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1.
The Holocene evolution of the Canning Coast of Western Australia has largely been overlooked so far mainly due to its remoteness and low population density. We report on new data from a sequence of foredunes inside the macro‐tidal Admiral Bay, 110 km southwest of Broome. Based on sediment cores, differential global positioning system (dGPS)‐based elevation transects, and stratigraphical analyses on outcrops of the relict foredunes, we aim at reconstructing Holocene coastal changes and relative sea levels (RSLs), as well as identifying and dating imprints of extreme‐wave events. Sedimentary analyses comprise the documentation of bedding structures, foraminiferal content and macrofaunal remains, grain size distribution, and organic matter. The chronological framework is based on 26 carbon‐14 accelerator mass spectrometry (14C‐AMS) datings. Marine flooding of the pre‐Holocene surface landward of the 2.5 km‐wide foredune barriers occurred 7400–7200 cal bp , when mangroves colonized the area. After only 200–400 years, a high‐energy inter‐tidal environment established and prevailed until c. 4000 cal bp , before turning into the present supralittoral mudflat. During that time, coastal regression led to beach progradation and the formation of aligned foredunes. Drivers of progradation were a stable RSL or gradual RSL fall after the mid‐Holocene and a positive sand budget. The foredunes overlie upper beach deposits located up to >2 m above the present upper beach level and provide evidence for a higher mid‐Holocene RSL. Discontinuous layers of coarse shells and sand are intercalated in the foredunes, indicating massive coastal flooding events. One such layer was traced over three dune ridges and dated to c. 1700–1550 cal bp . However, it seems that most tropical cyclones induce net erosion rather than deposition at aligned foredunes and thus, they are only suitable for reconstructing temporal variability if erosional features or sedimentation reliably tied to these events can be identified and dated accurately. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

2.
Beach ridge stratigraphy can provide an important record of both sustained coastal progradation and responses to events such as extreme storms, as well as evidence of earthquake induced sediment pulses. This study is a stratigraphic investigation of the late Holocene mixed sand gravel (MSG) beach ridge plain on the Canterbury coast, New Zealand. The subsurface was imaged along a 370 m shore-normal transect using 100 and 200 MHz ground penetrating radar (GPR) antennae, and cored to sample sediment textures. Results show that, seaward of a back-barrier lagoon, the Pegasus Bay beach ridge plain prograded almost uniformly, under conditions of relatively stable sea level. Nearshore sediment supply appears to have created a sustained sediment surplus, perhaps as a result of post-seismic sediment pulses, resulting in a flat, morphologically featureless beach ridge plain. Evidence of a high magnitude storm provides an exception, with an estimated event return period in excess of 100 years. Evidence from the GPR sequence combined with modern process observations from MSG beaches indicates that a palaeo-storm initially created a washover fan into the back-barrier lagoon, with a large amount of sediment simultaneously moved off the beach face into the nearshore. This erosion event resulted in a topographic depression still evident today. In the subsequent recovery period, sediment was reworked by swash onto the beach as a sequence of berm deposit laminations, creating an elevated beach ridge that also has a modern-day topographic signature. As sediment supply returned to normal, and under conditions of falling sea level, a beach ridge progradation sequence accumulated seaward of the storm feature out to the modern-day beach as a large flat, uniform progradation plain. This study highlights the importance of extreme storm events and earthquake pulses on MSG coastlines in triggering high volume beach ridge formation during the subsequent recovery period. © 2019 John Wiley & Sons, Ltd.  相似文献   

3.
The results from three years of surveying and monitoring a dynamic foredune and dunefield restoration effort on Vancouver Island, Canada is presented. Complete removal of foredune vegetation occurred in three phases spaced a year apart in an effort to control invasive Ammophila spp. The collection of airborne LiDAR, orthophotographs, and bi‐monthly topographic surveys provided a means to quantify and examine sediment budgets and geomorphic responses. Three survey swaths, corresponding with each phase of vegetation removal, were established to provide detailed topographic coverage over the impacted beach, foredune, and dunefield landscape units. The swath corresponding with the first phase of removal recorded a positive sediment budget of 1·3 m3 m?2 after three years. A control swath, with data collected for a year prior and two years following removal, exhibited a distinct pulse of sediment delivery into the dunefield unit with a maximum gain of 0·03 m3 m?2 pre‐removal compared to 0·11 m3 m?2 post‐removal. Vegetation analysis zones, associated with each of the three swaths, demonstrate a range of vegetation responses due to variation in the vegetation removal and subsequent re‐invasion or removal methods employed. The first site to be cleared of vegetation, received ongoing invasive re‐growth control, and three years following removal vegetation cover dropped from 57% in 2009 to 13% in 2012 (?44%). An adjacent site was cleared of vegetation two years later (only one year of recovery) but experienced rapid Ammophila re‐invasion and percent cover changed from 61% in 2009 to 26% in 2012 (?35%). The data presented provides insights for improving the application of sediment budget monitoring in dynamic restorations and discusses the potential for detailed spatial–temporal survey data to improve our understanding of meso‐scale landscape morphodynamics following foredune disturbance. Overall, the vegetation removal treatments reduced the extent of invasive grass and increased dunefield mobility and dynamic activity. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

4.
A series of airborne topographic LiDAR data were obtained from May 2008 to January 2014 over two coastal sites of northern France (Bay of Wissant and east of Dunkirk). These data were used with wind and tide gauge measurements to assess the impacts of storms on beaches and coastal dunes, and particularly of the series of major storms that hit western Europe during the fall and early winter of 2013. Our results show a high variability in shoreline response from one site to the other, but also within each coastal site. Coastal dune erosion and shoreline retreat occurred at both sites, particularly on the coast of the Bay of Wissant where shoreline retreat up to about 40 m was measured. However, stability or even shoreline advance were also observed despite the occurrence of an extreme water level with a return period >100 years during the storm Xaver in early December 2013. Comparison of shoreline change with variations of coastal dune and upper beach volumes revealed only weak relationships. Our results nevertheless showed that shoreline behavior seems to strongly depend on the initial sediment volume on the upper beach before the occurrence of the storms. According to our measurements, an upper beach volume of about 30 m3 m?1 between the dune toe and the mean high water level is sufficient at these sites to protect the coastal dunes from storm waves associated with high water levels with return periods >10 years. The identification of such thresholds in terms of upper beach width or sediment volume may represent valuable information for improving the management of shoreline change by providing an estimate of the minimum quantity of sand on the upper beach necessary to ensure shoreline stability in this region. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

5.
Vegetation surveys were conducted on a variety of coastal foredunes in a largely natural region along the Gulf County region of the Florida panhandle. Species presence, absence and percentage cover were surveyed on 12 foredune profiles during different seasons. The vegetation data were analyzed using the Shannon–Wiener Diversity Index and Sørensen Index. Uniola sp. and Andropgon sp. were the dominant species on foredunes. Uniola sp. was found predominantly on the gulfward facing or stoss slopes, and Andropgon sp. was found to be dominant on the inland or lee slopes of foredunes. While they are present on all foredunes, their presence and percentage cover are dominant on rapidly prograding coasts. Prograding/accretional beaches had higher Sørensen Index values (i.e. higher similarities) than did the foredune‐vegetation profiles on eroding beaches. Diversity as indicated by the Shannon–Wiener analysis (H’) is greatest on the highest, and generally eroding dunes. Foredune diversity increased with foredune height, and the tallest foredunes were found on shorelines with relatively low erosion rates, where dunes were slowly translating landwards, cannibalizing older dunes, and moving into areas colonized by late successional species, such as Quercus sp. These observations of foredune species richness, diversity, profile similarities, and the use of ecological indices can provide excellent proxy evidence of shoreline dynamics, and in particular the degree of beach erosion and accretion, in the absence of historical erosion/accretion data. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

6.
This paper documents application of an established geostatistical methodology to detect significant changes in a foredune–transgressive dune complex where Parks Canada Agency (PCA) implemented a dynamic restoration program to remove invasive marram grasses (Ammophila spp.) and enhance dynamic dune habitat for an endangered species. Detailed topographic surveys of a 10 320 m2 site in the Wickaninnish Dunes in Pacific Rim National Park, British Columbia, Canada for the first year post‐treatment are compared to a pre‐restoration LiDAR baseline survey. The method incorporates inherent spatial structure in measured elevation datasets at the sub‐landscape scale and models statistically significant change surfaces within distinct, linked geomorphic units (beach, foredune, transgressive dune complex). Seasonal and annual responses within the complex are discussed and interpreted. All geomorphic units experienced positive sediment budgets following restoration treatment. The beach experienced the highest differential volumetric change (+1656 m3) and net sediment influx (+834 m3, 0 · 19 m3 m–2) mostly from supply to the supratidal beach and incipient dune. This sediment influx occurred independent of the restoration effort and was available as a buffer against wave erosion and as supply to the landward dunes. The foredune received +200 m3 (0 · 13 m3 m‐2) and its seaward profile returned to a similar pre‐restoration form following erosion at the crest from vegetation removal and scarping by high water events. Sediment bypassing and minimal change was evident at the mid‐stoss slope with appreciable extension of depositional lobes in the lee. The transgressive dune complex experienced high accretion following restoration activity (+201 m3) and over the year (+284 m3, 0 · 07 m3 m–2) mostly from depositional lobes from the foredune, precipitation ridge growth along the downwind boundary, and growth of existing lobes within the complex. Further integration of this methodology to detect significant geomorphic changes is recommended, particularly for applications where sampling densities are limited or logistically defined. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

7.
In this study, the decadal evolution of a washover fan on the west coast of Denmark is examined from its initial generation in 1990 until 2015. Since its inception, the bare and flat washover fan surface has recovered and accreted slowly due to re-activation by overwash during surges and due to aeolian activity and dune formation, stimulated by vegetation growth. The volume of sand on the washover has increased steadily at an average rate of about 23 m3/yr per unit length of shoreline, and a total of 175,000 m3 of sand is now deposited on the fan, while at the same time the shoreline has receded by some 250 m. The evolution can be divided into three stages: 1) An initiation phase when storm surge levels and energetic wave conditions caused a breach in the foredunes and overwash processes formed a washover fan with a relatively low elevation above mean sea level; 2) An initial recovery phase during which waves supplied sand to the fan during frequent overwash activity and winds transported this sand into marginal dunes surrounding the fan; and 3) A later recovery phase when the surface of the fan had accreted to a level where vegetation could survive and trap sediment into new foredune growth across the fan. The rate of accretion has been overall linear but scales with neither annual overwash frequency, nor with aeolian transport potential. Instead, the linear accretion is more closely related to the steady onshore migration of nearshore bars that weld to the beach and provide a sand supply for transfer to the fan. The fan evolution demonstrates the importance of washover fans in preserving barrier resilience during transgressional phases caused by increasing mean sea level. © 2019 John Wiley & Sons, Ltd.  相似文献   

8.
The Oceano Dunes near Pismo Beach, California is part of a large transgressive dune system that extends up to 5 km inland and hosts a state park that has been managed for off-highway vehicle recreation since 1982, although vehicle activity has existed in the dunes for almost 90 years. As a result, foredunes have been largely obliterated and sand surfaces in vehicle use areas are highly emissive of dust-sized particles, causing frequent exceedances of state air quality standards. To reduce dust emissions from the dunes, a nature-based foredune restoration strategy using five different treatments over a 20-ha site was implemented in February 2020. The research hypothesis is that treatments will differ in their ability to promote deposition and dune development and that more intensive planting-based treatments will outperform simpler treatments. We test this using biannual high-resolution uncrewed aerial system (UAS) surveys to quantify sediment budgets, sand exchanges between beach, foredune, and backdune components, changes in plant cover, and related dune development over a 2-year period (October 2019–2021). After two full wind and plant-growth seasons, results show that all treatments are maintaining a positive sediment budget (net accumulation), most are developing sizable nebkha dunes (an important stage in foredune development in this region), and some are increasing plant cover and species richness. There is no clear winner, yet two treatments (broadcast native plant and sterile grass seeds, and a high-density straw planting node with native seedlings) are performing well toward developing an incipient foredune. These results will inform an adaptive management process that could entail further modifications to enhance foredune development. Based on this experience, and with reference to other types of restoration projects, we refine existing criteria used to assess the performance of “dynamic” dune restoration efforts to include settings that involve revegetation (vs. devegetation) as a means for foredune re-establishment.  相似文献   

9.
The recovery of Santa Rosa Island in northwest Florida is characterized following Hurricane Katrina (September 2005), which was preceded by Hurricanes Ivan (2004) and Dennis (2005). Beach and dune recovery were quantified to the east and west of Pensacola Beach through a comparison of LiDAR data collected immediately following Hurricane Katrina and in July 2006 after almost a year of recovery. East of Pensacola Beach (the Santa Rosa Unit), the shoreline retreated by an average of 64 m during the 2004–2005 hurricane season and recovered by an average of 19 m. To the west of Pensacola Beach (the Fort Pickens Unit), the shoreline retreated by an average of 30 m, and while no significant shoreface recovery was observed, the presence of vegetation on low‐profile dunes promoted backshore accretion. It is found that beachface recovery in the Santa Rosa Unit and backshore accretion in the Fort Pickens Unit occurred at the widest sections of the island where the pre‐storm profile volume had been relatively large and overwash penetration was at a minimum. The narrow sections of the island (between cuspate headlands) had a smaller profile volume before the storms, leading to greater overwash penetration and in some cases island breaching in both sections, which limited the volume of sediment available for shoreface recovery. The alongshore variation in recovery is not only related to the island width, but also the offshore bathymetry, height of the pre‐storm dunes and the overwash penetration. If sufficient time is allowed for the return of vegetation and the recovery of the dunes, the variations in storm impact observed during Hurricane Ivan will be reinforced during subsequent storms. In this respect, the level of impact during subsequent storms and the ability of the island to recover will depend on the frequency of storm events. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Coastal dunes provide essential protection for infrastructure in developed regions, acting as the first line of defence against ocean-side flooding. Quantifying dune erosion, growth and recovery from storms is critical from management, resiliency and engineering with nature perspectives. This study utilizes 22 months of high-resolution terrestrial LiDAR (Riegl VZ-2000) observations to investigate the impact of management, anthropogenic modifications and four named storms on dune morphological evolution along ~100 m of an open-coast, recently nourished beach in Nags Head, NC. The influences of specific management strategies – such as fencing and plantings – were evaluated by comparing these to the morphologic response at an unmanaged control site at the USACE Field Research Facility (FRF) in Duck, NC (33 km to the north), which experienced similar environmental forcings. Various beach-dune morphological parameters were extracted (e.g. backshore-dune volume) and compared with aeolian and hydrodynamic forcing metrics between each survey interval. The results show that LiDAR is a useful tool for quantifying complex dune evolution over fine spatial and temporal scales. Under similar forcings, the managed dune grew 1.7 times faster than the unmanaged dune, due to a larger sediment supply and enhanced capture through fencing, plantings and walkovers. These factors at the managed site contributed to the welding of the incipient dune to the primary foredune over a short period of less than a year, which has been observed to take up to decades in natural systems. Storm events caused alongshore variable dune erosion primarily to the incipient dune, yet also caused significant accretion, particularly along the crest at the managed site, resulting in net dune growth. Traditional empirical Bagnold equations correlated with observed trends of backshore-dune growth but overpredicted magnitudes. This is likely because these formulations do not encompass supply-limiting factors and erosional processes. © 2019 John Wiley & Sons, Ltd.  相似文献   

11.
Coastal dunes are dynamic features that are continuously evolving due to constructive (e.g., wind- and wave-driven sediment transport) and destructive (e.g., elevated total water levels during storm events) processes. However, the relative importance of these processes in determining dune evolution is often poorly understood. In this study, ten lidar datasets from 1997 to 2016 are used to determine the relative role of erosion and accretion processes driving foredune change on the coast of Cape Lookout National Seashore, North Carolina, USA. Beach and dune morphometrics reveal that dune toe locations have generally retreated since 1997, while dune crest heights accreted by 0.01–0.02 m/year. We develop three univariate metrics that represent (1) the potential for erosion, i.e., total water level impact hours per year, (2) accretion, i.e., dune building hours per year, and (3) the relative net effect of foredune accretion and erosion processes, i.e., constructive–destructive dune forcing (CDDF) ratio, and test the correlative power of these metrics in explaining changes in foredune morphology. The total water level impact hours per year metric explained as much as 66% and 67% of the variance in dune crest and toe elevations, respectively, across the nearly two decades of dune evolution. The greatest number of dune building hours per year and largest dunes within the study site co-occurred at locations exposed to the dominant cross-shore wind direction as a result of varying shoreline orientation. The CDDF ratio was positively correlated to changes in the dune toe elevation in approximately 70% of dunes within the study site, outperforming the impact and dune building hours per year metrics. Our results show that these three metrics can provide first-order estimates of dune morphometric change across multiple spatial and temporal scales, which may be particularly useful at sites where lidar acquisition is intermittent.  相似文献   

12.
Since the end of the post‐glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south‐eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid‐Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid‐Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
Beach erosion poses significant threat to small island economies which are generally highly dependent on coastal tourism. This work investigates the evolution of the low-lying sandy coast of Boa Vista through an integrated characterization of coastline and shoreline indicators (over the past four decades) based on aerial imagery. It was found that tandem use of the two indicators was important to obtain a reliable perspective of the Boa Vista low-lying coastal evolution across a wide range of coastal environments. Results indicate that between 1968 and 2010 the coast was relatively stable, although some spatial variability was recognized. The largest changes were observed at the tips of embayed beaches and a clear coastal progradation was found at the southern (downwind) coastal sectors. Coastal evolution has been dominated by sediment budget and the results put in evidence the sedimentary connections between the beaches across the island, either through bypass and overpass processes. Findings show that understanding coastal evolution at low-lying islands should be supported on island-scale observations, being the only scale capable to capture the sedimentary connections between beach systems, that often control coastal evolution. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

14.
St. Vincent Island, on the northwest Gulf coast of Florida, USA, preserves a well-developed beach ridge plain that began to form 4000 years ago based on inferred ages of pottery artefact assemblages. Seven vibra cores up to 2.88 m in length (uncompacted sediment) were retrieved from six ridges across the island. The single-aliquot regenerative-dose (SAR) procedure was used to obtain two optically stimulated luminescence (OSL) ages per core at different depths within each core.

The geographically youngest ridges gave ages of 370±50–1900±300 years, yielding an inter-ridge accumulation time of 150 years assuming uniform sediment accumulation. The oldest ridge sets yielded ages of 2700±400–2800±300 years. Dating results are evaluated in terms of equivalent dose (DE) distributions and other characteristics in relation to aspects of the burial environment, including pedogenesis.  相似文献   


15.
Quartz optically stimulated luminescence (OSL) dating has been applied to sandy beach ridge systems from the Magdalen Islands in the center of the Gulf of St. Lawrence (Quebec, Canada) to provide the first chronological framework for these features. Nineteen beach ridges (22 samples) from four different sites throughout the archipelago were investigated. At one of the sites, samples were taken at 9 m and 7.5 m depth using a vibracore. The quartz is dominated by the fast OSL component and a single-aliquot regenerative-dose (SAR) protocol was used to measure the equivalent doses; a low preheat (180°C/10 s) was chosen to avoid the influence of thermal transfer. The average dose recovery ratio of all samples is 1.02 ± 0.02 (n = 130) suggesting that the SAR protocol works satisfactorily on this material. The OSL ages are internally consistent and supported by independent age control (radiocarbon). The OSL ages indicate that the ridges were built between 2.6 ± 0.2 ka and 0.40 ± 0.10 ka, i.e. during a period of sea level rise. This rise eroded adjacent sandstone cliffs, which contributed a significant sediment supply to the littoral drift and beaches. Some low-lying coasts in the archipelago are still prograding, despite a relative sea level increase of ∼1.6 mm/a over the last 600 years. The late Holocene ages obtained in this study indicate that these processes have been active for at least the past two thousand years. This study demonstrates for the first time that OSL dating using quartz has great potential in this area, and is an appropriate method for establishing precise chronologies for coastal sediments in this region of the Gulf of St. Lawrence.  相似文献   

16.
Optical dating, sedimentological analysis and soil profile development have been used to develop a chronology for, and an understanding of, the geomorphic evolution of the Holocene coastal plain between Otaki and Te Horo, North Island of New Zealand. The coastal plain has prograded 0·48 m a?1 since sea‐levels reached their post‐glacial maximum 6500 years ago. Dune development on the plain, which is dependent on the supply of sediment suitable for dune building, has been episodic. Three periods of dune activity have been identified – the Foxton, Motuiti and Waitarere phases – the last two of which are believed to have resulted from anthropogenic activities. The dunes north of the Otaki River and south of Mangaone Stream are typical of a coastal dune system that extends from Patea in the north to Paekakariki in the south. However, this system is disrupted by the Otaki River and the gravels it delivers to the coastal zone. Immediately south of the river mouth the dunes are significantly smaller, coarser, and contain significantly more magnetic material. The character of the landforms is the result of: the reworking of the last glacial deposits; ongoing coastal progradation; variation in the input of sediment suitable for dune formation; the change in beach character because of gravel input; and the position of the Otaki River mouth. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
Although dunes fronted by sandy beaches constitute approximately 80 per cent of South Africa's coastline, few studies have addressed the formation and life cycle of coastal foredunes, the small, ephemeral shore‐parallel dune ridges typically less than 5 m high and 20 m wide, which form seaward of the storm line. This study used regular, detailed topographic surveys of embryo and foredunes at Tugela mouth, an aggrading stretch of shoreline on the subtropical east coast of South Africa, over a 32‐month period, to gain insight into the formation and motion of these highly mobile landforms over the short term. Average wind drift potential at Tugela mouth during the study period, at 2·35 m s?1, was an order of magnitude lower than that typical of most parts of the eastern South African coast. The dominant sand‐moving wind for the region was from the southwest to west‐southwest at 10·7 to 13·8 m s?1, with a secondary vector from north to north‐northeast at 10·8 to 13·8 m s?1. Signi?cant shoreline retreat, a result of the low sediment yield of the Tugela River during the study period, was one of the main results. This provided the context for redistribution of sand from the inland to the seaward side of the study area, a consequence of the dominant wind direction, and for frequent creation and destruction of short‐lived embryo dunes. Those foredunes which survived the whole study period tended to increase in height, but there was no consistent directional trend in foredune crest movement throughout the 32 months. The study results generally supported Psuty's model of foredune development, but could not con?rm his contention of landward retreat of dune forms under conditions of shoreline erosion. This may be due to the relatively short duration of the study, or possibly to low wind drift potential at the site. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

18.
Optically stimulated luminescence (OSL) dating was performed on Late Quaternary deltaic sequences from a 55-m-long core sampled from the Nakdong River estuary, Korea. OSL ages obtained from chemically separated fine (4–11 μm) and coarse (90–212 μm) quartz grains ranged from 29.4 ± 2.6 to 0.4 ± 0.04 ka, revealing clear consistency between the grain-size fractions. The De values from the standardized growth curve (SGC) are consistent with those from the single-aliquot regenerative-dose (SAR) procedure, which suggests that the SGC is valid for the Nakdong deltaic sediments. The 14C ages of shells and wood fragments ranged from 11 to 2.9 ka, demonstrating reasonable agreement with the OSL ages, within the error range. However, the limited number and random sampling interval of the 14C age data (10 ages) result in a simple linear and exponential trend in the depth–age curve. In contrast, OSL ages obtained by high-resolution sampling show down-section variations in the depth–age curve, indicating the occurrence of rapid changes in sedimentation rate. It is suggested that the high-sampling-resolution OSL ages provide a more realistic and detailed depth–age curve and sedimentation rate. The Nakdong deltaic sediments were divided into five units based on sedimentation rate. The lowest (unit 5) shows a break in sedimentation between the last glacial maximum (LGM) and the Holocene. The sedimentation rate increased in units 4 and 3, presumably corresponding to the early to middle Holocene sea level rise and high stand. Unit 2 shows a gradually decreasing sedimentation rate following the regression of the shoreline, until about 2 ka. The progradation of the Nakdong River delta resulted in the rapid accumulation of unit 1 during the last 2000 years.  相似文献   

19.
Spatial patterns of multidecadal shoreline changes in two microtidal, low-energetic embayments of southern Zealand, Denmark, were investigated by using the directional distribution of wave energy fluxes. The sites include a barrier island system attached to moraine bluffs, and a recurved spit adjacent to a cliff coast. The barrier island system is characterized by cross-shore translation and by an alignment of the barrier alongshore alternating directions of barrier-spit progradation in a bidirectional wave field. The recurved spit adjacent to the cliff coast experienced shoreline rotation through proximal erosion and distal lateral accretion in a unidirectional wave climate. The multidecadal shoreline changes were coupled to a slope-based morphological coastal classification. All erosive shores occurred within a narrow range of onshore and offshore coastal slopes. The alongshore variability of directional distributions of wave energy fluxes furthermore outlined potential sediment sources and sinks for the evolution of the barrier island system and for the evolution of the recurved spit.  相似文献   

20.
Probably the largest regular shoreline fluctuations on Earth occur along the 1500 km-long wave-exposed Guianas coast of South America between the mouths of the Amazon and Orinoco Rivers, the world's longest muddy coast. The Guianas coast is influenced by a succession of mud banks migrating northwestward from the Amazon. Migrating mud banks dissipate waves, partially weld onshore, and lead to coastal progradation, aided by large-scale colonization by mangroves, whereas mangrove-colonized areas between banks (inter-bank areas) are exposed to strong wave action and undergo erosion. On large tracts of this coast, urbanization and farming have led to fragmentation and removal of mangroves, resulting in aggravated shoreline retreat. To highlight this situation, we determined, in a setting where mangroves and backshore freshwater marshes have been converted into rice polders in French Guiana, shoreline change over 38 years (1976-2014) from satellite images and aerial orthophotographs. We also conducted four field experiments between October 2013 and October 2014, comprising topographic and hydrodynamic measurements, to determine mechanisms of retreat. The polder showed persistent retreat, at peak rates of up to -200 m/yr, and no recovery over the 38-year period of monitored change. Notwithstanding high erosion rates, mangrove shorelines show strong resilience, with recovery characterized by massive accretion. Retreat of the polder results in a steep wave-reworked shoreface with a lowered capacity for bank welding onshore and mangrove establishment. Persistent polder erosion is accompanied by the formation of a sandy chenier that retreats landwards at rates largely exceeding those in inter-bank situations. These results show that anthropogenic mangrove removal can durably modify the morphodynamics of muddy shorefaces. This limits the capacity for shoreline recovery and mangrove re-establishment even when there is no sustained long-term deficit in mud supply, as in the case of the Amazon-influenced Guianas coast. © 2019 John Wiley & Sons, Ltd.  相似文献   

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