26 May 2008
Tangjiashan (Beichuan) Lake – update
Posted by Dave Petley
China Daily is today reporting that real progress is being made with the lake above Beichuan, which is now known as Tangjiashan Lake:
Amid low visibility, a Russian helicopter successfully delivered a large bulldozer and other eight sets of big machineries near the swelling Tangjiashan lake, in order to dig a diverting channel to prevent a flood, said Minister of Water Resources Chen Lei. “I feel relieved this morning as we have reached the area,” said Chen, who is in charge of dealing with the emergency situation of dozens of quake lakes in Sichuan. Quake lakes were formed after landslides blocked rivers.“All these mean we can officially start to deal with swelling lakes now,” said Chen, adding it first time he spoke to the media, as he was not in the mood the previous days to do failures to deal with the possible flooding situation. Other helicopters managed to airdrop materials for the operation by 7:49 am Monday at the top of the quake lake at Tangjiashan in Beichuan County. It was formed by landslides that blocked a local river known as Jianhe after the May 12 earthquake. While military forces are preparing to dynamite the swelling Tangjiashan quake lake, rescue headquarters have urged the expert team to quickly give “risk evaluation” reports on bursting of the barrier. Chen asked the provincial and Mianyang municipal governments to draw “three red lines” on the map until Tuesday noon, with each representing which area will be affected if the barrier bursts by one third, half or to nothing.
This sounds much more promising. The best way to deal with this issue is a combination of a channel constructed using heavy equipment, backed up with an assessment of the areas likely to be inundated by a flood. Blasting the dam should be avoided if at all possible – it is a real last gasp approach. However, great care will be needed in the construction of the channel, in particular in preventing rapid erosion of the channel bed and thus a rapid collapse event.
This is progress for sure but they are not yet out of the woods. The lake now has a volume of >128 million cubic metres of water.
Wouldn’t siphoning via hundreds of pipes do the job ? This would prevent the threat of channel colapse and permit water to exit at a safer location so that the base of the dam isn’t threatened.Once the water is at a safe level, the channel could be dug.
Dear Anonymous,In theory yes, but the volumes of water that need to be handled are large. Xinhua has reported that the volume of the lake yesterday was 128 million cubic metres, which gives an inflow rate of about 9.1 million cubic metres per day. This corresponds to about 106 cubic metres per second. This is the equivalent to the volume of a 25 m swimming pool every 3 or 4 seconds. Dealing with this type of volume with syphons would be difficult. Note also that:1. This volume would only prevent the lake from filling further. To drain it in a reasonable time the authorities would need to double that.2. This is still not the wet season. The rate of filling will increase dramatically when the rains start properly.Finally, the distance over which the water would need to be transported might preclude their use.I am no expert on syphons so would welcome further comments.
Thanks for your reply. At 106,000 litres per second of inflow, there’s not much chance that a siphon would do the job. A 40mm siphon pipe could handle only around 1 litre per second, so about100,000 of them would be required !
I suspect that some siphons can deal with a greater capacity than that, but your point about the number needed is well made, thanks. It appears that they are building a channel now.
They are so screwed!! With the rate of rise they are not going to be able to safely handle that amount of water spilling over as it continues to feed itself from the river. It is sad but the best thing to do is to evacuate the area in the path of the water and blow the entire damn all at once.
Dear anonymous,I agree that evacuating all areas that might be inundated downstream is essential. I caution against blowing the dam all at once as the flow would be massive. Even if everyone is out of the way the damage to infrastructure and properties would be extraordinary.
sooner or later your not going to have to worry about it anymore, theres no way to do anything in a controlled fashion with that much water flow, start a channel and hope for the best,and prepare for the worst and move the people.
Get out your floaties…