Sediment problem eased at Three Gorges
The build-up of silt, a key problem
threatening the huge Three Gorges Reservoir, the largest one of its kind on the
Yangtze River, is under control.
About 40 per cent of sediment
flowing into the reservoir at its dam site has been washed away, with the amount
of remaining suspended silt getting smaller.
Annual sediment passing through the
dam site totals 530 million tons, and has been reduced to 200 million tons,
China Three Gorges Project Corporation announced Tuesday.
"Most sediment can be discharged
from the reservoir to ensure its long-term ambition of controlling floods,
improving navigation and generating hydropower," the corporation said at the
release of its latest monitoring report Tuesday during the ongoing ninth
International Symposium on River Sedimentation (ISRS).
One effective way is to lower water
levels and flush away the higher sediment brought into the reservoir during
flood periods by releasing floodwater through the huge sluice gates at the
bottom of the dam between June and September -- the peak period of summer
By the end of the flood season in
October, the huge reservoir then stops releasing floodwater and starts to store
water with a low sediment content to generate hydropower and improve navigation
up and down the dam site.
In the past, the sediment discharge
operation has enabled effective operation of the Three Gorges reservoir and
prevented drastic shrinkage of its water storage capacity resulted from
increasing sand filling -- a chronic problem plaguing most reservoirs on high
sediment-laden rivers, according to the report.
Meanwhile, to reduce sediment
upstream, a massive water and soil conservation programme has been launched to
rehabilitate the ecosystem of the Yangtze River.
Over the past two to three decades,
a score of major reservoirs were built along major tributaries upstream of the
Yangtze to prevent sediment from entering the Three Gorges Reservoir.
In the next 10 years, a group of
large reservoirs and key hydropower stations are scheduled to be completed
upstream of the reservoir to further reduce sediment from flowing downstream,
Since 2002 three projects have been
launched in the north to prevent the further rising of the Yellow River's bed
which has risen 10 metres above its levees for hundreds of kilometres in the
Henan and Shandong sections of its lower reaches.
During these operations,
high-sediment laden waters were released from key reservoirs upstream in
man-made waves to flush away millions of tons of sand downstream to deepen the
river bed and enlarge the flood discharge capacity.
Such experiments, the only ones of
their kind ever done, have been proven effective for the Yellow River -- the
world's muddiest river -- to keep its balance between water and sediment and
sustainability of its ecosystem.
These major achievements in sediment
control and reduction have been submitted to the ongoing ninth ISRS for
Sponsored by the Ministry of Water
Resources and supported by the United Nations Educational, Scientific and
Cultural Organization (UNESCO), some 500 Chinese and foreign experts are
attending the symposium to share their latest research and achievements made all
over the world in erosion control and river-related sedimentation reduction.
To further promote international
co-operation in the field, all delegates applauded the symposium's decision for
the official inauguration of the World Association for Sediment and Erosion
Research, which was also announced Tuesday.
Experts and officials from UNESCO
said they were confident that association and co-operation with it can further
improve global conservation, utilization of soil and water resources and the
betterment of ecosystems along the world's river basins.
Sedimentation problems are a matter
of global concern as they include issues arising from land erosion,
desertification, sediment yield, transport and deposition in reservoirs and
lakes, process of river course, estuary and coast and interactions between
sediment and hydropower projects, experts warned Tuesday.
According to preliminary statistics,
the annual erosion of surface soil from global river basins amounts to 60
billion tons with 5 to 7 million hectares of farmland lost each year.
About 1 per cent of the precious
storage capacity of the world's reservoirs is annually lost due to river-related
sedimentation with more floods and droughts induced and ecosystems deteriorated
as a result.
Experts attending the seminar made it clear that the problem has become a global challenge with adverse impacts on the worst problems facing humankind this century -- rapid population increase, a worsening shortage of resources and the rehabilitation of ecosystems.