Cellular automata to understand the prograding limit of deltaic tidal flat

Deltaic tidal flat is an important ecosystem that supports the livelihoods of millions of people globally. Due to the complexity and stochastic nature of tidal flat development, it is a challenging task to predict the tidal flat prograding limit accurately. In this study, we fill out this gap from the perspective of numerical simulation by focusing on the tidal flat extension limit of Nanhui Shoal (NHS), the largest marginal shoal of the Changjiang Estuary. Our results suggest that the tidal flat of NHS extended 29.16 m/y seaward in 1989–2002, had a rapid progradation of 158.08 m/y in 2002–2012 and a retreat rate of 26.88 m/y in 2012–2019, while the fluvial sediment supply decreased 56.78% from 1989–2002 to 2003–2012 and another 20.68% from 2003–2012 to 2013–2019. During the 1989–2002 period, the advancing rate of the −5 m isobaths followed a Gaussian frequency distribution, in agreement with simulation results of cellular automata. We suggest that in the absence of human perturbations, as the sediment input to NHS declines to the threshold of 0.01 × 10⁸ t/y, the seaward advance of tidal flats may significantly slow down. However, human interferences generated large uncertainties on the development tendency of the NHS.