Draining the Tangjiashan Barrier Lake

This paper documents the emergency breaching of an approximately 110-m-high landslide dam that was created at Tangjiashan during the 2008 Wenchuan earthquake of China’s Southwest Sichuan Province. As an emergency measure, a 13-m-deep channel was excavated to reduce the volume and head of the released water during the dam breaching and to create a controlled flood that was intended to prevent catastrophic consequence for 1.2 million people downstream. From detailed monitoring and survey works carried out during and after the dam break it was found that (1) the breaching initiated at a pool water level of 742.1 m with an estimated flow velocity of 2.4 m/s, and virtually terminated at a pool water level of 720.3 m with an estimated velocity of 2.5 m/s, which can be regarded as the incipient velocity that initiated erosions for the soils at this particular site; (2) a controlled flood released 167×106-m3 reservoir water with a peak flow rate of 6,500 m3/s , during which a maximum velocity of about 5.0 m/s was measured; (3) in 12 h, the channel bed was eroded 30 m down and the sidewall was enlarged 145 m wide at an elevation of 719 m, which was expanded to 200–350 m at the top of the channel due to subsequent landslips of the temporary slopes; and (4) the measured data indicated an incipient velocity that initiated soil erosions, which is found to be conceptually in general agreement with the existing approaches of this threshold value.