Landslides (2011) 8:91–98 DOI 10.1007/s10346-010-0236-6 Received: 5 January 2010 Accepted: 19 July 2010 Published online: 3 August 2010 © Springer-Verlag 2010
Xiao Qing Chen I Peng Cui I Yong Li I Wan Yu Zhao
Emergency response to the Tangjiashan landslide-dammed lake resulting from the 2008 Wenchuan Earthquake, China
Abstract Natural landslide dams triggered by earthquakes are a common feature and a significant hazard in high-relief, tectonically active areas. The great Wenchuan Earthquake of May 12, 2008 created 256 natural dams, of which 34 presented significant risks to downstream areas in the event of their uncontrolled failure. Out of the 34 large landslide dams that warranted mitigation, we discuss Tangjiashan landslide dam in detail. Emergency response to the Tangjiashan landslide-dammed lake in the following weeks and months successfully reduced the risk, and the advantages and disadvantages of various countermeasures that were applied are summarized here. Successful strategies relied on accurate scientific assessments, on timely execution of the countermeasures, and on the correct design of sluiceway (spillway) channels across the landslide dams. Retrospective assessment indicates that the following improvements would be more beneficial: (1) Sluiceway channels utilizing a combination of cross-section types, rather than simply trapezoidal in shape; (2) increased channel slope, which is more than the original gradient of the river; (3) better protection of inlets and outlets to control the planned incision rates; and (4) channels lined to better control the incision rate. We discuss applications of the concept of artificially controlled failure, and we submit these observations for the benefit of those responding to future seismic catastrophes.
Keywords Wenchuan Earthquake . Landslide-dammed lake .
Emergency response . Mitigation . Artificially controlled failure
Introduction Earthquake-induced dams are comprised of loose, unconsolidated material. They may collapse as the level of impounded water rises, and most will inevitably be overtopped, with probable cata- strophic breaching, in many cases causing large losses of life and property downstream. As the size and frequency of past disasters have become recognized, researchers have categorized and archived case histories (SSB 1983; Liu et al. 1987; Chen et al. 1992; Clague and Evans 1994; Chai et al. 1995; Xie 1995; Casagli and Ermini 1999; Natural Hazard Mitigation Research Center 1999; Chai et al. 2000; Nie et al. 2004; Dai et al. 2005; Sassa et al. 2005). Costa and Schuster (1991) are compiling the most comprehensive inventory and creating a database of 463 landslide dams throughout the world. Clague and Evans (1994) described 16 existing and 22 historical landslide dams in the Canadian Cordillera. Casagli and Ermini (1999) presented an inventory of 68 present and historic landslide dams in the northern Apennines. In China, Chai et al. (1995) compiled an inventory of 147 present and historic landslide dams. On August 25, 1933, a large earth- quake (∼M 7.5) near Diexi, Mao County, Sichuan Province, triggered three landslide dams, with a maximum height of 160 m, on the Minjiang River (a major tributary of the Yangtze River) and nine landslide dams on its tributaries. The three lakes behind
the landslide dams on the main river combined into a single huge lake, with continued inflow of water impounded behind the highest, downstream dam. The dam was overtopped and breached 45 days later, and a flood rushed 250 km down the valley, taking at least 2,500 lives (SSB 1983; Chai et al. 2000). More recently, the Yigong River in southeastern Tibet was dammed on April 9, 2000 by a huge landslide with a volume of 300×106 m3. The landslide dam, ca. 130-m high and 1.5-km long, was formed in about 8 min, partially failing on June 10, 2000 (Shang et al. 2003). The resulting flood traveled more than 500 km downstream, damaged many bridges, and caused numerous landslides along the downstream channel. The flood is known to have taken 30 lives, but more than 100 people remained missing; more than 50,000 were made homeless in the five districts of Arunanchal Pradesh, India (Zhu and Li 2001).
The Wenchuan Earthquake resulted in 256 landslide-dammed lakes, some with the potential to cause disasters on the order of the Diexi calamity (Cui et al. 2009). To date, research on landslide dams has focused on their distribution and failure potential (e.g., Costa and Schuster 1988; Casagli et al. 2003; Ermini and Casagli 2003; Korup 2004; Dunning et al. 2005). Little attention has been given to their emergency treatment. The simplest and most commonly used emergency treatment has been the construction of spillways across either adjacent bedrock abutments or the crest of the dam (Harrison 1974; Li et al. 1986; Schuster 2000). An example of a carefully engineered spillway across a landslide dam was that constructed by the US Army Corps of Engineers on the Madison Canyon landslide dam (Harrison 1974). In a few cases, large-scale blasting has been used to excavate new stream channels across landslide dams. This technique was used in 1964 to open a channel across a 15-million-m3 landslide that dammed the Zeravshan River in Tajikistan, upstream from the ancient city of Samarkand (Engineering News-Record 1964). Other methods of emergency treatment include drainage by means of siphon pipes, pump systems, and tunnel outlets and diversions (Sager and Chambers 1986; Cambiaghi and Schuster 1989; Govi 1989).
Immediately after the Wenchuan Earthquake, by selecting dam height and structure and the maximum reservoir capacity as indexes, we rapidly compiled a risk assessment for the 21 landslide-dammed lakes that we believed to be the most danger- ous examples (Chen et al. 2008a). Engineering countermeasures were subsequently applied to the dams with the highest potential for future failure, including the largest and most dangerous example, the Tangjiashan landslide-dammed lake. That lake is now greatly reduced in size, and the blockage increased in stability following the emergency countermeasures taken between May 26 and June 10 that triggered the resultant planned breaching on June 11. No lives were lost and property damage was minimized in this example of a successful emergency response. Nevertheless, possible improvements in that response are now
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evident, and these should be considered in future responses to large, rapidly filling, and extremely dangerous landslide-dammed lakes.
Background and characteristics of Tangjiashan Lake The Wenchuan Earthquake and its aftershocks were distributed along the Longmenshan Fault Zone, extending northeast–south- west from Tianquan, Sichuan, in the south, through Dujiangyan, Wenchuan, Maoxian, Beichuan, Qingchuan, and Ningqiang, Shaanxi in the north. The fault zone is 500 km long and 30– 40 km wide (Fig. 1). The earthquake directly affected an area of more than 1.0×105 km2 and created a homeless population of
more than 45 million people. The Tangjiashan landslide dam is located in the northern part of the fault zone, 6 km northwest of Beichuan County (Fig. 2).
Background The Tangjiashan landslide-dammed lake formed in the valley of the upper Jianjiang (Tongkou River) (Fig. 2). The Tangjiashan landslide is a rock slide damming a drainage area of 3,550 km2, with the potential to have created a lake with a maximum volume of 3.16×108 m3. The backwater caused by the dam inundated the upstream Zhicheng hydropower station on May 19, 7 days after
Fig. 1 Location of Tangjiashan landslide dam
Fig. 2 ADS40 aerial image of Tangjiashan landslide dam, from Chinese State Bureau of Surveying and Mapping, photographed in June 2008
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the earthquake. The water level reached 719.8 m on May 23 (11 days after the earthquake), an increase of 2.33 m per day. The volume of the reservoir reached 1.1×108 m3.
The dammed lake is located in the mountainous Beichuan County, located between Minshan Mountain to the northwest and Longmenshan Mountain to the southeast. The highest point is 4,769 m in altitude, at the summit of Chaqi Mountain; the lowest is 540 m in altitude in the Xiangshui River valley. Elevation decreases abruptly from northwest to southeast at a rate of 46 m/ km. The dam lies in an asymmetrical, V-shaped valley with a slope of 60° on the right valley-side slope and 30° on the left. Relief to the southeast of the dam is 928 m (below an elevation of 1,568 m) and 882 m to the northwest.
Tangjiashan Mountain lies northwest of the Longmenshan fault zone (Fig. 3), which includes the Qingchuan–Maowen fault, Beichuan–Yingxiu fault, and the Jiangyou–Dujiangyan fault, and has an overall attitude of approximately NE 40° in strike and NW 50–80° in dip. Previous investigations report that the region has risen about 5 to 6 km in the last 10 Ma, at a rate of 0.5 to 0.6 mm/ year (Liu et al. 1995). Bordered by the Yingxiu–Beichuan fault, the area southeast of the fault zone is part of the Longmenshan fold zone. The area northwest of the fault zone is part of the Maowen– Danba fold in the Songpan–Ganzi structural terrane. Tangjiashan Mountain is located on the hanging wall of the Beichuan–Yingxiu fault only 2.8 km from the faultline.
Outcropping bedrock belongs to the Precambrian Qingping formation. The Tangjiashan landslide is composed of gray, thinly laminated feldspar-, mica-, and quartz-bearing siltstone, and calcareous muddy siltstone. The sequence from which the landslide was derived consists of interbedded and both well and poorly consolidated strata with an overall attitude of N 60° E, NW 60°.
Beichuan County is located in the transition zone between the Qinghai–Tibet Plateau and the Sichuan Basin. The regional climate changes frommonsoonal in the west to a hot, dry climate in the east. The weather is mild and wet; records from a meteorological station
3 km away from the landslide indicate a mean annual temperature of 15.6°C and mean annual rainfall of 1,287.5 mm (Beichuan 2008).
The Jianjiang River, a tributary of Fujiang River, has a drainage area of 3,350 km2. The 50-, 20-, 10-, and 5-year return peak discharges are 5,120, 3,920, 3,040, and 2,190, respectively. Themean annualflood has a discharge of 1,600.
Characteristics of the lake The landslide dam is 803.4 m in length (along the river) and 611 m in width, the dam area is 0.31 km2, and the volume is 2,037× 104 m3(Liu et al. 2009). The height of the dam above the pre- landslide river channel varies from 82 to 124 m. Elevation of the highest point is 793 m and of the base 669 m. Weak, weathered bedrock is encountered beneath pre-existing valley fill, about 31 m below the base of the dam.
Yingxiu-Beichuan Fault
Tangjiashan Landslide
Jiangyou-Dujiangyan Fault
Fig. 3 Geologic map of the Tangjiashan region: P2, upper Permian; P1, lower Permian; C1zn, Zonghchanggou group of lower Carboniferous; D3tn, Tangwangzhai group of upper Devonian; D2gn, Guanwushan formation of middle Devonian; S2–3mx1, S2–3mx2, S2– 3mx3–1, the three formations of Maoxian Group of Silurian; S1ln, lower Silurian; O2b, Baota formation of middle Ordovician; ∈1c, Qingping formation of lower Cambrian
See Fig.6
See Fig.5
Fig. 4 The Tangjiashan landslide dam, looking upstream—cross-sections indicated by dashed lines are shown in Figs. 5 and 6
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Three rills developed on the dam during the month prior to breaching—on the right, middle, and left sections of the mass (Fig. 4). The rill on the right extended from the upper section of the dam to the river, with a width of 20–40 m at an elevation of 752 m. The other two rills are located on the lower part of the dam, with lengths of ca.400 m and widths of 10–20 m.
The upstream slope of the dam is about 20°. The downstream side is steep at the top and the lower part and gentle in the middle—the upper 50 m of elevation has a gradient of 1:0.7, the lowest 20 m a gradient of 1:0.5, and the middle section a gradient of 1:2.5.
The right bank, source of the landslide, is a dip slope while the left bank is an up–dip slope (Figs. 5 and 6). Fracturing is intense at the base of the failed mass. A highly weathered layer on the valley-side slopes is 5–10 m in thickness. The channel alluvium is up to 15-m thick, which on local floodplains consists of silty loam (60%), rock debris 5–20 cm in diameter (30–35%), and crushed rock derived from slopes (5–10%) (Chen et al. 2008b).
Bedrock structure is preserved on the dam surface. Figure 7a illustrates the stratified rock on the dam. En masse movement is recorded by vegetation on the dam that was moved by landslide and remains upright (Fig. 7b) and by planted trees that remain in rows.
Dam stability can be inferred from its structure, texture, and morphology. Given that the 1933 coseismic landslide dam(s) only partially failed on June 15, 1986 (Cheng 1988) and again on June 29, 1992, their similarity in those characteristics to the Tangjiashan landslide dam suggests that the latter is unlikely to fail catastroph- ically again in the future. By analogy, we can infer that the dam will fail partially several more times and that the mode of its failure is likely to be erosion of the channel developed across the landslide deposits. Furthermore, we infer that the proportion of large boulders 3 to 4 m in diameter and the proportion of non-disaggregated
bedrock in the dam will reduce the possibility of an en masse collapse even at high water levels with overtopping.
Emergency response and engineered breaching At 8:00 on May 23, the water level behind the Tangiashan landslide dam had risen to 719.8 m alt, 32.2 m below the low point on the dam, and the water volume was 1.1×108 m3. Before the end of May 2008, the water volume had risen to 2×108 m3 and had inundated the upstream power station. The need for maximum emergency response was clearly urgent. So, in order to reduce the risk of dam break, the Chinese government decided to excavate a spillway on the top of the Tangjiashan landslide dam and gave a warning to the people downstream the river.
Between May 26 and 31, a trapezoidal channel was excavated to trigger breaching of the dam. The channel was 475 m long, with the inlet at 740 m alt and the outlet at 739 m alt. The bottom was 7 m wide at the inlet and 50 m wide at the top; the depth at the outlet was ∼12 m. A second channel was cut to the left of the first channel in order to accelerate the water release (Fig. 8). To create the main channel, ∼13.55×104 m3 of soil and rocks was removed, 4,200 m3 of rock-in-wire gabion was created, an area of 14,040 m2
was leveled, 17 km of road was created, and 35,000 m3 of terrain was cleared (Sina 2008).
Overtopping began at 7:08 on June 7 when the water level rose to 740.0 m. During the first 2 days, the discharge was small because of the low channel slope, and the water level behind the dam continued to rise. Figure 9 shows the huge block that also slowed the rate of channel erosion, and this block was removed at 8:30 AM on June 8. Then, outflow increased rapidly, and discharge peaked at 6,420 m3/s on June 10 (Fig. 10). By 12:00 AM of June 12, the channel had stabilized, rapid incision had ceased, and the partial breaching of the dam had been accomplished. Downstream areas were inundated, including parts of Beichuan County (Fig. 11). People in these areas, however, had been evacuated. Sediment transported by the released flood was deposited throughout downstream reaches (Fig. 12).
Successes and flaws in the emergency response
Successes The emergency response to the formation of the Tangjiashan lake resulted in the successful release of the impounded water, protecting lives, property, and key infrastructure downstream. The main elements in this success include the following:
(1) The basic strategy was correct and its execution was timely, based on accurate risk assessment.
800 800
Elevation (m)
5.4 A
xe s
Original ground line
Fig. 5 Cross-section of the drainage channel, indicated on the photo in Fig. 4, showing suggested addition of a small triangular channel in the base of the main trapezoidal design channel
Strong weathered rock
752.2m
The base of the landslide
Fig. 6 Cross-section through the Tangjiashan landslide dam, shown on the photo in Fig. 4, along the Jianjiang River
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(2) Many systems and people cooperated, including those involved in rainfall forecasting and dam and lake monitor- ing, and communication was successfully achieved and maintained throughout the affected area. Experts were available for consultation throughout the response.
(3) The excavation of the sluiceway and its planned gradual incision during overtopping were based on the rock and soil composition of the dam. Breaching occurred at a rate that did not lead to excessive headward erosion and a cata- strophic discharge.
Flaws In retrospect, flaws are evident that, if recognized, will improve future responses and countermeasure planning, We emphasize that (1) the initial efficiency of water release was so low that the lake continued to rise 2 days after overtopping first occurred and (2) the outflow discharge rose more rapidly than what was desirable on June 10, resulting in local flooding downstream.
(1) Design of the sluiceway cross-section could have been improved Hydraulically, the trapezoidally shaped channel ini- tially conveyed a small discharge, which can be partially ascribed to its large wetted perimeter and roughness ratio (Wu 2003). We suggest a compound cross-section in future responses (Fig. 5). When the initial discharge is small, the compound cross-section has a smaller wetted perimeter and roughness, resulting in an increased discharge and thus to reduction of lake level instead of its continued increase, as in the case of the Tanjiashan lake. On the other hand, if the initial release discharge is large, a compound cross-section has a larger wetted perimeter and roughness and thus can effectively control the discharge. Compared with this suggested, easily accomplished modification, additional factors are less important.
(2) Sluiceway slope was too low Low sluiceway gradient is another flaw in the response. The gradient of the Jianjiang River at the site is 0.6%, in contrast to that of the channel, from 740 m alt to 739 m, over a distance of 300 m, of only 0.33% between the inlet and outlet. Because of numerous huge blocks in the channel, a much bigger channel gradient was required to achieve a dynamic balance in the case of a discharge of 10-year recurrence, the design specification. Then, the balance gradient must be
Fig. 7 Composite elements of the dam, showing in situ movement of the landslide. a Masses of non-disaggregated bedrock with preserved stratification, b large blocks up to 4 m in diameter, showing vegetation remaining upright, indicating movement en masse
Fig. 8 Aerial view of drainage channel on top of the Tangjiashan landslide dam
Fig. 9 Large block in channel-inhibiting incision, Tangiashan landslide dam. The block was later removed
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greater than the original value and the channel gradient must be 0.6% or greater.
(3) Lack of controls on rate of channel incision The dam is highly heterogeneous because of the content of poorly sorted loose soil and rock. In some places, erosion was slow because of the large boulders in the channel, but in other sections of the sluiceway erosion was uncontrolled.
Improving countermeasures The goal of the emergency response was to release water steadily, at a rate below that of a flood with a return period of 20 to 50 years. We suggest the following areas for potential improvement:
(1) Design of a channel using a compound cross-sectional shape The addition of a triangular-shaped cross-section is proposed for the bottom of the original trapezoidal channel (Fig. 5). Considering a sluiceway discharge at one third the rate of inflow in mid-May, 75 m3/s, the additional cross-sectional area
needed is 15 m2, and thus the added triangle should be 6-m wide and 5-m deep. This will increase the volume of excavated material by only 7,000 m3, 5% of the original total.
(2) Increase in sluiceway gradient Increased slope will increase flow velocity and channel erosion, further lowering the water level and reducing the risk of dam failure. If we increase the gradient to 3% (Cui et al. 2005), the elevation near the outlet, 326.0 m from the inlet, should be lowered to 730.97 m, with an extra 5% of excavation volume required.
(3) Controlling sluiceway incision The inlet and outlet are important in controlling the rate of channel discharge. Stone-in- wire gabions are commonly used for inlet protection. Huge boulders in the channel, which prevent channel incision, should be dynamited. Removing all large boulders will result in excessive incision and a potentially dangerous escalation of discharge. Man- made structures, such as concrete blocks, rough block stones as well as stone-in-wire gabions can be helpful in stabilizing the channel and limiting erosion.
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Fig. 10 Lake level and discharge hydrograph, Tangjiashan landslide dam
Fig. 11 Outlet flood passing through Beichuan County, downstream from Tangjiashan landslide dam (by Li Gang of XINHUA News Agency)
Fig. 12 Channel aggradation after the flood in Beichuan County, downstream from Tangjiashan landslid dam
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Conclusions and discussion The professional and timely emergency countermeasures at Tangjiashan Lake are a general model for response to landslide- dammed lakes at imminent risk of catastrophic failure or unplanned breaching. Success can be attributed to accurate scientific assessment, timely execution of the countermeasures, and the specific design and technique of sluiceway excavation that resulted in planned, controlled breaching of the dam 1 month after its emplacement.
In retrospect, we suggest consideration of areas of improve- ment because (1) low discharge in the sluiceway during the initial 2 days of overtopping allowed the lake level to continue to rise, increasing the risk, and (2) the discharge of the breaching on June 10 was somewhat, but not significantly, in excess of optimal, and local flooding occurred downstream in Beichuan County. The following improvements are suggested: (1) use of a combined trapezoidal and triangular channel cross section, instead of only a trapezoidal configuration, (2) increase in the channel gradient, and (3) better control on the rate of incision. In addition, to avoid the initial low sluiceway convey- ance and the eventual incision in excess of the optimal rate, as what occurred at Tangjiashan, the channel should be designed to result in significant but stable and uniform flow immediately upon overtopping.
In terms of the aforementioned analysis, the concept of artificially controlled failure is proposed, which includes following three steps: (1) optimizing the design of the sluiceway to minimize the volume of excavated material and maximize the release of water, (2) clearing significant impediments to flow (large boulders or masses of bedrock, whose diameter is more than 3 m generally, that were not disaggregated during dam emplacement), using explosives or mechanical means, and (3) limiting the potential for excessive rates of sluiceway incision with the use of concrete blocks, natural rocks emplaced as riprap, and rock-in-wire gabions.
The rate of planned discharge during breaching should reflect local environmental, societal, and economic conditions. A discharge corresponding to a flood with a return period of 20 or 50 years is appropriate. Finally, we suggest the creation of a specific, detailed protocol for engineering design and execution that can be rapidly applied in responding to future landslide-dammed lakes.
Acknowledgements The research is supported by the National Basic Research Program of China (Grant No. 2008CB425802) and the National Science Foundation of China (Grant No. 40501008). We appreciate the editorial suggestions from Prof. Kevin M. Scott, Senior International Scientist, CAS. We would like to thank the two anonymous reviewers for their comments on the manuscript.
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X. Q. Chen ()) : P. Cui : Y. Li Key Laboratory of Mountain Hazards and Surface Process, CAS( Chengdu, 610041, China e-mail: xqchen@imde.ac.cn
X. Q. Chen : P. Cui : Y. Li :W. Y. Zhao Institute of Mountain Hazards and Environment, CAS( Chengdu, 610041, China
Original Paper
Abstract
Introduction
Background
Successes and flaws in the emergency response
Successes
Flaws