6 June 2008

Tangjiashan – delayed again

Posted by Dave Petley

Xinhua is now reporting that the flow through the sluice has now been delayed through the use of a 60 cm high temporary dam (see below). This is an interesting development – I wonder why they have decided to do this at this late stage. Delaying the flow has a risk associated with it as the body of water will be larger, and thus the flood is potentially more damaging. Presumably they feel that the risks posed by this delay are more than compensated for by the larger channel, which reduces the chances of the dam eroding away quickly. I have noted before that in my view the channel is too small, but I am surprised that they have done this at this late stage.

The decision not to use dynamite, noted at the bottom of the text below, is undeniably sensible!

Original Xinhua text:

Water level of China’s main quake lake Tangjiashan topped the lowest point of 740meters by 0.20 meters on a sluice channel at 4:00 a.m. Saturday, but it was prevented from overflowing by a temporary dam added to protect workers on emergency construction downstream. More than 100 armed police were airlifted to Tangjiashan to broaden and deepen the sluice channel on Friday afternoon, said an expert at local commanding center on early Saturday morning. The temporary dam was 0.6 meters high, said the expert. Local headquarters said rainfall was heavy at the lake, with an average of two millimeters per hour. The swollen lake was formed by a massive landslide that blocked the flow of the Jianjiang River in Beichuan County, Mianyang City, following the May 12 earthquake in the country’s southwest. It held more than 220 million cubic meters of water and posed a threat to about 1.3 million people downstream. Some 600 armed police and soldiers worked for six days and nights to dig a 475-meter channel to divert water from the lake. More than 250,000 people in low-lying areas in Mianyang have been relocated under a plan based on the assumption that a third of the lake volume breached its banks. Two other plans require the relocation of 1.2 million people if half the lake volume is released or 1.3 million if the barrier fully opened. The swollen quake lake has put China’s longest oil pipeline at risk. The pipeline, winding from Lanzhou via Chengdu to Chongqing, was 60 kilometers downstream from the lake. With a capacity of transferring six million tons of oil each year, the pipeline provides 70 percent of product oil to Sichuan and neighboring Chongqing Municipality. If the line was cut, refined oil in storage could only supply Sichuan for three days, whereas repair work would take 30 days. Emergency plans have been drafted, according to colonel Liu Yongzhan. “Gate valve of the pipeline will be shut to prevent potential pollution, and a float bridge shall be later set up, so that a temporary line would ensure resumption of oil supply within48 days,” he said. Engineers have feared that the lake could burst and cause a flood, citing unforeseen factors such as fresh landslides, heavier than expected rain and the instability of the mud and rock dam. The plan of using dynamite to blast the lake had once been under discussion but was later announced as given up, according to Liu Ning, chief engineer of the Ministry of Water Resources. “We will not use dynamite if water in the lake could spill over naturally,” he said. “Blast might incur unexpected result.”