Flow regulation by water reservoirs

The Aral Sea basin has over 60 reservoirs with a usable capacity of over 10 million m³ each. Their total volume is 64.5 km³, of which 46.5 km³ is usable capacity, including 20.2 km³ in the Amudarya and 26.3 km³ in the Syrdarya basins. In the countries of Central Asia, there are 45 operational hydropower stations with a total capacity of 34.5 GW, and with individual capacities ranging from 50 to 2,700 MW. The largest are Nurek hydropower plant on the Vakhsh river in Tajikistan (2,700 MW) and Toktogul hydropower plant on the Naryn river in Kyrgyzstan (1,200 MW). Hydropower accounts for 27.3% of the entire power consumption in the Aral Sea basin. However, this value varies between countries, with Tajikistan generating the most (98% of its the total power generation), Kyrgyzstan ranking second (91%), and Turkmenistan last, with just 1%. The region is capable of meeting up to 71% of its potential demand for electric power with hydropower.

The existing reservoirs have brought the run-off control rate to 0.94 for the Syrdarya (i.e. close to its maximum), and to 0.78 for the Amudarya (i.e. with capacity for further increases). Upstream flow regulation in the Amudarya basin is provided by three reservoirs: the Nurek and Baypasin on the Vakhsh river and the Tuyamuyun on the Amudarya, as well as by a network of off-river reservoirs associated with canals, including four on the Karakum Canal, two on the Amubukhara Canal and one on the Karshin Canal, with a total volume of 6 km³. These can only be filled only where release schedules are closely coordinated with the water-intake limits for their associated canals. Most reservoirs are over 25 years old. During the years of operation, nearly all of them were silted up, gradually losing their useful storage capacity. The above usable capacity values of the reservoirs should, therefore, be reduced by at least 30%, and the actual run-off regulation rate adjusted accordingly.

While the dams and hydropower stations in Central Asia are solid structures with a proven safety record, their age and significant cuts in funding for maintenance give rise to concerns. It is, therefore, essential to develop activities related to checking and upgrading the safety of large dams and providing them with modern equipment.

The problem of the so-called rock-dammed lakes should also be mentioned. The largest of them, Sarez, in Tajikistan, has a volume of nearly 16 km³. It was formed in the Pamir Mountains in Tajikistan following an earthquake in 1911 at the site located over 3,000 metres above the sea level. This natural dam, 600 m high and 5 km wide, has all but blocked the Murgab river.

In recent years geological processes have considerably complicated the situation in the Sarez lake area. In 1987, 20 million m³ of rock slid into the lake 12 km upstream from the Usoy Dam creating a 6-metre wave. Seepage through the dam has increased significantly, and the canyon is eroding at a rate of 30-40 metres a year.

The Tajik Government launched an International Safety Programme for this Lake. It calls for:

• Facilitating the development of early-warning arrangements in connection with threats from Lake Sarez;
• Developing and implementing a joint international programme to solve Lake Sarez’s problem and also establishing an organizational framework for joint action.

The collapse of rock dams on three lakes in the Shakhimardansay river basin, killing many people in Kyrgyzstan and Uzbekistan, was a reminder of the serious threats associated with natural reservoirs. Difficult access to and poor knowledge of mountain lakes makes it difficult to forecast and prevent bursts which could well develop into a regional disaster. Thus, a burst from Lake Sarez may affect over 55,000 km² with a population of 6 million.