Morphodynamic Resilience of the Tide-Dominated Estuary With Interference From Tidal Flat Reclamations

Most tide-dominated estuaries worldwide are among the morphodynamic shrinking types heavily disturbed by anthropogenic interventions like embankments and reclamations. A typical example is North Branch (NB), the northern first-order distributary of Yangtze (Changjiang) Estuary, showed a rapid alteration from river-dominated to tide-dominated character in less than 50 years after intensive reclamations, resulting in serious morphodynamic shrinkage, thus attracted great concern of its evolutionary fate. Here, we explored this issue based on historical bathymetry data and hydro-morphodynamic modeling to examine the morphology-hydrology interactions and the suggested equilibrium re-adjustment mechanism after intensive reclamations. Results demonstrated a mechanism of geomorphological resilience for the tide-dominated estuary, evolving from preliminary estuarine shrinking to dynamic equilibrium after reclamations. Although reclamation could destabilize the system, leading to temporary positive feedback between tidal pumping and morphology shrinking, driving the estuary vanishing quickly in a short time. A substantial loss in the intertidal areas, changing the estuary geometry from a highly-curved bank to a highly-constrained channel, weakened flood tidal asymmetry of NB, which in turn altered sediment dynamics from deposition to erosion. Such negative feedback process tends to drive the estuary toward a new dynamic equilibrium morphology. The discovered nonlinear feedback mechanism is supported by the evidence of system efficiency adjustment, which was substantially gained during the first 30 years, then stayed in a low entropy production and minimum work state, indicating a more efficient dynamic equilibrium. If further estuary preservation policies are implemented, the dynamic equilibrium of NB is expected to continue for a long time in the future.