The influence of wind waves and tidal currents on sediment resuspension in Middle Chesapeake Bay

Resuspension of bottom sediments by waves and tidal currents was investigated in three characteristic environments of middle Chesapeake Bay (shallow platform, deep platform, and main channel). In the shallow near shore platform wind waves frequently resuspended significant amounts of sediment, some of which was transported offshore. In both the shallow and adjacent deep platform regions, tidal currents were too weak (<20 cm cm/sec) to resuspend bottom sediments. In the main channel, peak current velocities were substantially stronger (40 cm/sec), but were still not competent to erode the bottom. The stability of the bottom in this area is related to the activities of the benthic organisms which are influenced by seasonal anoxia. University of Maryland Center for Environmental and Estuarine Studies (CEES) Contribution No. 1572.     

Modelling of sediment movement in the surf and swash zones

Under the action of marine currents, non-cohesive sediments evolve by bed-load, by saltation or suspension depending on their granulometry. Several authors have considered that the movement of sediment...     

黄河三角洲孤东海域沉积物及水动力

根据黄河三角洲孤东近岸海域表层沉积物取样、水文泥沙观测和风浪资料推算,分析沉积物特征和运移趋势,并通过水动力条件(潮流和波浪)探讨沉积物起动和输移特征。结果表明,孤东海域沉积物多为粉砂类物质,由内向外逐渐变细,分选变差,丁坝的修建对周围粒径分布影响明显;沉积物运移趋势受风成余流、岸线轮廓和丁坝工程修建的影响,不同区域表现为不同的输移方向;研究区水动力表现为波浪掀沙、潮流输沙的特征,由于潮流较小,不足以引起泥沙的起动,泥沙起动主要由波浪引起。     

潮流场对渤、黄、东海陆架底质分布的控制作用

运用二维潮流数学模型，模拟了渤、黄、东海陆架的M2潮汐、潮流。结果表明，渤、黄、东海陆架的潮流有强弱之分以及往复流和旋转汉之别。在此基础上，计算了8种粒径沙的湖平均悬移输沙率、潮平均推移输沙以及相应的输沙率散度。根据输沙率散度的正负，划分了海底冲刷区与淤积区。根据不同粒径泥沙输沙率散度的相对大小，确定出海底的主要底质类型为砂质沉积、粉砂质泥沉积和以粉砂为主的混合沉积。计算结果表明，海底3种主要底负类型的分布格局与海底的冲淤格局以及与输沙率矢量的发散和聚合状况基本一致。在渤、黄、东海陆架，沙脊主要在强往复流区形成，沙席主要在强或较强的旋转流区形成，泥质沉积主要在弱潮流区形成。砂质沉积、泥质沉积以及混合沉积这3种主要底质类型并非孤立存在，而是受渤、黄、东海陆架潮流场控制而形成的一个完整的潮流沉积体系。渤、黄、东海陆架的砂质沉积与泥质沉积并非残留沉积，而是潮流沉积。在没有冷涡的情况下，黄、东海陆架的典型泥质沉积在弱潮流环境中同样可以形成，因此，认为冷涡并非黄、东海陆架典型泥质沉积形成的必要条件。     

潮流作用下洋山港水域悬沙和底沙的交换

通过对泥沙起动流速、淤积流速和不淤流速的计算,划定了悬沙和底沙交换的各个时段,提出了冲刷悬沙浓度、淤积悬沙浓度和不淤悬沙浓度的概念及其计算方法;继而导出了潮流作用下悬沙和底沙在交换层面上单位面积的日交换量计算式,并用于洋山港水域的冲淤计算。结果表明:自然状况下洋山港水域北部年淤积量为37.5 kg/m2,淤高了1.77 cm/a,属微淤;中部年冲刷量为10.0 kg/m2左右,刷深不超过0.52 cm/a,可认为冲淤平衡;南部年冲刷量为55.5 kg/m2,刷深了2.60 cm/a,属微冲。这一结果与该水域海床的自然冲淤状况相吻合。     

Structure and sediment distribution in the western Bering Sea

Eleven seismic reflection profiles across Shirshov Ridge and the adjacent deep-water sedimentary basins (Komandorsky and Aleutian Basins) are presented to illustrate the sediment distribution in the western Bering Sea. A prominent seismic reflecting horizon, Reflector P (Middle—Late Miocene in age), is observed throughout both the Aleutian and Komandorsky Basins at an approximate subbottom depth of 1 km. This reflector is also present, in places, on the flanks and along the crest of Shirshov Ridge. The thickness of sediments beneath Reflector P is significantly different within the two abyssal basins. In the Aleutian Basin, the total subbottom depth to acoustic basement (basalt?) is about 4 km, while in the Komandorsky Basin the depth is about 2 km.Shirshov Ridge, a Cenozoic volcanic feature that separates the Aleutian and Komandorsky Basins, is an asymmetric bathymetric ridge characterized by thick sediments along its eastern flank and steep scarps on its western side. The southern portion of the ridge has more structural relief that includes several deep, sediment-filled basins along its summit.Velocity data from sonobuoy measurements indicate that acoustic basement in the Komandorsky Basin has an average compressional wave velocity of 5.90 km/sec. This value is considerably larger than the velocities measured for acoustic basement in the northwestern Aleutian Basin (about 5.00 km/sec) and in the central Aleutian Basin (5.40–5.57 km/sec). In the northwestern Aleutian Basin, the low-velocity acoustic basement may be volcaniclastic sediments or other indurated sediments that are overlying true basaltic basement. A refracting horizon with similar velocities (4.6–5.0 km/sec) as acoustic basement dips steeply beneath the Siberian continental margin, reaching a maximum subbottom depth of about 8 km. The thick welt of sediment at the base of the Siberian margin may be the result of sediment loading or tectonic depression prior to Late Cenozoic time.     

The role of current velocity in structuring eelgrass (Zostera marina L.) meadows

Measurements of velocity profiles, bathymetry, and surface sediment characteristics across eelgrass (Zostera marina L.) meadows yielded information on community development processes and functional attributes of this ecosystem. Height/length ratios of the meadows were positively correlated with tidal current velocity. Low, medium, and high current regimes were separated by surface current velocities of approximately 50 and 90 cm s^?1. Z. marina can tolerate approximately 120–150 cm/sec current velocities in the areas studied. Per cent silt-clay and organic matter content of the surface sediments are negatively associated with shear velocity, suggesting that meadows in high current areas are sources while meadows in low current areas are sinks of autochthonous detritus. Current velocity maintains seagrass meadows at different equilibrium levels (relative climaxes). We theorize these different equilibrium levels provide unequal habitat utilization potentials for the associated faunal community.     

海底沉积物实验室剪切波速度及其与沉积物的物理性质之间的关系

采用压电陶瓷弯曲元法和共振柱试验的方法对采自我国海域的一些典型海底浅表层沉积物样品进行了剪切波速测试,获得首批可信数据.两种方法所测得的剪切波速数据具有很好的一致性,且在数赫兹至数十千赫兹频段范围内剪切波速不具明显弥散性.剪切波速与沉积物类型关系密切,不同海区和不同类型海底沉积物的剪切波速有明显差异.近海较细颗粒沉积物粉砂的剪切波速在100m/s左右,细颗粒沉积物的剪切波速在100m/s以下;陆架较粗颗粒沉积物的剪切波速最大,超过100m/s;深海、半深海细颗粒沉积物的剪切波速最低,小于50m/s.剪切波速与含水量、密度、孔隙度、塑限和液限等沉积物物理参数之间具很好的相关性,反映了剪切波速和物理性质之间的密切关系.剪切波速与压缩波速呈正相关性,但在不同的波速范围剪切波速随压缩波速的变化有很大不同.     

Control of pelagic sediment distribution by internal waves of tidal period: Possible interpretation of data from the southern East Pacific Rise

Sediment depositional patterns were observed on acoustic-reflection profiles at 36 and 42°S across the East Pacific Rise, near 100°W longitude. The sediment thickness as a function of distance from the crest shows a remarkable linearity on the east side of the rise, where the bottom topography is unusually subdued. The rate of sedimentation is 3.8 m/m.y at 42°S and 7.2 m/m.y. at 36°S. Disturbance to the even sedimentation appears to be correlated with topographic features more than 300 m high, and may therefore be associated with the interference between the barotropic tides and the topography. The group velocity of internal waves of semi-daily period is 15 cm sec⁻¹ in this area for a vertical wave number of 300 m⁻¹, and the characteristic slopes at 9° to the horizontal. The waves travel faster than the flow velocity of the tides and at an angle less than the slopes associated with the larger topographic features. Therefore a typical tidal velocity of 3 cm sec⁻¹ can be magnified substantially before the particle velocity approaches the group velocity and breaking occurs. Less magnification is possible near smaller topography because the group velocity is proportional to wavelength for internal waves of constant period. The tidal flow is magnified most near the boundary where the internal waves are reflected, and the higher velocities should cause settling sediment particles to remain in suspension locally. Thick boundary layers caused by breaking and mixing can shield the smaller-scale topography from the tidal motion.     

北戴河海滩泥沙捕获实验及其初步结果分析

介绍了利用泥沙捕获器观测破波带泥沙垂直分布结构的现场实验方法和基本程序，以及利用实验结果计算泥沙通量的方法。研究表明，近岸带泥沙运移通量及其在垂向上的分布受破波带相对位置和海滩地形变化的影响。在破波点附近，波浪的搅动和流场作用强，泥沙运移通量增大，泥沙在波浪的作用下可以大量进入垂直水体以悬移和跃移的方式运移。在本实验中，破波点附近的泥沙在距海底100cm的垂直水体中运移，通量垂向向上逐渐减小。远离破波点，泥沙运移通量和进入垂直水体的高度明显下降。在地形变化复杂的有坝海滩，沙坝顶部的泥沙运移通量最大，泥沙进入垂直水体运移的机率增加，而在沙坝问的沟槽内，波浪和海流作用减弱，泥沙通量和垂向进入水体运移的比例下降。     

The glacimarine sedimentary environment of Expedition Fiord, Canadian High Arctic

Expedition Fiord is a small, shallow inlet on the west coast of Axel Heiberg Island near 80°N latitude. It receives runoff and sediment at its head from a 1079 km² drainage basin, 72% of which is glacier-covered. Subbottom acoustic survey and cores from the fiord floor were used to assess the sedimentary environment. Most of the sediment is deposited within 3 km of the inflow from suspension in the overflowing cap and by gravity flows on the foreset beds of the delta. Occasionally, weak turbidity currents reach the mid fiord where they deposit fine-grained sediments. Icebergs from a large calving glacier in an adjacent fiord raft additional sediment, especially to the outer part of the fiord. They also scour the seafloor, although the persistent ice cover and slow currents in the fiord restrict this process. Except near the inflow, the total sediment accumulation since deglaciation is less than 20 m, and the rates of 0.5–1 mm/yr have not varied significantly to the present. A thicker deposit in the outer fiord is probably related to an early Holocene glacier margin near that location.     

Variability in in situ shear strength of gassy muds

Torque measurements are made by divers with a vane apparatus to a sediment depth of 136 cm in the high-porosity, gassy sediments of Eckernförde Bay. Corrected shear strength values calculated from torque measurements are quite variable in the Eckernförde Bay sediments, varying from less than 0.5 kPa in the top 10 cm of sediment to 4–5 kPa at 136 cm sediment depth. Variability increases markedly below 60 cm sediment depth, probably because of the presence of methane gas bubbles within the sediment fabric.     

Distribution and morphology of large submarine sediment slides and slumps on Atlantic continental margins

Abstract

Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km² in area, at least 600 km³ in volume of translated sediments. No apparent consistent relationship has yet been observed between the presence of the slides and the sedimentary environment in which they occurred. The slides off Southwest Africa and Spanish Sahara occurred in pelagic sediments rich in planktonic organic matter. In contrast, the slides off North America, Senegal‐Mauritania, and Brazil (Amazon Cone) occurred in sediments containing a high percentage of terrigenous material from nearby landmasses. Large sediment slides have also occurred in pelagic sediments on isolated oceanic rises such as the Madeira Rise (East‐Central Atlantic) and the Ontong‐Java Plateau (Pacific), where sedimentation rates are less than 2 cm/1000 years. The failure mechanism of the slides initiated near the shelf edge can probably be explained by sediment overloading during low glacio‐eustatic sea level, which allowed rivers to debouch sediments directly onto the outer shelf or upper slope. Possible mechanisms of failure of the deepwater slides and slumps include earthquakes, undercutting of the slope by bottom currents, and changes in porewater pressures induced as a direct or indirect result of glacio‐eustatic changes in sea level.     

Across-shelf sediment dispersal, Hauraki Gulf, New Zealand

Side-scan, seismic and surficial sediment data accompanied by current meter records highlight across-shelf sediment transport in Hauraki Gulf, an island-studded embayment off northern New Zealand. Calm weather currents are locally dominated by the tides, with periodic incursions of oceanic water from detached meanders of the East Auckland Current. Under these conditions, bedload transport occurs mainly in three 15–20 km-wide channels, where bathymetric intensification of the flow brings about near-bottom speeds of up to 82 cm s⁻¹ for Colville Channel and 33–44 cm s⁻¹ in Jellicoe and Cradock Channels. Surficial sediments are gravelly to muddy sand, winnowed in places, leaving a lag deposit of mainly biogenic carbonate gravel. Modelling results suggest that in Colville Channel, dominant fine to medium sand modes are mobile for 20–60% of the time, with a net eastward movement for fine sand. In Jellicoe and Cradock Channels, the prevailing direction of transport is southwards across the shelf, with sand mobile for up to 33% of the time. Oceanic incursions have the potential to boost flow in the western Gulf, however such incursions are transitory, and there is no measurable expression of oceanic water in the sedimentary record. Because of their association with prolonged periods of calm weather, the incursions are unlikely to accompany storm events, where their cumulative effect might be important for sediment transport. Near-bottom currents resulting from oceanic incursion may reinforce peak tides inside the Gulf by up to 2–4 cm s⁻¹. Enhancement of prevailing water motions occurs during periods of extreme weather. During cyclone Drena (January 1997), measured flow speeds in Jellicoe Channel reached 48 cm s⁻¹. Furthermore, the disturbance generated large waves that stirred bottom sediments down to over 100 m water depth. Such events are probably the major agent of sediment redistribution in the Hauraki Gulf. The net effect of storm and calm weather currents is to move sediment across the outer to middle shelf where, in the western and central Gulf it accumulates, and in the eastern Gulf it escapes eastward via Colville Channel.     

基于ADP-XR和OBS-3A的潮滩水文泥沙过程研究以胶州湾北部红岛潮滩为例

于2004年8月17~24日在胶州湾北部红岛潮滩上用OBS-3A和ADP-XR观测了水深、浊度、水平和垂直流速、回声强度、波浪、盐度、水温等水文泥沙要素,同时采集了悬沙和底沙样品作粒度分析.结果和结论为:(1)潮流动力较弱,表层和近底层最大流速分别只有31和26cm/s;(2)弱潮流动力导致潮周期大部分时间的悬沙浓度小于30mg/dm3,但浅水阶段近底悬沙浓度为100~1000mg/dm3;浅水阶段的短暂高悬浮泥沙浓度和其余长淹没时段的低悬沙浓度共同构成悬沙浓度的“U”形潮周期过程线;(3)悬沙浓度的垂直成层分布主要发生在潮周期的深水阶段和平静天气;(4)由于潮流弱和风浪的干扰,悬沙浓度未呈现大小潮周期的变化规律;(5)水体盐度为23.6~29.5;(6)淹没期的温度(21.4~28.6℃)比出露期的(19.3~30.9℃)稳定,温度极高值出现在午后出露期,而极低值出现在凌晨出露期;(7)“浅水效应”是弱动力潮滩泥沙运动的重要特点.     

A preliminary study on fingerprinting approach in marine sediment dynamics with the rare earth elements

Locating the quantitized natural sediment fingerprints is an important work for marine sediment dynamics study.The total of 146 sediment samples were collected from the Shelf of the East China Sea and five rivers,including Huanghe (Yellow),Changjiang (Yangtze),Qiantang,Ou and Min River.The sediment grain size and the contents of rare earth elements (REEs) were measured with laser particle size analyzer and ICP-MS technology.The results show that absolute REE content (ΣREE) and the concentration ratio of light REEs to heavy REEs (L/HREE) are different in the sediments among those rivers.There are higher REE contents in being less than 2 m and 2–31 μm fractions in the Changjiang Estuary surface sediments.The REE contents of bulk sediment are dominated by the corresponding values of those leading size-fractions.REE of sediment is higher close to the estuaries and declines seaward on the inner shelf of the East China Sea (ECS).The L/HREE ratio has a tendency of increase southward from 28 ? N.Hydrodynamic conditions plays a predominate role on spacial distributions of the surficial sediment’s REE parameters.In some situations,the currents tend to remove the coarser light grains from initial populations,as well as the deposit of the finer heavy mineral grains.In other situations,the currents will change the ratio of sediment constituents,such as ratio between silts and clays in the sediments.As a result,the various values of REE or L/HREE ratio in different bulk sediments are more affected by the change of size-fractions than source location.Under the long-term stable hydrodynamic environment,i.e.,the East China Sea Shelf,new sediment transport model based on the size and density gradation concept may help to understand the spatial distribution patterns of REE parameters.     

A flood-dominated mesotidal inlet

Tidal inlets along the mesotidal coast of Maine contrast with those from other parts of the world by being dominated by flood-tidal currents. Analysis of the factors responsible for flood or ebb dominance indicates factors external to the backbarrier environment. We suggest that the flood dominance is caused by both a steepening of the tidal wave in the Gulf of Maine and the shallow depth of the ebb-tidal delta and spit platform. Flood currents are typically 10–20 cm/sec stronger than the ebb at the inlet throat. The flood dominance results in a significant net landward transport of sediment into the backbarrier.     

Sediment concentrations,flow conditions,and downstream evolution of two turbidity currents,Monterey Canyon,USA

The capacity of turbidity currents to carry sand and coarser sediment from shallow to deep regions in the submarine environment has attracted the attention of researchers from different disciplines. Yet not only are field measurements of oceanic turbidity currents a rare achievement, but also the data that have been collected consist mostly of velocity records with very limited or no suspended sediment concentration or grain size distribution data. This work focuses on two turbidity currents measured in Monterey Canyon in 2002 with emphasis on suspended sediment from unique samples collected within the body of these currents. It is shown that concentration and grain size of the suspended material, primarily controlled by the source of the gravity flows and their interaction with bed material, play a significant role in shaping the characteristics of the turbidity currents as they travel down the canyon. Before the flows reach their normal or quasi-steady state, which is defined by bed slope, bed roughness, and suspended grain size, they might pass through a preliminary adjustment stage where they are subject to capacity-driven deposition, and release heavy material in excess. Flows composed of fine (silt/clay) sediments tend to be thicker than those with sands. The measured velocity and concentration data confirm that flow patterns differ between the front and body of turbidity currents and that, even after reaching normal state, the flow regime can be radically disrupted by abrupt changes in canyon morphology.     

A flood-dominated mesotidal inlet

Tidal inlets along the mesotidal coast of Maine contrast with those from other parts of the world by being dominated by flood-tidal currents. Analysis of the factors responsible for flood or ebb dominance indicates factors external to the backbarrier environment. We suggest that the flood dominance is caused by both a steepening of the tidal wave in the Gulf of Maine and the shallow depth of the ebb-tidal delta and spit platform. Flood currents are typically 10–20 cm/sec stronger than the ebb at the inlet throat. The flood dominance results in a significant net landward transport of sediment into the backbarrier.     

Physical control of sediment carbon content in an estuarine tidal flat system (Nanakita River, Japan): A mechanistic case study

Temporal variations in sediment carbon content were tracked by sampling every 2 weeks for 6 to 33 months at 6 tidal flat stations with different carbon content levels (0.05–1.64%) in a single estuary. Three temporal series of current velocity at 5 cm above the sediment were also obtained. Non-cohesive and cohesive sediment stations differed in patterns of temporal variation in sediment carbon content, suggesting the difference in processes controlling sediment carbon content. In the stations of non-cohesive sandy sediment with relatively low carbon content (0.05–0.15%), sediment carbon content fluctuated within ranges specific to each station. In these stations, current velocity data suggested that frequency of sand resuspension washing out sediment carbon controls sediment carbon content level. In the stations of cohesive sediment with relatively high carbon content (0.77–1.64%), sediment carbon contents showed some unusual upward and downward peaks, recovered to usual levels specific to the stations, and was always kept higher than that in the non-cohesive sediment stations. We speculate that in the cohesive sediment stations, sand inputs may significantly control processes establishing the consolidated sediments with various carbon content levels. Spatial transitions from non-cohesive to cohesive sediments and from low to high sediment carbon contents likely occur as sand resuspension at spring tides become more irregular and less frequent. Based on these results, conceptual models describing physical processes controlling sediment carbon content at the studied stations were proposed.