IV. PROTECTION OF REGIONAL NATURAL ENVIRONMENT
A. Sea level rise and ground base subsidenceShanghai, a city bordered by both sea and river and built on a delta, is sensitive to sea level rise and land subsidence which are directly related to urban flood prevention, the draining of waterlogged fields and resistance to some natural disasters, such as wind storms, and to economic construction.
In the Shanghai urban areas, the ground level is generally less than 4.5 m and the lowest is between- 3.0 and 3.5 m. The level in most of Jing'an District and some parts of Putuo District is less than 3.0 m. The lowest is 2.3 m. At Huangpu Park hydrometric station, the perennial mean high tidal level is 3.11 m, while the flood tidal level can be 4.0-4.35 m during the flood season (June-September). The highest tidal level on record is 5.22 m (1 September 199l), which means that all urban ground level is lower than the higher tidal levels during the flood tidal season.
The Shanghai meteorological station forecasts that absolute sea level in Wusong will rise at an accelerating rate. The rising rate is forecast to be 2.5mm/a in the period 1999-2030, and 5.00 mm/a in the 2030-2050 period. The absolute rising height is 5cm, 10cm and 20cm in 2010, 2030 and 2050 respectively.
Moreover, since land subsidence was first discerned in Shanghai city
in 1921, the accumulative mean subsidence in urban areas had reached 1.76
m by 1995. The maximum subsidence is 2.63 m. The fastest descending rate
is 287 mm/a (table 22), and the coverage reached is 400 km2. Since 1965,
Table 23. Ground base subsidence in Shanghai
downtrend has been basically controlled through reducing ground water exploitation to a large extent and the recharging of ground water. So there was little land subsidence in the 1970s, but in 1980s the problem of land subsidence became serious again because of accelerating urban infrastructure construction associated with the rapid economic development. The mean land subsidence was 6.2mm/a in 1984-1987. The largest accumulative subsidence is 3m. Urban construction of infrastructure in Shanghai has developed substantially since Pudong New Area was established in 1990. Investment for infrastructure utilities increased by 474% over that of 1990, and number of high-rise buildings of over 20 storeys reached 407 in 1995 which was 2.88 times that of 1990 and 80 times that of 1980. Land subsidence will be a very serious problem in a relative short time. According to estimates, taking into account all the factors of regional geological structure in Shanghai, the mean land subsidence will be 10cm, 15cm and 17.3cm in 2010, 2030 and 2050 respectively. If there is any overlap of the respective absolute sea level rises, the relative sea level rises will be 16cm, 29cm and 43 cm. However, the extent of land subsidence varies from area to area, and according to the research of the geological environment station of the Geology Bureau, the land subsidence in the Suzhou River may reach 36.6m by 2050, and the relative sea level rise will amount to 60cm.
Sea level rises will present many challenges to the achievement of urban sustainable development in Shanghai in the 21st century. This subject is discussed in more detail below.
(a) Prevention and treatment of urban waterlogging and flood disasters
Shanghai city is lower in relief. The river network crosses and the density of the drainage network is 6-7 km/km2 with the total water surface area accounting for 11-12% of the whole urban area. There are about ten rivers besides the Huangpu River and the Suzhou River, both large and small, contributing to the natural urban drainage system. Furthermore, the Huangpu River also drains 40% of water in the Tai Lake. But all these rivers are sensitive to tidal rivers with lower vertical water differences. Moreover, they have become silted up and have not been dredged for many years. If typhoons, rainstorms and astronomically high tides occur at the same time, the water from upstream of the rivers will go down and be withstood by the floods in the Yangtse River and the tidal water. The water level will then rise rapidly, which will cause serious damage from flooding and waterlogging. For example, in August 1962, hit by a typhoon and with the water level in Huangpu Park reaching 4.76m, the storm tide impacted against the flood-prevention walls causing about 46 breaches. Half of the urban areas were inundated with water which caused serious economic loss.
Figure 59. A sketch map of the reconsolidated flood walls in Shanghai urban area
Table 24. Influences upon the defence standard of flood walls in Shanghai by sea level rise and ground level subsidence
The 17th typhoon in 1994 occurred at Wenzhou, Zhejiang Province, and resulted in a rare tidal level of 6.88 m. 520.7 km of dikes were destroyed. There were 5,698 breaches. Seawater broke in for several kilometers, inundated 4,600,000 mu of farmland, and destroyed 4,681 km of electrical power lines and 2,397 km of telecommunication lines. 13,920,000 people were seriously affected and the direct economic loss was 17.76 billion yuan. This disaster caused people to re-estimate the defence ability of dikes in Shanghai. Only the embankment with a protected wall top near the Baoshan steel plant and the Jinshan petro--chemical plant that was less than 60 km long could resist such a strong typhoon. However, according to the survey and summary of the 17th typhoon in 1994, the tidal level surpassed the historic record but did not surmount the embankment top. The wave stirred up by the typhoon surmounted the wall top and reached 10 m high, then dropped to the top and the inner slope, damaged the base of the wave-prevention walls and scoured the inner slope, causing a collapse of the walls and resulting in the calamity. So the defence criteria of dikes should consider wind speed and tidal level at the same time. Then the implication of the criteria of once in a hundred years should include two aspects: tidal level of once in a hundred years, plus corresponding wind speed, and wind speed of once in a hundred years, plus corresponding tidal level.
The dike along the Baoshan steel plant and the Jinshan petro-chemical
plant and sea walls to protect coastal industry, ports, airports and areas
for travel and holidays, belong of first-level dikes. They should adopt
the defence criteria of once in a thousand years. The remainder should
adopt the criteria of once in a hundred years. The example of the dike
in Wenzhou indicated that: facing the reality of sea level rise, while
taking relevant engineering measures, we might not need to raise the height
of the embankment in complete accordance with the sea level rise. We should
consolidate the top of the dike and strengthen the wave-prevention walls
and anti-wave walls. But only raising a simple wave--prevention wall of
0.8 m high on the current dike without top protection needs an investment
of 0.8 billion yuan. Considering the protection of the bank top and slope
(outside and inside), we need to consolidate about 400 km of dikes, for
which the accumulative investment would be 3.3 billion yuan. On these conditions,
the project could satisfy the requirements of flood prevention up to 2050,
but it would be a huge and difficult project.
The urban relief is low in Shanghai. Facing the average tidal level
of 2.2 m at Huangpu Park, many areas are not conditioned to discharge naturally.
Domestic sewage, industrial wastewater and rainwater all need raising by
water pumps and then can be discharged outside. If rainstorm and tides
occur at the same time; it is easy for waterlogged fields to form. Furthermore,
the ground base in Shanghai is subsiding. The ground level subsided from
2.4 m to the current 1.4 m. The loss of relief resource is more than 40
per cent. Over the next several decades, the sea level rise will increase
the loss of such resource.
Table 25. The status of the drainage system in Shanghai urban area
At the same time, the drainage criteria are relatively low and the runoff coefficient is relatively small. The drainage criteria has been advanced to once a year (rainfall 36 mm/h and runoff coefficient 0.5-0.6), but this requirement cannot be met yet. What is more worrying is that the actual discharge capability is much lower than the criteria. As of May 1994, there were 417 pumping stations for drainage in Shanghai (165 in the urban areas). There was about 960 m3/s runoff, which was discharged directly into the river. But the equipment in the pumping stations was too old and could not be used. So the only real drainage capability exists in Wusong. Minhang and Taopu Industry Zone could meet the criteria (once in a year), while there are some parts in Nanshi, Luwang, Xuhui, Yangpu, Hongkou, Changning and Zhabei districts where the drainage capability is less than the criteria of once in half year. Once rainstorms and tides come, it will continue to be difficult to deal with the outside floods and the inner waterlogged areas.
(b) Water level rises, transport capacity of river decreases and depressed areas expand
Sea level rises result in water level rises in the rivers. The relative height of works in the port and wharves along rivers becomes lower which affects the handling capacity of the ports. The net spatial height of bridges becomes lower which reduces the transportation capacity of the ships passing under them. At present, even small barges cannot pass some bridges on the Suzhou River during high tides. Further water level rises could make the bridges float dangerously. Thus, the newly built large bridge on the Huangpu River should take these important aspects fully into account.
The water level rise inevitably leads to a rise of groundwater table. According to the above estimates of sea level rises, through the decades 2010, 2030 and 2050, the current depression area in Shanghai will extend from present 680,000 ha to 730,000, 1,030,000 and 1,330,00 ha respectively, representing about one-third of all cultivated land, which will increase the demand for the construction of water conservancy projects in heavily silted areas. On the other hand, the groundwater table rise will seriously endanger agricultural production. If the river level rises 43 cm, the groundwater table of 60 per cent of the farmland in the suburbs of Shanghai will escalate to 50 cm, about half of which land is unsuitable for dry cultivation. There are 80,000 ha farmland in the three islands of the Yangtze River and some parts of Minhang, Jiaxing and Baoshan region whose cultivation capacity will worsen. As a result, the production of summer grain crops may reduce by up to 420,000 tons every year, which will adversely affect the life of the urban people.
(c) Invasion of salt water, pollution of water sources and salinization of cultivated land
The mixed belt of fresh and salt water in Shanghai generally resides near Jiuduansha at the Yangtze River estuary. If a serious great drought and huge tides occur at the same time, however, the belt can reach near the Baogang and Chenhang reservoirs and even farther. According to current hydraulic conditions at Wusong Estuary and the predicted sea level rises, and considering the future dredging of the Yangtze River estuary and sand bar, salt water isoline of 2 per cent will move upward with increasing speed, surpassing the Chenhang Reservoir and reaching the west reaches of the Liu River mouth. The Huangpu River will be most affected by the invasion of salt tides, which will not only increase the possibility of undrinkable water but also affect water for industrial use. Especially in the Chenhang Reservoir, the second fresh water source of Shanghai, the dam will be endangered. The period for drawing and storing fresh water will be shortened (principally because its practical storage capacity will be reduced), which will not only directly affect Shanghai’s normal water supply, but also influence the protection of water sources and further development and utilization.
Sea level rises will exacerbate the situation with regard to seawater invasion in the coastal zones in Shanghai and will increase the salt content of groundwater. At present, there are 72,000 ha of reclaimed arable tidal flat along the 170 km-long Shanghai coastline. Of this, 20,000 ha has not been desalinized effectively. Some harnessed cultivated land may be desalinized by seawater invasion. Until 2050, there is likely to be over 50,000 ha of salinized farmland in the Shanghai suburbs which will seriously affect plant structure, variety structure and the production levels of suburban crops.
(d) Slow-down of deposits of tidal flat resource
The sedimentation of the Yangtze River at Shanghai has been going on for some 6,000 years ago. Thus the formation of Shanghai is closely related to the Yangtze River. At present, Shanghai city obtains precious land from the reclaimed tidal flat. During 1980-1990, 7,100 ha of land was added through tidal flat reclamation. Now the sediment carried by the Yangtze River is continually deposited at the eastern sand beach in Congming, Hengsha, the southern sand beach in the Yangtze River estuary and the beaches along Hangzhou Bay. These beaches advance with a speed of -1 km/40 years, with the result that there will be 17,000, 38,000 and 50,000 ha of tidal flat along these beaches by the years 2010, 2030 and 2050 respectively.
However, the sea level rise will submerge some current tidal flats. As the estuary marine hydraulics become relatively stronger and the sediment-carrying capability of the water body increases, the erosion of the estuary and coastal beaches will be intensified. As a result, a large amount of tidal flat may be fully eroded. According to the estimates (16 cm in 2010, 30 cm in 2030 and 43 cm in 2050) of sea level rises, there could be as much as 2,100, 4,000 and 6,000 ha of tidal flat lost respectively. In addition, the increase in size of tidal flats also depends on the amount of sediment in the Yangtze River. In the 150 years following the closure of the Three Gorges Great Dam, the sediment carried by the Yangtze River will decrease 0.114 billion tons every year. 0.5 billion tons of sediment will be deposited in 50 years and the sand content will be about 3.5 billion m3. The sediment can accumulate to a height of 5 m and cover an area of 700 km2. If all the sediment were to be deposited at the estuary, there will still be 1,400 ha of tidal flat lost every year in the following 50 years than might be expected under natural conditions. Considering all these factors, more than 1,400 ha tidal flat will be lost every year in Shanghai.
In synthesizing the above aspects, sea level rise presents a serious
challenge to the environmental protection of Shanghai in the coming century.
All administrative departments in Shanghai are paying much attention to
this problem and are working out appropriate strategies and measures. The
key will be to emphasize the role of the sciences in the prediction and
decision-making process, to master the regional model of global changes
and to respond with sound technical information and a scientific approach.
This is the intelligent approach to solving the specific and concerted
problems of environment and development in developing countries. All the
people in the Netherlands pay attention to sea level rises and the influence
of ice-snow melting in the Alps on the floods of the Rhine River. Its national
hydrological station network automatically monitors and measures these
influences once every 10 seconds, forecasts the conditions and situations
of water bodies every 10 minutes, and then disseminates this information
in great detail to the whole nation. This abundance of information is a
prerequisite in avoiding severe disasters. The Tai Lake watershed is the
hinterland of resources and the economy in Shanghai. The flood prevention
and water quality protection of the Tai Lake directly affects the safety
of Shanghai in its lower reaches. The Tai Lake Watershed Management Bureau
of National Department of Water Conservancy is building an automatic system
for the monitoring of watershed water and water quality with a loan from
the World Bank. It can collect all data of the watershed water situation
and water quality every 15 minutes and provide unprecedented informational
support for watershed scientific management.