Ebb Tidal Currents Research Articles

Three-dimensional modelling of sediment transport and the effects of dredging in the Haihe Estuary

The Haihe Tide Lock was constructed on the Haihe River in 1958 to stop salty and muddy water intrusion. Nevertheless, tidal currents carry sediment, which is eroded from the surrounding silty coast, into the river mouth and, thus siltation of the channel downstream of the tide lock becomes a major problem. Employed are trailer dredges, which stir up the silt and subsequently moves it out of the mouth with ebb tidal currents. While the application of this method is encouraging there are still problems to be studied: how high is the dredging efficiency, how far can the resuspended sediment be transported by the ebb currents, and is the sediment carried back by the next flood tide? This paper develops a 3-D model to answer these questions. The model employs a special element-interpolating-function with the σ-coordinate system, triangle elements in the horizontal directions and the up-wind finite element-lumping-coefficient matrix. The results illustrate that the efficiency of dredging is high. Sediment concentration is 4–20 times higher than the flow without dredging. About 40–60% of the resuspended sediment by the dredges is transported towards the sea 3.2 km off the river mouth and 10–30% is transported 5 km away from the mouth. Calculations also indicate that the rate of siltation of the river mouth is about 0.6 Mm 3 per year. If the average discharge of the river runoff is 0, 200 or 400 m 3 s −1 the mouth has to be dredged for 190, 99 or 75 days every year so to maintain it in equilibrium. The dredging efficiency per day is 0.53–1.31%.

Dinâmica da desembocadura do corpo lagunar do Ararapira

The Ararapira lagoonal mouth connects the ocean to Ararapira lagoon (Ararapira Sea), and has a SSW-NNE orientation, parallel to the coastline. The mouth has had a southwestward continuous lateral migration. The dominant direction of longshore currents is NE, related to the passage of cold fronts. Lasting absence of this meteorological effect causes temporary changes on wave features, leading to inversion of longshore currents. Considering this inversion, two circulation models of tidal currents and sediments are suggested acting alternately to induce migration to SW, opposite to longshore currents. Under the influence of NE longshore currents the acting processes on the erosive margin during ebb tide are similar to those on the concave margins of meandering rivers; sand is deposited on the longshore drift delta. However when longshore currents are inverted, the sands of the longshore drift delta are eroded through wave action and deposited, during flood tides, on the growing margin. This sand is here deposited because of drift blocking by ebb tide currents. The Ararapira lagoonal margins show critical erosion portions, mainly on the internal margins of the sandy barrier that separates coastal lagoon from ocean; this erosional process can lead to a new lagoonal mouth. This, together with progressive elongation of the coastal lagoon caused by lateral sandy barrier growth would lead to the opening of a new mouth to the NE of the present one, and to the filling of the portion of the coastal lagoon between the two mouths.

Morphostratigraphy of an ebb-tidal delta system associated with a large spit in the Piedras Estuary mouth (Huelva Coast, Southwestern Spain)

The Piedras Estuary is one of the most significative estuarine systems on the mesotidal Huelva Coast, in the Northwestern portion of the Cadix Gulf. The river mouth is presently an estuarine lagoon partially closed by a large spit constructed from an old barrier island system. This estuary is in an advanced state of infilling and its tidal prism has decreased during the Holocene causing instability and clogging of old inlets and transforming the barrier island chain into a spit. Sedimentation is controlled by the interaction of ebb tide currents and the prevailing SW waves. The main sediment supply is provided by an intensive West-to-East longshore current, transporting sand material from Portuguese cliffs and the Guadiana River. Tidal range is mesotidal (2.0 m) and the mean significant wave height is 0.6 m with an average period of 3.6 s. A boxcore study allowed five depositional facies to be distinguished in the Piedras Estuary mouth: (1) main ebb channels; (2) marginal flood channels; (3) ebb-tidal delta lobes; (4) marginal levees; and (5) curved spits. The recent evolution studied in this area suggests a cyclic evolutionary model for the ebb-tidal delta system. The architectural facies relations shown by the vibracore/boxcore study confirm that the apical growth of the spit occurred over the innermost of these ebb-tidal deltas. Consequently the preserved sequence shows the ebb-tidal delta facies under the spit facies.

Patterns of bottom current flow deduced from dune asymmetries over the Gulf of Cadiz shelf (southwest Spain)

Abstract The analysis of dune-like morphologies on the Gulf of Cadiz continental shelf, using high-resolution seismic reflection profiles and sediment samples, reveals a well-defined distribution pattern controlled by the area's hydrodynamics. These bedforms are considered to be modern features from the Holocene, and therefore they provide information about the bottom circulatory patterns established over the continental shelf. Most of the submarine dunes have been identified over a shallow physiographic feature named the Barbate High. The current flows that generate the bedform fields can be attributed to a complex interaction of several hydrodynamic agents, in which the process of current direction reversal related to the tidal cycle in the Straits of Gibraltar seems to be involved. Eastward-oriented submarine dunes located on the onshore zones of the inner shelf are generated by the Atlantic inflow, which over these shallow zones is mainly orientated towards the east during high-tide conditions in the Straits of Gibraltar. Bedforms oriented westwards and west–northwestwards in the offshore zones indicate that a previously undescribed flow occurs on the Gulf of Cadiz continental shelf. This flow is attributed to the influence of ebb tidal currents, which move preferentially towards the west over these zones. As a result of this complex bottom flow pattern, a clockwise sand transport pattern over the Barbate High is established.

Transport of fish larvae through a tidal inlet

In the South‐eastern USA many species of estuarine‐dependent fishes spawn offshore and their larvae are transported into estuaries. The present study combined physical measurements and zooplankton sampling at the Beaufort Inlet (North Carolina) to determine: (i) whether fish larvae enter the estuary inlet in different horizontal sections of the tidal stream and (ii) whether larvae use selective tidal stream transport (STST) for ingress through the inlet and for movements up the estuary. Larvae of Atlantic menhaden (Brevoortia tyrannus), spot (Leistomus xanthurus), pinfish (Lagodon rhomboides), Atlantic croaker (Micropogonias undulatus), gulf flounder (Paralichthys albigutta), summer flounder (P. dentatus), and southern flounder (P. lethostigma) were sampled over two 25‐h periods (13–14 March, 25–26 March 1996) at three locations across the Beaufort Inlet and in the three main channels receiving water from different locations in the inlet. All species, except Atlantic croaker, primarily entered the inlet on the east side with up to an order of magnitude difference in abundance across the inlet from East to West. The gradient continued in the corresponding channels in the estuary. In most cases equal sizes of larvae were caught in the East and West channels of the estuary. Most larvae were caught at night. A pattern of abundance relative to tide was least evident in the inlet, but STST was evident in the estuary. At the location of greatest larval abundance (Shackleford Channel), the greatest numbers of all species of larvae were caught during rising tides at night. This relationship was not due to passive suspension, since abundance was higher during the faster nocturnal flood tide currents than during to ebb tide currents of equal magnitude. In the Shackleford Channel, southern/gulf flounder and pinfish were distributed throughout the water column during STST, but primarily at the surface. Atlantic menhaden, spot and Atlantic croaker were distributed throughout the water column, and summer flounder were near the surface.

Process-response analysis for Hörnum tidal inlet in the German sector of the Wadden Sea

The Hörnum tidal inlet is located in the German sector of the Wadden Sea between the barrier islands Sylt and Amrum. On the basis of seven bathymetric surveys of the tidal inlet covering a 55-yr period from 1939 to 1994 and long-term records from two tide gauges, a process-response analysis for the inlet was carried out. Following the method described by Dean and Walton (1975), the volume of sediment stored in the ebb–tidal delta (Vebd) was calculated for each survey. Furthermore, the cross-sectional area of the tidal inlet throat (Ac), and the area-height distribution (hypsographic curve) of the study area were established. Between 1939 and 1994, Ac increased by about 32%, whereas Vebd diminished by about 18%. Earlier investigations by, e.g., O'Brien (1931), Walton and Adams (1976), Dean (1988), and Eysink and Biegel (1992) documented a strong positive correlation between these two morphological parameters and tidal currents in the inlet. In conformity with the literature, regression of the values of Ac with the corresponding fall velocities of the water level in the inlet (a direct indication of ebb–tidal currents) produced a correlation coefficient (r) of 0.87. However, Vebd shows a weak negative correlation (r=−0.74) with fall velocities of the water level. Apparently, other factors than changes in ebb–tidal currents must have been responsible for the decrease in Vebd. It is suggested that one factor might be the observed considerable increase in storminess, i.e. wave action, in the region since approximately 1960. Consequently, storm wave action on the ebb–tidal delta intensified and erosion was here initiated. As a result, the swash bars of the ebb–tidal delta, in particular, suffered from severe erosion between 1959 and 1994. As the ebb–tidal currents increased simultaneously, sand supply to the terminal lobe of the ebb–tidal delta increased as well, thereby balancing the loss of sediment from the terminal lobe caused by the increasing storminess. The results of this case study indicate that the ratio of ebb–tidal currents to (storm) wave action determines, in large part, the value of Vebd.

Importations et remaniements de thanatofaunes dans les sables de la plate-forme profonde des approches occidentales de la Manche

Macro- and micro-faunas are reported from six gravity cores collected at a mean depth of 150 m in the surficial deposits of the Kaiser sand bank in the Southern Western Channel Approaches. These are bivalves (mainly), gastropods, echinids, crustaceans and foraminifers. These fauna present a very rich association of numerous, well preserved species from various ecological settings. Such an association is commonly reported from the continental shelf of the last transgressive cycle. In addition, the sediment comprises two imported faunal associations. The first one corresponds to thanatocaenoses of Pliocene/lowermost Pleistocene and Weichselian age, reworked respectively from the underlying celtic incised valley fills and the Celtic Sand Banks. The absence of Quaternary fauna prior to the Weichselian suggests that the introduction of Pliocene sources occurred during the last climatic cycle. The second stage of importation took place after the last transgression in the Western Channel, and corresponds to the enrichment of the sediment in coastal fauna derived from western Brittany by the the predominant ebb tidal current. The variations of taxonomic diversity from core to core suggest a sediment transport around the bank that matches the one deduced from the study of tidal bedforms. At the present day, the wave action is highlighted by accumulation of species of the same shape, size and density inside storm beds. However, the survival within these beds of very small, juvenile shells indicate that each storm reworking was short in duration.

Nearshore circulation and synthetic aperture radar: An exploratory study

We use a sequence of synthetic aperture radar (SAR) images to map differences in the flood and ebb tidal currents in a cove along the coast of Nova Scotia, Canada. The asymmetry in the tidal flow determines the flushing rate of the cove which, in turn, has a significant effect on biological production within the cove and its potential for aquaculture. We find significant differences in the SAR images collected on flood and ebb tides. Specifically there are well-defined lines on the flood images and large whorls on the ebb. A three-dimensional hydrodynamic model of the tidal currents is used to interpret the SAR images. In particular we use the model flow fields to wind back the SAR images to an earlier stage of the tide in an attempt to determine the physical origin of the features in the images. We conclude that the most likely explanation for the ebb-tide whorls is the advection of surface slicks.

Dinâmica sedimentar e feições sedimentares identificadas na superfície de fundo do sistema Cananéia-Iguape, SP

O sistema Cananeia-Iguape e separado do oceano pela Ilha Comprida, com aproximadamente 70 km de extensao, tendo como limites ao sul e ao norte, respectivamente, as desembocaduras de Cananeia e Icapara. Estudos sobre as modificacoes na configuracao das margens do sistema, no padrao de distribuicao sedimentar, bem como sobre o conhecimento do padrao hidrodinâmico evidenciaram a interrelacao existente entre os mecanismos de circulacao geral, condicionados principalmente pelas correntes geradas pelas mares e a variacao temporal de descarga de agua doce, e a distribuicao espacial dos sedimentos de superficie dos fundos dos canais. Dentre as feicoes sedimentares identificadas nas superficies de fundo destacam-se as ocorrencias de marcas onduladas de grande porte (1 a 2 metros de altura e comprimentos de onda de 20 a 30 metros) concentradas no canal entre a Ponta do Frade (Mar de Cananeia) e a foz do Rio Subauma (Mar Pequeno). Estas feicoes de fundo resultam da dinâmica junto aos fundos dos canais, sob forte influencia das correntes de mare, sobre sedimentos predominantemente arenosos, cujas fontes atuais sao os depositos quaternarios de planicie costeira depositados durante os eventos transgressivos/regressivos quaternarios. A analise das orientacoes das correntes de mare, indicadas pelas marcas onduladas assimetricas, permite concluir que o local que representa, no processo hidrodinâmico da area, o ponto de inversao do sentido das mares vazantes, anteriormente atribuido a area da Pedra do Tombo, apresenta-se, na verdade, deslocado cerca de 10 km no sentido SW.

Clay minerals in recent sediments of the continental shelf and the Bay of Cádiz (SW Spain)

AbstractThe distribution of clay minerals in recent sediments on the continental shelf and in the Bay of Cádiz has been analysed as part of a wider study of sedimentary exchange between the continent and the continental margin. The clay minerals included in the muddy sediments consist mainly of illite, kaolinite, smectite, randomly mixed-layered illite-smectite, chlorite, vermiculite and randomly mixed-layered illite-chlorite. Distribution of the minerals indicates that inheritance from the adjacent continental areas is the most important process involved.The relations between the clay minerals were established by Q-mode and R-mode factor analysis. The main clay mineral associations are: (1) illite-kaolinite, (2) smectite-random mixed-layered illitesmectite, and (3) kaolinite-random mixed-layered illite-smectite-vermiculite. The main sources of sediment supply to the continental shelf are the Guadalquivir and Guadiana rivers. Sediment from these rivers and from the rivers flowing into the Bay of Cádiz (mainly the Guadalete) is transported to the part of the shelf outside the bay by ebb-tide currents. The path of this outflow to the continental shelf, where the sediment is partially deposited, is influenced by the morphology of the coast and by hydrodynamic processes.

The evolution of Buctouche Spit, New Brunswick, Canada

Buctouche Spit is an 11-km long sandy spit on the northeast shore of New Brunswick, Canada. It is a typical flying spit, having a narrow proximal section which is characterized by a single foredune that is prone to overwash and transgression, and a much wider prograding distal section which is characterized by a well-developed foredune backed by a series of relict foredunes. An optical-luminescence dating method in which infrared stimulation is used on potassium feldspars was developed for this study. It is a very promising method for dating Holocene deposits, yielding good resolution and having simplicity of measurement. The luminescence ages obtained indicate that Buctouche Spit extended at a rate of ~4 m yr −1 between 715 ± 45 years ago and 240 ± 25 years ago. This extension rate equates to a distal end accumulation rate of ~ 56,000 m 3 yr −1 for this period. At this rate of accretion, the present Buctouche Spit would have formed during the past 2000 years. The core data indicate that the spit is ⩾2200 years old, which is consistent with the preceding estimate. Since 240 ± 25 years ago, the rate of sediment accumulation at Buctouche Spit has apparently been falling. The accumulation rate between 1839 and 1945 was ~23,000 m 3 yr −1, less than half of the historic rate of ~56,000 m 3 yr −1. Since 1945, the distal end of Buctouche Spit has not prograded/extended measurably. This is because Buctouche Spit's present vertical growth rate is, at most, equal to the rate of relative sea-level rise at the spit and may be as little as half of the rate of relative sea-level rise. Present sediment supply to Buctouche Spit is probably < 16,000 m 3 yr −1. It is likely that the much larger sediment supply required to sustain an historic accretion rate of ~ 56,000 m 3 yr −1 was derived from the reworking of a former sandy barrier located seaward, and possibly updrift of, the present spit. Buctouche Spit is now in a limited sediment supply situation and it has become a constrained spit. Sediment is being eroded from the centre section of the spit, transported to the distal end, and then carried away by ebb-tidal currents. The centre section of the spit will likely breach within the next hundred years. If a breach (or breaches) becomes permanently established in the centre section of the spit, the distal end will suffer significant erosion and may be destroyed. After this, the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise. It is concluded that the key to understanding Buctouche Spit's long-term evolution is recognizing that it is migrating landward both by continuous processes like overwash, and by the more discrete and catastrophic process of overstepping.

Tidal Sedimentation in Part of the Late Silurian Grampians Basin, Southeastern Australia

ABSTRACT Thick quartz arenite formations in the Upper Silurian Grampians Group of southeastern Australia have been previously interpreted as fluvial deposits, but detailed study of one of these formations, the Mount Difficult Sandstones, shows that shallow-marine conditions existed during their deposition in the southeastern part of the Grampians Basin. The Mount Difficult Sandstones are a succession of cross-bedded quartz arenites with an overall fining-upward trend. Four lithofacies associations are recognized: the two lower associations record tidally dominated sedimentation, whereas the two upper associations represent deposition in wave-dominated environments. Boundaries are gradational, and the associations arc interpreted as four spatially and genetically related environments within a t dal-inlet and barrier-beach system. The basal tidal-inlet channel-fill complex overlies an erosion surface on tidally influenced coastal-plain deposits of the Silverband Formation. Vertically stacked fining-upward channel fills record lateral migration of tidal channels during transgression. The overlying association was deposited in mixed tide-wave conditions, and tidal-channel margin to ebb-tidal delta environments seaward of the inlet are proposed. The development of tidal-inlet channels and a tidal delta suggests deposition of the Mount Difficult Sandstones in a mesotidal setting (tidal range 2-4 m) with a time-velocity asymmetry dominated by ebb-tidal currents. The increasing influence of waves is recorded by barrier-beach deposits that migrated over the inlet-delta system. The barrier beach is represented by two lithofacies associations, interpreted as upper-shoreface and overlying foreshore deposits, an arrangement that signals a change from transgressive to regressive (progradational) conditions. Thus, the Mount Difficult Sandstones represent a single transgressive-regressive cycle (most likely third-order) that was controlled mainly by the rate of subsidence in the tectonically active Grampians Basin, under conditions of a relative highstand of sea level and high rates of sediment supply.

Murre foraging, epibenthic sound scattering and tidal advection over a shoal near St. George Island, Bering Sea

MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published July 2 Murre foraging, epibenthic sound scattering and tidal advection over a shoal near St. George Island, Bering Sea Kenneth 0. Coylel, George L. Hunt, ~ r Mary ~ , Beth ~ e c k e r Thomas J. weingartner1 'Institute o l Marine Science, School of Fisheries and Ocean Sciences. University of Alaska Fairbanks, Fairbanks. Alaska 99775-1080, USA 'Department of Ecology and Evolutionary Biology, University of California, Irvine. Irvine, California 92717, USA ABSTRACT: High concentrations of 2 murre species (Uria aalge and U. lomvia) were observed forag- ing over a submarine ridge on the east side of St. George Island. Bering Sea, usually in the morning. Simultaneous measurements of tidally generated current velocity and acoustically determined biomass along transects over the ridge demonstrated that the murres were concentrating over a dense epi- benthic sound-scattering layer near the ridge crest on southward ebb tide currents. Samples taken by net tows through the layer contained high numbers of euphausiids (Thysanoessa inerrnis and T. raschii). Stomachs of murres collected in the area contained predominantly euphausi~ds. When tidal currents reversed from southward to northward, the sound-scattering layer was carried northward off the ridge and the murres left the area. We hypothesize that the euphausiid aggregations resulted from the interaction of the die1 downward migration of euphausiids in the morning, bottom topography and tidal currents. INTRODUCTION Energy flow through marine ecosystems depends on a number of variables, one of which is prey concentra- tion. While high primary production may result in favorable conditions for grazers, the efficiency of energy transfer from grazer stocks to higher trophic levels is sensitive to physical processes which influ- ence the concentration of prey. The complex inter- action of animal behavior patterns and physical processes can produce a patchy distribution of zoo- plankton and micronekton (Omori & Hamner 1982, Sameoto 1983), thereby affecting feeding conditions of high level consumers. Physical processes can transport weakly swimming or drifting organisms horizontally (Banse 1986, Schel- tema 1986, Stancyk & Feller 1986). Tidal currents and wind-induced turbulence can vertically disperse hori- zontal layers (Haury et al. 1990, Incze et al. 1990), or interact with bathymetry to upwell organisms to the surface (Vermeer et al. 1987, Brown & Gaskin 1988). In addition, physical processes and behavior may interact O Inter-Research 1992 to affect the horizontal or vertical distribution of some species (Norcross & Shaw 1984, Epifanio 1987, Genin et al. 1988). Schneider et al. (1987) hypothesized that the interaction of flow gradients with bathymetry would result in concentrations of prey that were pre- dictable in time and space. Whatever the origin of zooplankton aggregations, they are an important source of energy to predators. For example, baleen whales seek out and prey on zoo- plankton aggregations (Kenney et al. 1986, Wishner et al. 1988) and the diets of fish are known to be influ- enced by prey distribution (Owen 1981, Genin et al. 1988). A variety of marine birds prey upon zooplankton aggregations associated with frontal zones (Bradstreet & Brown 1985, Schneider et al. 1987, 1990, Hunt 1990, Hunt & Harrison 1990), and tidal upwellings (Vermeer et al. 1987, Brown & Gaskin 1988). Spatially and temporally predictable prey aggregations may be of particular importance to foraging birds (Schneider et al. 1987). In this paper we examine the effects of tidal currents, bathymetry and the diurnal vertical migra- tion of euphausiids on the feeding behavior of thick-

Environmental conditions which lead to increase in cell density of the toxic dinoflagellates Dinophysis spp. in nutrient-rich and nutrient-poor waters of the French Atlantic coast

From 22 April to 19 June 1990, 7 stations located off La Rochelle, France, from near-shore ('bassin Marennes-Oleron') to offshore waters (50 m depth contour) were visited 8 times, with the aim of determining whether the toxic dinoflagellates Dinophysis spp. increase in cell density in nutrient-poor offshore water, in nutrient-rich inshore waters, or both, and to determine their vertical distribution in relation to stratification. Temperature stratification developed offshore, but not inshore. In April, few Dinophysis spp. were present offshore and none inshore. In late May, up to 15 X 10(3) cells l(-1) were recorded in the 10 to 15 m layer offshore, whereas inshore waters contained only a few cells l(-1). In late May, a short period of wind partly modified the vertical structure, and Dinophysis spp. concentration fell, increasing again when marked stratification was again established. Through the strait, flood-tide currents moved Dinophysis spp. cells to the inshore area, and ebb-tide currents brought some of them back out. No relationship was found between Dinophysis spp. growth and availability of dissolved nutrients. We concluded that: (1) stratification of sufficient magnitude (DELTAt 5-degrees-C) and duration ( 2 wk) are the factors necessary for Dinophysis spp. cell increase, and (2) inorganic nutrient input of terrestrial origin does not directly promote their growth.

INFLUÊNCIA DA DERIVA LITORÂNEA NO DESENVOLVIMENTO DO ESPORÂO DE CAIXA-PREGOS (BAÍA DE TODOS OS SANTOS/BA)

Monitoring the topographic variations of beach profiles along a spit located at the southern end of the Itaparica Island (Todos os Santos Bay, Bahia), it has been possible to show that, developing since the beginning of the Holocene, this feature has stopped growing in the present. Its growth was due to longshore drift of sediments from eastern beaches, induced by NE, E and SE winds. In the present days, instead of being deposited on the spit front, the sediments are being carried into an adjacent channel by strong ebb-tide currents.

Precambrian cyclic rhythmites: solar-climatic or tidal signatures ?

For more than 60 years geologists have sought evidence of solar-climatic cyclicity in rhythmically laminated sedimentary rocks, but claims in general have not been persuasive. Three Precambrian rhythmite sequences in Australia that comprise varve-like laminae recently have received attention, however, as their conspicuous cycles ofca. 10-14 and/or 20-25 laminae have been ascribed a sunspot-cycle origin. They are the 2500 Ma old Weeli Wolli Formation, the 1750 Ma old Wollogorang Formation, and the 650 Ma old Elatina Formation. New observations for the Weeli Wolli Formation, a siliceous banded iron-formation, suggest a cycle period exceeding the 23 microband couplets proposed by Trendall, casting doubt on the solar interpretation. The Weeli Wolli cyclicity may record Earth-tidal rhythms that modulated the discharge and composition of silica- and iron-bearing fumarolic waters. The structure of the cycles of silty dolomite and mudstone in the Wollogorang Formation does not support a sunspot-cycle origin, and a tidal control on sedimentation should be considered. The Elatina sequence of cyclic sandstone and silts tone laminae displays several empirical similarities to the sunspot series. The discovery of thicker, more complex lamina-cycles in the correlative Reynella Siltstone has, however, caused reappraisal of the solar interpretation of the Elatina rhythmites. The Reynella cycles consistently contain 14 or 15 laminae, with many laminae comprising pairs of‘semi-laminae’; the overall structure of the cycles is similar to the mixed (semidiurnal and diurnal) fortnightly tidal growth patterns of modern bivalves. Also, recent observations that fine material can be transported in suspension to deeper waters offshore by ebb-tidal currents provides a tidal mechanism that can explain the deposition of thin, laterally extensive, graded laminae whose thickness would be a measure of tidal range and current speed. By this tidal model, the Elatina laminae are regarded as diurnal increments, and the lamina-cycles as commonly abbreviated fortnightly groupings. The Elatina series so interpreted may encode unique information on lunar orbital periods and the Earth’s palaeorotation: the data indicateca. 30.5 days per lunar month, 13.1 lunar months andca. 400 days per year, and lunar apsides and lunar nodal cycles of 9.7 andca. 19.5 years respectively some 650 Ma ago. Evidence for significant modulation of terrestrial climate by the solar activity cycle in the geological past may prove as elusive as have convincing indications of solar signals in modern patterns of weather and climate.

Fair- and foul-weather shell accumulations on a Georgia beach

Wrack-line accumulations of beach shells are governed primarily by longshore drift and secondarily by local geomorphic variations along the strand. During low-energy conditions of summer on Sapelo Island beaches, most shells move in the direction of prevalent wave incidence and accumulate in beach re-entrants. Very few shells accumulate on beach protrusions, which are focal points for refracting waves, or on estuary-inlet beaches, where ebb-tidal currents predominate over longshore currents. During high-energy conditions associated with a minor tropical storm, virtually all trends in shell accumulation were reversed. Most shells were removed from the beach; yet more shells remained along relatively straight beach segments than within beach re-entrants, at a point midway between major northward and southward components of longshore drift. These contrasting assemblages thus help demonstrate the importance of comparative taphonomy as (1) a measure of fluctuating coastal conditions-especially the shift from background to episodic stratinomy-and (2) a prerequisite to paleoecological and paleoenvironmental reconstructions. Most beach shells, storm and nonstorm, were derived originally from the nearshore shelf. Major differences between the structure of nearshore populations and that of beach-shell assemblages, aside from physical stratinomic effects, are attributable to hermit crab utilization of select gastropod valves and to differential rates of productivity among standing crops of mollusks.

Evidence of superimposition of storm waves on tidal currents in rocks from the Tithonian-Neocomian Umia Member, Kutch, India

The general opinion is that hummocky cross-stratification is generated from superposition of storm waves on a unidirectional current. However, constraints on the type of the current field have not been clearly established. Sedimentary structures preserved in rocks of the Tithonian-Neocomian Umia Member, Kutch, India, indicate hummocky cross-stratification developed from storm wave interference with ebb-tide currents. Facies indicate repeated progradation of the shoreline and the increasingly important role of sand transport by currents in offshore direction. Stratigraphic analyses identify two major stages of shoreline progradation. Each is composed of distinctive shelf to shoreface transitions. In the tidally influenced shoreface parts of these transitions, the azimuths of units with nearly unidirectional hummocky cross-stratification are consistent with the direction of ebb-tidal cross-stratification, suggesting offshore sand transport by superposition of waves on an ebb-tidal current.

Barrier island migration and onshore sediment transport, southwestern Delaware Bay, Delaware, U.S.A.

A detailed sediment budget for part of a rapidly migrating (3 m yr −1) barrier system of southwestern Delaware Bay demonstrates that longshore drift and other principal sources of sediment are insufficient to balance the sediment budget during 1926–1981. Changes in bathymetry from 1964 to 1984 indicate that erosion is occurring in shallow areas outside the surf zone. If approximately half of the sediment eroded offshore is transported onto the beach, the sediment budget balances. These results suggest that sand and gravel is supplied to the barriers of the study area from offshore. Several processes probably combine to carry sediment landwards. Tidal currents are weak, and probably rarely entrain sediment without the assistance of waves. Once entrained, sediment may be carried onshore by the combined influence of dominant ebb-tidal currents and frequent northwest winds. This process is probably also aided by a residual onshore shear stress associated with 2nd-order Stokes waves.

Untersuchungen an Plattfischen und Nordseegarnelen(Crangon crangon) im Eulitoral des Wattenmeeres nach dem Übergang zum Bodenleben

In spring and early summer,Pleuronectes platessa, Platichthys flesus, Solea solea, andCrangon crangon transform to benthic postlarvae in the shallow water region of the North Frisian Wadden Sea (eastern North Sea). During the first weeks after metamorphosis, they remain in the puddles and drainage gulleys on the tidal flats that are exposed during low tide. Positive rheotaxis prompting the juveniles to swim against the ebb tide currents is an important mechanism for maintaining their position on the flats. When sunlight intensity increases, they continue their locomotory and feeding activity, even during daytime, in the pools still present at low tide. On warm, cloudless days, temperature levels in these pools may increase to lethal thresholds. Consequently, the animals attempt to escape from higher tidal flats near the shore through the drainage gulleys. Such migrations bring about changes in their distribution patterns. Plaice can scarcely be found in near-shore areas, and they are absent at both high and low tide. The responses observed were correlated to physical and chemical data recorded during the observation period. The ecological significance of tidal pools as habitats is discussed.