Introduction

The Wangjiawan landslide occurred on the 1st of July 2017 near Weishan Town in Ningxiang County, Hunan Province, China, and it was triggered by up to 338 mm of rainfall between 29 June and 1 July. In all, nine people were killed and another 19 were injured, most during search and rescue efforts.

Ningxiang County is located in central east Hunan Province, China (Fig. 1a). It covers an area of about 2906 km2 (Changsha Municipal Bureau of Statistics 2015) and has a population of 1.39 million. The area is in a subtropical monsoon climate zone, with four distinctive seasons and abundant rainfall. The average annual temperature varies from 16.1 to 17.8 °C, and temperature extremes range from − 9.2 to 43.6 °C. Annual precipitation totals 1418 mm, most of which falls from June to August in the summer. Rainfall records, which began in 1973, show the maximum monthly rainfall in the area was 566.4 mm for June 1998, and maximum hourly rainfall of 54.4 mm occurred on 15 September 1998.

Fig. 1
figure 1

a Location of Ningxiang County, China. b Location map of Wangjiawan landslide in Ningxiang County. The digital elevation model (DEM) in this figure is derived from 1:10000 cartography contour lines and is based on China’s National Geodetic Coordinate System 1980

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The topography of Ningxiang County is complex and diverse and includes low mountains, hills, and flat plains. The terrain decreases from the southwest to the northeast (Fig. 1b). The highest mountain, with an elevation of 1071 m, is located in the western part of the county, where there are hundreds of landslides, mainly slides and rock falls. A preliminary landslide inventory map of Ningxiang County has been produced, but its resolution and level of information are highly variable. Therefore, detecting landslides and issuing accurate early warnings are very difficult at a regional scale.

Our field investigation focused on the Wangjiawan landslide, which is located on the south-eastern side of Xiaolongtan Reservoir, which is around 65 km west of Ningxiang County. In the preliminary study, the aim was to understand the landslide mechanism and the factors that triggered the landslide, based on field investigation, eyewitness accounts, and rainfall data analysis. Photos for the site investigation were taken using a DJI drone (Phantom 3).

Study area

The Wangjiawan landslide occurred at about 16:10 on the 1st of July 2017 in Weishan Town, on the south-eastern side of Xiaolongtan Reservoir. The landslide is about 100 m from the reservoir. The landslide was triggered by up to 338 mm of rainfall from the 29th of June to the 1st of July, 2017. In that period, many other landslides were also generated by the continuous rainfall in the area. For example, before the catastrophic event, a small landslide located near the Wangjiawan landslide occurred at about 6:00 am on July 1, 2017, which was 10 h earlier than the major landslide. The relative location of the source areas of these two landslides is shown in Fig. 2.

Fig. 2
figure 2

a Topography of the study area (modified from Google Earth) with red lines outlining the source areas of the Wangjiawan landslide (WL) and small landslide (SL) and b a DEM of the area in relation to Xiaolongtan Reservoir

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Landslide features

The Wangjiawan landslide occurred in two stages. First, a volume of 1.7 × 104 m3 in the lower part of the landslide gave way at about 14:00 on July 1, 2017. About 2 h later, at around 16:10, the main landslide occurred.

Photographs (Fig. 3) of the frontal part of the Wangjiawan landslide taken from a drone show the source and accumulation area after the final landslide. The landslide has a tongue-shape plan, with a length of 140 m, width of 80 m, and an elevation ranging between 404 and 329 m. The average thickness of the sliding mass is about 7 m. The final total volume, including the earlier landslide, amounts to 7.8 × 104 m3. The main sliding direction was initially 138° towards the southeast, but changed towards 180° (south) afterwards due to obstruction by the opposing mountain at the landslide toe, as shown in Fig. 3. It moved about 250 m in the southern direction in less than 10 s. It was an extremely rapid landslide, moving at a speed in excess of 25 m/s with a travel angle of 10.6°. The irregularly shaped landslide deposit covers an accumulation area of 2.8 × 104 m2 to an average thickness of 3 m. The thickness decreases from 5 m at the landslide toe to 0.5 m at the terminus. This landslide also includes materials deposited by a small landslide down the hillside to the east, which has an estimated volume of 2.5 × 103 m3 with a length of 85 m and a width of 30 m. The small landslide is mainly composed of recent unconsolidated fine sediments. Based on the investigation of the boulder sizes and bedrock lithology, it is inferred that some boulders of up to 1.3 m in the accumulation area came from the Wangjiawan landslide (Fig. 4).

Fig. 3
figure 3

a Photograph taken facing north-east, using a drone. It shows the source areas of the two landslides and accumulation area after the final sliding. b Photo showing the source area of the Wangjiawan landslide taken by a drone and facing east

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Fig. 4
figure 4

This boulder in the accumulation zone (photo taken facing east) is inferred to have come from the Wangjiawan landslide

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From a geomorphological point of view, the Wangjiawan landslide is a translational and rotational slide that degraded to an earth flow as it moved downslope. The landslide moved in a southerly direction after coming out of the slope and was finally deposited on nearby flat ground. The map view and cross section (Fig. 5) show that the gradient of the initial slope is steep and ranges from 35° to 40°. The geological structure of the landslide was determined by field survey. The bedrock is slate, overlain by Quaternary deposits consisting of silty clay, and the main landslide is parallel to the bedrock bedding plane. The mobilized material is composed of Quaternary deposits and slate. From top to bottom, the sliding body’s profile includes (1) soft, yellow, and brown silty clay with rubble, 4 m thick; (2) 3-m thick slate; and (3) a mud-rich slate intercalation, 0.2–0.5 m thick. The depth of the sliding surface is not constant and decreases from the middle to both flanks of the landslide.

Fig. 5
figure 5

a Map view and b cross-section profile of the Wangjiawan landslide

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Triggering factors and motion mechanisms

A field investigation was carried out after the major failure to discover the landslide mechanism and the causes. The likely cause of the landslide was a combination of 338 mm of rainfall 2 days prior to the landslide event, the steep slope, a buildup of pore water pressure, and the existence of a soft layer in the slope structure.

No rain gauge station exists at the site. The nearest one available is located about 4 km away, and the rainfall data from that station was used in this study. Figure 6 presents the hourly and cumulative rainfall data prior to the final failure. The average annual rainfall was 1418 mm, while the cumulative 2-day rainfall from 29 June to 1 July, 2017, totaled 338 mm. The sustained heavy rain may have saturated the sliding mass. In addition, the water table rose rapidly in the highly permeable layer when the infiltrated rainwater reached the impermeable sliding surface (Ebel et al. 2007; Wei et al. 2016). Therefore, this resulted in a rapid increase of pore pressure in the landslide.

Fig. 6
figure 6

Monitored rainfall data before the small landslide failure (SL) and the final Wangjiawan (WL) failure

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The Wangjiawan landslide failed about 8 h after the last major rainfall event, which had ended at about 8:00 a.m. This delay is attributed to the subsurface flow of water in the sliding mass. For the small landslide to the east, the cumulative rainfall at the time of failure was 277 mm, and the failure occurred 10 h earlier than the main Wangjiawan landslide. The reason was that the sliding mass of the small landslide was thinner, and this resulted in rapid subsurface infiltration and rise in the groundwater table. Therefore, it was inferred that the heavy rainfall and the rising pore water pressure caused by rainfall played dominant roles in triggering the landslide.

The extremely rapid main landslide is parallel to the bedrock bedding plane. The slope, which ranged from 35° to 40° was steep, which increased the shear stress and caused the rapid mass movement. In addition, the continuous rainfall rapidly seeped into the sliding mass due to its loose structure, increasing the water content of the sliding zone. The mud-rich slate intercalation disintegrated as water dissolved the mineral cement, resulting in a major loss in shear strength in the sliding zone. Therefore, the mechanism of the fast-moving landslide was mainly dictated by the integrated control of the steep slope and shear strength loss.

Landslide losses and initial rescue efforts

The Wangjiawan landslide resulted in serious loss of life and illustrated the dangers of rescue efforts. An old lady was buried by an initial local failure of the landslide. During the rescue operation to find her, eight local residents were then killed and 19 were injured by the fast-moving main Wangjiawan landslide, which occurred 2 h after the local failure. A resident who survived the incident said: “At the timing of the final failure, I saw more than 30 people participating in the rescue to find the buried old lady at the scene of the landslide. It was terrible that the landslide pouring down from the mountain moved so fast like a flash flood. I saw it rushing towards us and it was hard for me to run to safer place. I was pushed more than 100 meters away by the earth flow. I am lucky that I was not hit by the boulders in the earth flow. However, it was tragic accident for 9 fatalities and 19 others injured.”

Although the residents had advance warning from the local government to evacuate the area because of frequent slope failures nearby, they had little knowledge of landslide behavior, and the later and larger failure, which was fast-moving, could not be forecast. Therefore, the main landslide resulted in more fatalities.

In terms of economic losses, seven houses and 20 ha of rice paddy were destroyed (Fig. 7), and the village road in the middle of the landslide was also seriously damaged.

Fig. 7
figure 7

a A photograph, facing north, taken before the landslide, shows the Wangjiawan landslide source area (WL). b View of neighborhood buildings destroyed, taken after the landslide

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Emergency response measures

After the major failure, rescue services, including dozens from a disaster response force and locals, were immediately mobilized to search for survivors of the initial rescue efforts. A major focus during the rescue was the prevention of a secondary landslide disaster in and around the landslide site and ensuring the safety of the rescuers. Firstly, emergency field investigation and ground monitoring of the landslide were conducted by landslide experts. During the field investigation, several cracks continued to grow and a faint rumbling sound from the sudden cracking of the village road was noticeable. Because it was possible that a further failure of the nearby slopes would occur due to continuous rainfall, observation of landslide warning signs and monitoring of cracks were carried out. Several simplified displacement monitoring piles were set up at the landslide boundary. At least nine cracks of 3 to 100 m length and 5 to 15 cm width were found in the crown and both flanks of the landslide.

In order to reduce the landslide risk and prevent potential failure in the locality, several emergency measures were adopted:

  1. 1.

    The survivors of the landslide accident were ordered to move to another locality by the local authority. Because the nearby slopes were considered to be dangerous and prone to further sliding, people in another 14 households in this area were requested to relocate to safer ground.

  2. 2.

    Only landslide experts and rescue workers were permitted to enter the landslide area, and warning signs were set up at the landslide boundary. In addition, the relief workers were advised to avoid the mountain foot during the disaster relief operations. Rescue work ceased in the evening. The resumption of rescue operations was suspended if crack development continued or heavy rainfall occurred. In such conditions, the relief workers would be requested to move out of the area.

  3. 3.

    Plastic sheets were used to cover the landslide cracks. This prevented surface water from seeping through the narrow cracks and limited the increase of pore water pressure in the landslide.

Although the adopted emergency measures played an important role in reducing the landslide risk, there were still some limitations, as these measures could not prevent future sliding.

The following measures should be taken in the future:

  1. 1.

    To improve the slope stability, the residual mass should be removed from the landslide crown.

  2. 2.

    Local governments are responsible for land planning in order to find a safe place for the local people and for land-use policies and regulations that will reduce the landslide risk.

  3. 3.

    There is a need to provide training to local people. Local people can benefit from the professional training of engineering geologists to acquire the knowledge of landslide potential dangers.

Conclusions

A field investigation was carried out on the Wangjiawan landslide in Ningxiang County, Hunan Province, China, which occurred on 1 July 2017 after 2 days of continuous rainfall. The mechanisms for the landslide, especially the triggering factors, were studied. Based on the fieldwork and analysis thereafter, the following conclusions could be drawn:

  1. 1.

    The Wangjiawan landslide, which occurred along a pre-existing weak layer consisting of mud-rich slate intercalation, is a translational and rotational slide that degraded downslope to an earth flow. The travel angle of the landslide is 10.6°, and its estimated velocity was more than 25 m/s.

  2. 2.

    The prolonged heavy rainfall of up to 338 mm from 29 June to 1 July 2017 was the main triggering factor for the landslide. It caused the sliding mass to become saturated, with a rise of pore water pressure in the landslide.

  3. 3.

    The possible reasons for the extremely rapid movement were movement on a steep slope and the rapid reduction in shear strength of the sliding zone, which consisted of a mud-rich slate intercalation, which readily disintegrated under the effect of heavy rainfall.

  4. 4.

    Nine people were killed and 19 were injured, most during an initial local rescue attempt when rescuers were struck by the main landslide. For the second and larger operation by rescue services, officials were aware that a secondary landslide disaster could occur when heavy rainfall hit the nearby slopes again. Thus, several emergency measures including evacuation of the survivors, setting up warning signs, monitoring cracks, and covering them with plastic sheeting were adopted.