RECENT EVOLUTION OF OUTLETS IN ZHUJIANG RIVER DELTA AND THE PROSPECT FOR LAND RECLAMATION
Liu Yuefeng, Han Mukang, Wu Lun, Nobuo Mimura
1998, 53 (6):
Zhujiang River empties into the South China Sea through eight outlets in the delta. The evolution of the outlets, the rates of delta reclamation and the changes in the coast line in 1966～1996 are quantitatively studied. The results revealed by remote sensing and GIS techniques are compared with the data taken and digitized from the 1966 topographic map at the scale of 1∶10 000 . The total reclaimed area in the entire delta during the period has been calculated to be 344 km2, or at the average rate of 11 47 km2/a which is much greater than that in the historical period. Of this total, 146 km2 have been reclaimed in the Lindingyang district where four eastern outlets (Humen, Jiaomen, Hongqili and Hengmen, denoted as A, B, C, D in Fig.1) are found. In the Modaomen and Jitimen districts (E and F in Fig. 1) in the western part of the delta, 115 km2 have been reclaimed, and around the Yamen and Hutiaomen (G and H in Fig.4) in the Huangmaohai district 73 km2 have also been reclaimed. Through our analysis of the digital TM images for five years (1986, 1988, 1992, 1994 and 1996), three areas with different levels of sediment concentration——relatively high, medium and relatively low——can be differentiated in the Lingdingyang estuary (Figs. 1 and 2). Here, the presence of three closely located outlets (B, C. and D in Fig. 1), where large amounts of sediments are deposited in the western part of the estuary, has made it the most rapidly expanding reclamation area. In addition, the blocking of sediment movement by the rocky Qi’ao Island has also facilitated rapid reclamation in the peripheral areas of the Hongqili and Henmen outlets to the north of the island. Beside, because of the presence of rich sediment coming from the north, active and strong siltation has occurred in the sea area to the west of the island and between the island and Zhuhai/Macao, including the harbor of Zhuhai City. By contrast, the eastern coast of the Lingdingyang estuary has grown much more slowly, with some sections even experiencing slight retreat due to erosion. Thus the prospect for large scale reclamation does not look bright, but this should not adversely affect the development of landfill projects for industrial use along the rocky coast. In the western part of the delta (Figs. 3 and 4) during the same period, due to rapid reclamation, the coast line has moved seaward by 4.7 km, at the rate of 156 m/a, seriously affecting the hydrological conditions and sediment dynamics in each outlet and its adjoining channel. This has caused the channels to lengthen greatly and resulted in channel splitting, a redistribution of runoff and sediment load, and frequent river flooding. Rapid reclamation during 1966～1996 led the Modaomen outlet to rapidly advance seaward for 4.7 km (Fig. 3). After the completion of a planned reclamation project, shown in the area with dashed line, the present Modaomen outlet will advance further seaward by more than 11 km, while in the Huangmaohai estuary (Fig. 4), the Yamen and Hutiaomen outlets are merging into a new outlet due to rapid reclamation. Should the current trend continues, the prospect for delta reclamation in the future would be adversely affected by the following two factors∶1. The source of sediment generation will become progressively small as increasing amount of sediment is trapped in the reservoirs on the middle and lower reaches of the river and as sand excavation from the river intensifies to satisfy the growing demand for sand by the booming construction in the delta. 2. In the next 50 years, the delta region’s relative sea level will rise by 0 5 m, which will increase the water depth and weaken sedimen tation inside as well as outside of the outlets. These issues should be taken seriously in making land use decisions and in the management of the delta.
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