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1.
This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.  相似文献   

2.
The Ouémé River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km2 circular central basin, Lake Nokoué, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km2). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood‐tidal delta sand deposited in association with non‐preserved transgressive sand barriers. The change in estuary‐mouth sedimentation from a transgressive barrier‐inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross‐barrier tidal flux associated with barrier‐inlet systems. As barrier formation west of the Ouémé River led to an increasingly rectilinear shoreline, the longshore drift cell matured, ensuring voluminous eastward transport of sand from the Volta Delta in Ghana, the major purveyor of sand, to the Ouémé embayment, 200 km east. Concomitantly, the number of tidal inlets, and the tidal flux associated with a hitherto interlinked lagoonal system on this coast, diminished. Complete sealing of Lake Nokoué has produced a large, permanently closed estuary, where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east. Since 1885, tides have entered the estuary directly through an artificial outlet cut across the sand barrier. Although precluding the seaward loss of fluvial sediments, permanent estuary‐mouth closure has especially deprived the highstand estuary of marine sand, a potentially important component in estuarine infill on wave‐dominated coasts. In spite of a significant fluvial sediment supply, estuarine infill has been moderate, because of the size of the central basin. Estuarine closure has resulted in two co‐existing highstand sediment suites, with limited admixture, the marine‐derived, estuary‐mouth barrier and upland‐derived back‐barrier sediments. This situation differs from that of mature barrier estuaries characterized by active fluvial‐marine sediment mixing and facies interfingering.  相似文献   

3.
The literature on incised river valley sedimentology is dominated by studies of sediment‐rich systems in which the valley has been filled during and/or shortly after drowning. In contrast, the Holocene evolution of the Kosi Lagoon, South Africa (an incised coastal plain river valley) took place under very low sedimentation rates which have produced a distinctive stratigraphy and contemporary sedimentary environments. The findings are based on a synthesis of the results of studies of seismic stratigraphy, sediment distribution, morphodynamics and geomorphology. Barrier migration was prevented by a high pre‐Holocene dune barrier against which Holocene coastal deposits accumulated in an aggradational sequence. Holocene evolution of the back barrier involved: (i) drowning of the incised valley; (ii) wave‐induced modification of the back‐barrier shoreline leading to segmentation during the highstand; and (iii) marine sedimentation adjacent to the tidal inlet. Segmentation has divided the estuary into a series of geochemically and sedimentologically distinctive basins connected by channels in the estuarine barriers. The seismic stratigraphy of the back barrier essentially lacks a transgressive systems tract, shoreline modification and deposition having been accomplished during the highstand. The lack of historical geomorphological change suggests that the system has achieved morphological equilibrium with ambient energy conditions and low sediment supply. This study presents a classification for estuarine incised valley fills based on the balance between sea‐level rise and sedimentation in which Kosi represents a ‘give‐up’ estuary where much of the relict incised channel form is drowned and preserved. It exhibits a fundamentally different set of evolutionary processes and stratigraphic sequences to those of the better known incised valley systems in which sedimentation either keeps pace with sea‐level (‘keep‐up’ estuaries) or occurs after initial drowning (‘catch‐up’ estuaries).  相似文献   

4.
High-resolution sedimentological and rock magnetic analyses from sediment cores collected in the Seine estuary record changes in coastal sedimentary dynamics linked to climatic variations during the late Holocene. Using AMS 14C and paleomagnetic data we present a first attempt in developing a reliable age model on macrotidal estuarine archives, with a decadal resolution. Correlations between sedimentary successions from the outer Seine estuary document the main sedimentary infilling phases of the system during the last 3000 years. Between 3000 and 1150 cal. BP sedimentary patterns reveal that sequence deposition and preservation are predominantly controlled by marine and tidal hydrodynamics while severe storm events are recorded at ca. 2700 and 1250 cal. BP in the outermost estuary. Conversely, the Medieval Warm Period (MWP; 900–1200 AD) is characterized by a drastic waning of the influence of marine hydrodynamics on sediment preservation. Pronounced episodes of Seine river floods indicate a much stronger impact of continental inputs on sedimentary patterns during this period. The onset of the Little Ice Age marks a diminishing influence of continental inputs while tidal and open marine hydrodynamics again exerted a primary control on the sedimentary evolution of the system during 1200–2003 AD. Coastal sedimentary dynamics as preserved within sedimentary successions appear to have been largely influenced by changes in storminess during the last 3000 years. We have matched the preservation of MWP Seine river floods, as revealed by sedimentological and rock magnetic proxy data, to a prolonged interval of weakened storminess in Normandy permitting the preservation of estuarine flood deposits within a context of reduced coastal erosion in northern Europe. The preservation of sedimentary successions in the Seine estuary is therefore maximal when climate conditions resembled those of the preferred low phase of the NAO on multidecadal timescales such as during 800–1200 AD (MWP). In contrast, increased removal and transport of estuarine sediments occur when winter storm activity greatly intensified over northwestern France. We report four prominent centennial-scale periods of stronger storminess, occurring with a pacing of ～1500 years, which are likely to be related to the last four Bond's Holocene cold events. Our results documenting a close link between coastal sedimentary dynamics, millennial-scale variations in Holocene climate and North Atlantic atmospheric circulation are fairly consistent with other records from Scandinavia, central Greenland and southern Europe.  相似文献   

5.
Future estuarine geomorphic change, in response to climate change, sea-level rise, and watershed sediment supply, may govern ecological function, navigation, and water quality. We estimated geomorphic changes in Suisun Bay, CA, under four scenarios using a tidal-timescale hydrodynamic/sediment transport model. Computational expense and data needs were reduced using the morphological hydrograph concept and the morphological acceleration factor. The four scenarios included (1) present-day conditions; (2) sea-level rise and freshwater flow changes of 2030; (3) sea-level rise and decreased watershed sediment supply of 2030; and (4) sea-level rise, freshwater flow changes, and decreased watershed sediment supply of 2030. Sea-level rise increased water levels thereby reducing wave-induced bottom shear stress and sediment redistribution during the wind-wave season. Decreased watershed sediment supply reduced net deposition within the estuary, while minor changes in freshwater flow timing and magnitude induced the smallest overall effect. In all future scenarios, net deposition in the entire estuary and in the shallowest areas did not keep pace with sea-level rise, suggesting that intertidal and wetland areas may struggle to maintain elevation. Tidal-timescale simulations using future conditions were also used to infer changes in optical depth: though sea-level rise acts to decrease mean light irradiance, decreased suspended-sediment concentrations increase irradiance, yielding small changes in optical depth. The modeling results also assisted with the development of a dimensionless estuarine geomorphic number representing the ratio of potential sediment import forces to sediment export forces; we found the number to be linearly related to relative geomorphic change in Suisun Bay. The methods implemented here are widely applicable to evaluating future scenarios of estuarine change over decadal timescales.  相似文献   

6.
Vegetated mid-channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide-dominated deltas. Meinmahla Island is a mangrove-forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid-channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi-continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single-entrance (dead-end/blind) and multi-entrance tidal channels of the island. The morphological evolution was evaluated using grain size, 210Pb geochronology, remote sensing and channel surveys. The data show that ebb-dominant, single-entrance channels along the island exterior import sediment year-round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multi-entrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal-channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi-entrance channel is responding to a drainage-network change by narrowing and shoaling. Overall, mid-channel islands trap sediment and associated nutrients at the river–ocean interface, and these resilient landscape features evolve in response to changes in drainage-network connectivity.  相似文献   

7.
Many shoreline studies rely on historical change rates determined from aerial imagery decades to over 50 years apart to predict shoreline position and determine setback distances for coastal structures. These studies may not illustrate the coastal impacts of short-duration but potentially high-impact storm events. In this study, shoreline change rates (SCRs) are quantified at five different sites ranging from marsh to sediment bank shorelines around the Albemarle-Pamlico estuarine system (APES) for a series of historical (decadal to 50-year) and short-term (bimonthly) time periods as well as for individual storm events. Long-term (historical) SCRs of approximately ?0.5 ± 0.07 m year?1 are observed, consistent with previous work along estuarine shorelines in North Carolina. Short-term SCRs are highly variable, both spatially and temporally, and ranged from 15.8 ± 7.5 to ?19.3 ± 11.5 m year?1 at one of the study sites. The influence of wave climate on the spatial and temporal variability of short-term erosion rates is investigated using meteorological observations and coupled hydrodynamic (Delft3D) and wave (SWAN) models. The models are applied to simulate hourly variability in the surface waves and water levels. The results indicate that in the fetch-limited APES, wind direction strongly influences the wave climate at the study sites. The wave height also has an influence on short-term SCRs as determined from the wave simulations for individual meteorological events, but no statistical correlation is found for wave height and SCRs over the long term. Despite the significantly higher rates of shoreline erosion over short time periods and from individual events like hurricanes, the cumulative impact over long time periods is low. Therefore, while the short-term response of these shorelines to episodic forcing should be taken into account in management plans, the long-term trends commonly used in ocean shoreline management can also be used to determine erosion setbacks on estuarine shorelines.  相似文献   

8.
Sedimentation in a river dominated estuary   总被引：2，自引：0，他引：2  
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.  相似文献   

9.
Oceanic islands – such as the Azores in the mid‐North Atlantic – are periodically exposed to large storms that often remobilize and transport marine sediments along coastlines, and into deeper environments. Such disruptive events create deposits – denominated tempestites – whose characteristics reflect the highly dynamic environment in which they were formed. Tempestites from oceanic islands, however, are seldom described in the literature and little is known about storm‐related sediment dynamics affecting oceanic island shelves. Therefore, the geological record of tempestite deposits at oceanic islands can provide invaluable information on the processes of sediment remobilization, transport and deposition taking place on insular shelves during and after major storms. In Santa Maria Island (Azores), a sequence of Neogene tempestite deposits was incorporated in the island edifice by the ongoing volcanic activity (thus preserved) and later exposed through uplift and erosion. Because it was overlain by a contemporary coastal lava delta, the water depth at the time of deposition could be inferred, constituting an excellent case‐study to gain insight on the still enigmatic processes of insular shelf deposition. Sedimentological, palaeontological, petrographic and palaeo‐water depth information allowed the reconstruction of the depositional environment of these sediments. The sequence typifies the characteristics of a tempestite (or successive tempestites) formed at ca 50 m depth, in a steep, energetic open insular shelf, and with evidence for massive sediment remobilization from the nearshore to the middle or outer shelf. The authors claim that cross‐shelf transport induced by storm events is the main process of sediment deposition acting on steep and narrow shelves subjected to high‐energetic environments, such as the insular shelves of open‐sea volcanic islands.  相似文献   

10.
The purpose of this paper is to analyze downstream effects of freshwater flow diversion from a small, active-continental-margin river basin. The Skokomish River delta is a tributary estuary to Hood Canal in Washington state that receives drainage from the southeastern side of the Olympic Mountains. Its drainage basin is steep, and rainfall is high. Approximately 40% of the annual average runoff of the entire system has been diverted from the North Fork Skokomish River for power production since completion of two dams in 1930; this water does not pass through the lower river or over the delta. Extensive logging has occurred in the remainder of the basin. Comparison of prediversion (1885) and postdiversion (1972) bathymetric surveys shows that deposition (about 0.013 to 0.022 m yr?1) has occurred on the inner delta and erosion (up to 0.033 m yr?1) on the outer delta. This steepening of the delta surface has apparently been caused by a loss of sediment transport capacity in the lower river and estuary combined with an increased sediment supply due to logging. Although the total area of unvegetated tidal flats thas decreased by only about 6%, there has been a more than 40% loss of highly productive low intertidal surface area. A conservative estimate of loss of eelgrass (Zostera marina) beds is 18%; a reduction in the size of mesohaline mixing zone has also occurred. These habitat losses are similar to those observed elsewhere in the world in larger river basins that have suffered water withdrawals of the same magnitude, but their impacts cannot either be evaluated or understood causally through consideration of simple measures like changes total estuarine deltaic area. Evaluation of estuarine effects of anthropogenic modification must, therefore, include consideration of both changes in habitat function and in the physical processes. These must be evaluated within the totality of the river basin-estuary system that cause these changes. In this case, sediment transport constitutes the critical link between fluvial alterations and the remote downstream, estuarine consequences thereof.  相似文献   

11.
The Mono estuary is an infilled, microtidal estuary located on the wave-dominated Bight of Benin coast which is subject to very strong eastward longshore drift. The estuarine fill comprises a thick unit of lagoonal mud deposited in a ‘central basin’between upland fluvial deposits and estuary-mouth wave-tide deposits. This lagoonal fill is capped by organic-rich tidal flat mud. In addition to tidal flat mud, the superficial facies overlying the ‘central basin’fill include remnants of spits resting on transgressive/washover sand, an estuary-mouth association of beach, shoreface, flood-tidal delta and tidal inlet deposits, and a thin sheet of fluvial sediments deposited over tidal flat mud. After an initial phase of spit intrusion over the infilled central basin east of the present Mono channel, the whole estuary mouth became bounded by a regressive barrier formed from sand supplied by the Volta Delta during the middle Holocene eustatic highstand. Barrier progradation ceased late in the Holocene following the establishment of an equilibrium plan-form shoreline alignment that allowed through-drift of Volta sand to sediment sinks further downdrift. Over the same period, accretion, from fluvially supplied sediments, of the estuarine plain close to the limit of spring high tides, or, over much of the lower valley, into a fluvial plain no longer subject to tidal flooding, induced marked meandering of the Mono and its tidal distributaries in response to confinement of much of the tidal prism to these channels. The process resulted in erosion of spit/washover and regressive barrier sand, and in reworking of the tidal flat and floodbasin deposits. The strong longshore drift, equilibrium shoreline alignment and the year-round persistence of a tidal inlet maintained by discharge from the Mono and from Lake Ahémé have resulted in a stationary barrier that is reworked by a mobile inlet. The Mono example shows that advanced estuarine infill may result in considerable facies reworking, obliteration of certain facies and marked spatial imbrication of fluvial, estuarine and wave-tide-deposited facies, and confirms patterns of sedimentary change described for microtidal estuaries on wave-influenced coasts. In addition, this study shows that local environmental factors such as sediment supply relative to limited accommodation space, and strong longshore drift, which may preclude accumulation of sediments in the vicinity of the estuary mouth, may lead to infilled equilibrium or near-equilibrium estuaries that will not necessarily evolve into deltas.  相似文献   

12.
A tidally averaged model of estuarine dynamics is used to estimate sediment transport in the Hudson River estuary over the period 1918 to 2005. In long-term and seasonal means, along-channel gradients in sediment flux depend on the estuarine salinity gradient and along-channel depth profile. Lateral depth variation across the estuary affects the near-bottom baroclinic circulation and consequently the direction of net sediment flux, with generally up-estuary transport in the channel and down-estuary transport on the shoals. Sediment transport capacity in the lower estuary depends largely on river discharge, but is modified by the timing of discharge events with respect to the spring–neap cycle and subtidal fluctuations in sea level. Sediment transport capacity also depends on the duration of high-discharge events relative to the estuarine response time, a factor that varies seasonally with discharge and estuarine length. Sediment fluxes are calculated with the assumption that over long periods, the system approaches morphological equilibrium and sediment accumulation equals sea level rise. The inferred across- and along-channel distributions of sediment erodibility correspond with observations of bed properties. Equilibrium is assumed at long time scales, but at annual to decadal time scales the estuary can develop an excess or deficit of sediment relative to equilibrium. On average, sediment accumulates in the estuary during low- and high-discharge periods and is exported during moderate discharge. During high-discharge periods, maximum export coincides with maximum sediment supply from the watershed, but the nearly cubic discharge dependence of fluvial sediment supply overwhelms the roughly linear increase in estuarine transport capacity. Consequently, sediment accumulates in the estuary during the highest flow conditions. Uncertainty remains in the model, particularly with sediment properties and boundary conditions, but the results clearly indicate variability in the sediment mass balance over long time scales due to discharge events.  相似文献   

13.
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14.
Estuarine and beach deposits in the vicinity of the present coastline at Pakarae River record the infilling of an estuary and subsequent development of a sequence of seven marine terraces during Holocene time.

At the maximum of the last glaciation about 18,000 years ago the shoreline at the ancestral Pakarae River was approximately 20 km east of the present shoreline. By about 9000 years BP the sea had transgressed across most of that coastal plain to lie within a few hundred metres of the base of the present coastal hills. Seventeen radiocarbon ages from estuarine deposits record the overall rise in post-glacial sea level, but in the period c. 9500-7000 yrs BP there are reversals to the overall rising trend. Between 9500 and 8500 yrs BP there appears to have been a eustatic fall in sea level of at least 4 m. This observation is supported by data from several other localities around New Zealand. Maximum transgression occurred about 6500–7000 yrs BP when the sea reached the base of hillslopes and an extensive estuary existed behind a barrier bar.

Since that time the barrier bar disappeared, probably due to stranding in an uplift event, and the coastline advanced progressively outward toward its present position. Coastal progradation (sea level regression) and subsequent erosion have occurred in association with episodic large earthquakes at about 6700, 5400, 3910, 2450, 1570, 1000 and 600 yrs BP. The present distribution of terraces has been influenced by coastal erosion, which has removed all trace of some terraces from some areas, and river erosion has modified the marine terraces near the river.  相似文献   


15.
《Sedimentology》2018,65(4):1170-1212
Barrier‐island system evolution is controlled by internal and external forcing mechanisms, and temporal changes in these mechanisms may be recorded in the sedimentary architecture. However, the precise role of individual forcing mechanisms is rarely well understood due to limited chronological control. This study investigates the relative role of forcing conditions, such as antecedent topography, sea‐level rise, sediment supply, storms and climate changes, on the evolution of a Holocene wave‐dominated barrier‐island system. This article presents temporal reconstruction of the depositional history of the barrier‐island system of Rømø in the Wadden Sea in unprecedented detail, based on ground‐penetrating radar profiles, sediment cores, high‐resolution dating and palynological investigations, and shows that ca 8000 years ago the barrier island formed on a Pleistocene topographic high. During the initial phase of barrier evolution, the long‐term sea‐level rise was relatively rapid (ca 9 mm year−1) and the barrier was narrow and frequently overwashed. Sediment supply kept pace with sea‐level rise, and the barrier‐island system mainly aggraded through the deposition of a ca 7 m thick stack of overwash fans. Aggradation continued for ca 1700 years until sea‐level rise had decreased to <2 mm year−1. In the last ca 6000 years, the barrier prograded 4 to 5 km through deposition of a 10 to 15 m thick beach and shoreface unit, despite a long‐term sea‐level rise of 1 to 2 mm year−1. The long‐term progradation was, however, interrupted by a transgression between 4000 years and 1700 years ago. These results demonstrate that the large‐scale morphology of the Danish Wadden Sea shoreline influences the longshore sediment transport flux and the millennial‐scale dispersal of sediment along the shoreline. On decadal to centennial timescales, major storms induced intense beach and shoreface erosion followed by rapid recovery and progradation which resulted in a highly punctuated beach and shoreface record. Major storms contributed towards a positive sediment budget, and the sustained surplus of sediment was, and still is, instrumental in maintaining the aggradational–progradational state of the barrier island.  相似文献   

16.
AMS 14C ages of post-glacial core sediments from the subaqueous Yangtze delta, along with sedimentary structures and distributions of grain size, pollen spores, and dinoflagellate cysts, show an estuarine depositional system from 13 to 8.4 cal ka BP and a deltaic system from 5.9 cal ka BP to the present. The estuarine system consists of intertidal to subtidal flat, estuarine, and estuarine-front facies, characterized by sand–mud couplets and a high sedimentation rate. The deltaic system includes nearshore shelf and prodelta mud featured by lower sedimentation rate, markedly fewer coastal wetland herbaceous pollens, and more dinoflagellate cysts. We explain the extremely high sedimentation rate during 9.2–8.4 cal ka BP at the study site as a result of rapid sea-level rise, high sediment load due to the unstable monsoonal climate, and subaqueous decrease of elevation from inner to outer estuary. A depositional hiatus occurred during 8.2–5.9 cal ka BP, the transition from estuarine to deltaic system, caused possibly by a shortage of sediment supply resulting from delta initiation in paleo-incised Yangtze valley and strong tidal or storm-related reworking in offshore areas. The subsequent development of deltaic system at the study site indicates accelerated progradation of Yangtze delta post-5.9 cal ka BP.  相似文献   

17.
Material transfer between estuaries and the nearshore zone has long been of interest, but information on the processes affecting Pacific Northwest estuaries has lagged behind other areas. The west coast of the U.S. is a region of seasonally variable upwelling that results in enhanced phytoplankton production in the nearshore zone. We examined estuarine-nearshore links over time by measuring physical oceanographic variables and chlorophylla concentration from an anchor station in South Slough, Oregon. Data was collected during 24-h cruises conducted at approximately weekly intervals during summer 1996 and spring 1997. The results demonstrate that the physical oceanography of this estuarine site was strongly influenced by the coastal ocean. Marine water reached the estuarine site on every sampled tide, and chlorophylla was clearly advected into the estuary with this ocean water. In contrast, phytoplankton concentrations were comparatively reduced in the estuarine water. There were, however, large fluctuations in the import of chlorophyll over the course of the summer. These variations likely reflect upwelling-generated phytoplankton production in the coastal ocean and subsequent cross-shelf transport to the estuary. Suspension feeding organisms in South Slough likely depend on the advection of this coastally-derived phytoplankton. The large allochthonous chlorophyll input measured for this system appears dissimilar from most estuaries studied to date. Previous investigations have focused on the outwelling and inwelling of materials in estuaries. We must now consider the influence of coastal upwelling and downwelling processes on estuarine material exchange.  相似文献   

18.
This paper examines sediment transport, sedimentation and properties of suspended matter and sediments in the Ra?a River estuary, a small, rock-bounded, microtidal, low-wave-energy karstic estuary in the north-eastern Adriatic. The Ra?a River is characterized by large variation in water flow and variable load of mineral particles. More than 90% of this load is brought into the estuary as fine-grained suspended matter, consisting of only 24–36% of carbonates, the rest being clays. Sedimentation occurs at the salt wedge, resulting in a prograding estuarine delta. Salt-induced flocculation is the predominant process of sediment deposition. The Ra?a estuary is infilling with sediment, and classifies as a disequilibrium estuary. We propose a modification of Cooper's (1993) classification scheme to include river-dominated, disequilibrium estuaries, with the Ra?a River as an example.  相似文献   

19.
Hurricanes 2004: An overview of their characteristics and coastal change   总被引：2，自引：0，他引：2  
Four hurricanes battered the state of Florida during 2004, the most affecting any state since Texas endured four in 1884. Each of the storms changed the coast differently. Average shoreline change within the right front quadrant of hurricane force winds varied from 1 m of shoreline advance to 20 m of retreat, whereas average sand volume change varied from 11 to 66 m3 m−1 of net loss (erosion). These changes did not scale simply with hurricane intensity as described by the Saffir-Simpson Hurricane Scale. The strongest storm of the season, category 4 Hurricane Charley, had the least shoreline retreat. This was likely because of other factors like the storm's rapid forward speed and small size that generated a lower storm surge than expected. Two of the storms, Hurricanes Frances and Jeanne, affected nearly the same area on the Florida east coast just 3 wk apart. The first storm, Frances, although weaker than the second, caused greater shoreline retreat and sand volume erosion. As a consequence, Hurricane Frances may have stripped away protective beach and exposed dunes to direct wave attack during Jeanne, although there was significant dune erosion during both storms. The maximum shoreline change for all four hurricanes occurred during Ivan on the coasts of eastern Alabama and the Florida Panhandle. The net volume change across a barrier island within the Ivan impact zone approached zero because of massive overwash that approximately balanced erosion of the beach. These data from the 2004 hurricane season will prove useful in developing new ways to scale and predict coastal-change effects during hurricanes.  相似文献   

20.
Harbor-estuaries are complex sedimentary environments where both natural and anthropogenic processes influence the distribution and accumulation of sediments, which in turn is of importance for maintenance dredging and pollution control. This paper uses sediment characteristics from the Göta älv estuary, Sweden to evaluate the natural estuarine processes that persist despite the extensive human impact on the estuary and to separate and characterize depositional sub-environments. The most important harbor processes include maintenance dredging and turbulence from ship traffic, and their influence upon shallow stratigraphy and sediment distribution and deposition is significant. Polymodal particle-size distributions are interpreted to reflect the natural estuarine transport processes. Suspension transport is generally predominant, except in the river and inner harbor where bottom transport and selective deposition of sand occur. Particle-size characteristics divide the estuary into four depositional areas: the river and the inner, middle, and outer harbor. The shallow stratigraphy reveals a hiatus between the firm silty clays and the loose recent sediments. The hiatus in the stratigraphy is probably related to dredging in most areas of the harbor. Significant maintenance dredging and turbulence from ship traffic also influence the accumulation (0–61 cm) of recent sediments, during the 20th century.  相似文献   

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