Over the past 40 years, the Aral Sea on the border between Kazakhstan and Uzbekistan has split into two. Four decades of syphoning water from feeder rivers for irrigation have caused its surface to shrink by 60%, and its volume to plummet by 80%.
The resulting dust bowl around the sea's edges is toxic and, when whipped up in high winds, poisons the population in the region. Increasing salinity has also killed fish stocks and the local shing industry.
During this same period development on the flood plains anking one of the Aral's tributaries, the Syr Darya, has prevented flood waters being passed down to the sea. Engineers are working to increase the river's capacity.
Falling water levels resulted in a split appearing in the Aral in the late 1980s, explains Mike Haigh, international director at consultant Mott MacDonald.
Water still owed intermittently from the North to South Aral, but it merely slowed the larger sea's rate of desiccation.
The Kazaks decided to replenish the northern half of the sea by sealing it from the southern, Uzbeki, half and by bringing more water down the Syr Darya.
'The Kazakh government saw an opportunity to build a dyke across the spit of land now separating the North Aral Sea from the south, and to raise the water level in the north.' Its aim was to raise water levels, replenishing the sea's once abundant sheries. Higher water levels would also revive local agriculture and improve the lives of over 100,000 people.
Two attempts at building the 12.8km long earth embankment, in 1992 and 1996, were washed out, but the failed structure retained water long enough to prove that water levels would rise, says Haigh.
This helped the Kazakh government convince the World Bank to put up 65% of the £57M funding needed for construction of new structure. The five-year project also included works to increase the capacity of the 1,000km long Syr Darya river within Kazakhstan.
In 2002, Mott MacDonald in joint venture with Turkish firm Temelsu International, and help from local consultants, was appointed to supervise dyke construction and to nd ways of reducing water loss from the Syr Darya River.
Early last year, Russian contractor Zarubezhaodstroy completed a massive four-year earth moving operation to complete construction of the North Aral Sea Dyke using local sands and gravels.
The northern face of the dyke has a shallow gradient mimicking the Aral's natural shore. The embankment also incorporates a 395m 3/second concrete spillway to ensure the structure cannot be eroded by overtopping.
Water levels in the North Aral Sea have already risen by more than 8m - faster than expected.
As a result the local fishing industry is beginning to recover.
Attention is now focused on improving water ow down the cascade of dams and barrages along the Syr Darya River.
'When we started looking at the Aral Sea in the late 1990s it became clear that we had to get more water into it, ' explains Mott MacDonald technical director Jack Meldrum.
'That involved looking at Chardara Dam at the top of the Syr Darya River system, and at three major barrages which were impeding ow.' These barrages are supposed to regulate ow to help with irrigation and maintain constant water ows down the river, preventing flooding.
But they had been badly maintained and were in poor condition. As a result water was held back in the Chardara reservoir and excess water tipped on to a nearby low-lying area known as the Arnasai Depression so it could evaporate.
Allowing this excess water to flow downstream is being achieved by refurbishing the dam and barrages, increasing their capacity.
At the same time some 320km of flood dykes have been repaired and raised, so the river can pass higher water volumes.
'Once we've completed the final phase of works on the Syr Darya later this year they will be able to pass more spare water down the river rather than pouring it uselessly away, ' Meldrum notes.
Contractor China GeoEngineering Corporation (China Geo) has already carried out major refurbishment of two of the barrages at Kzyl-Orda and Kazalinsk and reconstruction of a barrage at Aklak which had been undermined by scour.
'Abstraction higher up the river changed the flow regime at Aklak. The river bed became steeper and exposed the downstream toe of the weir, ' says Haigh. It was replaced with a reinforced concrete structure two years ago.
With bottlenecks along the Syr Darya eased if not eliminated, increasing flow into the North Aral Sea now hinges on completion of critical strengthening and re-engineering works on Chardara Dam.
This holds back a 4.6km 3 reservoir, 70km long. 'It was built by prison labour in the early 1960s for irrigation, and has a hydropower station that generates mainly from irrigation releases. It also serves a flood protection function, ' Meldrum says.
Water flow to the Syr Darya River is regulated by the hydropower station and four spillway gates, arranged two either side of the hydropower plant.
The dam needed extensive refurbishment because flaws in the original design prevented it from releasing excess water quickly enough, forcing operators to tip it into the Arnasai Depression (see box).
The next phase of the North Aral Sea project, now getting under way, is focused on water management along the 1,000km of the Syr Darya, says Haigh.