Long-term (1842-2006) morphological change and equilibrium state of the Changjiang (Yangtze) Estuary, China

The 165 year morphological evolution of the Changjiang (Yangtze) Estuary
and its long-term equilibrium characteristics have been investigated
using 16 selected maritime charts from 1842 to 2006. These charts were
digitized and analyzed with a GIS to provide quantitative estimates of
geomorphologic changes below selected bottom elevations. The results
show that the Changjiang Estuary has experienced overall erosion over
this 165 year period, having been affected by several large erosion and
deposition episodes. It is found that the overall volume changes are
less than 10% and aspect ratios of <5 have increased from 2% to ca.
30%. During this period, both coastlines and thalwegs have become
straighter and more aligned with the propagation direction of offshore
tidal current, indicating that the estuary is approaching dynamic
equilibrium. Based on variations of aspect ratios and thalweg movements,
five types of stable channels were identified. Significant
topographical changes, including infilling after 1958 and erosion after
1980, are principally associated with engineering work around the
estuary and a reduction in sediment supply. Although human influence
only notably occurred at certain locations in the estuary, such as
upstream node of an estuary, it had a significant impact on the overall
geomorphologic evolution. In addition to these anthropogenic effects,
freshwater flow, sediment discharges, tide propagation and Coriolis
force are also found to play important roles in channel development
within the estuary. By contrast, changes in other factors such as sea
level changes and wave climate were considered to be of relatively minor
importance in the morphological evolution process of the estuary.