首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 734 毫秒
1.
The formation of a shoal was investigated in the Piscataqua River, New Hampshire, which is a well-mixed channel with low freshwater flow and tidal currents up to 2.3 m s?1. Observations of sediment characteristics, bathymetry, and bottom current were made, and theory was used to predict bedload transport. Sediment sampling showed the bottom material to be coarse sand and gravel, and sidescan sonar revealed large sand waves directed upriver at the shoal. Bottom current measurements were made along transects upriver and downriver of the shoal and downriver of an adjacent deepwater area that was also studied for comparison. Bedload flux inferred from current measurements using the Brown-Einstein theory indicated that transport is generally directed upriver. Sediment budget calculations showed the shoal area to be depositional before, immediately after, and subsequent to a dredging operation at rates of 0.36 m yr?1, 1.06 m yr?1, and 0.35 m yr?1, respectively. Predredge and subsequent rates were consistent with the historical record of removal by dredging at the shoal.  相似文献   

2.
  相似文献   

3.
The Weddell Sea Embayment (WSE) sector of the Antarctic ice sheet has been suggested as a potential source for a period of rapid sea-level rise – Meltwater Pulse 1a, a 20 m rise in ～500 years. Previous modelling attempts have predicted an extensive grounding line advance in the WSE, to the continental shelf break, leading to a large equivalent sea-level contribution for the sector. A range of recent field evidence suggests that the ice sheet elevation change in the WSE at the Last Glacial Maximum (LGM) is less than previously thought. This paper describes and discusses an ice flow modelling derived reconstruction of the LGM ice sheet in the WSE, constrained by the recent field evidence. The ice flow model reconstructions suggest that an ice sheet consistent with the field evidence does not support grounding line advance to the continental shelf break. A range of modelled ice sheet surfaces are instead produced, with different grounding line locations derived from a novel grounding line advance scheme. The ice sheet reconstructions which best fit the field constraints lead to a range of equivalent eustatic sea-level estimates between approximately 1.4 and 3 m for this sector. This paper describes the modelling procedure in detail, considers the assumptions and limitations associated with the modelling approach, and how the uncertainty may impact on the eustatic sea-level equivalent results for the WSE.  相似文献   

4.
Detailed marine geological–geophysical survey of the submarine groundwater discharge (SGD) system at Stoupa, Messinia (Greece) was conducted as part of an offshore study aiming at the evaluation of the discharge rate, the quality of the water and the investigation of potential ways for exploitation. Systematic mapping of the seafloor included swath bathymetry, seismic profiling and side-scan sonar imaging in order to reveal the precise morphology of the submarine discharge site, to better understand the structure of the SGD system and the nature and thickness of the sedimentary cover, and finally to provide the necessary data for a potential exploitation design. The SGD system is located in an E–W trending ellipsoidal depression characterized by two depth maxima at 27 and 29 m. This depression has been developed on the hanging wall of a N–S trending fault, whilst the groundwater discharges occur at the base of a 10-m-high steep and faulted rocky slope developed on conglomerates or limestone formations, also occurring in the coast. Recent sand deposits cover the seabed around the depression. The complex morphology of the discharge site, the steep slopes, and the rapid changes (due to erosion with subsequent slope collapse) during enhanced water flow periods, do not favor submarine constructions for the exploitation of the SGD system.  相似文献   

5.
Foraminifera have been successful inhabitants of every aquatic environment from deep oceans to brackish water lagoons, estuaries and even rarely in freshwater streams, lakes, etc. In order to know their distribution in different offshore habitats, the present study has been undertaken between Tuticorin and Tiruchendur, wherein 31 bottom sediment and water samples were collected. Benthic foraminiferal taxa belonging to four suborders (Textulariina, Miliolina, Lagenina and Rotaliina), 14 superfamilies, 25 families, 45 genera and 66 species have been identified; three planktonic species (suborder Globigerinina) have also been identified. An increase in total foraminiferal distribution with depth has been observed, except off Tiruchendur. Living/dead ratios range from 1.57% to 10.02%, indicating a low to moderate rate of sedimentation in this region. Off Punnaikayal and Tiruchendur, the rate of sedimentation is more at greater depths; at other stations, however, it is higher at relatively shallower depths. Based on this study, it is concluded that fluctuations in the distribution of foraminifera are mainly controlled by coastal morphology, wave dynamics, bathymetry and environmental parameters.  相似文献   

6.
Linkages among density, flow, and bathymetry gradients were explored at the entrance to the Chesapeake Bay with underway measurements of density and flow profiles. Four tidal cycles were sampled along a transect that crossed the bay entrance during cruises in April–May of 1997 and in July of 1997. The April–May cruise coincided with neap tides, while the July cruise occurred during spring tides. The bathymetry of the bay entrance transect featured a broad Chesapeake Channel, 8 km wide and 17 m deep, and a narrow North Channel, 2 km wide and 14 m deep. The two channels were separated by an area with typical depths of 7 m. Linkages among flows, bathymetry, and water density were best established over the North Channel during both cruises. Over this channel, greatest convergence rates alternated from the left (looking into the estuary) slope of the channel during ebb to the right slope during flood as a result of the coupling between bathymetry and tidal flow through bottom friction. These convergences were linked to the strongest transverse shears in the along-estuary tidal flow and to the appearance of salinity fronts, most markedly during ebb periods. In the wide channel, the Chesapeake Channel, frontogenesis mechanisms over the northern slope of the channel were similar to those in the North Channel only in July, when buoyancy was relatively weak and tidal forcing was relatively strong. In April–May, when buoyancy was relatively large and tidal forcing was relatively weak, the recurrence of fronts over the same northern slope of the Chesapeake Channel was independent of the tidal phase. The distinct frontogenesis in the Chesapeake Channel during the increased buoyancy period was attributed to a strong pycnocline that insulated the surface tidal flow from the effects of bottom friction, which tends to decrease the strength of the tidal flow over relatively shallow areas.  相似文献   

7.
The continental margin of southern South Africa exhibits an array of emergent marginal marine sediments permitting the reconstruction of long-term eustatic sea-level changes. We report a suite of optical luminescence ages and supplementary amino acid racemization data, which provide paleosea-level index points for three sites on this coastline. Deposits in the Swartvlei and Groot Brak estuaries display tidal inlet facies overlain by shoreface or eolian facies. Contemporary facies relations suggest a probable high stand 6.0-8.5 m above modern sea level (amsl). At Cape Agulhas, evidence of a past sea-level high stand comprises a gravel beach (ca. 3.8 m amsl) and an overlying sandy shoreface facies (up to 7.5 m amsl). OSL ages between 138 ± 7 ka and 118 ± 7 ka confirm a last interglacial age for all marginal marine facies. The high stand was followed by a sea-level regression that was associated with the accumulation of eolian dunes dating to between 122 ± 7 ka and 113 ± 6 ka. These data provide the first rigorous numerical age constraints for last interglacial sea-level fluctuations in this region, revealing the timing and elevation of the last interglacial high stand to broadly mirror a number of other far-field locations.  相似文献   

8.
极浅水边界层的沉积环境效应   总被引：1，自引：1，他引：0  
高抒 《沉积学报》2010,28(5):926-932
极浅水环境是水深远小于正常边界层厚度的环境,潮滩滩面和潮水沟就经常处于这种环境之中。来自江苏潮滩的观察资料表明,落潮后期滩面和潮水沟在薄层低速水流作用下可形成浅水波痕、平床和次级潮水沟形态,而边界层内的流速结构仍然得以维持,物质输运强度也与Von Kármán-Prandtl模型所定义的u100数值相一致。涨急时段中潮位附近形成的滩面涌潮是极浅水边界层的另一种动力学行为,它代表薄层高速水流作用下发生的底部边界层系统的崩溃,此时Von Kármán-Prandtl模型所刻画的流速结构不复存在。滩面涌潮高度是系统崩溃的临界水深,它可以定量地表示为Hb=4z0。极浅水边界层过程对潮滩沉积和地貌形成具有独特的作用。  相似文献   

9.
Today, below 2500 m, benthonic foraminiferal faunas in the North Atlantic are dominated by a few species. Faunal composition changes slowly with increasing depth and decreasing temperature. Surface sediment and down-core counts of benthonic foraminifera reported by Phleger, Parker, and Peirson (1953) in the reports of the Swedish Deep-Sea Expedition have been supplemented by additional bottom sediment and piston core samples. Present-day benthonic foraminiferal assemblages from the deeper portions of the North Atlantic appear to be controlled more by the distribution of bottom water types than by bathymetry. In most piston cores, the assemblages vary greatly during the last 150,000 yr, suggesting depression and elevation of faunas at the core site through a depth range of several hundred meters. This would indicate that bottom water characteristics have shifted back and forth in this interval of time and, therefore, that bottom circulation partakes in the well-documented shifts recorded for surface waters of the North Atlantic. It appears that dense water, similar to present-day North Atlantic Deep Water, was produced over a wide area north of 45° N during cooler intervals and that it spread widely at depth.  相似文献   

10.
Submarine landslides can generate local tsunamis with high run-ups, posing a hazard to human lives and coastal facilities. Both ancient (giant Storegga slide off Norwegian coast, 8200 B. P.) and recent (Papua New Guinea, 1998) events show high potential danger of tsunamigenic landslides and the importance of mitigation efforts. This contribution presents newly discovered landslides 70 km off Padang (Western Sumatra, Indonesia) based on recent bathymetry measurements. This highly populated city with over 750,000 inhabitants exhibits high tsunami vulnerability due to its very low elevation. We model tsunamis that might have been induced by the detected landslide events. Estimations of run-up heights extrapolated from offshore tsunami amplitudes for Padang and other locations in the northern Mentawai fore-arc basin yield maximum values of about 3 m. We also provide a systematic parametric study of landslide-induced tsunamis, which allows us to distinguish potentially dangerous scenarios for Padang. Inside the fore-arc basin, scenarios involving volumes of 0.5–25 km³ could endanger Padang. Apart from slide volume, the hazard distribution mainly depends on three landslide parameters: distance to Padang, water depth in the generation region, and slide direction.  相似文献   

11.
Integrated classification maps were produced by combining sediment grain-size and hydrological data (water renewal time, WRT) from two Mediterranean lagoons, Lesina (LL) and Varano (LV), Italy. The geophysical characteristics of the two basins, derived from detailed bathymetric charts, are quite distinct: ～30% of LL (mean depth ～1 m) but only 3% of LV (mean depth ～3 m) is shallower than 1 m. The sediments of both lagoons are mainly composed of mud (～80%). A detailed multivariate analysis of grain-size data by EntropyMax classified the lagoon beds of LL and LV into five sedimentary facies. WRT data, computed by a hydrodynamic model, indicated different hydrological conditions in the two lagoons: LL showed a sharp west–east gradient, with a basin-wide average of ～190 days, whilst LV showed a fairly uniform distribution and a higher basin-wide average (～260 days). The distribution of sedimentary facies and water renewal times were combined in a composite map representing the distribution of environmental patterns. The approach outlined in this study can be used to improve zonation schemes by providing a hydromorphological perspective on transitional and coastal environments.  相似文献   

12.
Digital echo sounding, SeaBeam swath bathymetry data and sediment cores were collected on the continental slope (1500–3700 m water depth) off southeastern Tasmania in order to study sedimentary processes in the vicinity of an ocean disposal site. The new bathymetry data show that the shallower limits of the disposal site are positioned on the seaward edge of a gently dipping (3°) mid‐slope shoulder, between 1200 and 2100 m water depth. The slope below the disposal site is relatively steep (6.5°) and is cut by submarine canyons which lead into the adjacent East Tasman Saddle. The SeaBeam bathymetry data show a small submarine canyon traversing the slope in 2400 m water depth directly downslope from the disposal site, with local slopes of up to 22°. The canyon feeds into a perched basin at 2450 m, which could be acting as a local sediment trap. Short (<90 cm) gravity cores indicate that indurated erosional surfaces characterise the slope environment. The cores contain Upper Cretaceous (upper Campanian) sandstones and siltstones, which in places crop out on the sea floor where they are locally draped by a thin (0–30 cm), modern layer of hemipelagic calcareous ooze. Five cores collected from the vicinity of the disposal site had lead and zinc concentrations in the surface 1 cm of 10.3 ± 5.0 and 39.5 ± 19.6 mg/kg, respectively, significantly greater than the background values (2.9 ± 1.4 for lead and 21.2 ± 5.4 for zinc) which characterise the underlying unit that is composed of the same hemipelagic calcareous ooze. Lead and zinc are constituents of the dumped material, jarosite, which, after mixing with slope sediments, can be used as sediment tracers. One core contains a fining‐upwards bed which is also elevated in lead and zinc. This is interpreted as evidence for dispersal of the jarosite from the disposal site downslope to depths >3000 m via turbidity flows sometime during the past 24 years. Current meter data collected from 30 m above the sea floor over one year at the disposal site show that bottom currents attain speeds of up to 0.46 m/s. The current events are attributed to eddies shed by the East Australia Current. The measured bottom currents are capable of transporting fine‐grained hemipelagic muds and could provide a trigger mechanism for turbidity flows.  相似文献   

13.
Ollier  Cochonat  Lénat  & Labazuy 《Sedimentology》1998,45(2):293-330
A volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores, camera surveys, high-resolution sonar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Réunion Island, a basaltic shield volcano in the SW Indian Ocean. Three main areas are identified from the study: (1) the proximal fan extending from 500 m water depth down to 2000 m water depth; (2) the outer fan extending from 2000 m water depth down to 3600 m water depth; (3) the basin extending beyond 3600 m water depth. Within these three main areas, seven distinct submarine environments are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep-water deltas, debris-avalanche deposits, and eroded floor in the valley outlets; the outer fan is characterized by a discontinuous fine-grained sedimentary cover overlying coarse-grained turbidites or undifferentiated volcanic basement; the basin is characterized by hemipelagic muds and fine-grained turbidites interbedded with sandy and gravelly turbidite lobes. The evolution of the deep-sea volcaniclastic fan is strongly influenced by sector collapses, such as the one which occurred 0·0042 Ma ago. This collapse produced a minimum of 6 km3 of debris-avalanche deposit in the proximal area. The feeding regime of the deep-sea fan is ‘alluvial dominated’ before the occurrence of any sector collapse and ‘lava-dominated’ after the occurrence of a sector collapse. The main deep-water lava-fed delta is prograding among the blocks of the debris-avalanche deposits as a result of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave-action, earthquakes and accumulation of new volcanic debris on top of the deltas. Both turbidity currents triggered on the deep-water delta slope, and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line.  相似文献   

14.
The South-East Karst Province of South Australia is an extensive area of low relief with dolines, cenotes, uvalas, and a variety of cave types developed in the soft, porous, flat-lying Tertiary Gambier Limestone and also as syngenetic karst in the overlying calcarenite dunes of the Pleistocene Bridgewater Formation. The most spectacular surface karst features are the large collapse dolines, especially those that extend below the water table to form cenotes. Shallow swampy hollows occur in superficial Quaternary sediments. These are an enigmatic feature of the Bool Region, where all gradations appear to occur between definite karst dolines and nonkarstic hollows. Some depressions may be polygenetic—involving a combination of: (1) primary depositional hollows on coastal flats or in dune fields, (2) deflation, and (3) karst solution and subsidence. There are extensive underwater cave systems in the southern part of the province, and the bulk of the cave development there may well lie below the present water table, although these systems would have been at least partly drained during the lower sea levels of the last glacial period. Systematic variations within the province reflect differences in the parent rock types, the extent and nature of the cover and, most importantly, the hydrology—in particular the depth to the water table and its gradient.  相似文献   

15.
用VOF方法模拟静止浅水环境中的垂向紊动射流   总被引：2，自引：0，他引：2       下载免费PDF全文
以k-ε紊流模型为基础,针对浅水环境垂向射流的水面会有明显突起情况,采用VOF(Volume of Fluid)方法追踪自由面,建立了适合浅水环境垂向平面紊动射流的二维数学模型.对不同入射速度、水深、入射口宽度组合进行了大量的数值模拟实验,对整个流场的分布作了分析,发现自由面的分布特点和速度矢量场的分布特点相对应;中轴线附近的自由面隆起具有自相似性,文中给出了拟合表达式;由于水面的阻滞,中轴线上垂向速度的衰减较自由射流快,衰减规律随水深和射流宽度而异.  相似文献   

16.
Shallow estuarine lagoons characterize >70 % of the eastern Alaskan Beaufort Sea coastline and, like temperate and tropical lagoons, support diverse and productive biological communities. These lagoons experience large variations in temperature (?2 to 14 °C) and salinity (0 to >45) throughout the year. Unlike lower latitude coastal systems, transitions between seasons are physically extreme and event-driven. On Arctic coastlines, a brief summer open-water period is followed by a 9-month ice-covered period that concludes with a late-spring sea ice breakup and intense freshwater run-off. From 2011 to 2014, we examined interannual variations in water column physical structure (temperature, salinity, and δ18O) in five lagoons that differ with respect to their degree of exchange with adjacent marine waters and magnitude of freshwater inputs. Temperature, salinity, and source water composition (calculated using a salinity and δ18O mixing model) were variable in space and time. During sea ice breakup in June, water column δ18O and salinity measurements showed that low salinity waters originated from meteoric inputs (50–80 %; which include river inputs and direct precipitation) and sea ice melt (18–51 %). Following breakup, polar marine waters became prevalent within a mixed water column over the summer open-water period within all five lagoons (26–63 %). At the peak of ice-cover extent and thickness in April, marine water sources dominated (75–87 %) and hypersaline conditions developed in some lagoons. Seasonal runoff dynamics and differences in lagoon geomorphology (i.e., connectivity to the Beaufort Sea) are considered key potential drivers of observed salinity and source water variations.  相似文献   

17.
The vulnerability of the shallow aquifer system for saline water intrusion has been evaluated using the classical tools at a coastal area, southern India. Groundwater samples (N=144) from Quaternary aquifer system within 25 km2 area in pre- and post-monsoon seasons were analyzed for major ion chemistry including Electrical Conductivity (EC). The hydrochemical parameters are examined applying classical irrigation suitability tools. Based on their weight percentages (ratios in meq/l) the dominance of cations and anions was established as Na-Cl and Ca-SO4 type. Results show that high hydraulic conductivity (10?2 to 1 cm/s) of the sandy aquifer enhanced the vertical recharge leading to major spatial distribution suitable for irrigation use in post-monsoon. The overexploitation of groundwater resources has generated reversal of hydraulic gradient enhancing salinity intrusion from marine sources in pre-monsoon. Further, the Differential Global Positioning System (DGPS) survey and water level measurements are assessed to demarcate the study area into zones of water table ‘above mean sea level (amsl)’ and ‘below mean sea level (bmsl)’. It was deduced that, the industrial effluent and seawater were the prime sources of groundwater salinity of water table ‘amsl’ and ‘bmsl’ zones, respectively. The area up to 600 m from marine source is found vulnerable which is falling under ‘Unsuitable’ category of irrigation classifications. The remedial measures are also framed to protect further extension of aquifer vulnerability for sustainable agriculture.  相似文献   

18.
Fluid dynamical simulations were carried out in order to investigate the effect of the large-scale mantle flow field and the depth of the plume source on the structure of the Iceland plume through time. The time-dependent location and shape of the plume in the Earth's mantle was calculated in a global model and it was refined in the upper mantle using a 3D Cartesian model box. Global flow was computed based on density heterogeneities derived from seismic tomography. Plate motion history served as a velocity boundary condition in both models. Hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the North Atlantic. If a plume source in the lowermost mantle is assumed, the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle. Depending on tomography model, assumed plume age and buoyancy the southward component is more or less dominating. Plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion. Many whole-mantle plume models result in a V-shaped track, which does not match the straight Greenland–Iceland–Faroe ridge. Models without strong southward motion, such as for a plume source at 660 km depth, match actual bathymetry better. Plume tracks were calculated from both plume conduits and plume heads. A plume head of 120 K anomalous temperature gives the best match between plume head track and bathymetry.  相似文献   

19.
随着声纳渗流检测技术在水文地质勘探中的应用,其测量结果精度的高低对获取的水文地质参数的准确性起着至关重要的作用。文章依托南宁某基坑工程,采用现场试验方法,同时考虑天然流场和人工流场,分三个阶段对基坑止水帷幕进行声纳渗流检测,对比不同阶段渗漏缺陷暴露的情况,同时根据渗透流速量级变化判断其检测结果精度的高低,并在实际工程中验证其准确性。研究结果表明:不同降水阶段的渗透流速大小变化显著,基坑地下水位降深每增加10 m,声纳检测到的渗透流速平均提高1~2个量级。其原因为降水导致基坑内外水头差增大,水力坡度的增加使渗漏缺陷附近的渗流场发生变化,高水头作用下渗透流速变化明显,渗漏缺陷定位更加精准,声纳检测精度也越高。可见,抬高基坑内外水头差对墙体渗漏缺陷的精准定位十分必要。因此,在对基坑止水帷幕采取声纳渗流检测时,建议将地下水位降至基坑底板以下,以提高检测结果的精度,有效探测止水帷幕渗漏风险。  相似文献   

20.
This study provides new insights in the relative role of tidal creeks and the marsh edge in supplying water and sediments to and from tidal marshes for a wide range of tidal inundation cycles with different high water levels and for marsh zones of different developmental stage. Net import or export of water and its constituents (sediments, nutrients, pollutants) to or from tidal marshes has been traditionally estimated based on discharge measurements through a tidal creek. Complementary to this traditional calculation of water and sediment balances based on creek fluxes, we present novel methods to calculate water balances based on digital elevation modeling and sediment balances based on spatial modeling of surface sedimentation measurements. In contrast with spatial interpolation, the presented approach of spatial modeling accounts for the spatial scales at which sedimentation rates vary within tidal marshes. This study shows that for an old, high marsh platform, dissected by a well-developed creek network with adjoining levees and basins, flow paths are different for tidal inundation cycles with different high water levels: during shallow inundation cycles (high water level <0.2 m above the creek banks) almost all water is supplied via the creek system, while during higher inundation cycles (high water level >0.2 m) the percentage of water directly supplied via the marsh edge increases with increasing high water level. This flow pattern is in accordance with the observed decrease in sedimentation rates with increasing distance from creeks and from the marsh edge. On a young, low marsh, characterized by a gently seaward sloping topography, material exchange does not take place predominantly via creeks but the marsh is progressively flooded starting from the marsh edge. As a consequence, the spatial sedimentation pattern is most related to elevation differences and distance from the marsh edge. Our results imply that the traditional measurement of tidal creek fluxes may lead in many cases to incorrect estimations of net sediment or nutrient budgets.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号