The Three Gorges Project.  Part IV:  Siltation of TGP.
 
Let us now get to the technical part of TGP.  Siltation will be discussed first
because it is generally considered as the hardest as well as the most common
problem for every dam.  Siltation is often regarded as the "cancer" of a dam
because it largely determines the lifetime of a project. This is especially true
for those dams in tropical areas, where rivers are rich in silt.  The Yangtze
River is the 4th most silt-laden river in the world.  Every year, it carries
about 680 million tons (the average of the recent 6 years) of silt to the East
China Sea.  Therefore siltation is a serious problem for TGP.  Whether it can
be solved properly will directly affect the performance of the dam, as
already proved by the case of the Sanmenxia Reservoir.  Although it is
difficult to predict the rate of sedimentation, it is generally agreed that,
under normal operating conditions (to be described in more detail later),
more than 70 % (i.e., about 0.5 billion tons) of silt will be trapped behind the
dam every year during the early period of TGP.  This implies that, within 50
years, the sediments of the Yangtze River will silt up more than 10 billion
cubic meters of the reservoir volume.  For the 150-meter scheme, that means
all the dead storage will be silted up.
 
        Siltation is closely related with deforestation and soil erosion.
Deforestation also dramatically increases the risk of flooding which will in
turn aggravate siltation.  Soil erosion not only deprives the area of nutrition
and deteriorate the quality of the land,  but also silts up dams and dykes,
elevates the river bed and therefore the water level, and aggravate flood
devastation.  The outcomes of deforestation are well explained by Goldsmith
and Hildyard (1984, p125):
        "When the catchment area of a river is heavily forested, the elaborate
root system of the trees acts as a vast sponge which soaks up rainfall,
releasing it only very slowly to the river below.  Once a catchment area has
been deforested, however, the run-off (as a proportion of rainfall) is vastly
increased.  Thus, a recent UNESCO study found that, when forested, the
watershed of one selected river only released between 1 and 3 % of the total
rainfall; by contrast, once the area was deforested, between 97 and 99% was
released to the river.  During periods of heavy rainfall, therefore, the volume
of water carried by rivers in deforested areas can be massive."
        "In a heavily forested area, the soil's organic content - and hence its
structure - is maintained by the decomposition of the forest litter.  At the
same time, the soil is held together by the elaborate network of roots which
underlies the forest floor and is, thus, subject to minimum erosion.  Once the
forest cover has been removed, however, the roots rot away and there is no
longer anything to hold the soil together.  In addition, the soil - deprived of
the forest litter - rapidly loses its structure, becoming very vulnerable to
erosion by wind and water.  A study . . . shows that a natural rain forest lost
only 1 ton of soil per hectare to soil erosion; soil losses increased to between
20 and 30 tons once the trees had been removed and the land put to
cultivation."
 
        The above statements have been proved by the situation in China.
The upper reach areas of the Yangtze and its tributaries used to be the
second largest forest reserve area of the country.  During (and after) the
Great Leap period, the forests in this area have been seriously devastated:
for more than 30 years, the extinction of forests has been 10 times faster
than its regeneration.  The forest coverage of the southwestern area (e.g.,
Sichuan, Yunnan and Guizhou provinces) has been reduced by more than
half, from 28 % in the early 50's to only 13 % in the 80's.  The forest
coverage of the upper reach of the Jinsha Jiang (in west Sichuan) has also
decreased from 40 % to 19 % within 30 years.  The average forest coverage
of the middle Sichuan hills is only 3.5 % [1].
        As a result, in the recent years, the threat of flooding of the Yangtze
and its tributaries has been more often than ever.  In 1981, the middle
Sichuan area was hit by a record-breaking flood, which killed more than
1,000 people.  At the mean time, according to the measurements at the
hydrometric station at Yichang, the amount of silt carried by the Yangtze
River in the recent 6 years has increased by 30 %.  The soil erosion of the
upper reach of the Yangtze River has reached 1.8 billion tons every year, in
which Sichuan alone contributes 640 million tons.  Every year, the three
major rivers in Sichuan carry about a quarter billion tons of silt,
representing 5 centimeters of topsoil over an area of 166,000 hectares of
cultivated land.  It will many years to recover the quality of the eroded land.
 
        We have seen from above that the environmental condition of the
Yangtze River has reached a critical state.  Although the government has
planned to recover the forest and plantation in 30-40 years, the situation is
still deteriorating because, due to the shortage of fuel and electricity in the
vast rural area, firewoods are the only energy source of the local peasants.
It is noticeable that, despite the devastation of forests in the upper reach of
the Yangtze, the silt in the river did not seem to increase until 1981. But the
silt did not disappear.  It silted up many dams built during the 60's and the
70's.  For the 10 reservoirs in Wushan (Mount Wu), a county located
between the Three Gorges, 62 % of the total volume were silted up within
the last 10 years.
        All these facts have indicated that the increasing siltation caused by
the devastation of environment has started to exceed the capacity of the
dams already built on the Yangtze and the tributaries.  Taking into account
the fact that no large dams have been built so far on the Jinsha Jiang (a
mainstream of the upper Yangtze) to effectively reduce the downstream
siltation, it seems to be only a matter of time to silt up the future TGP.  The
experts estimated that it takes between 50 years (for the 150-meter
scheme) to 80 years (for the 180-meter scheme) for siltation of TGP to reach
equilibrium (a state when there is roughly the same amount of silt that
enters as it leaves the reservoir).  The siltation of TGP could be very serious
by that time, if no measures are taken.
 
        Of course there is always a way out.  Under normal conditions, for the
sake of power generation, improvement of navigation, and flood control of
the downstream areas, TGP is supposed to store water during flood season
and to release water during dry season.  Well, if this won't work (due to
siltation), there is always the Sanmenxia "experience",i.e.,"to store the clear
and drain off the muddy (water)" (Xu4 Qing1 Pai2 Hun3).  But that means
TGP has to "drain off the muddy" during muddy (i.e., flood) season and give
up flood control, a major benefit of TGP repeatedly emphasized by its
proponents.  More disturbingly, the opponents of TGP argued that such a
scheme is unlikely to solve the accelerated siltation near the tail of the
reservoir [2], where silt tends to accumulate and is hard to be flushed away. If
that happens, TGP will seriously deteriorate the navigation condition of the
upper Yangtze during dry season.  Here we already start to see the conflicts
among the various benefits of TGP claimed by the proponents of the project.
(to be continued)
 
Notes:
[1]     Data from Chinese Academy of Science, 1988.
[2]     More details will be found in Part VI. Navigation.