A historical review of sediment export–import shift in the North Branch of Changjiang Estuary

Abstract

Net sediment transport is predominantly seaward in fluvial-dominated estuaries worldwide. However, a distributary branch in the Changjiang Estuary, the North Branch, undergoes net landward sediment transport, which leads to severe channel aggradation. Its controlling mechanism and the role of human activities remain insufficiently understood, although such knowledge is necessary for better management and restoration opportunities. In this study we revisit the centennial hydromorphodynamic evolution of the North Branch based on historical maps, field data, and satellite images and provide a synthesis of the regime change from ebb to flood dominance. The North Branch was once a major river and ebb-dominant distributary channel. Within which alternative meandering channels and sand bars developed. Deposition of river-borne sediment leads to infilling of the branch, while tidal flat embankment reduces the bankfull width and modifies the channel configuration, resulting in a profound decline in the sub-tidal flow partition rate. The North Branch then becomes tide-dominant with an occurrence of tidal bores and elongated sand ridges. Once tidal dominance is established, extensive tidal flat reclamation enhances the funnel-shaped planform, amplifying the incoming tides and initiating a positive feedback process that links tidal flat loss, sediment import, and channel aggradation. Overall, the shift in branch dominance is a combined result of a natural southeastward realignment of the deltaic distributary channels and extensive reclamation. One management option to mitigate channel aggradation is to stop the aggressive reclamation and allow tidal flats to build up, which might reduce the sediment import and eventually lead to a morphodynamic equilibrium in the longer term. Understanding the impact of tidal flat reclamation is informative for the management of similar tidal systems under strong human interference.

Unraveling The Impacts of Meteorological and Anthropogenic Changes on Sediment Fluxes Along an Estuary-Sea Continuum

“…The shift from total sediment export to import can result from bathymetric changes (estuary deepening and narrowing; Figure 1b and c), as observed by Guo et al 23 . Nonetheless, it may also result from changes in river flow and wave forcing (Figure 2a and b).…”

“…Human activities can drastically change the upstream river supplies (e.g., through dam construction ), the local sediment nature (e.g., through dredging activities 21 ), and the estuary morphology (e.g., through harbor extension and channelization 22 ). Guo et al 23 recently investigated a centennial hydro-morphodynamic evolution of the Changjiang Estuary (China) to highlight the influence of anthropogenic pressures on estuary sediment import-export. More specifically, they observed that a narrower funnel-shaped estuary resulting from intensive human activities induced a shift from ebb to flow dominated estuary, leading to increase sediment import and channel aggradation.…”

Unraveling the impacts of meteorological and anthropogenic changes on sediment fluxes along an estuary-sea continuum

“…The shift from total sediment export to import has also happened in other systems (e.g., Changjiang Estuary 27 ) and can be the result of bathymetric changes (estuary deepening and narrowing; Fig. 1 b,c, Supplementary Figure S2 ).…”

“…4 i (i.e., 2nd period minus 1st period), characterizing more import (or less export) of mud in 2005–2015 than in 1990–2000. Such behavior can be attributed to human-induced changes, which impacted the system functioning via the estuary deepening and narrowing, as observed by Guo et al 27 . It is also supported by Grasso and Le Hir 26 who simulated an intensification of density-induced circulation due to bathymetric changes in the Seine Estuary from 1960 to 2010.…”

“…Human activities can drastically change the upstream river supplies (e.g., through dam construction 24 ), the local sediment nature (e.g., through dredging activities 25 ), and the estuary morphology (e.g., through harbor extension and channelization 26 ). Guo et al 27 recently investigated a centennial hydro-morphodynamic evolution of the Changjiang Estuary (China) to highlight the influence of anthropogenic pressures on estuary sediment import–export. More specifically, they observed that a narrower funnel-shaped estuary resulting from intensive human activities induced a shift from an ebb- to flood-dominated estuary, leading to increased sediment import and channel aggradation.…”