Morphodynamic response of a macro-tidal estuary to large-scale land reclamation

Abstract

Many estuaries worldwide have been significantly modified by land reclamation, influencing hydrodynamics and ultimately bed level changes. However, detailed observations documenting how hydrodynamic and morphological changes interact, especially revealing positive feedback mechanisms strengthening the original intervention, remain scarce. This study investigates the morphodynamic response in the macro-tidal, highly turbid Hangzhou Bay (China) over the past three decades to interventions, using unique datasets of annual bathymetry (1991–2019) and synchronous hydrography before and after intensive reclamation (1999, 2019). The analysis reveals severe accretion within the inner bay, with an average bed level rise of 1.21 m (total volume ∼ 2.2 × 10⁹ m³) since 1999, concurrent with a drastic 54.6% reduction in intertidal storage volume. Two key morphological parameters, the ratio of intertidal storage volume to channel volume (Vs/Vc) and the relative tidal amplitude (a/h) systematically decreased by 43% and increased by 6.5%, respectively, thereby enhancing flood dominance. All key hydrodynamic and sediment transport parameters, current velocity, suspended sediment concentration (SSC), tidal volume and sediment flux, showed strong correlations with the tidal range at the bay mouth. Despite a ∼ 30% reduction in mean current velocity and tidal volume from 1999 to 2019, SSC paradoxically increased by 47–133%, and sediment flux rose by ∼44%. The changes are driven by a positive feedback mechanism: the loss of accommodation space and increasing flood dominance promote sediment import and accelerated deposition, which in turn further reduces channel volume and reinforces the tidal asymmetry. This work reveals a key feedback loop accelerating the transition of macro-tidal estuaries towards flood dominant, rapidly infilling and highly turbid systems in response to land reclamation. Such knowledge is key to sustainable sediment management in turbid macrotidal estuaries.