Process Of High-concentrated Sediment Hyperpycnal Flow At The Huanghe River Mouth: In-situ Observations And Numerical Simulation

Estuary is a link between river and coastal ocean. The terrestrial material,delivered by suspended sediments from river to the sea in complicated transmissionways. Hyperpycnal flows are not common patterns of the riverine sediment in theestuaries. They are characterized by their quick discharge and high sediment load,which make them important factors for the deltaic and coastal environment evolution,and the transport processes of the fluvial heavy metal and toxic pollutant particles.Huanghe is famous for its high annual sediment load delivered to the sea, historicallyconsidered as a typical area for the observations and researches of hyperpycnal flows.Impacted by climate change and extensive human activities, the transmission way ofhyperpycnal flows is transformed, which causes a series effect of geomorphology andbiogeochemical. This dissertation bases on the in-situ observations of hyperpycnalflows during Cruise1995and Cruise2010. The spatio-temporal variations and thedynamic mechanisms of hyperpycnal flows are illustrated. A three-dimensionalhydrodynamics model (EFDC model) is used to simulate hyperpycnal flows in anidealized highly turbid estuary. Four experiments are conducted in order to examinethe influential factors to the formation and maintenance of hyperpycnal flows in anidealized highly turbid estuary. Comparing the in-situ observations with the results ofnumerical simulations, new knowledge of the periodic variation and main controllingmechanisms, is significant in formation and variation of hyperpycnal flows and alsohave important biogeochemical effect.The conclusion of the dissertation shows:1) The median grain size and the suspended sediment concentration (SSC) of theHuanghe to the sea are controlling elements in the formation and attenuation ofhyperpycnal flows. The hyperpycnal flows in Huanghe mouth with stable river effluent are periodically modulated by tidal cycles, when start at the slack water inflood tides, enhance in ebb tides, attenuate at the slack water in ebb tides, and thendisappear in flood tides. During the formation of hyperpycnal flows, fresh water withdecreasing salinity and increasing SSC controls the bottom boundary layer.Accelerated by the down-slope force (Fg) greater than the shear stress (Fr),hyperpycnal flows scour the sea-bed. Because of low turbulence frequency, themedian grain sizes are reduced by the deposition of the coarser sediment from thewater. On the contrary, during the attenuating process of hyperpycnal flows, thebottom boundary layer is controlled by ocean water with increasing salinity anddecreasing SSC. Hyperpycnal flows are decelerated by Fr greater than Fg, whichleads to the nearshore deposition of hyperpycnal flows. The median grain sizes isincreased by the high coarser sediment with increasing turbulence frequency. Basedon the in-situ observations and numerical simulations, the transporting pathways ofhyperpycnal flows are non linear but possibly curvilinear dragged by the tidalcurrents.2) Results of the numerical experiments show that: the variation characters ofhyperpycnal flows are according with the in-situ observations and the previousresearch. The SSC of the river effluents is of primary importance to the formation ofhyperpycnal flows, but not the sufficient factor. The median grain size is a key factorto the maintenance of hyperpycnal flows in estuary. Even the SSC is large enough, thehyperpycnal flows can’t occur with coarser sediment. The enhance turbulencetriggered by tidal cycles or waves leads to suppression in the formation and deliveryof hyperpycnal flows.3)Recent years, the water and sediment discharges from the Huanghe to the seaare significantly decrease, accompanied by the increasing median grain size. Underthe circumstances, the transporting pattern of the riverine sediment to the sea verifiedquickly from hyperpycnal flows to hypopycnal flows. The diffusion and deposition ofheavy metal and nutrients, carried by the fine-grain size sediments, is changed by thetransformation of the transporting pattern of sediment, which consequently affect thebiogeochemical cycles in the estuary and coastal sea. Based on the in-situ observations, together with EFDC model, several crucial factorsin formation and attenuation of hyperpycnal flows are refined in this dissertation. Theeffect of the crucial factor on the process of hyperpycnal flows is important. The studyhere is helpful for the future study in the Huanghe hyperpycnal flows transformation,the geomorphology and the coastal biogeochemical cycles.