A parametric model for barred equilibrium beach profiles

Environment predictions for locations for which bathymetric data is missing, poor or outdated requires the use of some sort of representative bathymetric form, usually one that is concave up but monotonic. We propose and test a parametric form that superimposes realistic sand bars (Ruessink et al., 2003b) on a background profile that mixes a concave up nearshore with a planar far field behavior. Implementation at any new site involves estimation of five parameters, three that can be found from approximate information from climatology or old offshore charts, one that can be estimated by almost any remote sensing modality and one, h_sea that is less well understood but mostly affects deeper bathymetry that has little impact on the resulting surf zone hydrodynamics. Tests against several hundred surveys at three diverse locations show that bathymetry is better estimated by the new barred form than with a previous monotonic profiles in about 80% of cases. The remaining cases are usually associated with the parametric prediction of bars that look realistic but are out of phase. The presence of parametric bars has an even greater impact on predicted hydrodynamics since wave breaking is concentrated at sand bar locations. Modeled cross-shore transects of alongshore current and wave height over the measured survey profile are well represented by modeled transects over the barred parametric form but not for results over a Dean profile. The peak alongshore current strength and location are particularly sensitive to the presence of a sand bar.     

Experimental study of the instabilities of alongshore currents on plane beaches

A laboratory experiment on alongshore currents was conducted for two plane beaches, with gradients 1:40 and 1:100, to investigate the instability of alongshore currents. Complicated and strongly unstable alongshore current motions were observed. In order to clearly examine the spatial and temporal variations of the shear instability of the currents, digital images from a charge-coupled device (CCD) recorded the deformations of dye batches released in the surf zone. Some essential characteristics of the shear instability were obtained from analyses of images showing the temporal variation of the dye patches.A high-resolution spectral analysis technique (the maximum entropy method, or MEM) was used to analyze the dominant frequency of the observed oscillation, along with the trigonometric regression method for determining the variations of the oscillation strength in the cross-shore direction. The propagation speed of the dye patch was obtained by tracking the movement over time of fixed locations in the dye patch, such as its peak, in the longshore direction. This data was then fitted linearly.Alongshore and cross-shore velocity time series acquired from sensors showed clearly that large-amplitude, long-period (about 50 s or 100 s) oscillations were present for all sensors deployed in the cross-shore direction under regular and irregular wave conditions. The analysis found that the maximum shear wave amplitude was approximately one-sixth of the maximum for the mean alongshore current, and occurred approximately at the position of the maximum of the mean alongshore current for irregular waves. The spatial structure of the shear waves was studied by analyzing collected images of the dye patches. The phase velocity of the meandering movements was obtained by measuring the magnitude of the oscillations of the dye patches in the alongshore direction with respect to time. The results suggest that the propagation speed of the shear instability was approximately one-half to three-quarters of the maximum mean longshore current for both regular and irregular waves.Linear instability analysis theory was applied to the characteristics of alongshore current instability, which suggested that there are two instability modes related to the observed oscillations: the frontshear mode observed for the 1:100 slope, and the backshear mode observed for the 1:40 slope. Theoretical analyses agreed with the experimental results in both cases. The velocity profile of the mean longshore current was found to affect the instability mode significantly, leading to further investigations on the influence of the velocity profiles and to provide support for the above conclusions.     

Coastline sand waves on a low-energy beach at “El Puntal” spit, Spain

Coastline sand waves have been observed at “El Puntal” spit, located on the north coast of Spain. The spit has been monitored by an Argus video system since 2003 and the formation and destruction of sand waves has been observed. Coastline data from the video images are analyzed by means of principal components analysis, obtaining a mean sand wave length of 125–150 m and a maximum amplitude of ≈ 15 m. It is also observed that sand waves reach their maximum amplitude at about 15 days. No propagation of these sand waves is noticed during the approximately two-month-long events analyzed. Sand wave formation and evolution are examined in relation with the prevailing local wave conditions during that period. Incident waves at the west end of the spit approach from the east–northeast, with a very high angle with respect to the shoreline. Field observations suggest that sand waves may result from an instability in alongshore sediment transport caused by moderate-energy waves with a high-angle incidence.     

Modeling regional sediment transport and shoreline response in the vicinity of tidal inlets on the Long Island coast,United States

A new numerical model was developed to simulate regional sediment transport and shoreline response in the vicinity of tidal inlets based on the one-line theory combined with the reservoir analogy approach for volumetric evolution of inlet shoals. Sand bypassing onshore and sheltering effects on wave action from the inlet bar and shoals were taken into account. The model was applied to unique field data from the south coast of Long Island, United States, including inlet opening and closure. The simulation area extended from Montauk Point to Fire Island Inlet, including Shinnecock and Moriches Inlets. A 20-year long time series of hindcast wave data at three stations along the coast were used as input data to the model. The capacity of the inlet shoals and bars to store sand was estimated based on measured cross-sectional areas of the inlets as well as on comprehensive bathymetric surveys of the areas around the inlet. Several types of sediment sources and sinks were represented, including beach fills, groin systems, jetty blocking, inlet bypassing, and flood shoal and ebb shoal feeding. The model simulations were validated against annual net longshore transport rates reported in the literature, measured shorelines, and recorded sediment volumes in the flood and ebb shoal complexes. Overall, the model simulations were in good agreement with the measured data.     

Breakpoint generated surf beat induced by bichromatic wave groups

This paper presents new experimental data on 2-D surf beat generation by a time-varying breakpoint induced by bichromatic wave groups. The experimental investigation covers a broad range of wave amplitudes, short wave frequencies, group frequencies and modulation rates. The data include measurements of incident and outgoing wave amplitudes, breakpoint position, shoreline run-up and the cross-shore structure of both the short and long wave motion. Surf beat generation is shown to be in good agreement with theory [Symonds, G., Huntley, D.A., Bowen, A.J., 1982. Two dimensional surf beat: long wave generation by a time-varying breakpoint. J. Geophys. Res. 87, 492–498]. In particular, surf beat generation is dependent on the normalised surf zone width, which is a measure of the phase relationship between the seaward and shoreward breakpoint forced long waves, and linearly dependent on the short wave amplitude. The cross-shore structure of the long wave motion is also consistent with theory; at maximum and minimum surf beat generation, the mean breakpoint coincides with the nodal and anti-nodal points, respectively, for a free long wave standing at the shoreline. A numerical solution, using measured data as input, additionally shows that the phase relationship between the incident bound long wave and the outgoing breakpoint forced wave is consistent with the time-varying breakpoint mechanism.     

Surf zone dynamics simulated by a Boussinesq type model. Part II: surf beat and swash oscillations for wave groups and irregular waves

This is the second of three papers on the modelling of various types of surf zone phenomena. In the first paper the general model was described and it was applied to study cross-shore motion of regular waves in the surf zone. In this paper, part II, we consider the cross-shore motion of wave groups and irregular waves with emphasis on shoaling, breaking and runup as well as the generation of surf beats. These phenomena are investigated numerically by using a time-domain Boussinesq type model, which resolves the primary wave motion as well as the long waves. As compared with the classical Boussinesq equations, the equations adopted here allow for improved linear dispersion characteristics and wave breaking is modelled by using a roller concept for spilling breakers. The swash zone is included by incorporating a moving shoreline boundary condition and radiation of short and long period waves from the offshore boundary is allowed by the use of absorbing sponge layers. Mutual interaction between short waves and long waves is inherent in the model. This allows, for example, for a general exchange of energy between triads rather than a simple one-way forcing of bound waves and for a substantial modification of bore celerities in the swash zone due to the presence of long waves. The model study is based mainly on incident bichromatic wave groups considering a range of mean frequencies, group frequencies, modulation rates, sea bed slopes and surf similarity parameters. Additionally, two cases of incident irregular waves are studied. The model results presented include transformation of surface elevations during shoaling, breaking and runup and the resulting shoreline oscillations. The low frequency motion induced by the primary-wave groups is determined at the shoreline and outside the surf zone by low-pass filtering and subsequent division into incident bound and free components and reflected free components. The model results are compared with laboratory experiments from the literature and the agreement is generally found to be very good. Finally the paper includes special details from the breaker model: time and space trajectories of surface rollers revealing the breakpoint oscillation and the speed of bores; envelopes of low-pass filtered radiation stress and surface elevation; sensitivity of surf beat to group frequency, modulation rate and bottom slope is investigated. Part III of this work (Sørensen et al., 1998) presents nearshore circulations induced by the breaking of unidirectional and multi-directional waves.     

An experimental study of the spatial structures of the instabilities of longshore currents on plane beaches

A laboratory experiment on alongshore currents is conducted for two plane beaches with slopes 1:40 and 1:100 to investigate the instability of alongshore currents.The dye release experiment is also performed synchronously in surf zone.Complicated and strongly unstable motions of alongshore currents are observed in the experiment.To examine the spatial and temporal variations of the shear instabilities of longshore clearly,dye batches are released in surf zone.The deformation of the dye patch is observed efficiently and effectively with charge coupled device(CCD) system.Some essential characteristics of the shear instability are validated from the results of image analyses of the temporal variation of the dye patch.The influences of alongshore currents,Stokes drift,large-scale vorticity and the shear instabilities on the transport of dye are analyzed using the collected images.The spatial structure of the instabilities of longshore currents is studied by analyzing collected images of the dye patch.And the phase velocity of the meandering movements is obtained through measuring the movement distances of the oscillations of dye patch in alongshore direction with time.The results suggest that the propagation speed of the shear instability is approximately 50%-75% of maximum of mean alongshore currents for irregular and regular waves.The calculated propagation speed using a linear instability analysis theory is compared with the experimental results.The comparison shows agreements between them.     

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.     

A statistical-dynamical method for predicting long term coastal evolution

A new approach to the problem of predicting long term coastline evolution is described. The cumulative effect of shoreline processes is inferred by inverting an extended form of one-line equation. Results from the inversion process are then used to predict future coastal evolution on the basis of past historical changes. The data required are charts showing the historical changes in coastline position. By way of validation, the model is applied to a region on the east coast of the United Kingdom where a 12 year forecast is made and compared against recent aerial survey data. The results are encouraging and suggest that the method has a predictive capability, suitable for strategic studies of regional shoreline evolution.     

Wavenumber–frequency analysis of the landslide-generated tsunamis at a conical island

In this paper we investigate the propagation of tsunamis generated by landslides around the coast of an island to understand the propagation and trapping mechanisms of the waves. Records of shoreline displacement have been processed using the wavenumber–frequency analysis (k–f). We identify the dispersion relation followed by the waves that propagate alongshore. It appears that the 0th-order edge wave mode is the only one relevant for shoreline run-up. Furthermore, it is shown that the edge wave dispersion relation is a quantitative tool to estimate the phase and group wave celerities. A very good agreement is found when comparing the wave celerities as calculated from the experimental records, against those predicted by the edge wave theory. Furthermore the analysis of the tsunami around the island has been carried out by means of the two-dimensional k–f. Using as input data a high-space resolution dataset of free surface elevations it is found that other modes, both trapped (1st-order edge waves) and non-trapped (free radiating waves), occur during the propagation.     

江苏海岸中部近岸冬季潮汐和潮流特征

南黄海西侧的江苏海岸近岸区域,素以地形复杂、潮流强劲、悬沙输运剧烈著称,但是较长期的同步潮位和潮流观测数据仍然缺乏,尤其是在近岸(20 km)浅水(20 m)区域。2014年1月在大丰港附近开展了连续潮位和潮流观测,获得的数据揭示了一系列特征。此地潮汐潮流为正规半日潮,浅水分潮显著。平均潮差为3.05 m,最显著的两个分潮为M2和S2分潮,振幅分别为1.45 m和0.52 m。潮流最显著的半日分潮M2分潮和最显著的浅水分潮M4分潮在沿岸方向上振幅分别为0.84m/s和0.12m/s,在跨岸方向上振幅分别为0.24 m/s和0.01 m/s,沿岸方向占绝对优势。潮波的沿岸传播介于前进波和驻波之间,驻波的特征稍强。M2分潮潮流椭圆最大流(长轴)方向为南偏东7.4°。存在冬季沿岸向北的余流,垂向平均值的大小为2.2 cm/s。以上潮汐潮流特征为该区域海洋物质输运研究提供了基础资料。     

On the genesis of some bottom and coastal features in the Eastern Gulf of Finland

An investigation undertaken recently by the Division of Regional Geoecology and Marine Geology of the Karpinsky All-Russian Research Geological Institute in the coastal zones of the Eastern Gulf of Finland allowed finding some specific relief forms of both near-shore bottom topography and shoreline shape. First of all, among the most interesting objects, the sand ridges on the surface of submarine terrace (between Repino locality and Cape Lautaranta) should be mentioned. These ridges are elongated at an angle to the shoreline and are located beyond the limits of wave action. The other interesting morphological type is represented by longshore sand waves up to some hundreds of meters long and some tens of meters wide near the Bol’shaya Izhora locality. Longshore sand waves move along the southern coast of the gulf, this causing alteration of erosion and accretion zones and leading to formation and degradation of the sand spits. Shore-face-connected ridges are believed to develop under the action of drift currents generated during the passage of deep west cyclones. It is shown that the ridge turned toward the current gives rise to a convergence of the cross-shore flows over the crest and provokes a shift of the maximum velocity toward the front side of the structure. Associated changes in sediment discharges result in accumulation and growth of the ridge. The origin of wavelike features in the shoreline contour (longshore sand waves) is due to a very oblique wave approach caused by predominance of the west winds blowing along the axis of the gulf. Under these conditions a small perturbation of the shoreline contour is shown to manifest a trend to increase with time.     

Forecast of coastal changes based on morphodynamical modeling

A new version of the prognostic model is suggested, which differs from the previous version by its structure and method for including long-term variables into the process of coastal modeling. A fundamental prerequisite of the model is the assumption of the quasistationary state of the coast at any arbitrary stage of its evolution. The displacement of the coast under the influence of external factors at given moment is predicted on the basis of the obtained equation for the balance of sediments. The model takes into account a number of properties of the coastline. In particular, the distinguished dependence of the coastline shape on the regime of sediment transport is used to calculate the gradient of the alongshore sediment flux. The model also uses the empirical dependence found earlier, which makes it possible to estimate the second most important component of the sediment balance for selected types of coasts — the cross-shore matter flux. Special attention is focused on adjustment of the model to Arctic conditions. The results obtained are illustrated by the example of forecasting the development of accumulative coasts of the Western Yamal. It is shown that the major part of the Sharapovy Koshki sand islands could disappear within the next 700 years due to erosion.     

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.     

Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.     

Artificial beach growth for breakwater protection at the Port of Timaru, east coast, South Island, New Zealand

The paper concerns beach growth by trapping longshore drift to form a protective beach seaward of the principal “weather” breakwater at the Port of Timaru, east coast, South Island. This “spending beach” concept was aproached by evaluating downdrift extension and considerable progradation of an existing accumulation at South Beach which is a product of harbour development since 1879 and which was held in quasi-stability by ongoing extractions of the net surplus littoral drift of coarse sands and gravels (averaging 60,000 m³ yr⁻¹).A one-line model was adapted from sand beach conditions and scaled to the morphology and processes of the mixed sand and gravel beaches at Timaru. Calibration of the model was performed from related research into the rates and temporal pattern of longshore drift on South Beach. A hundred year history of shoreline progradation against the harbour structures was utilised to verify the model.The concept offered a high benefit: cost ratio for a small engineering intervention provided shoreline forms and behaviour could be specified sufficiently for planning, statutory consent, engineering, economic and environmental impact assessment purposes. A 150 m long spur groin near the harbour entrance would trap about 12 ha of sand and gravel in about 8 years. The new shore would be better aligned to the dominant swell and storm waves than the present shore, so reducing long term net drift rates.Construction of the scheme commenced in May 1987 and progress to date is detailed.     

The influence of tides,wind and waves on the redistribution of nourished sediment at Terschelling,The Netherlands

A calibrated morphodynamic model of the barrier island of Terschelling, The Netherlands [Grunnet, N.M., Walstra, D.J.R., Ruessink, B.G., 2004. Process-based modelling of a shoreface nourishment. Coastal Eng. 51/7, 581–607], comprising the island and its two adjacent tidal inlet systems, is applied to identify the relative contribution of tides, wind and waves to the cross-shore and alongshore redistribution of a 2 Mm³ nourishment supplied to the nearshore zone along the island. Several model simulations with varying combinations of horizontal and vertical tide, wind and wave forcing were designed to investigate the effect of each individual forcing on a large spatio-temporal scale (order of kilometres and months, respectively). As expected, stirring and transport by waves and wave-induced currents are predicted to be by far the dominant contributor to the net sediment transport along the coast of Terschelling. Because of the strong obliquity of the winds and the relatively small tidal currents in front of the island (≈ 0.5 m/s), alongshore wind-driven currents increase sediment transport rates and horizontal tides virtually have no net transport capacity. This motivated a local model of the study area along the closed coast of Terschelling, not including tidal inlets and further simplifying tidal boundary definitions by omitting the horizontal tides: morphodynamic simulations of the local model show virtually identical results as the larger model predictions. The reduction in complexity in setting up a local model instead of a regional model coupled with the corresponding significant reduction in computational time points to an increasing applicability of complex process-based models.     

Cross-shore hydrodynamics within an unsaturated surf zone

This paper concerns the hydrodynamics induced by random waves incident on a steep beach. New experimental results are presented on surface elevation and kinematic probability density functions, cross-shore variation in wave heights, the fraction of broken waves and velocity moments. The surf zone is found to be unsaturated at incident wave frequencies, with a significant proportion of the incident wave energy remaining at the shoreline in the form of bores. Wave heights in both the outer and inner surf zones are best described by a full Rayleigh distribution [Thornton, E.B., Guza, R.T., 1983. Transformation of wave height distribution. J. Geophys. Res. 88, 5925–5938], rather than a truncated Rayleigh distribution as used by Battjes and Janssen (1978) [Battjes, J.A, Janssen, J.P., 1978. Energy loss and setup due to breaking of random waves. Proc. 16th Int. Conf. Coastal Eng. ASCE, New York, pp. 569–588]. A new parametric wave transformation model is outlined which provides explicit expressions for the fraction of broken waves and the energy dissipation rate within the surf zone. On steep beaches, the model appears to offer improved predictive capabilities over the original Battjes and Janssen model. Cross-shore variations in the velocity variance and velocity moments are best described using Linear Gaussian wave theory, with less than 20% of the velocity variance in the inner surf zone due to low frequency energy.     

高分影像砂质海岸线精细提取及校正方法

通过遥感影像稳定获取大范围、连续性海岸线数据,是开展海岸带研究的重要手段之一.针对传统边缘检测算法处理高分辨率遥感影像存在的噪声敏感性、阈值不稳定性等问题,引入一种强鲁棒性的结构森林边缘检测(Strected?Forests?Edge?Detection,?SE)算法,对海口市西海岸砂质岸线进行识别,并提出基于Bruu...     

Influence of bathymetric fluctuations on coastal storm surge

Natural events constantly alter nearshore bathymetric properties. Hurricanes particularly affect bathymetry as they pass over a body of water. To compute an accurate forecast or recreate a hurricane's effects through hindcasting techniques, an operational bathymetry data set must be known in advance. However, obtaining and maintaining current and accurate bathymetric data can be costly and difficult to manage. In this paper we examine the extent to which variations in nearshore bathymetry affect the storm surge at the coast. A common question for wave and surge modeling is, “how good is the bathymetric data?” If we can allow for a range of fluctuations in the bathymetry without significantly adjusting the results of the surge predictions, we can potentially save months of field work and millions of dollars. A one-dimensional (1D) analytical solution for waves and water level is developed for initial testing. In the 1D case we find that as long as the amplitudes of the bathymetric fluctuations are less than 60% of the original depth, the surge at the coast is within ± 10% of the surge generated on the initial bottom slope. If the fluctuation produces a hole, a deepening of the local bathymetry, within 80% of the local water depth, the coastal storm surge calculated is still within 10% of the unperturbed value computed for bottom slopes shallower than 1:20. In addition, we find there is an optimum distance offshore for each sloped profile that corresponds to a depth between 25 and 40 m, beyond which the effects of bathymetric fluctuations begin to decrease. A coupled 2D modeling system is implemented to test our hypothesis along a realistic coastline. After selecting three study sites, we vary the bathymetry at the selected locations by ± 20%. Consistent with the 1D tests, the storm surge at the shoreline varies by less than 5%.