The role of bore collapse and local shear stresses on the spatial distribution of sediment load in the uprush of an intermediate-state beach

Spatial variations in sediment load in the swash uprush and textural properties of sediment in transport were evaluated to investigate the mechanisms responsible for sediment transport during wave uprush. Four streamer traps were deployed at 2.0-m intervals across the swash zone of a sheltered, microtidal sandy beach at Port Beach, Western Australia, over a 4-day period. During these trapping experiments, offshore significant wave heights were 0.3–0.5 m and wave periods were about 10 s. The average width of the uprush zone was 6.9 m and the average uprush duration was 5.9 s. Cross-shore distributions of sediment load for 70 uprush events reveal a maximum in sediment load landward of the base of the swash (at about 20% of swash width) during single events and a maximum closer to mid-swash (at about 40% of swash width) during multiple events characterized by swash interactions. Settling velocity distributions of trap samples during individual uprush events are similar to distributions found on the beach surface, with the lowest settling velocities (finest sediments) near the base of the swash zone and maximum settling velocities (coarsest sediments) around the mid-swash position. It was found that sediment transport during wave uprush occurs through two distinct mechanisms: (1) sediment entrainment during bore collapse seaward of the base of the swash zone and subsequent advection of this bore-entrained sediment up the beach by wave uprush; and (2) in situ sediment entrainment and transport induced by local shear stresses during wave uprush. Both mechanisms are considered important, but the first mechanism is considered most significant during the early stages of wave uprush when sediment is transported mainly in suspension, while the second mechanism is likely to dominate the mid- to later stages of wave uprush when sediment is transported mainly by sheet flow. The relative importance of the two mechanisms will vary between different beaches with the morphodynamic state of the beach (reflective versus dissipative) expected to play a major role.     

High-frequency sediment-level oscillations in the swash zone

Sediment-level oscillations with heights of about 6 cm and shore-normal lengths of order 10 m have been measured in the swash zone of a high-energy, coarse-sand beach. Crests of oscillations were shore parallel and continuous alongshore. The oscillations were of such low steepness (height-to-length ratio approximately 0.006) that they were difficult to detect visually. The period of oscillation ranged between 6 and 15 min and decreased landward across the swash zone. The sediment-level oscillations were progressive landward with an average migration rate in the middle to upper swash zone of 0.8 m min⁻¹. Migration was caused mostly by erosion on the seaward flank of the crest of an oscillation during a period of net seaward sediment transport. Thus, the observed migration was a form migration landward rather than a migration involving net landward sediment transport. The observed sediment-level oscillations were different than sand waves or other swash-zone bedforms previously described.     

杭州湾淤泥质海岸岸线变化及其动态模拟

通过对杭州湾北岸南竹港-龙泉岸段实测岸滩断面资料的统计分析,进而对淤泥质海岸岸线变化、影响因素及其动态模拟预报进行探讨。结果表明:因长江来沙减少以及侵蚀/淤积波自东向西移动,导致近10a来该岸段岸滩处于侵蚀状态,其中岸线整体后退;因季节性的波侯作用,岸滩具有冬淤夏冲的特征,岸线则表现为冬涨夏退,因局部工程作用影响的岸线则具有冬退夏涨的特征。此外,基于-3m和-8m等深线构建的径向基函数神经网络模型预报岸线的变化是可行的。     

Relationships Between Community Structure of the Intertidal Macroinfauna and Sandy Beach Characteristics Along the Chilean Coast

Abstract. Eight sandy beaches were seasonally sampled along the coast of Chile, from ca. 21 to 42° S (about 3000 km) to study the relationship between community structure of the intertidal macroinfauna and beach characteristics. Sediment samples (0.1 m², 30 cm deep) were collected (July – September 1998 and December 1998 – January 1999) with plastic cylinders at 15 equally spaced levels along three replicated transects extending from above the drift line to the swash zone. The sediment was sieved through a 1 mm mesh and the organisms collected stored in 5 % formalin. To define beach types, Dean's parameter (Ω) was calculated from wave heights and periods, and fall velocity of sand particles from the swash zone. Crustaceans (mainly peracarids) were the most diverse group with 14 species, followed by polychaetes with 5 species. The talitrid amphipod Orchestoidea tuberculata , the cirolanid isopods Excirolana braziliensis and E. hirsuticauda and the anomuran decapod Emerita analoga were the most widely distributed and common species. Regression analyses between species richness, abundance and biomass of the whole macroinfauna versus sediment characteristics, beach face slopes and morphodynamic beach states showed no significant relationships. Thus, macroinfaunal community characteristics did not increase linearly from lower intermediate to higher intermediate or dissipative beach states as had been found before in Chile or in other coasts. A comparative analysis with data from sandy beaches of other world regions showed that the number of species inhabiting Chilean sandy beaches was generally lower, whereas total population abundances were generally higher compared with values reported elsewhere.     

Coastal Placer Deposits of Central Tamil Nadu, India

Placer mineral exploration has been undertaken along the beach of Central Tamil Nadu coast from Pondicherry to Vedaranyam. On the basis of the drainage network, geomorphology, and the coastal environment, the study area has been grouped into three sectors, North, Central, and South. Heavy mineral by Wt% shows a slightly higher abundance in the Northern sector, an enrichment of opaques in the Central sector from stations between Poompuhar and Karaikal, and a total depletion in the Southern sector. An abundance of heavies in the Northern sector is considered to be the result of a higher wave energy, and the cymatogenic downwarping of the basin during the present transgression. An enrichment of heavies in the Central sector from Poompuhar to Karaikal is attributable to the reworking of the beach ridges, which were submerged during the present transgression, and to the role of density sorting. A depletion of heavies in the Southern sector is accounted for by the absence of a terrigenous supply and the prevalence of wave shadow conditions throughout the year. The nature of the heavy mineral assemblage reflects the derivation of sediments principally from khondalites, granites, metamorphic rocks and paleo-sediments. Factor analysis also supplements the predominant role of a density factor in the segregation of heavy minerals in the study area. A five-stage model is proposed for the formation of placer deposits in the study region. The present study has disclosed rich concentrations of ilmenites in the central sector between Poompuhar and Karaikal that can be commercially exploited.     

海滩层理中的反递变纹层

本文提出海滩反递变纹层自下而上粒度由小到大,重矿物富集于纹层底部,它是前滨冲流“剪切分选”的产物。原生反递变纹层常被激浪破坏和再造,遇到后期加积海滩过程时,才能保存于海滩层理中。海滩层理的现场观测是研究海滩层理反递变纹层形成机理的重要方法之一。     

Coastal Placer Deposits of Central Tamil Nadu,India

ABSTRACT

Placer mineral exploration has been undertaken along the beach of Central Tamil Nadu coast from Pondicherry to Vedaranyam. On the basis of the drainage network, geomorphology, and the coastal environment, the study area has been grouped into three sectors, North, Central, and South. Heavy mineral by Wt% shows a slightly higher abundance in the Northern sector, an enrichment of opaques in the Central sector from stations between Poompuhar and Karaikal, and a total depletion in the Southern sector. An abundance of heavies in the Northern sector is considered to be the result of a higher wave energy, and the cymatogenic downwarping of the basin during the present transgression. An enrichment of heavies in the Central sector from Poompuhar to Karaikal is attributable to the reworking of the beach ridges, which were submerged during the present transgression, and to the role of density sorting. A depletion of heavies in the Southern sector is accounted for by the absence of a terrigenous supply and the prevalence of wave shadow conditions throughout the year. The nature of the heavy mineral assemblage reflects the derivation of sediments principally from khondalites, granites, metamorphic rocks and paleo-sediments. Factor analysis also supplements the predominant role of a density factor in the segregation of heavy minerals in the study area. A five-stage model is proposed for the formation of placer deposits in the study region. The present study has disclosed rich concentrations of ilmenites in the central sector between Poompuhar and Karaikal that can be commercially exploited.     

Sedimentology and morphodynamics of a macrotidal beach, Pendine Sands, SW Wales

The foreshore of Pendine Sands forms the seaward part of an extensive, sandy coastal barrier in a shallow Carmarthen Bay, SW Wales. The sedimentological features of the macrotidal foreshore reflect a tide-induced modification of nearshore wave characteristics. As the tide ebbs, the breaker height may decrease, the surf zone widens and becomes increasingly dissipative, and swash/backwash velocities diminish. A concomitant change from plunging to spilling breakers and increasingly symmetrical swash zone flows are associated with a decreasing beach gradient.

A zero net transport model demonstrates that the beach profile is self-stabilising in the short-term, and periodic levelling has shown that the beach is in long-term equilibrium with prevailing conditions, though this does not preclude a significant dynamic response to changing tides and waves.

The flow regimes of wave-generated currents decline as the tide ebbs, and normal beach processes do not usually affect the lower foreshore. Accordingly, there is an overall seaward-fining of the primary framework component of the sands. In more detail, this framework component displays a slight seaward-coarsening across an upper foreshore dominated by high water swash and surf; a rapid seaward-fining across the mid-foreshore in response to the ebb-attenuating swash zone flow velocities; and a slight seaward-fining across the lower foreshore under the action of nearshore shoaling waves. Bedforms vary from a swash/backwash emplaced flat bed across the upper foreshore to the small ripples of nearshore asymmetric oscillatory flows across the lower foreshore.

The surface sediment veneer is not representative of the subsurface sediments which form in response partly to fairweather conditions, partly to storms. The upper foreshore is characterised by swash/backwash emplaced plane bedding in fine sands frequently disrupted by bubble cavities. The mid-foreshore is composed of coarser-grained shelly traction clogs arranged as landward- and seaward-dipping large-scale cross bedding and/or plane bedding; these are probably storm breaker/surf deposits. The lower foreshore, though partially and sometimes totally bioturbated, shows landward-dipping small-scale cross bedding in very fine sands sorted by nearshore shoaling waves.

Tide- and storm-induced modification of the nearshore flow regimes therefore produces a distinctive shore-normal array of sedimentary facies. Each facies is characterised by diagnostic textural and structural signatures. A prograding sequence of such macrotidal deposits would be similar to, but more extensive than, a comparable microtidal sequence.     

Morphodynamic zone variability on a microtidal barred beach

This paper builds on the work of Masselink [Masselink, G., 1993. Simulating the effects of tides on beach morphodynamics. J. Coast. Res. SI 15, 180–197.] on the use of the residence times of shoaling waves, breaking waves and swash/backwash motions across a cross-shore profile to qualitatively understand temporal beach behaviour. We use a data set of in-situ measurements of wave parameters (height and period) and water depth, and time-exposure video images overlooking our single-barred intertidal measurement array at Egmond aan Zee (Netherlands) to derive boundaries between the shoaling zone, the surf zone and the swash zone. We find that the boundaries are functional dependencies of the local relative wave height on the local wave steepness. This contrasts with the use of constant relative wave heights or water levels in earlier work. We use the obtained boundaries and a standard cross-shore wave transformation model coupled to an inner surf zone bore model to show that large (> 5) relative tide ranges (RTR, defined as the ratio tide range–wave height) indicate shoaling wave processes across almost the entire intertidal profile, with surf processes dominating on the beach face. When the RTR is between 2 and 5, surf processes dominate over the intertidal bar and the lower part of the beach face, while swash has the largest residence times on the upper beach face. Such conditions, associated with surf zone bores propagating across the bar around low tide, were observed to cause the intertidal bar to migrate onshore slowly and the upper beach face to steepen. For RTR values less than about 2, surf zone processes dominate across the intertidal bar, while the dominance of swash processes now extends across most of the beach face. The surf zone processes were now observed to lead to offshore bar migration, while the swash eroded the upper beach face.     

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.     

The Beach Environment in Mexico as a Potential Source of Placer Minerals

The potential of Mexico for placer minerals in beach sediments has been analyzed, taking into consideration the geomorphic, tectonic, and lithologic conditions of nine different coastal regions. Besides, the textural characteristics of average size and degree of classification, carbonate content was also considered as a measure of terrigenous influence. It is concluded that mineral bearing rocks in the source area, and well-classified fine sands, poor in carbonates, in the beach area represent the major potential for placer minerals. Favorable conditions for placer minerals may be present in Regions 2 and 3 in the Central and Southern portion of the Gulf of Mexico coast, and have their highest potential in Regions 6, 7, 8, and 9 in the Gulf of California and along the Western mainland coast from the Colorado River in the North to the Guatamalan border in the South.     

The Beach Environment in Mexico as a Potential Source of Placer Minerals

The potential of Mexico for placer minerals in beach sediments has been analyzed, taking into consideration the geomorphic, tectonic, and lithologic conditions of nine different coastal regions. Besides, the textural characteristics of average size and degree of classification, carbonate content was also considered as a measure of terrigenous influence. It is concluded that mineral bearing rocks in the source area, and well-classified fine sands, poor in carbonates, in the beach area represent the major potential for placer minerals. Favorable conditions for placer minerals may be present in Regions 2 and 3 in the Central and Southern portion of the Gulf of Mexico coast, and have their highest potential in Regions 6, 7, 8, and 9 in the Gulf of California and along the Western mainland coast from the Colorado River in the North to the Guatamalan border in the South.     

Mineralogical Characterization in the Marine Environment of Kalpakkam,Southeast Coast of India

Sediments from Kalpakkam in the southeast coast of India were analyzed to characterize heavy mineral distribution. The mineral deposits are fine-grained in nature, and temporal and spatial variations are observed in mineral deposits. Iron mineral peaks observed indicate that hematite is present in the samples. The abundance of iron-based minerals explains the occurrence of black grains in the beaches of Sadras and Kokillamedu. In the rest of the region, the mineral distribution is found to be poor and patchy. Uranium-based radioactive minerals are present in the study area, viz., Uranmicrolite, Dessauite, Dumontite, and Sedovite. The common light minerals are quartz and caminite and the heavy minerals that are not iron-rich are tungusite, charoite, glagolevite, mangazeite, falkmanite, augelite, and struvite. Heavy minerals concentrated in the intertidal tide zone show well-developed swash marks by their typical alignment during the process of swash and backwash. Caminite is formed due to the reaction of seawater with hydrothermal fluids in a midoceanic ridge; its presence in the beach indicates the extent of onshore-offshore sediment transport. The heavy mineral distribution pattern along the Kalpakkam-Mahabalipuram stretch reveals the influence of energy conditions and energy fluctuations that are capable of transporting high-density minerals.     

粤东后江湾冲流带滩面高频振动及动力作用分析

利用涌浪影响下短时段内的冲流带滩面高频高程数据和碎波带波流资料,在奇异谱分析(SSA)的基础上,对比研究了不同形态滩面的冲淤变化趋势、趋势分布形状、冲淤变化周期和冲淤变化强度,以及同一条剖面不同桩点间各因素间的变化关系;用交叉谱方法探索了每分钟滩面高频冲淤变化与碎波带长重力波间的作用关系。分析结果表明,滩角韵律地形引起的冲流分流作用促进了滩脊向滩谷的泥沙转运,冲流带滩面存在明显的长重力波频段的周期性冲淤振动,滩面冲淤振动强度由滩面下部向上部递减,碎波带长重力波对滩面高频冲淤变化起重要作用。     

Structures in deposits from beach recovery, after erosion by swell waves around the southwestern coast of Aruba (Netherlands Antilles)

Hurricane- or storm-generated swell waves may cause erosion and deposition along coasts which are situated thousands of kilometers outside the generating wind field. Marked beach erosion, caused by such swell waves, was observed along the micro-tidal west coast of Aruba. During the process of erosion a swash bar was formed, which moved up-beach during the waxing part of the swell event. The swash bar welded to the beach during the waning part of the event. Rapid sedimentation occurred on the upper beach. Finally, recovery of the beach was observed. The formation of a swash bar was attributed to an erosive, dissipative interval of a normally accretionary reflective beach. The sedimentary structures, although generally in line with observations on other beaches, show several peculiar characteristics: (1) the great thickness of the laminae in these calcareous sands; (2) the succession of low-angle sigmoidal and tangential sets in the swash bar; (3) the relatively steep erosional lower set boundaries and the wedge-shaped lamination in the successive stages of beach recovery; and (4) the several types of deformation structures.     

Distribution patterns of interstitial polychaetes in sandy beaches of southern Brazil

This study describes the distribution patterns of interstitial polychaetes along morphodynamic gradients on six exposed sandy beaches in Santa Catarina and Paraná (South Brazil). Three random transects were sampled at two points on each beach, one at the swash and another at the surf zone, in winter and summer conditions. Six sediment replicates were collected at each sampling point using a corer of 4.6 cm internal diameter that removed 10 cm into the sediment. Abundance and composition of interstitial polychaete were correlated to wave height, slope, grain size, CaCO₃, chlorophyll a , omega indexes, temperature and relative tide range using a canonical correspondence analysis (CCA). A factorial ANOVA showed that taxa richness, mean density and Shannon's diversity were significantly higher at the reflective beaches, but average values differ significantly between transects and these differences change according to the beach zones on both sampling dates. PERMANOVA showed that polychaete associations differ among transects according to the beach zones. The composition of interstitial polychaete associations was significantly correlated to beach morphodynamics and features (P < 0.01). Polychaete associations of reflective beaches were more diverse than in other morphodynamic states. Intermediate beaches may also sustain diverse associations due to temporal variability of the morphodynamic patterns. Beaches presenting extreme dissipative morphodynamics and compacted sediments appear to be unfavourable for the occurrence of interstitial polychaetes.     

Measurements and modelling of the advection of suspended sediment in the swash zone by solitary waves

Novel laboratory experiments and numerical modelling have been performed to study the advection scales of suspended sediment in the swash zone. An experiment was designed specifically to measure only the sediment picked up seaward of the swash zone and during bore collapse. The advection scales and settling of this sediment were measured during the uprush along a rigid sediment-free beach face by a sediment trap located at varying cross-shore positions. Measurements were made using a number of repeated solitary broken waves or bores. Approximately 25% of the pre-suspended sediment picked up by the bores reaches the mid-swash zone (50% of the horizontal run-up distance), indicating the importance of the sediment advection in the lower swash zone. The pre-suspended sediment is sourced from a region seaward of the shoreline (still water line) which has a width of about 20% of the run-up distance. An Eulerian–Lagrangian numerical model is used to model the advection scales of the suspended sediment. The model resolves the hydrodynamics by solving the non-linear shallow water equations in an Eulerian framework and then solves the advection–diffusion equation for turbulence and suspended sediment in a Lagrangian framework. The model provides good estimates of the measured mass and distribution of sediment advected up the beach face. The results suggest that the correct modelling of turbulence generation prior to and during bore collapse and the advection of the turbulent kinetic energy into the lower swash is important in resolving the contribution of pre-suspended sediment to the net sediment transport in the swash zone.     

Radioelement distribution in river,beach, and offshore areas and their significance to Chavara placer deposit,Southern Kerala Coast of India

The radioelement and heavy mineral distribution in river, beach and innershelf areas of the southern Kerala coast is related to placer mineral concentration on the beaches at Chavara. Southern Kerala rivers—Neyyar, Karamana and Vamanapuram—transport higher amounts of radioactive elements than the larger Kallada River due to higher radioactive minerals in the hinterland rocks. Coastal configurations and the seasonal longshore current pattern seems to control along-shore distribution of minerals. The proposed model for placer concentration suggests that the energy difference and seasonal current direction along this coast is important.     

The influence of seaward boundary conditions on swash zone hydrodynamics

The influence of the seaward boundary condition on the internal swash hydrodynamics is investigated. New numerical solutions of the characteristics form of the nonlinear shallow-water equations are presented and applied to describe the swash hydrodynamics forced by breaking wave run-up on a plane beach. The solutions depend on the specification of characteristic variables on the seaward boundary of the swash zone, equivalent to prescribing the flow depth or the flow velocity. It is shown that the analytical solution of Shen and Meyer [Shen, M.C., Meyer, R.E., 1963. Climb of a bore on a beach. Part 3. Runup. J. Fluid Mech. 16, 113–125] is a special case of the many possible solutions that can describe the swash flow, but one that does not appear appropriate for practical application for real waves. The physical significance of the boundary conditions is shown by writing the volume and momentum fluxes in terms of the characteristic variables. Results are presented that illustrate the dependence of internal flow depth and velocity on the boundary condition. This implies that the internal swash hydrodynamics depend on the shape and wavelength of the incident bore, which differs from the hydrodynamic similarity inherent in the analytical solution. A solution appropriate for long bores is compared to laboratory data to illustrate the difference from the analytical solution. The results are important in terms of determining overwash flows, flow forces and sediment dynamics in the run-up zone.     

辽东湾表层沉积物碎屑矿物组合分布及其对物源和沉积物扩散的指示意义

对取自渤海辽东湾的128个表层沉积物样品进行了碎屑矿物鉴定,对矿物成分含量应用log-ratio法处理后进行统计分析。结果表明,研究区重矿物质量百分含量平均值为6.3%,高值区位于六股河口和湾西北近岸。共鉴定出7种轻矿物,38种重矿物,轻矿物以石英、斜长石和钾长石为主,重矿物以普通角闪石、绿帘石、磁铁矿、石榴子石、钛铁矿为主。根据碎屑矿物的组合分布特征,可将研究区划分为两个矿物组合区,其中又可细分为6个矿物亚区。六股河和湾西北近岸矿物亚区的碎屑矿物主要来源于六股河、湾西北入海小溪和海岸侵蚀物质,湾东北部矿物亚区沉积物主要来源于双台子河和大辽河,湾中部矿物亚区为多源混合区,湾南部矿物亚区受邻近海域和复州河沉积物影响大,湾东南部的辽东浅滩矿物亚区为潮流作用的产物。与河流入海沉积物相比,该区表层沉积物的石英/长石比值和ZTR指数明显增大。研究认为,物源是控制研究区碎屑矿物的组合与分布的主要因素,同时也受到海洋水动力和矿物性质等因素影响。