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Level pegging

Central Asia: Kazakhstan

After decades of depletion, the Aral Sea in Kazakhstan is saline, toxic and half its former size. Andrew Mylius finds out how engineers will restore it.

Since 1960, when the Aral Sea was the fourth largest freshwater body in the world, its level has fallen by 16m and its surface area has halved. Agricultural irrigation has reduced the two principal rivers feeding the great Kazakh lake to the point where they cannot sustain it. The consequences have been catastrophic.

Winter and summer temperatures have steadily become more extreme, dramatically shortening growing seasons.

Added to this, wetland and pasture on the lake's margins and in the river deltas have been lost. Agricultural productivity has been decimated.

And as the Aral Sea has shrunk it has become poisonous. Higher concentrations of pesticides and salts from fields have wiped out many plant and animal species, and all but destroyed what was until a few years ago a major fishery. Meanwhile, salt and toxins blown up from the dry lake bed are causing cancers and respiratory diseases which have taken a heavy toll on life expectancy.

But the first steps towards restoring the Aral are now being taken. Under a ú57M scheme financed 65/35 by the World Bank and Kazakh government, major civil engineering works are being undertaken to revive the northern portion of the Aral Sea.

Work consists of two distinct halves - closing up a natural constriction in the Aral Sea to divide the northern part permanently from the greater southern body, and improving water flow down the 900km long Syr Darya river.

The logistical challenge of managing projects over such a vast geographical area will be immense, observes Mike Haigh, international director at consultant Mott Macdonald, which is undertaking detailed design and construction supervision.

The Russian contractor Zarubezhaodstroy is now mobilising to build the 12.7km long north Aral dyke. Barriers have been constructed twice before, in 1992 and 1996, but both times the sand and rock structures have been overtopped and washed away. Zarubezhaodstroy will incorporate remaining parts into its new structure, but this time will equip it with erosion protection and a 395m 3/second capacity concrete spillway to ensure its durability.

On the Syr Darya River several major structures impede flow, resulting in loss of valuable water. At the head of the river, the path of snow melt and rainfall runoff is blocked by the ageing and poorly maintained Chardara Dam. Discharge gates have become inoperable, reducing peak flows from 1,800m 3/s to 1,000m 3/s. As a result, in the last couple of decades floodwater has been diverted into the low lying Arnasai Depression, where it is lost to the Aral.

Repair and modernisation of Chardara dam will see structural repair and a refit of mechanical and electrical equipment, says Haigh. Geotechnical investigation of the embankments will gauge the danger of liquefaction under seismic loading.

Further obstructions downstream at Aklak, Aitek and Koraozek must also be removed.

Chinese contractor

China Geo-Engineering Group will begin work shortly on replacement of the Aitek headworks, built in the mid-1980s to take off water for irrigation. The reinforced concrete structure is close to collapse, and has too little capacity to cope with the 1,500m 3/s floods that surge down the river in spring, which instead inundate the city of Kzyl Orda.

China Geo-Engineering will also be responsible for a major overhaul of mechanical and electric systems at Kazalinsk and Kzyl Orda, and rehabilitation of 320km of dykes.

The Aklak weir is the last control structure on the Syr Darya before it discharges into the Aral Sea, constructed in the 1980s to maintain river levels upstream.

However, a steep gradient developed in the river, and resulting downstream erosion undermined the weir, bringing it to the point of collapse. A new structure will be built downstream with a 500m 3/s capacity.

It will take a decade to bring the north Aral up to its desired level, says Haigh, restoring the livelihoods of 150,000 people.

INFOPLUS

www. worldbank. org www. dfd. dlr. de/app/land/aralsee

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