CASE HISTORY

Characterizing the spatial distribution and fundamental controlsof landslides in the three gorges reservoir area, China

Songlin Li1 & Qiang Xu1& Minggao Tang1

& Javed Iqbal2,3 & Jie Liu1& Xing Zhu1

& Fangzhou Liu4& Dongxue Zhu1

Received: 6 April 2018 /Accepted: 9 October 2018# Springer-Verlag GmbH Germany, part of Springer Nature 2018

AbstractA comprehensive field investigation and analysis of the spatial distribution of landslides was conducted in the Three GorgesReservoir Area (TGRA) along the Yangtze River and its first-order tributaries. A landslide inventory for this area was preparedusing regional-local geological investigations and other available information. The analysis of the extensive inventory data revealsthat the occurrence of landslides is mainly affected by four primary factors: (1) Lithology—the highest incidence of landslides isassociated with the lithological combination of marl and shale intercalated with mudstone (MSM), and a number of landslidesoccurred in an area dominated by sandstone andmudstone intercalated with shale and coal seams (SMSC), which cover amajor part(approximately 71%) of the bank slopes; (2) Structure—slope structure fundamentally controls the development of the sliding zoneand the mechanism of formation of landslides occurring on the same lithological combinations at subregional scales. Landslideconcentrations in consequent slopes are approximately 5–17 times greater than those of reverse slopes and diagonal slopes; (3)Topography—landslides are particularly abundant between elevations of 100 m and 600 m. The elevations of the headscarps of thelandslides gradually decrease from the head to the tail of the reservoir due to the topography, and almost all of the toes of thelandslides are below the highest historical floodwater level. The analysis of the correlation between landslide occurrence andhillslope gradients shows that 50% of all landslides occurred on slopes between 10° and 15°, despite slopes in this range representingonly 30% of the total hillslope gradients; and (4) Water level fluctuation and precipitation—the fluctuation of reservoir water levelexerts a fundamental effect that controls the occurrence and reactivation of landslides during the period of initial impoundment,whereas the precipitation becomes even more important than water level fluctuation after the first impoundment at 175 m above sealevel. In addition, the comprehensive analysis performed in this study provides broad insight into the landslide distribution in theTGRA. This work will improve our understanding of the role of morphological and geological conditions that affect the occurrenceof landslides, which may provide useful guidance for regional risk assessment and landslide susceptibility studies.

Keywords Landslides . Three gorges reservoir area . Lithology . Structure .Water level fluctuation

Introduction

The high frequency of the occurrence of landslides in the ThreeGorges Reservoir Area (TGRA) in China is due to the effects ofboth natural factors and human activities (He et al. 2008; Yinet al. 2010; Du et al. 2013). These landslides occur over largeareas, have remarkable characteristics and result in propertydamage and causalities. The Jipazi landslide of July 1982 rapidlyfell into the Yangtze River and seriously affected the navigationthat had occurred before the construction of the Three GorgesDam (Wei et al. 2006; Huang 2009). In June 2003, a trial im-poundment to 135 m above sea level (ASL) was begun, and amonth later, theQiangjiangping landslide occurred, which result-ed in 24 fatalities and nearly 1000 injuries. Several catastrophiclandslides even occurred during the stable operation period of the

* Songlin Lilsl68@foxmail.com; 597177914@qq.com

1 State Key Laboratory of Geohazard Prevention and GeoenvironmentProtection, Chengdu University of Technology, Chengdu 610059,China

2 Department of Earth Sciences, Abbottabad University of Science &Technology, Abbottabad, Pakistan

3 Institute of Mountain Hazards and Environment, Chinese Academyof Sciences, Chengdu 610041, China

4 School of Civil and Environmental Engineering, Georgia Institute ofTechnology, Atlanta, GA, USA

Bulletin of Engineering Geology and the Environmenthttps://doi.org/10.1007/s10064-018-1404-5

reservoir. Although the number of landslides has been dramati-cally reduced compared to the initial impoundment period (Yinet al. 2016), the challenges of geological hazard prevention areongoing, and the landslides have posed serious threats to navi-gation and the inhabitants in the reservoir area.

Landslide occurrence is usually triggered by external fac-tors and the interaction of geological conditions in a particularregion; earthquakes, precipitation, reservoir level fluctuation,and human activities are considered to be the main factors(e.g., Guzzetti et al. 2008; Borgomeo et al. 2014; Timilsinaet al. 2014). In particular, the water level fluctuation and pre-cipitation are considered to be the main factors leading to thereactivations of landslides in reservoirs (Jian et al. 2009;Zhang et al. 2010; Ahmadi and Eslami 2011; Xia et al.2013; Tang et al. 2016; Javed et al. 2017; Javed et al. 2018).The geological conditions could be classified into the geo-mechanical properties of bedrock, geomorphic conditions,tectonic controls, etc. (Guzzetti et al. 1999; Guzzetti et al.2005; Korup et al. 2007; Korup et al. 2010; Larsen et al.2010). Both mechanical properties and slope structure affectthe slope stability (Yamasaki and Chigira 2011; Miao et al.2014). Landslides are typically susceptible to the presence ofgeologic structures that dip towards the slope free-face (i.e.,consequent slope), whereas a few failures tend to occur wherestructures dip into the slope (i.e., reverse slope) (Chigira 1992;Godt et al. 2008; Qi et al. 2010; Javed et al. 2017). The fluvialerosion of the slope toe is also a driver of landslides (Larsenand Montgomery 2012; Deng et al. 2017).

The characterization of landslide distribution and deductionare the prerequisites for determining landslide susceptibility andrisk control (Ayalew and Yamagishi 2005; Van westen et al.2008; Guzzetti et al. 2012). Many researchers have studied wellthe failure mechanism and characteristics of deformation formany landslides that have comprehensive monitoring data(e.g., the Qiangjiangping, Huangtupo and Shuping landslides)(Wang et al. 2004; Wang et al. 2008; Liu et al. 2013; Jiao et al.2014; Hu et al. 2015; Tang et al. 2015; Ma et al. 2016; Huanget al. 2017; Sun et al. 2017). Additionally, some studies on thelandslide distribution in the subregional areas, such as theXiangxi River catchment (Bi et al. 2014), the Qinggan Rivercatchment (He et al. 2012; Wu et al. 2014) and the Badong–Zigui River channel (Chen et al. 2013; Wu et al. 2013; Penget al. 2015) have been reported. However, relatively little re-search on the spatial distribution and controlling factors forlandslide occurrence in the entire area has been specificallyexplored. Moreover, the relative importance of controlling fac-tors, such as lithology, topography, slope structure, precipitationand water level fluctuation, in determining landslide distribu-tion, type and frequency are not fully understood.

The purpose of this study is to analyze the spatial-temporaldistribution and the fundamental factors of landslide occur-rence and reactivation in the entire TGRA. This study focuseson the distribution and deformation characteristics of 865

landslides that occurred on the two sides of the river channel;of these, 576 landslides occurred along the Yangtze River, and289 landslides occurred along its first-order tributaries.Moreover, the bank slopes were divided into segments forquantitatively characterizing the various controls on landslideoccurrence. We are able to understand the spatial distributionand fundamental controls of landslides, which could provideguidance for regional risk assessments. On top of this founda-tion is further comprehensive research for the failure mecha-nism of landslides, which will also facilitate the study of land-slide reactivation. This research is of great interest and rele-vance, because it will allow the identification of controls onlandslide activity through field monitoring, in the event ofrapid reservoir drawdown or impoundment.

Study area

The study area is located in the transition zone between the sec-ond and third steps of China’s topography (see Fig. 1), and in-cludes complex geological environments (Zhang et al. 2009;Wang et al. 2014b). The total lengths of the bank slopes of themainstream and its tributaries are 670 km and 5000 km, respec-tively. Most of the rainfall occurs between May and September.The geomorphology is controlled by the lithology, tectonic eventsand significant geologic structures. The rocks in TGRA haveundergone intense tectonic events (Sun et al. 2017). Tectonicevents include the Jinning orogeny before the Sinian period, theYanshan orogeny in the Late Jurassic and theHimalayan orogenyin the Neogene (Li et al. 2009). These tectonic events formed thehilly or mountainous landscape of the Sichuan Basin west ofFengjie city and the east valley erosional landscapes.

The hillslope gradient ranges from less than 10° betweenFuling and Jiangjin, to more than 70° in some of the steepestvalleys, such as the Wu Gorge and the Qutang Gorge. Thegeological strata from the Pre Sinian to the Quaternary periodare all exposed excluding the Upper Silurian and UpperCarboniferous stratum. The TGRA contains an extensive areaof sedimentary strata and a small area of magmatic rocks andmetamorphic complexes. Clastic and carbonate rocks are thedominant components of the sedimentary strata. Separated byFengjie, the TGRA is mainly composed of Jurassic red-stratain the west, and carbonate rocks partly interlayered with mud-stone, shale and coal seams in the east (Yin et al. 2016).

Data and methods

Data preparation

There is no universal rule for selecting and classifying the con-trolling factors or influencing factors for landslide initiation;however, the selection and classification must be based on

S. Li et al.

physical relationships to produce reliable results (Timilsinaet al. 2014). Seismic activity is filtered out in this study becauseno deep faults or other geological features that can cause strongearthquakes have been found. In addition, these landslides arerarely caused by earthquakes (Chen 1999; Bai et al. 2010). Thebedrock lithology, slope structure and topography are selectedas the fundamental controlling factors, while the fluctuation ofthe reservoir and precipitation are selected as the critical trig-gering factors (Wu et al. 2004; Ward and Day 2011).

A digital elevation model (DEM) is available and is usedfor extracting topographical parameters (e.g., elevation andhillslope gradient) using GIS. The DEM, which has a resolu-tion of 30 × 30m, was prepared by the Geospatial Data Cloud,Chinese Academy of Sciences.

Two geological parameters, bedrock lithology and slopestructure types, are considered. The 1:200,000 and 1:250,000 scale geological maps that were mapped by theChina Geological Survey show strata, tectonic features, andthe attitude of bedding planes. All of the geological maps arescanned and digitized in ArcGIS, and the polygons andpolylines are constructed and coded according to their litho-logical and faults properties.

The Three Gorges reservoir impoundment was finished ona timeline in three stages. In the first stage, the water level rosefrom 65 m to 135 m in June 2003. In the second stage, thewater level rose to 156 m in October 2006 and 172 m inOctober 2008. The reservoir water level reached its maximumelevation of 175 m in October 2010 in the last stage. Then, the

reservoir level fluctuated from 145 m in summer (May toSeptember) to 175 m in winter (October to April), resultingin the formation of the water level fluctuation zone with a totalheight of 30 m (Zhang et al. 2014). The water surface leveldata from June 2003 to June 2015 used in this study wasprovided by the China Three Gorges Corporation.

Landslide inventory and mapping

Landslide occurrence in a given area can be analyzed with fieldinvestigations. The resultant landslide inventories could be usedby geological surveys around theworld to provide frameworks inwhich to gather, display, and analyze spatially explicit informa-tion about landslides (Galli et al. 2008; Pradhan 2013; Santangeloet al. 2015). The comprehensive and systematic investigation ofgeohazard and geoenvironment in the TGRA started in the 1950sandwas accelerated during 1985–1990.After the research projecttitled BBank slope Stability of Three Gorges Reservoir^ wasimplemented, the Chinese geologists conducted an in-depth in-vestigation (Chen 1999). Besides, a series of research projects,such as the BMonitoring and Early warning for Control ofGeological Hazard in the Three Gorges Reservoir Area^, wereimplemented later (Yin et al. 2010; Miao et al. 2014).

Therefore, preexisting inventories and a great deal of de-tailed information about many landslides are available to iden-tify and map the extent of landslides. The information on land-slides that includes location, volume, geological engineeringexploration, mitigation construction, monitoring data, the

Fig. 1 Location of the Three Gorges Reservoir Area, Central China (from Three Gorges Dam to Jiangjin City)

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

evolution of deformation, etc., are collected from theHeadquarter of Geological Hazard Prevention and Mitigationin the Three Gorges Reservoir.

Post-fieldwork, our landslide inventory map is refined andupdated with observations from the existing inventories. Thecurrent information is used as a base guide for fieldwork thatwas undertaken to; (1) identify subregional geological environ-ments such as topography, stratum, geological structure, etc.;(2) investigate the geological conditions of slopes (e.g., thestructural type of slope, bedrock lithology, microtopography);and (3) reconfirm detailed information about the features oflandslides (e.g., type of material involved in slope failures,landslide boundaries, hillslope gradients) based on a 1: 2000scale topographic map.

The inventory presented in this paper is historical, meaningthat the landslides we mapped are not differentiated based ontheir ages, and it consists of both bedrock landslides and ac-cumulated landslides. A significant amount of informationconcerning landslide inventories and relevant geological sur-veys is obtained, and features of the landslides are observedand documented well in the field. Only the landslides thatoccurred on the two sides of the river channel of the YangtzeRiver and its first-order tributaries are included in this inven-tory.Moreover, because the deformation history had beenwelldocumented, the reactivated landslides are identified;reactivated means that the motion of the landslides could beobserved within recent years (Yin et al. 2016). This inventoryprovides the basis for the analysis of the inducing factors onlandslide reactivation.

Analysis of controls and influencing factorson landslide occurrence

There are approximately 30 strata (see Fig. 4) that could bedivided into four lithological combinations (Table 1) based onthe percentage of hard, weak and soft rocks, as ascertained from

geological maps and other bibliographical data. In this study,hard rocks include granite, diorite, gneiss, limestone, dolostone,and sandstone; weak rocks include marl, shale, clayey siltstone,mudstone, etc.; and soft rocks include clay, coal seams andcarbonaceous shale (Bell 1992). Generally, the entire bankslopes are composed of the four lithological combinations.

The analysis of the correlation between landslide occur-rence and topography, as well as landslide occurrence andgeological conditions, is performed using GIS. To investigatethe control of lithological combination on landslide occur-rence, the stream bank of the Yangtze River and its tributariesare divided into 18 and 36 segments, respectively. As illustrat-ed in Fig. 5A, each segment represents one type of lithologicalcombination. Subsequently, the landslide concentration ineach segment or each lithological combination is calculated.Concentration, the number of landslides divided by the lengthof bank slope, describes the density of the spatial distributionof landslide in a region.

The slope structure, mainly relevant in mountainous areaswith sedimentary rocks, refers to the combined spatial relation-ships between the local slope aspect and the attitude of thebedding-plane, and it is affected synchronously by the faults,joint sets, and foliation (Guzzetti et al. 2008). The formationmechanism and the disintegration of the sliding body are affect-ed by the slope structure. In this study, a considerable amount ofdata about bedding attitude are extracted from the 1: 10,000geological map, and the map of the bedding plane, which al-lows access of the bedding attitude data from each grid, isgenerated in ArcGIS with boundary conditions of faults andfolds using the Kriging spatial interpolation method (seeFig. 7). Considering bedding attitude and slope aspect, threeslope structural types are identified (Guzzetti et al. 2008), name-ly: (1) the bedding dipping into the slope (i.e., reverse slope),(2) bedding dipping towards the slope free-face (i.e., conse-quent slope) and (3) bedding dipping transverse to the slope(i.e., diagonal slope). The stream bank of the Yangtze River and

Table 1 The main lithologicalcombinations of bedrock lithological combination Geological formation Main lithology

Limestone and dolostoneintercalated with shale(LDS)

Z,∈,O,S1–2,T1–2j,T1d,T3xj,T3j thick massive limestone, dolostone,argillaceous limestone and chertylimestone occasionally intercalatedwith sandstone and shale

Sandstone and mudstoneintercalated with shaleand coal seams(SMSC)

J1z,J1–2z,J1t,J2s,J1b,J2x,J3p,J3s,K1–2 medium thick layer or massivesandstone, clayey siltstone, feldspathicquartz–sandstones interlayered withmudstone, shale, and partly intercalatedwith coal seams

Marl and shale intercalatedwith mudstone(MSM)

T2b,T2l,D2c,P,C1–2 marl, limestone, interlayered withmudstone, shale, sandstone, andoccasionally intercalated with coalseams

Magmatic rocks andmetamorphic rocks(IM)

Pt, Ar, Nh granite, diorite, gabbro, and gneiss, etc.

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its first-order tributaries are divided into 65 and 38 segmentsaccording to their slope structural types. The structural types onthe left and right of the channel are usually different; hence, thelandslide concentrations are calculated separately.

The correlation between landslide occurrence and hillslopetopography is analyzed. Landslide hillslope gradient valuesare divided into six intervals to obtain a hillslope gradientfrequency distribution for landslides in the area. The correla-tion between the elevation of the landslide and water levelfluctuation was analyzed as well. To explore the key inducingfactor on landslide reactivation, considerable attention is de-voted to the landslides with severe deformation within recentyears. Therefore, the specific deformation time, displacementand triggers of these landslides are analyzed.

Results

Landslide characteristics and spatial distribution

There are 865 landslides along the 1151 km of bank slopes in thestudy area (see Fig. 2) (with an average landslide concentration(ALC) of 0.752 LS/km), with a total volume of approximately3.2 × 109 m3. Landslide volumes are grouped into four catego-ries: small (sub–105 m3), medium (105–106 m3), large (106–107 m3), and mega (>107 m3) (Whalley 1984; Fell 1994).There are approximately an equal number of medium (44.2%)and large (39.6%) landslides; 67 landslides (7.7%) with small

volumes; and 73M landslides (8.4%). Landslide thickness typesconsist of shallow (31%), medium thick (32%), thick layer(26%) and deep-seated (11%), following prevalent terminology(Varnes 1978; Hungr et al. 2001; Hungr et al. 2014).

Figure 3 shows the average landslide concentrations andvolume concentrations of landslides versus the dam site alongthe Yangtze River, with distances binned at 10-km intervals.Accordingly, the entire area is divided into three distinct sec-tions as follows:

Zigui-Badong section(I)

This section consists of 129 landslides, which is nearly 15% ofthe landslides distributed along the 60 km of river channel fromthe dam site to Badong city. In particular, there are 21 large andmega landslides along the right channel from Shazhenxi toFanjiaping, and 59 landslides along the Xiangxi River, whichis the first-order tributary with the most numerous landslides.The typical mega landslides, e.g., the Baishuihe, Shuping, andHuanglashi landslides occurred in this region.

Wushan-Wanzhou section(II)

This section shows broad valleys from Wushan to Wanzhouand contains several areas with high incidences of landslides,such as Fengjie, Yunyang and Wanzhou cities. Many largelandslides occurred in this section, such as the Liangshuijing,Ou’tang, and Sifangbei landslides.

Fig. 2 Spatial distribution of landslides in the TGRA

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

Zhongxian-Jiangjin section(III)

This section is characterized by low mountains and a hillylandscape, and the landslide concentrations and scale are low-er than in the previous two sections. The number of small andmedium-sized landslides accounts for 82% of the total land-slides, and the landslide concentrations are uniform.

In general, the concentrations and scale of landslideseast of Wanzhou are higher than in the west. Additionally,the stability of partial bank slopes is good; for instance,only a few, small landslides occurred along the 25 kmchannel in Wu Gorge. The distribution of landslidesshows significant regional and subregional differences inthe entire region (Fig. 4).

Fig. 3 Distribution of landslide concentrations and volume concentrations along the Yangtze River. I: Zigui-Badong section; II: Wushan-Wanzhousection; III: Zhongxian-Jiangjin section

Fig. 4 Distribution map of main strata and faults/fold belts in the TGRA

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Correlation with lithological types of bedrock

The lithological combinations of bedrock and segment divi-sions in the study area are shown in Fig. 5. The landslide locusand concentration in each division are obtained, and theseresults show that the landslides are not evenly distributed,and the lithological controls play an important role in deter-mining landslide occurrence.

As shown in Fig. 5B, the ALC varies significantly amongthe lithological combinations. The #1 bank slope has a goodstability, and few landslides occurred. The #2 to #6 bankslopes exhibit a higher ALCwhen they occur over SMSC thanover LDS. The ALC of the #9 bank slope is the highest in theentire region because the underlying stratum, T2b, is a typical

landslide-prone stratum. No landslide occurred in the #12bank slope, which is mainly composed of LDS.

Landslides frequently occur over SMSC, which makes up amajor proportion (71%) of the bedrock of the studied area(Table 2). However, the highest density of landslides (1.3LS/km) over MSM is approximately twice that found overSMSC, which implies that MSM is more prone to landslideoccurrence than SMSC.

The ALC over LDS bedrock is the lowest (0.283 LS/km).The large difference in ALC between MSM and LDS resultsin the regional zonation characteristics observed from Badongto Fengjie (see Fig. 5B). Consequently, SMSC and MSM arethe most influential lithological combinations for landslides inthe TGRA (Figs. 6 and 7).

Fig. 5 The correlation between lithological types of bedrock and landslides. (A) Lithological combination map of the study area and divided segments,(B) Subregional area amplification of landslide distribution

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

Correlation with slope structure types

The bank slopes of the Yangtze River and its tributariesare divided into 65 and 38 segments, respectively, ac-cording to the slope structural types. The most commonbedding setting is that of the bedding plane dippingtowards the slope free-face. The ALC of the left andright bank slopes in each segment are calculated sepa-rately (see Fig. 8).

The bank slopes of the Yangtze River from Fengjie toYunyang (#29–#35 segments) are assumed to be typical casesfrom which to characterize the control of slope structure onlandslides (see Fig. 7b). The ALC is 1 LS/km on the rightchannel, which has consequent slopes, whereas it is only0.06 LS/km on the left channel, which has diagonal or reverseslopes; i.e., nearly 94% of the landslides are distributed inconsequent slopes. This shows that slope structure is the fun-damental control on landslides in a local region, e.g., the left orright bank slope.

As shown in Table 3, the landslides distributed onconsequent slopes account for nearly 60% of the total,while the landslides on diagonal slopes account for only16% of the total. This means that the consequent slopesare more susceptible to landslides than other slopestructures.

Topography and geomorphology

The elevation of the toes and headscarps of the landslides thatoccur along the Yangtze River are shown in Fig. 9. The toeelevations of most landslides (93%) are concentratal between100 m and 175 m, which was the water level fluctuation zonebefore the establishment of the reservoir, and the landslidesare closely related to the effects of the fluvial erosion of theslope toe. The headscarp elevations are mainly between 180mand 600 m, and they gradually decrease from the head to thetail of the reservoir due to the topography.

Figure 10 illustrates that the average hillslope gradient east ofWanzhou is between 10° and 30°, and it is mostly under 20° tothe west. Some high and steep valley slopes are formed in local-ized regions with hard bedding rocks, for instance, the steep-sided valley of 70°–80° gradients formed in carbonate rocks.

The correlation between landslide concentration and hill-slope gradient is explored by binning hillslope gradient valuesinto several intervals and plotting the concentrations of land-slides within these binned hillslope gradients. As shown inFig. 11, the ALC is the highest on slopes between 16° and20°, despite this interval representing only 12% of the hill-slope gradients of the stream bank. These data suggest thatmost landslides occur on slopes between 8° and 30°. Thelower range of angles is associated with low shear stress on

Fig. 6 The ALC along the Yangtze River over different lithological combinations

Table 2 Landslide distributioncharacteristics over differentlithological combinations in theTGRA

Lithological combination Bank slope Landslide

Length (km) Percentage (%) Number Percentage (%) Density (/km)

LDS 130.7 11.2 37 4.3 0.283

SMSC 828 71.2 575 66.5 0.694

MSM 188.4 16.2 245 28.3 1.3

IM 16.2 1.4 8 0.9 0.495

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the slope materials, and thus a gentle slope is expected to bemore stable.

Influence of water level fluctuation and precipitation

Analysis of the influence of water level fluctuation and precip-itation on the frequency of reactivated and new landslides that

exhibited severe deformation, where severe means that the mo-tion of the landslide could be observed (Yin et al. 2016) fromJune 2003 to June 2015, is conducted as shown in Fig. 12. Thereare 34 and 27 reported cases of landslide deformation in Juneand July 2003, respectively, exceeding the historical peak forthosemonths. Seventy reactivated landslideswere observed dur-ing the first drawdown period following impoundment to 156m

Fig. 7 The correlation between slope structural types and landslides. (A) Slope structural types map of the study area and segments of bank slopes, (B)Amplification of landslide distribution in subregional areas

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

ASL, and 78 landslides were observed from October toDecember 2008 during the 175 m ASL impoundment.Relatively less landslide activitywas identified after six reservoircycles. The landslide reactivation response to precipitationshowed an increase in the rainy season from late June to earlyAugust. For instance, 20 landslides were reactivated in Augustand September 2014 due to the B8.31 heavy rainstorm,^ whichreached more than 350 mm on 31 August in Fengjie, Yunyangand Wushan County (Meng et al. 2017).

It can be shown that the fluctuation of the reservoir is thefundamental trigger on most landslide reactivations during theinitial impoundment; however, this effect decreases and thenis replaced by precipitation in the later period. However, forcertain landslides, both precipitation and reservoir water affectthe process of deformation.

Furthermore, the fluctuation rate of reservoir water is thecritical controlling factor on landslides deformation. Using theShuping landslide as a case study, there were four steps oflandslide deformation from 2011 to 2015 as shown inFig. 13. Reservoir drawdown leads to the most significantincrement of landslide displacement.

During the slow drawdown period from February toMay 2012, the displacement rate increased slightly to 2 mm/day. However, when the drawdown rate increased to 1.0 m/day on June 10, the landslide began to move rapidly later, andthe displacement rate reached its maximum of 30 mm/day on

June 13. As the drawdown rate decreased to 0.3 m/day onJune 11, two days later, the movement reduced rapidly andreached 1 mm/day on June 20, which means that there is apositive correlation between the drawdown rate and the dis-placement rate. Moreover, many landslides have shown a sim-ilar trend of deformational response to the fluctuation of thereservoir, such as the Woshaxi landslide and the Baijiabaolandslide(Wu et al. 2016).

The primary reactivation mechanism of most (70%) land-slides is hydrodynamic pressure, whereas 15% of the land-slides were affected by buoyancy in the TGRA. The hydrody-namic pressure appeared when the water level drawdown in-side the landslide lagged behind the reservoir because thepermeability of the landslide was lower than the drawdownrate. (Haneberg 1991; Yan et al. 2010). Due to the action of theupward pressure, the weight of the foot-of-slope rocks dimin-ishes, the passive forces decreases, and stability is disturbed(Lindenmaier et al. 2005). From this point of view, the firstfilling of the reservoir is considered to be the most hazardous.

The influence of reservoir water fluctuation is not onlyrelated to the fluctuation rate but also to the affected part ofthe landslide. This influence can be calculated as the inunda-tion ratio of the landslide, expressed as:

p ¼ hd−h1h2−h1

� 100% ð1Þ

Fig. 8 The ALC along the Yangtze River within different slope structures on the right channel (the upper bar) and the left channel (the lower bar)

Table 3 Landslides distributioncharacteristics on different slopestructural types in the TGRA

Slope structure Bank slope Landslide

Length (km) Percentage (%) Number Percentage (%) Density (/km)

Consequent slope 1085 48.9 496 57.3 0.457

Diagonal slope 360 16.2 138 16 0.383

Reverse slope 776 34.9 231 26.7 0.298

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where h1 and h2 are the elevations of the toe and headscarp of thelandslide, respectively, hd is the elevation of the reservoir waterlevel when the landslide is deformed. The value of p is groupedinto three categories: 0–30% (toe submergence), 30%–60%(main body submergence), and 60%–100% (headscarp submer-gence). As shown in Fig. 14, the toes are submerged in most ofthe landslides (>90%), compromising the slope stability by re-ducing the resistance provided by the lower part of the slope.

Discussion

Lithological control of bedrock on landslideoccurrence

In previous research, the analysis of landslides in the TGRAconcentrated on individual groups of landslides at a more

localized scale (Fourniadis et al., 2007; Bai et al. 2010; Penget al. 2014). Only a few studies focused on the occurrence anddistribution of landslides and quantitative statistical analysisusing GIS in the entire region. We attribute the high landslideconcentrations in certain lithological combinations character-ized by high proportions of weak and soft rocks to the poormechanical properties of the materials.

In the case of the bank slope of Anping, all of the landslidesoccurred in stratified rocks that are composed of a hard rocklithology intercalated with a high percentage of soft and weaklayers (e.g., carbonaceous shale and purple-red mudstone)(Chen 1999). The intercalations of shale and mudstones arereadily swelled and weakened under saturation due to the highcontent of clay minerals, including illite and montmorillonite,which exhibit expansion, disintegration and softening undercyclical wetting-drying conditions. In consequence, these stra-ta are prone to slide (Zhang et al. 2012; Torres-Suarez et al.

Fig. 10 The gradient of hillslope in the TGRA

Fig. 9 The elevations of the toesand headscarps of the landslidesalong the Yangtze River. The Aand B lines are the historicalaverage flood-water and low-water level, respectively, of theYangtze River before the ThreeGorges Damwas established. TheC and D dashed lines are the175 m ASL and 145 m ASL ofreservoir water level, respectively

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

2014; Wang et al. 2014a), as demonstrated in the Baota land-slide (Chen et al. 2008).

The structural features of the rock mass play an importantrole in the occurrence of landslides. For example, the T2bstratum, which was deformed during the Indo-China event,contains clay and shale mainly with a chaotic structure.These preexisting fissures can expand into dissolution gapsand karst collapses in argillaceous limestone and marl by pro-viding a channel for ingress of groundwater. The permeabilityof the underlying mudstone is low, which prevents groundwa-ter flow along the bedding plane. In an area with strong tec-tonic events, mudstone can be transformed into a more clayeyinterlayer with low strength under cyclical wetting-drying

conditions, and the potential slip surface can be formed fromlong-term corrosion, mudding and softening.

Slope structure control on main landslide types

Researches on the effects of slope structure on landslides inthe TGRA were started in the 1980s (e.g., Chen 1999), andqualitatively described slope structure. In this study, approxi-mately 10,000 bedding attitude data are extracted from thegeological map, and the database is established. Using thebedding attitude and localized slope aspect, the slope structurecould be identified for each grid in ArcGIS, and the landslide

Fig. 11 Bank slope frequencyand the ALC in different slopeintervals

Fig. 12 The correlation between the number of landslides and the variation of the water surface level in the reservoir

S. Li et al.

occurrence in regions of different types of slope structure canbe evaluated in such a manner.

In the cases observed, the sliding-bending landslides typi-cally originate from creep-slide motion of a rock mass alongits bedding plane in consequent strata that dip moderatelyoutwards, or steeply without a free-face. The movement isimpeded near the toe of the slope, and subsequently causesbending and breakage until it cuts through by shear (seeFig. 15). In the obsequent or mild-dipping slopes, the strataexhibit bending-tensile motion, resulting in tensile fracturingnear the crown area, and develop a creep-slip- shearing frac-turing type of landslide (see Fig. 16). The source area of thesliding-bending type of landslide is typically chair-shapedwith a large width and a small opening connected to the zoneof depletion, whereas the sliding-tensile fracturing type has amore arc-shaped source area. This morphological difference isrelated to the reactivations of landslides under reservoir levelfluctuation. Most of the landslides with arc-shaped sourceareas deform when the reservoir level decreases, whereas the

chair-shaped landslides deform when the reservoir level in-creases. The disintegration of the sliding bodymay vary great-ly for different failure mechanisms.

Effect of fluctuation of reservoir water level

Both occurrences, as well as the reactivations of landslides,show evident correlation with the variation in reservoir level.This effect is related to the fluctuation rate and location ofsubmergence of the landslide, and therefore, reducing thedrawdown rate during the nonflood period can be an effectivemeasure for slope stabilization in the TGRA.

The initial impoundment triggered a large number of land-slides, but the data shows a decreasing trend as impoundmentcontinued. In the first three years of the trial impoundmentphase, the frequency of landslides reached its peak, but ithas decreased dramatically after the 175 m ASL impound-ment. In the twelve-year period of the reservoir operation,508 landslides were reported, and 321 of them (63.2%)

Fig. 14 Distribution map of theinundated extent of landslidesalong the Yangtze River

Fig. 13 Cumulative displacement of the Shuping landslide and fluctuation of reservoir water level versus time

Characterizing the spatial distribution and fundamental controls of landslides in the three gorges...

occurred during the first period of reservoir level variation.More attention should be paid to the long-term evolution ofbank slope deformation affected by reservoir operations (fill-ing and drawdown).

Conclusions

Landslides are widespread in the TGRA, and they are formedunder very complex conditions. This study developed a land-slide inventory along 1151 km of river channel of the reservoirusing a combination of existing information and detailed fieldinvestigations. The spatial distribution of landslides is charac-terized by significant regional and subregional differences,which is examined concerning the morphological and litho-logical combinations of bedrocks, slope structure, reservoirlevel fluctuation, and precipitation.

The bedrocks exert a primary control in determining thedistribution of landslides, and can be divided into four mainlithological combinations in the area. The majority of land-slides occurred within the sandstone, shale, and mudstonewith coal seams combination. The marl and limestone withmudstone lithological combination are found to be the mostprone to landslides. The highest number and densest concen-tration of landslides occur where bedding dips towards theslope free-face (i.e., consequent slope); landslide concentra-tions could be up to an order of magnitude greater for conse-quent slopes than for reverse or diagonal slopes. In addition,the length of the consequent slopes accounts for nearly half ofthe slopes in the entire area, thus resulting in regional distri-bution differences.

Landslides are particularly abundant between 100 m and600 m in elevation, and between 8° and 30° hillslope gradi-ents. Almost all toes of the landslides are below the highesthistorical floodwater level, showing that the fluvial erosion ofthe slope toes affects the landslide occurrence; while the ele-vations of headscarps gradually decrease, coupled with thetopography. The toe elevations of the reactivated landslidesare below the highest reservoir water level (i.e., 175 m). Thefrequency of landslide reactivation indicates that the

fluctuation of reservoir water level was the fundamental trig-gering factor during the initial impoundment, and the funda-mental triggering factor becomes the precipitation after 5–6impoundment-drawdown cycles. Most of the landslides aredeformed when their toes are submerged, and the displace-ment rate is mainly affected by the reservoir fluctuation rate.

Acknowledgements This research was financially supported by theGeological Hazard Prevention Project of Three Gorges (Grant No.000121 2015C C60 005), the National Basic Research Program ofChina (Grant No. 2013CB733202, 2013CB733206), and the State KeyLaboratory of Geohazard Prevention and Geoenvironment Protection(Grant No. SKLGP2015Z008). The headquarter of Geological HazardPrevention and Mitigation in the Three Gorges Reservoir is greatly ac-knowledged for providing required datasets for this study. Finally, theauthors would like to express their sincere appreciation and gratitude tothe reviewers and the handling editor for their valuable comments, sug-gestions, corrections, and encouragement for improving the originalmanuscript.

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