Grain-Size Distribution of Surface Sediments in the Bohai Sea and the Northern Yellow Sea: Sediment Supply and Hydrodynamics

The grain-size distribution of surface sediments in the Bohai Sea(BS) and the northern Yellow Sea(NYS), and its relationship with sediment supply and hydrodynamic environment were investigated based on grain-size compositions of surface sediments and modern sedimentation rates. The results showed that the surface sediments in the BS and the NYS were primarily composed of silty sand and clayey silt with a dominant size of silt. In addition, the Yellow River delivered high amount of water and sediments to the BS, and they are dominated in surface sediments(mainly silt) in the Bohai Bay, the Yellow River mouth, the center of the BS, and the north coast of Shandong Peninsula. The coarse-grained sediments were mainly deposited at the river mouth due to the estuarine filtration and physical sorting. Meanwhile, there was a significant relationship among the modern sedimentation rate, the surface sediment grain size distribution and sediment transport pattern. The areas with coarser surface sediments generally corresponded low sedimentation rates because of strong erosion;whereas the sedimentation rate was relatively high at the place that the surface sediments were fine-grained. Furthermore, the grain-size trend analysis showed that the areas with fine-grained surface sediments such as the mud area in the central BS and the upper Liaodong Bay were the convergent centers of surface sediments, except for the Bohai Bay and the subaqueous Yellow River Delta where offshore sediment transport was evident.

Impact of Sediment Supply on Spartina Salt Marshes

During the past century,natural and human modifications of environmental systems have greatly accelerated coastal salt marsh deterioration and shoreline retreat in many regions worldwide. Field investigation,profile analysis,geographical information analysis,and remote sensing were employed in combination to study the effect of sediment on Spartina alterniflora salt marshes of the coast in Jiangsu Province,East China. The results indicated that the propagation of Spartina alterniflora salt marshes was closely related to regional sediment conditions,especially the supply of fine-grained materials. Additionally,because of the dense and high grass in Spartina alterniflora salt marshes,wave energy and tidal currents were baffled and weaker than those of the adjacent,unvegetated mud flats. Fine sediment was hardly resuspended under the low energy conditions in the Spartina alterniflora salt marshes.

Flow variability along a vegetated natural stream under various sediment transport rates

The influence of vegetation and sediment on flow characteristics in open channels cannot be neglected. To study the flow variability under the effects of the instream natural vegetation and sediment supply, experiments were conducted with varied water and sediment supply in a movable bed of a river prototype. The instantaneous threedimensional velocities near two types of vegetation patches(the shrub and the weed) and along the centerline of the main channel with vegetation belts were measured using a 3-D side-looking acoustic Doppler velocimetry. The experimental results show that both the instream vegetation and sediment supply strongly affect the flow and turbulence characteristics. In the case of vegetation patches, both the shrub and weed have a considerable influence on the distribution of the streamwise velocity and turbulence intensity of their surrounding water. The streamwise velocity distribution followed as J-shape and linear shape around the weed and shrub under different experimental conditions. The turbulence intensity was large at the top of the weed and shrub;the shrub had its greatest influence on the downstream water flow. In the case of vegetation belts,the streamwise velocity along the centerline of the main channel exhibited an S-shape, J-shape and linear shape at different locations under varied water,vegetation structures and riverbed configurations.The turbulence intensity along the centerline of the main channel ranged from 0.0 to 0.1. The upstream turbulence intensity was affected considerably by a sediment supply, while the downstream turbulence intensity changed with the varied vegetation characteristics and riverbed topography. The second flow coefficient M-value increased longitudinally and was almost positive along the centerline of the main channel, implying that the rotational direction of the secondary current cell was clockwise.

Local hydraulic jump effects on sediment deposition in open-channel flume experiments

When a river channel is narrow,bifurcated,or intersected,or when extreme weather or geological disasters cause shed rock masses to occupy a river flood channel,local hydraulic jumps may develop in the channel.Natural disasters such as landslides,floods,and debris flows occur upstream,will result in large transport rate of large-sized gravel particles.Those particles may be blocked in hydraulic jump areas,causing river channel water depth to rise.In this study,the effect of local hydraulic jumps on the sediment deposition rate was investigated in flume experiments.The ratio of upstream and downstream Froude numbers,particle size,Sediment supply intensity,and flow discharge all affected the sediment deposition rate.With increases in the ratio of upstream and downstream Froude numbers,particle size,and sediment supply intensity,the sediment deposition rate increased.The sediment deposition rate decreased with an increase in flow discharge.Approach hydraulic conditions and particle properties jointly determined the sediment deposition rate in a hydraulic jump section,and an empirical formula was developed using those parameters to calculate the sediment deposition rate.Thus,to identify risks and prevent disasters in mountain rivers,local changes in hydraulic conditions and particle properties need to be jointly evaluated.

Sediment provenance of a carbonate bioclastic pocket beach—Le Dune(Ionian Sea,South Italy)

This study focuses on the analysis of a carbonate bioclastic pocket beach located along a coastal sector of the Apulia Ionian Sea,Le Dune beach,South Italy.The beach develops for about 800 m and it is exposed to the south-westerly and southerly seas.Coastal sediments range from very coarse to medium-fine sands and they are mainly composed of bioclasts(more than 90%)which include molluscs,foraminifers,echinoderms,algae branched,bryozoans,spicules of sponges and arthropods.The study area is one part of a marine protected reserve characterised by 15 different habitats of the typical Mediterranean submerged populations and the presence of Posidonia oceanica meadows.The aim of our research is to highlight the correlation between physical and biological processes influencing Le Dune beach dynamics and its sediment provenance by analysing the textural and compositional characteristics of beach sands,which is fundamental for pocket beach conservation.The beach sand analysis,deriving from textural,compositional and bioclast investigations,underlines that one of the main indicators of the beach dynamics is the bioclast component,which provides relevant information about sand provenance and sediment transport.The beach constitutes a semi-close system only nourished by the shells of organisms and by the erosion of headlands and dunes without important sediment interchange with adjacent littoral sectors.

1984-2019年环渤海滩涂遥感提取及时空演化分析

Tidal flats, a precious resource that provides ecological services and land space for coastal zones, are facing threats from human activities and climate change. In this study, a robust decision tree for tidal flat extraction was developed to analyse spatiotemporal variations in the Bohai Rim region during 1984–2019 based on 9539 Landsat TM/OLI surface reflection images and the Google Earth Engine(GEE) cloud platform. The area of tidal flats significantly fluctuated downwards from 3551.22 to 1712.36 km^(2) in the Bohai Rim region during 1984–2019, and 51.31% of tidal flats were distributed near the Yellow River Delta and Liaohe River Delta during 2017–2019. There occurred a drastic spatial transition of tidal flats with coastline migration towards the ocean. Low-stability tidal flats were mainly distributed in reclamation regions, deltas, and bays near the estuary during 1984–2019. The main factors of tidal flat evolution in the Bohai Rim region included the direct impact of land cover changes in reclamation regions, the continuous impact of a weakening sediment supply, and the potential impact of a deteriorating sediment storage capability. The extraction process and maps herein could provide a reference for the sustainable development and conservation of coastal resources.

Controlling factors on the submarine canyon system: A case study of the Central Canyon System in the Qiongdongnan Basin, northern South China Sea

Based on an integrated analysis of high-resolution 2D/3D seismic data and drilling results, this study analyzes the tectonic- sedimentary evolution of the Qiongdongnan Basin （QDNB） since the late Miocene, and discusses the controlling factors on the formation and development of the Central Canyon System （CCS）. The sediment failures caused by the relative sea level falling might have discharged deposits from the slope to the canyon. The two suits of the infillings, i.e., turbidites and mass transport complex （MTC）, were derived from the northwestern source and northern source, respectively. The sediment supplies, which differ significantly among different areas, might have led to the variations observed in the internal architectures. Tectonic transformation around 11.6 Ma had provided the tectonic setting for the CCS and formed an axial sub-basin in the central part of the Changchang Depression, which could be called the rudiment of the CCS. The tectonic activity of the Red River Fault （RRF） at about 5.7 Ma might have strengthened the hydrodynamics of the deposits at the junction of the Yinggehal Basin （YGHB） and the QDNB to trigger a high-energy turbidity current. The MTC from the northern continental slope system might have been constrained by the Southern Uplift, functioning as a barrier for the infillings of the CCS. Thanks to a sufficient sediment supply during the Holocene period and the paleo-seafloor morphology, the relief of modern central canyon with the starving landform in the eastern Changchang Depression might have been accentuated by deposition of sediments and vertical growth along the canyon flanks, where collapse deposits were widely developed. Corresponding to the segmentation of the CCS, the forming mechanisms of the canyon between the three segments would be different. The turbidite channel in the head area had likely been triggered by the abundant sediment supply from the northwestern source together with the fault activity at about 5.7 Ma of the RRF. The formation and evolution of the canyon in the western segment were caused by combined effects of the turbidite channel from the northwestern source, the MTC from the northern continental slope, and the paleo-seafloor geomorphology. In the eastern segment, the canyon was constrained by the tectonic transformation occurring at approximately 11.6 Ma and the insufficient sediment supply from the wide-gentle slope.

High-Resolution Transgressive-Regressive Sequence Stratigraphy of Chang 8 Member of Yanchang Formation in Southwestern Ordos Basin,Northern China

The reservoirs within the Chang 8 Member of Yanchang （延长） Formation in the south- western Ordos basin are characterized by low permeability. It is significant to build a high-resolution sequence stratigraphic framework for the exploration and development of these reservoirs. Based on an integrated investigation of well logs, seismic data, cores and outcrops, the Chang 8 Member is interpreted as a 3rd-order transgressive-regressive （T-R） sequence, which consists of six 4th-order T-R se- quences, namely, K1, K2, K3, K4, K5 and K6, respectively, from base to top. Each 4th-order sequence comprises a transgressive systems tract overlain by a regressive systems tract. From K1 to transgressive systems tract of K4, it is characterized by the landward shifts of facies and overall retrogradational sequence stacking pattern due to accommodation increase more than sediment supply during the 3rd-order base-level rise. The distributary channels are mainly deposited in the transgressive systems tracts, and the interdistributary bays, sheet sands, and small-scale mouth bars are primarily developed in the regressive systems tracts in response to the 4th-order base-level fluctuations. From the regressivesystems tracts of K4 to K6, it is characterized by the basinward shifts of facies and overall progradational sequence stacking pattern because of accommodation increase less than sediment sup- ply during a fall in 3rd-order base level. The distributary channels are mainly accumulated in the transgressive systems tracts, and distributary channels and mouth bars are deposited in the regressive systems tracts as a result of the 4th-order base-level changes. Detailed stratigraphic and sedimentoiogical analyses indicate that sequences K1, K2, K5 and K6, as well as the basin edge of K3 and K4, remain potential for exploration and development due to their high reservoir quality in response to the low accommodation and more sediment supply.