Research Of Water Quality Assessment And Improvement Strategy In Semi-enclosed Bay

Semi-closed bay is the key area where gulf red tides occur in China,so it is of great significance to improve its water quality.However,the water quality of semi-closed bays is affected by various factors such as inlet pollution source,self-purification capacity and tidal dynamics,so there is a lack of research on the evaluation and improvement of water quality of semi-closed bays.The existing marine water quality assessment methods are often used in open bays with strong hydraulic exchange,but when they are applied to semi-closed bays,there are some problems,such as weak pertinence in data selection,weak orientation in source identification,limited monitoring frequency and limited monitoring points.In this paper,a semi-closed bay in the south of S bay is taken as the research object.By optimizing the existing water quality evaluation methods and constructing the water quality model of the semi-closed bay EFDC(The Environmental Fluid Dynamics Code),The optimization of data selection,targeted identification and characteristic analysis of inlet pollution sources,and research on the hydrodynamic force and water quality distribution in the bay area are realized.Combined with EFDC model,the measures to improve the water quality in the bay area were proposed,and the expected environmental benefits were analyzed,so as to realize the research from water quality evaluation to the water quality improvement of the semi-closed bay.Optimizing the existing marine water quality assessment methods and applying them to S bay.Based on the characteristics of semi-closed bay,a three-level index collection system was established for data collection.The results showed that the water quality of S bay was unevenly distributed and fluctuated greatly at different water stages and different sites.For example,The water quality at the outlet is superior to that in the middle,Water quality in low water period is better than that in normal water period,and water quality in normal water period is better than that in high water period.The overall water quality of the bay area needs to be improved,and the potential exceeding indicators of the bay area are inorganic nitrogen and active phosphate.Based on this,the indexes of monitoring and characteristic analysis of pollution sources flow into gulf are nitrogen and phosphorus.The identification results of rainfall runoff pollution in The results showed that the concentration of pollutants was the highest in residential areas and the lowest in public management areas.The days of semi-saturation and saturation drought were 3 and 5 days respectively,and the rainfall scour ability was positively correlated with the intensity.The hydrodynamic model and water quality model of semi-enclosed bay EFDC were constructed and applied.Identification results based on the pollution into the bay for model building,the results showed that the established EFDC hydrodynamic model of nash coefficient over 0.8,the water quality model of nash coefficient over 0.7,the hydrodynamic model and water quality model precision is good.Based on the hydrodynamic model and water quality model with good precision,the hydrodynamic distribution and water quality distribution in the bay area were studied.The results showed that the bay water quality is related to water power,In other words,the point that has the better hydrodynamic has better water quality.A water quality improvement scheme and environmental benefit assessment model for semi-enclosed bay were established and applied to S bay.Based on the EFDC model,the key contribution form that have a greater impact on the potential exceeding indicators in the bay area are identified,and the scheme is constructed based on the improvement of the key contribution form.The results showed that key contribution forms are the C water purification plant and the rainfall runoff pollution into the bay.The EFDC model was used to evaluate the environmental benefits of the water quality improvement scheme.The results showed that the scheme with the sponge assembly facility and the water plant marking modification scheme had the highest expected environmental benefits.