Impacts of channel dredging on hydrodynamics and sediment dynamics in the main channels of the Jiaojiang River Estuary in China

Channel dredging in estuaries increases water depth and subsequently impacts sediment dynamics and morphology. The Jiaojiang River Estuary is dredged frequently owing to heavy shipping demands. In this study,the effects of different dredging schemes on siltation were assessed through numerical modeling. The sediment model of the Jiaojiang River Estuary utilized an optimized bottom boundary layer model that considered the bed sediment grain size and fluid mud, and this model was calibrated using field data. Result reveal that channel dredging modifies the flow velocity inside and around the channel by changing the bathymetry;subsequently, this affects the residual current, bed stress, suspended sediment concentration, and sediment fluxes. Increasing the dredging depth and width increases the net sediment fluxes into the channel and dredging depth has a greater influence on the channel siltation thickness. When the dredging depth is 8.4 m or11.4 m, the average siltation thickness of the channel is 0.07 m or 0.15 m per mouth respectively. The parallel movement of the channel has small effects on the siltation volume during the simulation period. The sediment deposits in the channel primarily originates from the tidal flats, through bottom sediment fluxes. Vertical net circulation has a dominant impact on siltation because the difference of horizontal current of each layer on the longitudinal section of the channel increases, which intensifies the lateral sediment transport between the shoal and channel. The influence of vertical frictional dissipation on the lateral circulation at the feature points accounts for more than 50% before dredging, while the non-linear advective term is dominant after dredging. Tidal pumping mainly affects the longitudinal sediment fluxes in the channel. These results can be used for channel management and planning for similar estuaries worldwide.

Dynamics of Phaeocystis globosa bloom and implications for its seed sources in the Beibu Gulf,China

Algal blooms of haptophyte Phaeocystis globosa have been recorded in the Beibu Gulf,South China Sea(SCS)since 2011.Recent evidence suggests that TypeⅠand TypeⅣare the two dominant genotypes of P.globosa coexisting in this area,and their marker pigment profiles,colony sizes,and genetic information differ.Two genotype-specific quantitative polymerase chain reaction(qPCR)assays were used to clarify the distribution and dynamics of P.globosa bloom in the Beibu Gulffrom November 2018 to February 2019.Results show that the patterns of spatial-temporal distribution of TypeⅠand TypeⅣwere similar,the abundance of TypeⅣwas higher than that of TypeⅠduring the algal bloom,and thus TypeⅣcells dominated the P.globosa bloom in winter 2018.The relationships between environmental factors and P.globosa abundance indicated that temperature and nitrate were critical to the distribution and dynamics of P.globosa blooms in this area.The distribution of P.globosa cells in different water masses before the bloom revealed that the westward water via the Qiongzhou Strait,the SCS bottom water,and the current in the northwestern waters of Hainan Island could transport P.globosa cells into the Beibu Gulf serving as the seed sources triggering the intense blooms in the Beibu Gulf.

Three-dimensional circulation in northern South China Sea during early summer of 2015

Using the hydrographic data obtained during two nearly simultaneous surveys in June 2015,we carried out semi-diagnostic calculations with the help of a finite element model and a modified inverse method,to study the circulation in the northern South China Sea(NSCS)during the early summer of 2015.A number of new circulation features were found.(1)In most of the observation region,a large,basin-scale anticyclonic gyre appeared south of the 50-m isobath,which contained anticyclonic eddies.One anticyclonic eddy existed from the sea surface to 50-m depth,whose center showed no tilt,while the center of another eddy tilted eastward from the sea surface to 500-m depth.In the eastern part of the observation region,which is west of the Dongsha Islands,there was a sub-basin-scale cyclonic gyre containing a cyclonic eddy whose center tilted southward from the sea surface to 200-m depth.(2)There was a cross-continental slope current(CCSC)in the area southwest of the Dongsha Islands.Its volume transport was about 2.0×10^(6) m^(3)/s.(3)From the estimated order of magnitude of the stream function equation,the joint effect term of the baroclinity and relief(JEBAR)andβ-effect term are two important dynamic mechanisms affecting the variation of the circulation in the NSCS.(4)The JEBAR,as a transport-generating term,resulted in the dynamic mechanism determining the pattern of the depth-averaged flow across the contours of potential vorticity fH–1.Furthermore,we show that the negative values of the JEBAR were the most dominant dynamic mechanism,causing the CCSC southwest of the Dongsha Islands to deflect from the isobaths and veer toward the deep water.The CCSC around the Dongsha Islands was located further southwest during the early summer of 2015 than during the fall of 2005(revealed by a published study),which suggests that the location of the CCSC around the Dongsha Islands may vary with season.

Seasonal Difference of the Deep Currents in the Luzon Strait during October 2008-January 2009 and July-August 2009

Based on direct current measurements from two separated cruises in October 2008-January 2009 and July-August 2009, we obtained a valuable deep current observation of the Luzon Strait (LS). Rectified wavelet power spectra analysis (RWPSA) and the geostrophic current calculation are used to study the deep current. We find that the deep current differs in different seasons. The current is strongest in autumn (October-November) and weaker in summer (July-August) and in winter (December-January). The cyclonic and anti-cyclonic meander with different subtidal current directions plays an important role in the seasonal difference of the deep current in the LS. The observed seasonal difference of the deep current in the LS is connected with the deep current observed at the western boundary of the northern Philippine Basin and is also linked with the overflow near the central Bashi Channel and Luzon Trough. The RWPSA of the long observation suggests the dominant periods of 8 d, 19 d in the deep current. The dynamical cause of the resulting velocity distribution at 1850 and 1760 m is the pressure field and bottom topography steering. The observed deep current agrees well with the geostrophic current calculation.

波浪作用下岬湾海滩蚀积特点:以澳大利亚Narrabeen海滩为例

岬湾海岸因其特殊的岬角弧形地形,导致海岸线的冲淤与顺直海岸有明显区别.本文分析了1997～1998年澳大利亚东南部Narrabeen岬湾海滩实测数据.以1次典型的大浪过程为例,详细分析了大浪对海滩的侵蚀及其后的常浪修复期海滩岸线的演变情况,以及岬湾海滩蚀淤沿岸差异与波浪要素之间的关系.为讨论海滩岸线旋转的非偶然性及其原因,还分析了1990～2000年6次大浪事件前后的海滩旋转情况,这6次大浪事件后均有8个月以上的常浪期.研究结果表明:大浪期间海滩普遍侵蚀,常浪期间沿岸蚀积的差异性导致海滩旋转;在常浪恢复期间北部至中部剖面南方涛动指数(southern oscillation index,SOI)负相关,南部剖面与SOI指数正相关,导致北蚀南淤,从而岸线会发生旋转;伴随着SOI指数的增减,南北冲淤特性随之发生改变,海滩岸线旋转呈现周期性.

Ecological wetland paradigm drives water source improvement in the stream network of Yangtze River Delta

Jiaxing created a precedent using bypass riparian marshes to purify micro-polluted water sources in China.Pond-wetland complex with constructed root channel technology becomes a paradigm which can be analogized as"human-body wetland model"based on bionics or biomimetics.Heterogeneous plant-bed/ditch system with highly active land/water ecotone interfaces,especially meandering boundaries,breeds many biochemical reactions"living areas".Optimization of hydraulic regulation promotes redox environment alternations and wetland treatment efficiency.Here we reported a series of upgrades and performances in Guanjinggang wetland after the Shijiuyang prototype.Morphological reform of plant-bed/ditch system played a vital role.Spatially root channel zone was main force of wetland purification,and temporally the treatment effect was higher in low-temperature seasons indicating non-temperature dependent mechanisms worked.Water pollution comprehensive index improved steadily from IV to III,and comprehensive pollution load was reduced by ca.40%–60%.Comprehensive evaluation function value further showed the gradients purification effect of the upgraded wetland.Ecological wetlands ameliorated source water quality,and reduced drinking water treatment reagents,thereby bringing about economic benefits.Through wetlands operation,people can see how the micro-polluted surface water becomes clear and clean,so promoting a significant social benefit.As a viable component of urban green space,wetlands could beautify regional eco-environment,freshen the air,increase urban ecological taste,and enhance the eco-environmental protection publicity.Thus,the multifunctional service values and indirect benefits are substantial.Jiaxing ecological wetlands provide a typical paradigm for water pollution remediation in developing countries and plays a leading role in technology engineering radiation effect.