Morphodynamic classification of sandy beaches in low energetic marine environment

Morphodynamic classification of beaches has achieved widespread acceptance in both geological and geomorphological literature. In this sense, the present work classifies twelve Mediterranean low energetic beaches according to the dimensionless fall parameter (Ω) parameter in the Island of Mallorca. Propagation of 44 yr of wave data as well as a detailed sediment study allows to provide probabilities for morphodynamical beach state on an annual and seasonal basis. Consequently, beaches in Mallorca fall between three major categories which are (a) truly reflective, (b) reflective skewed to intermediate and (c) intermediate beaches. The Mallorcan beach position in the morphodynamical scheme is close related to the physiographical and geological framework. Comparison of observed values with those obtained in the analysis leads that for gross beach classification there is agreement between predicted and real state. However on a seasonal classification, mainly during summer, there is no agreement between the predicted state and the real one. As the model does not incorporate the role of summer sea breezes, beach reflective states are highlighted. Real beach configurations correspond to more energetic wave dynamic conditions and to intermediate state scenarios.     

Community structure of macrobenthos in two tropical sandy beaches with different morphodynamic features, Rio de Janeiro, Brazil

The macrobenthos of two exposed tropical sandy beaches in Rio de Janeiro (Brazil) were compared in relation to density, species richness, and vertical zonation. Biological and sediment samplings were carried out in the austral winter of 2002 and the austral summer of 2003. The sampling design consisted of 10 transects perpendicular to the water line, evenly divided into strata. A sampling unit was taken in each stratum with a 0.04 m² quadrat sampler. Beaches were also compared according to physical features, such as slope, wave period, wave height, and grain size. According to Dean's Ω morphodynamic index the Pontal is a dissipative beach while the Costa Azul is a reflective one. The mean grain size ranged from median to coarse sand in Costa Azul, whereas in Pontal it ranged from median to very fine sand. Eleven species were collected in the two beaches. Crustaceans were the dominant in the Costa Azul Beach, while the polychaete Scolelepis squamata dominated the Pontal beach. A negative correlation was found between the density of the macrobenthos and mean grain size, and beach slope. On the other hand, the Dean's parameter correlated positively with faunal density. Based on the results of ANOSIM, in both beaches, two groups of stations were identified, defining an upper and a lower beach zone along the vertical distribution of the macrobenthos.     

Macrobenthic zonation patterns along a morphodynamical continuum of macrotidal,low tide bar/rip and ultra-dissipative sandy beaches

The species composition, densities, biomass and zonation patterns of the macrobenthos of sandy beaches are greatly influenced by the morphodynamics and morphology of the beaches. Macrobenthic zonation patterns along a small-scale morphodynamic gradient, comprising eight Belgian beach sites, were investigated. By taking into account the dimensionless fall velocity (Ω) and the relative tidal range, the beach sites were ordered along the gradient from the ultra-dissipative beach type (UD) to the low tide bar/rip beach type (LTBR). The resulting beach state index varied between 1.8 and 4.2 and the beach profiles were related with the beaches' morphodynamic state.In total 35 macrobenthic species, mainly polychaetes and crustaceans, were encountered, varying between 19 and 23 species per beach site. The species composition was quite similar among beach sites, with Scolelepis squamata being abundant at all eight sites. Furthermore, the macrobenthic distribution patterns were mainly related to elevation at all beach sites. Some remarkable difference in metrics, largely related to the beach morphodynamics and the consequent hydrodynamics, were found. At the hydrodynamically benign and consequently macrobenthos-rich UD beaches, the highest macrobenthic densities and biomass occurred on the upper beach, while at the hydrodynamically harsh and thus macrobenthos-poor LTBR beaches, the maximum densities and biomass occurred lower on the beach. Species, typically occurring on the upper UD beaches, such as Eurydice pulchra, S. squamata, and Bathyporeia sarsi, were restricted to the sub-optimal middle and lower beach zone at LTBR beaches. Only Bathyporeia pilosa was found on the upper beach of both UD and LTBR beaches. The more robust polychaete Ophelia rathkei and the interstitial polychaete Hesionides arenaria were exclusively found in the hydrodynamically harsh conditions of the middle LTBR beach zone.     

华南水东湾波控、中等潮差岬湾海滩地形动力分类

海滩地形动力分类在国外的海岸地貌研究中已经被广泛接受。本文使用了华南粤西水东湾切线带、过渡带和遮蔽带海滩连续16个月32次大潮期间同步获取的波浪、潮汐、泥沙和海滩地形数据,分别按照无量纲沉降参数、相对潮差参数和无量纲海湾尺度参数对这一岬湾海滩不同岸段的海滩类型进行了研究,研究发现：（1）水东湾切线带海滩的主要状态为有裂流的低潮台地状态和沙坝型海滩状态,过渡带海滩主要状态是低潮沙坝/裂流海滩和沙坝消散型状态,遮蔽带海滩主要状态是有或无沙坝的消散型状态;（2）海湾不同岸段海滩状态的顺序变化与差异体现了岬湾海滩状态的时空变化性,与现场观测的海滩地形的变化基本一致,说明了对波控中到强潮海滩进行研究时,需要考虑潮汐的影响。同时,本文主要给出了海滩状态研究的一个框架体系,由于海滩不同的状态伴随不同的侵蚀模式,要求我国今后需加强在这一方面研究,以进一步丰富我国海滩地形演变、海滩地形动力过程和海滩防侵蚀的理论基础。     

Morphodynamics of structurally controlled headland-bay beaches in southeastern Brazil: A review

This paper presents a comprehensive review on the interaction between hydrodynamic processes, beach morphology and sedimentology at large scale coastal behaviour along the coastline of Santa Catarina, between Laguna and São Francisco Island, a microtidal east coast swell environment with headland and bay geomorphologies. The parabolic bay shape equation has proven to be a convenient and practical tool for studying the stability of the headland-bay beaches, tombolos, and salients in Santa Catarina. The beaches exhibit different patterns of sediment removal as a function of the degree of beach curvature. In highly curved beaches, there is a well-developed shadow zone and a range of morphodynamic conditions, from a sheltered low-energy beach adjacent to the downdrift headland to a high-energy exposed beach on the straight end of the headland-bay beach. The less curved beaches instead, tend to show more uniform behaviour since they are directly exposed to incident waves. There is no obvious relationship between average wave height and mean grain size, showing the importance of sediment source to characterize the sedimentary distribution patterns in the study area. The analysis of beaches showed that beach morphodynamics and sequence profiles for a bay–headland coast in a microtidal east coast environment is a function of geological inheritance (e.g., distance between headlands and orientation, nearshore and inner shelf morphology, coastal plain morphology, and sediment source), and hydrodynamic factors (wave conditions, oceanic wave exposure and relative tidal range).     

Formation of beach cusps in a wave tank

Beach cusps with a longshore spacing of 20 to 150 cm have been built by the continuous action of incident waves on a steep laboratory beach floor covered uniformly with a thin bed of glass beads. Breaking of incident waves was observed to induce vortices on the bed by interacting with swash motion along the beach face. Beach cusps formed when the value of a dimensionless parameter H_b/sgT_i² became smaller than 0.042; H_b is the breaking height of the incident waves, T_i their period, s the beach slope and g the acceleration due to gravity. This critical value occurred at a nearly central part of the generation region 0.003 < H_b/sgT_i² < 0.068 for plunging breakers presented by Galvin (1968). Breaking-wave-induced vortices rather than breaker types controlled the movement of bed material in the nearshore zone. Most of the measured spacings of beach cusps, including previous observations, were in good agreement with half a wavelength of the zero-mode subharmonic edge wave, which is generated on the beach by the refraction of incident waves and has twice the period of the waves. The role of edge waves at each stage of cusp formation still remains as an important problem to be clarified.     

Dean模型在山东半岛西北岸滩动力地貌研究的应用

根据山东半岛西北部25条实测岸滩剖面及相关表层沉积物、波浪、潮流资料,通过计算浪潮作用指数K和波浪-沉积物参数Ω,确定研究区主控动力因素并对海滩动力地貌类型进行划分,在此基础上应用Dean平衡剖面模型对实测剖面进行拟合,探讨了该模型的适用性及其参数物理意义等问题。山东半岛西北部整体为浪控海岸,刁龙嘴-三山岛岸段、海北嘴-石虎嘴岸段和三山岛-海北嘴岸段均属过渡型海滩,屺坶岛-栾家口岸段属消散型海滩。研究区实测剖面与Dean模型的拟合结果整体较好,部分剖面(剖面1、剖面2、剖面3、剖面5、剖面8)由于发育离岸沙坝,使其形态与Dean模型预测的平滑上凹的形态特点存在一定差别。屺坶岛-栾家口岸段(剖面19~剖面24)受外海侧桑岛及沿岸龙口人工岛群影响,与Dean模型预测结果存在较大差别,Dean模型对发育槽-坝体系的海滩的拟合仍具有局限性。Dean模型中m和A分别反映了海滩剖面反射性和泥沙沉速,与海滩动力地貌类型判别公式计算结果吻合较好。     

Tide-induced flow signature in rip currents on a meso-macrotidal beach

On rip-channelled beaches, intense rip currents are driven by waves due to alongshore variations in breaking-induced wave energy dissipation. This study addresses for the first time the potential development of tidal currents superimposed onto the wave-driven circulation. This phenomenon is observed on a rip-channelled meso-macrotidal beach (Biscarrosse, SW France). Field measurements show 20 to 45% stronger mean rip velocities during ebb than during flood. Numerical experiments reveal that this asymmetry is the signature of tidal currents developing over the rip channel morphology. This asymmetry is found to increase roughly linearly with increasing tidal range. These results are significant to beach safety and lifeguarding and stimulate further numerical exercises.     

Abundance,Tidal Movement,Population Structure and Burrowing Rate of Emerita analoga (Anomura,Hippidae) at a Dissipative and a Reflective Sandy Beach in South Central Chile

Abstract. To evaluate the effects of beach morphodynamics upon the abundance, tidal movement, population structure and burrowing rate of the crab Emerita analoga (Stimpson) (Anomura, Hippidae) we sampled two beaches in south central Chile (ca. 42° S), Mar Brava and Ahui with dissipative and reflective characteristics, res­pectively. The swash zone at the dissipative beach was 5 – 6 times wider than that of the reflective beach. A at the dissipative beach, upwash speeds were higher and the number of effluent line crossings were lower by more than an order of magnitude. To examine the tidal movement of E. analoga, we collected crabs from 5 to 6 tidal levels of each beach every 2 h across 12 h of the tidal cycle. The intertidal distribution of crabs differed between beaches; i. e., at the dissipative beach they were primarily located at the swash zone, while at the reflective beach they were mostly located at the low tide level and shallow subtidal. The change in position of crabs was pronounced across the tidal cycle at the dissipative beach (Mar Brava), with most of the animals remaining in the active swash zone. Body size data were used to construct size frequency distributions for each population. Crabs from the dissipative beach reached larger sizes than those at the reflective beach. Sediments were coarser at the latter versus the former beach. Crabs burrowed at similar rates in the sand from both beaches, a result which supports the idea that E. analoga is a “sediment generalist” capable of burrowing successfully in a wide range of sediment types. This characteristic is likely a key to the broad success of this species on the full range of beach morphodynamic types along the coasts of South and North America.     

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.     

The influence of a research pier on beach morphology and the distribution of Emerita talpoida

The spatial distribution of the common Atlantic Coast mole crab, Emerita talpoida, is strongly influenced by inshore processes and the grain size and morphology of sand beaches. E. talpoida increase in density across the foreshore toward the lower part of the beach (step), with high concentrations occurring within areas of reduced wave energy. Engineering structures extending across the foreshore, such as the Coastal Engineering Research Facility (research pier) at Duck, North Carolina, significantly alter wave action and thus modify the distribution of beach sediment and beach morphology. This, in turn, results in modifications of E. talpoida distributions. The pier's impact on the natural processes and patterns of both physical and ecological elements of the beach introduces an “uncertainty principle” that must be taken into consideration when designing experiments at research facilities.     

Application of the equilibrium planform concept to natural beaches in Northern Ireland

The equilibrium planform concept (EPC) for bayed beaches has achieved wide currency in coastal morphodynamics. The north coast of Ireland comprises a series of discrete headland-embayment beaches within which waves and currents recycle a finite sediment volume. It is therefore an ideal setting in which to explore the applicability of the concept. Application of the approach to 9 embayment beaches on the north coast of Ireland provides some insights into the application of the concept. The planform of some beaches does correspond to that predicted while others do not. Those whose measured planform does not correspond to the predicted planform can be interpreted through, (a) difficulty in identifying the wave diffraction point, (b) disequilibrium on the beach (sediment scarcity or excess), (c) geological control of beach morphology. The subjectivity in selecting the diffraction point renders alternative explanations difficult and reduces the utility of the approach on natural shorelines, where significant irregularities render identification of such points difficult.     

Wave height parameter for damage description of rubble-mound breakwaters

In this paper it will be shown that the wave height parameter H₅₀, defined as the average wave height of the 50 highest waves reaching a rubble-mound breakwater in its useful life, can describe the effect of the wave height on the history of the armor damage caused by the wave climate during the structure's usable life.Using Thompson and Shuttler (Thompson, D.M., Shuttler, R.M., 1975. Riprap design for wind wave attack: A laboratory study on random waves. HRS Wallingford, Report 61, UK) data it will be shown that H₅₀ is the wave parameter that best represents the damage evolution with the number of waves in a sea state. Using this H₅₀ parameter, formulae as van der Meer (van der Meer, J.W., 1988. Rock slopes and gravel beaches under wave attack. PhD Thesis. Technical University of Delft) and Losada and Giménez-Curto (Losada, M.A., Gimenez–Curto, L.A., 1979. The joint effect of the wave height and period on the stability of rubble mound breakwaters using Iribarren's number. Coastal Engineering, 3, 77–96) are transformed into sea-state damage evolution formulae. Using these H₅₀-transformed formulae for regular and irregular sea states it will be shown how damage predictions are independent of the sea state wave height distribution.To check the capability of these H₅₀-formulae to predict damage evolution of succession of sea states with different wave height distributions, some stability tests with regular and irregular waves have been carried out. After analysing the experimental results, it will be shown how H₅₀-formulae can predict the observed damage independently of the sea state wave height distribution or the succession of sea states.     

A new predictive formula for inception of regular wave breaking

Efforts are made to enhance the predictive formula for the inception of wave breaking. To achieve success, the existing formulas are extensively reviewed. They are categorized into four types, i.e., the McCowan type, the Miche type, the Goda type and the Munk type. The inherent relations among the different types are then exploited. The differences among each formula within a group are also discussed. Four representative formulas from the different types are chosen to compare with the measured data for a total number of 1193 cases reported in literatures. It is shown that Goda's and Ostendorf and Madsen's formulas are advantageous in general among the selected ones. Goda's formula, however, is found to be inaccurate as the beach slope becomes steeper than 1/10. Ostendorf and Madsen's formula is fairly good even for cases of very steep slopes, but its accuracy for the cases of ordinary slopes is not as good as Goda's. A new predictive formula for the inception of wave breaking is proposed. The unique index, defined by ψ′_b = (1.21 − 3.30λ_b)(1.48 − 0.54γ_b)ψ_b, where ψ_b = gH_b/C_b², H_b is the breaking wave height, C_b is the breaking wave celerity, λ_b is the breaking wave steepness, γ_b is the relative breaking wave height, and g is the gravity acceleration, is introduced. The incipient condition of wave breaking is then given by ψ′_b = 0.69. This formula is a significant improvement to the existing ones in terms of the accuracy. In addition, it is a local relation. Further verification shows that the proposed formula performs similarly well when applied to the field and to the waves over permeable bed.     

A morphodynamic model to simulate the seasonal closure of tidal inlets

Seasonally open tidal inlets usually occur in microtidal, wave-dominated coastal environments where strong seasonal variations of streamflow and wave climate are experienced. These inlets are closed to the ocean for a number of months every year due to the formation of sand bars across their entrances. The annual closure of these inlets inhibits ocean access for boats and could also cause deterioration of water quality in the estuary/lagoon connected to the inlet. As these estuaries/lagoons are commonly used as harbours or recreational facilities there is increased interest in keeping the inlets permanently open. A process-based numerical model capable of simulating inlet closure is invaluable in terms of identifying the natural processes governing inlet closure. As a further step, this type of model could also be used to determine the effect of any proposed engineering solutions to keep the inlet open on the adjacent beaches. A morphodynamic model capable of simulating the seasonal closure of inlets, which includes both longshore (LST) and cross-shore transport (CST) processes, was developed in this study. Application of the model to two idealised scenarios indicated that cross-shore processes govern inlet behaviour when LST rates were low. The Dean's criterion [Dean, R.G., 1973. Heuristic models of sand transport in the surf zone. Proc. Conf. on Eng. Dynamics in the Surf Zone, Sydney, pp. 208–214.] for on–offshore transport was employed to show that, for small offshore wave incidence angles, onshore transport aided inlet closure when the offshore wave steepness (H_o/L_o) was less than the critical wave steepness (H_o/L_o)_crit, while offshore transport helped to keep the inlet open when (H_o/L_o) was greater than (H_o/L_o)_crit. LST was found to be the dominant process leading to inlet closure when (H_o/L_o) was much larger than (H_o/L_o)_crit or when the offshore wave incidence angle was large.     

Statistical simulation of wave climate and extreme beach erosion

Recent developments in extreme values modelling have been used to develop a framework for determining the coastal erosion hazard on sandy coastlines. This framework quantitatively reproduced the extreme beach erosion volumes obtained from field measurements at Narrabeen Beach, Australia. This encouraging finding was achieved using Kriebel and Dean's [Kriebel, D.L. and Dean, R.G., 1993. Convolution method for time-dependent beach profile response. Journal of Waterway, Port, Coastal and Ocean Engineering, 119(2): 204–226.] simple beach erosion and accretion model. The method includes allowances for joint probability between all basic erosion variates including; wave height, period and direction, event duration, tidal anomalies and event spacing. A new formulation for the dependency between wave height and period has been developed. It includes the physical wave steepness limitation. Event grouping, where significantly more erosion can occur from two closely spaced storms is handled by temporally simulating the synthetic wave climate and the resulting beach erosion and accretion.     

Field evaluation of beach profile response to wave steepness as predicted by the Dean model

An evaluation of the Dean (1973) model of shore-normal sediment transport, using field data (N = 111) including daily beach profiles, suggests that the model correctly predicts 98% of the 45 erosional events when K = 2.0 and using breaker steepness. Only 45% of the 52 depositional beaches were correctly predicted by the model, whereas 14 beach profiles displayed essentially no change.     

Variations in grain‐size and sorting on two kaikoura beaches

Two exposed, high‐energy beaches on the Kaikoura coast of New Zealand are composed of sand and gravel derived from a greywacke terrain. Both beaches can be classified as mixed beaches although the sediment varies from dominantly gravel at the ends of the beach to dominantly sand at the centre, through transition zones in which sand and gravel are mixed. Sixty‐four surface samples were analysed for grain size; two sediment parameters, mean grain size (M_z) and sorting (σ_I), were calculated.

A striking feature of the cumulative frequency curves is that both unimodai and bimodal distributions include median sizes over the whole range of sampled material, even though bimodal samples display two strong modes in the sand and gravel grades. The general deficiency lof sediment dn the very coarse sand and granule classes (0 to — 2 _F ) noted by numerous authors in many parts of the world is apparent in the poorly‐sorted bimodal samples. However, the best‐sorted samples also occur in these two classes.

Mean grain size of samples ranges from medium sand (1.820) to medium pebbles (—4.7 _F ), and sorting ranges from very well sorted (0.250) to very poorly sorted (2.69 _F ). Mean erain size on the northern beach is significantly greater than on the southern beach, but values of sorting are comparable. The greater mean size on one beach compared with the other is thought to be a function of the grade of material supplied by local rivers; the similarity in sorting presumably reflects the similarity of the processes acting on the two beaches.

Mixed sand‐shingle beaches are relatively rare on a world scale but common in New Zealand. Sediment distributions along the Kaikoura beaches do not reveal a regular decrease in size away from the rivers which supply material to shore at present. Instead, the beaches are differentiated into a number of sediment zones composed of either sand, or mixed sand‐gravel, or gravel. On each beach a gravel zone is located furthest from the river outlets. Sorting generally improves toward the Kaikoura Peninsula. Explanations for these trends are not given. Variations in size and sorting across the two beaches do not show a well developed zonation because of the high level of wave energy which continually mixes the material across the beach.