Numerical modelling of erosion and accretion of plane sloping beaches at different scales

The paper focuses on the numerical simulation of erosion of plane sloping beaches by irregular wave attack in three wave flumes of different scales. One of the prime objectives of the tests was to provide a consistent data set for the improvement of numerical beach profile models. A practical application of this research with wave attack on plane sloping beaches is the erosion of the plane beaches after nourishment. Three models (CROSMOR, UNIBEST-TC and DELFT3D) have been used to simulate the flume experimental results focusing on the wave height distribution and the morphological development (erosion and deposition) along the beach profiles. Overall, the model predictions for wave heights show consistent results. Generally, the computed wave heights (H_rms and H_1/3) are within 10% to 15% of the measured values for all tests (under-prediction of the largest wave heights close to the shore). The three models can simulate the beach erosion of the wave flume tests (erosive tests) reasonably well using default values of the sand transport parameters. The model performance for the accretive tests is less good than that for the erosive tests. A practical field application of this research is the erosion of nourished beaches, as these beaches generally have rather plane beach slopes immediately after nourishment. Various graphs are given to estimate the beach erosion of nourished beaches.     

Morphodynamic response of a two-barred beach to a shoreface nourishment

The response to a shoreface nourishment of the two-bar system at Noordwijk (the Netherlands) is analyzed based on a daily data set of time-exposure video images collected during about 6 years, complemented with topographic and bathymetric surveys. The 1.7 Mm³ nourishment, implemented as a 3 km alongshore bump seaward of the outer bar, migrated more than 300 m onshore in 4 years before losing its integrity. Furthermore, the nourishment interrupted the autonomous seaward migration of both bars for the entire duration of the study period and, allaying earlier fears, did not intensify the three-dimensional patterns in the bars, such as the crescentic plan-shape and rip channels. The nourishment did result in clear head effects on both flanks, with the bar becoming discontinuous and the flank section decaying or becoming attached to an offshore-located bar, while the section of bar landward of the nourishment became attached to a landward-located bar. This sequence of morphologies is known as bar switching. Each switching episode took almost one year to complete and can therefore not be ascribed to individual wave events. We suspect that shoreface nourishments enhance the possibility of bar switching by creating alongshore variability in the position and depth of the outer bar and in its cross-shore migration rate and direction. The Noordwijk nourishment did not influence the shoreline position as its trend did not undergo distinctive variations after 1998. Differences in the response of the Noordwijk sandbars to the shoreface nourishment compared with other Dutch nourishments are attributed to the location and size (volume per unit length) of the nourishment with respect to the sandbars and to the median grain size of the nourished material.     

不同补沙方案对海滩剖面影响的数值模拟对比分析

大量工程实践表明,海滩养护是当前抵御海岸侵蚀的常见措施。通过人为地向海滩补充沙源,以达到海岸防护、修复沙滩等目的。本文建立了XBeach一维海滩剖面演变数值模型,并与物理模型试验结果进行对比验证;计算常浪条件下不同方案补沙后海滩剖面达到平衡状态的速率;通过对比补沙工况与未补沙工况的风暴后剖面,分析风暴作用下的防护效果。结果发现,补沙量较大时,沙坝向海侧位置补沙在常浪条件下补沙效率高于滩肩补沙,在风暴条件下防护效果优于滩肩补沙。此次研究对于在实际海滩养护过程中节约施工成本、提高施工效率有着重要意义,同时对于评估沙滩养护工程效果有一定的参考价值。     

Bars formed by horizontal diffusion of suspended sediment

The mechanism responsible for the ubiquitous presence of convex beach profiles and shoreward migration of linear bars is examined using numerical circulation and sediment transport models. The models are validated against laboratory measurements and observed natural beach cross-sections. While not discounting the importance of infragravity and advective horizontal circulation or bed-return flow mechanisms, a robust diffusive process explains the convex profile shape and bar formation. In the presence of concentration gradients across the surf zone, a diffusive sediment flux from high to low concentration results in the transfer of sediment outwards from the breakpoint, both onshore and offshore, and the subsequent formation of a “diffusion bar” and “diffusion profile”. The profiles are characterised by single- and double-convex dome-like shapes, developing during shoreward migration of the bars by the diffusion mechanism. The mechanism explains several phenomena observed on natural beaches, including (i) convex beach profiles; (ii) shoreward migration of the bar with concomitant beach accretion under narrow-band swell; (iii) reduced propensity for bar formation on low-gradient, fine-sand beaches or under wide-band wave spectra (even though multiple bars are common on some low-gradient beaches) and (iv) offshore migration of the bar during periods of increasing wave height. The diffusion mechanism can be dependent on orbital motion alone and, as such, requires no frequency selection or strong correlation between multiple processes for bar formation.     

Morphodynamics of intertidal bars on a megatidal beach, Merlimont, Northern France

The morphology, bedforms and hydrodynamics of Merlimont beach, in northern France, characterised by intertidal bars and a spring tidal range of 8.3 m, were surveyed over a 10-day experiment with variable wave conditions that included a 2-day storm with significant wave heights of up to 2.8 m. The beach exhibited two pronounced bar-trough systems located between the mean sea level and low neap tide level. Waves showed a cross-shore depth modulation, attaining maximum heights at high tide. The mean current was characterised dominantly by strong tide-induced longshore flows significantly reinforced by wind forcing during the storm, and by weaker, dominantly offshore, wave-induced flows. Vertical tidal water-level variations (tidal excursion rates) showed a bimodal distribution with a peak towards the mid-tide position and low rates near low and high water. The two bar-trough systems in the mid-tide zone remained stable in position during the experiment but showed significant local change. The absence of bar migration in spite of the relatively energetic context of this beach reflects high macro-scale bar morphological lag due to a combination of the large vertical tidal excursion rates in the mid-tide zone, the cross-shore wave structure, and the pronounced dual bar-trough system. The profile exhibited a highly variable pattern of local morphological change that showed poor correlation with wave energy levels and tidal excursion rates. Profile change reflected marked local morphodynamic feedback effects due mainly to breaks in slope associated with the bar-trough topography and with trough activity. Change was as important during low wave-energy conditions as during the storm. Strong flows in the entrenched troughs hindered cross-shore bar mobility while inducing longshore migration of medium-sized bedforms that contributed in generating short-term profile change. The large size and location of the two pronounced bars in the mid-tide zone of the beach are tentatively attributed respectively to the relatively high wave-energy levels affecting Merlimont beach, and to the cross-shore increase in wave height hinged on tidal modulation of water depths. These two large quasi-permanent bars probably originated as essentially breakpoint bars and are different from a small bar formed by swash and surf processes in the course of the experiment at the mean high water neap tide level, which is characterised by a certain degree of tidal stationarity and larger high-tide waves.     

Process-based modelling of a shoreface nourishment

A state-of-the-art process-based model is applied to hindcast the morphological development of a shoreface nourishment along the barrier island of Terschelling, The Netherlands. The applied morphodynamic model is Delft3D for fully three-dimensional flow and sediment transport in coastal environments. Owing to a complex geomorphological setting of the study area, the curvi-linear model includes adjacent tidal inlets and covers 40×70 km with an increasing grid size resolution towards the nourishment site in the center of the island. In 1993, a total of 2 Mm³ of sand was supplied to the nearshore bar zone off Terschelling, filling up the trough between the middle and outer bar. By spring 1994, most of the nourished sediment had been redistributed onshore and welded onto the middle bar where it remained in the following years. The morphodynamic model is applied to the prediction of this rapid nearshore profile behaviour. The calibration of the model against an extensive set of full-scale hydrodynamic data at several locations throughout the nearshore bar zone shows a good representation of the measured hydrodynamics. Morphodynamic results show a dependency on spatial scale: on the scale of precise bed level evolution with respect to bar migration and growth, model predictions are poor as the nearshore bars are predicted to flatten out. Resorting to bulk volumes integrated over larger spatial scales, the model clearly has skill in predicting the overall effects of the nourishment. The lack of phase shift between sediment transport and bathymetry is identified as the key controlling factor for the poor sandbar predictions.     

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.     

Morphodynamic response of nearshore bars to a shoreface nourishment

The autonomous nearshore bar behaviour along the barrier island of Terschelling, The Netherlands, is characterised by the presence of net seaward cyclic migrating sand bars generated near the shoreline. In 1993, a perturbation of the cyclic bar system was introduced by the implementation of a 2 Mm³ shoreface nourishment supplied to the nearshore bar zone, filling up the trough between the middle and outer bar. The morphodynamic response of the nearshore bars to the nourishment perturbation is investigated using a bathymetric data set with an alongshore extent of 12 km and sampled for 10 years. Bar behaviour is quantified in terms of bar crest position in relation to morphometric parameters such as bar depth, height, width and volume. Along with a pronounced development of a three-dimensional bar system unseen in the autonomous behaviour, the nearshore bars exhibited a 6–7 year arrest in their migrational behaviour during which bar morphology remained stable at immediate pre-nourishment morphometric values. At the subsequent onset of bar movement, bars resumed their migration at a rate predicted by autonomous behaviour in parallel development with morphometric parameters along their predicted trends. It is shown that the observed onshore transport of nourished sediment in the 6–7 year arrest results from a gradual deepening of troughs. Cross-shore sediment transport modelling is used to assess the effect of the nourishment on yearly averaged onshore (short-wave nonlinearity) and offshore (undertow) sediment transport rates. The gradual reappearance of the pre-nourishment bar-trough morphology is shown to engender a normalisation in the cross-shore distribution of sediment transport rates to pre-nourishment rates.     

近岸人工沙坝保滩促淤的试验研究

借助正态波浪模型,对北戴河海滩某横断面有、无沙坝布置进行了3种水位的极限波高入射试验,测取了坝内外波高过程线及稳定后的断面地形。试验资料及理论分析显示:有无沙坝时沿岸流及波浪的挟沙能力之比与水下沙坝的波浪透射系数的平方成正比,按试验最大透射系数计算的挟沙能力亦可降50%左右,且有沙坝上涌带破波回流对岸滩的冲刷水深可以维持在平均海平面以下1 m内,说明人工沙坝的填筑很大程度促使水体挟带泥沙于掩护区域落淤有效的阻止了泥沙的离岸亏损。试验将为保滩促淤工程设计与实施提供科学依据。     

海岸沙坝剖面和滩肩剖面特征研究

通过物理模型实验研究了海岸沙坝剖面和滩肩剖面的形成和演化过程,给出了稳定的沙坝剖面和滩肩剖面的几何特征。实验中考虑了两种初始坡度(1∶20和1∶10)和不同波高的规则波和不规则波,讨论了不同初始坡度海岸上破碎波空间分布特征。结果表明,沙坝产生后存在向岸和离岸两种运动形态,但最终将停留在稳定位置。稳定的沙坝剖面对应不同初始坡度和波浪存在不同的大沙坝和小沙坝分布。沙坝剖面由长时间小波高波浪序列作用后可转化为稳定滩肩剖面,该剖面不依赖于波浪和初始坡度。实验也给出了平衡剖面与理论曲线的对比以及剖面上泥沙粒径的分布。     

Multigrain sedimentation/erosion model based on cross-shore equilibrium sediment distribution: Application to nourishment design

In the light of global warming and sea level rise there are many coastal beaches that suffer from erosion. Beach nourishment has become a common practice to maintain the sediment balance on a shore-face. In this paper, a three-dimensional numerical model for evaluating long-term impact of beach nourishment projects has been developed. The model addresses the longstanding complex issue of coastal morphology and sediment grain size distribution from an unconventional angle, which exploits the strong links between grain size distribution and the prevailing transport direction of each sediment constituent under ‘average’ wave and storm action. The present model predicts the redistribution of nourished sediment according to the subtle clues implied by equilibrium distribution curves and latest coastal wave transformation theories. After verification against recent field observations in Terschelling, The Netherlands, the model was used to predict long-term effects of different beach nourishment strategies. It was found that: (a) given the source sediment available in Terschelling the tactics of large volume and less frequent implementation are better than otherwise; and (b) from a pure engineering point of view, waterline nourishment outperforms offshore trough nourishment.The model offers an additional tool for coastal engineers to evaluate the feasibility, effectiveness and the optimization of dumping locations for beach nourishment projects. It is also a useful tool for stratigraphic modelling of shallow-marine sedimentation in conjunction with sea level changes.     

The morphological response of a nearshore double sandbar system to constant wave forcing

Results of 2DH morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing. Sixteen different conditions varying both offshore wave height and angle of wave incidence were applied. The mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars. For similar longshore velocities, the length scales of the outer bar are larger than of the inner bar. This is explained by accounting for the difference in water depth above the bar crest. The variable morphological response times can be explained by including additional bathymetrical parameters. The active volume of the bar, defined by the breaker index, plays an important role in this response time. With larger active volumes the bar responds more rapidly to identical boundary conditions. Also, bars with a smaller total volume respond more quickly. This faster response is due to the steeper active volume of the bars. Different initial perturbations resulted in different locations of the emerging features, showing that their location is sensitive to the initial bathymetry. However, the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions. Based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing.     

Appraisal of storm beach buffer width for cyclonic waves

The loss of beach sand from berm and dune due to high waves and surge is a universal phenomenon associated with sporadic storm activities. To protect the development in a coastal hazard zone, hard structures or coastal setback have been established in many countries around the world. In this paper, the requirement of a storm beach buffer, being a lesser extent landward comparing with the coastal setback to ensure the safety of infrastructures, is numerically assessed using the SBEACH model for three categories of wave conditions in terms of storm return period, median sand grain size, berm width, and design water level. Two of the key outputs from the numerical calculations, berm retreat and bar formation offshore, are then analysed, as well as beach profile change. After having performed a series of numerical studies on selected large wave tank (LWT) test results with monochromatic waves using SBEACH, we may conclude that: (1) Berm erosion increases and submerged bar develops further offshore as the storm return period increases for beach with a specific sand grain size, or as the sand grain reduces on a beach under the action of identical wave condition; (2) Higher storm waves yield a large bar to form quicker and subsequently cause wave breaking on the bar crest, which can reduce the wave energy and limit the extent of the eroding berm; (3) A larger buffer width is required for a beach comprising small sand grain, in order to effectively absorb storm wave energy; and (4) Empirical relationships can be tentatively proposed to estimate the storm beach buffer width, from the input of wave conditions and sediment grain size. These results would benefit a beach nourishment project for shore protection or design of a recreational beach.     

Submarine bars on sandy coasts

A brief review of the studies concerning the problem of submarine bars over the past decade is presented. Various types of bars are distinguished, and the mechanisms responsible for the formation thereof are discussed. The short-term (days, weeks) behavior of the bars is described in relation to the changes in the local wave parameters. The bar systems are shown to demonstrate cyclic behavior of two different types on time scales of years and decades. In the former case, the bars arise near the shore, migrate towards the sea, and degrade in the external margin of the coastal zone. The other cycle’s type is characterized by the landward migration of the bars and their welding with a subaerial beach, which results in creating a conveyor delivering material to the beach and eventually to the foredune by Aeolian transport. In the former case, the bar zone behaves as a closed system, while, in the latter, as a transition zone. It is noted that the long-term evolution of a bar system is controlled by a feedback mechanism tending to return the system to the initial state.     

ONWET: A Simple Integrated Tool for Beach Morphology and Wave Dynamics Analysis

The analysis of beach profile combined with wave data provides a clear understanding of beach topography and physical behavior of the coast. Numerous commercial and noncommercial computer programs and add-on tools are available to analyze beach profile and wave data. But, most of them are specifically designed for particular coastal application. Moreover, searching of open source software for diverse coastal application is a tedious process. To make it easily accessible, we have developed an integrated Matlab GUI for beach morphology and wave analysis. The tool “ONWET (OceaN WavE Tool)” mainly includes wave processes, beach morphology, spatiotemporal variation in beach profile, wave refraction, and sediment transport rate. The tool has automated inbuilt functions and is designed to accommodate importing/exporting of data (results both in data and graphical format). It includes user friendly options and can be freely download from our institutional website. The tool is very useful for researchers, scientists, engineers, and environmentalists who are working in the field of coastal studies and beach nourishment.     

不同沉积物养护海滩对台风响应的差异性研究

本文通过对厦门天泉湾人工卵石滩和会展人工沙滩在1614“莫兰蒂”超强台风影响前后的典型剖面监测，结合水文动力要素的观测和数值模拟，计算了台风影响过程的波浪场、总水位，分析了剖面形态和台风过程的剖面平均变化量。结果表明，强潮海岸人工卵石滩与人工沙滩对台风响应的特征明显不同，人工卵石滩横向上大部分卵石向岸输移堆积，滩面侵蚀，滩肩堆积形成更高的风暴滩肩，坡度明显变陡。而人工沙滩则表现为明显的沉积物离岸输运，上部滩面侵蚀，下部滩面淤积，滩面坡度明显变缓，受台风登陆后的强烈向岸风作用，滩肩顶有所夷平，滩肩高度变化很小。海滩滩肩在台风过程中是否侵蚀与台风登陆和影响过程的总水位(天文潮、风暴增水、波浪爬高)密切相关，两个人工海滩的风暴响应模式均为冲蚀；台风影响过程中，波浪能量相对强、滩面坡度相对陡的人工卵石滩比人工沙滩的剖面平均变化量小，对于台风的响应程度小，在强侵蚀高能海岸采用砾石等粗粒径沉积物进行海滩养护是减缓砂质海滩侵蚀的一种有效手段。     

Formation of Equilibrium Beach Profile of the Abandoned Yellow River Delta Coast in North Jiangsu

The abandoned Yellow River Delta coast is a typical erodible silty and muddy coast in China. The paper analyses the marine dynamic characteristics and the mechanism of beach erosion of this area. Analysis and calculation show that in this sea area wave and tidal current action should be considered. Based on the above analysis, an equilibrium beach profile calculation model is developed, in which the wave-current interaction is considered while sediment supply and sediment re-deposition are neglected. The model consists of four parts: (1) calculation of wave parameters, (2) calculation of velocity due to wave-current interaction at different water depth, (3) calculation of friction velocity and shear stress at different water depths, and (4) calculation of the amount of sediment erosion, erosion intensity and variation of beach profile. Calculated results are in good agreement with observed data. Finally, the evolution tendency is discussed and the equilibrium beach profile of this coast is calculated. B     

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.     

台风"艾利"对福建沙质海滩影响过程研究

通过对福建沙质海滩地形地貌现场调查,对比分析0418号台风“艾利”登陆福建前后各沙滩剖面的蚀淤变化,探讨了福建沙滩对台风“艾利”响应规律。研究结果表明,2004年8月底台风“艾利”过境期间,福建沿海砂质海岸显出了较强烈的地貌响应。沙滩剖面地形变化,冲淤相间,以冲蚀为主;后滨沙丘蚀退、滩肩蚀低、岸线向陆迁移是地貌响应的主要特征;沙滩沉积物的总收支趋于亏损。不同海岸类型的沙滩剖面对台风“艾利”的响应特征不同。岬湾型沙滩剖面地貌响应以前滨滩面侵蚀为主要特征。夷直型的长乐江田下沙剖面,对“艾利”台风的主要地貌响应特征为后滨沙丘、高潮带侵蚀,岸线后退超过16m,出现高度近1 m的直立侵蚀陡坎。沙坝-泻湖型沙滩剖面冲淤变化的主要特征是中潮带形成次一级沙坝。各沙滩的地貌风暴效应有自北往南依次减弱的趋势。     

Chronic offshore loss of nourishment on Nice beach,French Riviera: A case of over-nourishment of a steep beach?

The 4.5 km-long gravel beach fronting the exclusive resort of the city of Nice, on the French Riviera, in southeastern France, was artificially nourished from 1976 to 2005 to the tune of 558,000 m³, making this long-term operation one of the most significant for gravel beaches in the world. Nourishment has ranged from nil in certain years (1979, 1980, 1983–85, and 2001–2002) to a peak of over 97,000 m³ in 2000. Analyses of 50 transects covering the beach highlight no significant change in net beach width over this 30-year period of massive gravel nourishment. A Principal Components Analysis and a Cluster Analysis used to detect patterns in the 87 beach-width measurement dataset show no clear spatial trends in transect groups that can be interpreted in terms of the morphology of the beach and the steep inner shoreface. Significant wave height off Nice shows no change over the period 1979–2005. Since there is no possibility for alongshore gravel leakage on the strongly embayed Nice beach, the relative stability in beach width clearly implies loss of recharged gravel offshore. Gravel loss following nourishment is favoured by: (1) the steep inner shoreface inherited from the geological context of Nice beach at the flanks of the southern Alps, and (2) the practise of artificial beach widening through flattening, in summer, of a narrow (5–15 m-wide) mobile zone of the profile in order to enhance the ‘carrying’ capacity of this highly touristic beach. Beach widening and flattening following nourishment bring close to the very steep inner shoreface zone several cubic metres of gravel for each metre of beach that may be permanently lost downslope during autumn and winter storms. Recharged gravel is redistributed alongshore and offshore leakage is probably enhanced where small narrow submarine canyon heads impinge on the beach, resulting in a very narrow shoreface. Mean beach width shows an oscillating alongshore pattern that may be due to the influence of these canyons as pathways of gravel loss offshore. However, there is no correlation between mean beach width and distance to the 10-m isobath, used as a surrogate for inner shoreface width. Storms are associated with plunging waves that are particularly effective and concentrated, on this almost tideless shore, over the narrow mobile zone of the beach profile where a series of steep reflective berms are built during storms. The high dynamic pressures associated with this narrow zone of concentrated wave breaking, and energy reflection from the steepened profile, are deemed to contribute to the permanent downslope loss of gravel. This situation of long-term gravel loss is probably accepted by the beach management authority because of the low cost of obtaining nourishment material and the advantages derived from a temporarily wider beach in terms of recreational space.