Seasonality of modern pollen and sediment deposition in an estuarine context: the Severn Estuary Levels,southwest England

Recent sedimentological and palynological research on subfossil Holocene banded sediments from the Severn Estuary Levels suggested seasonality of deposition, registered by variations in mineral grain‐size and pollen assemblages between different parts of the bands. Here we provide data that strengthen this interpretation from sampling of modern sediments and pollen deposition on an active mudflat and saltmarsh on the margin of the Severn Estuary, and comparison with a vegetation survey and contemporary records of climate, river and tidal regimes. The results of grain‐size analysis indicate deposition of comparatively coarse‐grained silts during the relatively cool and windy conditions of winter and comparatively fine‐grained sediments during relatively warm and calm summer months. Pollen analysis demonstrates the significance of long‐term storage of pollen grains and fern spores in the estuarine waterbody, superimposed on which seasonal variations in pollen inputs from local and regional vegetation remain detectable. Copyright © 2007 John Wiley & Sons, Ltd.     

Discovery of the Avon – Solent Fracture Zone and its relationship to Bath hot springs

 The hot springs of Bristol and Bath and two geothermal wells at Southampton are located on a 155-km-long Avon–Solent Fracture Zone extending in a NW–SE direction from the Severn Estuary to the English Channel. Initiated during the Variscan earth movements and reactivated in Miocene times, the structure, which extends across the English Channel to France, is still active. With this discovery, it should now be possible to throw fresh light on the origin and movement of the thermal water at Bath and thus to protect the hot springs from derogation by limestone quarrying. Received: 16 October 1995 · Accepted: 13 February 1996     

Evolution of an ancient (Lower Cretaceous) marginal‐marine system from tide‐dominated to wave‐dominated deposition,McMurray Formation

Regionally extensive parasequences in the upper McMurray Formation, Grouse Paleovalley, north‐east Alberta, Canada, preserve a shift in depositional processes in a paralic environment from tide domination, with notable fluvial influence, through to wave domination. Three stacked parasequences form the upper McMurray Formation and are separated by allogenic flooding surfaces. Sediments within the three parasequences are grouped into three facies associations: wave‐dominated/storm‐dominated deltas, storm‐affected shorefaces to sheltered bay‐margin and fluvio‐tidal brackish‐water channels. The two oldest parasequences comprise dominantly tide‐dominated, wave‐influenced/fluvial‐influenced, shoreface to bay‐margin deposits bisected by penecontemporaneous brackish‐water channels. Brackish‐water channels trend approximately north‐west/south‐east, which is perpendicular to the interpreted shoreline trend; this implies that the basinward and progradational direction was towards the north‐west during deposition of the upper McMurray Formation in Grouse Paleovalley. The youngest parasequence is interpreted as amalgamated wave‐dominated/storm‐dominated delta lobes. The transition from tide‐dominated deposition in the oldest two parasequences to wave‐dominated deposition in the youngest is attributed mainly to drowning of carbonate highlands to the north and north‐west of the study area, and potentially to relative changes in accommodation space and deposition rate. The sedimentological, ichnological and regional distribution of the three facies associations within each parasequence are compared to modern and Holocene analogues that have experienced similar shifts in process dominance. Through this comparison it is possible to consider how shifts in depositional processes are expressed in the rock record. In particular, this study provides one of few ancient examples of preservation of depositional process shifts and showcases how topography impacts the character and architecture of marginal‐marine systems.     

Lateral Saltwater Intrusion in the North Channel of the Changjiang Estuary

Saltwater intrusion typically develops in the along-channel direction but exceptions can be found in bifurcated estuaries. Based on the observational data, we found that the saltwater intrusion in the upper reaches of the North Channel (NC) of Changjiang Estuary is dominated by the lateral saltwater intrusion from a small northern outlet (denoted as NONC) of this channel. This phenomenon has severe effects on the freshwater usage in this region. To investigate the underlying mechanisms of this pattern of intrusion, numerical experiments were conducted using a well-validated model. A flux decomposing method was used to decompose the process of saltwater intrusion into several mechanisms. During the neap tide, the saltwater begins to intrude landward into the NONC through shear transport induced by estuarine circulation. During the transition period between the neap tide and the following spring tide, the saltwater that previously reached the NONC further intrudes into the NC via Lagrangian and tidal pumping transports, causing a significant salinity increase in the middle and upper reaches of the NC. During the spring tide and the subsequent middle tide, saltwater intrusion in the NONC retreats. The impacts of the topography of the NONC and the wind stress on this lateral saltwater intrusion were also evaluated in this study.     

Early to middle Holocene palaeoenvironmental reconstruction of the Beagle Channel (southernmost Argentina) based on terrestrial and marine palynomorphs

This paper presents the main palaeoenvironmental results obtained from a site located in the western sector of the Beagle Channel, Isla Grande de Tierra del Fuego, southernmost Argentina. The palynological analysis performed on marine sediments from Aserradero‐Lapataia 2 (latitude 54°51′22.7″S, longitude 68°34′22.8″W) allowed us to document the evolution of palaeoenvironmental conditions during the beginning of the Holocene marine transgression into the Beagle Channel. The results indicate that, prior to the marine incursion, the coastal areas were characterized by the presence of open‐grown shrubs and herbs along with woodland palaeocommunities. The aquatic environments were dominated exclusively by freshwater taxa. Around 8300 cal. a BP, seawater gradually flooded the channel while cold and high effective moisture conditions favoured the development of an arboreal vegetation with dominance of Nothofagus forest and scarcity of shrub and herbaceous communities. Gradually increasing salinities allowed the development of freshwater/marine transitional environments indicated by aquatic palynomorphs able to tolerate stressed conditions under fluctuating salinities. The increasing dinocyst diversification with dominance of heterotrophic taxa corroborates the establishment of a fully marine environment during the middle–late Holocene in the Beagle Channel.     

Flood Hazard Assessment for a Hyper-Tidal Estuary as a Function of Tide-Surge-Morphology Interaction

Astronomical high tides and meteorological storm surges present a combined flood hazard to communities and infrastructure. There is a need to incorporate the impact of tide-surge interaction and the spatial and temporal variability of the combined flood hazard in flood risk assessments, especially in hyper-tidal estuaries where the consequences of tide and storm surge concurrence can be catastrophic. Delft3D-FLOW is used to assess up-estuary variability in extreme water levels for a range of historical events of different severity within the Severn Estuary, southwest England as an example. The influence of the following on flood hazard is investigated: (i) event severity, (ii) timing of the peak of a storm surge relative to tidal high water and (iii) the temporal distribution of the storm surge component (here in termed the surge skewness). Results show when modelling a local area event severity is most important control on flood hazard. Tide-surge concurrence increases flood hazard throughout the estuary. Positive surge skewness can result in a greater variability of extreme water levels and residual surge component, the effects of which are magnified up-estuary by estuarine geometry to exacerbate flood hazard. The concepts and methodology shown here can be applied to other estuaries worldwide.     

Characteristics of sediment in transport in the swash zone of a steep estuarine foreshore

Cross‐shore grading of sediment has been observed on the surface of estuarine beaches but the swash zone processes responsible for this grading have not been measured. This study was conducted to provide an explanation for the cross‐shore grading of sediment on a predominantly sandy estuarine foreshore. Data on wave and swash characteristics and sediment trapped in the uprush and backwash during 25 swash events were gathered from mid‐rising to mid‐falling tide on a small transgressive barrier in Delaware Bay, New Jersey, USA. Sediment is predominantly quartz and feldspar, medium to coarse sands with a gravel fraction of granules and pebbles. Wave energies increased with tidal rise. The percentage of gravel in transport in the uprush and backwash is similar (11% and 13%) during the rising tide when the swash zone is at mid‐foreshore, decreases in the uprush (9%) and increases in the backwash (18%) when the swash zone is on the upper foreshore. When the swash zone is at mid‐foreshore on the falling tide, the quantity of gravel in the backwash (30%) is greater than in the uprush (24%). The low proportion of gravel within the foreshore prior to trapping, and the increase in the percentage of gravel when the waves and swash are on the upper foreshore, suggests that the step is the primary source of gravel high on the foreshore. The size of the step increases as wave heights increase with tidal rise. The rate of delivery of gravel into the swash is enhanced by sediment entrained during wave breaking and interaction of the uprush with the previous backwash. The lag in the rate of step migration relative to breaker migration during the falling tide increases the likelihood of mining gravel from the step and subsequent transport in the uprush and backwash. These findings are important for low energy estuarine beaches sensitive to small changes in tidal range and wave energy that cause sedimentological change across the foreshore.     

Seasonal deposition of Holocene banded sediments in the Severn Estuary Levels (southwest Britain): palynological and sedimentological evidence

Banded sediments outcrop widely in the intertidal zone of the Severn Estuary and have been suggested, on the basis of textural analysis, to have formed in response to seasonal variations in sea temperature and windiness (Holocene, 14 (2004) 536). Here palynological and sedimentological analyses of banded sediments of mid-Holocene date from Gold Cliff, on the Welsh side of the Severn Estuary, are combined to test and further develop the hypothesis of seasonal deposition. Pollen percentage and concentration data are presented from a short sequence of bands to establish whether textural variations in the bands coincide with variations in pollen content reflecting seasonal flowering patterns. It is shown that fine-grained band parts contain higher total pollen concentrations, and a higher proportion of pollen from late spring- to summer-flowering plants, than coarse-grained band parts. Pollen in the coarser deposits appears primarily to reflect deposition from the buffering ‘reservoir’ of suspended pollen in the estuarine water-body and from rivers, when there is little pollen in the air in winter, while the finer sediments contain pollen deposited from the atmosphere during the flowering season, superimposed on these ‘background’ sources. The potential of such deposits for refining chronologies and identifying seasonality of coastal processes is noted, and the results of charcoal particle analysis of the bands presented as an example of how they have the potential to shed light on seasonal and annual patterns of human activity.     

The significance of organic matter degradation in the interpretation of historical pollution trends in depth profiles of estuarine sediment

The quantitative significance of organic matter degradation in bringing about the early diagenetic mobility of anthropogenic trace metals (Cu, Zn, Pb) is assessed specifically in relation to the use of estuarine sediments as historical records of pollution. A 1,500 mm salt-marsh sediment depth profile from Tites Point, Severn Estuary, England, was sampled at 10-mm intervals. Organic carbon determinations were carried out by a wet oxidation technique, and ‘organic fraction’ metals were separated by sequential leaching. Results demonstrated that organic phase metals are quantitatively significant in Severn Estuary sediments, particularly Cu and Zn (Cu>Zn), and that metals are probably released from this fraction during early diagenesis. The degree of release, and the apparent loss of the released trace metals from the sediment, would suggest that the use of estuarine sediments as historical records of pollution requires qualification.     

A study of the net flux of nitrates from estuaries of the eThekwini Municipality of Durban, KwaZulu-Natal, South Africa

Estuaries, the interface of interaction of fluvial discharge and marine action serve as temporary repositories of materials (solid and dissolved) before finally exporting them to sea. This interchange of material is dependant on a range of factors such as those due to tidal variation, fluvial flows and estuarine morphodynamics. The efficacy of transfer of materials to the marine environment is important for estuarine health particularly in estuaries located in highly developed areas such as the major coastal metropolitan areas of many countries. This study assesses this efficacy for three estuaries of the eThekwini Municipality (TM) of the city of Durban, South Africa which maintains an open mouth status, ensuring tidal exchange through the year. The net flux of nitrates was measured for these estuaries on a seasonal basis for both spring and neap tides. Results indicate that although there is a net export of nitrates to the nearshore, there were instances, particularly on the spring tide, when a net import of nitrates into the estuary occurred. Data analysis reveal summer and neap tide flux dominance for the Tongati and Mgeni estuaries whilst the Isipingo Estuary exhibited larger flux variance for spring tides and the spring season. The origin of the latter is likely derived from unusually high biotic decomposition at sea and/or the longshore transport of decomposing sewage outfall. This creates an added dimension for consideration in estuarine management plans. Taking all three estuaries studied into consideration, a net export of nitrates for all seasons for the TM was measured with a clear seasonal influence detected where high rainfall seasons led to greater export as a consequence of greater fluvial flows, erosion and leaching of agricultural lands and, longer ebb duration and flows.     

台风浪作用下长江口深水航道骤淤数值模拟

极端天气条件下,波浪引起的高含沙水体会引起航道骤淤,增大航道回淤量。以"马勒卡"台风期间现场水文资料为基础,建立了波流共同作用下的长江口三维潮流泥沙数学模型,研究北槽航道骤淤问题。数学模型验证了"马勒卡"台风期间的北槽水沙过程,相似性较好,通过分离波浪,探讨了潮流与波流共同作用下的含沙量及航道淤积分布差异,研究了台风浪对航道骤淤的影响。研究表明,台风浪对-15 m等深线以浅水域含沙量影响较大,北槽含沙量自上游至口外逐渐增大,改变了常态天气北槽"中间高、两头低"的含沙量分布趋势。台风浪对北槽口内航道回淤分布影响较小,北槽口外航道淤积量剧增,即航道骤淤主要发生在北槽口外。     

Interglacial marine deposits of the Somerset Levels, South West England

Sands and gravels underlain by silts and clays, known as the Burtle Beds, from the Somerset Levels of South West England have been examined in a specially excavated pit. The Mollusca, Foraminifera and the Ostracoda, and the sparse macroscopic plant remains included in the deposit, all indicate a prgressive marine transgression in which water temperatures obtained approximating to those of the present sea in the adjacent Bristol Channel. The deposits are shown to be estuarine/marine near shore and intertidal sediments of interglacial age. Radiometric assays and studies of palaeomagnetism have failed to indicate clearly which interglacial marine transgressive phase is involved. Geomorphological considerations supported to some extent by the Ostracoda suggest that a last interglacial (Ipswichian) age is more likely than an earlier (Hoxnian?) date. Estimates of the height of sea levels involved are, of necessity, based on evidence which cannot be regarded as wholly reliable. Having regard to the environmental conditions indicated by the included fauna, and making allowance for post depositional erosion, it is suggested that mean sea level at the height of the ‘Burtle’ transgression was between 9 and 12 m above that of the present (O. D.). Assuming a similar tidal range to that of today, a MHWST (Mean High Water of Spring Tides) level of between 15 and 18 m above O. D. is indicated     

Process Contribution to the Time-Varying Residual Circulation in Tidally Dominated Estuarine Environments

In tide-dominated environments, residual circulation is the comparatively weak net flow in addition to the oscillatory tidal current. Understanding the 3D structure of this circulation is of importance for coastal management as it impacts the net (longer term and event-scale) transport of suspended particles and the advection of tracer quantities. The Dee Estuary, northwest Britain, is used to understand which physical processes have an important contribution to the time-varying residual circulation. Model simulations are used to extract the time-varying contributions of tidal, riverine (baroclinicity and discharge), meteorological, external and wave processes, along with their interactions. Under hypertidal conditions, strong semi-diurnal interaction within the residual makes it difficult to clearly see the effect of a process without filtering. An approach to separate the residual into the isolated process contribution and the contribution due to interaction is described. Applying this method to two hypertidal estuarine channels, one tide dominant and one baroclinic dominant, reveals that process interaction can be as important as the sub-tidal residual process contributions themselves. The time variation of the residual circulation highlights the impact of different physical process components at the event scale of tidal conditions (neap and spring cycles) and offshore storms (wind, wave and surge influence). This gives insight into short-term deviation from the typical estuarine residual. Both channels are found to react differently to the same local conditions, with different short-term change in process dominance during events of high and low energy.     

Sharp-based, tide-dominated deltas of the Sego Sandstone, Book Cliffs, Utah, USA

The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east‐central Utah contains three 10‐ to 20‐m thick layers of tide‐deposited sandstone arranged in a forward‐ and then backward‐stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave‐influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide‐influenced river deltas during a time of fluctuating relative sea‐level. Shale‐dominated offshore marine deposits gradually shoal and become more sandstone‐rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea‐level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp‐based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic‐sandstone are exposed, a sharp‐based upward‐coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide‐eroded estuaries, formed during relative sea‐level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward‐stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward‐stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.     

Sedimentation in a river dominated estuary

The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta. Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 10³ m³ s^?1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level. The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents. During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change. If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology.     

Sediment dispersal and quantitative stratigraphic architecture across an ancient shelf

Two large (200 to 300 km), near‐continuous outcrop transects and extensive well‐log data (ca 2800 wells) allow analysis of sedimentological characteristics and stratigraphic architecture across a large area (ca 60 000 km²) of the latest Santonian to middle Campanian shelf along the western margin of the Western Interior Seaway in eastern Utah and western Colorado, USA. Genetically linked depositional systems are mapped at high chronostratigraphic resolution (ca 0·1 to 0·5 Ma) within their sequence stratigraphic context. In the lower part of the studied interval, sediment was dispersed via wave‐dominated deltaic systems with a ‘compound clinoform’ geomorphology in which an inner, wave‐dominated shoreface clinoform was separated by a muddy subaqueous topset from an outer clinoform containing sand‐poor, gravity‐flow deposits. These strata are characterized by relatively steep, net‐regressive shoreline trajectories (>0·1°) with concave‐landward geometries, narrow nearshore belts of storm‐reworked sandstones (2 to 22 km), wide offshore mudstone belts (>250 km) and relatively high sediment accumulation rates (ca 0·27 mm year^?1). The middle and upper parts of the studied interval also contain wave‐dominated shorefaces, but coeval offshore mudstones enclose abundant ‘isolated’ tide‐influenced sandstones that were transported sub‐parallel to the regional palaeoshoreline by basinal hydrodynamic (tidal?) circulation. These strata are characterized by relatively shallow, net‐regressive shoreline trajectories (<0·1°) with straight to concave‐seaward geometries, wide nearshore belts of storm‐reworked sandstones (19 to 70 km), offshore mudstone belts of variable width (130 to >190 km) and relatively low sediment accumulation rates (ca ≤0·11 mm year^?1). The change in shelfal sediment dispersal and stratigraphic architecture, from: (i) ‘compound clinoform’ deltas characterized by across‐shelf sediment transport; to (ii) wave‐dominated shorelines with ‘isolated’ tide‐influenced sandbodies characterized by along‐shelf sediment transport, is interpreted as reflecting increased interaction with the hydrodynamic regime in the seaway as successive shelfal depositional systems advanced out of a sheltered embayment (‘Utah Bight’). This advance was driven by a decreasing tectonic subsidence rate, which also suppressed autogenic controls on stratigraphic architecture.     

Conservative behaviour of riverine dissolved organic carbon in the Severn Estuary: chemical and geochemical implications

The distribution, variability and chemical behaviour of dissolved organic carbon (DOC) was investigated over 2$\text{1}\text{2}$ years in the Severn Estuary and Bristol Channel, UK. The concentrations of riverine DOC (3.1–7.8 mg C l^?1) covaried with river flow and were invariably conservative in this turbid slowly flushing (~200 days) estuary, indicating that any microbial degradation, chemical flocculation or adsorption processes do not affect the flux of riverine DOC through the estuary. The DOC inputs from the Severn (1.7–2.7 × 10¹⁰ g Cyr^?1) and other rivers (2.6–3.4 × 10¹⁰ g Cyr^?1) are the principal sources of DOC in the estuary and correspond to an export of 0.7–1.1% of the terrestrial productivity from the river catchment to the ocean. This export rate is in accord with recent predictions derived from global compilations of organic inputs from rivers and would imply that the global flux of riverine DOC could be as high as 7.8 × 10¹⁴ g Cyr^?1 which is 5 times greater than some previous estimates.The geochemical significance of a conservative delivery of riverine DOC to the ocean is that irrespective of which flux estimate is considered, such river inputs would make a significant contribution (~SO%) to oceanic DOC, and that the steady-state oceanic DOC flux would have to be significantly greater than present estimates (2.9 × 10¹⁴ g Cyr^?1) which are based on a mean radio carbon age of 3400 yr.An alternative, more realistic DOC flux model, which assumes a polydisperse age distribution about the mean age, is shown to yield the higher oceanic DOC fluxes required. Flocculation and adsorption processes would remove less than 10% and 0.2% respectively of riverine DOC in estuaries.     

Sedimentological and ichnological signatures of changes in wave,river and tidal influence along a Neogene tropical deltaic shoreline

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.     

磨刀门河口洪季波生流及其泄洪影响

磨刀门已由"径流型"向"径流-波浪型"河口转变,波浪已是该河口主要动力之一,但波浪对河口洪季水流及泄洪的影响缺少研究。在2-D潮流数学模型中添加随潮位实时变化的波浪辐射应力,建立波浪潮流耦合数学模型;波浪求解采用缓坡方程,背景水深由潮流模型实时提供,可通过比较考虑和未考虑波浪影响的河口流场来分析波浪对泄洪的影响。在年均常浪作用下,磨刀门河口洪季涨落潮阶段均有明显的波生环流结构。由于波浪作用方向向陆,波生流减弱了浅滩区的向海余流,增大了浅滩向陆余流;受浅滩向海余流减弱影响,河口动力自调整后形成归槽水流,促使深槽内向海余流增大。波浪有顶托河口泄洪之势,可改变滩槽泄洪分配比例;年均常浪的波高较小,其对潮流及泄洪的影响区域限制在浅水区,故对泄洪的负面影响有限。     

长江口12.5 m深水航道回淤特征

针对长江口12.5 m深水航道的回淤问题,收集整理了2010—2012年的航道回淤资料和水文测验资料,研究了航道回淤的时空变化特征及其与径流、潮流和含沙量的关系。结果表明:航道回淤呈洪季大、枯季小的年内变化特征,大风骤淤明显;航道回淤沿程主要集中在南港—圆圆沙段和北槽中下段,其回淤量占全航道的80%以上。南港—圆圆沙段回淤的泥沙颗粒以细砂为主,回淤强度与径流的关系不密切,与潮流的关系表现为大潮大、小潮小。北槽航道回淤泥沙颗粒以粉砂为主,回淤部位随径、潮流变化而变化,表现为径流增大,回淤部位下移;潮动力减弱,回淤部位上提。长江口拦门沙水域泥沙的再悬浮,可能是航道淤积最主要的泥沙来源。