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River discharge, tide, wind, topography and other factors all have great impacts on the saltwater intrusion of Modaomen Waterway (MW), a major outlet of the Pearl River Estuary. A coupled 1D-3D numerical model was applied in this study to account for the dynamic characteristics of saltwater intrusion in the MW, and the impacts of tide and river discharge on the length of saltwater intrusion were uncovered. Results are as the followings: (1) River discharge from upstream induces an obvious dilution of salinity along the MW, whereas tide can exert a positive force that pushes salt water landward. The effects of river discharge and tide on the length of saltwater intrusion can be well described by a regression function; (2) the saltwater intrusion along the MW is generally aggravated by increases in tidal range from the South China Sea. The length of saltwater intrusion usually reaches a maximum 2 or 3 days before spring tide, and the hourly length of saltwater intrusion along the MW usually slows the tidal process for approximately 4 h, which can provide important information that the pumping operation along the MW to store freshwater in the backup storages needs to be at least 3 days ahead of the spring tide so as to avoid serious impact from saltwater intrusion; (3) the length of saltwater intrusion generally decreases with increasing river discharge. In 2005, 2009 and 2010, the average river discharge from upstream was 2680, 2630 and 3160 m/s, respectively, with corresponding average lengths of saltwater intrusion of 32.7, 42.3 and 21.4 km. The inverse correlation between the water flow and the length of saltwater intrusion may provide some guidance for operations to maintain enough upstream flow to dilute the salinity and therefore satisfy the domestic water supply. [ABSTRACT FROM AUTHOR]

Abstract: The Modaomen Waterway (MW), a major outlet of the Pearl River Estuary, is a crucial source of water supply for Macao and Zhuhai City in Southern China. This waterway is frequently affected by saltwater intrusion from the South Sea, which has caused serious water shortage in recent years. For the planning and management of water supply at this waterway it is necessary to understand the characteristics of saltwater intrusion particularly in the low flow period of the year. However, as a result of multiple impacts from tide, river discharge, wind, topography, channel bathometry, and some other factors, the saltwater intrusion into this waterway is highly non-linear. In order to account for the non-linear characteristics, the wavelet method was used to analyze the period characteristics of tidal range, river discharge, and salinity and the impact of tidal range and river discharge on the salinity. Results show that the tidal range at Sanzao station and the salinity at Pinggang station both exhibit a stable 14.8-day period and the salinity in the MW is largely impacted by the tidal range during low stream flow periods. River flows from upstream of the waterway don't show any stable period characteristic, it, however, is critical in mitigating the salinity for water supply during saltwater intrusion. A minimum total flow of 2 500 m3/s from upstream at Sanshui and Makou stations has been practiced as a threshold for safe water supply. This study has found that the percent time when salinity is below the maximum permissible salinity of 250 ml/g for water supply increases as the river discharge from upstream increases and up to 50–70% of the time the salinity at water supply pumping station is below 250 mg/l when the upstream flow is greater than 2 500 m3/s. Planning and management of water supply operation at the MW is politically, economically and mostly importantly technically challenging, findings from this study can provide some guidance for operation and further research. For example, the phase lag of average salinity to tidal range was quantified to be 3.4–3.8 days. This phase lag time would suggest that the pumping operation to store freshwater in the backup storages needs to be at least 4–5 days ahead of the spring tide so as to avoid serious impact from saltwater intrusion. Safer water supply strategies can be developed by scheduling reservoir operations to maintain the threshold stream flow and the pumping. [Copyright &y& Elsevier]