Jicai Zhang

Sediment dynamics in an offshore tidal channel in the southern Yellow Sea

Abstract The geomorphology of the southern Yellow Sea (SYS) is characterized by offshore radial sand ridges (RSR). An offshore tidal channel (KSY Channel) is located perpendicular to the coast, comprised of a main and a tributary channel separated by a submarine sand ridge (KSY Sand Ridge) extending seaward. In order to investigate the interactions among water flow, sediment transport, and topography, current velocity and suspended sediment concentration (SSC) were observed at 11 anchor stations along KSY Channel in RSR during a spring tide cycle. High resolution bottom topography was also surveyed. Residual currents and tidally averaged suspended sediment fluxes were calculated and analyzed by using the decomposition method. Results suggested that the water currents became stronger landward but with asymmetrical current speed and temporal duration of flood and ebb tides. Residual currents showed landward water transport in the nearshore channel and a clockwise circulation around the KSY Sand Ridge. Tidally-averaged SSC also increased landward along the channel. The main mechanisms controlling SSC variations were resuspension and horizontal advection, with spatial and temporal variations in the channel, which also contributed to sediment redistribution between channels and sand ridges. Residual flow transport and the tidal pumping effect dominated the suspended sediment flux in the KSY Channel. The KSY Sand Ridge had a potential southward migration due to the interaction between water flow, sediment transport, and topography.

Optimization of Open Boundary Conditions in a 3D Internal Tidal Model with the Adjoint Method around Hawaii

Based on the theory of inverse problem, the optimization of open boundary conditions (OBCs) in a 3D internal tidal model is investigated with the adjoint method. Fourier coefficients of internal tide on four open boundaries, which are regarded as OBCs, are inverted simultaneously. During the optimization, the steepest descent method is used to minimize cost function. The reasonability and feasibility of the model are tested by twin experiments (TEs). In TE1, OBCs on four open boundaries are successfully inverted by using independent point (IP) strategy, suggesting that IP strategy is useful in parameter estimation. Results of TE2 indicate that the model is effective even by assimilating inaccurate “observations.” Based on conclusions of TEs, the internal tide around Hawaii is simulated by assimilating T/P data in practical experiment. The simulated cochart shows good agreement with that obtained from the Oregon State University tidal model and T/P observations. Careful inspection shows that the major difference between simulated results and OSU model results is short-scale fluctuations superposed on coamplitude lines, which can be treated as the surface manifestation modulated by the internal tide. The computed surface manifestation along T/P tracks is comparable to the estimation in previous work.

Simulation of sedimentary dynamics in a small-scale estuary: the role of human activities

Human activities, such as tidal sluice gate construction and reclamation, are prevalent along northern coastline of China and influence significantly sedimentary environments. The Sheyang River estuary is a small estuary situated on the Jiangsu coast. For the objective of storing fresh water, maintaining river channel depth and managing the land resource, a tidal sluice gate was constructed. At the same time, estuarine wetlands have been reclaimed extremely in recent decades. A numerical model based upon Delft3D was developed to quantify the impact of these human activities, and four simulation schemes were designed (a) tidal sluice gate present, but always closed; (b) tidal sluice gate not present; (c) tidal sluice gate present, but closed during flood and open during the ebb; and (d) intertidal area reclamation taking place in the estuary. A bathymetric survey confirmed the erosion and deposition results of the simulation. The rate of sediment deposition increased significantly after the gate was closed. However, during the third scheme, the sediment deposition rate reduced considerably; that was because the ebb current increased the scouring force when the gate was open, especially within the channel area close to the gate. Meanwhile, reclamation lessened erosion behind the gates and may have been enhanced by siltation. The coastline shifted seaward as the siltation area increased; it extended farther because of reclamation. Appropriate coastal management planning strategies must be developed to reduce negative effects of the channel siltation, resulting from the various forms of human activity. The results of this study can provide input to governmental recommendations on more effective tidal gate management strategies.

A Methodology for Estimating the Parameters in Three-Dimensional Cohesive Sediment Transport Models by Assimilating In Situ Observations with the Adjoint Method

AbstractThe model parameters in the suspended cohesive sediment transport model are quite important for the accurate simulation of suspended sediment concentrations (SSCs). Based on a three-dimensional cohesive sediment transport model and its adjoint model, the in situ observed SSCs at four stations are assimilated to simulate the SSCs and to estimate the parameters in Hangzhou Bay in China. Numerical experimental results show that the adjoint method can efficiently improve the simulation results, which can benefit the prediction of SSCs. The time series of the modeled SSCs present a clear semidiurnal variation, in which the maximal SSCs occur during the flood tide and near the high water level due to the large current speeds. Sensitivity experiments prove that the estimated results of the settling velocity and resuspension rate, especially the temporal variations, are robust to the model settings. The temporal variations of the estimated settling velocity are negatively correlated with the tidal elevati...

The High Order Conservative Method for the Parameters Estimation in a PM2.5 Transport Adjoint Model

We propose to apply Piecewise Parabolic Method (PPM), a high order and conservative interpolation, for the parameters estimation in a PM2.5 transport adjoint model. Numerical experiments are taken to show the accuracy of PPM in space and its ability to increase the well-posedness of the inverse problem. Based on the obtained results, the PPM provides better interpolation quality by employing much fewer independent points. Meanwhile, this method is still well-behaved in the case of insufficient observations. In twin experiments, two prescribed parameters, including the initial condition (IC) and the source and sink (SS), are successfully estimated by the PPM with lower interpolation errors than the Cressman interpolation. In practical experiments, simulation results show good agreement with the observations of the period when the 21th APEC summit took place.

Trajectory Estimation of Aircraft in a Double-Satellite Passive Positioning System with the Adjoint Method

A double-satellite passive positioning system is constructed based on the theory of space geometry, where two observation coordinate systems and a fundamental coordinate system exist. In each observation coordinate system, there exists a ray from the observation satellite to the aircraft. One difficulty lies in that these two rays may not intersect due to the existence of various errors. Under this situation, this work assumes that the middle point of common perpendicular between two rays is the actual position of aircraft. Based on the theory of space geometry, the coordinates of aircraft in the fundamental coordinate system can be determined. A dynamic model with the adjoint method is developed to estimate the trajectory of aircraft during the process of rocket propulsion. By assimilating observations, the trajectory of aircraft can be calculated. Numerical experiments are designed to validate the reasonability and feasibility of this model. Simulated results indicate that even by assimilating a small number of observations, the trajectory of aircraft can be estimated. In addition, the trajectory estimation can become more accurate when more observations are assimilated to the model.

Application of Surface Spline Interpolation Method in Parameter Estimation of a PM2.5 Transport Adjoint Model

A new method for the estimation of initial conditions (ICs) in a PM2.5 transport adjoint model is proposed in this paper. In this method, we construct the field of ICs by interpolating values at independent points using the surface spline interpolation. Compared to the traditionally used linear interpolation, the surface spline interpolation has an advantage for reconstructing continuous smooth surfaces. The method is verified in twin experiments, and the results indicate that this method can produce better inverted ICs and less simulation errors. In practical experiments, simulation results show good agreement with the ground-level observations during the 22nd Asia-Pacific Economic Cooperation summit period, demonstrating that the new method is effective in practical application fields.