Turbidity Current And Morphodunamics In Submarine Canyon Of Northern South China Sea

Northern slope of the northern South China has abundant natural gas hydrate and oil and gas resources,The Liwan 3-1 pipeline project and the gas hydrate test area are all within this scope.Therefore,the study of turbidity current activity in the canyon can not only explore the genesis mechanism of the canyon,but also be of great significance to the development of Marine energy engineering and the protection of submarine cables.In this paper,a 2-D turbidity current numerical model is based on Navier-Stokes equations coupled with turbulence k-epsion model,combine with the multi-beam data,geological borehole data and the regional geological data et,al.The characteristics of current and deposition/erosion of turbidity current with different input particle-size under different inflow mechanisms(sustain inflow and sudden release)in the continuous slope break of northern south China sea and other similar terrain.The conclusion of this dissertation are as follows:(1)The turbidity current accelerates in the slope and on the nearly horizontal bed,velocity drops obviously and gradually deposits at the horizontal bed.The turbidity current deposition area may be restricted to the continental shelf slope break zone.(2)The turbidity current's head is differentiated,there is a maximum of density and velocity at the bottom where the particle size is coarser,and transported forward by drift or suspension.Partical gradually deposited on the bottom with the flow,which may be associated with baumar sequence A.The fine particles are evenly distributed at the turbidity current's head and trapping the environmental fluid into head with the flow,resulting decrease in the density,which may be associated with baumar sequence E.The head of the continuous inflow turbidity current has a higher speed and lower concentration than the body.During the flow,the ambient fluid is displaced and pushed forward by the body of the turbidity current with high speed and high concentration.The concentration and velocity of sustain inflow turbidity current arestable,and foemed continuous sedimentary layer in the sea floor.In this process,turbidity current occurs mechanical sedimentary differentiation along the direction of turbidity current,the grift was deposited on the slope,and the deposition particle size decreases gradually along the downstream direction,the particle size changes little in vertical.The longitudinal deposits of multi-frequency sustain inflow turbidity current are continuously deposition,the subsequent turbidity current erode the previous sedimentary,which are characterized by the rhythmic deposits of multi-layer discontinuous baumar sequence on the stratum,and the sediments migrate upstream with the flow of turbidity current.The turbidity current head with sudden release has the advantages of velocity and concentration compared with the turbidity body,deposited on the bottom bed as the flow force decreases.Turbidity current occurs mechanical sedimentary differentiation in the flow,the coarse sand was basically deposited in the accelerated belt on the slope,and the particle size gradually decreased along the downstream direction.The sedimentary layer shows inverse stratification in the vertical direction.(3)Dynamic geomorphic process of turbidity current presents erosion on the slope and deposition under the slope.Fine particles(clay and silty sand)drives turbidity current erosion obviously,while coarse particles(coarse sand)are mainly deposited in the bed floor.The erosion of turbidity current is concentrated on the turbidity current head,and the erosion is proportional to the particle concentration and velocity.The formation of submarine canyons outside the pearl river estuary on the northern continental slope of the south China sea may be under the background of the ancient pearl river,and its topographic development is controlled by many factors such as tectonic subsidence,sea level fluctuation,river source material supply,climate change and so on.The early denudation process of the canyon may be dominated by landslides and persistent hydrodynamics.After the sea level drops,the channel gradually deepens under the persistent hydrodynamics,forming the prototype of the canyon.After sea level rise,it may be dominated by sudden activities,such as sudden release of gravity flow and landslides,which further erode the valley floor and valleyside,deepen the width or depth of the canyon,and form the existing NW-NWW trending canyon in the process of continuous collapse,erosion and slip.