Morphodynamic Feedback Dominates Seasonal Sediment Dynamics in a Bore-Affected Macrotidal Estuary

Abstract

While the seasonal sediment signal in estuaries has been largely attributed to river-modulated advective processes, the potential dominance of internal morphodynamic feedback, particularly in systems with highly mobile beds, remains poorly quantified. Using an extensive data set comprising over 180 tidal cycles of suspended sediment concentration (SSC) measurements (April, July and November 2018) alongside concurrent hydrographic and bathymetric surveys, we investigated seasonal SSC variations in the upper Qiantang Estuary, a macrotidal system in China with active morphological changes. The results showed that the observed SSC exhibited distinct seasonal patterns superimposed on tidal fluctuations. SSC showed a strong positive correlation with tidal range under normal river discharge conditions. Summer SSC values exceeded those in spring and winter by 2–3 times. Three distinct seasonal regimes are driven by a morphodynamic feedback loop. This loop initiates as high river discharges erode the bed and amplifies tidal (and bore) energy, which subsequently drives intense sediment resuspension and rapid accumulation; which in turn leads to a low-energy adjustment phase, setting the stage for the next cycle. The system exhibited moderate flood-ebb SSC asymmetry under normal conditions but extreme flood dominance during tidal bore events. Furthermore, high discharges reduced SSC by ∼50% through dilution effects while simultaneously inducing bed erosion and tidal amplification, which subsequently elevates SSC in the following months. This study establishes that active morphological evolution is the fundamental driver of seasonal sediment dynamics, providing a new mechanistic framework for similar high-energy estuarine systems.