Leading edge

In four decades the Aral Sea has shrunk 60% in area, stranding ships

Soviet agricultural planners robbed the Aral Sea of water, all but killing it. Some visionary engineering is saving it from extinction.

In 1960 the Aral Sea, straddling the border between Kazakhstan and Uzbekistan, was the world’s fourth largest inland water body. Four decades on, the sea’s surface area has shrunk by 60% and its volume by 80%. Impacts on biodiversity, economic productivity and human health have been profound.

Yet the outlook is not entirely bleak. Some visionary remedial engineering, now in its final phase, is restoring water to a small area of the once mighty Aral. The Aral Sea’s precipitous decline started when Soviet agricultural planners diverted water from its principal tributaries, the Syr Darya and Amu Darya, to irrigate vast cotton plantations. Without replenishment, the sea’s level fell – by roughly 200mm a year during the 1970s, accelerating to nearly 800mm a year in the 1990s. As the freshwater Aral evaporated, it became saline, killing fish stocks on which local fishermen relied. Ships foundered in the shallow waters. Wetland and pasture on the sea’s margins and in the river deltas were lost.

As the Aral’s water level fell, the sea divided in the late 1980s, explains Mott MacDonald water director Mike Haigh. “The Kazakh government saw an opportunity to build a dyke across the spit of land separating the North Aral Sea from the South and to raise the water level in the North.” Its aim was to bring the sea back across 100km of parched sea bed to the North Aral Sea’s former economic hub, Aral Harbour, to replenish the sea’s once abundant fisheries and revive local agriculture, turning around the lives of over 100,000 people.

The World Bank has put up 65% of the £57 million needed for construction of the new dyke structure and to increase the conveyance capacity of 1000km of the Syr Darya River within Kazakhstan. Mott MacDonald in a joint venture with Turkish firm Temelsu International, and with help from local consultants, was appointed to supervise dyke construction and to design the Syr Darya River works. After a massive earth moving operation, Russian contractor Zarubezhaodstroy completed construction of the North Aral Sea dyke early in 2006 using local sands and gravels. The upstream face has a shallow gradient mimicking the Aral’s natural shore. A 395m3/second concrete spillway into the South Aral ensures the structure cannot be eroded by overtopping.

Water flow down the Syr Darya was impeded by the Chardara Dam at the top of the river system and three major barrages, built for flood control and to regulate water flow for irrigation. Mott MacDonald technical director Jack Meldrum says that the barrages had been badly maintained. Contractor China Geo-Engineering Corporation (China Geo) has already carried out major refurbishment of two of the barrages at Kzyl-Orda and Kazalinsk, and rebuilt a barrage at Aklak, while 320km of flood dykes have been repaired and raised, enabling the river to pass higher volumes of water.

With bottlenecks along the Syr Darya eased, increasing flow into the North Aral Sea has hinged on completion of re-engineering works on Chardara Dam, which holds back a reservoir of 4.6 billionm³. The dam’s hydropower station and four spillway gates regulate water flow to the river. “There was a vibration problem with the gates, which forced the operator to restrict opening to about 40% for fear of causing them to fail,” Meldrum reports. “It was expected that replacement of the aged gates would solve the problem. But monitoring the spillways revealed that the hydraulics of the structure were wrong. Water was rebounding from baffle piers in the spillways’ stilling basins and hitting the tops and sides of the structures, stopping air flow. This was creating pressure waves behind the gates, causing the vibration,” says Meldrum.

Baffle piers remove energy from water to prevent erosion downstream of the dam. They were oversized and badly profiled for the flow volume and velocity, and replaced with smaller baffle piers. Work was completed by China Geo in September, enabling both sets of spillway gates to be fully opened.

Water levels in the North Aral Sea have already risen by more than 8m – faster than expected – enabling significant growth of the fishing industry.

Key sustainability facts

• Replenishment of an inland sea
• Restoration of fisheries, shipping and wetland agriculture
• Reduction of environmental pollution and resulting ill health