Numerical Study On The Tidal Prism And Water Exchange Ability Of Qinzhou Bay And Maowei Sea

The Qinzhou Bay and Maowei Sea are major bays along coastal area of southernChina, playing an important role in the economic development of the Beibu Gulf Rimarea and the trade between China and the ASEAN. Besides, there is a largedistribution in the Bay mangroves, with a high value of fisheries and ecologicalresources. However, serious siltation and atrophy of the sea area, water qualitydeclining and other serious problems appear because of the disorderly developmentactivities in recent years, which has adversely affected the local economic and socialdevelopment. To solve this problem and take more advantage of Qinzhou Bay andMaowei Sea for social development, the Qinzhou government proposed acomprehensive treatment plan on Maowei Sea in2011. In this context, we conduct acomprehensive simulation study on the tidal prism and water exchange ability ofQinzhou Bay and Maowei Sea, expecting to provide a theoretical reference forimproving the ecological environment quality and more rational exploitation ofQinzhou Bay and Maowei Sea.Combining the comprehensive treatment plan on Maowei Sea, a two-dimensionaltidal current model of Qinzhou Bay and Maowei Sea is established to study the tidalcurrent characteristics and tidal prisms of this area. On this basis, a conservativesubstance transport-diffusion model is coupled with the tidal model to simulate thewater exchange of the two seas before and after the comprehensive treatment plan,with conditions of with and without river discharge.1. A two-dimensional tidal current model of Qinzhou Bay and Maowei Sea isestablished with MIKE21FM, based on the sea chart and measured data of coastlinesand water depths. It is concluded that:(1)This area is controlled by irregular diurnal tidal current. There are somecharacteristics of semidiurnal current in about two days of the neap tide period, while diurnal tidal current characteristics are presented in other days. It is found that thetidal range of this bay can be up to5.61m.(2)The tidal current in Maowei Sea and the neck of Qinzhou Bay are reversingcurrent with a direction of NNW-SSE, while the current in southern Qinzhou Bay isrotary current. At the time of maximum flood, the current velocity in the area fromthe shipping lane in middle Maowei Sea to Longmen, neck of Qinzhou Bay are higherthan other parts. The maximum velocity is1.15m/s, while the mean velocity inMaowei Sea and Qinzhou Bay are0.32m/s and0.35m/s respectively. The currentdirection at the time of maximum ebb are opposite with that of maximum flood, whilethe maximum current velocity reaches1.57m/s. The mean velocity of ebb momentin Maowei Sea and Qinzhou Bay are0.40m/s and0.46m/s respectively, which aresignificantly higher than that of flood moment.(3)After the comprehensive treatment plan, maximum current velocity in most partsof Maowei Sea is decreased, while that in neck of Qinzhou Bay is incresed. AtLongmen, the maximum flood and ebb velocity can reach1.50m/s and1.78m/s. Themean flood velocity in Maowei Sea and Qinzhou Bay are0.31m/s and0.35m/srespectively, while the mean ebb velocity are0.35m/s and0.42m/s respectively. Thetidal current direction is much close to that before the plan.2. Based on the results of tidal current model, the tidal prism of Qinzhou bay andMaowei Sea, which are defined with mouth lines of21.75°N and Qisha-Wulei, arecalculated. The results show that, for Qinzhou Bay, the maximum and minimumtidal prism are17.1×108m3and1.89×108m3respectively, with an average of9.20×108m3; while for Maowei Sea, the maximum and minimum tidal prism are5.5×108m3and0.62×108m3respectively, averages2.96×108m3.After the comprehensive treatment plan, the maximum and minimum tidal prism ofQinzhou Bay are17.1×108m3and1.89×108m3respectively, with an average of9.20×108m3; while for Maowei Sea, the maximum and minimum tidal prism are5.5×108m3and0.62×108m3respectively, averaging2.96×108m3. The tidal prism ofQinzhou Bay and Maowei Sea are significantly increased by the comprehensive treatment plan.3. On basis of the tidal current model, a conservative substance transport-diffusionmodel is coupled to simulate the water exchange of the two seas, with conditions ofwith and without river discharge. After that, the half-time and water exchangeabilities of the two seas before and after the comprehensive treatment plan aresimulated contrasted.(1) Before the comprehensive treatment plan, the half-life time of Qinzhou Bay andMaowei Sea with the condition of river discharge are15.5days and7.7daysrespectively; while without river discharge, the half-life time of Qinzhou Bay andMaowei Sea are60.8days and45.4days respectively.(2) After the comprehensive treatment plan, with the condition of river discharge,the half-life time of Qinzhou Bay and Maowei Sea are20.6days and13.6daysrespectively; while without river discharge, the half-life time of Qinzhou Bay andMaowei Sea are71.7days and56.7days respectively.(3) It is shown that, the water exchange ability is stronger if the current velocity ofthe area is higher, or the area is near bay mouth or estuaries. With the condition ofriver discharge, the strongest areas of water exchange ability are located at the estuaryof Maolingjiang River and Qinjiang River in top of the sea and the area near themouth of the Qinzhou Bay, while the weakest areas are located at Jingujiang Riverand west side of Longmen. For Maowei Sea, the weakest areas are located at thesoutheast part near the coast.The results indicates that although the comprehensive treatment plan increases thetidal prism of Qinzhou Bay and Maowei Sea, it also has some negative influences.As the project increases the volume of water in the sea, the plan implementation hassome adversely affect for the transport and diffusion of pollutants in the bay, in caseof the pollutant concentration remain unchanged.