Numerical Study On The Effect Of Regulation On Hydrodynamic Environment In Xincun Lagoon

Xincun Lagoon is located in Lingshui County in the southeast of Hainan Island,which is adjacent to the South China Sea in the east and Sanya City in the south.It is a typical semi-closed lagoon with sand bar at entrance.Recent years,with the gradual increase of marine exploitation activities,Xincun Lagoon has faced problems such as the lake area shrinking,hydrodynamics weakening,ecosystem degradation,and water pollution.It is necessary to remediate the lagoon.Since most of the substances in lagoon are transported and diffused by sea water,their biochemical process and movement process are closely related to the hydrodynamic conditions of lagoon.In order to adapt the lagoon 's hydrodynamic environment to development needs,it is a common practice to renovate the lagoon through dredging the entrance and the part inside.Although these ways aim to maintain and improve the hydrodynamic environment of lagoon in order to facilitate the sustainable development,artificial projects may cause huge changes in the physical parameters.Therefore,it is of great significance to discuss the impact of remediation on the lagoon hydrodynamic environment.This article mainly focuses on the lagoon hydrodynamic environment from three aspects: tidal current field,water exchange capacity,and residual current field.Based on the MIKE model,a hydrodynamic numerical model of Xincun Lagoon and nearby waters was established in this paper.The hydrodynamic module was coupled with the material transport model(convection-diffusion model)and particle tracking model.According to the model simulation results,the hydrodynamic environment of Xincun Lagoon was analyzed.On the basis of the characteristics of the hydrodynamic environment of Xincun Lagoon,this paper first put forward a basic remediation plan based on dredging projects of water channels in ebb delta.Then numerical models of Xincun Lagoon under the remediation plan were established,whose results were used for analyzing hydrodynamic environment.After that,the typical project which were beneficial to the exchange capacity were selected.By changing the dredging depth,increasing the internal dredging area and comprehensively considering both of them,the impact of different dredging projects on the hydrodynamic environment was discussed.On the whole,the quarterly average water level has little effect on the exchange capacity of Xincun Lagoon,while the tidal range and the change of the tidal range over time have agreater impact.On the time scale,Xincun Lagoon shows stronger water exchange capacity in winter and summer;on the spatial scale,the water exchange capacity is unevenly distributed in a fan shape generally,and the closer the distance from the entrance is,the stronger the exchange capacity becomes.There is no obvious change in the tidal current field inside after dredging the entrance,however,different dredging projects of entrance lead to different current fields outside.The diversion effect is obvious when there are multiple ebb channels,where the current velocity in the channel is smaller than the one of single ebb channel,and from the perspective of water exchange,the single tide channel is more favorable.Renovation in entrance has limited changes to the water exchange capacity of the area away from the area of entrance,and the dredging depth of the ebb tidal channel outside the entrance has little effect on the exchange of water in the Xincun Lagoon as well.Thus,if merely considering dredging the entrance and tidal channel,it is recommended to design dredging plan with a single channel which is close to the original state.Dredging internal parts of Xincun lagoon has a limited improvement in the overall water exchange capacity,but it can affect the spatial distribution of the water exchange capacity.Since the influence on the spatial distribution of water exchange capacity cannot be simply judged,it needs to be judged according to the specific situation.