Last month a magnitude eight earthquake ripped through China’s mountainous Sichuan province, killing nearly 70,000 and leaving many hundreds of thousands homeless (NCE 21 May).
A devastating event without question, but one which could have been far worse had the many hydroelectric dams in the region failed. Initially fears were high, with widespread cracking immediately visible to the 158m high Zipingpu dam which towers over the devastated town of Dujianyan. The quake damaged joints and seals in the rock filled dam’s concrete clad face, but fortunately the dam was quickly made safe by lowering the water level.
However, it is estimated that there are around 400 dams in the area of the earthquake, with four dams being classed as major with heights over 100m, and up until now little has been known about the condition of these structures.
"No single dam was breached," reported China’s leading dam expert, Professor Chen Houqun of the China Institute of Water Resources and Hydropower Research (IWHR) to last week’s International Commission on Large Dams’ (ICOLD) seismic committee meeting.
"Sichuan is rich in water, hydropower and a large number of reservoirs," said Chen.
"However after the earthquake the safety of all dams is under control, and all dams that are higher than 100m are structurally sound."
The potential impact of a dam failure is huge, as the surge of water it could produce would wipe out any communities living downstream.
Twenty five teams of experts were dispatched by the ministry of water resources after the earthquake to inspect the reservoirs and sluice gates were opened on high risk dams to avoid breach.
Zipingpu dam was 17km from the epicentre. After the earthquake, cracks were reported on the face of the dam. Investigations are ongoing and have shown that the 158m high concrete faced rock fill dam (CFRD) had a maximum settlement of 734.6mm and a horizontal deflection downstream of 179.9mm. The 101.6mm cross canyon deflection indicated that the dam had been squeezed.
The change in the level of seepage is a critical indicator to the stability of the dam. The seepage of the dam changed from 10.38l/s to 15.07 l/s.
"There wasn’t a significant change in seepage which is important in showing stability," said Chen.
"There are several waterstops: The first is a copper sheet and at the surface there is another softer material to fill the joints with surface protection. Then there is a surface water stop system followed by a clay overburden. We think the waterstop system is undamaged but the copper sheet may be damaged. We don’t know as it’s inside the dam."
Water levels have been kept low at Zipingpu since the earthquake. Chinese dam design regulations specify that all CFRDs must be equipped with a bottom outlet which allows the lowering of a reservoir in the case of a severe earthquake or other critical incident. The dam is stable but is being inspected and will need repairing.
"The investigation is ongoing," says Chen. "We need to evaluate the deformation and carefully discuss the reconstruction."
The gates for the diversion channel have been repaired as a matter of urgency before the flood season. The hydropower generators are working as normal, helping to push the water downstream.
Next under the spotlight was the 132m high Shapai dam. An arch RCC (roller compacted concrete) dam, it is located only 12 km from the epicentre. Here a close investigation of the dam is not yet possible as local infrastructure leading to the dam was destroyed by the quake. However a helicopter survey of the structures has reported that there is no immediate cause for concern.
"There is no water overflow or significant leakage," said Chen.
"It looks like a stable situation. It must have been damaged of course, but as a whole, it looks structurally stable."
The dam will be of interest to experts as it was near the epicentre, and it was the only dam in the area to be nearly full.
"It will be an interesting dam to look at for the Chinese and other countries."
However the earthquake was so strong that some of the monitoring equipment was destroyed in the quake. The instruments at the base of Zipingpu and Shapai were destroyed so the only information is from the crests.
"The Zipingpu dam was designed for an acceleration of 0.26g but the acceleration at the crest during the earthquake overtook 2g. The RCC dam was designed for 0.0138g but was subjected to much higher acceleration but we have no records.
Bikou, a 101.8m high clay core dam built in 1976 with a capacity of 0.5BM3 and Baozhusi dam, a 137m high gravity dam built in 1997 with a capacity of 2.52 BM3 both experienced the quake with an intensity of 7, but both were designed for magnitude 7 according to Chen, and no damage has been reported.
Many smaller dams are not designed according to design codes and are in rural areas. Chen attributes the fact that none of these failed as due to the low levels of water in the reservoirs as it was not yet the flood season. There has been no loss of life due to dam failure following this earthquake, thanks to the low water levels. The earthquake was much stronger than what the dams had been designed for.
"In China the fault was not so active and there were no large historical events in these areas," said Chen.
"Up until now, it’s not been significant, but we underestimated the activity."
Quake under control
As NCE went to press, Chinese news agencies reported that the war was being won with the Tangjiashan quake lake.
Tangjiashan was formed 6km from Beichuan and at its peak was 803m long by 612m wide and 124m high. It was formed after quake-triggered landslides from the Tangjiashan Mountain created a dam blocking the Tongkou River which runs through Beichuan County. Fresh landslides, aftershocks and higher than expected rainfall continued to thwart efforts to reduce levels in the dam and reports of seepage through the dam body and visible cracks indicated the earth dams vulnerability.
However on Tuesday the water level in the lake had been reduced to around 72m, reducing the number of people at risk downstream from 1.3M to less than 50,000 and reducing the pressure on the dam. More than half of the lake’s 250M m3 of water has been discharged from the lake.
A diversion canal was created as an emergency measure and drainage of the quake lake proceeded through a man- made spillway from Saturday morning. Military engineers fired short-range missiles several times on Sunday and Monday to blast boulders in the channel and speed up the outflow.