Seasonal changes in river-tide dynamics in a highly human-modified estuary: Modaomen Estuary case study

Many estuaries worldwide are subject to significant human interventions, such as dam construction, channel deepening, and land reclamation. However, the impact of these interventions on the spatiotemporal river-fide dynamics is insufficiently understood. In this study, we explored the seasonal changes in river-fide dynamics in the Modaomen Estuary of the Pearl River, where a significant increase in tidal range by approximately 0.14 m on average was observed after substantive channel deepening mainly due to human-induced sand excavations. Using the double mass curve approach with regard to the river discharge and the absolute value of tidal damping rate (or residual water level slope), we identified three stepwise phases (i.e. 1960-1990 representing the prehuman intervention period, 1991-2000 representing the transitional period, and 2001-2016 representing the post-human intervention period) corresponding to high to low residual water level slopes, and qualitatively matched the river-fide dynamics shift observed in the Modaomen Estuary. It was shown that the increase in tidal range was primarily due to the reduction in residual water level slope (and thus the weakening of the tidal damping effect) caused by the channel deepening along the Modaomen Estuary (with length of 124 km) where the residual water level slope was decreased by 46% from the pre-human period (1960-1990) to the post-human period (2001-2016), while the absolute value of the tidal damping rate was decreased by 32% due to the overall impacts from human interventions. Based on the regression coefficients obtained from the double mass curve approach, we showed that the river-fide dynamics in terms of tidal damping rate and residual water level slope were more sensitive to human interventions during the dry season than during the wet season, owing to the combined impacts of riverbed deepening and freshwater regulation by dam's operation. The underlying mechanism could be primarily attributed to the considerable seasonal variation in the lateral storage area (such as tidal flats or salt marshes). The results also showed that larger freshwater discharge is required to alter the critical value of maximum tidal damping (indicating the strongest damping effect along the channel) owing to the increase in tidal range due to substantial riverbed deepening and hence significant increase in channel storage. The method proposed in this study to quantify the overall impacts of human interventions on seasonal changes in river-fide dynamics is particularly useful for setting scientific guidelines for sustainable water management and conservation planning for engineering works in the Modaomen Estuary and other estuaries with significant human interventions.