If sea level rose one meter, inundation or more frequent flooding
would cover 125,000 square kilometers, home to 73 million people
on China's long coastline. Protective dikes are already overtopped
during typhoons, coasts are eroding and saltwater intrudes along
the four large coastal plains. Natural subsidence is exacerbated
by excessive drawing of groundwater. But protection of these coasts
would be both affordable and economically effective, according
to the authors.
China began constructing dikes 1,000 years ago, and the rate of
protection has been increasing. Ten to 1 5 typhoons a year bring
storm surges which, if superimposed on high tides, can cause disastrous
flooding on Chinese low-lying plains, causing significant economic
losses. All of the low-lying areas are now protected to varying
degrees. But typhoons may be intensified by global warming. The
typhoons have been taking more northerly paths since the 1970s,
possibly as a result of global warming, but research is needed
to confirm this.
In the four major plains:
-
Sea level rise of one meter, plus a one meter storm surge
and 2 meter high tide would overwhelm existing dikes of the
North China Plain and submerge the eastern margin where
subsidence has occurred because of over-pumping of groundwater.
Tianjin, the country's third largest city with a population
of more than eight million, lies in this plain.
-
Storm surges frequently flood Shanghai, China's largest city
with a population of 12.5 million (1987). It is situated in
the East Plain, where long-term subsidence has been
exacerbated also by excessive pumping of groundwater. Recently,
a tidal gate at the mouth of the Suzhou River and a two-level
dike along the Huangpu have been built to protect the city,,
at the level of a 1,000-year storm surge, assuming existing
sea level. But unless there were additional protection, a
one meter sea rise with an additional one meter surge plus
a two-meter high tide would threaten a vast area of the East
Plain, up to 180 km from the coast and would permanently inundate
nearly 35 percent of the land area including most of the central
city. The effectiveness of the new flood defenses around downtown
Shanghai would then only protect against a 100-year instead
of a 1,000-year surge.
-
Typhoon-driven waters frequently flood the whole of the Zhujiang
(Pearl) River Deltaic Plain. With a one meter rise plus
surge plus high tide, the water level would be nearly three
meters above the existing mean sea level, threatening more
than 7,000 square kilometers.
-
Frequent flooding also threatens a 3,000 square kilometer
area of the Liao River Deltaic Plains, including Yingkou,
with a population of two million and some mid-size cities.
Coastal Erosion During the last 40 years, most
of the nation's rivers have been dammed at the middle or upper
reaches and many also at their mouths by sluices to prevent the
intrusion of salt. This has blocked sediment from reaching the
coasts, leading to erosion. The Huang He (Yellow River) changed
its course in 1855, leaving a large area of the East China plain
with no sediment supply. The coastline retreated more than 22
kilometers since then, leading to a loss of 1400 km2 of land.
Along the coral and mangrove shores n the mountainous coastal
zones, the mining of beach sand and gravel for building houses
and dams plus tectonic subsidence has lead to serious erosion
and retreat of the coastline by three meters a year in the last
30 years. So far city planners have ignored these processes.
Salt Intrusion Tides have a large range in most
of China, and they can penetrate long distances up the main rivers,
carrying salt water 60, 70 even 250 kilometers upstream. Sluices
have been built across most river mouths to prevent this and develop
irrigation. Since the 1980s, excessive pumping of ground water
has led to the intrusion of salt and the deterioration of groundwater,
salt intruding at the rate of 90 meters a year in the southern
margin of the Bohai Sea in the early 80s, rising to more than
400 meters in the late 80s. It is now very difficult to obtain
drinking water , and agriculture has been damaged. Sea level rise
will intensify these problems.
Mangrove Loss The forests have been over-exploited, especially
for fuel. However, mangroves have been successfully planted to
protect farmland along the Fujian coast and extended into new
areas. While further research is needed, this experience suggests
that if there is not excessive exploitation, the mangroves might
survive a one meter sea rise.
Economic Impacts All of the coasts of China are used in
some way. The coastal products of agriculture and industry account
for more than 50 percent of the country's GNP. China has the largest
marine aquaculture industry in the world, and agriculture and
aquaculture sites are being developed based on present sea levels
with little regard for possible rising ocean waters.
Sea level rise would threaten the economies of China's most developed
cities and regions, its important industries, harbors, commercial
businesses, etc. Coastal tourism, including recently developed
tourist beaches are vulnerable to erosion, and future planning
has not taken into consideration possible future threats. The
value of land, buildings and warehouses could change, even if
high-cost dikes were constructed, and less vulnerable sites inland
would become more attractive.
Response Strategies As it would be impossible
for the large and vital population and industries of the coasts
to move inland, the country's only alternative is to raise and
consolidate existing dikes and build additional ones as necessary.
Although this is an expensive proposition, recent experience suggests
provincial and municipal governments and large, state-run enterprises
could afford it. The total length of the mainland dikes amounts
to 13, 000 km. The North China coastal plains, as an example,
would require 500 km of standard dikes to prevent flooding from
a one meter sea level rise, costing about US $370 million, equivalent
to 0.82 percent of the GNP of two related provinces and one municipality.
Other regions are likely to have similar costs. Cities, towns
and industrial structures would need to be relocated and replanned,
and sewage systems would need to be reconstructed. The most urgent
requirement would be establishment of a government authority to
start planning and coordination of response strategies and counter
measures, including fund-raising.
(Mukang Han, Jainjun Hiou, and Lun Wu, Peking University,
China)
