Investigating Salt Intrusion Characteristics Considering Changesin River Beds, Water Intake from Rivers, and Dam Supply in theSeomjin River using a Numerical Model

Abstract

Saltwater intrusion in river estuaries poses significant challenges for water quality management and ecosystem sustainability. In this study, we investigate the characteristics of saltwater intrusion in the Seomjin River by constructing an Environmental Fluid Dynamics Code (EFDC) model for the Seomjin River and Gwangyang Bay, incorporating environmental changes and basic data.  After validating the model through calibration and verification processes, we conduct numerical experiments to explore 28 scenarios for the Songjeong flow rate and Daap intake rate. We begin by constructing and validating the numerical model using historical data of river discharge, tidal levels, and salinity measurements. The main income of this area is collecting a corbicula from the river. However, since the 1970s, many projects such as dam construction, river aggregate collection, and Gwangyang Bay reclamation have been carried out, and now the fishery is suffering salt damage. To understand the long-term trends and seasonal variability of salt intrusion, we analyze historical datasets spanning multiple years. This analysis helps identify potential shifts in salt intrusion patterns over time, which could be attributed to natural variations or anthropogenic influences. We analyze salinity concentrations at four key points (Dugog, Sinbi, Mogdo, and Hwamog) during the entire period, spring, and neap periods. The spatial plane and stratified distribution of salinity are examined, and a salinity and flux model is developed. The longitudinal distribution of saltwater intrusion from the estuary is analyzed, and a salinity and saltwater intrusion distance model is constructed. Our findings reveal that the changes in salinity concentration range from 4.7 psu for Dugog to 28.2 psu for Hwamog at different Songjeong flow rates. In the spring period, salinity  changes increase, but average concentrations decrease, while in the neap period, salinity changes decrease, but average concentrations increase. Salinity stratification is observed in Sinbi, Mogdo, and Hwamog during the neap period due to significantly increased salt concentrations. Additionally, the effect of the Daap intake rate on salt concentration is found to be small, with a salinity difference of less than 1 psu. Spatially, the maximum salinity concentration decreases as the Songjeong flow rate increases, and the influence of the Songjeong flow rate is more pronounced in the spring period compared to the neap period. Furthermore, we construct quantitative prediction models for salinity reduction scenarios at different points, determining the instream flow required to achieve target salinity concentrations. The study indicates that Hwamog requires a Songjeong flow rate of 100 cms or more to achieve 20 psu during the neap period.In conclusion, this research sheds light on the complex interactions governing salt intrusion in the Seomjin River, facilitating informed decision-making for water resource management and environmental conservation. The integration of the EFDC model with deep learning techniques offers a comprehensive understanding of saltwater intrusion dynamics and contributes to informed decision-making for water quality enhancement inestuarine ecosystems