Lake Shinji and Lake Nakaumi are coupled brackish lakes on the Japan Sea coast of Japan. Seawater from the Japan Sea frequently intrudes into Lake Nakaumi through the bottom. Salinity flux at Nakaura Watergate was analysed. Effects of river discharge, tide level, and meteorological forcing variables makes the seawater intrusion non-linear. To account for the non-linear characteristics, partial wavelet coherence (PWC) was used to analyse the impact of river discharge, tide level, and meteorological forcing variables on salinity flux after controlling the effect of all other forcing variables. PWC results show that tide level has impact on salinity flux over short cycles (limited to less than 4 days) throughout the analysis period with an average time lag of about 2 days. This suggests that astronomical tides are the dominant factor in the salinity intrusion processes. The river discharge largely influences salinity flux over medium cycles (5-16 days) during the months of June and July with an average time lag of 0.3 days. Atmospheric pressure occasional affect tide level, indirectly driving salinity transport. The study findings provide guidance for factors affecting salinity intrusion processes and further research.

Lakes Shinji and Nakaumi form a coupled brackish lake system on the Japan Sea coast of Japan, and seawater frequently intrudes into these lakes. The study analyzed the influence of river discharge, tide level, and meteorological forcing on seawater flux at Nakaura Watergate. Continuous wavelet transform (CWT), wavelet coherence (WTC), crosswavelet transform (XWT), and partial wavelet coherence (PWC) were used to analyze seawater intrusion. CWT reveals the characteristics of seawater flux, river discharge, tide level, and meteorological forcing. WTC and XWT showed the correlation between forcing variables and seawater flux. PWC analyzed the impact of river discharge, tide level, and meteorological forcing variables on seawater flux after controlling the effect of all other forcing variables. The results showed that tide level has an impact on seawater flux over 0.5-day and 1-day cycle throughout the analysis period, suggesting that astronomical tides play an important role in the salinity intrusion processes. The river discharge characteristics and its influence on seawater flux were clearly revealed especially during high river flows. Sometimes, atmospheric pressure and wind velocity affect tide level, thereby driving salinity transport. The study reveals the power of wavelet analysis in examining nonlinear time series such as salinity intrusion processes.

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2 正会員 大成建設株式会社 技術センター(〒245-0051 神奈川県横浜市戸塚区名瀬町344-1) 3 フェロー会員 大成建設株式会社 技術センター(〒245-0051 神奈川県横浜市戸塚区名瀬町344-1)4 港湾空港技術研究所 沿岸環境研究グループ(〒239-0826 神奈川県横須賀市長瀬3-1-1)

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