Morphodynamic couplings between the Biandan Shoal and Xinqiao Channel, Changjiang (Yangtze) Estuary

Channel-shoal systems (CSS) are some of the significant morphodynamic cells in estuaries, which provide immeasurable economic, ecological and environmental value. However, most CSS in mega-estuaries of the world are facing great challenges from serious risk of degradation due to intensive human activities and sea-level rise. Here, we focus on the tradeoff between the Biandan Shoal (BDS) and Xinqiao Channel (XQC), which is the largest CSS of the South Branch, Changjiang Estuary. The results show that the BDS area above the 5 m water depth had an observable increasing tendency with an interannual increasing rate of 1.43 km²/yr during 1996–2016. The corresponding XQC volume exhibited an obvious reduction trend with an annual decreasing rate of 3.79 × 10⁶ m³/yr. Meanwhile, the BDS presented lateral and downward extension with more than 4.7 km of downward movement, while the XQC indicated shrinking with the width of the upper reaches decreasing by nearly half since 1998. Furthermore, the morphodynamic evolution of the BDS-XQC system during 1998–2016 could be divided into four stages: preflood stage with the joint extension of the BDS and XQC (1996–1998); conversion stage due to impacts from the extreme flood (1998–2000); development stage with obvious deposition throughout the system (2000–2009); stabilization stage with deposition on the high flats but no lateral expansion of the BDS, and continuous shrinkage of the XQC (2009–2016). The dynamic balance of the BDS-XQC system was mainly determined by the tradeoffs between the BDS and XQC, even though the occasional extreme hydrological events disrupted the coupling of the BDS-XQC. Meanwhile, the local channel blocked by the construction of the Dongfengxisha Reservoir induced a decrease in water discharge into the XQC, which was responsible for the recent XQC recession. In light of the impact of future climate change and increasing human interference in the anthropogenic era, fragile CSS distributed in estuaries should be holistically considered with policies to support sustainable integrated coastal zone management.