Effects of human-induced riverine sediment transfer on deposition-erosion in the South Passage of the Changjiang (Yangtze) delta

Deltaic channels, as the essential part in system of river deltas where river and ocean connect, are influenced profoundly on its deposition and erosion processes, by changes of riverine sediment due to human activities from catchment to mouth of large rivers and sea-level rise worldwide. Here, based on a series of hydrological, sedi-mentary and morphological records in the main outer channel South Passage (SP) of the river-tide dominant Changjiang (Yangtze) Delta (CJD) for more than half a century, regional deposition-erosion process and the implication mechanism were examined. The results showed that fluvial sediment supply declined by over 70% stepwise during the past decades and local suspended sediment concentration in the SP reduced by half within recent years. Correspondingly, three distinct stages of morphodynamic process came out one after another, which were phases of rapid deposition (1959-1979), moderate deposition (1979-1997) and dramatic erosion (1997-2021), respectively. Channel and shoal had asynchronous performance during this deposition-erosion transition, especially after 1979, when channel entered a rapid erosion period earlier after a stable volume period, while shoal suffered slow erosion after high-speed deposition. Moreover, the current erosion phase was furtherly explained as two main modes by the Empirical Orthogonal Function analysis, accounting for 61% and 14%, respectively. The dominant mode describes the continuous scouring in channel contributed by transfer in riverine sediment regime, consisting of the reducing upstream sediment, local increasing flow diversion, dredging activities and sea-level rise. The second mode suggests decadal shift from deposition to slight erosion of shoal zones, which was also adjusted to episodic events and local engineering projects like shoal embankment and reclamation. It's predictable that the SP would constantly suffer long-term erosion under riverine sediment reduction, sea-level rise and artificial waterway regulation, together with aggradation of the fringing shoals by reclamation. This highlights channels in the river deltas are faced with the erosion risk caused by anthropogenic activities, which calls for compelling attention.