Distribution of riverine sediment chemistry on the shelf,slope and rise of the Gulf of Papua

The tectonically active islands of the Indo-Pacific Archipelago deliver much sediment to the ocean margins. In the Gulf of Papua on the south coast of Papua New Guinea (PNG), the chemical composition of surface sediment from grab samples indicates that Fly River muds are dispersed to the north and east, where they are joined by sediment plumes from the other large rivers along the south coast of PNG. This is the likely source of terrestrial sediment on the Papuan Plateau and the northern Coral Sea Abyssal Plain. The sediment is transported through submarine troughs and canyons offshore, far to the east of the riverine inputs. Immediately south and 30–50 km offshore from the Fly and Purari deltas is a platform of algal and reef carbonate materials, containing little or no terrestrial surface sediment.     

New evidence of slope instability in the Outardes Bay delta area, Quebec, Canada

The Outardes Bay delta constitutes one of the best sites to study the formation of failure deposits in a modern lowstand environment. These deposits are located in a pseudo-shelf-edge position along the northern part of the Laurentian Channel in the St. Lawrence Estuary. The site has been investigated over the past 20 years with a Raytheon model RTT1000 boomer (3.5 kHz, 400 J) on the shelf, and most recently with a Simrad model EM 1000 multibeam sonar (95 kHz) on the slope to provide high-resolution seismic and bathymetric data. The seismic data show wavy, chaotic and contorted reflectors which are typical in marine environments characterised by instability features. The multibeam sonar data have revealed many slope instability features such as creep folds, channel incisions, debris flows, and rotational slide scars. Thus, these interpreted features are in direct relationship with the seismic interpretation of the data collected upslope. These geomorphological and geophysical signatures express both past and present sedimentological processes. Some of the mass movement signatures observed in the surveyed area are believed to be related with the great MS~7 Charlevoix earthquake in 1663.     

New evidence of slope instability in the Outardes Bay delta area, Quebec, Canada

The Outardes Bay delta constitutes one of the best sites to study the formation of failure deposits in a modern lowstand environment. These deposits are located in a pseudo-shelf-edge position along the northern part of the Laurentian Channel in the St. Lawrence Estuary. The site has been investigated over the past 20 years with a Raytheon model RTT1000 boomer (3.5 kHz, 400 J) on the shelf, and most recently with a Simrad model EM 1000 multibeam sonar (95 kHz) on the slope to provide high-resolution seismic and bathymetric data. The seismic data show wavy, chaotic and contorted reflectors which are typical in marine environments characterised by instability features. The multibeam sonar data have revealed many slope instability features such as creep folds, channel incisions, debris flows, and rotational slide scars. Thus, these interpreted features are in direct relationship with the seismic interpretation of the data collected upslope. These geomorphological and geophysical signatures express both past and present sedimentological processes. Some of the mass movement signatures observed in the surveyed area are believed to be related with the great MS~7 Charlevoix earthquake in 1663.     

Characteristics of sediment transport in the Zhoushan Archipelago sea area

Based on the field data including tidal current,suspended sediment concentration,grain size of surface sediments,the transport mechanism and movement trend of sediments are analyzed using the method of flux decomposition and Grain Size Trend Analysis.The results show that the concentration and flux of suspended sediments decrease sharply from the Hangzhou Bay to the offshore area.The suspended sediment transport is mainly controlled by advection transport including average current transport and Stokes drift-induced transport and the gravitational circulation transport.The surface sediments are transported from the entrance of the Hangzhou Bay to the offshore area in general; meanwhile,the sediment transport has two obvious transport trends in the offshore area.The interaction of tidal currents,residual currents,the East China Sea coastal current,Taiwan Warm Current and wind waves appear to play important roles in the sediment transport.Furthermore,the sediment distribution and transport are significantly affected by the Zhoushan Archipelago.     

黄河三角洲埕北海域底质沉积物粒度特征及泥沙输运分析

通过黄河三角洲埕北海域表层沉积物的粒度资料、实测潮流数据以及50 a一遇的波浪要素,分析了埕北海域表层沉积物的分布类型以及粒度参数特征,并计算了表层沉积物的临界起动应力、日平均单宽输沙通量以及波流共同作用下的单宽推移质输沙率。结果表明:研究区表层沉积物主要是粉砂质砂、砂质粉砂、粉砂和泥四种,分选中等偏差,近对称到正偏,中等尖锐到尖锐峰态;对大潮期间埕北海域潮流底应力和表层沉积物的临界起动应力的计算说明潮流在大部分区域具有起动和输运泥沙的作用;秋季潮流作用下的日平均单宽输沙通量均值为372.32 kg/（m·d）,方向为涨潮流向。利用贝克尔（Bijker）和经典的Van Rijn公式计算出的50 a一遇的波流共同作用下单宽推移质日平均净输沙率结果近似相等。     

基于水下自主航行器（AUV）的神狐峡谷谷底块体搬运沉积特征及其对深水峡谷物质输运过程的指示

海底峡谷是陆源物质向深海运移的重要通道。对于远离陆地的海底峡谷,通常认为浊流是物质搬运的主要营力。受限于探测精度和复杂作业环境影响,使用常规地球物理资料对深水海底峡谷尤其是对谷底沉积体的形态和结构特征的刻画不够精细。基于水下自主航行器（AUV, Autonomous Underwater Vehicle）采集的高分辨率多波束、旁扫声呐和浅地层剖面资料,对神狐峡谷群中的一条峡谷的谷底表面及部分浅部地层的沉积特征进行了分析。结果表明,峡谷谷底浅部地层并不像它平滑的表面那么简单,而是由大量内部杂乱弱反射、厚度在8.4 m及以下的块体搬运沉积体组成。峡谷中下游块体搬运沉积体大都沿峡谷走向整体呈条带状展布,不是直接来源于相邻的峡谷脊部。研究认为在特定沉积环境下（例如高海平面时期）,陆坡限定性峡谷谷底的块体搬运沉积过程的重复进行是峡谷谷底物质输运的重要途径,与浊流共同雕刻了峡谷的地形地貌。基于AUV的地球物理探测技术将是研究海底浅表层沉积过程和保障海底工程施工的重要手段。     

Erosion and slope instability on Horizon Guyot,Mid-Pacific Mountains

Seismic-reflection profiles, sediment cores, and current velocities were assessed to study the impact of erosion and sediment redistribution on the pelagic sediment cap of Horizon Guyot, a flat-topped submarine volcanic ridge in the Mid-Pacific Mountains. These processes seem to concentrate their effect around the rim of the sediment cap. Sediment slumping occurs on the northwest perimeter of the guyot's sediment cap. Slope stability analysis suggests that if overconsolidation on Horizon Guyot is the result of current reworking or if local undercutting by bottom currents steepens the sea floor declivity, the sediment cap may be unstable during infrequent earthquake loading, transporting sediment from the guyot summit to the abyssal sea floor.     

Variable deposition rates on the slope and rise off the Mid-Atlantic States

Pronounced variations in slope topography and offshelf spillover have produced a three-fold difference (9 to 25 cm/1,000 yr) in Late Quaternary deposition rates among slope cores southeast of the Mid-Atlantic States. Upper rise cores revealed a high rate but lower core-to-core variability (20 to 44 cm/1,000 yr), largely the result of slope bypassing, i.e., effects of gravity-controlled downslope transport. A transport model suggests temporary slope sediment storage and higher accumulation of mud in more distal rise regions. Eustatic change accounts for the four-fold decrease in slope deposition rate between the Late Pleistocene and Holocene.     

东碇倾倒区疏浚泥悬沙输移和海床冲淤数值模拟

采用区域海洋模型ROMS(Regional Ocean Modeling System)在东碇倾倒区建立三维水动力与泥沙输移数值模型,在水动力验证的基础上模拟预测了在冬、夏季节不同背景潮流和风场作用下的疏浚泥悬沙输移规律及海床沖淤变化。结果表明:在纯潮流作用下,悬沙扩散以抛泥点为中心,呈圆对称分布,且冬季泥沙输移扩散范围比夏季大;加入风场驱动后,悬沙扩散呈下风面拖拽的椭圆形,同时水体混合加强更有利于高浓度悬沙的扩散,但对悬沙输移的作用不明显;抛泥点位置从倾倒区中心变为西北角后,其作用主要体现在冬季较高浓度区面积的减小和由于局地水槽引导而形成的向北延伸的浓度舌;在海床冲淤方面,潮流和风场的作用主要是改变地形增量较大的面积,而更改抛泥点位置则会导致地形增量最大值出现较大的差异。     

A new formula for the total longshore sediment transport rate

A new predictive formula for the total longshore sediment transport (LST) rate was developed from principles of sediment transport physics assuming that breaking waves mobilize the sediment, which is subsequently moved by a mean current. Six high-quality data sets on hydrodynamics and sediment transport collected during both field and laboratory conditions were employed to evaluate the predictive capability of the new formula. The main parameter of the formula (a transport coefficient), which represents the efficiency of the waves in keeping sand grains in suspension, was expressed through a Dean number based on dimensional analysis. The new formula yields predictions that lie within a factor of 0.5 to 2 of the measured values for 62% of the data points, which is higher than other commonly employed formulas for the LST rate such as the CERC equation or the formulas developed by Inman–Bagnold and Kamphuis, respectively. The new formula is well suited for practical applications in coastal areas, as well as for numerical modeling of sediment transport and shoreline change in the nearshore.     

Movement of sediment on the Gulf of Mexico,continental slope and upper continental shelf

Abstract

Grain size, coarse fraction analyses, and depositional environment as interpreted from microfauna are related to the character of sparker reflections at the location of core holes drilled by Exxon, Chevron, Gulf, and Mobil on the continental slope of the northern Gulf of Mexico. Continuous sparker reflections are correlated with slowly deposited, evenly bedded sediments containing bathyal faunas. The coarse fraction is dominated by the tests of foraminifera. Discontinuous, discordant reflections and diffractions are correlated with sediments more rapidly emplaced in the bathyal environment of the continental slope by slumping and sliding from the continental shelf. Their coarse fraction is dominated by terrigenous sand grains. A large portion of the volume of continental slope sediments appears to consist of these “displaced”; sediments, including an area 3–24 km wide and 80 km long, southeast of Corpus Christi, Texas. Comparable processes of movement of sediments are interpreted on the continental shelf south of the Southwest Pass of the Mississippi River. Bathymetry in this area is characterized by a series of subaqueous “gullies”; radiating from the river mouth and leading to terraces at their southern extremities. Side‐scan sonar and PDR surveys show a rough bottom in these “gullies”; and terraces, as contrasted with a relatively smooth bottom elsewhere. The rough bottom is interpreted as indicative of slump and creep of the sediments from shallower water. Some foundation soil borings in this area south of Southwest Pass find a low‐strength material gradually increasing in strength with depth. Other borings find a “crust”; of anomalously strong material 8–15 m below the mudline. The microfauna recovered from the “crust”; has moved to its present position by slump or creep from shallower water along a pattern comparable to the gullies shown in the present‐day bathymetry.     

Process-based model for nearshore hydrodynamics, sediment transport and morphological evolution in the surf and swash zones

Hydrodynamics and sediment transport in the nearshore zone were modeled numerically taking into account turbulent unsteady flow. The flow field was computed using the Reynolds Averaged Navier–Stokes equations with a k–ε turbulence closure model, while the free surface was tracked using the Volume-Of-Fluid technique. This hydrodynamical model was supplemented with a cross-shore sediment transport formula to calculate profile changes and sediment transport in the surf and swash zones. Based on the numerical solutions, flow characteristics and the effects of breaking waves on sediment transport were studied. The main characteristic of breaking waves, i.e. the instantaneous sediment transport rate, was investigated numerically, as was the spatial distribution of time-averaged sediment transport rates for different grain sizes. The analysis included an evaluation of different values of the wave friction factor and an empirical constant characterizing the uprush and backwash. It was found that the uprush induces a larger instantaneous transport rate than the backwash, indicating that the uprush is more important for sediment transport than the backwash. The results of the present model are in reasonable agreement with other numerical and physical models of nearshore hydrodynamics. The model was found to predict well cross-shore sediment transport and thus it provides a tool for predicting beach morphology change.     

Geophysical evidence for cyclic sediment deposition on the southern slope of Qiongdongnan Basin,South China Sea

Gravity flow deposits form a significant component of the stratigraphic record in ancient and modern deep-water basins worldwide. Analyses of high-resolution 3D seismic reflection data in a predominantly slope setting, the southern slope of Qiongdongnan Basin, South China Sea, reveal the extensive presence of gravity flow depositional elements in the Late Pliocene−Quaternary strata. Three key elements were observed: (1) mass transport deposits (MTDs) including slumps and debris flows, (2) turbidity current deposits including distributary channel complexes, leveed channel complexes and avulsion channel complexes, and (3) deep-water drapes (highstand condensed sections). Each depositional element displays a unique seismic expression and internal structures in seismic profiles and attribute maps. Based on seismic characteristics, the studied succession is subdivided into six units in which three depositional cycles are identified. Each cycle exhibits MTDs (slump or debris) at the base, overlain by turbidities or a deep-water drape. The genesis of these cycles is mainly controlled by frequent sea-level fluctuations and high sedimentation rates in the Late Pliocene–Quaternary. Moreover, tectonics, differential subsidence, and paleo-seafloor morphology may have also contributed to their formation processes. The present study is aimed to a better understanding of deep-water depositional systems, and to a successful hydrocarbon exploration and engineering-risk assessment.     

Evidence for the sediment circulation along the coast of East Anglia

McCave (1978) suggested that, based on sediment budget considerations and coarsening of sand-size away from cliff sources, the “nesses” on the East Anglian coast of England are sites where sediment is lost to the shoreline. This view is directly opposed to that of Robinson (1966, 1980) who believed that material reached the beach from offshore at nesses largely through the effect of residual currents.

The present paper examines these two opposing views, mainly in the light of recent data from the Easton Bavents-Aldeburgh area of East Anglia. It is concluded that neither argument is proven although it is more probable that nesses are places where sediment is lost from the foreshore rather than gained from offshore. The variability of trends between one ness and another, and at different sites offshore, implies that generalisations may not be valid.     

Indirect evidence for sediment transport on the continental shelf off Israel

The geographical distribution of coal particles that fell during nine years to the sea bottom from the open sea coal terminal off Hadera, Israel, shows a consistent northerly transport path, and it is inconceivable that the sand at that location would move in an opposite direction. Semicircular, current-scoured moats associated with small mounds found at the edge of the continental shelf off Ashdod and Haifa, Israel, also suggest a general northward-directed sediment transport. The northward sediment flow on the Israeli continental shelf inferred from this evidence therefore supports the model of Emery and Neev about general sand transport patterns along the Israeli coastline and continental shelf.     

Geotechnical properties of sediment on the Kodiak continental shelf and upper slope,gulf of Alaska

Abstract

Fine‐grained sedimentary deposits on the Kodiak continental shelf and upper slope comprise three distinct compositional types: terrigenous mud, diatomrich mud, and ash‐rich sandy mud. The sediment types can be distinguished on the basis of geotechnical properties as well as by composition. The terrigenous mud has properties largely within the normal range for fine‐grained marine sediment, except for the low compressibility of many samples. This sediment underlies the walls of canyons that incise the upper slope, and analyses of undrained static and cyclic loading indicate potential instability in the steepest areas. The diatom‐rich mud has high water content, plasticity index, and compression index but low grain specific gravity. The ash‐rich sandy mud is nonplastic and has low water content and compressibility. It has high drained and undrained static strength but is extremely weakened by cyclic loading. Extensive deposits of sedimentary bedrock and coarsegrained glacial sediment in the region apparently are relatively stable, but low sediment strength or high compressibility may be encountered at the local sites of soft sedimentary deposits.     

北部湾环流和沉积物输运的数值模拟

研究环流和沉积物输运对北部湾的环境保护和资源开发十分重要。本文使用区域海洋模型（ROMS）,研究了北部湾海域的季节环流、沉积物输运以及长期的地形演变过程。展示了湾内冬季和夏季都存在逆时针环流,冬季的风生环流比夏季强的特征。悬沙浓度较大值主要分布在琼州海峡、海南岛西侧海域,以及越南沿岸、雷州半岛沿岸等近岸地区。沉积物在琼州海峡附近表现为在冬季由东向西输运,夏季则相反;在琼州海峡西口处,沉积物全年以西向输运为主。结果显示,北部湾内沉积物输运主要由潮流引起的再悬浮控制。侵蚀和淤积的分布模式为：（1）湾内大部分区域侵蚀淤积不明显,（2）琼州海峡两侧口门附近淤积比较明显,（3）琼州海峡内深槽侵蚀严重,（4）海南岛西侧海域存在侵蚀和淤积交替发生区域。     

A validation concept for cohesive sediment transport model and application on Lianyungang Harbor,China

Morphology evolution and hydro-sedimentological interactions in muddy coastal environments are long term processes. These processes are closely related to suspended sediment transport driven by waves and tidal currents. In the traditional calibration/verification methodology for cohesive sediment transport models, time-series data of suspended sediment concentration (SSC) measured during one or several tidal periods are commonly used as major validation references. But the disadvantage of this approach includes that it cannot filter the noises caused by the stochastic nature of short term hydrodynamics induced by waves and the varying properties of bottom sediments; besides a phase-lag phenomenon is often observed between sediment transport and hydrodynamics in the short term. On the contrary, a stable relationship between sedimentation and hydrodynamics is usually found in the long term. For a specific weather, the SSC values often agree well with local wave heights for muddy coasts. Therefore, in this paper a conceptual quantity defined as “representative SSC” was brought forward, and a model validation concept, including calibration and verification approaches, was proposed, in which calibration is performed against yearly-representative SSC values and the long term transport trend, and verification by using representative SSC values corresponding to different wave conditions. A numerical simulation was set up, and a real-life engineering application, Lianyungang Harbor, China, was executed to elaborate the proposed validation concept. Finally, the characteristics of SSC distribution around Lianyungang Harbor were discussed.     

New evidence for widespread mass transport on the Northeast Newfoundland Shelf revealed by Olex single-beam echo sounding

Based on Olex single-beam sounder data, multibeam sonar surveys, and sparse seismic reflection profiles, we recognize a large area of anomalous bathymetry on the Northeast Newfoundland Shelf as having formed as a result of mass-transport processes. Transported masses include (1) an arcuate ridge of deformed material with an area of 430 km², which has moved distances of ~20 km; (2) a 70-km² mass of deformed material displaced 50 km along a nearly horizontal track flanked by 90-m-high berms. The movement of these and other sediment bodies has created a 150-m-high headwall escarpment extending 110 km along the north flank of the Notre Dame glacial trough. In addition, a 35-km² block of undeformed material has moved 5 km to the southeast, away from the headwall, creating a gap of the same dimensions, while a smaller block of material originating in this vicinity has been displaced 24 km in the opposite direction, creating a 20-m-deep groove on the seafloor. There is evidence for mass transport and headwall formation elsewhere on the Northeast Newfoundland Shelf. Analysis of seismic reflection data indicates that the transported material most likely consists of stacked Quaternary till sheets that overlie Cenozoic, Mesozoic and older sedimentary rocks. Given the very low gradients involved, glaciotectonism is the most likely process to account for transport and deformation of the large sediment masses. However, some mass transport may have resulted from submarine sliding away from the headwalls that were created by the glacial transport.