Study On Water And Sediment Movement And Siltation Of Alongshore Basin In Jiulongjiang Estuary

The estuary is located in the transition zone between river mouth and sea,which is an important area for harbor development.The hydrodynamics and sediment transport in estuaries are complicated and have been widely studied.The characteristics of hydrodynamics,sediment transport and siltation in the Jiulongjiang Estuary are investigated.A siltation formula of the alongshore basin is deduced,which is applied to calculate the siltation in alongshore basin of Jiulongjiang Estuary.Furthermore,a three-dimensional hydrodynamics and sediment transport model is developed,in which the settling velocity formula of cohesive sediment is improved.The interactions among the hydrodynamics,sediment transport and salinity are studied based on the numerical model.The influence factors of siltation in alongshore basin of Jiulongjiang Estuary are analyzed.The main results are summarized as follows.1.Jiulongjiagn Estuary is affected by multiple hydrodynamic factors,and the current and mixing of salt water and fresh water take great influences on the sediment transport and siltation of alongshore basin.2.On the basis of the numerical hydrodynamic model,the relationship between the current velocity in alongshore basin and the length-width ratio is presented.Then,a siltation formula in alongshore basin is proposed and the calculated reselts agree with the measurement.According to the calculated results of siltation in alongshore basin in Jiulongjiang Estuary by the formula,it is shown that the siltation in the east basin decreases when the length-width ratio of the basin increases.The siltation in the extended basin is large because of the shallow water and the high concentration.3.Including the typhoon model,SWAN wave model,EFDC hydrodynamic and sediment transport model,the three-dimensional numerical models for hydrodynamics and sediment in estuaries are developed.Considering the effects of sediment concentration,salinity and flow turbulence on the settling velocity,a new formula is obtained to improve the sediment model.It can be shown that the numerical model is more reasonable to the hydrodynamics,salinity,sediment transport and siltation after the verification by the observed data..4.Based on the improved three-dimension numerical model the interaction mechanism among salinity,water and sediment of Jiulongjiang estuary is studied.It is shown that the salinity distribution is higher in north area and east area than south area and west area because of the difference between the flood and ebb current.The salinity gradient in vertical is more uniform in normal hydrology condition than that in typhoon condition,and that in flood discharge condition is most nonuniform.In flood discharge condition,there are obvious frontal surface of salt water and fresh water in the area of estuary mouth and alongshore basin where forms a vertical circulation due to the influence of mixing.5.When considering salinity effect,the simulated sediment concentration becomes larger and the distribution difference in vertical depth increases and the simulated results agree more closely to measured data.The calculated siltation of alongshore basin in Jiulongjang estuary agree with to the survey data by the formula considering the influence of salinity and sediment concentration.6.The retaining flow into the basin with high sediment concentration from the north channel and middle channel is the main factor to the siltation in the Jiulongjiang alongshore basin,and the vertical circulation in basin formed by the salinity gradient accelerates the sediment to deposit that increase the siltation of the extended basin.