Sand Ridge Field Research Articles

Sedimentology and evolution of the Holocene radial tidal sand ridge in the south Yellow Sea, China

The radial tidal sand ridge (RTSR) off the Jiangsu Coast in the south Yellow Sea is one of the most fantastic and unique natural landscapes in the world. A large number of investigations on geomorphology and marine geology have been implemented during the last several decades. The formation and evolution of the RTSR, however, are still under debate. To resolve this issue, 10 ∼30–60 m-long cores were retrieved from the Jiangsu coastal plain and offshore sand ridge field in 2018–2019. Stratigraphic transects chronologically constrained by abundant radiocarbon dates were built based on sedimentological and paleoecological (i.e., foraminifer) data from selected cores. Combined with well-documented core stratigraphy from earlier studies, an age–depth plot with a local sea-level change curve revealed that the RTSR initiated approximately 9,000 cal a BP in a sandy bedform shape under transgressive conditions, continued throughout the Holocene, but developed mostly after 1128 AD, when the Yellow River flowed southward into the Yellow Sea. Abundant sediment supply, thus, appears to have played a major role in the formation and evolution of the RTSR in the south Yellow Sea.

Dynamics of a Tidal Current System in a Marginal Sea: A Case Study of the Yellow Sea, China

Tidal currents belong to the main driving forces shaping the bathymetry of marginal seas. A globally unique radial sand ridge field exists in the South Yellow Sea off the central Jiangsu coast, China. Its formation is related to the distinctive “radial tidal current” pattern at that location. A generally accepted hypothesis is that the “radial tidal current” is a consequence of the interference between the northern amphidromic tidal wave system and the southern incoming tidal wave. In this study, a schematized numerical tidal model was designed to investigate the tidal current system and the factors of influence in the South Yellow Sea. Concepts of the tidal current amphidromic point (CAP) and the tidal current inclination angle are utilized to analyze the inherent structure of the tidal current system. By conducting a series of numerical experiments, it is found that the Poincaré modes are necessary for the existence of “radial tidal current,” and the e-folding decay length should be smaller than the basin length. In the Yellow Sea, cross-basin phase differences due to lateral depth differences as well as open boundary conditions favor the emergence of the “radial tidal current.” Further analyses indicate that the CAP system (i.e., the co-inclination lines, the CAPs, and the tidal ellipticity) deepens the understanding on the dynamic structure of a tidal current system, and therefore, it deserves more attention in future studies.

Mud deposits at Cassino beach: role of dredging

Mud deposits on Cassino beach are associated with waves generated from storms during cold front passages and extratropical cyclones from the southern quadrant. During such episodes fluid mud deposited at the shoreface and surf zone is transported towards the surf zone and the sandy beach prism. The first mention on the origin of mud has always attributed a source exclusively natural for the deposits. However, new interpretations and recent data indicate that the increase in dredged volumes over the years increased the frequency of mud depositions. In order to verify this association, records of dredged and discarded volumes in the Port of Rio Grande and Cassino beach shoreface were analyzed. Maps of bottom types, silt and clay ratio, and fluid mud thickness obtained from superficial samples and sediment cores between 4 and 15 m water depth at the time of the 1998 impact were also reanalyzed. Current conditions near the dumping sites in the estuary and inner shelf in 1998 were reconstructed by hindcast based on reanalysis data and an ADCP anchored 15 m off the Rio Grande coast. In order to verify the effect of the passage of a cold front on the bottom near the ocean dumping site, data from two mooring stations made on the crest and swale of a sand ridge field were considered. To further understand the disposal of the dredged material, a particle transport model was created for two periods related to the depositional events. In the period of 1998, it was simulated the disposal of dredged material on two locations, one with the disposal of 640,000 m3 of sediment inside the estuary at 8 m of depth and, the other, with 2.3 million m3 of disposal material, at 13 m depth in the shoreface. The simulation at depth of 19 m, for the dumping site of 2014 considered the disposal of 1,000.000 m3 at 19 m water depth. The occurrence of massive concentrations of an estuarine gastropod associated with the deposits at the beach led to a literature search on their occurrence in the area after researches carried out for more than three decades on the fauna ecology of sandy beaches. From the 1970s onwards, dredged volumes intensify and were deposited into the estuary and ocean using Hopper dredges. There was an increase in episodes of mud deposition at CB. The sedimentation patterns after dredging operations indicate that natural sedimentary processes are altered in a transient way. Currents generated by winds and storms in both quadrants north and south provide conditions of instability of fine sediments in the disposal sites with flow components parallel and also direct to the coast regardless of wave action. Hydrodynamic modeling indicates that sediments disposed into both the estuary and the shoreface during northeast and southern winds have the potential to be driven towards the beach. The massive deposition of millions of organisms after dredging events is not a natural process and is an indication of the dredging activities. Dredging can be an important part of the environmental impact because the natural accumulation process is enhanced by intense pulses of fine sediments in short time favoring more frequent depositions since the historical and current disposal sites are not appropriate. Negative environmental impacts are related to beach usage since it affects sunbathers, surfers, vehicle traffic, morphodynamic changes, and massive mortality of the benthic fauna.

Evolution of the radial sand ridge field in the southwestern Yellow Sea, China since the late MIS 3

Evolution of the radial sand ridge field in the southwestern Yellow Sea, China since the late MIS 3

Stratigraphic framework and evolution of the mid–late Quaternary (since marine isotope stage 8) deposits on the outer shelf of the East China Sea

The East China Sea (ECS) is a vast river-dominated depositional basin because of its broad shelf and extensive river sediment supply. This study investigated the mid–late Quaternary stratigraphic framework and evolution of the outer ECS shelf based on a borehole core (DH02) and seismic profiles. Seven sedimentary facies were recognized in the core sediments according to lithology, grain-size composition, sediment structure, and microfossil assemblages, while eight unconformity-bounded major stratigraphic units were identified in the seismic profiles. Based on seismic–core correlations, dating data (radiocarbon and optically stimulated luminescence), and comparison between sedimentary cycles and sea-level curves, the 100 m thick strata below the seafloor were divided into three sequences corresponding to glacial–interglacial sea-level cycles of marine isotope stage (MIS) 1–2, MIS 3–6, and MIS 7–8 (Sq1–Sq3 from top to bottom). In particular, Sq1 is developing in the present sea-level highstand and consists mainly of deposits of transgressive systems tract (TST), including tide-influenced river, tidal flat, and tide-dominated estuarine facies. Sq2 and Sq3 consist mainly of prodelta and related inner-shelf facies of forced regressive systems tract (FRST) and transgressive deposits similar to those of Sq1. For all three sequences, a thin-sand sheet layer of open-shelf facies represents deposits of highstand systems tract (HST). Deposits of lowstand systems tract (LST) were typically represented by river facies. In addition, comprehensive analyses of seabed sand ridge distribution, core sediment characteristics, and ridge deposition mechanism suggest that the occurrence of a paleoestuary with transgression determined the development of a tidal sand ridge field. Based on all these results, a conceptual stratigraphic sequence model in association with the deposition of LST, TST, HST, and FRST was constructed. Overall, our stratigraphic framework, along with a comparison of the sequence stratigraphy to that of other shelves, suggested that glacial–interglacial sea-level fluctuations, huge amounts of river sediment supply, active subsidence, gentle shelf topography, and marine erosion processes (transgression and regression) controlled the mid–late Quaternary stratigraphic development of the ECS shelf.

Abstract #4308 Genetic variation of IL-6 expression modulates age-related memory decline

An integrated approach combining swath bathymetry, high-resolution seismic reflection and seabed sediment sampling was performed to characterize a field of sand ridges on the Murcia continental shelf in the western Mediterranean Sea. The aim of this work was to improve knowledge about the formation, evolution and present-day dynamics of these bedforms on a narrow, tideless continental shelf receiving little sediment input. Sand ridges are observed on the middle shelf at water depths ranging between 58 and 78 m. The sand ridges are 1.5 to 3 m high and show a predominant E-W orientation oblique to the present-day shoreline. Smaller-scale subaqueous dunes (0.3–1.3 m high) appear superimposed on the sand ridges and beyond the ridge field. The subaqueous dunes show a NW-SE orientation and asymmetric profiles, with the lee side facing southwest. The comparison of two bathymetric surveys 10 years apart revealed that the subaqueous dunes are migrating towards the southwest at very low rates (~ 3 m yr− 1). Internally, both sand ridges and dunes display southwestward dipping oblique reflections, indicating long-term migration in that direction.The morphology, architecture and distribution of the Murcia sand ridges suggest that they were formed in a shallow-water environment during the Holocene transgression and were later detached from the coast due to the subsequent sea level rise. The shallow architecture of the sand ridges reveals the presence of small, mound-like features within the sand ridges, most probably associated with coastal deposits, which could have served as a precursor for the development of the sand ridges. The preservation of these deposits within the sand ridges evidences limited offshore ridge migration. At present, active subaqueous dunes superimposed on the sand ridges suggest that they may play a significant role in the ridge evolution. At a regional scale, the comparison of the Murcia sand ridges with those described in the western Mediterranean allows us to propose a gradation from partially evolved to fully evolved sand ridges with increasing water depth.

Modelling tidal-induced sediment transport in a sand-silt mixed environment from days to years: Application to the Jiangsu coastal water, China

In the present study a new multi-fractional, depth-averaged sediment transport module was developed and embedded into a morphodynamic model for a sand-silt mixed shallow water environment. Subsequently, the model was applied to the case of the Jiangsu coast, which features a silt enriched sedimentary environment bordered by two large-scale geomorphological units: the Old Yellow River Delta (OYRD) in the north and the Radial Sand Ridge Field (RSRF) in the south. Based on this case, the predictive abilities of the present model are assessed on both the short-term and the long-term. Comparisons with measurements over two successive tidal cycles indicate that the present model produces very good results on short-time scales. The model performance is extended and further validated by comparing the overall annual Suspended Sediment Concentration (SSC) pattern, the annual morphological changes, the annual sediment budget and the evolution trend of the bed composition. Also, these long-term results agree well with existing observations over the past several decades. Hence, an essential feature of the present modelling approach is the ability to simulate sediment transport and morphological changes over a relatively long time span (i.e., time scale of years) in a sand-silt mixed sedimentary environment, based on its validated short-term performance.

Enigmatic holocene sand ridges: complex meandering to anastomosing bedforms in the North Sea (German Seabight)

An intricate field of NNW-SSE trending sand ridges was discovered for the first time in 2010 and mapped in detail over the past 6 years during multiple campaigns in the vicinity of the islands of Helgoland and Dune in the German Seabight. The tramline-shaped, elongate, linear and meandering sand ridges are up to 1 km in length, over 10 m in height and spaced 10–50 m apart. The ridges exhibit complex anastomosing patterns, converging and disappearing to the south of the study area and diverging, becoming more subdued and disappearing further to the north. Seismic profiling, surface samples and multibeam data show that the ridges are stationary, while the sediment wave fields to the north and west are migrating westwards. Based on our mapping results, the most likely hypothesis for the development and maintenance of these enigmatic structures is a combination of tidal currents, tides and sediment influx. Whether these structures are associated with the Mesozoic subcrop or are glaciofluvial in origin and have been modified by the last glacial-interglacial intervals or are sedimentary structure linked to Holocene sea level rise in the southern North Sea is a matter of debate. Further studies, including numerical modelling of these sand ridges structures, would provide clues to past sedimentary and hydrodynamic conditions and future changes in the highly dynamic and complex tidal environment of the southern North Sea.

Magnetic characteristics of sediments from a radial sand ridge field in the South Yellow Sea, eastern China, and environmental implications during the mid- to late-Holocene

Proxy records from radial sand ridge fields are essential for understanding sedimentary environmental changes forced by climate-driven coast and sea level variations that affect large river inlets. We conducted a systematic environmental magnetic study on sediments from a radial sand ridge field in the South Yellow Sea, eastern China. From 41.6 to 0 m, the sediments of core YZ07 derive from river mouth, littoral and modern tidal flat environments. Magnetic measurements indicate that magnetite and hematite dominate the studied magnetic mineral assemblages. Magnetic mineral contents are highest in the river mouth environment and lowest in the modern tidal flat environment. The magnetic minerals in the core YZ07 sediments can be attributed primarily to detrital inputs rather than biogenic sources. The highest χ and SIRM values and lowest χARM/SIRM and χARM/χ values reflect relatively warm and humid climate conditions respectively during the period ∼7000 to 2900 a B.P. (41.6–24.73 m). Compared with the period ∼7000 to 2900 a B.P. (41.6–24.73 m), the period between ∼2900 and 870 a B.P. (24.73–5.14 m) is characterized by lower χ, χARM and SIRM values and higher χARM/SIRM and χARM/χ values, which indicate cool and dry climatic conditions. Our results reconfirm known regional climatic events that demonstrate Holocene climatic instability. Gradual decreases in χ and SIRM and increases in χARM/SIRM, χARM/χ and χfd characterize the sediments after 870 a B.P. (5.14–0 m). As indicated in previous studies, these changes were influenced by diversion of the Yellow River. Additionally, spectral analysis of the χ and χARM/SIRM records reveals rapid climatic fluctuations on millennial and centennial scales that coincide with those recorded in marine sedimentary records from adjacent regional seas. These results are important for investigations into the interactions between regional systems and global change in monsoonal climatic regions, thus providing an example of the evolution of a large scale geomorphic feature resulting from river-sea interaction.

Formation, evolution and present-day activity of offshore sand ridges on a narrow, tideless continental shelf with limited sediment supply

An integrated approach combining swath bathymetry, high-resolution seismic reflection and seabed sediment sampling was performed to characterize a field of sand ridges on the Murcia continental shelf in the western Mediterranean Sea. The aim of this work was to improve knowledge about the formation, evolution and present-day dynamics of these bedforms on a narrow, tideless continental shelf receiving little sediment input. Sand ridges are observed on the middle shelf at water depths ranging between 58 and 78m. The sand ridges are 1.5 to 3m high and show a predominant E-W orientation oblique to the present-day shoreline. Smaller-scale subaqueous dunes (0.3–1.3m high) appear superimposed on the sand ridges and beyond the ridge field. The subaqueous dunes show a NW-SE orientation and asymmetric profiles, with the lee side facing southwest. The comparison of two bathymetric surveys 10years apart revealed that the subaqueous dunes are migrating towards the southwest at very low rates (~3myr−1). Internally, both sand ridges and dunes display southwestward dipping oblique reflections, indicating long-term migration in that direction.The morphology, architecture and distribution of the Murcia sand ridges suggest that they were formed in a shallow-water environment during the Holocene transgression and were later detached from the coast due to the subsequent sea level rise. The shallow architecture of the sand ridges reveals the presence of small, mound-like features within the sand ridges, most probably associated with coastal deposits, which could have served as a precursor for the development of the sand ridges. The preservation of these deposits within the sand ridges evidences limited offshore ridge migration. At present, active subaqueous dunes superimposed on the sand ridges suggest that they may play a significant role in the ridge evolution. At a regional scale, the comparison of the Murcia sand ridges with those described in the western Mediterranean allows us to propose a gradation from partially evolved to fully evolved sand ridges with increasing water depth.

Contemporary genesis of sand ridges in a tideless erosional shoreface

A sand ridge field located over a retreating deltaic lobe in the Ebro Delta (NW Mediterranean) is characterized using three sets of co-located multibeam bathymetric data acquired in 2004, 2013 and 2015, measurements of near-bottom currents, high-resolution seismic profiles, and aerial photographs. The aim of this study is to illustrate the processes and timing involved in the initial stages of sand ridge development. The sand ridge field extends from the 5 to 15m isobaths and the ridges have maximum heights and wavelengths of 2.5 and 400m, respectively; they have straight crests that are arranged obliquely to the shoreline and are composed of fine sand. In general, the sand ridges are symmetric, although asymmetries with the lee side of the ridge facing to the SE and to the NW are also present. The genesis of the sand ridge field is closely related to the contemporary evolution of the Ebro River mouth. The change of the main river channel in the 1940s led to progressive abandonment of the former river mouth and to severe coastal retreatment (~37m·yr−1), making large amounts of sediment available for ridge formation. The persistent Mistral winds (NW) induce near-bottom currents flowing towards the SE, which are able to rework and transport sandy sediments. The sand ridges are currently active, with mean SE migration rates of ~10m·yr−1 most likely in pulses, when high-speed currents occur. Wave-storm events induce reverse flows (E-SE), which reshape the ridges to symmetric or opposite asymmetry geometries.The bedform morphologies, the oblique arrangement with respect to the shoreline, the angle between the strongest current and the crestlines and the sediment grain size match well with those of shoreface-connected (attached) sand ridges and, particularly, with the initial stages of sand ridge development on storm-dominated continental shelves. Sediment availability, shoreline retreat, relatively strong near-bottom currents induced by winds, seafloor irregularities and relative sea-level rise on the Ebro Delta combine to provide a suitable transgressive environment for sand ridge development. Time-scales related to sand ridges are usually of hundreds/thousands of years, but here it is demonstrated that the genesis of sand ridges can take place within a few decades. The studied bedforms are unlikely to persist in the absence of rapid sea-level rise, leading to sediment scarcity and wave and current reworking.

Change in morphology and modern sediment thickness on the inner continental shelf offshore of Fire Island, New York between 2011 and 2014: Analysis of hurricane impact

Seafloor mapping investigations conducted on the lower shoreface and inner continental shelf offshore of Fire Island, New York in 2011 and 2014, the period encompassing the impacts of Hurricanes Irene and Sandy, provide an unprecedented perspective regarding regional inner continental shelf sediment dynamics during large storm events. Analyses of these studies demonstrate that storm-induced erosion and sediment transport occurred throughout the study area in water depths up to 30m. Acoustic backscatter patterns were observed to move from ~1m to 450m with a mean of 20m and movement tended to decrease with increasing water depth. These patterns indicate that both of the primary inner continental shelf sedimentary features in the study area, linear sorted bedforms offshore of eastern Fire Island and shoreface-attached sand ridges offshore of central and western Fire island, migrated alongshore to the southwest. The migration of the sorted bedforms represents the modification of an active ravinement surface and is thought to have liberated a significant volume of sediment. Comparison of isopach maps of sediment thickness show that the volume of modern sediment composing the lower shoreface and shoreface-attached sand ridges decreased by ~2.8×106m3 across the ~73km2 of common seafloor mapped in both surveys. However, a similar analysis for the relatively calmer 15-yr period prior to 2011 revealed significant accretion. This allows speculation that the shoreface-attached sand ridges are maintained over decadal timescales via sediment supplied through erosion of Pleistocene outwash and lower Holocene transgressive channel-fill deposits exposed on the inner continental shelf, but that the sand ridges also periodically erode and move to the southwest during large storm events. Analyses show that significant storm-induced erosion and sediment transport occurs far seaward of the 5 to 9m depth of closure assumed for Fire Island, where it is thought that an onshore-directed sediment flux from the inner continental shelf to the littoral system is required to balance the coastal sediment budget. It is also thought that the morphology of the shoreface-attached sand ridges controls the persistent shape of the adjacent shoreline through modification of incident waves. Thus, we suggest that the sediment dynamics of the inner continental shelf and both storm-induced and anthropogenic modification of the field of shoreface-attached sand ridges be considered in future coastal resiliency planning.

Paleoenvironmental reconstruction of the radial sand ridge field in the South Yellow Sea (east China) since 45 ka using the sediment magnetic properties and granulometry

Sediments of radial sand ridge field are commonly represented by a set of flood plain, paleosol, and tidal sand ridge facies. In this paper, measurements of magnetic properties and particle size were obtained from Core Y2 from the radial sand ridge field in the South Yellow Sea. The results show that each facies has specific magnetic minerals and particle size distributions. In the flood plain, sand and coarse silt are the main grain sizes, and magnetite and subordinate hematite comprise the magnetic minerals. The clay and fine silt are the main grain-size distributions of paleosol, the magnetic minerals are dominated by hematite, and the soft-magnetic mineral content is very low. In the tidal sand ridge facies, sand is the predominant grain-size distribution, and magnetite dominates the bulk magnetic properties, with small amounts of hematite. The strong relationships between χ and the >63-μm fraction suggest that magnetite is enriched in the coarse sand fraction. Based on the combined mineral-magnetic, particle size and loss-on-ignition data, the low χ value of paleosol mainly resulted from the decrease in the magnetic mineral inputs and post-depositional dissolution of the magnetic minerals. The results show that the magnetic minerals were controlled by changes in the climate and hydrodynamic environment. This study not only provides a basis for a radial sand ridge field paleoenvironmental reconstruction by integrating detailed logs of magnetic properties and grain-size parameters, but it also provides constraints for exploring the land–sea interactions recorded in the sediment cores as a part of the earth system, and thus demonstrates the effectiveness of this approach.

Mathematical model of wave transformation over radial sand ridge field on continental shelf of South Yellow Sea

According to a deformed mild-slope equation derived by Guang-wen Hong and an enhanced numerical method, a wave refraction-diffraction nonlinear mathematical model that takes tidal level change and the high-order bathymetry factor into account has been developed. The deformed mild-slope equation is used to eliminate the restriction of wave length on calculation steps. Using the hard disk to record data during the calculation process, the enhanced numerical method can save computer memory space to a certain extent, so that a large-scale sea area can be calculated with high-resolution grids. This model was applied to wave field integral calculation over a radial sand ridge field in the South Yellow Sea. The results demonstrate some features of the wave field: (1) the wave-height contour lines are arc-shaped near the shore; (2) waves break many times when they propagate toward the shore; (3) wave field characteristics on the northern and southern sides of Huangshayang are different; and (4) the characteristics of wave distribution match the terrain features. The application of this model in the region of the radial sand ridge field suggests that it is a feasible way to analyze wave refraction-diffraction effects under natural sea conditions.

Sediment dynamics over sand ridges on a tideless mid-outer continental shelf

Abstract A sand ridge field with crests oriented NE–SW (52°) located between 55 and 85 m water depth on the Valencia continental shelf (Spain) was mapped with multibeam swath bathymetry and characterized with seismic profiling and sediment sampling. Boundary-layer hydrodynamic measurements conducted over a sand ridge at 66 m depth show evidence of wave and current sediment resuspension during a major storm event with a three-year recurrence period. From the measured intrawave sediment concentration profiles it is shown that the suspended sediment time evolution is related not only to mean currents but also to the wave field. The near-bottom shear stress generated during storm conditions reached around 1 N/m2 and was able to resuspend and transport sandy and muddy sediments for more than 32 h. The total suspended sediment transport in a layer of 1 m above the bottom was around 4000 kg per meter width and directed 160°. It is argued that these sand ridges were developed in a shallow environment and there is no evidence of present-day ridge migration or morphological degradation. It is hypothesized that sediment dynamics affecting sand ridges during storms favors the maintenance of their morphology.

Hydrological and Sediment Characteristics of Sand Ridge Field in Jiangsu Coastal Area of China

The sand ridges field in Jiangsu coastal area has been the key region of tidal flat reclamation, waterway regulation and port construction. Adopting the hydrological and sediment data in winter, the tidal patterns were revealed. In addition, the depth-averaged velocity, sediment grain size and sediment concentration in each station were analyzed and compared in detail. The result shows that the tidal in study area is regular semidiurnal and diurnal inequality. The velocity has a decreasing tendency with the Tiaozini water area being centered towards the surrounding areas. The grain size gradually decreases and then increases from north to south, while the sediment concentration gradually increases and then decreases from north to south.

Seabed characterization for the development of marine renewable energy on the Pacific margin of Canada

An inventory of Canada's marine renewable energy resources based on numerical modeling of the potential tidal, wave and wind energy has been published that identifies areas with maximum resource potential. However, the inventory does not consider the seabed geological conditions that will control the safe development of seabed installations and cable corridors. The Geological Survey of Canada (Natural Resources Canada) has therefore undertaken an assessment of seafloor geological characteristics and physical environmental parameters that will be encountered during any extensive deployment of marine renewable energy systems for the Pacific offshore of Canada. Here we present an overview of seabed characterization for key sites for each of the three energy types.Narrow passages exiting the Salish Sea near the Canadian boundary with the United States and northwards out of the Strait of Georgia provide very promising sites for tidal generation. Here, elliptical fields of very large subaqueous dunes, from 12 to 28m in height, present a significant challenge to site development. Along the exposed continental shelf of Vancouver Island focused wave-energy close to shore (40–60m water depth) offers significant energy potential, but any engineering systems would have to be founded on a seafloor made up of a mobile gravel lag and an extensive boulder pavement. A large wind farm proposed for the Pacific North Coast would be built on an extensive shallow bank that has active sediment transport and a large field of sand ridges that have developed within a macrotidal environment. A significant challenge is providing for a safe seafloor cable corridor of over 100km that crosses a large subaqueous dune field to connect to the electrical grid on the mainland. These examples show how geoscience has and will provide critical information to project proponents and regulators for the safe development of marine renewable energy.

Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea

Zhang C. K., Yang Y. Z., Tao, J. F., Chen, Y. P., Yao, P., Su. M., 2013. Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea.The radial sand ridge at the South Yellow Sea is one of unique geomorphic units in the world. It is famous of its radial current pattern and high suspended sediment concentration, forming abundant tidal flat resources which could be reclaimed for the land use. In order to make a scientific plan for the land reclamation, it is necessary to quantitatively understand the distribution of suspended sediment fluxes in this field. In this study, a 2D tidal current and suspended sediment transport model is developed and validated by the hydrologic survey data measured in 2006. The suspended sediment fluxes are calculated by the period-averaged multification of the simulated tidal current and sediment concentration in two consecutive tidal periods. The numerical results show that, in either winter or summer season, there has a positive sediment flux comes from the north and the south boundaries, indicating a net gain of sediment for the sand ridge field from both boundaries. The numerical results also show that a clockwise sediment flux system exists in the middle of the sand ridge field, but a small portion of sediment leaves the sand ridge area at the north wing. The overall gain of the sediment in this field is positive, implying that the radial sand ridge is still in the growing process.

The sand ridge field of the South Yellow Sea: Origin by river–sea interaction

The origin of a large field of sand ridges offshore from the north Jiangsu coast in the South Yellow Sea has been investigated on the basis of bathymetry, seabed samples, high-resolution seismic profiles and 31 to 71m deep boreholes. Sediment composition including heavy minerals and clay minerals sampled from surface sediments shows that most of the sand was derived from the Changjiang (Yangtze) River, but in the northern part of the field particularly the clay came from the Huanghe (Yellow) River. Seismic profiles show late Pleistocene distributary channels of the ancient Changjiang River underlying much of the southern part of the sand ridge field. The Holocene transgression cut a widespread ravinement surface reworking abundant late Pleistocene sandy sediment. The available sand was reworked by tidal currents into large ridges. The location of the ridges is strongly influenced by relict channels in the southern part of the field and by tidal currents in the northern, and especially the northeastern part of the field, resulting in a radiating ridge pattern. These ridges provide an example of the evolution of a large scale geomorphic feature resulting from river–sea interaction, in which climatic and sea level changes played an important role.

Sediment-starved sand ridges on a mixed carbonate/siliciclastic inner shelf off west-central Florida

High-resolution side-scan mosaics, sediment analyses, and physical process data have revealed that the mixed carbonate/siliciclastic, inner shelf of west-central Florida supports a highly complex field of active sand ridges mantled by a hierarchy of bedforms. The sand ridges, mostly oriented obliquely to the shoreline trend, extend from 2 km to over 25 km offshore. They show many similarities to their well-known counterparts situated along the US Atlantic margin in that both increase in relief with increasing water depth, both are oriented obliquely to the coast, and both respond to modern shelf dynamics. There are significant differences in that the sand ridges on the west-central Florida shelf are smaller in all dimensions, have a relatively high carbonate content, and are separated by exposed rock surfaces. They are also shoreface-detached and are sediment-starved, thus stunting their development. Morphological details are highly distinctive and apparent in side-scan imagery due to the high acoustic contrast. The seafloor is active and not a relict system as indicated by: (1) relatively young AMS 14C dates (<1600 yr BP) from forams in the shallow subsurface (1.6 meters below seafloor), (2) apparent shifts in sharply distinctive grayscale boundaries seen in time-series side-scan mosaics, (3) maintenance of these sharp acoustic boundaries and development of small bedforms in an area of constant and extensive bioturbation, (4) sediment textural asymmetry indicative of selective transport across bedform topography, (5) morphological asymmetry of sand ridges and 2D dunes, and (6) current-meter data indicating that the critical threshold velocity for sediment transport is frequently exceeded. Although larger sand ridges are found along other portions of the west-central Florida inner shelf, these smaller sand ridges are best developed seaward of a major coastal headland, suggesting some genetic relationship. The headland may focus and accelerate the N–S reversing currents. An elevated rock terrace extending from the headland supports these ridges in a shallower water environment than the surrounding shelf, allowing them to be more easily influenced by currents and surface gravity waves. Tidal currents, storm-generated flows, and seasonally developed flows are shore-parallel and oriented obliquely to the NW–SE trending ridges, indicating that they have developed as described by the Huthnance model. Although inner shelf sand ridges have been extensively examined elsewhere, this study is the first to describe them in a low-energy, sediment-starved, dominantly mixed siliciclastic/carbonate sedimentary environment situated on a former limestone platform.