Withering of the Abandoned Yellow River Delta
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WITHERING OF THE ABANDONED YELLOW RIVER DELTA Dayuan Yang and Yongzhan Zhang School of Geographic and Oceanographic Sciences Nanjing University Nanjing, CHINA ABSTRACT A large scale delta, called the Abandoned Yellow River delta (AYRD), was built at the middle of Jiangsu coast while the Yellow River, the largest river in the world as for river sediment concentration and discharge, entered into the Yellow Sea there during 1128-1855. The coastline of AYRD began to retreat rapidly suffering from the coastal erosion after the Yellow River turned back to enter into the Bohai Sea at 1855. On the contrary, the long coastline south to AYRD was still keeping on progressing fast with the huge sediment supply from the littoral drift. Convex profile was formed along the erosional coast, and beach was formed back to the dyke. Recently, following the construction of dikes along the erosional part, beach materials there are getting coarser, and the progression processes are becoming slow at accumulating part along the coast. BACKGROUND There are lots of studies on the geomorphologic processes about the formation of the large river deltas, but less detailed research for the geomorphologic processes about the withering of the large river deltas so far. A large scale delta, with the area more than 7160 km2 called Abandoned Yellow River Delta (AYRD) (Fig. 1), was built at the middle of Jiangsu coast while the Yellow River, the largest river in the world as for river sediment concentration and discharge, entered into the Yellow Sea there during 1128-1855 (Yang et al., 2000, 2003; Zhang, 1984). On the contrary, after the Yellow River turned back to enter into the Bohai Sea at 1855, the decay geomorphologic processes of AYRD has begun at the middle Jiangsu coast (Table 1). Table 1. Progradation and regressive processes of AYRD during the different period Progradation processes at the river mouth of AYRD during 1128-1855 A.D. Period 1128-1578 1579-1591 1592-1700 1701-1747 Velocity (m/a) 33 154 119 319 Period 1748-1776 1777-1803 1804-1810 1811-1855 Velocity (m/a) 190 111 500 311 The average velocity (m/a) for progradation progress during 1128-1855 122 New land formed (km2) during 1128-1855 7160 Regressive processes at the different parts of AYRD since 1855 Different parts Period 1855-1890 1891-1921 North to river mouth Velocity (m/a) 300 250 South to river mouth Velocity (m/a) 400 200 River mouth Period 1922-1958 1959-1971 Velocity (m/a) 75-80 70 -10 m isobath Period 1937-1965 1966-1994 Velocity (m/a) 285 233 288 Figure 1. The geomorphologic processes of AYRD during 1194 – 1855 WITHERING OF THE AYRD Continuous Retreat of the Delta Front AYRD grew fast shaped in bird-mouth extended to the Yellow Sea during the constructional period. Since 1855, lost the huge sediment supply from the Yellow River suddenly, AYRD turned into destruction period dramatically. The coastline of AYRD began to retreat rapidly suffering from the coastal erosion, it has withdrew for more than 23 km so far. Prior to 1940, the average velocity of drawing back processes reached 172 m/a at the front point of AYRD, and reduced to 55 m/a after 1940 gradually (Zhang et al., 2004). On the other hand, the coast turned from a tidal flat to a beach coast while wave dynamics became more and more active and got to be the dominant dynamic process. Consequently, the width of the intertidal zone became narrower and narrower and the surface sediment of the eroded delta got coarser and coarser. A small cliff was formed, chenier (Wang and Ke, 1989) began to develop consisting of shell fragments and “gravel” (mass of muddy materials) at the foot of the cliff, and a narrow coarser sandy beach with more marine shell debris developed (Fig. 2). Comparing the bathymetry maps of 1960, 1980 and 2005 (Fig. 3), it is obvious that 0 m, -5 m and -10 m isobaths retreated paralleled keeping the relatively certain distance between each other. It means that the submarine coastal slope (the slope of the subtidal zone) is relatively stable (Fig. 4) while it receded landward. Southern End of the Erosion Coast Moving Southwards Accelerando Since the 20th century, the position of the turning point between the erosion and accumulation coastal parts of AYRD is moving continuously southward (Fig. 5). In the early 20th century it is located at the mouth of the Bian-Dan-Gang river but changed southwards to the mouth of the Kua-Tao River in the 1940s, then to the mouth of the Shuang-Yang River in the 1960s, to the 289 mouth of the Yun-Liang River in the 1970s, to the mouth of the La-Ba River in the 1980s, and then to the southern of the She-Yang River mouth at the end of the 20th century. Now, the average receding speed is up to 20-30 m/a at the La-Ba River mouth. Thus, the southern end of the eroded coast kept on moving southwards accelerando. Figure 2. Cliff and chenier along the coast of AYRD (Taken in 1988) Figure 3. Regressive processes along the coast of AYRD since 1960 290 Figure 4. Vertical profiles across AYRD at the year of 1960, 1980, 1994 and 2004. Profiles of 1960 and 1980 from bathymetry map in the scale of 1:250,000, and profiles of 1994 and 2004 from the map in the scale of 1:10,000 Figure 5. Changes of the coastlines (1-6) at Sheyang county, Jiangsu Province during recent several centuries. 1-6 show the coastlines during different period. 1-Jia-jing (1522-1567) of Ming dynasty, 2- Wan-li (1573-1620) of Ming dynasty, 3-Shun-zhi (1644-1662) of Qing dynasty, 4-Qian-long (1736-1796) of Qing dynasty, 5-Xian-fen (1851-1862) of Qing dynasty, 6-present. 291 It could be seen that the position (A-F) of the turning point between the erosion and accumulation parts of the coast is moving southwards continuously during the recent century. A-F shows the positions of the turning points during the different period. A-early of 20th century, B-1940s, C-1960s, D-1970s, E-1980s, F-end of 20th century. South to the Erosion Coast Keeping on Progressing Seawards Fast Figure 1 shows the progradation processes of AYRD during 1194-1855. It is obviously that the southern part of AYRD accumulated relatively slow. At the northeastern Yanchen area, the mean progressive velocity was 41 m/a during 1194-1855 (Wang et al., 2004). Several dykes were built to reclaim the intertidal area at 1930s, 1950s, 1970s-1980s and around 2000. As the result, after 1855, the coastline of Yanchen area progressed bit fast with the average speed reaching to about 66 m/a. The average velocity for coastline progression at eastern Dafeng county was 8.57 m/a during 10-16 century, 40 m/a during 16-18 century, and up to 100 m/a after 1855 (Fig. 6). According to repeat level measures along several profiles crossing tidal flat during 1980-1984, it is clear that the coastlines progressed in the annual average velocity of 150 m/a at Xin-Yang-Gang, and the altitude raised up in the annual average speed of 2.4 cm/a with the fast siltation processes; south to Xin-Yang-Gang, the velocities were 155 m/a and 3.3 cm/a; and they were 154 m/a and 4.5 cm/a at Wang-Gang. Figure 6. Continuous progressive processes along the coast south to AYRD. 1-5 show the coastlines during the different period. Recent Changes After the dikes were set up along the erosion coast at the river mouth of AYRD and its nearby later of 20th century, regressive processes of AYRD was stopped, and coarser sandy beach is developed at supratidal zone back to the foot of the dikes (Fig. 7). Correspondingly, progressive processes were slowdown along the coast south to AYRD. The average speed reduced from 100 m/a to 30-40 m/a at southern part of Sheyang county. Furthermore, -15 m isobath around AYRD is moving landwards rapidly (Fig. 3). 292 Figure 7. Coarser sandy beach at supratidal zone back to the foot of dike constructed at the end of the 20th century (Taken in 2006) CONCLUSION During the flood season of the Yellow River, its delta was formed and extended rapidly with huge sediment supply. On the contrary, while the sediment supply was reduced or stopped, its delta regressed fast suffering from heavy coastal erosion. During the regression processes, the slope of delta front receded paralleled landwards, which is one of the main characteristics in AYRD. Even sediment supply from littoral drift is almost same during progradation and regression period of AYRD, the siltation velocities became fast at the coast south to AYRD since 1855 cause of the dike construction. ACKNOWLEDGEMENT Mr. Jun Xia and Peidong Lu studied on the evolution processes along middle Jiangsu coast during historical period at 2006, thanks for their help while we are preparing this paper. 293 REFERENCES Wang, Y. and X.K. Ke. 1989. Cheniers on the east coastal plain of China. Marine Geology. 90:321-335. Wang, Y.H., R.S. Zhang, P.D. Lu et al. 2004. Effects of sediment supply on coastal salt marsh evolution. In Proceeding of 9th International Symposium on River Sedimentation, volume 4, edited by C. 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