**1. Introduction**

214 Hydrodynamics – Natural Water Bodies

Zhao, Z. G. (2007). Discussion on Several Techniques of Immersed Tunnel Construction.

Chinese)

(in Chinese)

*and Engineering)*, Vol. 25, No. 1, (March 2001), pp. 16-20, ISSN 1006-2823 (in

*Modern Tunnelling Technology*, Vol. 44, No.4, (August 2007), pp. 5-8, ISSN1009-6582

The Yangtze River originates in the Qinghai-Tibet Plateau and extends more than 6300 km eastward to the East China Sea, a tectonic subsidence belt (Li & Wang, 1991). It is one of the largest rivers in the world, in terms of suspended sediment load, water discharge, length, and drainage area. The Yangtze River Estuary is located in the east China. There are three main islands including Chongming Island, Changxing Island, and Hengsha Island as well as several shoals in the Yangtze River Estuary (Fig. 1). These islands once are shoals emerged from the water and merged to the north bank or coalesced together. In the Yangtze River Estuary, most of the sediments from the drainage basin are suspended. The spatial and temporal variations of the suspended sediment concentration in the estuarine field survey indicate that the sediment is suspended, transported, and deposited under riverine and marine processes, such as river flow, waves, tidal currents, and local topography (Cao et al., 1989; Chen, 2001; Gao, 1998; Li et al., 1995, Huang and Chen, 1995; Xu et al., 2002; Pan and Sun, 1996). In longitudinal section, these islands and shoals stand out on the link between the -10 m isobathic line (the zero elevation means the 1956 Yellow Sea Water Surface in Qingdao Tidal Station, Qingdao, Shandong Province, China) from the upper reach section to the lower reach section, it is a convex geomorphic unit in the Yangtze River Estuary (Fig.2 A-A´ and Fig. 3), in transverse section, these shoals and islands sit in between the channels and distributaries (Fig.2 B-B´ and Fig. 4). In order to analyze the formation and evolution of the wetland and landform of the Yangtze River Estuary, related sea maps from 1945 to 2001 and satellite images from 1975 to 2001 are collected and analyzed. Water and sediment discharge from 1950 to 2003 at the Datong Hydrologic Station 640 km upstream from the estuary mouth are also collected. Datong Hydrologic Station is the most downstream hydrologic station on the free-flowing Yangtze River, where the tidal influence can affect flows hundreds of kilometers upstream. All related sea maps are digitized using Mapinfo7.0, and the sediment volume deposited in this area is calculated from a series of processes dealt in Surfer7.0. The relation between formation and evolution of the wetland and landform of the Yangtze Estuary over the past 50 years were analyzed via Geographical Information System technology and a Digital Elevation Model.

Formation and Evolution of Wetland and Landform in the Yangtze River Estuary Over

1990

Jiuduansha Shoal

1997

1979



Estuary from 1953 to 2001


Nanhui Marginal Tidal Flat

1953

South Passage


0

Elevation(m)

3

6

of the Jiuduansha Shoal from 1959 to 2001




Elevation(m)


0

3

the Past 50 Years Based on Digitized Sea Maps and Multi-Temporal Satellite Images 217

2001

121.7 121.9 122.1 122.3 122.5

1953 1959 1965 1979 1986 1990 1997 2001

North Passage

1979

31 31.1 31.2 31.3 31.4

Fig. 4. Sketch map of the cross section at 122°E (shown on Fig. 2 as B-B') of the Yangtze River

Jiuduansha Shoal

2001

1965

1986

1990

Fig. 3. Longitudinal section at 121°35′E, 31°16′N-122°25′E, 31°5′N (shown on Fig. 2 as A-A')

1959

long.

North Channel

Eastern Hengsha Tidal Flat

1959

Fig. 1. The sketch map of the Yangtze River Estuary

Fig. 2. Location of the Jiuduansha Shoal in Yangtze River Estuary


( ) E







0

(m)

Fig. 1. The sketch map of the Yangtze River Estuary

Chongming

C

Fig. 2. Location of the Jiuduansha Shoal in Yangtze River Estuary

hangxing Is.

121 121.3 121.6 121.9 122.2 122.5

Hangzhou Bay

Island

A

Eastern Hengsha Tidal Flat

North Channel

A'

North Passage

B'

Jiuduansha Shoal

B

South Passage

Hengsha Is.

( ) N

31.7

30.8

31.1

31.4

Fig. 3. Longitudinal section at 121°35′E, 31°16′N-122°25′E, 31°5′N (shown on Fig. 2 as A-A') of the Jiuduansha Shoal from 1959 to 2001

Fig. 4. Sketch map of the cross section at 122°E (shown on Fig. 2 as B-B') of the Yangtze River Estuary from 1953 to 2001

Formation and Evolution of Wetland and Landform in the Yangtze River Estuary Over

East China Sea.

100

300

500

700

900

Annual water runoff (billion m3

)

1100

1300

1500

the Past 50 Years Based on Digitized Sea Maps and Multi-Temporal Satellite Images 219

90°-115°. The ebb tide current is not in a direction opposite to the flood tide direction; there is a 10°-35° angle between the extension line of the flood and ebb tidal currents because of the Coriolis force(Shen et al., 1995). Ebb tidal current is obviously diverted to the south, while the flood current is diverted to the north. Thus, between the flood and ebb tidal currents in the river mouth area there is a slack water region where sediment rapidly deposited to form shoals, and eventually coalesced to form estuarine islands (Chen et al., 1979). This is the evolutionary history of the three larger islands (Chongming Island, Changxing Island and Hengsha Island, respectively) in the estuary. These islands form three orders of bifurcation and four outlets in the Yangtze River Estuary. The first order of the bifurcation is the North Branch and the South Branch separated by Chongming Island. The South Branch is further divided into the North Channel and the South Channel by Changxing Island and Hengsha Island. The South Channel is further divided into the North Passage and the South Passage by the Jiuduansha Shoal (Fig. 1). Therefore, the Yangtze River Estuary has North Branch, North Channel, North Passage and South Passage four outlets through which the water and sediment from the Yangtze River discharge into the

From 1950 to 2003, the annual water discharge at the Datong Hydrologic Station did not substantially change. The total annual discharge is about 9481×108 cubic meters per year and the sediment load is about 3.52×108 tons/yr. The sediment discharge during the flood season (from May to October) constituted 87.2% of the annual sediment load before the 1990s, but decreased in the 1990s (Fig. 5). Most of the suspended sediment are silt and clay, which are transported to the East China Sea where they are carried away from the delta by the longshore currents. Part of the suspended load is deposited in mouth bars and a subaqueous delta area to form the tidal flats and mouth bars in the Yangtze Estuary. A broad mouth bar system and tidal flats were formed. The runoff and the sediment discharge

Annual water runoff Sediment load

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year

Fig. 5. Water and Sediment discharge from 1950 to 2003 at Datong Hydrologic Station.

100

200

300

400

500

Sediment load (billion kg)

600

700

### **2. Data and methodology**

In order to analyze the formation and evolution of the Yangtze River Estuary in past 50 years, related sea maps from 1945 to 2001 and satellite images from 1975 to 2001 are collected and analyzed. Landsat MSS (multi-spectral scanner) data acquired on 1975 and 1979, Landsat TM (Thematic Mapper) and Landsat ETM+ (Enhanced Thematic Mapper Plus) from 1990 to 2001, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data from2002 to 2005 were collected and analysed. All these remote sensing data were corrected geometrically. Image processing of these satellites remote sensing data were used ENVI4.6 and Erdas9.0. And formation and evolution of the landform over the past 50 years are analyzed in detail.
