Deep-sea sediment disturbance may occur when collecting polymetallic nodules,resulting in the creation of plumes that could have a negative impact on the ecological environment.This study aims to investigate the poten...Deep-sea sediment disturbance may occur when collecting polymetallic nodules,resulting in the creation of plumes that could have a negative impact on the ecological environment.This study aims to investigate the potential solution of using polyaluminum chloride(PAC)in the water jet.The effects of PAC are examined through a self-designed simulation system for deep-sea polymetallic nodule collection and sediment samples from a potential deep-sea mining area.The experimental results showed that the optimal PAC dose was found to be 0.75 g/L.Compared with the test conditions without the addition of PAC,the presence of PAC leads to a reduction in volume,lower characteristic turbidity,smaller diffusion velocity,and shorter settling time of the plume.This indicates that PAC inhibits the entire development process of the plume.The addition of PAC leads to the flocculation of mm-sized particles,resulting in the formation of cm-sized flocs.The flocculation of particles decreases the rate of erosion on the seabed by around 30%.This reduction in erosion helps to decrease the formation of plumes.Additionally,when the size of suspended particles increases,it reduces the scale at which they diffuse.Furthermore,the settling velocity of flocs(around 10^(-2) m/s)is much higher that of compared to sediment particles(around 10^(-5) m/s),which effectively reduces the amount of time the plume remains in suspension.展开更多
The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity...The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity and suspended sediment concentrations （TSSC） were measured in stations located within the bay using Aanderaa Recording Current Meter （RCM-9）, Turbidity Sensor mounted on RCM-9, Divers Gauges and Aanderaa Tempera- ture-Salinity Meter. The study established that Malindi Bay receives a high terrigenous sedi- ment load amounting to 5.7 x 106 ton.yr-1. The river freshwater supply into the bay is highly variable ranging from 7 to 680 m3.s-1. The high flows that are 〉 150 m3.s^-1 occurred in May during the South East Monsoon （SEM）. Relatively low peak flows occurred in November during the North East Monsoon （NEM） but these were usually 〈70 m3.s^-1. The discharge of highly turbidity river water into the bay in April and May occurs in a period of high intensity SEM winds that generate strong north flowing current that transports the river sediment plume northward. However, during the NEM, the river supply of turbid water is relatively low occur- ring in a period of relatively low intensity NEM winds that result in relatively weaker south flowing current that transports the sediment plume southward. The mechanism of advection of the sediment plume north or south of the estuary is mainly thought to be due to the Ekman transport generated by the onshore monsoon winds. Limited movement of the river sediment plume southward towards Ras Vasco Da Gama during NEM has ensured that the coral reef ecosystem in the northern parts of Malindi Marine National Park has not been completely destroyed by the influx of terrigenous sediments. However, to the north there is no coral reef ecosystem. The high sediment discharge into Malindi Bay can be attributed to land use change in the Athi-Sabaki River Basin in addition to rapid population increase which has led to clearance of forests to open land for agriculture, livestock grazing and settlement. The problems of heavy siltation in the bay can be addressed by implementing effective soil con- servation programmes in the Athi-Sabaki Basin. However, the soil conservation programmes in the basin are yet to succeed due to widespread poverty among the inhabitants and the complications brought about by climate change.展开更多
Sedimentary environments in the Hangzhou Bay are introduced by a three-subregional landform pattern and a sedimentation map with six major sedimentation types. The geographic setting, sediment transportation and sedim...Sedimentary environments in the Hangzhou Bay are introduced by a three-subregional landform pattern and a sedimentation map with six major sedimentation types. The geographic setting, sediment transportation and sedimentation in different subregions are discussed with sediment grain parameters. Sedimentary basin suffering from effect of plume front can be identified by the fine sediments with median ( Mdφ ) 7φ-8φ, clay content 30% - 40% and silt 60% - 70%. Sediment transportation along the plume front seems not to be a direct effect on sedimentary formation of the shoals fringing the south coast of the Hangzhou Bay.展开更多
Freshwater plume represents a key passage for the river to deliver sediment to the coastal ocean. A wa- ter-sediment regulation scheme was implemented for the Huanghe River in 2005; in order to examine the effect of s...Freshwater plume represents a key passage for the river to deliver sediment to the coastal ocean. A wa- ter-sediment regulation scheme was implemented for the Huanghe River in 2005; in order to examine the effect of such an activity on the river plume dynamics, three cruises were carried out off the Huanghe River mouth. The prelimi- nary results of the in-situ measurements suggested that the plume pathway shifted rapidly during the period of less than twenty days, which was confirmed by satellite remote sensing data in the same period. The rapid shifts were resulted from intensive interactions between river discharge and the vary- ing bathymetry in the river mouth area. A large amount of river sediment was delivered to the coastal region during this period and mostly deposited in the mouth bar area due to jet-flow deceleration, forming a cluster of mouth bars across the river mouth, which caused the rapid shifts of the plume pathway.展开更多
Numerical simulation is an important method for calculating the hydrodynamic performance of otter boards used in sea floor trawling.Although such simulations have been explored in prior studies,the effects of the prox...Numerical simulation is an important method for calculating the hydrodynamic performance of otter boards used in sea floor trawling.Although such simulations have been explored in prior studies,the effects of the proximity of the otter boards to the seafloor and the plume of upward-drawn sediment during bottom trawling have largely been ignored.In this study,we assessed these factors.The results show that within the angles of attack used during normal operations,the effect of the seafloor bottom boundary of the flow field on the hydrodynamic performance of an otter board is obvious.We found that when the ratio of the distance between the bottom of an otter board and the surface boundary of the flow field to the chord length of the board exceeds 0.4,the influence of the bottom boundary of the flow field on the hydrodynamic performance of the board is negligible.For values of less than 0.4,the seafloor bottom boundary has an increasingly obvious impact on the hydrodynamic performance as this ratio decreases.We also found that the turbid plume of ocean floor sediment raised during bottom trawling has an obvious effect on the lift and resistance coefficients of an otter board at high angles of attack.At low angles,this effect on the lift-to-drag ratio is reversed and less obvious.The simulation results show that the optimal lift-to-drag ratio decreases with an increase in the sediment concentration;however,beyond a certain threshold,an increasing concentration of sediments was not found to have an obvious impact on the lift-to-drag ratio.展开更多
基金supported by the National Natural Science Foundation of China(Nos.52225107,U2106224,U1906234,51822904,and U1706223)the Fundamental Research Funds for the Central Universities(No.202041004)
文摘Deep-sea sediment disturbance may occur when collecting polymetallic nodules,resulting in the creation of plumes that could have a negative impact on the ecological environment.This study aims to investigate the potential solution of using polyaluminum chloride(PAC)in the water jet.The effects of PAC are examined through a self-designed simulation system for deep-sea polymetallic nodule collection and sediment samples from a potential deep-sea mining area.The experimental results showed that the optimal PAC dose was found to be 0.75 g/L.Compared with the test conditions without the addition of PAC,the presence of PAC leads to a reduction in volume,lower characteristic turbidity,smaller diffusion velocity,and shorter settling time of the plume.This indicates that PAC inhibits the entire development process of the plume.The addition of PAC leads to the flocculation of mm-sized particles,resulting in the formation of cm-sized flocs.The flocculation of particles decreases the rate of erosion on the seabed by around 30%.This reduction in erosion helps to decrease the formation of plumes.Additionally,when the size of suspended particles increases,it reduces the scale at which they diffuse.Furthermore,the settling velocity of flocs(around 10^(-2) m/s)is much higher that of compared to sediment particles(around 10^(-5) m/s),which effectively reduces the amount of time the plume remains in suspension.
基金funded by START and implemented as part of IGBP-LOICZ AfriCat Pilot project on the ‘Coastal Impacts of Damming and Water Abstraction in Africa’
文摘The paper presents results of a study on the sediment supply and movement of highly turbid sediment plume within Malindi Bay in the Northern region of the Kenya coast. The current velocities, tidal elevation, salinity and suspended sediment concentrations （TSSC） were measured in stations located within the bay using Aanderaa Recording Current Meter （RCM-9）, Turbidity Sensor mounted on RCM-9, Divers Gauges and Aanderaa Tempera- ture-Salinity Meter. The study established that Malindi Bay receives a high terrigenous sedi- ment load amounting to 5.7 x 106 ton.yr-1. The river freshwater supply into the bay is highly variable ranging from 7 to 680 m3.s-1. The high flows that are 〉 150 m3.s^-1 occurred in May during the South East Monsoon （SEM）. Relatively low peak flows occurred in November during the North East Monsoon （NEM） but these were usually 〈70 m3.s^-1. The discharge of highly turbidity river water into the bay in April and May occurs in a period of high intensity SEM winds that generate strong north flowing current that transports the river sediment plume northward. However, during the NEM, the river supply of turbid water is relatively low occur- ring in a period of relatively low intensity NEM winds that result in relatively weaker south flowing current that transports the sediment plume southward. The mechanism of advection of the sediment plume north or south of the estuary is mainly thought to be due to the Ekman transport generated by the onshore monsoon winds. Limited movement of the river sediment plume southward towards Ras Vasco Da Gama during NEM has ensured that the coral reef ecosystem in the northern parts of Malindi Marine National Park has not been completely destroyed by the influx of terrigenous sediments. However, to the north there is no coral reef ecosystem. The high sediment discharge into Malindi Bay can be attributed to land use change in the Athi-Sabaki River Basin in addition to rapid population increase which has led to clearance of forests to open land for agriculture, livestock grazing and settlement. The problems of heavy siltation in the bay can be addressed by implementing effective soil con- servation programmes in the Athi-Sabaki Basin. However, the soil conservation programmes in the basin are yet to succeed due to widespread poverty among the inhabitants and the complications brought about by climate change.
基金This work is supported by both Grant-4860244 from the National Natural Science Foundation of China and Grant-188011 from the Zhejiang Provincial Natural Science Foundation
文摘Sedimentary environments in the Hangzhou Bay are introduced by a three-subregional landform pattern and a sedimentation map with six major sedimentation types. The geographic setting, sediment transportation and sedimentation in different subregions are discussed with sediment grain parameters. Sedimentary basin suffering from effect of plume front can be identified by the fine sediments with median ( Mdφ ) 7φ-8φ, clay content 30% - 40% and silt 60% - 70%. Sediment transportation along the plume front seems not to be a direct effect on sedimentary formation of the shoals fringing the south coast of the Hangzhou Bay.
基金supported by the Ministry of Science an d Technology(Grant No.2002CB412404)Nationa1 Natura1 Science Foundation of China(Grant No.403060081.
文摘Freshwater plume represents a key passage for the river to deliver sediment to the coastal ocean. A wa- ter-sediment regulation scheme was implemented for the Huanghe River in 2005; in order to examine the effect of such an activity on the river plume dynamics, three cruises were carried out off the Huanghe River mouth. The prelimi- nary results of the in-situ measurements suggested that the plume pathway shifted rapidly during the period of less than twenty days, which was confirmed by satellite remote sensing data in the same period. The rapid shifts were resulted from intensive interactions between river discharge and the vary- ing bathymetry in the river mouth area. A large amount of river sediment was delivered to the coastal region during this period and mostly deposited in the mouth bar area due to jet-flow deceleration, forming a cluster of mouth bars across the river mouth, which caused the rapid shifts of the plume pathway.
基金the National Natural Science Foundation of China(Grant No.31972845).
文摘Numerical simulation is an important method for calculating the hydrodynamic performance of otter boards used in sea floor trawling.Although such simulations have been explored in prior studies,the effects of the proximity of the otter boards to the seafloor and the plume of upward-drawn sediment during bottom trawling have largely been ignored.In this study,we assessed these factors.The results show that within the angles of attack used during normal operations,the effect of the seafloor bottom boundary of the flow field on the hydrodynamic performance of an otter board is obvious.We found that when the ratio of the distance between the bottom of an otter board and the surface boundary of the flow field to the chord length of the board exceeds 0.4,the influence of the bottom boundary of the flow field on the hydrodynamic performance of the board is negligible.For values of less than 0.4,the seafloor bottom boundary has an increasingly obvious impact on the hydrodynamic performance as this ratio decreases.We also found that the turbid plume of ocean floor sediment raised during bottom trawling has an obvious effect on the lift and resistance coefficients of an otter board at high angles of attack.At low angles,this effect on the lift-to-drag ratio is reversed and less obvious.The simulation results show that the optimal lift-to-drag ratio decreases with an increase in the sediment concentration;however,beyond a certain threshold,an increasing concentration of sediments was not found to have an obvious impact on the lift-to-drag ratio.
