Redai dili (Mar 2021)
Water Surface Profile Dynamics and Underlying Mechanism of Transverse Channel in Pearl River Channel Networks: A Case Study of the Dongping Channel
Abstract
Since the 1980s, the Pearl River Delta has undergone rapid development on economic and social fronts. Owing to the impacts of large-scale and intensive human activities, the river discharge and sediment loads entering the Pearl River channel networks along with its morphology have changed drastically, leading to substantial alterations in spatial-temporal tide-river dynamics. Accordingly, the estuarine water surface profile has also experienced stepwise evolution. However, little attention has been paid to the mechanisms underlying water-level profile dynamics. In this study, the characteristics of and factors influencing the spatiotemporal evolution of the water surface profiles and residual water level curvature in the Dongping Channel were systematically explored based on the abrupt change in the Sanshui and Makou hydrological stations' flow diversion ratio in 1993, with the Dongping Channel considered as a case study. The obtained results are useful for setting scientific guidelines for evaluating morphological changes, adjusting flood control strategies, preventing saltwater intrusion, and constructing large-scale water conservancy projects. First, according to the abrupt change in the Sanshui and Makou hydrological stations' diversion ratio at the apex of the Pearl River channel networks, the evolution of the surface water profile dynamics in the Dongping Channel was divided into two phases: the pre-human period (1960-1992) and the post-human period (1993-2016). Subsequently, the dependence of the residual water level slope or tidal damping rate on discharge before and after the abrupt change in diversion ratio was studied using the double accumulation curve method. The residual water level curvature before and after human intervention was used to understand the dynamics of the residual water level slope and the resulting water surface profiles; in particular, the larger the curvature, the faster was the change in the residual water level. Meanwhile, the change in curvature from positive to negative (or vice versa) indicated a change in the shape from concave to convex (or vice versa). The results showed that: 1) The slope of the double cumulative curve of the residual water level slope or the tidal damping rate with regard to the river discharge decreased, indicating that the dependence of the residual water level or the tidal damping rate on the river discharge decreased; 2) the water surface slope of the Dongping Channel decreased as a whole, and the water surface slope upper reaches (Zidong-Sanshui reach) decreased the most. The rate of change of the gradient among the four seasons decreased by 4% to 67%. Meanwhile, the curvature fluctuation amplitude decreased, ranging from 2×10-10 to 5.48×10-10 before 1993, and from -0.9×10-10 to -0.07×10-10 after 1993; and 3) the main reason for the substantial change in the water surface profile was that sand excavation occurred in the middle and upper reaches of the Dongping Channel, leading to a substantial deepening of the river bed, a decrease in the water level, and an increase in water discharge. The lower reaches were mainly affected by tidal flat reclamation, resulting in channel deposition. Owing to the geometric changes, both the river discharge and tidal discharge increased in the middle reaches, leading to conversion of the water surface profile into an upward convex shape. Meanwhile, owing to the regulations for the Feilaixia Reservoir in the upper reaches of the Beijiang River and the seasonal regulation for cross-sectional width-depth ratio and sea-level change, the variation in the water surface curvature of the Dongping Channel in winter was much more significant than that in summer.
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