共查询到20条相似文献，搜索用时 23 毫秒
1.
《Marine and Petroleum Geology》2007,24(6-9):388-405
High-resolution 3D seismic data of several subsurface examples reveal significant differences in internal architecture and evolution of fluvial and deep-water sinuous channel systems, although there are many similarities in external morphologies of both systems. Channel migrations or shifts in fluvial systems, with point-bar scrolls, are relatively continuous laterally and show a downstream component; they are commonly a single seismic phase thick, with flat tops. In deep-water systems, channel migrations or shifts, with or without point-bar scroll-like features, may be lateral, either continuous or discrete, and laterally to vertically aggrading, again either continuous or discrete; they are single to multiple seismic phases thick, with or without a downstream component. Even the most laterally migrated channel complex commonly aggrades, to varying degrees, from the inside to the outside of sinuous loops. Similarities between fluvial and deep-water sinuous channel systems discussed here imply that sinuosity enhancements in both cases are the result of gradual processes, involving interaction of flows, sediments and alluvial plain or seafloor in attempts to build equilibrium profiles. Flat gradients, high width to depth ratios of valleys/channel belts, fine sediment grain sizes, a certain degree of bank cohesiveness, and presence of secondary circulations in flows were pre-requisites in both systems. However, a number of factors appear to have caused major differences in the internal architecture and modes of evolution of fluvial and deep-water channels. These include differences in (1) density contrasts of flows relative to ambient fluids, (2) entrainments of ambient fluids into flows, (3) effects of centrifugal and Coriolis forces on flows, (4) frequency, volume and duration of steady vs. catastrophic flows, (5) modes of sediment transport, and (6) effects of sea level changes on deposition. Furthermore, within deep-water systems, changes in flow parameters and sediment grain size can cause erosion, bypassing or deposition in space and time and result, through cuts and fills, in sinuous channels with lateral migrations, vertical aggradations and combinations thereof. 相似文献
2.
In May of 2005, an observational program was carried out to investigate the along channel hydrodynamics and suspended sediment transport patterns at North Inlet, South Carolina. Along channel variability, which is important in establishing sediment transport pathways, has not been characterized for this system. Measurements of water column currents, salinity, bed sediment, suspended sediment concentration, and particle size distribution were obtained over a complete tidal cycle along the thalweg of the inlet entrance. Along channel currents, shear stress and bed sediment distributions vary significantly in space and time along a 3 km section bracketing the inlet throat. Most of the variability is consistent with geomorphic controls such as bed elevation variability and channel width. The highest velocities, shear stresses, suspended sediment concentration and bed sediment grain size are observed in the narrowest section of the inlet throat. Magnitudes systematically decrease along the channel toward the marsh as changes in channel geometry and branching reduces flow energy. Due to tidal asymmetry, the ebb phase contains significantly higher currents and associated sediment transport. Over the complete tidal cycle, depth integrated transport is directed towards the marsh landward of the intersection of Town and Debidue Creek. In contrast, net transport is out of the inlet seaward of this intersection. Sediment grain size distributions show 35% more material less than 63 μm on flood, suggesting net landward transport of fines. 相似文献
3.
Morphodynamic modelling of open-sea tidal channels eroded into a sandy seabed, with reference to the channel systems on the China coast 总被引：3，自引：0，他引：3
In this contribution, the morphodynamics of open-sea tidal channels eroded into sandy seabed in regions of flow constriction
is simulated by a one-dimensional model using the Bagnold formula for bedload transport rate, and accounting for the effect
of bed slope. The results show that equilibrium conditions for such channels can be reached over a period of 102–103 years. Sediment eroded from the channel floor is transported in the direction of the dominant current, and deposited beyond
the regions of flow constriction where the current looses competence due to spreading. In this way, the material remobilized
from older strata in the channels is deposited in younger sand banks near the channel heads. Where several successive channels
are incised along the current axis, they interact in their morphological evolution. The morphodynamic equilibrium of a tidal
channel is reached once the combined interacting sedimentological and hydrodynamic factors, such as sediment particle diameter,
tidal current velocity and flood/ebb dominance, are balanced. The model output shows that the equilibrium shape of the tidal
channels appears to be related mainly to flow field characteristics and, to a lesser extent, to particle size. A positive
correlation exists between the depth of the channels and their response times. The equilibrium water depths of the channels
are more sensitive to current speed than to either particle size or the time–velocity asymmetry of the flow field. The response
times for overall morphological equilibrium are sensitive to all of the above-mentioned parameters. In particular, sediment
characteristics associated with critical current velocities have far-reaching effects on the morphodynamic behaviour of tidal
channels. 相似文献
4.
The combined tidal and wind driven flow and resulting sediment transport in the ocean over a flat bottom at intermediate water depth has been investigated, using a simple one dimensional two-equation turbulence closure model. This model has been verified against field measurements of a tidal flow in the Celtic Sea. The tidal velocity ellipses and the time series of the horizontal velocity components at given elevations above the bottom are well predicted through the water column although there are some deviations between the predicted and measured velocities near the bottom due to the uncertainty of the bottom roughness. For the combined tidal and wind driven flows the velocity profiles, turbulent kinetic energy profiles and surface particle trajectories are predicted for weak and strong winds. Furthermore, the bottom shear stress and the resulting bedload transport have been predicted; the parts of the particle trajectories in the close vicinity of the bottom where the bedload transport exists are displayed. Finally, the direction and magnitude of the surface drift, the depth-averaged mean velocity and the mean bedload transport are given, and the effect of the bottom roughness on the sea surface drift is investigated. 相似文献
5.
The Arcachon Lagoon has an important network of tidal channels and well developed tidal flats covered by the marine grass Zostera marina. Based on 66 piston cores taken from the Graveyron tidal channel, and observations on the neighbouring channels, this paper documents the facies and geometry of the channel-fill deposits. In the inner lagoon (studied area) the tidal channels are 80 to 150 m wide and have a meandering morphology with sandy point bars 2 to 5 m thick. The channel-fill does not consist of the classic inclined heterolithic bedding typical of many channel-fills (Reineck, 1958), but of cross-stratified sandy deposits characterized by the absence of slack-water clay-drapes. These unusual facies characteristics are due to the low turbidity of the lagoonal waters which is caused by the lack of significant river inflow and the dense coverage of Zostera marina on the tidal flats. The overall geometry of the channel-fill deposits is characterized by a narrow sand-ribbon shape, a few kilometres long, 80 to 150 m wide and 1 to 5 m thick. This sand ribbon is made of elliptical sand bodies, deposited as point bars, that coalesce longitudinally along the channel axis. This narrow shape is due to the fact that the lateral migration of the channel is virtually nil (reduced to a few metres). In spite of their characteristic meandering morphology, these channels do not deposit extensive tabular sand sheets of amalgamated point bars like the tidal creeks on the North Sea tidal flats. Two factors are thought to control this lack of channel migration. (1) The tidal flats adjacent to the tidal channels are made of 3- to 5-m-thick cohesive muddy sediments covered by Zostera marina that prevents the erosion of the channel banks. This first mechanism is supported by the observation that the tidal creeks that drain the muddy tidal flats covered by Zostera marina do not migrate laterally, whereas those that drain the sandy tidal flats devoid of a dense coverage of marine grass do have active lateral migration. (2) The tidal channels are not fed by any river and therefore do not receive any fluvial sand influx during the winter floods. Their morphology is in equilibrium with the tidal discharge and represents a stable stage in the development of the channel. This second mechanism is supported by the fact that the only tidal channels that actively migrate laterally in the lagoon receive sandy fluvial influx from the River Leyre located in the southeastern corner of the lagoon. 相似文献
6.
Tidal sand bars and tidal sand ridges are extensively developed in the macrotidal Gulf of Khambhat, offshore western India. The inner and outer regions of the gulf are characterised by the development distinct tidal sand bodies with discrete geometries and dimensions. The outer gulf ridges are long, narrow, curvilinear and several metres high (∼20 m). They are asymmetric in cross-section and migratory in nature, forming ‘ribbon’ like sand bodies separated by tidal channels. Active dunes on these ridges indicate the presence of sand and their orientation parallel to palaeo-shorelines supports a tidal origin. In contrast to the outer gulf tidal sand ridges, sand bars associated with macrotidal estuaries flanking the Gulf of Khambhat typically have an elongate to diamond shape and are only hundreds of metres in width and a few kilometres length. These tidal sand bars occur in the estuary mouths and within the tidally influenced fluvial reaches of the rivers flowing into the gulf. The height of these sand bars is in the range ∼1–3 m. Due to high tidal ranges and bi-directional flow the sand bars do not develop significant height and are formed between the mutually evasive ebb and flow channels. Their bi-directional foresets and the presence of abundant mud drapes associated with the dunes within in-channel sand bars indicate a tidal origin.The Gulf of Khambhat acquired the present configuration in the last few thousand years since the Pleistocene sea-level lowstand (last glacial maximum, ∼18 ka) when the entire continental shelf was subaerially exposed and rivers down-cut into the coastal plain. With increasing sea-level rise, the exposed shelf was drowned, flooding parts of the Modern western Indian peninsula, and large tidal sand ridges formed in the outer gulf. After the fall of sea-level at 2 ka the gulf acquired the Modern configuration with multiple estuaries on both coastlines, rivers supplied the embayment with sandy sediment, and tidal sand bars formed in the Modern estuaries.Quantitative data gathered from the Modern Gulf of Khambhat indicates that for the P50 case, a vertical drill hole will encounter tidal sand bodies (ridges and bars combined) of approximate dimensions 1700 m long, 470 m wide and 1.5 m high, with a spacing of 400 m. In subsurface hydrocarbon reservoirs, where data is sparse and only limited amount of core is available, this quantitative dataset can be useful to constrain subsurface geocellular models. Also, the overall geometry, distribution and aspect ratio of the tidal sand ridges and tidal sand bars can be used to identify ancient counterparts through seismic geomorphology or in core. 相似文献
7.
The effect of a residual current on the combined tidal and wind driven flow and the resulting bedload sediment transport in the ocean has been investigated, using a simple one dimensional two-equation turbulence closure model. Predictions of the combined tidal and wind driven flow with given residual currents are presented, showing that the residual current has a substantial effect on both the depth averaged mass transport and the mean bedload transport directions; in some cases the effect of the residual current is to almost reverse the mean bedload transport direction. The residual current affects the rotation of the flow due to the Coriolis effect in the lower part of the water column (the near-surface flow is wind dominated), causing a larger or smaller clockwise rotation of the depth averaged mass transport, depending on the direction of the residual current. 相似文献
8.
Stratigraphic models for microtidal tidal deltas; examples from the Florida Gulf coast 总被引：1，自引：0，他引：1
Richard A. Davis Jr. C. Kelly Cuffe Katherine A. Kowalski Eric J. Shock 《Marine Geology》2003,200(1-4):49-60
Extensive vibracoring of both flood- and ebb-tidal deltas along the central Gulf Coast of the Florida peninsula reveals a strong overall similarity with subtle distinctions between flood and ebb varieties. Although the coast in question is microtidal, the inlets range from tide-dominated to distinctly wave-dominated. Both types of tidal deltas overlie a muddy sand interpreted to have been deposited in a back-barrier environment. The sharp contact at the base of the tidal delta sequence is typically overlain by a thin shell gravel layer. The ebb-tidal delta sequence is characterized by fine quartz sand with shell gravel in various concentrations; coarse and massive at the margins of the main ebb channel, and finer and imbricated at the marginal flood channels. The flood-tidal deltas are characterized by the same facies but with a small amount of mud. Shelly facies on the channels on flood deltas are not as well developed as on the ebb deltas. The combination of the stratigraphic sequence and the lithofacies make tidal deltas readily identifiable in the ancient record. The differences between flood and ebb varieties are subtle but consistent. 相似文献
9.
Seismic data and sediment cores collected offshore from the Sanaga River and Nyong River mouths were used to analyse a loose mantle of yellow to reddish sandy gravel with a range of fluvial and deltaic characteristics cropping out in the middle part (25–65 m water depth) of the continental shelf of Cameroon. Contrary to most of the Atlantic shelves, where the lowstand systems tract was destroyed by erosion, we found 80–120 ms (60–90 m) of sediment mainly in the middle part of the shelf, which correspond credibly to MIS 2–4. Fluvial paleodrainage systems are preserved beneath the shelf and individual filled channels with planar infillings were mapped that cross the shelf along two surfaces of erosion. These incisions suggested westerly and northwesterly drainage shifts during presumed lowered base level. The presence of closely spaced channel fills suggests repeated avulsion of a single stream during a long-lasting sub-aerial erosion period. The seismic facies of these Pleistocene deposits distinguish themselves clearly from well-stratified older strata showing deformation (Pliocene) or intense folding (Miocene). The orientation of the paleovalleys appears strongly controlled by the N60°E trending cross faults within Mesozoic–Cenozoic strata. 相似文献
10.
基于平面二维水沙床耦合地貌模型,反演了1958年1月至1964年12月连续枯水年期间钱塘江尖山河段的主槽摆动过程,揭示了河势由顺直到弯曲的主要演变规律和内在机制.结果表明,在低径流和强潮流作用下,丰水年形成的北部落潮槽逐渐淤积形成浅滩,南部涨潮槽冲刷发展形成南、北两支,两槽间江心滩发育壮大,形成弯曲河势.河床冲淤主要集中在前两年内,潮汐周期内涨潮初期冲刷、涨憩和落潮初期淤积,区域淤积泥沙主要来源于下游杭州湾,北槽前期淤积为落潮型淤积,后期为涨潮型淤积.河势变化使得区域潮差增大潮动力增强,南槽涨落潮流速显著增大.顺直河势下,江心滩南北两侧分流比差异不大,涨潮期间南侧略高、落潮期间北槽略高.弯曲河势下,南槽水深和纳潮量增加,涨、落潮分流比均显著增大至75％以上. 相似文献
11.
崇明岛南岸发育有六滧涨潮槽,研究其演变过程对于崇明世界级生态岛港口建设、通航安全和岸滩稳定具有重要意义。为此,在长江口北港中上段2000、2004、2008、2012和2018年海图水深数据的基础上,分别于2018年7月利用SeaBat 7125、2019年7月利用M80无人艇搭载的SeaBat T50-P多波束测深系统以及ADCP对六滧涨潮槽开展了水下地形和流速的测量工作,分析工程影响下六滧涨潮槽的演变特征。结果表明:六滧涨潮槽淤浅,外侧沙脊冲刷剧烈,槽内落潮优势明显。其原因为流域来沙减少、以青草沙水库为主体工程的南北港分流口整治及长江大桥的修建导致了北港中上段主槽冲刷、主流北移。同时,六滧涨潮槽局部水动力条件增强,槽内的丁坝坝头局部冲刷加剧,并且奚家港东侧岸滩出现侵蚀,需引起重视。 相似文献
12.
13.
Arnold H. Bouma Monty A. Hampton Robert C. Orlando 《Marine Georesources & Geotechnology》2013,31(1-4):291-308
Abstract Lower Cook Inlet in Alaska has high‐ tidal currents that average 3–4 knots and normally reach a peak of 6–8 knots. The bottom has an average depth of about 60–70 m in the central part of the inlet that deepens toward the south. Several types of bedforms, such as sand waves, dunes, ripples, sand ribbons, and lag deposits form a microtopography on the otherwise smooth seafloor. Each bedform type covers a small field, normally a few hundred to a few thousand meters wide, and usually several kilometers long parallel to the tidal flow. High‐resolution seismic systems, side‐scan sonar and bottom television were used to study these bedforms. Large sand waves with wavelengths over 300 m and wave heights up to 10 m were observed. Fields of ebb‐oriented or flood‐oriented asymmetric bedforms commonly grade into more symmetric shapes. Several orders of smaller sand waves and dunes cover the flanks of the very large bedforms. The crest directions of both size groups are normally parallel, but deviations of up to 90° have been observed; local deviations may occur where smaller forms approach the crests of the larger sand waves. Bottom television observations demonstrated active bedload transport in a northerly direction on crests and midflanks of southward asymmetric large sand waves, but not in their troughs. Movement of bedload occurs in the form of small ripples. Although the asymmetry of the large bedforms suggests that migration has taken place in the ebb or flood directions, the very low surface angles (2.5°‐8°) of these bedforms do not indicate regular movements. The large bedforms are probably relict features, or they migrate only under severe conditions, whereas active sand transport by ripples and smaller sand waves and dunes moves bedload back and forth with the tides. An understanding of such movements is essential for determining design criteria for offshore installations and in benthic‐faunal studies. 相似文献
14.
Understanding tidal and mean flow transport mechanisms that maintain the estuarine salt balance against seaward transport by river flow is one of the fundamental questions of estuarine oceanography. Previous salt transport studies have failed to resolve this problem for two principal methodological reasons, in addition to the inherent variability of estuaries. First, salt transport expansions used to represent the salt balance have included a large number of terms that could not be related to any theory of estuarine circulation and whose physical meaning was thus unclear. Second, it has proven difficult to measure small mean velocities in the presence of much larger tidal variations. A new analysis method that expresses Lagrangian scalar property transport in terms of time and space variations of Eulerian variables is much simpler than expansions previously used and yields new insights into estuarine transport processes. This approach (like previous expansions) is valid for narrow systems in which cross-channel transport processes are weak and the ratio of tidal amplitude to mean depth is small. A surface boundary condition is used to eliminate one class of troublesome terms. The equivalence of the tidal-cycle average transport of tidal variations in salinity and the Stokes transport of mean salinity is then employed to show that the vertically integrated tidal flow plays no role in the overall salt balance. That is, seaward transport of salt caused by the river flow can only be balanced by landward transport resulting from correlations between shear and stratification in the mean, tidal and overtide flows. In a hypothetical inviscid estuary, tide and overtide flows would be vertically uniform, and outward fluvial salt transport could only be balanced by shear and stratification in the mean gravitational circulation. In a more realistic example with strong friction, the gravitational circulation would be severely damped, and inward transport could only be accomplished by correlations of shear and stratification in the tide and overtide flows. 相似文献
15.
Nikuradse roughness (ks) is very important in the sediment transport prediction because it is related to the evaluations of the velocity distribution, shear stress and erosion depth. Dimensionless Nikuradse roughness (ks/D, where D is the sediment diameter) is usually given 1–2.5 on the immobile plan bed or at low shear stress. But it behaves differently on the mobile plan bed at high shear stress with much sediment picked up to movement when the Shields parameter (Θ) is larger than 0.8–1.0. The effective Nikuradse roughness on the mobile plan bed was derived indirectly from the erosion depth correlated to the mobile plan bed thickness considering the mass conservation in the present study. The proposed erosion depth confirmed the relation to the Shields parameters with an extra factor consisting of suspended sediment and its damping to turbulence. The decrement of the erosion depth caused by the increment of the sediment diameter at large shear stress was obtained, which was usually absent in classical empirical formulas based on the bedload theory. Good agreement with experiments was achieved by the present prediction of the Nikuradse roughness, erosion depth and sediment transport rate. Discussion was mainly focused on the prediction improvement caused by considering the impact of suspended sediment and its damping to turbulence. 相似文献
16.
河口地貌形态对潮汐不对称性的产生和发展有着至关重要的作用。本文根据英国Humber河口数据建立了概化模型,研究了在同一纳潮量情况下,主槽断面形态、平面形态和河口收缩率对河口潮汐不对称性的影响。结果表明,较深的主槽能使相位差峰值出现较晚且峰值更大,从而影响局部区域的涨潮流强弱,主槽越浅,最大落潮流速越小,落潮所需历时越长,河口更倾向于涨潮主导,窄潮滩倾向于涨潮主导型,宽潮滩倾向于落潮主导型;平面形态沿程收缩且长度较长的河口涨潮主导型最强,此外,河口宽度沿程缩窄会加大主槽的余流流速,减小潮滩的余流流速;随着河口平面收缩率的增强,主槽的余流流速减小,潮滩余流流速增大,潮滩更倾向于涨潮主导。本文进一步丰富了河口地形地貌变化对潮汐不对称性影响的认识,可为河口区工程建设和管理维护提供科学依据。 相似文献
17.
This study presents a review of extensive literature and reports new findings extracted from previously collected cores. Globally lowered sea level during the last glacial maximum (LGM) reduced the cross-sectional area in the Korea Strait, minimizing volume transport of the paleo-Tsushima Current and increasing freshwater input to the East Sea. The higher supply of freshwater played an important role in compositional changes of surface water in the sea, indicated by low sea surface salinity (down to about 20‰) and light d 18O of planktonic Foraminifera (lighter than 1‰) recorded in core sediments. The Korean fluvial systems (Nakdong and Seomjin rivers) emptying into the southeastern sea of Korea may have contributed substantially to freshwater supply to the surface layer of the LGM East Sea, although Chinese paleo-river (Huanghe and Yangtze rivers) waters, together with the paleo-Tsushima Current, also seem to have supplied some freshwater to the sea. The higher supply of river waters to the East Sea is strongly evidenced by the high amount of terrigenous material (quartz, feldspar and rock fragments) in core sediments. In addition, high magnetic susceptibility, high grain density, and high C/N ratios were documented in cores MB98PC-11 and 95PC-1. In contrast with earlier studies, we propose that Korean rivers played a more substantial role in supplying freshwater to the East Sea during the LGM than previously thought. 相似文献
18.
《Marine and Petroleum Geology》2007,24(6-9):470-486
Turbidity currents in sinuous submarine channels are an important mechanism for transporting terrestrial sediments to deep water, and their deposits are of increasing importance as hydrocarbon exploration targets. Despite this, the architecture and dynamics of submarine channel systems are not well understood. Analogies are often drawn with fluvial systems due to similarities between their planform shapes even though differences in channel evolution and hydrodynamics have been noted. A key question is the nature of deposition within submarine channel bends; in particular at inner bends where point bars form in alluvial meandering rivers. Recent experimental and numerical work has demonstrated that the fluid dynamics of submarine channel bend flow are markedly different from rivers. Notably, a reversal in the orientation of secondary (helical) flow at bend apices occurs in submarine channels. The potential influence of these differences in fluid dynamics on deposition within submarine channel bends is investigated herein. We report the results of a series of physical experiments in which solute-driven gravity currents were run through pre-formed sinuous channels containing mobile beds. These experiments reveal sedimentation patterns characterised by accumulation zones downstream of bend apices and erosion zones at outer bends. These patterns are broadly analogous to the point bars and outer-bank pools observed in meandering rivers, demonstrating that the longitudinal flow component dominates over the cross-stream component, as also occurs in rivers. However, the data suggest that the reversal in direction of the cross-stream flow component compared with subaerial flows is important in determining the position and morphology of ‘point bars’ relative to bend apices. From analogy with fluvial compound channels, and fluvial theory, this reversal in secondary flow cell orientation is also expected to influence the spatial variations of grain size in submarine channel ‘point-bar’ deposits. 相似文献
19.
The Bedload Movement in the Changjiang Estuary 总被引：6，自引：0，他引：6
- Sandwaves in the Changjiang estuary were measured with a shallow sediment profiler and an echosounder from 1978 to 1988. The data, together with grain size and bedform of sediment indicates that the bedload movement by rolling and saltation is of great significance to sediment transport and is the principal factor responsible for sandwave and sandbody development in the estuary. The sandwaves were found well-developed, which is related to the tidal range and the velocity of ebb current. However, the further growth is restricted by strong flood current prevailing in the estuary. Because of the significant bedload, the sandbodies shift obviously and frequently, and sometimes the exchange of position occurs between the sandbodies and tidal channels. As a result, ships are regularly forced to change their navigation course. 相似文献
20.
Channel constrictions within an estuary can influence overall estuary-sea exchange of salt or suspended/dissolved material. The exchange is modulated by turbulent mixing through its effect on density stratification. Here we quantify turbulent mixing in Hikapu Reach, an estuarine channel in the Marlborough Sounds, New Zealand. The focus is on a period of relatively low freshwater input but where density stratification still persists throughout the tidal cycle, although the strength of stratification and its vertical structure vary substantially. The density stratification increases through the ebb tide, and decreases through the flood tide. During the spring tides observed here, ebb tidal flow speeds reached 0.7?m?s?1 and the buoyancy frequency squared was in the range 10?5 to 10?3?s?2. Turbulence parameters were estimated using both shear microstructure and velocimeter-derived inertial dissipation which compared favourably. The rate of dissipation of turbulent kinetic energy reached 1?×?10?6?m2?s?3 late in the ebb tide, and estimates of the gradient Richardson number (the ratio of stability to shear) fell as low as 0.1 (i.e. unstable) although the results show that bottom-boundary driven turbulence can dominate for periods. The implication, based on scaling, is that the mixing within the channel does not homogenise the water column within a tidal cycle. Scaling, developed to characterise the tidal advection relative to the channel length, shows how riverine-driven buoyancy fluxes can pass through the tidal channel section and the stratification can remain partially intact. 相似文献