Coastal Erosion along the Changjiang Deltaic Shoreline
Wide delta plains along the coasts are important areas in which people live, carry on economic activities, grow or collect food, and so on. However, deltaic coasts around the world have recently become sites of serious environmental problems. One outstanding problem is the erosion of deltaic coasts. A major cause of such erosion is a decrease in sediment discharge from delta-forming rivers because of the construction of dams. This is also the case in the Changjiang River catchment area.
Contents
Introduction
Deltaic coasts are one of the principal coastal landforms and an important area for human activities. Deltaic coasts are affected by changes both on the land (in the drainage basin) and in the ocean. A sea level rise is a typical example of a change in the ocean that may severely affect the coastline. Decreases in sediment and water discharge caused by dam construction, sand dredging in river channels, and water usage in drainage basins are typical examples of changes on the land that also impact coasts.
Coastal erosion has been a world-wide problem in recent decades. An estimate made by Bird shows that about 70% of the worlds sandy beaches are retreating at a rate of 0·5–1·0m/y. The increasingly intensified human activities within river catchments and along coasts have aggravated the ongoing coastal erosion processes in many locations. In China, coastal erosion has become one of the major concerns to the future socio-economic developments in major coastal cities.
Historical background
Since the early 1980s, the sediment supply to the coastal system in North China has been reduced dramatically because of the extensive hydroengineering works. For example, after the construction of the water diversion scheme which transfers water from the Luanhe River to the Tianjin and Tangshan areas, the annual sediment discharge into the Bohai Sea decreased rapidly from 22·19 to 1·03 million ton/y. Such a decrease in sediment supply has resulted in a maximum coastal retreat of 300 m year"1 in the Luanhe River mouth and an average erosion of 25m/y in its offshore sandbars, accompanied by a rapid decrease in sand-bar size.
Within the Changjiang River catchment, soil erosion due to cultivation and hill farming has increased over the past decades. However, more and more eroded soil has been trapped by newly constructed reservoirs, and there is no net increase in sediment discharge from the upper catchment of the Changjiang. In fact, a slow decline in sediment supply into the deltaic area has become obvious recently, particularly along the south coast, adjacent to Hangzhou Bay, where shoreline retreat has been severe.
During the past decade, the total length of shoreline under erosion became longer than the accreting sectors. Such a trend of coastal erosion is expected to accelerate when the large engineering projects within the Changjiang catchment and its estuary are completed. Among these projects, the Three-gorge Dam, the south-to-north water diversion schemes and the deepwater navigation channel are considered most important to the shoreline movements in the deltaic area.
Geographical background
Being one of the largest rivers in the world, the Changjiang River extends about 6300 km from the Qinghai–Tibet Plateau eastwards to the East China Sea. Its drainage basin covers about 1.80×106 km2 in area, with a human population of about 427 million in 2000. The Yangtze River transports about 900 billion m3 of water into the sea each year and carried about 434×106 tons of sediment to the coastal sea. The Changjiang estuary is up to 90 km across at the river mouth, within which there are four branches of tidal channel characterized by the three-ordered bifurcation. The North and South Branches are the first order bifurcation, which is separated by Chongming Island. The South Branch is further divided into the North Channel and the South Channel by two islands, the Changxing and the Hengsha. The South Channel is again divided into the North Passage and the South Passage by the Jiuduan Shoal. At present, the South Branch, the South Channel, the North Passage and the South Passage are the four main water channels that carry the water and sediments from the Changjiang River into the East China Sea.
Future prospects of shoreline movement
Previous studies indicate that the changes in sediment input into the coastal system are dependent on the two factors: (1) the sediment output from the Changjiang River basin to the estuary; and (2) from the river estuary to the south coast system. The sediment output from the drainage basin is determined by the sediment source available and the hydrological processes, on which human activities have exerted more and more effects. The sediments are redistributed by currents through the four tidal channels after they enter into the estuary. The sediment transport direction altered as the principal sediment transport route shifts its way from one channel to another.The shorelines along main waterways and along the south coast around Jinshan are very sensitive to the changes in the suspended sediment supply from the Changjiang River. It is anticipated that the sediment supply will decrease significantly due to the following engineering projects.
The Three-gorge Dam as a sediment trap
The Three-gorge Dam is a key state hydro-electric engineering work which has recently come under construction. There is no doubt that the Three-gorge Dam will provide a huge amount of electrical energy, a great reduction in the number of flood disasters to the fluvial plains along the middle reach of the Changjiang River and easy access for water transportation to the upper reach area. However, great attention should be given to its adverse impacts on the environment in the lower reach and the estuarine area.
The proportion of suspended sediments trapped by the reservoir will be as high as 57·9–69·4% during the first 48 years after the completion of the dam construction. This means a reduction of 281–337 million tons of suspended sediment supply, 60–70% of the total annual sediment discharge from the catchment, to the estuarine–coastal system. This will result in a level of coastal erosion much greater than previously recorded. However, such a trapping of sediment within the reservoir can encourage river-bed erosion in the middle reach of the Changjiang. These river-bed processes will generate and supply a certain amount of sediment to the coast. There has been much controversy about how much sediment the middle reach of the Changjiang River could supply, and how long a time the middle reach Changjiang River could function as an important sediment source after the construction of the dam According to the experience of the sediment recovery process downstream of the Danjiangkou Dam in the upper Hangjiang River, a distributary of the Changjiang River (Figure 1), the Changjiang Drainage Planning Office estimated that the initial suspended sediment concentration may be recovered when the river flows to Chenglingji, about 820 km downstream from the dam. Therefore, the Three gorge Dam will exert no adverse effects on the sediment supply to the estuary.
However, any reliable, not to say accurate, estimate about the recovery of suspended sediment concentration downstream from the dam is still very difficult to obtain when considering present state-of-the-art numerical simulation and physical modelling, because the extent of sediment recovery is connected to a series of complicated hydro-dynamical, geomorphological and sedimentological processes interacting with increasingly intensified human activities over a large temporal–spatial scale. It can be argued that no reliable estimate can be reached if one only takes into consideration the sediment transport capacity downstream from the dam. More attention should be given to the source of sediment supply. On the other hand, the river channels in the middle and lower reaches of the Changjiang River have been stabilized by river embankments during recent decades. As a result, much less river bank material is likely to be eroded. Furthermore, as the downward scouring process proceeds, the sediment transport capacity of the river will be reduced by the increase both in water depth and sediment grain size of the riverbed load. About 56 million tons of suspended sediments from the upper basin are trapped annually within the mid-lower basin. The amount of such trapped sediments accounts for nearly 12% of the annual total sediment discharge from the Changjiang into the sea. It is clear that a downward trend of sediment supply to the deltaic area will be inevitably when the construction of the Three-gorge Dam is completed.
Impacts of the water diversion schemes on sediment transport capacity
Due to a great spatial difference in water resource distribution, a serious water shortage problem and an increasing demand for water in North China has long been a main concern to the central government, therefore, three large scale, south to north, water diversion schemes are constructed. In past decades, water consumption for agricultural and industrial productions in North China has been increased dramatically, resulting in a reduction in runoff discharge into the lower reaches of the three major rivers, the Yellow River, the Huaihe River and the Haihe River. For example, the annual mean runoff discharge in the Haihe River through Tianjin, one of the three largest municipalities in China, into the Bohai Bay, has shown a dramatic drop from 14 430 million m3 to 980 million m3. The same situation occurred in the Yellow River. A sharp decrease in the runoff discharge into the sea has resulted in a dramatic decrease of sediment discharge to the sea and consequently strong erosion processes in the vicinity of the river mouth. According to the water diversion scheme plan, the total amount of water to be transferred to North China from the Changjiang River basin is up to 80000-90000 million m3 by the year 2020, and about 10% of the Changjiang’s total runoff discharge into the sea.
The navigation channel through the North Passage(Figure2)will be dredged downwards to a depth of -12.5m to allow navigation for 50,000 tonnage vessels into Shanghai harbour. Two dikes will be built in the mouth-bar area in order to facilitate the maintenance of the deep-navigation channel. These two dikes will inevitably alter the sedimentary and hydro-dynamics of the north passage. However, experience suggests that the amount of dredging could reach 20-30 million tons each year,output from the Changjiang drainage basin. The impact of such a dredging seem to encourage more sediments be transported through the North passage into the sea, resulting in less sediments being deposited on the shoals along the North and South Passages and the south coast of the deltaic plain. This certainly reduces the prospect of the shoals being reclaimed in the coming decades. There are two major methods for the disposal of the dredged materials. The first is to dump the sediments into the sea far enough from the channel, in order to prevent these sediments being carried back into the channel by tidal currents. The major drawback of this method of disposal is the high cost of transportation and pollution of coastal sea water. The second is to integrate the channel dredging with coastal land reclamation. This method overcomes the above mentioned drawback and provides a valuable land resource to Shanghai. In this method, the dredged sediments will be dumped onto shoals along the South Passage. As a result, the problems of sediment shortage due to the Three-gorge Dam and the south-to-north water diversion schemes will be partly alleviated
Conclusion
The shoreline of the Changjiang River delta has been highly sensitive to the changes in sediment output from the river basin to its estuary. Both the Threegorge Dam and the south-to-north water diversion schemes could possibly cause a reduction of up to 12 and 10%, respectively in sediment discharge from the Changjiang River to its estuary. Each engineering project will certainly slow the development of the Changjiang deltaic plain and reduce intertidal land resources for reclamation. The combination of the two projects will possibly reverse the direction of the shoreline movement and cause coastal erosion in many parts of the estuary.
A shift in the major sediment transport route within the estuary also exerts significant influence to the continuation of coastal sedimentation. However, the deep-water navigation channel project will have little effect on the accretion of the shoals along the North and South Passages if the dredged materials are dumped back onto these shoals. The overall trend for the next few decades will be the stagnation of sedimentation within the Changjiang estuary, and some sections of the shoreline will be under erosion. Such a significant deficiency in sediment supply, together with the anticipated rise of relative sea-level, will pose a great problem to the potential coastal land resource and coastal protection. As a result, the availability of intertidal land for reclamation will be significantly reduced. Consequently, to sustain agricultural production, particularly peri-urban agricultural, will become a big challenge to the local communities. Furthermore, the coastal ecosystems will be under threat. It is necessary to further investigate the sediment budget within the estuarine system
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References
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