The Three Gorges Project.  Part V:  Flood Control.
 
(I)  TGP and the ecology of the middle reach of the Yangtze River.
        Let's leave aside the impossible siltation problem for now.  A major
purpose for building TGP is to alleviate the flood threat of the Yangtze River.
However, this claim needs to be examined more carefully.
        We have already discussed in Siltation of TGP that the vegetation of
the upper reaches of the Yangtze River and its tributaries has suffered
severe damage.  If anything, the situation of the middle reach is only worse.
The middle reach of the Yangtze, a rich plain "full of rice and fish" (Yu3 Mi3
Zhi1 Xiang1), is the hometown of most lakes of China.  For the last 30 years,
under the pressure of the population growth, large-scale relamation
activities have been converting the lakes into farmland in this area.  This,
together with siltation, has dramatically reduced the area and the active
storage of the lakes.  Many lakes had even disappeared permanently.  As a
result, the flood control capability of these lakes has been diminished.
 
        Hubei used to have the reputation of "a thousand-lake province".
During the early 50's, it had 1066 lakes with the total lake area of 830,000
hectares.  In 1977, only 326 lakes were found left, with the total lake area of
240,000 hectares, less than 30% of that in the 50's. The total area of the Four
Lakes (i.e., Hong Hu, Chang Hu, San Hu, and Bailu Hu, where Hu = Lake) has
been decreased from 200,000 to 84,000 hectares, accompanied by an increase
of low-productivity farmland of 100,000 hectares.  The active storage volume of
these lakes has been reduced by more than 1 billion cubic meters.  This area
usually has half billion cubic meters of excessive water during flood season,
which has been causing drought for almost every year [1].
        Lake Dongting, which is the second largest lake of China and is mostly
located in Hunan, is another typical example.  There the rate of population
growth has been as high as 2.86 % every year.  The following table has
shown clearly that the reclamation activity due to the population growth is
the major factor that has caused this lake to shrink by 40 % in volume.
 
Table I.  Population, farmland and lake area of Lake Dongting [1].
Year    Population (millions)   Farmland (Km^2)         Lake area (Km^2)
1949            2.66            3,960                   4,350
1979            5.98            5,790                   2,691
 
        Experts have predicted that Lake Dongting will be silted up before the
year 2,000.  By that time, this lake, now the second largest in China, will
disappear during dry season of a year.  The dramatic reduction of the
regulatory capacity of the lakes has made the middle reach of the Yangtze
River much more vulnerable to the threat of flooding.
 
        TGP is proposed to be built at the intersection of the upper and the
middle reaches of the Yangtze.  It will be able to adjust the water from the
upper reach, whose drainage area is 1.0 million square kilometers which is
55 % of the total drainage area of the Yangtze River.  Because most sediment
will be trapped behind the dam, much less siltation will occur to Lake
Dongting (which is downstream of TGP).  Therefore, TGP will extend the
lifetime of Lake Dongting.  However, the effective volume of TGP for
flood control is only between 7.3 billion (for 150-meter scheme) and 24.9
billion (for the 180-meter scheme) cubic meters, which implies that the
flood control capacity of TGP is rather limited.  The opponents of TGP pointed
out that two types of flooding, namely, the ones that occurred in the upper
and middle reaches (e.g., in 1870 and 1981) and the ones in the middle and
lower reaches (e.g., in 1931 and 1954), should be treated differently.  The
effects of TGP on the upper, the middle and the lower reaches of the Yangtze
River are discussed separately as follows.
(to be continued)
 
Notes:
[1]     Data taken from the Chinese Academy of Science, 1988.
 
 
                The Three Gorges Project.  Part V:  Flood Control.
 
(II)    Effects of TGP on the flood control of the upper, middle and lower
        reaches of the Yangtze River.
 
(1)     TGP may largely alleviate the flooding of the middle reaches, including
the Jing Jiang, the Four Lakes and Lake Dongting areas.
        As we have mentioned in Introduction, the Jing Jiang is the most
dangerous section of the Yangtze River.  It has become a "suspending river",
with the river bed about 2 meters higher than its north bank.  Due to
siltation, the south bank is already 7-10 higher than the north bank.  The
water level can be 6-10 meters higher than the north bank during flood
season.  The turbulent flow of the Yangtze is barely contained by the
Jingjiang Dyke, creating tense situations during the summer and the fall
every year.  Along the 182-kilometer Jing Jiang Dyke are inhabited more than 5
million farmers with over 200,000 hectares of farmland.  The Jing Jiang flood
diversion area was once built with huge cost.  But it can no longer be used for
flood control because the population there has grown from 170,000 to
410,000.
        The two largest floods in the upper reach occurred in 1870 and 1981,
both caused by heavy rainfall.  It was said that the flood in 1870 had been
the largest in this area for the recent 4,000 years, since the ancient time of
Da Yu.  Its peak flow broke the record of 100,000 cubic meters per second in
Chongqing and Yichang, almost twice of an average flood which is 52,000
cubic meters per second.  The 52.7 billion cubic meters of excessive water
drowned 240,000 people and more than 1 million hectares of land. If the flood in
1870 occurs again, because the 180-meter scheme can have a volume of
nearly 25 billion cubic meters for flood control, the Jing Jiang will have to
divert about 28 billion cubic meters of extra water.  The flood control
volume of the 150-meter scheme ( 7.3 billion cubic meters) is too small to
alleviate any large flood.
        The flood in 1981 was largely a result of deforestation and
reclamation on the sloping fields.  Due to the large scale land erosion [1]
triggered by heavy rainfall and landslide, the amount of silt measured at
Yichang hit the record of 840 million tons in that year.  The peak flow
reached 85,000 cubic meters per second at Chongqing and 72,800 cubic
meters per second at Yichang.  If the flood in 1981 occurs again, the 180-
meter scheme of TGP should be able to protect the Jing Jiang area from
severe flooding.  It should be noted that, in both years (1870 and 1981), the
middle and lower reaches around Wuhan only had moderate flooding.
 
(2)     TGP may provide limited help in the flood relief of the middle and
lower reaches of the Yangtze.  The limitation comes from the fact that TGP
cannot have any control on those large tributaries which join the Yangtze in
Hunan and Hubei provinces, both downstream of the dam.  For example, the
Han Shui, a silt-laden river which joins the Yangtze at Wuhan, has
contributed to the flooding in this area.
        In 1954, a serious flood hit the middle and lower reaches of the
Yangtze River, causing severe damage to the metropolitan city Wuhan.  It
took away 30,000 lives, drowned 3.2 million Ha of farmland, and interrupted
the Beijing-Canton railway (Jing1 Guang3 Tie3 Lu4), which is a key railway
connecting the north and the south of China, for nearly 3 months.  The
excessive amount of water reached 102 billion cubic meters.  But the peak
flow at Yichang was only 66,100 cubic meters per second, relatively small
compared to the floods of 1870 and 1981.  Therefore, the downstream
tributaries contributed significantly to the flood of 1954.  With the
improvement of the entrenchments and the flood plain management
program, the flood control capacity in this area has been increased by
30 billion cubic meters.  It is said that another 20 billion cubic meters
capacity can be obtained with further improvement.  If the flood of 1954
is encountered in the future, the 180-meter scheme should be able to
detain about half of the remaining 50 billion cubic meters of flood.
 
        It should be pointed out that, for either case (1) or (2), even the 180-
meter scheme can in no way eliminate the flood threat of the middle and the
lower reaches.  It is still important to improve the existing entrenchments
and the flood plain management programs, which, according to the
opponents of TGP, only takes less than 1 billion yuan to obtain an extra
capacity of 50 billion cubic meters.  Of course, the fundamental solution is to
restore the environmental conditions, i.e., to afforest the Yangtze and its
tributaries and to return the excessively reclaimed land (e.g., land around
the lakes and on the slopes) to the nature.
 
(3)     TGP will aggravate the flood threat in the upper reach of the Yangtze.
        The storage of water in the reservoir will elevate the water level
upstream of the dam, which is to make the flooding in the upper reach of the
Yangtze River more serious and long-lasting.  It is calculated that, for the
flood encountered once every 20 years, the water level at Chongqing will be
between 2.6 meters (for the 150-meter scheme) and 7.5 meters (for the 180-meter
scheme) higher than that before the dam is built [2].  That will cause more
flood damage to Chongqing, a city with the annual industrial output more
than 10 billion yuan.
 
        Here we have seen that TGP may cause a conflict between the upper
and the middle reaches regarding flood control.  When the reservoir stores
water to save the middle reach from flooding, it aggravates the flood in the
upper reach.  "To benefit the middle and lower reaches by sacrificing the
upper reach (of the Yangtze)" has always been the strategy of the central
planners in China.  What the Gezhouba project did to Sichuan province is just
a small example.  During the construction period of the Gezhouba project,
Sichuan suffered a loss of 30 million yuan due to the frequent interrupt of
the transport on the Yangtze River.  But so far it has not been able to get
subsidized.  The biased policy of the command economy is probably a major
force to maintain a rich east and a poor west. But for right now, let us not go
too far beyond TGP.
(to be continued)
 
Notes:
[1]     Knowledgeable readers may help me to find the English word for "Ni2
Shi2 Liu2".
[2]     The calculation has taken into account the siltation of the reservoir in
50 years after it is built.  (from The Chinese Academy of Science, 1988).