Decadal morphological evolution of the mouth zone of the Yangtze Estuary in response to human interventions

Abstract

The morphology of the Yangtze Estuary has changed substantially at decadal time scales in response to natural processes, local human interference and reduced sediment supply. Due to its high sediment load, the morphodynamic response time of the estuary is short, providing a valuable semi‐natural system to evaluate large‐scale estuarine morphodynamic responses to interference. Previous studies primarily addressed local morphologic changes within the estuary, but since an overall sediment balance is missing, it remains unclear whether the estuary as a whole has shifted from sedimentation to erosion in response to reduced riverine sediment supply (e.g. resulting from construction of the Three Gorges Dam). In this paper we examine the morphological changes of two large shoals in the mouth zone (i.e. the Hengsha flat and the Jiuduan shoal) using bathymetric data collected between 1953 and 2016 and a series of satellite images. We observe that the two shoals accreted at different rates before 2010 but reverted to erosion thereafter. Human activities such as dredging and dumping contribute to erosion, masking the impacts of sediment source reduction. The effects of local human intervention (such as the construction of a navigation channel) are instantaneous and are likely to have already resulted in new dynamic equilibrium conditions. The morphodynamic response time of the mouth zone to riverine sediment decrease is further suggested to be >30 years (starting from the mid‐1980s). Accounting for the different adaptation time scales of various human activities is essential when interpreting morphodynamic changes in large‐scale estuaries and deltas. © 2019 John Wiley & Sons, Ltd.

The Case for a Critical Zone Science Approach to Research on Estuarine and Coastal Wetlands in the Anthropocene

“…Along with more than 50,000 other reservoirs in the catchment, the Three Gorges Dam has reduced downstream suspended sediment concentrations by 70 to 90% and induced major changes in sedimentation patterns in the estuary (Yang et al 2018a). The markedly reduced fluvial sediment loads have, as expected, resulted in recession of the Yangtze delta (Luo et al (2017), although impacts on the wider estuarine environment have not always been as anticipated (Zhu et al 2019). Indeed, accretion of mudflats has actually increased in many localities around the Yangtze estuary due to lower annual discharge and reduced flood frequencies (Hu et al 2019), a situation that has been further promoted through land reclamation (Sengupta et al 2019).…”

A Computationally Efficient Shallow Water Model for Mixed Cohesive and Non-Cohesive Sediment Transport in the Yangtze Estuary

“…Not surprisingly, bed erosion and reduction in suspended sediment concentration (SSC) have been observed in various regions of the Yangtze Estuary [1][2][3][4]. However, it has been suggested that there might be a morphological lag response to riverine sediment supply changes of about 10-30 years in seaward regions [5,6]. In this regard, it is important to conduct highresolution 10-30 years prediction of the response of the hydro-sediment-morphodynamic system in the Yangtze Estuary.…”

Decadal link between longitudinal morphological changes in branching channels of Yangtze estuary and movement of the offshore depo‐center

“…Over past decades, more than 50000 dams have been constructed and a large number of local engineering projects undertaken in the river basin and its estuary. Surveys have comprehensively examined decadal to seasonal erosion-deposition processes affecting geomorphic units within estuarine and offshore areas of the Yangtze subject to significant variations in water dynamics and sediment supply caused by intense human activities (Liu et al, 2009;Yang et al, 2011;Dai et al, 2013Dai et al, , 2014Dai et al, , 2016Du et al, 2016;Luan et al, 2016;Yang et al, 2016;Zhu et al, 2017;Mei et al, 2018;Zhu et al, 2019;Zhu et al, 2020). It has been established that certain of these morphological changes have led to practical problems, including migration of navigation features (Liu et al, 2009;Dai et al, 2013), increased risk of embankment failure related to water abstraction projects (Ou et al, 2013), and recession of saltmarsh-wetland systems (Wei et al, 2015;Gu et al, 2018).…”

“…The latter processes are usually related to local erosion or deposition in estuarine systems, and are caused by variations in sediment load, runoff discharge and local hydrodynamics due to human interference. For example, reduction in fluvial sediment load due to the trapping effect of river dams has led to widespread recession of river deltas around the world (Rao et al, 2010; Maloney et al, 2018; Zhu et al, 2019), whereas runoff regulation of river dams has triggered depositional features (Frihy and Lawrence, 2004; Zamora et al, 2013) and altered spatial erosion–deposition distributions in estuaries (Zhu et al, 2017, 2018). Moreover, estuarine engineering projects interfere with local hydrodynamics and adjust nearby erosion–deposition patterns (Nitsche et al, 2007; Liu et al, 2009; Luan et al, 2016; El Jakani et al, 2019).…”

“…Consequently, offshore depo‐centers are useful indicators of offshore and continental evolution processes. Although researchers have investigated decadal to seasonal morphological changes caused by human disturbances within inland and near‐mouth areas (Frihy and Lawrence, 2004; Nitsche et al, 2007; Rao et al, 2010; Zamora et al, 2013; Luan et al, 2016; Zhu et al, 2017; Maloney et al, 2018; Zhou et al, 2018; El Jakani et al, 2019; Zhu et al, 2019), longitudinal erosion–deposition patterns within inland estuarine channels (i.e. differences in morphological behavior between upper and lower sub‐reaches of the channels) have not been systematically studied to date.…”