The effects of sediment characteristics and wave height on shape-parameter for representing equilibrium beach profiles

The shape parameter helps determining the shape of equilibrium beach profile in terms of offshore distance and water depth. The shape parameter therefore, should represent the effect of all the environmental factors involved in beach profile formation, such as wave climate and sediment properties. However, all the previous studies carried out to define shape parameter only consider the effects of sediment characteristics in their definitions. The aim of this study is to add the effect of wave climate also in the definition of shape parameter. This is achieved by integrating wave energy dissipation rate per unit volume at the surf zone. The result yields equilibrium wave energy dissipation rate that leads to theoretical definition of equilibrium beach profiles involving the effects of both the grain size and the wave climate parameters. It is found that the sediment grain size and the incoming wave height affect the value of shape parameter; whereas, the effects of wave period can be neglected. By means of energy equation, it is also possible to observe in macro scale the strength of wave energy on beach profile for different grain sizes. The findings also bring about the possibility of defining shape parameter such that any two arbitrary beach profiles owning the same sediment grain sizes can have the opportunity to have different beach profile formations. Finally, by adding the effect of wave height in the definition of shape parameter the graphical representation of the parameter, previously given by Moore (1982) is improved herein.     

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.     

Process-based model for nearshore hydrodynamics, sediment transport and morphological evolution in the surf and swash zones

Hydrodynamics and sediment transport in the nearshore zone were modeled numerically taking into account turbulent unsteady flow. The flow field was computed using the Reynolds Averaged Navier–Stokes equations with a k–ε turbulence closure model, while the free surface was tracked using the Volume-Of-Fluid technique. This hydrodynamical model was supplemented with a cross-shore sediment transport formula to calculate profile changes and sediment transport in the surf and swash zones. Based on the numerical solutions, flow characteristics and the effects of breaking waves on sediment transport were studied. The main characteristic of breaking waves, i.e. the instantaneous sediment transport rate, was investigated numerically, as was the spatial distribution of time-averaged sediment transport rates for different grain sizes. The analysis included an evaluation of different values of the wave friction factor and an empirical constant characterizing the uprush and backwash. It was found that the uprush induces a larger instantaneous transport rate than the backwash, indicating that the uprush is more important for sediment transport than the backwash. The results of the present model are in reasonable agreement with other numerical and physical models of nearshore hydrodynamics. The model was found to predict well cross-shore sediment transport and thus it provides a tool for predicting beach morphology change.     

广西万尾岛金滩沿岸输沙计算与分析

依据CERC公式,年内代表浪向作用下,广西万尾岛金滩平直岸滩中部泥沙分别向东西两侧净输沙,意味着金滩中部有淘刷趋势而两端有淤积趋势,而实际上岸滩中部滩面长年基本稳定、未有明显侵蚀现象。分析认为公式计算成果反映的输沙特征定性仍然是正确的,岸滩能够维持稳定是因为还存在自海向岸的横向输沙补给沙源。当岸滩并非平直且足够长时,应完整分析纵、横向输沙才能更为合理地反映岸滩泥沙运动特征。     

Field measurements of longshore sediment transport during storms

This paper presents an analysis of longshore sediment transport (LST) rates based on an accumulation of data obtained during five storms. Direct measurements of velocities and suspended sediment concentration were conducted at a minimum of nine positions across a barred profile in waves up to H_m0=3.5 m to provide a measure of the cross-shore distribution and total suspended-load sediment transport rates. The study was conducted at the US Army Engineer Waterways Experiment Station's Field Research Facility, located in Duck, NC. Measurements were made using the Sensor Insertion System (SIS) which provides an economical means to collect the required information. The largest LST rate computed from the measurements was 1780 m³ h⁻¹. Although the cross-shore distribution of the LST varied, it most often had two peaks associated with wave shoaling and breaking at the bar and near the beach. Comparisons of measurement results with predictions using the ‘CERC' LST formula show the predicted rates were sometimes higher and other times lower; suggesting that additional terms may be required for short term predictions during storms. Comparisons to a ‘Bagnold' type formulation, which included a velocity term that could account for wind and other effects on LST, show better agreement for at least one of the storms. These results are intended to help fill a void of information documenting the cross-shore distribution and LST rates, particularly during storms.     

Shoreline accretion and sand transport at groynes inside the Port of Richards Bay

The south-western shoreline along the entrance channel inside the Port of Richards Bay has experienced continued erosion. Four groynes were constructed to stabilise the shoreline. Monitoring of shoreline evolution provided valuable data on the accretion adjacent to two of the groynes and on the sediment transport rates at these groynes. Tides, beach slopes, winds, wave climate, current regime, and sand grain sizes were documented. The one site is “moderately protected” from wave action while the other is “protected” according to the Wiegel [Wiegel, R. L. (1964). Oceanographical engineering. Prentice Hall, Inc., Englewood Cliffs, NJ.] classification. The shoreline accreted progressively at the two groynes at 0.065 m/day and 0.021 m/day respectively. The shorelines accreted right up to the most seaward extremity of the groynes. Equilibrium shorelines were reached within about 3.5 years to 4 years, which compare well with other sites around the world. The mean wave incidence angle is large and was found to be about 22°. The median sand grain sizes were 0.33 mm and 0.37 mm. The groynes acted as total traps, the beach surveys were extended to an adequate depth, and cross-shore sediment transport did not cause appreciable net sand losses into the entrance channel. The net longshore transport rate along the study area, which is north-westbound, is only slightly lower than the gross longshore transport. The actual net longshore transport rates are 18 000 m³/year and 4 600 m³/year respectively at the two groynes. A rocky area limits the availability of sand at one groyne. There is fair agreement between the predicted and measured longshore transport rates at the other groyne.     

Physical modeling of the cross-shore short-term evolution of protected and unprotected beach nourishments

This paper illustrates the results of an experimental investigation (model-to-prototype length ratio equal to 12) carried out to reproduce the cross-shore evolution of nourished sandy beaches. New two-dimensional experiments were performed to study the short-term response of the cross-shore profile for both “soft” (unprotected) and “mixed” (protected by submerged breakwaters) beach fill projects. Due to the simplified reproduction of prototype conditions in a two-dimensional geometry, only cross-shore sediment transport is considered. The results are related to the immediate post-nourishment evolution and far from beach fill boundaries where long-shore gradients of long-shore sediment transport are likely to be negligible. Three different pseudo-random wave trains were generated in order to simulate both accretive and erosive conditions. A fourth wave train, characterised by time-varying incident wave spectrum was generated for the investigation of the beach response to simplified storm time evolution. Dimensionless experimental results are given in terms of wave parameters, key features of cross-shore profile evolution and sediment transport rates. Furthermore, being highly resolved in both time and space, experimental data are suitable for mathematical model validation. It was observed that submerged breakwater switches erosive conditions to slightly accretive, at least within the tested experimental range.     

A phase-resolving cross shore sediment transport model for beach profile evolution

A phase-resolving wave transformation module is combined with an intra-wave sediment transport module to calculate the on-/offshore sediment transport rates. The wave module is based on the Boussinesq equations extended into the surf zone. The vertical variation of the mean undertow and the intra-wave sediment concentrations are calculated. The net sediment transport rates are calculated, and the equation for conservation of sediment is solved to predict the beach profile evolution. The results of the present paper showed that the undertow contribution to the sediment transport rates is not dominating in all parts of the surf zone, even for eroding beaches, suggesting that other contributions should not be neglected. The present model also showed that for the same offshore wave energy the time series of the oscillatory motion is important and that the effect of wave groups cannot be disregarded.     

Development of a medium–long term beach evolution model

A new medium–long term beach evolution model is proposed. This model is based on an analytically integrated sediment conservation equation and on a beach profile evolution model. The sediment conservation equation provides the sediment supplies or losses. The beach profile evolution model redistributes the sediment supplies or losses along the beach profile. In the beach profile evolution model, the definition of the complete profile is incorporated (breaking zone, transition zone, exterior zone and geological zone). The proposed model has been applied to several theoretical cases and to field data, showing the advantages of this model compared to classical “one-line models”.     

Longshore current and sediment transport on composite beach profiles

Natural beaches tend to exhibit an equilibrium profile that is planar nearshore and nonplanar, concave-up offshore. The longshore current on this type of beach profile depends on the horizontal distance to the location of the intersection between the planar and nonplanar profiles. As the width of the planar beach face decreases, the location of the maximum longshore current moves closer to the shore. The dependency of the corresponding longshore sediment transport rate on the location of the intersection between the two profiles is demonstrated for two energetics-based sediment transport models. Again, a narrower beach face results in the maximum sediment transport being closer to the shore. Total sediment transport rates are also a function of the planar beach face width. This suggests that longshore transport rates are modulated by the tidal elevation.     

Establishing uniform longshore currents in a large-scale sediment transport facility

A large-scale laboratory facility for conducting research on surf-zone sediment transport processes has been constructed at the U.S. Army Engineer Research and Development Center. Successful execution of sediment transport experiments, which attempt to replicate some of the important coastal processes found on long straight beaches, requires a method for establishing the proper longshore current. An active pumping and recirculation system comprised of 20 independent pumps and pipelines is used to control the cross-shore distribution of the mean longshore current. Pumping rates are adjusted in an iterative manner to converge toward the proper settings, based on measurements along the beach. Two recirculation criteria proposed by Visser [Coastal Eng. 15 (1991) 563] were also used, and they provided additional evidence that the proper total longshore flow rate in the surf zone was obtained. The success of the external recirculation system and its operational procedure, and the degree of longshore uniformity achieved along the beach, are the subjects of this paper. To evaluate the performance of the recirculation system, and as a precursor to sediment transport experiments, two comprehensive test series were conducted on a concrete beach with straight and parallel contours (1:30 slope), one using regular waves and the other using irregular waves. In the regular wave case, the wave period was 2.5 s and the average wave height at breaking was approximately 0.25 m. In the irregular wave case, the peak wave period was 2.5 s and the significant breaking wave height was approximately 0.21 m. The longshore current recirculation system proved to be very effective in establishing uniform mean longshore currents along the beach in both cases. This facility and the data presented here are unique for the following reasons: (1) the high cross-shore resolution of the recirculation system and the ease with which changes can be made to the longshore current distribution, (2) the degree of longshore uniformity achieved as a percentage of the length of the basin (even near the downdrift boundary), (3) the scale of the wave conditions generated, and (4) the relatively gentle beach slope used in the experiments (compared to previous laboratory studies of the longshore current). Measured data are provided in an appendix for use in theoretical studies and numerical model development and validation.     

季节性波浪动力作用下南湾弧形岸滩泥沙横向输运特征

响应季节性波候作用的泥沙输运特征是研究弧形海滩地貌变化及港工建筑的重要内容。基于南湾弧形海滩实际测量的冬、夏各11条剖面高程变化资料,将其划分为低潮间带、低中潮带、中潮带、高潮间带、低冲流带、中冲流带及其海滩后滨等7带,在此基础上利用经验正交函数(EOF)方法对各个带的体积变化进行分析,结果表明:1)南湾弧形海滩的泥沙以单向输运为主,并具有季节性变化特征,其中冬季泥沙在东南浪作用下,自陆向海输运,夏季泥沙在西南浪作用下自海向陆输运;2)南湾弧形海滩的泥沙分别在高潮带与中潮带、低冲流带与中冲流带之间存在频繁的双向输运;3)南湾弧形海滩不同岸段泥沙的横向输运因岬角的遮蔽能力、地形以及波浪作用的方向而有所差异。     

Wave-induced water table fluctuations,sediment transport and beach profile change: Modeling and comparison with large-scale laboratory experiments

Coastal groundwater systems can have a considerable impact on sediment transport and foreshore evolution in the surf and swash zones. Process-based modeling of wave motion on a permeable beach taking into account wave-aquifer interactions was conducted to investigate the effects of the unconfined coastal aquifer on beach profile evolution, and wave shoaling on the water table. The simulation first dealt with wave breaking and wave runup/rundown in the surf and swash zones. Nearshore hydrodynamics and wave propagation in the cross-shore direction were simulated by solving numerically the two-dimensional Navier–Stokes equations with a k–ε turbulence closure model and the Volume-Of-Fluid technique. The hydrodynamic model was coupled to a groundwater flow model based on SEAWAT-2000, the latter describing groundwater flow in the unconfined coastal aquifer. The combined model enables the simulation of wave-induced water table fluctuations and the effects of infiltration/exfiltration on nearshore sediment transport. Numerical results of the coupled ocean/aquifer simulations were found to compare well with experimental measurements. Wave breaking and infiltration/exfiltration increase the hydraulic gradient across the beachface and enhance groundwater circulation inside the porous medium. The large hydraulic head gradient in the surf zone leads to infiltration across the beachface before the breaking point, with exfiltration taking place below the breaking point. In the swash zone, infiltration occurs at the upper part of the beach and exfiltration at the lower part. The simulations confirm that beaches with a low water table tend to be accreted while those with a high water table tend to be eroded.     

Swash zone sediment fluxes: Field observations

This paper describes newly obtained, high-frequency observations of beach face morphological change over numerous tidal cycles on a macrotidal sandy beach made using a large array of ultrasonic altimeters. These measurements enable the net cross-shore sediment fluxes associated with many thousands of individual swash events to be quantified. It is revealed that regardless of the direction of net morphological change on a tidal time scale, measured net fluxes per event are essentially normally distributed, with nearly equal numbers of onshore and offshore-directed events. The majority of swash events cause net cross-shore sediment fluxes smaller than ± 50 kg m^− 1 and the mean sediment flux per swash event is only O(± 1 kg m^− 1) leading to limited overall morphological change. However, much larger events which deposit or remove hundreds of kilograms of sand per meter width of beach occur at irregular intervals throughout the course of a tide. It was found that swash–swash interactions tend to increase the transport potential of a swash event and the majority of the swash events that cause these larger values of sediment flux include one or more interactions. The majority of the larger sediment fluxes were therefore measured in the lower swash zone, close to the surf/swash boundary where swash–swash interactions are most common. Despite the existence of individual swash events that can cause fluxes of sediment that are comparable to those observed on a tidal time scale, frequent reversals in transport direction act to limit net transport such that the beach face volume remains in a state of dynamic equilibrium and does not rapidly erode or accrete.     

Modeling sediment transport in the swash zone: A review

A critical review of conceptual and mathematical models developed in recent decades on sediment transport in the swash zone is presented. Numerous studies of the hydrodynamics and sediment transport in the swash zone in recent years have pointed out the importance of swash processes in terms of science advancement and practical applications. Evidently, the hydrodynamics of the swash zone are complex and not fully understood. Key hydrodynamic processes include both high-frequency bores and low-frequency infragravity motions, and are affected by wave breaking and turbulence, shear stresses and bottom friction. The prediction of sediment transport that results from these complex and interacting processes is a challenging task. Besides, sediment transport in this oscillatory environment is affected by high-order processes such as the beach groundwater flow. Most relationships between sediment transport and flow characteristics are empirical, based on laboratory experiments and/or field measurements. Analytical solutions incorporating key factors such as sediment characteristics and concentration, waves and coastal aquifer interactions are unavailable. Therefore, numerical models for wave and sediment transport are widely used by coastal engineers. This review covers mechanisms of sediment transport, important forcing factors, governing equations of wave-induced flow, groundwater interactions, empirical and numerical relations of cross-shore and longshore sediment transport in the swash zone. Major advantages and shortcomings of various numerical models and approaches are highlighted and reviewed. These will provide coastal modelers an impetus for further detailed investigations of fluid and sediment transport in the swash zone.     

Longshore sediment transport on non-planar beaches

For a concave-up $\text{2}\text{3}$ power Bruun beach profile, the following two energetics-based sediment transport models are developed: (1) a Bagnold-type model and (2) a combined wave-current stress model. The stress model is calibrated with the Bagnold model using observed transport rates on planar beaches. The sediment transport profiles for the two models are in agreement within the surf zone for the planar beach case; but the stress model is also applied seaward of the breaker line where the Bagnold model is not. A mean swash transport of sand is predicted by the Bagnold model for a $\text{1}\text{2}$ power least-squares approximation to total depth including setup/setdown on a Bruun beach profile. The total longshore transport of sand is determined for each transport model as a function of the turbulent lateral mixing strength. The total sand transport is found to be less on a concave-up beach profile than for the corresponding planar beach case.     

A predictive Mesoscale model of the erosion and profile development of soft rock shores

The development, application and behaviour of a generic model of retreating soft rock (e.g. clay) shores is described. This represents a broad system, in coastal modelling terms, comprising shore platform, beach, tidal range, wave transformation, cliff and talus. The coast is divided into a series of representative cross-shore profiles, each of which is discretised into a column of elements. Erosion of a platform element at each timestep depends on its gradient. Material strength is dealt with as a calibration constant, wave forces are averaged over durations of a tide or hour and sediment transport is represented in bulk terms. Attention has been focussed on interaction between system parts and the emergence of system properties, in particular profile shape. This is allowed to develop towards dynamic equilibrium and is the principal means of model validation. The emergence of the profile shape is dominated by the distribution of wave scour by the tide and by interaction with a beach, if present. Because the model is process-based, it may be used to model the effects of climate change and engineering intervention. Yet it is also computationally inexpensive, so may be used to explore uncertainty through probabilistic application. The breadth of the included system, coupled with short run-times, enables predictions over timescales of decades, which we refer to as the Mesoscale. The model is used to explore the dynamics of retreating soft rock shore profiles and to predict future behaviour of a study site.