Short-term shoreline changes due to cross-shore structures: a one-line numerical model

The proposed numerical model simulates the short-term temporal changes in shoreline position due to a structure interrupting the longshore sediment flux. The impacts of both the groin-type construction and underwater trench of arbitrary orientation relative to the shore are discussed. In order to estimate the sediment mass trapped by the structure, a submodel of the longshore sediment transport induced by a random wave field is developed. The contribution of the surface roller in momentum balance as well as in sediment suspension is included. The shoreline changes are computed from the equation deduced from the mass conservation. The perturbations in the longshore sediment discharge caused by a structure are assumed to concentrate within some boundary area of which the spatial scale is proportional to the structure's length until the latter is exceeded by the width of the sediment flux. It is shown in particular that the total effect of a long trench (channel) and a pier in its nearshore part results in general shoreline recession except for the vicinity of a pier. The model is tested against the laboratory data of Baidei et al. (1994) and applied to the Baidara Bay coast (Kara Sea) where a pipeline would be designed.     

Experimental simulation of shoreface nourishments under storm events: A morphological,hydrodynamic, and sediment grain size analysis

This study focuses on barred beach shoreface nourishments physically simulated in a wave flume. The attack of a schematic storm on three different nourishments is analysed. The apex and waning storm phases lead respectively to offshore and onshore sediment transports. Nourishments in the trough and on the outer bar feed the bar and increase wave dissipation offshore. The bar acts as a wave filter and reduces shore erosion (lee effect). In contrast, nourishment on the beach face leads mostly to shore feeding and reconstruction (feeder effect). With successive nourishments, the beach face clearly becomes steeper and onshore sediment transport is reduced during moderate wave climates. The surface grain size analysis reveals marked variations. Coarser sediments are sorted on the bar and the upper beach face. These locations correspond to large wave dissipation zones during the storm apex.     

Model study of a shoreline wave-power system

A wave-power system which combines the concept of a breakwater and a harbor resonance chamber was developed in this study. In the caisson chamber, a multi-resonant oscillating water column (MOWC) was formed to push or suck air through the air turbine and thus continuously generated the power. The proposed wave-power system has two aims in mind: one is shore protection and the other is to extract energy from the ocean. To achieve an optimal effect of harbor resonance when excited by incident waves of various periods, a 60° opening of the cylindrical chamber with an entrance section and an arc-shaped curve board in front of the caisson was designed. In order to assess the energy-conversion efficiency and the hydraulic performance, a 1/20 model of this system was constructed and tested in the wave tank under various wave conditions. Our experimental data for the amplification factor of the MOWC agree well with previous theoretical results [Lee, J.J., 1971. Journal of Fluid Mechanics 45, 375–394]. The curve board proves to be useful: it not only broadens the resonant period but also increases the energy-extraction rate. The reflection coefficient was found to be generally low and to decrease with increasing wave height. However, due to the relatively high energy loss of the MOWC, only 28.5% of the incident-wave energy was converted into air energy, indicating that there are still areas for further improvement. In any event, the experimental results provided a clear picture of the energy-transformation process, and demonstrated the preliminary feasibility of this wave-energy device.     

Modelling analysis of the sensitivity of shoreline change to a wave farm

Change of shoreline wave climate caused by the installation of a wave farm is assessed using the SWAN wave model. The 30 MW-rated wave farm is called the ‘Wave Hub’ and will be located 20 km off the north coast of Cornwall, UK. Changes in significant wave height and mean wave period due to the presence of the Wave Hub are presented. The results suggest that the shoreline wave climate will be affected, although the magnitude of effects decreases linearly as wave energy transmitted increases. At probable wave energy transmission levels, the predicted change in shoreline wave climate is small.     

Alongshore fluid motions in the swash zone of a sandy and gravel beach

Field experiments were conducted on a low-gradient, high-energy sandy beach (Truc Vert, France) and a steep, low-energy gravel beach (Slapton, UK) to examine alongshore-directed currents within the swash zone. At Truc Vert, data were collected over 33 tidal cycles with offshore significant wave heights of 1–4 m and periods of 5–12 s. At Slapton data were collected during 12 tides with wave heights of 0.3–1 m and periods of 4–9 s. The swash motion was predominantly at infragravity frequencies at Truc Vert and incident frequencies at Slapton.     

Wavelet analysis for processing of ocean surface wave records

Wavelet analysis is a relatively new technique and in the recent years enormous interest in application of wavelets has been observed. This modern technique is particularly suitable for non-stationary processes as in contrast to the Fourier transform, (FT), the wavelet transform (WT) allows exceptional localization, both in time and frequency domains. The wavelet transform has been successfully implemented in signal and image processing, ordinary and partial differential equation theory, numerical analysis, communication theory and other fields. On the other hand, the application of the WT to ocean engineering and oceanography is rare. In this paper the WTs capability to give a full time–frequency representation of the wave signals is demonstrated. The processing of the time series of the non-stationary deep water waves, waves breaking at the tropical coral reefs and mechanically generated waves in the wave flume demonstrates the ability of the wavelet transform technique to detect a complex variability of these signals in the time–frequency domain. Various spectral representations resulting from the wavelet transform are discussed and their application for wave signals is shown.     

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.     

关于登州浅滩作用的研究

１９９０－１９９３年对蓬莱西庄海岸附近海域进行了水文、水深、地质地貌和泥沙等１３项测量和调查。整理分析实测资料并结合数值计算，首次分析研究了登州浅滩的作用并着论述了浅滩被破坏后袭击岸边的波浪增大，波能增中到５－７倍，沿岸输沙量增中到３－５倍。文中指出西庄海岸侵蚀加剧的主要原因是，登州浅滩被破坏后失去防浪消波作用而造成的。     

Sediment transport and beach morphodynamics induced by free long waves,bound long waves and wave groups

New laboratory data are presented on the influence of free long waves, bound long waves and wave groups on sediment transport in the surf and swash zones. As a result of the very significant difficulties in isolating and identifying the morphodynamic influences of long waves and wave groups in field conditions, a laboratory study was designed specifically to enable measurements of sediment transport that resolve these influences. The evolution of model sand beaches, each with the same initial plane slope, was measured for a range of wave conditions, firstly using monochromatic short waves. Subsequently, the monochromatic conditions were perturbed with free long waves and then substituted with bichromatic wave groups with the same mean energy flux. The beach profile changes and net cross-shore transport rates were extracted and compared for the different wave conditions, with and without long waves and wave groups. The experiments include a range of wave conditions, e.g. high-energy, moderate-energy, low-energy waves, which induce both spilling and plunging breakers and different turbulent intensities, and the beaches evolve to form classical accretive, erosive, and intermediate beach states. The data clearly demonstrate that free long waves influence surf zone morphodynamics and promote increased onshore sediment transport during accretive conditions and decreased offshore transport under erosive conditions. In contrast, wave groups, which can generate both forced and free long waves, generally reduce onshore transport during accretive conditions and increase offshore transport under erosive conditions. The influence of the free long waves and wave groups is consistent with the concept of the relative fall velocity, H/w_sT, as a dominant parameter controlling net beach erosion or accretion. Free long waves tend to reduce H/w_sT, promoting accretion, while wave groups tend to increase the effective H/w_sT, promoting erosion.     

Calculation of beach change under interacting cross-shore and longshore processes

This paper presents a mathematical approach and numerical model that simulates beach and dune change in response to cross-shore processes of dune growth by wind and dune erosion by storms, and by gradients in longshore sand transport that will alter shoreline position. Sub-aerial transport processes are represented, whereas sub-aqueous transport is neglected. The system is tightly coupled morphologically, with the berm playing a central role. For example, the potential for sand to be transported to the dune by wind depends on berm width, and sand lost in erosion of the dune during storms can widen the berm. Morphologic equilibrium considerations are introduced to improve reliability of predictions and stability of the non-linear model. An analytical solution is given under simplification to illustrate properties of the model. Sensitivity tests with the numerical solution of the coupled equations demonstrate model performance, with one test exploring beach and dune response to potential increase in storm-wave height with global warming. Finally, the numerical model is applied to examine the consequences of groin shortening at Westhampton Beach, Long Island, New York, as an alternative for providing a sand supply to the down-drift beach. Results indicate that the sand will be released over several decades as the shoreline and dune move landward in adjustment to the new equilibrium condition with the shortened groins.     

Longshore sorting on a beach under wave action

Sorting of sediment on a beach under wave action takes several forms. Stratified layers of finer and coarser sediment, which depend on wave climate, grain size and beach slope are formed. This complex problem can be simplified by defining the cross-shore and longshore sorting according to the angle between the breaking wave and the coast. In the present study, longshore distribution of sediment as well as corresponding beach profiles was measured in a wave basin. Three-dimensional hydraulic model experiments were performed with regular waves. Eighteen sets of experiments performed in longshore sorting mechanism using two different sand beds. The sorting of the bed material and the formation of armour coats along the beach were defined by grain size distributions and dimensionless parameters for sandy beaches.The rate of sediment transport with grain size sorting was measured in a wave basin. A method introduced sorting process was presented in this study. The sediment rate based on sorting mechanism was also discussed with known methods. It has been found that the non-uniformity of the grain size and hence sorting of the beaches play a very important role in the sand transport due to wave motion in a similar way to the case of steady flow in alluvial channels.     

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”.     

Numerical study on wave-induced filtration flow across the beach face and its effects on swash zone sediment transport

A numerical model, coupling an analysis of beach groundwater flow with an analysis of swash wave motion over a uniform slope, is presented. Model calculations are performed to investigate the variations of swash-induced filtration flows across the beach face for different input parameters. Swash zone sediment transport under the influence of such filtration flow across the beach face is investigated through modification of effective weight of sediment particle and modification of swash boundary layer thickness. These effects are quantified based on a bed load transport model with a modified Shields parameter.     

Depth of activation on a mixed sediment beach

The relationship between wave height and depth of sediment activation is evaluated on an estuarine beach to determine whether activation depth is less in pebbles than sand. Rods with washers were used to monitor three excavated beach plots filled with (1) pebbles with mean grain size of 11.5 mm; (2) sand and granules; and (3) sand, granules and pebbles. Plots were monitored for 26 events over 27 days. Significant wave heights ranged from 0.18 to 0.40 m and activation depths from 0.02 to 0.12 m. Activation depths in the pebble plot were less than the other two plots when waves reworked sediment not activated during previous tidal cycles. Proportionality coefficients for activation depth to wave height, when net change was < 0.02 m, were 0.24 in the pebble plot and 0.30–0.31 in the other plots when experimental fill sediments remained and 0.22 to 0.23 in all three plots over the entire monitoring period, which included activation of newly deposited native sediment. Results suggest that for similar wave heights, activation depths in pebbles is lower than in sand, granules and pebbles or sand and granules, but once waves have reworked the sediment there is little difference in activation depths.     

Investigation of the effects of offshore breakwater parameters on sediment accumulation

Littoral sediment transport is the main reason for coastal erosion and accretion. Therefore, various types of structures are used in shore protection and littoral sediment trapping studies. Offshore breakwaters are one of these structures. Construction of offshore breakwaters is one of the main countermeasures against beach erosion. In this paper, offshore protection process is studied on the effect of offshore breakwater parameters (length, distance and gap), wave parameters (height, period and angle) and on sediment accumulation ratio, one researched in a physical model. In addition to the experimental studies, numerical model in which the formulae of the sediment discharge (i.e. the formulae of CERC and Kamphuis), was used was developed and employed. The results of the experimental and numerical studies were compared with each other.     

Wavelet bicoherence analysis of wind–wave interaction

A new technique in the analysis of wind–wave interaction, wavelet bicoherence, will be applied in this article. Wavelet bicohence has the ability to detect phase coupling and nonlinear interactions of the quadratic order with time resolution. It is used in this study to analyze wind–wave interaction during wave growth in a Mistral event. A selected record of simultaneously measured wind and wave data during Mistral is divided into five segments and the computations of the wavelet bicoherence are conducted for the whole record and for all divided segments. The results show that the phase coupling occurs between wind speed and wave height over a certain range of frequencies and that the range is different from one segment to another due to the non-stationary feature of the time series.     

拉格朗日拟序结构分析渤海湾岸线变化对物质输运的影响

科学评估围填海对海洋中物质输运的影响,对海洋环境保护和管理具有重要的意义。本文采用非结构网格有限体积海洋模型 （FVCOM） 模拟渤海湾 2004 年和 2018 年岸线变化前后水动力场,通过拉格朗日拟序结构的方法,评估岸线变化 对物质输运的影响。结果表明：岸线变化后,物质自近岸向湾内的扩散加强,且物质的输运通道和聚集区发生改变。曹妃甸附近原有清晰输运和聚集区域消失,物质向湾内输运;黄骅港阻碍了西部水体沿岸的南下路径,并在防沙堤顶部形成新的聚集区;渤海湾东南区域的物质输运通道向东延伸,输运能力加强,且聚集面积减小。     

Dynamics of a wave-dominated tidal inlet and influence on adjacent beaches,Currumbin Creek,Gold Coast,Australia

Currumbin Creek on the Australian Gold Coast is a wave-dominated tidal inlet which exhibits a particularly active morphology. The recent history of Currumbin Creek entrance has seen rapid growth of the entrance for access to the ocean by fishermen, as a world class surfing site, and as a recreational area. Before the construction of two groynes in the 70's, Currumbin Creek entrance was highly variable in terms of inlet location and sand bar characteristics due to a cyclical behaviour of spit migration. Nowadays, the entrance is stabilised. However, natural processes continue with the entrance infilling causing flood and navigation issues, resulting in a regular dredging program to maintain an open entrance and for regular beach nourishment plans.     

Numerical implementation and sensitivity analysis of a wave energy converter in a time-dependent mild-slope equation model

Several Wave Energy Converters (abbreviated as WECs) have intensively been studied and developed during the last decade and currently small farms of WECs are getting installed. WECs in a farm are partly absorbing, partly redistributing the incident wave power. Consequently, the power absorption of each individual WEC in a farm is affected by its neighbouring WECs. The knowledge of the wave climate around the WEC is needed to predict its performance in the farm. In this paper a technique is developed to implement a single and multiple WECs based on the overtopping principle in a time-dependent mild-slope equation model. So far, the mild-slope equations have been widely used to study wave transformations around coastal and offshore structures, such as breakwaters, piles of windmills and offshore platforms. First the limitations of the WEC implementation are discussed through a sensitivity analysis. Next the developed approach is applied to study the wave height reduction behind a single WEC and a farm. The wake behind an isolated WEC is investigated for uni- and multidirectional waves; it is observed that an increase of the directional spread leads to a faster wave redistribution behind the WEC. Further the wake in the lee of multiple WECs is calculated for two different farm lay-outs, i.e. an aligned grid and a staggered grid, by adapting the performance of each WEC to its incident wave power. The evolved technique is a fast tool to find the optimal lay-out of WECs in a farm and to study the possible influence on surrounding activities in the sea.