Numerical Study On Mass Transport And Energy In Qiantang Estuary

Estuaries are affected by both runoff and tides, so that the hydrodynamic characteristics are particularly sophisticated. The transport processes of sediment, salinity and pollutants and the change of energy in estuaries are important issues, which have significant effects on estuarine environment, ecology and drinking water quality, and estuarine engineering. Therefore, the study on mass transport and energy in Qiantang estuary is important not only for theory but also for engineering practice.The diversity, complicity and impacts of mass transport in estuaries are summarized, the main methods and the status quo of the research of mass transport both at home and abroad are introduced. In addition, a series of studies on numerical simulation of mass transport are carried out.Focusing on the impacts of salt-water intrusion to drinking water sources in macrotidal estuaries, a 2D hydrodynamic-salinity model was established and numerically simulated the process of hydrodynamics and salt transport in Qiantang estuary. It was found that when the effect of salinity transport in macrotidal estuary reaches the maximum, the river bend is significantly affected by the main streamlines of flood and ebb currents, which causes greater salinity at the river bank near the streamlines of flood current than another bank. As a result, in terms of mere salt-water intrusion, at the river bend of macrotidal estuary, water intake should not be close to the main streamlines of flood current. Once runoff reaches a certain extend, the effect of increasing runoff on restraining salinity would be reduced, which means the utilization rate of water resources is decreased, In this case, water supply from reservoirs would effectively save water resources. This provides technical evidence for the construction of reservoirs to store fresh water and avoid over-salinity.Based on the Navier-Stokes equation under the hypothesis of incompressibility and Boussinnesq constitutive relation in orthogonal curvilinear coordinates, pollutants tranport in a hypothetical pollution accident in Qiantang estuary were numerically simulated. It was found that pollutants in macrotidal estuary diffused due to turbulence and were transported by tide to and fro. The variation of pollutants concentration was opposite to that of water level during each tidal cycle. an increase in upstream flow would effectively reduce the impacts of pollutants to water quality, which demonstrates that increasing upstream flow is feasible to ensure water quality from pollution accidents as soon as possible.Based on Navier-Stokes equation, convection-diffusion equation for sediment transport, a model under orthogonal curvilinear coordinates was established and applied to simulate sediment transport during extreme flood. It was found that increasing runoff would bring greater extend of sediment transport. It took less time to reach drastic erosion status under high flow than under low flow. Sediment transport would tend stable after several days. Increasing runoff would not cause unlimited vertical erosion but lateral erosion. In addition, the location of deep channel would deviate owing to groins.Based on 3D mathematical model, the spatial distribution of energy in flow-field planes from Wenjiayan to Cangqiao were finely calculated and analyzed in view of Euler method. It was found that geopotential energy was larger order of magnitude than kinetic energy of unit water volume, and that the ratio of geopotential energy to kinetic energy became smaller with scour. The ratio in convex banks was smaller than that in concave banks. However, the distribution of energy at the last river bend gradually went to opposite. There would be greater potential energy losses in deeper area.Nowadays, people are paying more attention to estuarine environment. Numerical calculation of mass transport and energy in macrotidal estuaries will have practical significance in order to reserve environment in macrotidal estuary, to ensure water quality and to improve human being's living conditions.