Geomorphological evolution in a medium macrotidal estuary across 88 years: shift from natural to human-influenced states

Abstract

Since the 1970s, human activities such as navigational projects, land reclamations, sand mining, and upstream damming have significantly impacted the geomorphology of Oujiang River Estuary (ORE). This study utilized bathymetric surveys, river discharge data, tide records, historical current velocity and suspended sediment concentration, and historical satellite imagery to investigate the geomorphological evolution of ORE over the past 88 years. The results reveal a distinctive five-phase evolution: a period of pronounced erosion (1931–1964), followed by pronounced deposition (1964–1979), minor erosion/deposition fluctuations (1979–2007), rapid erosion (2007–2014) and fast back-siltation (2014–2018). In its natural state before the 1970s, the ORE exhibited morphodynamics characterized by wandering, braided, and meandering channels interspersed with shoals. Meanwhile, its erosion and deposition pattern featured a dynamic equilibrium: high river discharge induced erosion, while dominant flood tides facilitated net upward sediment transport and deposition during low river discharge. However, post-1970s human activities disrupted this natural equilibrium and led to various geomorphological responses. Navigational projects stabilized shoals and channels, affecting local sedimentation. Dams reduced the frequency and peak of floods, thus reducing the potential for erosion. Land reclamations narrowed channels and reshaped the coastline. Sand mining and dredging for reclamation contributed significantly to erosion, especially during 1979–2014. The Empirical Orthogonal Function analysis revealed two primary morphodynamic patterns. The first mode indicates long-term continuous erosion in the channel and siltation over the tidal flat, identifying navigational projects and sand mining as the dominant causes of the main morphological changes. The second mode describes the transition from erosion to siltation, highlighting land reclamations and reservoir dams as the key factors driving this transition. Furthermore, human activities changed hydrodynamics and sediment transport, likely enhancing tidal pumping and strengthening longitudinal circulation. Consequently, the net up-estuary sediment transport had been intensified, ultimately resulting in fast back-siltation during 2014–2018. This insight is essential for sustainably managing medium-sized macrotidal estuaries, especially as they shift from natural to human-influenced states.