Traffic on the Three Gorges Dam two-way ship lock was more than halved on Friday as a year-long project to raise the beds of the uppermost two tiers began.

Traffic will be cut to one-way movement, alternating every 24 hours.

Frogmen jumped into the top tier of the southern track to seal off the water flow at 8:00 a.m., while the northern track was left open to allow passage of shipping.

The project will see the beds on the two topmost tiers raised from 131 meters to 139 meters, said Pan Dazhong, deputy director of the construction department of the China Yangtze River Three Gorges Project Development Corporation.

The work was necessary to ensure safe navigation of ships when the water level behind the dam rose from 135 meters to the higher levels of 156 meters and 175 meters.

Work on the northern track will begin after the southern track was completed. The handling capacity with the ship lock will go down by 60 percent.

(Xinhua News Agency September 15, 2006)