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1.
Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.  相似文献   

2.
Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of São Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s?1 to 60 cm s?1 during flood tides and from 70 cm s?1 to 100 cm s?1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux.  相似文献   

3.
0-group sea bass,Dicentrarchus labrax, colonize intertidal marsh creeks of Mont Saint Michel Bay, France, on spring tides (e.g., 43% of the tides) during flood and return to coastal waters during ebb. Most arrived with empty stomachs (33%), and feed actively during their short stay in the creeks (from 1 to 2 h) where they consumed on average a minimum of 8% of their body weight. During flood tide, diet was dominated by mysids,Neomysis integer, which feed on marsh detritus. During ebb, when young sea bass left tidal marsh creeks, the majority had full stomachs (more than 98%) and diet was dominated by the most abundant marsh (including vegetated tidal flats and associated marsh creeks) resident amphipod,Orchestia gammarellus. Temporal and tidal effects on diet composition were shown to be insignificant. Foraging in vegetated flats occurs very rarely since they are only flooded by about 5% of the tides. It was shown that primary and secondary production of intertidal salt marshes play a fundamental role in the feeding of 0-group sea bass. This suggests that the well known nursery function of estuarine systems, which is usually restricted to subtidal and intertidal flats, ought to be extended to the supratidal, vegetated marshes and mainly to intertidal marsh creeks.  相似文献   

4.
A study of the dynamics of a marine sandwave   总被引：3，自引：0，他引：3  
The movement of the crest of a sandwave was studied using cross-sectional profiles obtained from lines of sea-bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest. The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea-bed by storm waves, caused major reductions in crestal heights. Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.  相似文献   

5.
Optical in situ chemical sensors enable sampling intervals and durations that rival acoustic techniques used for measuring currents. Coupling these high-frequency biogeochemical and physical measurements in estuaries to address ecosystem-scale questions, however, is still comparatively novel. This study investigated how tides affect ecosystem metabolism in a mesotidal estuary in central California (Elkhorn Slough). Dissolved oxygen measurements were used to estimate the terms in a control volume budget for a tidal creek/marsh complex at tidal timescales over several weeks. Respiration rates were 1.6 to 7.3 g O2 m?2 day?1; net community production approached 20 g O2 m?2 day?1. We found that aquatic NCP integrated throughout the creek complex varied significantly over the spring-neap cycle. The intertidal contribution to aquatic metabolism was net heterotrophic during spring tides and generally in balance during neap tides because spring-tide marsh inundation was limited to nighttime, and therefore the marsh could not contribute any primary production to the water column. At the estuary scale, the fortnightly export of oxygen from the main channel to the intertidal was largely balanced by an advective flux up-estuary.  相似文献   

6.
A field investigation of temporal and spatial changes in wind and wave characteristics, runup and beach water table elevation was conducted on the foreshore of an estuarine beach in Delaware Bay during neap (April 9, 1995) and spring (April 16, 1995) tides under low wave-energy conditions. The beach has a relatively steep, sandy foreshore and semi-diurnal tides with a mean range of 1.6 m and a mean spring range of 1.9 m. Data from a pressure transducer placed on the low tide terrace reveal a rate of rise and fall of the water level on April 16 of 0.09 mm s−1 resulting in a steeper tidal curve than the neap tide on April 9. Data from three pressure transducers placed in wells in the intertidal foreshore reveal that the landward slope of the water table during the rising neap tide was lower than the slope during spring tide, and there was a slower rate of fall of the beach water table relative to the fall of the tide. Wave heights were lower on April 9 (significant height from 17.1 min records <0.16 m). The water table elevation was 0.08 m higher than the water in the bay at the time of high water, when maximum runup elevation was 0.29 m above high water and maximum runup width was 2.0 m. The elevation of the water table was 0.13 m higher than the maximum elevation of water level in the bay 74 min after high water, when wave height was 0.12 m and wave period was 2.7 s. The use of mean bay water level at high tide will underpredict the elevation of the water table in the beach, and demarcation of biological sampling stations across the intertidal profile based on mean tide conditions will not accurately reflect the water content of the sandy beach matrix.  相似文献   

7.
Patterns of nekton occurrence on the salt marsh surface at high tide and in an adjacent intertidal creek pool at low tide were used to investigate movements of nekton in an intertidal basin. Paired collections were made in North Inlet estuary, SC on 67 dates over 9 years. Comparisons of high- and low-tide total abundance indicated that what remained in the creek pool at low tide was representative of the nekton on the flooded marsh. Of the 64 taxa collected, the same 8 species ranked in the top 10 in both the high- and low-tide collections. Abundances of most resident species were positively correlated with the area of marsh flooded, but mummichog (Fundulus heteroclitus), the most abundant resident, was not. Abundances of young-of-the-year transient species were not related to the extent of tidal flooding. Some transient species used the flooded marsh but did not occupy the pool at low tide, and others found in the pool did not use the marsh. Differences in abundance, biomass, and length between the marsh and pool collections indicated differences in the tendency of species and life stages to retreat downstream of the pool to the subtidal channel. Proportionately more of the nekton that were present on the flooded marsh left the intertidal basin when large changes in temperature and salinity occurred between high and low tides. More transients left the basin following higher tides, but more residents did not. The results demonstrate a wide range of taxonomic and ontogenetic patterns among nekton using intertidal salt marsh basins and the underappreciated importance of intertidal creek pools as alternative low-tide refuges.  相似文献   

8.
Turnagain Arm is a macrotidal fjord‐style estuary. Glacier Creek is a small, glacially fed stream which enters the estuary tangentially near Girdwood, Alaska. Trenches and daily sedimentation measurements were made in a mudflat along the fluvio–estuarine transition of Glacier Creek during several summers since 2003. Each year, the flats appear to erode during the winter and then accrete vertically in the spring and summer. In each of the years studied, tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap–spring tidal cycles. In 2004, bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats. Five laminae bundles alternated between thicker and thinner bundles, corresponding to the perigean (high spring) and apogean (low spring) tides. Well‐preserved apogean–perigean cycles have rarely been documented in modern tidal flat sediments. At this location, vertical accretion of tidal rhythmites with well‐developed neap–spring cyclicity is possible because of the near‐complete removal of the flat from the previous year, which creates accommodation space for vertical accretion without significant reworking. Macrotidal conditions, no reworking by infaunal invertebrates, protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation. The position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles. In the summer of 2004, apogee and perigee were closely aligned with the new and full moons, resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick–thin spring cycles encoding the apogean and perigean tidal cycle. The apogean–perigean signal was not observed in subsequent years.  相似文献   

9.
The Swartvlei estuary possesses a prolific growth of both intertidal and subtidal eelgrass,Zostera capensis. During 1984 less than 12% of the eelgrass beds were located in the upper half of the estuary, yet deposition ofZostera/macroalgal wrack in this region, when the estuary was linked to the sea (open phase), was similar to that in the lower half. Over a period of 20 semidiurnal tidal cycles there was a net gain of 2.5 tonnes dry mass of plant material into the upper reaches. Export of aquatic macrophytes and filamentous algae from the lower reaches toward the sea over 20 tidal cycles amounted to 1.6 tonnes dry mass. The amount of plant material transported during spring tides was 2 to 3 times greater than that carried during neap tides. Shallowing of the estuary mouth due to sand deposition resulted in a decline in the tidal prism and a decrease in macrodetrital flux. Total export ofZostera and associated algae amounted to 0.87 g ash-free dry mass m?2d?1 and represented a monthly export of 18% ofZostera bed biomass. Deposition of plant wrack during the 1984–1985 closed phase amounted to 63 g dry mass per meter of shore per day at the lower reaches site but only 10 g m?1d?1 was recorded at the upper reaches site. The relatively low latter value was attributed to the absence of tidal action which transports macrodetritus from the lower and middle reaches into the upper part of the system. During the 1984 open phase 70 g m?1d?1 was deposited at the lower reaches site and 68 g m?1d?1 at the upper reaches site. The role of tides in the redistribution of aquatic macrophyte primary production in the Swartvlei estuary was therefore clearly underscored.  相似文献   

10.
Using both the photosynthetically active chlorophylla (chla) content of the organic carbon fraction of suspended particulate matter (chla/POC) and the percentage of photosynthetically, active chla in fluorometrically measured chla plus pheophytina (% chla), we determined that under specified hydrodynamic conditions, neap-spring tidal differentiation in particle dynamics could be observed in the Columbia River estuary. During summer time neap tides, when river discharge was moderate, bottom chla/POC remained relatively unchanged from riverine chla/POC over the full 0–30 psu salinity range, suggesting a benign trapping environment. During summertime spring tides, bottom chla/POC decreased at mid range salinities indicating resuspension of chla-poor POC during flood-ebb transitions. Bottom % chla during neap tides tended to average higher than that during spring tides, suggesting that neap particles were more recently hydrodynamically trapped than those on the spring tides. Such differentiation supported the possibility of operation of a particle conveyor belt process, a process in which low-amplitude neap tides favor selective particle trapping in estuarine turbidity maxima (ETM)., while high-amplitude spring tides favor particle resuspension from the ETM. Untrapped river-derived particles at the surface would continue through the estuary to the coastal ocean on the neap tide; during spring tide some particles eroded from the ETM would combine with unsettled riverine particles in transit toward the ocean. Because in tensified biogeochemical activity is associated with ETM, these neap-spring differences may be critical to maintenance and renewal of populations and processes in the estuary. Very high river discharge (15, 000 m3 s−1) tended to overwhelm neap-spring differences, and significant oceanic input during very low river discharge (5,000 m3 s−1) tended to do the same in the estuarine channel most exposed to ocean input. During heavy springtime phytoplankton blooms, development of a thick bottom fluff layer rich in chla also appeared to negate neapspring differentiation because spring tides apparently acted to resuspend the same rich bottom material that was laid down during neap tides. When photosynthetic assimilation numbers [μgC (μgchl,a)−1h−1] were measured across, the full salinity range, no neap-spring differences and no river discharge effects occurred, indicating that within our suite of measurements the compositional distinction of suspended particulate material was mainly a function of chla/POC, and to a lesser extent % chla. Even though these measurements suggest the existence of a conveyor belt process, proof of actual operation of this phenomenon requires scalar flux measurements of chla properties in and out of the ETM on both neap and spring tides.  相似文献   

11.
Hydrographic patterns and chlorophyll concentrations in the Columbia River estuary were compared for spring and summer periods during 2004 through 2006. Riverine and oceanic sources of chlorophyll were evaluated at stations along a 27-km along-estuary transect in relation to time series of wind stress, river flow, and tidal stage. Patterns of chlorophyll concentration varied between seasons and years. In spring, the chlorophyll distribution was dominated by high concentrations from freshwater sources. Periods of increased stream flow limited riverine chlorophyll production. In summer, conversely, upwelling winds induced input of high-salinity water from the ocean to the estuary, and this water was often associated with relatively high chlorophyll concentrations. The frequency, duration, and intensity of upwelling events varied both seasonally and interannually, and this variation affected the timing and magnitude of coastally derived material imported to the estuary. The main source of chlorophyll thus varied from riverine in spring to coastal in summer. In both spring and summer seasons and among years, modulation of the spring/neap tidal cycle determined stratification, patterns of mixing, and the fate of (especially freshwater) phytoplankton. Spring tides had higher mixing and neap tides greater stratification, which affected the vertical distribution of chlorophyll. The Columbia River differs from the more tidally dominated coastal estuaries in the Pacific Northwest by its large riverine phytoplankton production and transfer of this biogenic material to the estuary and coastal ocean. However, all Pacific Northwest coastal estuaries investigated to date have exhibited advection of coastally derived chlorophyll during the upwelling season. This constitutes a fundamental difference between Pacific Northwest estuaries and systems not bounded by a coastal upwelling zone.  相似文献   

12.
淤长型泥质潮滩双凸形剖面形成机制   总被引：1，自引：0，他引：1       下载免费PDF全文
以江苏中部淤长型泥质潮滩为例,建立了基于过程的潮滩动力地貌演变数学模型,研究了在沿岸潮流作用、供沙充分情况下双凸形潮滩横剖面形成机制。在向岸方向,流速自潮下带至低潮位线附近急速减小,潮间带区域流速则缓慢减小,泥沙在流速急变区域迅速堆积形成上凸点。潮下带中部在小潮期的淤积量大于大潮期冲刷量,总体淤积率较高;加之潮间带中部较弱的落潮流不足以将其上风处底沙掀动并向海输运,导致落潮后期潮下带上部含沙量小、沉积率相对较低,最终在潮下带中部形成下凸点。随着滩面淤长抬升,上、下凸点位置逐步向高、低潮位线附近移动。与前人关于双凸形剖面形成机制的定性分析成果相比,尚有不一致之处,需通过现场观测等进一步探讨。  相似文献   

13.
High-resolution current velocity and suspended sediment concentration (SSC) data were collected by using an Acoustic Doppler Current Profiler (ADCP) at two anchor stations and a cross-section in the South Channel of the Changjiang River mouth during meso and neap tides on Nov. 16, 2003. In addition, tidal cycle (13-hour) observation at two stations was carried out with traditional methods during the spring tide. Results indicated that resuspension occurred not only at the flood and ebb maximum, but also in the early phase of ebb in the meso and neap tide. When tidal current transited from high to ebb phase, current speed accelerated. Subsequently, fine-grained sediment with low critical threshold was resuspended and increased concentration. The river mouth area remained in siltation in the meso and neap tidal phase during the observation season, with calculated resuspension flux in the order of magnitude of 10−4–10−7 kg·m−2/s. Suspended sediment transport in the South Channel was dominated by freshwater discharge, but the Storks drift, vertical circulation and vertical shear effect due to tidal oscillation also played an important role in resuspension and associated sediment transport. In contrast, resuspension sediment flux in the spring tide was larger than that in meso and neap tide, especially at the ebb maximum and flood maximum. The present study revealed that intensive resuspension corresponded well with the larger current velocity during winter. In addition, the ‘tidal pumping’ effect and tidal gravity circulation were also vital for forming the turbidity maximum in the Changjiang River estuary.  相似文献   

14.
Using stationary zooplankton nets that fished the tidal current we measured the daily abundance ofCancer crab megalopae near the mouth of Coos Bay, Oregon, during the 1997 spring settlement season. During the spring of 1997, the coastal waters were dominated by a significant El Niño event. Sea surface temperatures (SST) were higher than normal, upwelling indices were an order of magnitude smaller than during the two previous springs, and upwelling favorable winds were weak. Daily catches ofCancer magister megalopae ranged from 0 to 78 with 61% of the total catch occurring during four pulses. Peak catches tended to occur every 13.6 d close to 13.8 d average period between spring tides. Significant cross correlations were found between the maximum daily tidal range and the catch ofC. magister megalopae; large catches tended to occur 4 to 7 d after the spring tide. Daily catches ofCancer oregonensis andCancer productus ranged from 0 to 307 with catch significantly positively cross correlated to the maximum daily tidal range at a lag of ?5 days suggesting that the largest catches tended to occur after the spring tides. We hypothesize that a tidally-generated phenomenon internal waves, transportedCancer megalopae shoreward and caused the observed variation in their abundance in Coos Bay.  相似文献   

15.
This contribution presents a new perspective on water chemistry and its relation to tidal hydrology in marsh-dominated estuaries. Results are derived from both field and modeling experiments. A heuristic model based on a tidally-averaged advection-dispersion equation is used in conjunction with source-sink terms (for benthic, marsh surface, and open-water exchanges) to make predictions of nutrient concentrations in the water column. Spring-neap tidal contrasts are associated with significant changes in water-column chemistry for a variety of nutrients sampled during the growing season in the Parker River estuary (Massachusetts). For ammonium, phosphate, nitrate plus nitrite, total dissolved N, and total dissolved P, concentrations are significantly lower during spring tides (marshes flooded) than during neap tides (marshes unflooded). Model results indicate that physical changes and open-water processing are insufficient to produce the observed effect, and that explicit biogeochemical processing on marsh surfaces is required. Field observations of changes in nutrient to nutrient ratios with the onset of marsh inundation also support this conclusion. As tides progress from the neap to spring condition, a “spectrum” of trajectories emerges in salinity-nutrient plots developed from both observational datasets and model output. Care must therefore be exercised in designing sampling programs for water chemistry in marsh-dominated ecosystems and in interpreting the resulting mixing diagrams.  相似文献   

16.
The tube-building polychaete Lanice conchilega forms intertidal mounds in association with macroalgae. This assemblage produces structures that can record tidal and seasonal cycles in the stratigraphic record. They mark low-tide levels because the assemblage occurs below neap low tide and many of the structures form when water drains off the assemblage. Mounds are created by disturbance of the sediment surface around aggregations of the assemblage and accumulation of sediments within the aggregations. The initial aggregations may be relict patches of adult worms, new clumps of juveniles or both. Juvenile worms and drift algae easily settle and survive in high-density patches, whereas predators and waves more readily disturb low-density patches. Algae and tubes extend through accumulated sediment and create a new, higher surface. Through this interactive process the high-density patches increase height rapidly while the low-density patches erode. Regardless of density, when the initial distribution is regular the surface remains flat, rising evenly or eroding evenly. Accumulation and erosion increases during spring tides and decreases during neap tides. Mound development also follows a seasonal pattern. The recruitment of juvenile worms in spring facilitates algal settlement and initiates mound buildup. The rate of tube-building and algal growth increases in early summer as the erosive forces of storms decline. This leads to the greatest development of mounds in late summer just before the macroalgae begin to die. The loss of algae coincides with autumnal storms, and causes catastrophic erosion of the mounds into relict patches of dormant tube-worms. Patches of tubes that survive the winter enhance spring recruitment and renewal of mound development. The shape of the mounds is often obscured during burial and preservation. Because of this, it may be difficult to distinguish the original form of ancient mounds without contemporaneous cementing organisms. But cycles of deposition and erosion are recorded in Lanice tubes and other biogenic structures. These structures can be used in conjunction with physical structures to define tidal height, seasonality or current regime. In general however, the presence of tube aggregations per se is not diagnostic of a specific hydraulic environment.  相似文献   

17.
Laminated blue-green algal mats are forming in the intertidal zone of a clastic (non-carbonate), tropical, lagoonal tidal-flat in northwest Ceylon. On the basis of their surface morphology three distinct mat zones are recognised. From the high-water mark seawards these are: (1) a smooth rounded-mat zone with discrete structures; (2) a crinkled- and blistered-mat zone; and (3) a smooth flat-mat zone without any perceptible relief. These mats are formed by the trapping and binding of sediment particles on to a sticky and mucilaginous complex of algal filaments.The morphology and distribution of these mats appear to be controlled by the frequency and duration of their exposure, strength of tidal-current velocities and the rates of bottom sediment movement. The flat zone is characterised by tidal-current velocities of 10–15 cm/sec, greater rates of traction movement and twice daily tidal flooding. The crinkled zone is partly or completely exposed during neap tides, the rate of bottom sediment movement is less than in the flat zone and the tidal-current velocities vary from 6–12 cm/sec. The rounded-mat zone is flooded only during high-water spring tide, with no tidal exchange during the neap tidal phase, but there is a continuous water cover forming a tidal pool. Here the rate of bottom sediment movement is negligible and the tidal-current velocities vary from 1–10 cm/sec. The rounded—elliptical mat forms are elongated in the direction of tidal flow and with decreasing tidal velocities they tend to assume a more rounded shape. All three zones are characterized by laminations produced by the alternation of thick sediment-rich and thin algal-rich layers. They show an accretion rate of 5–15 mm/week. The above features indicate that they are potentially very useful environmental indicators.  相似文献   

18.
A radioisotope tracer (82Br) was injected into a sand aquifer adjacent to a tidal creek at Hat Head, New South Wales, Australia. The injection was timed to coincide with the falling limb of a spring tide in August 2001 and was repeated in July 2002 during a period of neap tides. The tracer movement was detected using gamma logging and fixed gamma detectors in a bore 0.9 m from an injection bore and in a line approximately perpendicular to the creek. Movement of the tracer was detected by comparing measured gamma activity with calibrated activities determined under laboratory conditions. Net movement of the tracer indicated approximately 0.001 m/day laterally towards the creek and 0.07 m/day vertically upward during spring tide conditions. This pattern is reversed during neap tide conditions with little net vertical movement but horizontal movement of 0.15 m/day. The measurements indicated an oscillatory motion coinciding with the tide but lagging the tide by approximately 4.5–6.5 h. The vertical flow during spring tides indicates that vertical movement of the saline interface occurs and results in mixing of different water types beneath the banks of the creek.  相似文献   

19.
《Sedimentary Geology》2006,183(3-4):159-179
In the macrotidal Severn estuary, UK, the dynamics of intertidal fine-gravel dunes were investigated. These dunes are migrating across a bedrock platform. Systematic observations were made of hydraulic climate, geometry, migration rates and internal sedimentary structures of the dunes. During spring tides, the ebb flow is dominant, dunes grow in height and have ebb orientated geometry with bedrock floors in the troughs. During neap tides, a weak flood flow may dominate. Dunes then are flood orientated or symmetrical. Neap dune heights decrease and the eroded sediment is stored in the dune troughs where the bedrock becomes blanketed by muddy gravel. During spring tides, instantaneous bed shear stresses reach 8 N m− 2, sufficient to disrupt a 9 mm-gravel armour layer. However, a sustained bed shear stress of 4 N m− 2 is required to initiate dune migration at which time the critical depth-mean velocity is 1 m s− 1. Ebb and flood inequalities in the bed shear stress explain the changes in dune asymmetry and internal structures. During flood tides, the crests of the dunes reverse such that very mobile sedimentary ‘caps’ overlie a more stable dune ‘core’. Because ebb tides dominate, internal structures of the caps often are characterised by ebb orientated steep open-work foresets developed by strong tidal currents and some lower angle crossbeds deposited as weaker currents degrade foresets. The foresets forming the caps may be grouped into cosets (tidal bundles) and are separated from mud-infused cores of crossbeds that lie below, by reactivation and erosion surfaces blanketed by discontinuous mud drapes. The cores often exhibit distinctive muddy toe sets that define the spacing of tidal cosets.  相似文献   

20.
The arrangement of sediment couplets preserved in Thalassinoides shafts suggests that tides regulated the passive filling of these trace fossils and, thus, represent tubular tidalites. The thickness variation in individual layers and couplets implies a mixed diurnal, semi‐diurnal tidal signature where packages of either thick‐layered or thin‐layered couplets alternate. Calcarenitic sediment accumulated when tidal current velocity was too high to allow deposition of mud, whereas a marly mud layer is interpreted to have formed during more tranquil times of a tidal cycle (in particular, low‐tide slack water). The tidal record within the burrows covers a few weeks and the corresponding spring–neap cycles. The fill of the Thalassinoides shafts is the only known record to decipher the tidal signature from otherwise totally bioturbated sediments. These deposits accumulated in a lower‐shoreface to upper‐offshore setting during the late Miocene on a shallow shelf extending from the Atlantic Ocean to the west into northern Patagonia. The fill of all investigated burrows started around spring tide and, thus, the behaviour of the burrow producers – probably crustaceans – is speculated to have been affected by tides or the high water level because all studied burrows became abandoned around the same period of a tidal cycle.  相似文献   

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