Numerical Study On Characteristics Of Water Exchange And Sediment Transport In Bohai Bay

Frequent human activities at coastal regions inevitably impact the ecologicalenvironment which result in coastal habitat degradation and loss of ecologicalfunctions situation. In order to protect and improve the ecological environment ofcoastal regions, the understandings of contaminants movement in the water and theself-purification capacity of water bodies are necessary. Since sediment (especiallyfine sediment) is a carrier of many pollutants, the research of sediment transportdiffusion for understanding the local mass transfer diffusion is of great significance.Depth-integrated two-dimensional current model is established in the first placewith analytical verifications on each term. Meanwhile, the current model is validatedthrough experimental results and calibrated through Bohai Sea measured data. Theresults show that the model is reliable when applied to Bohai Sea region.Secondly, the Lagrangian particle tracking method is established to simulate themovement of water mass particles in Bohai Bay. According to the numerical results,the whole and partial water exchange characteristics of Bohai Bay are evaluated withwater exchange time scale as an indicator. The water exchange capacity of the BohaiBay and the characteristics of the residual flow field are combined to illustrate theinherent characters of water exchange mechanism. Results show that the flushing timeof Bohai Bay is about2.41~2.54years and the water exchange capability is weak.Water exchange capacity has a strong correlation with local residual flow. Moreover,It turns when apply the return coefficient to large bay with weak water exchangecapability, it is inadequate to evaluate the contribution of each local water bodies tothe entire area by using return coefficient only. This is due to the one-to-multi casewhere residence time and exposure time are also needed for comprehensive analysis.Thirdly, Bohai Sea wave model is calibrated and validated through measured data,which is established based on SWAN model. The modified ECMWF wind field databased on Bohai Sea measured data is applicable in the Bohai Sea region. The windgrowth coefficient and bottom friction coefficient in the wave model SWAN need tobe calibrated and validated in response to this study area. The results show that withmodified ECMWF wind driven wave model SWAN after calibration, the outputresults are in good agreements with measured data either in different water depthsarea or different time scales and time range. Finally, suspended sediment transport model is applied to the Bohai Bay regionunder the action of waves and currents. Satisfactory results show that the modelvalidated through measured data is reliable. Furtherly, seasonal changes of suspendedsediment transport and the erosion or deposition variations are simulated and analyzed.The results show that winter sediment concentration in Bohai Bay is larger thansummer’s since that strong wind offen occurs in winter. Winter wave in the Bohai Bayplay a bigger role on sediment than summer wave due to the bottom shear stressincreases during winter time. When the wave height increases significantly during thewind season, sediment concentration shows little overall change. After the strongwind, the wave height quickly restored to the level before the advent, but sedimentconcentration remained at the level of transit. Compared with the currents effect,wave effect is confined in nearshore regions mostly.