Numerical Simulation Of Effects On Hydrodynamic Environment Of Intensive Sea Use Projects In Laizhou Bay

Firstly, a large-scale three-dimensional baroclinic model of Bohai Sea was constructed with unstructed triangular mesh on the basis of considering the factors of air temperature, relative humidity, cloud cover, precipitation, evaporation, river discharge and so on. The model was calibrated by the validation of measured data of 12 tidal stations and 5 current stations, co-amplitude line and co-phase line of M2、 S2、 K1、 O1, spatial and temporal distribution characteristics of temperature and salinity. The results demonstrated that the model could reproduce the hydrodynamic and thermohaline characteristics of Bohai Sea well, so it could be used to provide hydrodynamic and thermohaline background field for the small-scale module of Laizhou Bay.Depending on the large-scale module of Bohai Sea, a coupling module of hydrodynamic and water quality module for Laizhou Bay was built and then the simulated salinity and water quality were mainly analysed. The results show that basic law of horizontal salinity distribution for Laizhou Bay in summer is increasing from west to east, from inner bay to outer bay, the Yellow River estuary and Xiao Qinghe estuary are a low value area, north and east of the bay are a high value area. Vertically, moat parts of the sea is uniform with a certain gradient exists just in estuary for the input of fresh water. In winter, horizontal salinity distribution pattern is consistent with summer’s, vertical distribution tends to be more uniform, but the overall salinity value becomes obviously higher due to the reduction of precipitation and river discharge. In addition, the analysis shows that the dominant factor of salinity for Laizhou Bay in summer is the Yellow River runoff, tidal influence is weaker. Simulation of water quality reproduced typical bimodal phenomenon of phytoplankton in Laizhou Bay in 2005 and the mechanism of this phenomenon is explained combining the change process of nitrogen and phosphorus nutrients. The nutrient composition structure of Laizhou Bay was analysed according to Redfield number. The overall nutrient level is under phosphorus limit, but nitrogen limitation still exists in local area. Through two numerical experiments, we studyed the impacts of the two most important pollution sources on water quality in the gulf, and explored the composition ratio of nutrients in the role of phytoplankton growth:Impact of inorganic phosphorus on phytoplankton growth is more than inorganic nitrogen, the increasing of inorganic nitrogen can also promote phytoplankton’s growth, which again implyed that phosphorus restriction is prominent overal, whereas nitrogen limitation still exists in local area; influence of Xiao Qinghe point source on nitrogen, phosphorus and phytoplankton level is weaker than the Yellow River point source.With the application of the model, the impact of four main intensive sea use projects on hydrodynamic environment in Laizhou Bay is predicted from five aspects of flow field, salinity field, tidal prism and tidal flux, water exchange capacity and nutrient composition in Laizhou Bay. Impact on the overall flow field of Laizhou Bay caused by the projects is limited, affecting mainly concentrated in the vicinity of every project. Due to the decreasing of high water level at flood slack time during spring tide after the project, the ability of saline’s tracing back to the top of the gulf is weakened and salinity contours has a certain fallback to some degree, which is obvious in the inner bay. At flood slack time, the extent of seawater’s fallback becomes smaller, indicating that the salinity exchange is slowing down. So the salinity isolines of inner bay fall back and size of low salinity area gets larger. Reducing of bay area for projects results in the decreasing of tidal prism and tidal flux of bay mouth section in a way. Advection-diffusion model of conservative substance suggests that half－life time is 45 days more than before, to a increasing degree of 26.9%, the water exchange capacity decreases significantly. This leads to the increasing of overall phosphorus level and the decreasing of nitrogen level, making the ratio of N/P lower and the role of phosphorus restriction weakened.