Numerical Study On Dynamic Mechanism Of Maintaining The Deep Tidal Channel And Its Evolution Trend In The Lianzhou Bay

As a natural waterway of Beihai Harbour, the deep tidal channel in the Lianzhou Bay is theresult of dynamic balance in marine hydrodynamic field. It is not only an important channelconnecting the water inside to that outside the bay, but also the natural barrier which can protectthe water of Beihai Silver Beach National Tourism and Holiday Resort from being polluted.Comparing the historical chart depth, it can been seen that in overall, the underwater bank in theLianzhou Bay has been moving toward the sea, however, the depth of the tidal channel keepsconstant basically. Based on numerical simulation and the observational data, the dynamicmechanism that the depth of the tidal channel in the Lianzhou Bay keeps constant was analyzedin this study. In addition, the evolution trend of the tidal channel was also discussed. The resultscan provide scientific theoretical basis and guidance for rational utilization of marine resourcesin the Lianzhou Bay, and protection of the deep water channel and tourism resources in thesouth bank of the peninsula.First, a hydrodynamic model was applied to simulate the tide and tidal current in theLianzhou Bay based on Estuarine and Coastal Ocean Mode（lECOM）. The simulation results oftidal currents and elevation agreed quite well with measured data, indicating that thehydrodynamic filed provided by the model is reliable. Second, considering different runoffconditions in dry, normal and rainy seasons of the Nanliu River and spring-neap tide variation,the water age of the Nanliu River was calculated based on the Constituent-oriented Age andResidence time Theroy（CART）. Spatial and temporal distribution characteristics of the waterage in the Lianzhou Bay were obtained, aiming at discussions about the material transportcharacteristics of the Nanliu River. At the same time, particle tracking method was applied toobtain the track of water particle from the Nanliu River after entering the Lianzhou Bay.According to results of particle tracking and water age, transport and distribution characteristicsof freshwater and sediment from the Nanliu River in the Lianzhou Bay were also studied. Thenresearches were carried on sediment transport, erosion and deposition at the bottom of theLianzhou Bay, in order to analyze the evolution trend of the deep tidal channel in natural state.Finally, based on the marine function zoning of the Lianzhou Bay and actual demand, takingpossible land reclamation engineering into account, hydrodynamics field after land reclamation was simulated to make a forecast for the evolution trend of the deep tidal channel influenced byhuman activities, through contrast of the hydrodynamics fields and sediment transport beforeand after land reclamation.In terms of material transport, temporal and spatial distribution differences of freshwatertransport have an import impact on maintaining the deep tidal channel. The result shows that,from the viewpoint of time, it takes about10days for the freshwater to be transported westwardsoutside the bay from the mouth of the Nanliu River, while more than45days to Guantoulinginstead. During this period, most of sediment carried by freshwater has settled. From theviewpoint of space, the material carried by the Nanliu River are transported more to the NanliuRiver Delta, west and southwest of the Lianzhou Bay than the deep channel, which is beneficialto maintenance of the deep channel. Besides, the result of particle tracking agrees with that ofcalculation of water age, which shows that sediment carried by the Nanliu River mostly aretransported towards the west and southwest of the bay, instead of towards the deep channel andpiling up there. This is beneficial to maintain the deep tidal channel.In terms of erosion and deposition at the bottom, in the area of the deep tidal channel, themaximum shear stress in the tidal cycle is much larger than others. The deep channel is beingscoured and it is difficult for sediment to deposit. This is the main reason why the deep tidalchannel keeps the depth constant. The result shows that the deep tidal channel in the LianzhouBay will be eroded by0.2-0.4cm every year.The land reclamation engineering leads to significant change in the area close to theengineering, resulting in local scour near the island. However, if it is in rational design, theexistence of artificial islands does not change the state of erosion in the deep tidal channel, butweaken the level of erosion slightly.