The recent magnitude 8 earthquake in Sichuan was the strongest and deadliest earthquake to strike China in over 30 years. Dr Milan Simic describes the additional threat from manmade dams and lakes formed by earthquake-induced landslides.

The recent official start of the 2008 Atlantic hurricane season opened another six-month period when insurance underwriters, brokers, and catastrophe-modelling professionals closely monitor the many websites that offer real-time information on developing tropical cyclones. The accessibility of relatively routine weather forecasts has meant that the formation and intensification of these and other weather events worldwide – and their likelihood of affecting concentrations of insured exposures – can be easily monitored by non-specialists.

Earthquakes unfortunately very rarely provide such advanced warning; they happen suddenly, without lead-time for evacuation, and cause high numbers of deaths and injuries. A typical earthquake lasts seconds and causes virtually instantaneous damage. Secondary earthquake effects such as landslides and tsunamis then develop in hours or days.

It is therefore unusual that more than a month after the Wenchuan earthquake (named after the county in Sichuan Province where the epicentre was located on 12 May), a threat of major loss of life and property is still present. This threat is associated with the potential failure of both manmade dams and of the many “barrier lakes” that formed because of landslides induced by the earthquake.

The Wenchuan earthquake was the strongest and deadliest quake to strike China in more than 30 years. According to AIR’s China earthquake model, total economic losses are likely to exceed $20bn while insured losses could reach $1bn. Although there is a high level of uncertainty in estimating the potential insured losses. The China Earthquake Administration has estimated its magnitude at 8 on the Moment Magnitude Scale and a focal depth of 10km. It took place on the Longmen Shan fault that marks the eastern margin of the Tibetan Plateau, one of China’s most seismically active regions.

Over 400 dams affected

The region is also very rich in rivers that flow down from the plateau. Because of this, many dams and hydropower systems have been constructed here. About 400 dams in the area are estimated to have been affected by the earthquake. Four of these are classified as major and are over 100 meters high: the 156 metre concrete-faced rock-fill Zipingpu dam, the 137 metre Baozhusi concrete gravity dam, the 132 metre roller-compacted concrete Shapai dam, and the 102 metre clay-core Bikou dam. Since dam failures can have catastrophic effects because of downstream flooding, they are designed according to very strict criteria.

Designers of major dams are required to undertake a “dam-break analysis” in which they investigate different possible design failures, and the resulting impact flooding will have on downstream communities. This analysis is used in regional emergency planning. Additionally, in keeping with requirements for other critical infrastructure, every major dam has to undergo detailed “anti-seismic design”. This effort consists of probabilistic seismic hazard analysis (and in that respect is similar to the concepts employed in catastrophe modelling) and/or deterministic analysis that incorporates at least two specified earthquake design criteria. In dam engineering, the two design earthquakes are known as “Operating Basis Earthquake” (OBE) and “Maximum Credible Earthquake” (MCE).

The OBE design criteria are used to limit damage to the dam and therefore are of interest and concern mainly to the dam owner. In this analysis, although there are no fixed international criteria with respect to exceedance probabilities, it is typical to use a return period of about 145 years (50% probability of exceedance in 100 years), often increasing to 200 (0.5% exceedance probability) or 500 years (0.2% exceedance probability). In response to such an event, the dam must remain operable and experience only minor, easily repairable damage.

“Military engineers had to fire missiles to blast holes in the natural barrier to widen the drainage opening in the Tangjiashan barrier lake

The MCE analysis considers the largest reasonably credible earthquake that could happen along a recognised fault or within a tectonic framework in the area where a dam will be situated. In practice, the MCE is often adopted as an event with a return period of 10,000 years (0.01% exceedance probability). In case of such an event, the stability of the dam must not be compromised, although significant structural damage is acceptable (along with a possible lowering of the dam’s reservoir level).

Following the Wenchuan earthquake, attention was focused mainly on the 156m tall Zipingpu dam, which is situated only 17km from the quake’s epicentre. The reservoir behind this dam was designed to hold 1.1bn cubic meters of water. Fortunately, it had less than a third of that amount when the earthquake struck. Many cracks were observed on the dam’s face and although inspection is still continuing, a maximum settlement of 735mm has been recorded so far and also a permanent horizontal deflection of 180mm.

Close examination of other major dams has not yet been possible because local infrastructure leading to the dams was destroyed by the earthquake. Despite the official statement by Chinese authorities that no single dam had been breached, careful monitoring will continue during repairs and probably will last for months, if not for years.

Barrier lakes forming

In addition to man-made reservoirs and dams, another major problem has been 34 “barrier lakes” created by the many earthquake-induced landslides. The Tangjiashan lake that formed 6km from Beichuan received the most media attention. At its peak, the lake was roughly 800 metres x 600 metres in area and more than 120 metres deep and threatened more than a million people downstream. Such barrier dams are intrinsically highly unstable and the immediate priority of local authorities was to release the captured water in a controlled manner. Their first attempt was to dig a drainage channel. That effort proved to be inadequate, however, and military engineers had to fire missiles to blast holes in the natural barrier to widen the drainage opening.

Further study of these structures by Chinese and international dam engineers over the next months and years will provide researchers, catastrophe modellers, and emergency managers with a wealth of information on the performance of manmade and natural dams under extreme earthquake conditions.

The cost of the 12 May Wenchuan earthquake, both in terms of life and property, was enormous – recovery from the event will take months, if not years. While acknowledging the enormity of the tragedy, this earthquake will undoubtedly provide a laboratory for seismologists and earthquake engineers to expand their understanding of seismic risk in China and how China’s rapidly growing building inventory and infrastructure will respond in future to such events.

Dr Milan Simic is managing director of AIR Worldwide.

Topics