Mean water level setup/setdown in the inlet-lagoon system induced by tidal action-a case study of Xincun Inlet, Hainan Island in China

With the tides propagating from the open sea to the lagoon, the mean water level （MWL） in the inlet and lagoon becomes different from that at the open sea, and a setup/setdown is generated. The change of MWL （setup/setdown） in the system imposes a great impact on regulating the development of tidal marshes, on determining the long-term water level for harbor maintenance, on the planning for the water front development with the flood control for the possible inundation, and on the interpretation of the historical sea level change when using tidal marsh peat deposits in the lagoon as the indicator for open sea＇ s sea level. In this case study on the mechanisms which control the setup/setdown in Xincun Inlet, Hainan in China, the 2-D barotropic mode of Eulerian - Lagrangian CIRCulation （ELCIRC） model was utilized. After model calibration and verification, a series of numerical experiments were conducted to examine the effects of bottom friction and advection terms, wetting and drying of intertidal areas, bathymetry and boundary conditions on the setup/setdown in the system. The modeling results show that setup occurs over the inlet and lagoon areas with an order of one tenth of the tide range at the entrance. The larger the bottom friction is, a larger setup is generated. Without the advection term, the setup is reduced clue to a decrease of water level gradient to compensate for the disappearance of the advection term. Even without overtides, a setup can still be developed in the system. Sea level rise and dredging in the inlet and tidal channel can cause a decrease of setup in the system, whereas shoaling of the system can increase the setup. The uniqueness of the Xincun Inlet with respect to MWL change is that there is no evident setdown in the inlet, which can be attributed to the complex geometry and bathymetry associated with the inlet system.     

A numerical model study of barotropic subtidal water exchange between estuary and subestuaries (tributaries) in the Chesapeake Bay during northeaster events

Northeasters are storms that affect the Chesapeake Bay area more frequently, last for longer periods and impact larger areas than hurricanes. Their impacts on storm surge development and the water exchange between estuary and subestuaries (tributaries) in the Bay vary from one event to another. In this study, three different northeaster events were selected based on their tracks when passing through the Chesapeake Bay area. An unstructured grid finite volume model ELCIRC was utilized to examine the response of the water level of the Chesapeake Bay to three selected northeasters, and the barotropic subtidal water exchanges between the tributaries and the estuary in the Bay. Model sensitivity tests were conducted to examine various effects induced by, for example, tide–surge interaction, open boundary condition, river inflow, wetting-and-drying of the low-lying land area and the usage of 2-D or 3-D mode. The results show that excluding tide–surge interaction did not deteriorate the model performance in the lower Bay but it increased the model inaccuracy in the upper Bay and in the tributaries; using radiation boundary condition decreased the sea level variation in the Bay without appropriately specifying incoming wave; excluding wetting-and-drying of low-lying land area reduced the volume flux by approximately 5%; and using 3-D mode generally increased the water level variation in the Bay. The model predicted storm surges well for three northeaster events. Further diagnostic experiments show that the relative importance of the local and remote winds in generating storm surges in the Bay varied with different northeasters. The inverse barometeric effect played an important role in inducing storm surges for two selected northeasters. The interaction between the tributaries and the Bay proper is considerable. The impacts of the remote wind and Bay wind can be much larger than that of the tributary wind and, thus, control the hydrodynamics and mass transport in the tributaries. The Bay wind and tributary wind effects are largely affected by the wind direction and wind phase, and geographic locations of the tributaries in the Bay. The tributary wind can be dominant over the remote wind and Bay wind effects when the local wind stress and barometric pressure changes are large.     

A Three-Dimensional Nearshore Hydrodynamic Model with Depth-Dependent Radiation Stresses

For the simulation of the three-dimensional (3D) nearshore circulation,a 3D hydrodynamic model is developed by taking into account the depth-dependent radiation stresses.Expressions for depth-depeedent radiation stresses in the Cartsian coordinates are introduced on the basis of the linear wave theory,and then vertical variations of depth-dependent radiation stresses are discussed.The 3D hydrodynamic model of EICIRC (Eulerian-Lagrangian CIRCulation) is extended by adding the terms of the depth-dependent or depth-averaged radiation stresses in the momentum equations.The wave set-up,set-down and undertow are simulated by the extended ELCIRC model based on the wave fields provided by the experiment or the REF/DIF wave model.The simulated results with the depth-dependent and depth-averaged radiation stresses both show good agreement with the experimental data for wave set-up and set-down.The undertow profiles predicted by the model with the depth-dependent radiation stresses are also consistent with the experimental results,while the model with the depth-averaged radiation stresses can not reflect the vertical distribution of undertow.     

2012年“苏拉”和“达维”双台风影响的近海风暴潮过程

作为影响我国沿海的主要自然灾害之一,台风风暴潮的产生和影响机制与防灾减灾息息相关。双台风引起的风暴潮因台风强度、路径等相对关系复杂多变,目前双台风相互作用下的风暴潮研究还不充分。采用参数化台风模型对2012年典型双台风"苏拉"和"达维"的风场、气压场过程进行了模拟与融合,并采用ELCIRC模型对双台风作用下的风暴潮过程进行了模拟。引入单台风单独作用的假设算例,探讨了双台风之间对增水、流场的相互影响和影响区域。研究结果表明,虽然两个台风登陆强度相当,但台风"达维"在海州湾海域引起的增水要远大于"苏拉"在台湾、福建海域引起的增水。风暴潮引起的增水及流速变化与台风在海表的风应力密切相关,较大变化幅值分布在台风行进路径的右侧。与台风单独作用时相比,台风"苏拉"与"达维"引起的风暴潮增水与流速变化在两者相互作用下均有所削弱,其中"苏拉"引起的风暴潮受到的影响更大。双台风风暴潮之间的非线性效应在不同区域的强度存在差异,在台风"苏拉"主要影响区域内非线性效应较强,其他区域则相对较弱。以上结果表明产生风暴潮较弱的台风一方对气象环境敏感性更高,风暴潮的响应更显著。     

Feedback between tidal hydrodynamics and morphological changes induced by natural process and human interventions in a wave-dominated tidal inlet: Xiaohai, Hainan, China

The feed back between morphological evolution and tidal hydrodynamics in a wave-dominated tidal inlet,Xiaohai,China is investigated through data analysis and numerical model experiments.His-torically,Xiaohai Inlet had two openings,located at the north and south of Neizhi Island(a rocky outcrop),respectively.The evolution of Xiaohai Inlet was dominated by the natural process before 1972.In addition to the natural process,human interventions,including the closure of the north opening,50% of fresh water reduction,and increase of land reclamation,have altered tidal hydrodynamics and morphological evolution since 1972.A series of numerical model simulations were conducted to investigate the influence of morphological changes on the hydrodynamics and the influence of human activities on the inlet evolution.The natural process has caused narrowing and shoaling of the inlet throat,development of theflood-tidal delta,and shoaling of the tidal channel in side the lagoon.Human intervention shave accelerated these changes.Consequently,the tidal propagation from the offshore into the lagoon has been impeded and the tidal energy has been dissipated substantially.Tidal current has changed from ebb-dominant to flood-dominant in most parts of the inlet system where as the inlet throat has remained as ebb-dominant,the tidal prism has decreased consistently,and sediment has continued to deposit in side the inlet.As a result,the changes of morphology,hydrodynamics,and sediment transport show a positive feedback.The human interventions have had both advantageous and adverse influences on the stability of the inlet.The closure of the North Opening has decreased the longshore sediment input to the inlet,and increased the tidal prism,ebb velocity,and sediment transport in the south opening,thus enhancing the inlet’sstability.However,reducing the river discharge and landfill of the tidal flats has resulted in a decrease of the tidal prism,the ebb velocity,and the ability to export sediment,thus having the tendency to deteriorate the inlet’sstability.As tability analysis based on a closure curve methodology has shown that Xiaohai Inlet is in a state of dynamic equilibrium at present.