Estuary Processes and Managment - River Dynamics and Integrated River Management - page 464

River mouth has undergone significant geomorphological changes over the past 3,000 years. The river
discharged into the East China Sea at Yangzhong some 2000 years ago (Chen et al., 1988). However,
sedimentation has since moved the river mouth more than 200 kilometers downstream. The present
location of the largest city of China—namely Shanghai—was located in the sea and emerged only in the
13th century. Chongming Island, a delta sand bar, appeared in the river mouth some 800 years ago and
divided the river into the North Branch and South Branch. Shanghai is the largest industrial hub of China
and land demand is increasing for urban construction and harbor expansion. In the past decades Shanghai
has expanded by 11% by creating land in the river mouth and sea. The new created land was used for
agriculture in the early period and since 1987 has been used mainly for industrial and urban development.
An analysis of the deposits has shown that the Yangtze River mouth has been expanding for a long time,
with 4,000 km 2 of land being created during the period from the 4th century to the 12th century and
3,000 km 2 from the 13th century to the 20th century (Wang et al., 2003).
Prior to the 18th century the North Branch was the main discharge channel; since then the main flow
has shifted gradually to the South Branch and the North Branch discharge has decreased. The North
Branch accommodated 25% of the river runoff in the 18th century and now only discharges river runoff
during low tides in the flood season. The discharge ratio decreased to about 2% in the 1950s and is now
about -8% since more tidal water flows upstream. Furthermore, the South Branch was bifurcated into the
North Channel and the South Channel by Changxing Island, which emerged from the South Branch after
the 100-year flood in 1860 (Le et al, 1998). A sand bar, namely Jiuduan Shoal, appeared after the 1954
flood and is now growing in the South Channel, and further divides the South Channel into the North
Passage and South Passage.
Figure 8.4 shows the development process of the Jiuduansha Shoal, which is one of the shoals that
have recently emerged from the water. The longitudinal section of the Jiuduansha Shoal forms a convex
geomorphic pattern, which stands out on the link between the -10 m isobathic from the upper reach
section to the lower reach section of the Yangtze River Estuary (Fig. 8.3 A-A'). In transverse section,
the Jiuduansha Shoal is confined between the channels of the South Passage and the North Passage (Fig.
8.3 B-B'). The 1954-flood is a 100 years flood, having a period of 107 days with water discharge larger
than 60,000 m 3 /s. After the flood, the Jiuduansha Shoal was isolated from the Tongsha Tidal Flat and
formed a new island in the Yangtze River Estuary, and then the Tongsha Tidal Flat was renamed the
Eastern Hengsha Tidal Flat. The area of the Jiuduansha Shoal is the major place for the sediment deposition
in the South Branch and South Channel. The sediment is deposited during the flood season and eroded
during the dry seasons, with the sediment depositing at neap tides and eroding during spring tides under
the action of runoff and the tidal current.
Figure 8.5 shows several female deltas: (a) Rhein-Meuse-Scheldt delta; (b) Irrawaddy delta; (c) Pearl
delta; and (d) Volga delta. The Rhein-Meuse-Scheldt Delta and the Pearl River delta are female as well.
The Rhein-Meuse-Scheldt delta consists of a very complex channel network, as shown in Figure 8.5(a).
The Rhine River begins at the Rheinwaldhorn Glacier in the Swiss Alps and flows north and east
approximately 1,320 km. At the Netherlands frontier, it divides into two parallel distributaries, the Lek
and the Waal, as it crosses a wide, marshy plain and a great delta before entering the North Sea. The river
Meuse links with the Rhine at the delta and forms a complex channel network. The Scheldt River also
joins the channel network at the delta. There are many channels in the delta and many islands between
the channels.
The Pearl River has a drainage area of 450,000 km 2 , carrying 3.086 trillion m 3 of water and 87 million
tons of sediment load annually into the South China Sea. The sediment load/water ratio is only about
0.03 kg/m 3 , thus a female delta has developed in this location. The delta has a complex channel network