thus, it is concluded that the periodical current fluctuations observed in the inshore region along the Fukushima coast are due to motions associated with the second-and third-mode shelf waves.     

Numerical experiment on <Emphasis Type="Italic">Kyucho</Emphasis> around the Tango Peninsula induced by Typhoon 0406

A numerical experiment using a three dimensional level model was performed to clarify the mechanism generating a strong coastal current, Kyucho, induced by the passage of Typhoon 0406 around the tip of the Tango Peninsula, Japan in June 2004. Wind stress accompanied by Typhoon 0406 was applied to the model ocean with realistic bottom topography and stratification condition. The model well reproduced the characteristics of Kyucho observed by Kumaki et al. (2005), i.e., the strong alongshore current with maximum velocity of 53 cm s⁻¹ and its propagation along the peninsula with propagation speed of about 0.6 m s⁻¹ one half-day after the typhoon’s passage. Coastal-trapped waves (CTW) accompanied by downwelling were induced along the northwest coast of the peninsula by the alongshore wind stress. The energy density flux due to the CTW flowed eastward along the coast, and indicated scattering of the CTW around the eastern coast of the peninsula. In addition, significant near-inertial internal gravity waves were also caused in the offshore region from the west of the Noto Peninsula to the north of the Tango Peninsula by the typhoon’s passage. The energy flux density of the near-inertial fluctuations flowed southward off the Fukui coast, and part of the energy flux was trapped on the tip of the Tango Peninsula, flowing with the coast on its right. It was found that the strong current, Kyucho, at the northeastern tip of the Tango Peninsula was generated by superposition of the near-inertial internal gravity waves and subinertial CTW.     

台湾岛东西两岸的海流

本文收集、整理和分析台湾岛东、西两岸的海流资料，获得以下主要结果；（１）台湾东岸的黑潮路径，无论是表层或深层，都是冬季偏酉（距台湾东岸较近），夏季偏东，春、秋两季的介于冬、夏季的路径之间。（２）台湾东岸黑潮的流速，具有夏、春强而冬弱的特点。（３）台湾西岸近海的海流，除表层受风的影响较大外，１０ｍ层开始，尤其是近底层，冬、夏两季皆以北向或东北向流为主，呈现出一派北向流的路径。这与传统观念不同。