Study on the tidal variability related to flooding and hydroelectric operations in the Qiantang river estuary

Abstract

The Qiantang River estuary, located in southeastern China, displays abnormally large tidal ranges due to its funnel shape. Although numerous studies have discussed river-tide interplay in the Qiantang River estuary, these studies usually treat tidal waves with dramatically different periods as a whole and use ZhaKou as the upstream boundary. To date, the area upstream from ZhaKou to the Fuchun River Power Station (FRPS) remains unexplored, which motives the current study. This research systematically explores the spatiotemporal variations of major tidal constituents in the Qiantang River estuary using classical harmonic analysis (T_TIDE), non-stationary harmonic analysis (NS_TIDE), and variational mode decomposition (VMD). The hindcast of NS_TIDE explains 72.9%–93.9% of original signal variances, better than the T_TIDE. However, the improvement is less from ZhaKou to CangQian, with the hindcast explaining less than 80% of the original signal variance, possibly due to the assumption of a static riverbed topography in NS_TIDE misaligned with the Qiantang River's conditions. Long-period constituents such as MSf, Mf, and Mm have large amplitudes near FRPS. Furthermore, diurnal S1 and P1 tides show notable amplitude increases at TongLu due to hydroelectric operations. The VMD method successfully captured the seasonal changes of tidal parameters caused by nonlinear tide-river-morphology interplay. The amplitudes of semi-diurnal and quarter-diurnal tides increase significantly between July and September due to decreased bottom friction caused by runoff erosion; thereafter, they gradually decrease due to increased bottom friction induced by sediment accumulation. This study has implications for similar fluvial estuaries worldwide with significant variations in water depth.