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Cross-front transport is a vital process in the offshore transport of terrestrial materials. The factors influencing cross-front transport are multifaceted and exhibit regional variability. As important factors regulating material transport patterns in the shelf seas of eastern China, the East Asian Winter Monsoon (EAWM) and Kuroshio current (including its branches) vary on multiple time scales, yet their role in controlling offshore transport is not fully understood. In this paper, cross-front offshore transport in the North Yellow Sea is chosen to investigate the aforementioned issue using numerical simulation. By excluding other potential processes for offshore transport, frontal instability is identified as the primary mechanism for offshore transport, which is triggered by winter storms (bursts of the EAWM); hence, the intensity of offshore transport is highly associated with the strength of winter storms on synoptic scales. However, on the inter-annual scale, the intensity of offshore transport is more likely regulated by the strength of the Kuroshio current, and a warming Kuroshio facilitates offshore transport. Although the role of each factor in controlling the intensity of offshore transport is delineated, the interactive effects of the EAWM and Kuroshio current on offshore transport are quite intricate, and how to quantitatively estimate their roles on multiple time scales remains a challenge using modelling. The results obtained from this study can be applied to analyse cross-front transport throughout the eastern China seas and bear significant implications for future studies on pollutant diffusion, nutrient distribution, and sediment transport in the coastal area. • Cross-front transport around the Shandong Peninsula was revealed using a numerical model. • EAWM and Kuroshio affect the strength of cross-front transport on different time scales. • With global warming, the temperature of Kuroshio might gradually become the primary control of cross-front transport. [ABSTRACT FROM AUTHOR]

Fluvial sediment is the major source for the formation and development of the Mekong Delta. This paper aims to analyse the dynamics of suspended sediment and to investigate the roles of different processes in order to explore flux pattern changes. We applied modelling on two scales, comprising a large-scale model (the whole delta) to consider the upstream characteristics, particularly the Tonle Sap Lake's flood regulation, and a smaller-scale model (tidal rivers and shelf) to understand the sediment processes on the subaqueous delta. A comprehensive comparison to in-situ measurements and remote sensing data demonstrated that the model is capable of qualitatively simulating sediment dynamics on the subaqueous delta. It estimates that the Mekong River supplied an amount of 41.5 mil tons from April 2014 to April 2015. A substantial amount of sediment delivered by the Mekong River is deposited in front of the river mouths in the high flow season and resuspended in the low flow season. A sensitivity analysis shows that waves, baroclinic effects and bed composition strongly influence suspended sediment distribution and transport on the shelf. Waves in particular play an essential role in sediment resuspension. The development of this model is an important step towards an operational model for scientific and engineering applications, since the model is capable of predicting tidal propagation and discharge distribution through the main branches, and in predicting the seasonal SSC and erosion/deposition patterns on the shelf, while it is forced by readily available inputs: discharge at Kratie (Cambodia), GFS winds, ERA40 reanalysis waves, and TPXO 8v1 HR tidal forcing. [ABSTRACT FROM AUTHOR]

This introduction to the special issue entitled, “Sediment- and hydro-dynamics of the Mekong Delta: from tidal river to continental shelf”, describes the setting and program design of collaborative studies with integrated field and modeling experiments in 2014–2015, along with associated research in the region. The Mekong River is among the largest on Earth in terms of water discharge, and much of the sediment delivered from the Tibetan Plateau has accumulated in the subaerial and subaqueous components of the Mekong Delta. As a group, the papers in this special issue describe the portion of the system where the sediment source signal is altered along the tidally influenced river and is delivered to the shorelines and continental shelf. The linked studies provide a holistic view of the system, and emphasize the interactions between hydrodynamics and sediment transport processes in sub-environments as sediment makes its way along the path from source to sink. [ABSTRACT FROM AUTHOR]

Sea ice is an important vehicle for sediment transport in the Arctic Ocean. On the Laptev Sea shelf (Siberian Arctic) large volumes of sediment-laden sea ice are formed during freeze-up in autumn, then exported and transported across the Arctic Ocean into Fram Strait where it partly melts. The incorporated sediments are released, settle on the sea floor, and serve as a proxy for ice-transport in the Arctic Ocean on geological time scales. However, the formation process of sediment-laden ice in the source area has been scarcely observed. Sediment-laden ice was sampled during a helicopter-based expedition to the Laptev Sea in March/April 2012. Sedimentological, biogeochemical and biological studies on the ice core as well as in the water column give insights into the formation process and, in combination with oceanographic process studies, on matter fluxes beneath the sea ice. Based on satellite images and ice drift back-trajectories the sediments were likely incorporated into the sea ice during a mid-winter coastal polynya near one of the main outlets of the Lena River, which is supported by the presence of abundant freshwater diatoms typical for the Lena River phytoplankton, and subsequently transported about 80 km northwards onto the shelf. Assuming ice growth of 12–19 cm during this period and mean suspended matter content in the newly formed ice of 91.9 mg l −1 suggests that a minimum sediment load of 8.4×10 4 t might have been incorporated into sea ice. Extrapolating these sediment loads for the entire Lena Delta region suggests that at least 65% of the estimated sediment loads which are incorporated during freeze-up, and up to 10% of the annually exported sediment load may be incorporated during an event such as described in this paper. [ABSTRACT FROM AUTHOR]

Sediment transport and depositional processes in the Southwest Atlantic Inner Shelf are strongly influenced by the contribution from Río de la Plata and Patos Lagoon fluvial systems. Understanding the behavior of erosive and depositional processes and bed evolution in the region is of utmost importance to address the impacts of anthropogenic activities, climate change and for navigation security. Therefore, this work aims to improve the understanding of such processes, and their temporal and spatial scales of variability. We employed the SISYPHE morphodynamic model coupled with the TELEMAC-3D hydrodynamic model to simulate the resulting depositional and erosional fluxes and the bottom evolution during a period of 8 years (2005–2012). Results indicate that during this period, depositional fluxes mostly exceeded erosional fluxes in the south of the Patos Lagoon mouth, resulting in predominantly positive bottom evolution. On the other hand, erosional fluxes exceeded depositional fluxes to the north of the Patos Lagoon mouth, resulting in predominantly negative bottom evolution. Widespread positive values of bottom evolution are observed along the northern and southern coasts off the Río de la Plata mouth, indicating a general dominance of sediment deposition over erosion. A wavelet analysis showed that fluvial discharge influences the variability of depositional process on an interannual time scale, whereas the synoptic variability is influenced by wind action. Moreover, positive ENSO periods (El Niño) are related to increased depositional fluxes, and decreased depositional fluxes during the negative phase (La Niña), with pronounced effects off Río de la Plata. • The study area presented predominantly positive bottom evolution. • Erosional fluxes exceeded depositional fluxes north of the Patos Lagoon mouth. • Positive values of bottom evolution are observed along coasts off the Río de la Plata mouth. • The fluvial discharge influences the deposition process on interannual time scale, while the wind action is related to the synoptic time scale variability. • The ENSO effect can increase the depositional fluxes during El Niño and decrease during the Niña. [ABSTRACT FROM AUTHOR]

A 6-month deployment of instrumentation from April to October 2012 in 90 m water depth near the outer edge of the mid-shelf mud belt in southern Monterey Bay, California, reveals the importance regional upwelling on water column density structure, potentially accounting for the majority of the variability in internal tidal energy flux across the shelf. Observations consisted of time-series measurements of water-column currents, temperature and salinity, and near-bed currents and suspended matter. The internal tide accounted for 15–25% of the water-column current variance and the barotropic tide accounted for up to 35%. The subtidal flow showed remarkably little shear and was dominated by the 7–14 day band, which is associated with relaxations in the dominant equatorward winds typical of coastal California in the spring and summer. Upwelling and relaxation events resulted in strong near-bed flows and accounted for almost half of the current stress on the seafloor (not accounting for wave orbital velocities), and may have driven along-shelf geostrophic flow during steady state conditions. Several elevated suspended particulate matter (SPM) events occurred within 3 m of the bed and were generally associated with higher, long-period surface waves. However, these peaks in SPM did not coincide with the predicted resuspension events from the modeled combined wave–current shear stress, indicating that the observed SPM at our site was most likely resuspended elsewhere and advected along-isobath. Sediment flux was almost equal in magnitude in the alongshore and cross-shore directions. Instances of wave–current shear stress that exceeded the threshold of resuspension for the silty-clays common at these water depths only occurred when near-bed orbital velocities due to long-period surface waves coincided with vigorous near-bed currents associated with the internal tide or upwelling/relaxation events. Thus upwelling/relaxation dynamics are primarily responsible for variability in the internal tide, as well as transport of near-bottom sediment in the mid-self mud belt during the relatively quiescent summer months. [ABSTRACT FROM AUTHOR]

The vertical distribution of suspended sediment in continental shelf waters has significant implications for water quality, aquatic ecology, and sediment transport. Nevertheless, there have been relatively few systematic field studies aimed at determining whether a general vertical trend in suspended sediment concentration (SSC) exists, or how any such vertical trend can be expressed mathematically. In this study, based on 402 individual SSC profiles measured hourly during 16 dual-tide voyages at 8 stations in the outer Yangtze Estuary, we found that SSC followed an average depth profile that was smoothly concave-up. This average profile, and the relationship between SSC and height above the seabed, was exponential ( R 2 >0.99). In comparison, the traditional Rouse equation based on hydrodynamics describes the measured average SSC profile poorly, in that the greater the distance from the near-bed height (where the reference SSC is taken), the greater the deviation of the simulated SSC from the measured SSC. However, in this study, a new approach was developed to overcome this flaw in the Rouse equation, which divides the equation into two parts. One part uses a reference SSC from a near-bed level, and an upwards decreasing coefficient, in the same way as the conventional Rouse equation. Conversely, the other part uses a reference SSC from at the water surface, and a downwards decreasing coefficient. Our modified hydrodynamics-based equation expresses the measured SSC well, with an R 2 between the simulated and measured SSCs exceeding 0.99. According to the average SSC profiles, the near-bed SSC was 2.8 times greater than the SSC at the water surface (close to the value of 3 suggested for modeling studies in the absence of empirical data), and the depth-averaged SSC was 1.8 times greater than the SSC at the water surface. We also found that the average SSC profile showed a uniform (from water surface to seabed) decline in SSC (25%) in response to the dramatic decline in the suspended sediment load supplied by the Yangtze River over the past three decades. However, the shape and vertical ratio of the average SSC profile have changed little, which suggests that the general form of the SSC profile is determined mainly by local hydrodynamics and sediment properties, and not by fluvial sediment supply. [ABSTRACT FROM AUTHOR]

A one-year study examining water-column and near-bed time-series observations from benthic-boundary-layer tripods was undertaken in the Waipaoa Sedimentary System (WSS) on the Poverty Bay continental shelf, offshore of the Raukumara Ranges, New Zealand. Here, the nature of the adjacent post-glacial shelf deposits, fed by the Waipaoa River, and century-scale sediment accumulation is well established, but event-scale sediment dispersal and hydrodynamics had not been measured. Data from two tripods outfitted with acoustic and optical instrumentation, and x-radiograph images of seabed cores collected at 4-month intervals, were analyzed to quantify progressive sedimentological changes, and to document processes responsible for seabed strata formation, destruction, and preservation. At the Northern Tripod (NT) mid-shelf location 15 km southeast of the river mouth, we observed increases in near-bed suspended sediment concentration when shear velocities exceeded 0.03 m s −1 on 33 occasions over an 11.5-month period. Of these events, 9 were depositional, 10 were erosional, and 14 had no bed-elevation change at the 1-cm resolution of our instrumentation. Relatively high (2–5 m) and short-period (<11–12 s) waves, combined with current velocities <0.15 m s −1 , led to deposition on the seabed. Smaller-height (2–3 m) and longer-period (11–13 s) waves with stronger current velocities (>0.15 m s −1 ) typically induced erosion. Despite the short reach of this mountainous riverine dispersal system, we observed a consistent delay between river discharge and apparent deposition on the mid-shelf at the NT location. Following the three largest floods, ~120 h of strong, wave-driven bed shear stress preceded measurable deposition on the mid-shelf, suggesting that event duration and intensity are key for transporting material offshore from Poverty Bay. The largest flux event recorded over the study year was a 130-hour-long storm and associated river flood that resulted in near contemporaneous deposition at the NT site. Suspended-sediment concentration data suggest transport by a wave-supported fluid mud. X-radiographs from the NT location corroborate the instrumental record of 5 cm deposition, but only the two largest depositional events were preserved in the strata. On the southern shelf we observed no evidence of deposition in either the instrumental data or in the x-radiographs of cores. This study suggests that event duration, and spatial and inter-annual variability are critical considerations when evaluating along- and across-shelf sediment transport and seabed stratigraphic evolution associated with discharge from small mountainous rivers. [ABSTRACT FROM AUTHOR]

The Waipaoa River Sedimentary System in New Zealand, a focus site of the MARGINS Source-to-Sink program, contains both a terrestrial and marine component. Poverty Bay serves as the interface between the fluvial and oceanic portions of this dispersal system. This study used a three-dimensional hydrodynamic and sediment-transport numerical model, the Regional Ocean Modeling System (ROMS), coupled to the Simulated WAves Nearshore (SWAN) wave model to investigate sediment-transport dynamics within Poverty Bay and the mechanisms by which sediment travels from the Waipaoa River to the continental shelf. Two sets of model calculations were analyzed; the first represented a winter storm season, January–September, 2006; and the second an approximately 40 year recurrence interval storm that occurred on 21–23 October 2005. Model results indicated that hydrodynamics and sediment-transport pathways within Poverty Bay differed during wet storms that included river runoff and locally generated waves, compared to dry storms driven by oceanic swell. During wet storms the model estimated significant deposition within Poverty Bay, although much of the discharged sediment was exported from the Bay during the discharge pulse. Later resuspension events generated by Southern Ocean swell reworked and modified the initial deposit, providing subsequent pulses of sediment from the Bay to the continental shelf. In this manner, transit through Poverty Bay modified the input fluvial signal, so that the sediment characteristics and timing of export to the continental shelf differed from the Waipaoa River discharge. Sensitivity studies showed that feedback mechanisms between sediment-transport, currents, and waves were important within the model calculations. [ABSTRACT FROM AUTHOR]

Abstract: The influence of reef structures on seafloor surface sediments has implications for marine spatial planning and coastal development, including use of the coastal zone for offshore wind energy. We present results of interannual changes in seafloor surficial geology at the Redbird artificial reef site, located on the continental shelf offshore of Delaware Bay. The Redbird reef is composed of NYC subway cars, barges, tugboats, and other sunken objects. Since objects were added sporadically between 1996 and 2009, the survey area acts as a natural laboratory to study the evolution of the surrounding seafloor at a structural reef habitat through time. Annual side-scan surveys from 2008 through 2011, and one bathymetric survey in 2010 provide information about surface geology and morphology. Local wave and current data for this time period were analyzed to determine the main morphological agents. Automated backscatter segmentation show that three bottom types dominate and that these large-scale (>10m) surface sediment patterns persist from year to year. Grab samples reveal that the bottom types are silty sand with clay and sandy gravel. Clear sediment and biological patterns emerged revealing the influence of the objects on the seafloor. Comet-shaped moats of sandy gravel surround single objects and grow to form large-scale coalesced patches around groups of objects. Alignment of sediment patches suggests the periodic hydrodynamic influence of seasonal storms. The abundance and diversity of organisms increases with decreasing clay/silt content. Evidence of scour includes the removal of fine sediments, the formation of moats 1–30m in diameter and 0.5–1m deep around the reef objects, and the >1m settling of objects into the seafloor. Data suggest subway cars reached equilibrium with the environment in 6–7 years, but that larger objects or clusters of objects take a longer time to equilibrate and have farther-reaching effects. Knowledge of local wave and current climate and stratigraphy could inform decisions of object clustering and orientation to decrease scour impacts. [Copyright &y& Elsevier]

Abstract: An exceptional flood event, accompanying a marine storm, was investigated simultaneously at the entrance and the exit of the Gulf of Lion’s hydrosystem (NW Mediterranean) in December 2003. Cs, Cr, Co, Ni, Cu, Zn, Cd and Pb signatures of both riverine and shelf-exported particles indicate that continental inputs and resuspended prodeltaic sediments were intensively mixed with resuspended sediments from middle/outer shelf areas during advective transport. As a result, particles leaving the Gulf of Lion inherited the mean signature of shelf bottom sediments, exporting anthropogenic Pb and Zn out into the open sea. When assessing the particulate metal budget in relation with the event, it appears that the output fluxes accounted for between 15% and 60% of the input fluxes, depending on the element and the period of reference. This trend is also observed for annual budgets, which were drawn up by compiling the data from this study and the literature. Results evidenced that, except some element fluxes during extreme output scenario, outputs never counterbalance the inputs. In its current functioning, the Gulf of Lion’s shelf seems to act as a retention/sink zone for particulate metals. Regarding anthropogenic fluxes, the contribution of the oceanic flood of December 2003 to the mean annual scenario is considerable. Environmental impacts onto coastal and deep-sea ecosystems should therefore tightly depend on both the intensity and the frequency of event-dominated sediment transport. [Copyright &y& Elsevier]

Abstract: A three-dimensional coupled hydrodynamic-sediment transport model for the Texas–Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed. At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20–40km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal–annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave–current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf. [Copyright &y& Elsevier]

Abstract: The main coastal processes controlling water, nutrients and sediment transport are considered in the present issue, to emphasize the need for multidisciplinary approaches to achieve a proper assessment of the environmental status in coastal zones (such as the Galician area). Special emphasis has been placed upon the interpretation of local processes, within the context of a global perspective, especially for those regions with environmental properties similar to Galicia. [Copyright &y& Elsevier]

Abstract: Ocean surface waves are the dominant temporally and spatially variable process influencing sea floor sediment resuspension along most continental shelves. Wave-induced sediment mobility on the continental shelf and upper continental slope off central California for different phases of El Niño-Southern Oscillation (ENSO) events was modeled using monthly statistics derived from more than 14 years of concurrent hourly oceanographic and meteorologic data as boundary input for the Delft SWAN wave model, gridded sea floor grain-size data from the usSEABED database, and regional bathymetry. Differences as small as 0.5m in wave height, 1s in wave period, and 10° in wave direction, in conjunction with the spatially heterogeneous unconsolidated sea-floor sedimentary cover, result in significant changes in the predicted mobility of continental shelf surficial sediment in the study area. El Niño events result in more frequent mobilization on the inner shelf in the summer and winter than during La Niña events and on the outer shelf and upper slope in the winter months, while La Niña events result in more frequent mobilization on the mid-shelf during spring and summer months than during El Niño events. The timing and patterns of seabed mobility are addressed in context of geologic and biologic processes. By understanding the spatial and temporal variability in the disturbance of the sea floor, scientists can better interpret sedimentary patterns and ecosystem structure, while providing managers and planners an understanding of natural impacts when considering the permitting of offshore activities that disturb the sea floor such as trawling, dredging, and the emplacement of sea-floor engineering structures. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a method to investigate the relative influence of waves, wind-induced currents and tidal currents on sediment mobility in coastal environment. It is based on the assumption that, knowing high uncertainties linked to sediment dynamics studies, simple sediment mobility indicators are sufficient tools to provide useful information. This method is based on five main steps: (1) assessing hydrodynamic conditions and related errors spatially and temporally for an area; (2) evaluating the bottom shear stress induced by these hydrodynamic conditions; (3) providing simple sediment mobility indicators; (4) analysing the sensitivity of these indicators to the existing hydrodynamic uncertainties; (5) interpreting indicators knowing their limits (identified in the sensitivity analysis). This methodology is applied to a test site on the inner shelf, seaward of the Pertuis Charentais region in France. Using wave (SWAN) and sea-level and currents (MARS-2DH) modelling, we study the spatial distribution of the relative contribution of waves, tides and wind-induced current in the non-cohesive sediment mobility under storm conditions. The deduced sediment dynamics indicators reveal the dominant action of waves in sediment initiation in the offshore area, in water depths of up to 60m. The sensitivity study, performed in order to evaluate the validity of results, shows that the sediment mobility indicators still provide relevant information, even for hydrodynamic uncertainties of ±20% or furthermore when only a qualitative sedimentological knowledge of the area is available. [Copyright &y& Elsevier]

Abstract: X-ray diffraction (XRD) mineralogical and grain-size analyses indicate that inner continental shelf sediments in the East China Sea (ECS) represent a unique mixing of clays derived from the Yangtze River and silts/sands from small western Taiwanese rivers. Taiwanese (e.g., Choshui) clays (<2μm) display no smectite but the best illite crystallinity and are only distributed along southeastern Taiwan Strait. Both Yangtze and Taiwanese river clays are illite-dominated, but the poor illite crystallinity and the presence of smectite and kaolinite indicate that Taiwan Strait clays are mainly Yangtze-dominated. In contrast, medium silts (20–35μm) and very fine sands (63–90μm) in the Taiwan Strait are characterized by low feldspar/quartz, low K-feldspar/plagioclase and high kaolinite/quartz, indicating their provenance from Taiwanese rivers. Taiwanese silts and sands are introduced primarily by the way of typhoon-derived floods and transported northward by the Taiwan Warm Current during summer–fall months. Yangtze clays, in contrast, are widely dispersed southward about 1000km to the western Taiwan Strait, transported by the China Coastal Current during winter–spring months. Since most Taiwan Strait samples were collected in May 2006, clay results in this paper might only represent the winter–spring pattern of the dispersal of Yangtze sediments. [Copyright &y& Elsevier]

Abstract: Cross-shore exchange by strong (cross-shore wind stress, τsx >0.05Pa) diurnal (7–25h) sea breeze events are investigated using two years of continuous wind, wave, and ocean velocity profiles in 13m water depth on the inner-shelf in Marina, Monterey bay, California. The diurnal surface wind stress, waves, and currents have spectral peaks at 1, 2, and 3cpd and the diurnal variability represents about 50% of the total variability. During sea breeze relaxation (−0.05<τsx <0.05Pa), a background wave-driven inner-shelf Eulerian undertow profile exists, which is equal and opposite to the Lagrangian Stokes drift profile, resulting in a net zero Lagrangian transport at depth. In the presence of a sea breeze (τsx >0.05Pa), a uniform offshore profile develops that is different from the background undertow profile allowing cross-shore Lagrangian transport to develop, while including Lagrangian Stokes drift. The diurnal cross-shore current response is similar to subtidal (>25h) cross-shore current response, as found by . The seasonality of waves and winds modify the diurnal sea breeze impact. It is suggested that material is not transported cross-shore except during sea breeze events owing to near zero transport during relaxation periods. During sea breeze events, cross-shore exchange of material appears to occur onshore near the surface and offshore near the sea bed. Since sea breeze events last for a few hours, the long-term cross-shore transport is incremental each day. [Copyright &y& Elsevier]

Abstract: Tropical cyclones affect storm-dominated sediment transport processes that characterise Holocene shelf deposits in many shelf environments. A summary of cyclone-associated deposits in the Great Barrier Reef published by Larcombe and Carter [2004. Cyclone pumping, sediment partitioning and the development of the Great Barrier Reef shelf system: a review. Quaternary Science Reviews 23, pp. 107–135 indicates a pervasive northwards orientation of deposits on the lee side of reefs and other obstacles. In this paper, we describe the geomorphology of reef talus deposits found in the Gulf of Carpentaria and Arafura Sea, Australia, that we attribute to tropical cyclones. The orientation of these deposits is also indicative of a consistent, along-coast transport pathway. The deposits are located on the leeward side of submerged coral reefs; they are up to 10m in thickness, comprised of re-worked carbonate sand and gravel and radiocarbon dating indicates that they are of Holocene age. An explanation for the consistent along-coast cyclone transport pathway is presented based on previously published hydrodynamic modelling results. These models illustrate how currents generated by the passage of a cyclone are asymmetric in plan view, such that stronger flows are generated between the eye of the cyclone and the coast. The result of the passage of many cyclones over geologic timescales is a net along-coast sediment transport pathway located on the inner- to mid-shelf, possibly extending over the entire length of northern Australia''s coastline. This process provides an explanation for the observed sediment transport patterns on modern tropical continental shelves, as well as a basis for the interpretation of ancient tropical shelf deposits. [Copyright &y& Elsevier]

Abstract: The mechanisms driving the air–sea exchange of carbon dioxide () in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea flux for two locations representative for the North Sea''s biogeochemical provinces. In the seasonally stratified northern region, net uptake is high () due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in partial pressure () during the productive season. In contrast, the permanently mixed southern region is a weak net source (). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the appears to be controlled by temperature. [Copyright &y& Elsevier]

Abstract: In the summer of 2001, a coastal ocean measurement program in the southeastern portion of San Pedro Bay, CA, was designed and carried out. One aim of the program was to determine the strength and effectiveness of local cross-shelf transport processes. A particular objective was to assess the ability of semidiurnal internal tidal currents to move suspended material a net distance across the shelf. Hence, a dense array of moorings was deployed across the shelf to monitor the transport patterns associated with fluctuations in currents, temperature and salinity. An associated hydrographic program periodically monitored synoptic changes in the spatial patterns of temperature, salinity, nutrients and bacteria. This set of measurements show that a series of energetic internal tides can, but do not always, transport subthermocline water, dissolved and suspended material from the middle of the shelf into the surfzone. Effective cross-shelf transport occurs only when (1) internal tides at the shelf break are strong and (2) subtidal currents flow strongly downcoast. The subtidal downcoast flow causes isotherms to tilt upward toward the coast, which allows energetic, nonlinear internal tidal currents to carry subthermocline waters into the surfzone. During these events, which may last for several days, the transported water remains in the surfzone until the internal tidal current pulses and/or the downcoast subtidal currents disappear. This nonlinear internal tide cross-shelf transport process was capable of carrying water and the associated suspended or dissolved material from the mid-shelf into the surfzone, but there were no observation of transport from the shelf break into the surfzone. Dissolved nutrients and suspended particulates (such as phytoplankton) transported from the mid-shelf into the nearshore region by nonlinear internal tides may contribute to nearshore algal blooms, including harmful algal blooms that occur off local beaches. [Copyright &y& Elsevier]

Pollen and spores of terrestrial plants found in marine sediments are excellent proxies for paleovegetation and paleoclimate reconstructions of surrounding areas. Ascertaining the pollen transport mechanism from terrestrial to oceanic environments is crucial for the interpretation of pollen data from marine sediments. In this study, we present the distribution of suspended sediments and pollen from 59 surface ocean water samples collected during September and October of 2013, in the northern South China Sea (SCS). The results suggest that there is a difference between the transmission mechanisms of suspended sediment and pollen/spores. Suspended sediment (silt, coarse sand, and gravel) settle quickly in nearshore and continental shelf waters due to their density. Alternately, small, light-weight pollen grains can be carried over long distances in a marine setting. The density of suspended sediment has a high correlation with the concentrations of Pinus , evergreen Quercus , Poaceae and Cyperaceae pollen as well as fern spores (e.g., Dicranopteris), but no correlation with total pollen concentration. This finding will broaden the understanding of the relationships between suspended sediments and pollen in the northern SCS and help to better interpret pollen records extracted from marine sediment cores from the SCS. • 59 sea surface water pollen samples in the northern South China Sea were investigated. • Close relationship between the densities of suspended sediment and the percentages of Pinus , evergreen Quercus , Poaceae and Cyperaceae pollen and spores. • Relatively low values of Pinus , Podocarpus and the evergreen Quercus pollen seem to be correlated with the flowering season. [ABSTRACT FROM AUTHOR]

Abstract: Tidal current and elevation data were collected from five oceanographic moorings during October 2004 in Torres Strait, northern Australia, to assess the effects of large bedforms (i.e., sand banks) on the drag coefficient (CD ) used for estimating bed shear stress in complex shallow shelf environments. Ten minute averages of tidal current speed and elevation data were collected for 18 days at an on-bank site (<7m water depth) and an off-bank site (<10m). These data were compared to data collected simultaneously from two shelf locations (<11m) occupied to measure regional tidal behaviour. Overall CD estimates at the on- and off-bank sites attained 7.0±0.1×10−3 and 6.6±0.1×10−3, respectively. On-bank CD estimates also differed between the predominant east–west tidal streams, with easterly directed flows experiencing CD =7.8±0.18×10−3 and westerly directed flows CD =6.4±0.12×10−3. Statistically significant differences between the off-bank and on-bank sites are attributed to the large form drag exerted by the sand banks on the regional tidal currents, and statistically significant differences between the westward and eastward flows is ascribed to bedform asymmetry. Form drag from the large bedforms in Torres Strait comprises up to 65% of the total drag coefficient. When constructing sediment transport models, different CD estimates must therefore be applied to shelf regions containing steep bedforms compared to regions that do not. Our results extend the limited inventory of seabed drag coefficients for shallow shelf environments, and can be used to improve existing regional seabed mobilisation models, which have direct application to environmental management in Torres Strait. [Copyright &y& Elsevier]

Abstract: A 1-year survey of sediment dynamics on the Têt inner-shelf in the south-western part of the Gulf of Lions was conducted as part of the EUROSTRATAFORM program (European Margin Strata Formation) from October 2004 to November 2005. Several bottom instruments (ADCP, wave gauge and altimeters) were deployed at 28m water depth on the Têt prodelta to measure forcing responsible for sediment erosion and transport on the inner-shelf. The survey period was characterized by low inputs from the coastal rivers and the absence of severe E–SE marine storms, but intense cold dense shelf water formation along the inner-shelf due to strong and persistent N–NW continental winds. The strong (>20cm/s) and prolonged (4 months) southwards alongshore currents during the preconditioning and formation phases of dense shelf water generated continuous sediment erosion of several centimetres on the Têt prodelta and significant suspended-sediment transport. Hydrodynamical modelling shows that the core of the flow predominantly affects the inner-shelf, but occasionally spreads over the mid-shelf mud belt and the outer-shelf, due to the variable intensity of the NW wind (Tramontane). The impact of this N–NW wind-driven process in terms of resuspension rates and suspended-sediment transport clearly differs from that of extreme E–SE storms observed during the previous winter at the same site [Guillén, J., Bourrin, F., Palanques, A., Durrieu de Madron, X., Puig, P., Buscail, R., 2006. Sediment dynamics during wet and dry storm events on the Têt inner shelf (SW Gulf of Lions). Marine Geology 234 (1–4), 129–142.]. Bottom shear stresses and resuspension rates due to strong dense shelf water flow peaked at 0.7–1Pa and 0.3–0.4g/m2/s, respectively, while they reached values of 6–9Pa and 1.6–1.8g/m2/s, respectively, during the strong E–SE storms events. However, due to differing durations, both led to significant (few centimetres) erosion of the inner-shelf sediment. [Copyright &y& Elsevier]

Abstract: Instrumented tripods deployed at depths of 40 and 60m on the shelf off the mouth of the Waiapu River on the east coast of New Zealand''s North Island recorded data on waves, currents, and sediment fluxes from May 22 to August 10, 2004. Three major flood events and several wave events occurred during the deployment. Data from acoustic Doppler velocimeters and profilers revealed that downslope sediment fluxes accompanied a flood event of late June, during which near-bed downslope current speeds approached 0.5ms−1. The most pronounced downslope transport within the benthic layer occurred the day after peak flood but coincided with strong isobath-parallel currents. Suspended sediment concentrations about 1m above the bed were on the order of 2–4gl−1 at times of maximum seaward flow. Suspension of freshly discharged sediment within the hyperpycnal layer over much of the profile was maintained by the high bed stresses associated with the strong benthic currents, in contrast to the wave-supported sediment gravity flows recently reported on other river-nourished shelf systems. Nonetheless, observed concentrations and velocities were largely consistent with the dynamics of critically stratified sediment gravity flows based on equations previously applied elsewhere to wave-supported cases. Despite lower concentrations in current-supported gravity currents, the greater thickness results in similar total loads. In further contrast to earlier results, our data suggest that as sediment off the Waiapu flowed into deeper water across the seaward steepening bottom profile, auto-suspension may have aided in thickening and accelerating the gravity current. [Copyright &y& Elsevier]

Abstract: On the basis of bathymetric and seismic data and data from piston cores collected by the Chinese–French marine geology and geophysics investigation of 1996, we discuss the internal architecture and mobility of tidal sand ridges in the East China Sea (ECS). We characterized the sand ridges on the middle to outer shelf of the ECS as tide-dominated sand ridges with southwest dipping beds, indicating that the regional net sediment transport is toward the southwest. As the sand ridges gradually migrate toward the southwest, new sand ridges are continually replacing old ones, and several generations of sand ridges have developed in the study area. High-resolution seismic data, acoustic Doppler current profiler data, and two 14C-dated piston cores, DGKS9614 and DGKS9612—from a sand ridge swale and crest, respectively—show that these sand ridges, which are at water depths of 90–100m, have been migrating for the last ca. 2–3kyr at least, though these ridges have previously been interpreted as moribund or relict. Sequence stratigraphic interpretation of seismic profiles and core data show that tidal ridges in the ECS evolved from muddier sand ridges formed during the last transgression to sandier shelf sand ridges in response to the shoreline retreat, which resulted in a decrease of riverine muddy sediments and recycling of sandy materials by tidal currents. Most active sand ridge formation occurred during the last transgression, but the present sand ridges on the middle to outer shelf are still being influenced by the modern hydrodynamics. Therefore, these sand ridges on the ECS shelf should be referred to as “quasi-active sand ridges” rather than as moribund or relict sand ridges. [Copyright &y& Elsevier]

Abstract: Field observations of flow and sediment transport in a tributary channel through intertidal mudflats indicate that suspended sediment was closely linked to advection and dispersion of a tidal salinity front. During calm weather when tidal forcing was dominant, high concentrations of suspended sediment advected up the mudflat channel in the narrow region between salty water from San Francisco Bay and much fresher runoff from the small local watershed. Salinity and suspended sediment dispersed at similar rates through each tidal inundation, such that during receding ebbs the sediment pulse had spread spatially and maximum concentrations had decreased. Net sediment transport was moderately onshore during the calm weather, as asymmetries in stratification due to tidal straining of the salinity front enhanced deposition, particularly during weaker neap tidal forcing. Sediment transport by tidal forcing was periodically altered by winter storms. During storms, strong winds from the south generated wind waves and temporarily increased suspended sediment concentrations. Increased discharge down the tributary channels due to precipitation had more lasting impact on sediment transport, supplying both buoyancy and fine sediment to the system. Net sediment transport depended on the balance between calm weather tidal forcing and perturbations by episodic storms. Net transport in the tributary channel was generally offshore during storms and during calm weather spring tides, and onshore during calm weather neap tides. [Copyright &y& Elsevier]

Abstract: Across a limited depth range (5–10m) on many continental shelves, the dominant sediment size changes from sand to mud. This important boundary, called the sand–mud transition (SMT), separates distinct benthic habitats, causes a significant change in acoustic backscatter, represents a key facies change, and delimits more surface-reactive mud from less surface-reactive sand. With the goal of improving dynamical understanding of the SMT, surficial sediments were characterized across two SMTs on the Adriatic continental shelf of Italy. Geometric mean diameter, specific surface area (SSA), mud fraction (<63μm) and heavy metal concentrations were all measured. The SMT related to the Tronto River is identified between 15 and 20m water depth while the SMT associated with the Pescara River varies between 15 and 25m water depth. The sediment properties correlate with a new, process-based sedimentological parameter that quantifies the fraction of the sediment in the seabed that was delivered as flocs. These correlations suggest that floc dynamics exert strong influence over sediment textural properties and metal concentrations. Relative constancy in the depth of the SMT along this portion of the margin and its lack of evolution over a period during which sediment input to the margin has dramatically decreased suggest that on the Adriatic continental shelf energy is the dominant control on the depth of the SMT. [Copyright &y& Elsevier]

Abstract: A mooring and tripod array was deployed from the fall of 2002 through the spring of 2003 on the Po prodelta to measure sediment transport processes associated with sediment delivered from the Po River. Observations on the prodelta revealed wave-supported gravity flows of high concentration mud suspensions that are dynamically and kinematically similar to those observed on the Eel shelf [Traykovski, P., Geyer, W.R., Irish, J.D., Lynch, J.F., 2000. The role of wave-induced density-driven fluid mud flows for cross-shelf transport on the Eel River continental shelf. Continental Shelf Research 20, 2113–2140]. Due to the dynamic similarity between the two sites, a simple one-dimensional (1D) across-shelf model with the appropriate bottom boundary condition was used to examine fluxes associated with this transport mechanism at both locations. To calculate the sediment concentrations associated with the wave-dominated and wave-current resuspension, a bottom boundary condition using a reference concentration was combined with an “active layer” formulation to limit the amount of sediment in suspension. Whereas the wave-supported gravity flow mechanism dominated the transport on the Eel shelf, on the Po prodelta flux due to this mechanism is equal in magnitude to transport due to wave resuspension and wind-forced mean currents in the cross-shore direction. Southward transport due to wave resuspension and wind forced mean currents move an order of magnitude more sediment along-shore than the down-slope flux associated wave-supported gravity flows. [Copyright &y& Elsevier]

Abstract: Instrumented bottom tripods have provided important data on sediment transport processes on continental shelves and in estuaries for four decades. Since the initial deployment in a tidal channel in Puget Sound, WA, in 1965 numerous tripods have been constructed to investigate bottom boundary layer and sediment dynamics worldwide. Tripod data have led to new understanding of near-bottom wave and current flows in the coastal ocean, and have been crucial to the development of shelf circulation and sediment transport models. Calculations of bottom stress, bottom roughness, and sediment flux that resulted directly from tripod data have been compared to bottom boundary layer model results. Where these have differed, new or revised model components have been developed to improve the skill of the models. The many discoveries that have been made from tripod experiments include dense, near-bottom fluid mud layers that transport large quantities of suspended sediment offshore into deeper regions of the continental shelf. This process has been linked to the seaward progradation of subaqueous deltas and to the boundaries of mid-shelf mud deposits off rivers with high fine-sediment discharge. [Copyright &y& Elsevier]

Abstract: A model for the emplacement, bioturbation, and preservation of fine-scaled sedimentary strata has been used to evaluate the impact of interacting physical and biological sedimentary processes on preserved sedimentary fabric in three depositional settings. The settings, which have contrasting hydrodynamic, geological, and biological characteristics, are Eckernförde Bay, German Baltic Sea; the Eel Shelf, northern California, USA; and the northern Gulf of Alaska continental shelf, USA. We have compared field measurements of sedimentation and bioturbation (from 7Be, 234Th, 210Pb, and 137Cs measurements), and observations of sedimentary fabric derived from core X-radiographs with model predictions of preserved sedimentary fabric. The one-dimensional model is forced by fluctuations in the instantaneous sedimentation rate, which deposits sediment with primary sedimentary fabric. Primary sedimentary fabric is then modified by bioturbation, which is represented as a first-order depth-limited reaction term, the rate of which is derived from radioisotopic and time-series estimates of bioturbation depth and intensity. Model output includes depth in seabed and corresponding preservation quotient, a measure of relative primary versus biogenic fabric. Although measured rates and depth scales of sedimentation and bioturbation vary across the three study areas by more than a factor of ten, model results strongly resemble actual sedimentary fabric in core X-radiographs in each case. Our results support and expand on concepts that invoke episodic sedimentation and depth-dependent bioturbation as important competing factors in the preservation or destruction of primary depositional fabric, and also suggest that radioisotopic estimates of bioturbation rates can adequately portray natural conditions. [Copyright &y& Elsevier]

Abstract: Recent field observations from several shelf environments show that gravity-driven transport within negatively buoyant layers is an important mode of fine sediment transport across continental shelves. Specifically, Dick Sternberg, along with his students and colleagues, stimulated a paradigm shift by reporting strong evidence from the Amazon and Eel shelves that hyperpycnal layers do not require autosuspension for sustenance but can be initiated by sediment flux convergence and supported by wave and current-induced suspension within relatively thin near-bed layers. As these layers move downslope under the influence of gravity, they may deposit sediment in response to decreases in bottom orbital velocities, near-bed current velocity, and/or bed slope. Direct or indirect evidence for wave or current supported sediment gravity flows has recently been reported off other high-load rivers including the Atchafalaya, Fly, Ganges–Brahmaputra, Klamath, Mad, Mississippi, Po, Rhone, Waiapu, Waipaoa, Yangtze, and Yellow among others. Growing evidence from observational and modeling studies suggests that flux convergence followed by wave and current supported gravity driven transport is a primary cause of across-shelf transport and emplacement of flood deposits on many muddy shelves and may be a major contributor to and control on the large-scale formation and morphology of subaqueous deltas and shelf clinoforms. Recent and ongoing studies on this subject are synthesized in this paper and recommendations are offered for further study. [Copyright &y& Elsevier]

An idealized morphodynamic model is used to gain further understanding about the formation and characteristics of shoreface-connected sand ridges and tidal sand banks on the continental shelf. The model consists of the 2D shallow water equations, supplemented with a sediment transport formulation and describes the initial feedback between currents and small amplitude bed forms. The behaviour of bed forms during both storm and fair weather conditions is analyzed. This is relevant in case of coastal seas characterized by tidal motion, where the latter causes continuous transport of sediment as bed load.The new aspects of this work are the incorporation of both steady and tidal currents (represented by an M2 and M4 component) in the external forcing, in combination with dominant suspended sediment transport during storms. The results indicate that the dynamics during storms and fair weather strongly differ, causing different types of bed forms to develop. Shoreface-connected sand ridges mainly form during storm conditions, whereas if fair weather conditions prevail the more offshore located tidal sand banks develop. Including the M4 tide changes the properties of the bed forms, such as growth rates and migration speeds, due to tidal asymmetry. Finally a probabilistic formulation of the storm and fair weather realization of the model is used to find conditions for which both types of large-scale bed forms occur simultaneously. These conditions turn out to be a low storm fraction and the presence strong tidal currents in combination with strong steady currents during storms. [Copyright &y& Elsevier]

We combine direct observations, longer-term wave data, and model calculations to characterize resuspension and transport of fine-grained, effluent-affected sediment on the Palos Verdes shelf. Near-bed waves, currents, and suspended sediment concentrations were monitored during the winter of 1992–93 with a bottom tripod and current-meter mooring at a 63-m-deep site. Wave conditions that winter were moderate (∼2 year recurrence interval), and mean current was alongshelf to the northwest; currents were not significantly correlated with wave conditions. Seven wave events during the winter (December–March) produced near-bed wave orbital velocities at the study site in excess of 14 cm s−1, the observed threshold for significant resuspension. Three of these events occurred during the bottom tripod deployment and are characterized by the highest persistent suspended sediment concentrations in the tripod record. Suspended sediment flux was alongshelf to the northwest for 5 of the 6 wave events for which current data were available; one event occurred during low southeast currents. Measured suspended sediment concentration and grain size generally agree with values that were calculated using a shelf sediment transport model with no adjustment of parameters from values determined for two muddy sites on the northern California shelf. We extend our seasonal observations to a period of almost 2 decades by applying the observed thresholds for wave-driven resuspension to near-bed wave conditions calculated from NDBC Buoy 46025 surface wave data. An average of 10 resuspension events per year, with an average duration of 1.6 days, were identified at a water depth of 60 m; the number of events dropped to 3 per year at 90 m, beyond the shelf break. For the majority of these events, calculated net suspended sediment flux is toward the northwest (alongshelf) at an average rate of 140 kg m−1 h−1; about a third of the events have net southeastward flux at an average rate of 30 kg m−1 h−1. The calculated thickness of the resuspended surface layer of the bed was less than 1 cm for all events at 60 m. [Copyright &y& Elsevier]

Ripples and small-scale bedforms are ubiquitous in shallow water environments under the combined action of currents and waves. Small scale processes linked to their formation and migration are interconnected with sediment transport at larger scales (e.g. tens of metres to kilometres), both resulting in and being affected by large scale sediment transport and geomorphological evolution. The lower shoreface provides a key link between coasts and continental shelves, but the contribution of ripples and small-scale bedforms to sediment transport in this region has yet to be fully addressed. This work presents a study of sediment dynamic processes on the lower shoreface in the presence of small-scale bedforms. Observations were made during the winter of 2017 on the lower shoreface of Perranporth Beach, which is in the south west of the UK and exposed to Atlantic waves. The analysis of morphological expressions and the variability of ripples under waves, currents and wave-current conditions are assessed. Ripple morphology and associated dynamics are analysed for their potential contribution to the exchange of sediment between the lower and the upper shoreface. In the present study it was observed that even though ripples were evolving depending on the wave-current forcing, little ripple migration was observed due to low wave skewness. The implication is that ripple migration and bedload transport are only a small contribution to onshore sediment transport under low to moderate energy conditions. However, during more energetic conditions, ripples were washed out and the wave skewness increased, resulting in onshore sediment transport under a sheet flow regime. This suggests that ripple formation and migration can have little impact on the cross-shore supply of sediment from the lower shoreface to the upper shoreface and that more energetic wave conditions are required to significantly transport sediment towards the beach. • High-resolution observations on the seabed reveal 2D and 3D wave vortex ripples in a macrotidal sandy lower shoreface. • The morphological ripple transitions provide assessment of sediment transport regime thresholds. • The hypothesis of ross-shore sediment supplied from the lower to the upper shoreface through ripple migration is not supported. [ABSTRACT FROM AUTHOR]

While the role of shore-parallel transport is extensively documented, its contribution to source-to-sink and provenance studies is often overshadowed by tectonic variations, sediment influx, and sea-level change. To isolate the sedimentary contribution of shore-parallel transport, this study examines the central Levant continental margin (easternmost Mediterranean), where the other three factors are well constrained. New high-resolution multibeam and seismic reflection data collected in a dedicated cruise aboard the RV Bat Galim are used for analyzing current-indicating morphologies formed since the Last Glacial Maximum (LGM). Results show six different current indicators, which attest to the ongoing shore-parallel Levant Jet System (LJS) flow. Throughout the post-LGM transgression and coastal retreat, the LJS shifted landward to flow over the submerged shelf and upper slope. This shift, along with periodic/seasonal alterations in the flow intensity, regulated the sediment transport, distribution and erosion, along and across the margin, during the lowstand, transgression, and highstand conditions. This case study emphasizes the central role of shore-parallel currents in regulating the sedimentary budget. It provides a reference case for continental margins where the shore-parallel role is obscured by more dominant tectonic-eustatic-sedimentation factors. • New high-resolution multibeam and Chirp data from the central Levant continental margin (easternmost Mediterranean). • The tectonic, eustatic, and sedimentation factors are well constrained in the study area. • The study examines the shore-parallel sediment transport role in shelf build-up. • Shore-parallel currents regulate sediment transport, distribution, and erosion, along and across the margin. [ABSTRACT FROM AUTHOR]