Tangjiashan Flood Alert Lifted And Latest Satellite Images

The outflow from the Tangjiashan quake lake has slowed to 56 cu. meters a second and the water level has dropped nearly 30 meters from its peak, officials said Wednesday. They have also lifted the flood alert as fears of downstream flooding receded. At those outflow rates the lake is likely to fill up a bit again, though the sluicing operation will keep the level at around 20 meters below the peak.

NASA satellite images (pdf), snapshots below, show the Jiangjiang River downstream from the quake lake before and after the sluicing. The top image, taken on June 8th, shows the river slow-moving and hampered by landslides. The lower image, taken on June 10th, shows the river as a torrent, submerging land along its banks, and flooding over the landslide that was previously visible in the upper picture. Its color indicates that it carries considerable sediment, including sediment from the breached landslide upstream.

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Tangjiashan Quake Lake Drains

The Tiangjiashan quake lake is draining, and officials have declared a “decisive victory”. Like Wellington said after the Battle of Waterloo, it was a damned close run thing.

Without demeaning the 10-day effort put into cutting drainage channels in extremely trying  conditions, it was in the end the crude tactic of blasting holes in the rock and mud holding back the waters that released the torrent of water needed to get the quake lake’s levels subsiding.

The flow of water through the spillway peaked, as far as one can tell, at 80 cu. meters a second, while the inflow was at least 115 cu meters a second. After two massive blasts on Monday evening which broke through what Xinhua called the “bottleneck” in the spillway, the water outflow surged to  6,420 cubic meters per second before slowing to o a steady 3,888 cubic meters per second by early Tuesday afternoon, dropping the lake’s level by 20 meters and emptying half the water that was in it at its peak. The outflow, though, was still much faster than originally planned, and as TV shots of the river well down stream showed, a river bed that was nearly dry an hour and a half away form Tangjiashan, was in full flow.

This is an important propaganda victory for the authorities and the army, too. Attempts to drain the lake have been front and center of state media reports, displacing concerns about the shoddy construction of schools. For a while it looked as if the story might have a catastrophically bad ending in all regards. And the story isn’t fully over. There is still a lot of lake water being held back by no more than an unstable landslide, and rainy season ahead.

New Satellite Images Of Tangjiashan Quake Lake As Waters, Worries Rise

New satellite pictures via NASA (.pdf), from which the snapshots below are taken, comparing the Tangjiashan lake on May 14 and June 3.

Officials had hoped that the water levels in the lake would peak on June 6, but despite the opening of the spillway, they continue to do so. By Monday the waters were reported to be 2 meters above the lip of the spillway. The outflow via spillway had reached 50 cu. meters a second after military engineers fired anti-tank missiles to smash boulders in the channel and blasted with dynamite to widen it, but the flow is half the planned 100 cu. meters a second flow and less than half the 115 cu. meters a second flowing into the lake from the river upstream further swollen by Sunday’s rains. With Xinhua reporting another strong aftershock on Monday raising concern about the stability of the landslide dam, the situation is becoming more not less acute.

Aftershock Creates Fresh Threat To Tangjiashan Quake Lake

The level of the Tangjiashan quake lake appears to be continuing to rise despite the spillway being opened on Saturday to sluice it. Another strong aftershock on Sunday caused new landslides and fresh fears about the stability of the one that created the lake in the first place by damming the Tongkou River. Crews were working Sunday to deepen and widen the spillway to increase the flow of water from the lake, and work started on cutting a second channel on the other side of the dam. The situation remains critical.

Sluicing Of Tangjiashan Quake Lake Starts

Sluicing of the Tangjiashan quake lake has started. Water levels reached the lip of the spillway cut earlier in the week on Saturday. Xinhua reports that work crews are trying to improve the flow so water can drain faster. The sluicing will stop the level of the lake rising further and hold the waters at a volume that engineers calculate won’t burst the dam created by landslides triggered by the May 12 earthquake.

Two key pieces of infrastructure lie downstream from the lake: the Fujiang river bridge on the Baoji-Chengdu Railway, an important part of the rail network in western China; and PetroChina’s Lanzhou-Chongqing pipeline which supplies 70% of Sichuan and Chongqing’s oil. If the pipe was ruptured, Sichuan has only three days of oil in store.

Tangjiashan Remains A Catastrophic Threat

Its all gone relatively quiet over the draining of Tangjiashan, the quake-created lake that is top of the list of those at risk of catastrophic bursting.

Sluicing was meant to have started last week following a well-publicized effort to cut a drainage channel. The latest aftershock, a 5.3 that occurred on Thursday and sent waves across the lake, reminds us that the draining has still to start.

The waters, swollen by heavy rains, are reported to have risen to within 2 meters of flooding over the banks. As of 2 pm Wednesday, Xinhua reported, the lake contained 211.6 million cubic feet of water. Prime Minister Wen Jiabao visited the area on Thursday, saying “Now it’s a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there is no casualty of the people.”

This Bystander’s sense is that the situation in on a knife-edge. More evacuations are underway of the million people at risk should he worst happen.

Barrier Lakes Are New Quake Concern

Barrier lakes — those formed by earthquake induced landslides — are the new concern. The Sichuan quake on May 12 created 35 of them, 34 in Sichuan, according to E Jingping, vice-minister for water resources. The situation is still grim, Xinhua reports him as saying, with heavy rains forecast for early this week. Evacuation plans have been drawn up for the 19 at greatest risk of bursting.

I posted a snapshot of some satellite photos of how these lakes form a couple of days back.

The biggest concern now is Tangjiashan, the largest barrier lake, just 3 kms upstream on the Jianhe River from a what’s left of a town in Beichuan. The lake is now 723 metres deep and now less than 30 meters below the lowest part of the barrier, threatening to inundate as many as 1.3 million people downstream with the 130 million cubic metres of water it is estimated to contain.

A Russian helicopter successfully delivered a large bulldozer and other eight sets of big earthmoving machines to near the swelling lake, the China Daily reports, something that seems to have defeated the PLA. The plan is to dig a diverting channel. Eighteen hundred PLA troops and armed police have made it on foot to help sluice the lake by blasting holes in it before it bursts or floods. Continuing aftershocks are complicating the work. The new shaking can further destabilize already unstable barriers.

Thirty thousand people have already been moved to higher ground and another 80,000 are likely to have to be evacuated.