Modeling assessment of a saltwater intrusion and a transport time scale response to sea-level rise in a tidal estuary

This paper documents a modeling investigation to comprehend the effect of future sea-level rise (SLR) on estuarine salinity and transport time scales, including the residence time and the water age of dissolved substances in a partially mixed estuary. A three-dimensional semi-implicit Eulerian–Lagrangian finite-element model was established and applied to the Tamsui River estuarine system and the adjacent coastal sea in northern Taiwan. The modeling results indicated reasonable agreement with the observed water levels, tidal currents, and salinity. The model was then applied to calculate the salt intrusion, residence time, and water age between the baseline (without SLR) and different scenarios, including SLRs of 0.34, 1.05, and 1.40 m for the year 2100. The numerical model results reveal that the average salt content and salt intrusion length will increase as the sea level rises. The 1 psu isohaline moves toward upstream reaches with an increase in SLR. The results reveal that the maximum increment of tidal-averaged and depth-averaged salinity would be 1.1, 2.4, and 3.0 psu, respectively, for the SLRs of 0.34, 1.05 and 1.40 m at the middle estuary under mean flow conditions. The regression between salt intrusion length and freshwater discharge are established corresponding to different SLR scenarios. The residence time of the entire Tamsui River system would increase from 6.3 to 23 % compared to the baseline under low flow conditions. The concentration of dissolved substances would have a longer transport time from upstream to downstream because water volume increases with SLR. This indicates that the water age will increase in the main Tamsui River estuary as the sea level rises.