Landslides

, Volume 13, Issue 3, pp 603–616 | Cite as

Risk management study on impulse waves generated by Hongyanzi landslide in Three Gorges Reservoir of China on June 24, 2015

Recent Landslides
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Abstract

On June 24, 2015, Hongyanzi slope located in Wushan County of the Three Gorges Reservoir collapsed, generating 5–6-m-high impulse waves, which overturned 13 boats, killed 2 persons, and injured 4 persons. It is the second incident of landslide-generated impulse waves since the 175-m experimental impoundment in 2008. The emergency investigation shows that Hongyanzi landslide is a bedding soil landslide with a volume of 23 × 104 m3 induced by a series of triggering factors such as rainfall, flooding upstream, and reservoir drawdown. The nonlinear Boussinesq water wave model is used to reproduce the impulse waves generated by the landslide of June 24th. The numerical simulation results suggest that the wave propagation process was influenced by the T-shaped geomorphic conditions of river valley, and the coastal areas in the county seat were the major wave-affected areas, which is opposite to the landslide. The numerical wave process accord well with the observed incident, and the investigation values were in good agreement with the calculated values. Moreover, the worst-case scenario of the 7 × 104 m3 deformation mass beside Hongyanzi landslide is potential to generate impulse waves, which was predicted with the same numerical model. This adjacent deformation mass will probably generate impulse waves with maximum height and run-up of 2.2 and 2.0 m, respectively, and only a very few areas in the water course had waves rising to a height of 1 m or above. The research results provide a technical basis for emergency disposal to Hongyanzi landslide and navigation restriction in Wushan waterway. More importantly, it pushes the risk management of the navigation based on the impulse wave generated by landslide. It is advised that the Three Gorges Reservoir and other reservoirs around the world should put more efforts in performing special surveys and studies on the potential hazards associated with landslide-generated impulse waves.

Keywords

The Three Gorges Reservoir Hongyanzi landslide Impulse wave risk management Nonlinear Boussinesq model 

Introduction

At about 6:23 p.m., June 24, 2015, Hongyanzi slope on the opposite bank of the seat of Wushan County in the Three Gorges Reservoir collapsed. The 23 × 104 m3 landslide generated impulse waves of about 6 m high that attacked the coastal areas of Wushan County, overturning 13 boats, killed 2 persons, and injured 4 persons.

Towns have been hit by a couple of landslide-generated impulse wave incidents since the impoundment of the Three Gorges Reservoir in June 2003. For example, Qianjiangping slope on the opposite bank of Shazhenxi Town collapsed on July 14, 2003, generating impulse waves that caused the knocking over of 22 fishing boats and missing of 11 fishermen (Wang et al. 2004, 2008; Yin et al. 2015). The Nierwan landslide failed in November 5, 2008, which generated small-scale impulse waves that endangered Shuitianba Town on the opposite bank (Tian and Lu 2012). The incident of Hongyanzi landslide-generated impulse waves on June 24, 2015 is the first time that a county was attacked by landslide-generated waves in the Three Gorges Reservoir. Therefore, it has aroused wide public concern at home and abroad. Several (potential) tsunamis generated by landslides have made the navigation closed in the history of Yangtze River, resulting in large economic losses.

In this paper, five major aspects of Hongyanzi landslide are researched based on the emergency investigation and risk disposal: (1) the geologic setting and engineering characteristics are described; (2) the deformation, failure process, and triggers are described; (3) the wave generation process and associated hazards are reproduced using the nonlinear Boussinesq water wave model to analyze the influence of local geomorphic conditions on wave hazards; (4) the wave hazards associated with the adjacent deformation mass are predicted; (5) based on the spatial distribution of the potential impulse wave, the risk management of the navigation is discussed. By combining all aspects mentioned above, the paper provides a complete picture of the incident from deformation to landslide and then to generated impulse wave. The research results provide a direct technical basis for risk control and disposal of this landslide. The Hongyanzi landslide incident will have a profound influence on the prospective planning for prevention and control of geologic hazards in the Three Gorges Reservoir. The findings presented in this paper will offer valuable insights into the planning of prevention and control of geologic hazards in the Three Gorges Reservoir and also offer guidance on predictive studies on landslide-generated impulse waves in reservoirs around the world.

Geologic setting and overview of Hongyanzi slope

Hongyanzi slope is located on the opposite bank of the seat of Wushan County in the Three Gorges Reservoir and on the east bank of Daning River, merely 800 m away from the Daning River bayou. This area features a geomorphic transition from low mountains to alpines and gorges. The elevation of the top of the failed slope is 449.2 m above sea level (a.s.l.), and the lowest elevation of Daning River valley is around 92 m a.s.l. Tectonically, the slope lies on the southeast wing of Wushan syncline (Yichang Institute of Geology and Mineral Resource 2001). The area around Hongyanzi slope is a monocline geomorphy, in NE strike direction, with a dip of 310–315° and an average gradient of 30–40°. The Daning River joins the Yangtze River here, leaving a broad valley at estuary with a width ranging from 0.88 to 1.3 km. The left lower part of Fig. 1 describes the rocks exposed on the slope:
Fig. 1

The location and geological setting map of Hongyanzi Slope. The black lines in the left lower part are geological boundary lines, red lines are the boundaries of Hongyanzi slope, and black dotted line is axis line of Wushan syncline. The right lower photo is the former scene of Hongyanzi slope taken in June 2006 when the water level was 135 m

Eluvium silty clay (Q4el+dl): purple-red hard-plastic silty clay, interbedded with 10∼30 % gravel of limestone and dolomitic limestone; mainly distributed at the top of Jiangdongzui slope with the thickness of 5∼10 m.

Colluvium (Q4col+dl): mainly consisting of silty clay with gravel and gravel soil. The gravels contain limestone and marlstone; the gravel soil make up 30∼80 % of the gravel, with an average and maximum grain size of 3∼20 cm and 1 m respectively; fine materials observed are yellow silty clays. The debris is widely distributed on the surface layer of monoclinic slope.

Alluvium (Q3apl): mainly distributed in the washland of Daning River with a maximum elevation of 125 m a.s.l. Flood plain and terrace consist primarily of muddy-silty clay, mixed with silty-fine sand in some places, currently deposited underwater.

Jialingjiang Formation, earlier Triassic System (T1j): massive and medium bedded rock, consisting mainly of light-grey microcrystal-fine crystalline limestone. Light-grey thinly-bedded dolomitic limestone is observed at the bottom, while grey dolomitic limestone, limestone, and breccia limestone are exposed on the top. It is the dominant formation where rocks are underlying and exposed in the slope region, with a strike of 320° and a dip of 35°.

Hongyanzi slope is bounded by gullies on both sides, and is a slightly ridged slope in micro-topographically. The slope has an average gradient of 34°. There is no house resting on the slope, and the periphery of both sides of the slope is concentrated residential areas. With a bedding structure, the slope stretches in the direction of 310°. Before the impoundment, the flood plain of Daning River lied at the slope toe, which is rather broad and flat, with an elevation of 100∼125 m a.s.l. (Fig. 1).

At around 6:23 p.m., June 24, 2015, Hongyanzi slope collapsed. The landslide geomorphy is in the shape of the letter “U”. It was about 180 m in length, 130 m in width, and 2 × 104 m2 in area, with a sliding mass volume of 23 × 104 m3. It generally slid in the direction of 310°, forming a medium-scale soil landslide. The crown was bounded by sliding scarp, arc-shaped, and had an elevation of 275 m a.s.l. The left and right sides were bounded by the landslide ridge. On the day of failure, the water level in the Three Gorges Reservoir was 145.3 m a.s.l. The water depth around the slope toe was 20 m, which was much shallower than those in Daning River main channel and Yangtze River channel. Figure 2 is a photo taken on June 25, 2015, which depicts the whole view of Hongyanzi landslide. Meanwhile, this aerial photo shows clearly the outcrop of bedrocks in places of the hydro-fluctuation belt on both sides of the landslide. By comparing with the right lower photo shown in Fig. 1, we can find that these bedrocks were originally covered in clay which was later eroded by the fluctuating water. Moreover, the photo indicates that the bedrocks in the sliding mass were buried deeper than those in the slopes on both sides; namely, the soil mass of Hongyanzi slope was thicker than that of the slopes on both sides.
Fig. 2

Whole view of Hongyanzi Slope, taken on June 25, 2015

On June 25, 2015, the Hydrogeololgical and Environmental Geological Survey Center of China Geological Survey conducted a topographic scanning over the landslide area using a 3D laser scanner (Fig. 3). In fact, the Chongqing Institute of Geological Disaster Prevention and Control ever had made a survey on these slopes (including Hongyanzi slope) on the opposite bank of Wushan County in June 2012 (Chongqing Institute of Geological Disaster Prevention and Control 2012) in an attempt to get the preparatory work on protective projects of the reservoir bank in the future. Five boreholes were drilled on Hongyanzi slope during the survey of 2012 (Fig. 3). The boreholes are arranged in a straight line, with elevations ranging from 129 to 232 m a.s.l. The borehole data show that the colluvium soil mass of Hongyanzi slope was 12 m thick on average. It was also revealed by the borehole data that limestone karst was highly developed in the weathered rock zone, evidenced by many dissolution fissures and pores; the rock core is rather broken, mostly in block or short-column shape; the thickness of the intensely weathered rock zone was 1.4∼6.8 m. The soil mass was prone to slide along the boundary of the weathered rock zone. The survey report showed that Hongyanzi slope was almost stable at that time (FOS = 1.07). By digging further into the survey report, it was found that the erosion of soil mass on the underwater bank slope by reservoir water fluctuation was not taken into account when analyzing the slope stability. The soil mass on the underwater bank slope formed an important anti-sliding section at the toe of Hongyanzi bedding slope. In addition, the landslide-induced impulse waves were not considered when analyzing the potential hazards. Such impulse waves would endanger the safety of waterway and lives and properties of the people residing in the coastal regions of the county town on the opposite bank.
Fig. 3

Engineering geological map of Hongyanzi landslide. The tooth line delineates the deformed zone formed after the landslide, the carmine line delineates the landslide area, the carmine dotted line delineates the sliding range in June 23rd, 2015, the red line denotes tension crack on the slope, the black circles denote the boreholes drilled in 2012, 1-1’ represents the section line, and the grey line represents the underwater contour line measured during 2012 survey; the landslide area was plotted by the Hydrogeololgical and Environmental Geological Survey Center of China Geological Survey using a 3D laser scanner on June 25, 2015

After failure, there are different degrees of deformation in the C and D zone. Most deformations in zone D were invisible, and no macro-fracture was observed. The zone D was predominantly affected by the trailing edge of the landslide. Zone C had three major tension cracks (LF1-LF3) and a great many of longitudinal cracks (Fig. 3). LF1 was 31 m long and 7∼12 cm wide, in the direction of 45° and almost perpendicular to the sliding direction. LF2 was 45 m long and 10–37 cm wide, slumping to a displacement of 2.6 m, with an arc shape strike and northward dip. LF3 was 48 m long and 3∼47 cm wide, slumping to a displacement of 0.12 m, with an arc shape strike and northward dip. The zone C was 165 m long and 35 m wide, with an average thickness of 12 m and volume of 7 × 104 m3. As it continues to deform, the soil mass of zone C looks like another “Hongyanzi landslide.”

Landslide process, triggers and losses

Hongyanzi slope did not collapse all at once. Instead, it underwent a period of macroscopic deformation before sliding as a whole. On June 21, 2015, a small-scale collapse was observed in the foot of the slope, with very small volume of sliding mass. On June 23, 2015, the foot collapsed further (Fig. 4) with a small volume. On the morning of June 24, 2015, the middle and trailing parts of the slope deformed more radically, producing arc-shaped cracks extending 25∼80 m. At noon, the local government organized the local residents to evacuate in emergency. At 4:00 p.m. or so, the cracks deformed and widened radically, linking up and penetrating each other gradually. Tension fracture, subsidence, and warping were found in the concrete road on the trailing edge.
Fig. 4

Photo of the failure of the foot of Hongyanzi slope, taken on June 23, 2015

At around 6:23 p.m., June 24, 2015, Hongyanzi slope slid as a whole along the cracks. Through calculation based on the elevation difference of the mark point of concrete block found after the slope failure and before, the sliding drop displacement was 100 m and the sliding distance was 150 m. Several witnesses were interviewed, including the local people of mass monitoring and preparedness, the local officer, and the local people living adjacent to the landslide, who witnessed the landslide event just outside the landslide. As recalled by the witnesses, Hongyanzi slope slid for 2 min or so. Figure 5 depicts the endangerment the county seat, and the photo was taken from the crown of the landslide, from which we can see that the impulse waves generated by the landslide could attack the docks and coastal regions of the county directly.
Fig. 5

Photo of the coastal regions opposite the landslide, taken on June 25, 2015 on the crest of Hongyanzi landslide

Figure 6 is two photos of the dock attacked by impulse waves taken by passerby. The left photo shows a couple of wave crests moving toward the bank; the boats’ bows upwarped under the action of the wave crest closest to the bank and the boats were pushed toward the bank by the waves. The right photo shown in Fig. 6 displays the convergence and superposition of the waves reflected from the bank slope and those propagated toward the bank slope. As the passerby swayed his mobile phone when taking the photo and missed the periods when the slope collapsed, it is rather difficult to figure out the maximum height of the wave and time from these photos.
Fig. 6

Transient photos of the dock attacked by the running-up waves, where the arrows of different colors denote different wave crest line

Residents around the sliding mass area were given early warning and evacuated timely before the landslide, so no fatality or injury was caused inside the slope. According to the witnesses, Hongyanzi landslide generated impulse waves of 5–6 m height after sliding into water. As revealed by an investigation conducted by Maritime Administration of Wushan County, affected by the impulse waves, 13 boats were knocked over including one 14-m-long coastal patrol vessel berthing beside the bank, 2 persons on the riverside were killed and 4 injured, 3 wireropes broken, and 8 wireropes damaged more or less. The landslide and landslide-induced impulse waves caused a direct economic loss of some RMB 5 million or 0.8 million U.S. dollars. Daning River and Wu Gorge of the Yangtze River are hot tourism routine lines of the Three Gorges, and the Three Gorges watercourse serve as a very important logistics watercourse, through which many cruises and cargo ships navigate every day. As a result of the landslide-induced impulse waves, temporary closure of navigation were performed on the reach of Yangtze River in Wushan County and Daning River, leading to an indirect economic loss of RMB 70 million (11.3 million U.S. dollars) or more.

The run-ups of the landslide-generated impulse waves were surveyed on June 25 (Fig. 7). The maximum run-up of 6.2 m was observed in the gully between Yushan Dock and Tourist Dock. The run-up around Maple Hotel was 4.7–5.4 m, and the run-up nearby the Maritime Dock was 2.9 m. The length of waterway affected by waves with above 1 m was approximately 4 km long. As such most of the affected section lied right in the coastal regions of Wushan County where many boats were docked and residents gathering; thus, this tsunami tragedy came.
Fig. 7

Run-ups in the coastal regions of Wushan County (remote sensing data from MAP WORLD)

Then how did Hongyanzi landslide happened? Figure 8 shows the engineering geological section of Hongyanzi landslide plotted on the basis of 2012 survey profile and 2015 emergency investigation results. By analyzing the lithology and thickness, it was discover that the soil was very thinly deposited in the elevation of 130–175 m a.s.l., which was also the toe of the slope. For a bedding slope, the debris deposited at the toe forms an important anti-sliding section; the higher the strength, and the longer and thicker the such anti-sliding section, the more stable the landslide will be (Zhang et al. 2014; TerZaghi 1950). However, the toe of Hongyanzi slope lies right in the hydro-fluctuation belt of the reservoir. When the water level fluctuates, the toe is eroded and the soil carried away by water. Meanwhile, the strength of soil was reduced by repeated immersing and air-drying (Wang et al. 1996; Wang et al. 1997). The structural and strength deterioration of the hydro-fluctuation belt will exert an adverse influence on the stability of Hongyanzi slope, reducing the slope stability gradually.
Fig. 8

Engineering geological section 1-1’ of Hongyanzi Landslide; the boreholes were drilled in 2012, the locations of which are shown in Fig. 3. The sliding plane and underwater terrain line after the landslide were both drawn by inference

On June 16–17, 2015, most areas of Wushan County were hit by rainstorms with daily precipitations of 56.9 and 44.6 mm, respectively (Fig. 9). Rainfall tended to produce seepage pressure, which is adverse to the stability of colluvial landslides (Liu et al. 2013; TerZaghi 1950). Wuxi County at upstream Daning River had been experiencing heavy rainfall since June 16. As a consequence, Daning River experienced a sharp increase in the rate of flow, elevating the water level to 1.1 m above the warning level on June 23–24 in Wuxi Course. Hongyanzi slope lies on the concave bank downstream of Longmen gorge, making it constantly exposed to flow impact. Due to erosion of loose debris at the slope toe by flood at a high rate of flow, the bank tended to collapse (Wang et al. 1996; Wang et al. 1997).
Fig. 9

Precipitation histogram in Wushan County in June, 2015

Meanwhile, with the arrival of flood season, the water level is drawn down to reserve the capacity for flood control in the Three Gorges Reservoir. The water level had been declining constantly since June 8 (Fig. 10). The water level declined at an average rate of 0.7 m/day during June 8–24. The drop of water level in the reservoir resulted in the lagging fall of underground water level within the colluvium slope, thus forming excess pore water pressure detrimental to the slope stability (Duncan et al. 1990).
Fig. 10

The water level histogram of the Three Gorges Reservoir in June, 2015

Therefore, rainfall, watercourse flooding, and reservoir drawdown may be the triggers contributing to the small-scale bank failures on the foot of Hongyanzi slope during June 21–23. The bank failure zone on the foot was an important anti-sliding section of Hongyanzi bedding slope. As such anti-sliding section disappeared, the slope deformed rapidly and slid along the boundary of the intensely weathered zone. The sliding mass slid into relatively shallow water (20 m deep) at a high rate with large volume, so large-scale impulse waves were generated.

Analysis on the process of impulse waves generated by Hongyanzi landslide

A wide range of research methods can be used to calculate landslide-generated wave data, which did have been applied properly with good results (Ataie-Ashtiani and Yavari-Ramshe 2011; Fritz et al. 2004; Pastor et al. 2009; Serrano-Pacheco et al. 2009; Zhao et al. 2015; Yin et al. 2015). However, the numerical simulation method of water wave model has many advantages when responding to the emergency, such as rapid computing speed, long calculating region, and accurate results. In this study, Open source code GEOWAVE is used to simulate the generation process of Hongyanzi landslide-generated impulse waves. The GEOWAVE software consists of the tsunami open and progressive initial conditions system (TOPICS) and nonlinear Boussinesq water wave model FUNWAVE. TOPIC is designed to produce the wave source field based on physical experimental formula, while FUNWAVE is adopted to calculate the subsequent propagation and run-up of the waves. GEOWAVE is capable of effectively simulating a large number of impulse wave disasters of different types (see Watts et al. 2003; Walder et al. 2003; Tappin et al. 2008; Applied Fluids Engineering Inc. and Center for Applied Coastal Research 2008 for details).

The purpose of this study were as follows: Hongyanzi landslide-generated impulse waves on June 24th were reproduced to examine the generation process of impulse waves; then, the potential impulse waves induced by the deformation mass of the Zone C are analyzed to help the local government make reasonable decisions on disposal of the emergency.

Supposing the slope slid at a constant acceleration (a1) and deceleration (a2), then the slide mass reached the maximum velocity (Vmax, impact velocity) after the duration of t1 from the velocity of zero, the slide mass stop after duration of t2 from the velocity of the maximum. According to the Newton’s Law, the following Eqs. 1 and 2 can be obtained.
$$ {a}_1{t}_1={a}_2{t}_2={V}_{\max } $$
(1)
$$ S=\frac{1}{2}{a}_1{t_1}^2+\frac{1}{2}{a}_2{t_2}^2=\frac{1}{2}\left({a}_1{t}_1\right){t}_1+\frac{1}{2}\left({a}_2{t}_2\right){t}_2=\frac{1}{2}{V}_{\max }{t}_1+\frac{1}{2}{V}_{\max }{t}_2=\frac{1}{2}{V}_{\max }t $$
(2)

Where, S is the total traveling distance, t1 is the duration of acceleration movement, t2 is the duration of deceleration movement, and the sum of t1 and t2 is the duration of the sliding movement.

According to the above description, given that the sliding distance of the road concrete debris was 150 m and the slip duration was 2 min, the maximum sliding speed was calculated to about 2.5 m/s with Eq. 2. As the toe of the slope is originally a very gentle flood plain of Daning River, the rock-soil mass would mostly rest here after sliding into the water. If these parameters are performed based on the equal-volume method and Newton’s Law, the centroid of the stopping mass should be at elevation of 135 m a.s.l. and the water depth of the centroid of the stopping mass is about 10 m, then the sliding distance was 178 m, the maximum kinematic velocity of the landslide is estimated to be 2.97 m/s; as the underwater sliding distance is 45 m, the underwater slip duration is estimated to be 36 s. The maximum kinematic velocity calculated using the mark point and that calculated based on the equal-volume method are nearly identical. The maximum kinematic velocity of 2.5 m/s is chosen for this study for existing visible evidence.

Table 1 summarizes the input parameters required for calculating Hongyanzi landslide-generated impulse waves using TOPICS, including the volume and kinematic velocity of the sliding mass. In light of the location of Hongyanzi landslide relative to Daning River, subaerial landslide-generated wave source is used as a wave source model, which is one of the initial wave sources set by TOPICS. This impulse wave model has been applied in calculation and analysis of impulse waves generated by landslides with proven results in the Three Gorges Reservoirs (Huang et al. 2012; Wang et al. 2015b). When the parameters are input into TOPICS, a series of parameters of the initial wave field will be figured out, thus forming a wave source field.
Table 1

Hongyanzi tsunami source parameters

Input

Output

Submerged landslide volume (m3)

230,000

Landslide deceleration (m/s2)

−0.07

Landslide width at shore (m)

130

Wave length (m)

96.3

Impact velocity (m/s)

2.5

Impact Froude number

0.25

Typical final depth (m)

10

Peak amplitude (m)

6.14

Run-out length under water (m)

45

Trough u velocity (m/s)

−2.42

Run-out time under water (s)

36

Peak v velocity (m/s)

2.03

The computational domain of wave propagation and run-up using FUNWAVE is almost identical with that region in Fig. 7. With a length of 5.4 km and width of 5.4 km, this computational domain is divided into 270 rows and 270 columns by a 20 m × 20 m grid. The river in the computational domain stretches in a “T” shape, and the landslide is located around the confluence reaches. FUNWAVE module calculates 8000 time steps in total, 0.20 s for each time step. Thus, by simulating the impulse waves for propagation duration of 1600 s, various data of wave propagation and run-up in the watercourse are obtained.

According to the simulation, the maximum height of waves in the watercourse generated by the landslide is 6.3 m observed near the point where the slope slides into the water (Fig. 11). This maximum wave height is close to 5–6 m as recalled by the witnesses. The maximum run-up of the impulse waves is 6.0 m observed near Yushan Dock. The maximum run-up remains above 1 m all the way from Longtan gully to the coastal regions in Xiping Village; the run-up in the region from Yushan Dock to Maritime Dock is extremely high, reaching 2.5 m or above. This region is the right place where boats were overturned, and persons were killed or injured during the landslide incident on June 24th. The run-up of waves along the shoreline of Hongyanzi slope is lower than that of waves on the opposite bank, and large run-ups in the landslide bank side are only observed in some places. By comparing the investigating run-up shown in Fig. 7 and the calculated ones (Fig. 12), we find the correlation coefficient (R2) of the two value sets is 0.92 and the difference varies from 1.8 to 25 %.
Fig. 11

Distribution map of maximum wave height calculated in watercourse and in coastal regions

Fig. 12

Comparison of survey value and calculated results, the deep color columns are investigating values, the light color columns are calculations

As seen from the distribution of maximum height of waves in the watercourse, the landslide incident on June 24th primarily affected Daning River reach between Yangtze River bayou and Longmen Gorge with a distance of 4 km and had little impact on the Yangtze River channel. The waves in Nanling Village on the south shore of Yangtze River ran up to 1 m in only some places, thus doing little harm to the village. This evaluation result accords well with the observed landslide-induced impulse wave incident, and the calculated run-ups are in good agreement with the survey value. It suggests that these numerical analysis results are valid, and the numerical model reasonably and accurately mirror the actual process of the impulse waves generated by Hongyanzi landslide.

By analyzing the transient propagation process of the waves, we find that the highest wave reached the slope of revetment of Maple Hotel at 96 s after the landslide failed (Fig. 13a). One hundred sixteen seconds later, the waves reached the gully near Yushan Dock on the opposite bank, with a maximum run-up of 6.0 m (Fig. 13b). One hundred sixty seconds later, the waves arrived at Maritime Dock, with a maximum run-up of 2.8 m (Fig. 13c). The waves propagated out of estuary within 60 s after the landslide failed. The waves were still undulating in Daning River at 27 min after the incident (Fig. 13d). The waves propagated at an average velocity of 9.8 m/s in Daning River. With such a high velocity, the impulse waves left very short time for the coastal residents to evacuate safely after the landslide failed.
Fig. 13

View of instantaneous wave propagation in waterway

The propagation process of impulse waves generated by Hongyanzi landslide was greatly influenced by the “T” shaped valley geomorphic. First of all, the waves propagating in a circle directly hit the shoreline from Yushan Dock to Maple Hotel Dock. The slope at Maple Hotel is a protruded artificial platform, protruding 300 m from the Yushan Dock shoreline. This protruded slope impeded partly the propagation of waves downstream. In the propagation direction of the waves at upstream Daning River valley, a 100-m-wide riverway in Longmen Gorge was the only place reserved for the waves entering into. Affected by such geomorphic features, Longmen Gorge in Daning River is slightly affected by this tsunami, while the waves undulated for long time in the waterway from Maple Hotel to Longtan gully (Fig. 14a). When some waves rounded through the protruded platform at Maple Hotel, they moved forward rapidly in the form of alongshore currents along the straight shoreline from Maritime Dock to Xiaping Village downstream where a straight reservoir bank is built manually, the alongshore currents also last long (Fig. 14b). In the waterway, the amplitude of the waves dropped significantly as the waterway is wider largely in the propagating direction after flowing into the Yangtze River. After reaching Yangtze River, the waves propagated upstream and downstream respectively from Daning River, with low amplitude and little effect.
Fig. 14

Run-up process of impulse waves at A and B points. A is located on the bank of Longtan gully, and B is located on the bank of Maritime Dock, which can be found in Fig. 11. For they both are located on the banks, so in the tsunami events, the run-up lines are inconsecutive

This T-shaped valley is similar to the Lituya T-shaped Bay, Alaska, U.S. Lituya Bay is a T-shaped inlet that cuts through the coastal low land and foothills belt flanking the Fairweather Range of the St. Elias Mountains, on the south coast of Alaska. Lituya mega tsunami happened in 1958, July, 9, generated by about 40 million cubic yards of rock-soil mass. The largest run-up was about 524 m, and the main influencing range was the Lituya bay in this tsunami (Miller 1960). So as the similar T-shaped valley worked well in the incident of Hongyanzi landslide-generated impulse wave. At the intersection of “T” rivers, the impulse wave decays rapidly, this landform favors the decay of the wave.

Potential tsunami analysis on the adjacent deformation mass

Hongyanzi landslide exerted a tractive force on the soil mass in zones C and D (Fig. 3), resulting in the deformation of the two zones. According to the monitoring data, zone D had no macroscopic deformation and it collapsed toward the landslide trough (zone B). If zone D collapsed, the rock-soil mass would fall into the zone B and then into the water. Thanks to a platform erected by some residuals of Hongyanzi landslide in the lower section of zone B, the rock-soil mass produced following failure of the zone D will slid into the water at a low velocity and with small volume; that is to say, the impulse waves generated were rather limited. On the contrary, zone C has a volume of 7 × 104 m3, where horizontal and longitudinal cracks were highly developed. Similar with Hongyanzi landslide, the failure of zone C is also likely to generate dangerous impulse waves. Emergency closure of navigation was performed in Daning River waterway and Yangtze River channel after the Hongyanzi landslide. The Yangtze River channel was re-opened for navigation soon as the local government realized that it was less influenced in the impulse wave incident of June 24th, and the potential tsunami should not exceed the impulse wave generated by the Hongyanzi landslide. Therefore, the key of whether Daning River waterway could be re-opened for navigation lied in the potential hazards associated with the soil mass of the zone C, which was also the key problem to be dealt with in emergency response to Hongyanzi landslide. On this regard, this study made the analysis on the worst-case scenario of impulse waves generated by zone C based on the same numerical model of Hongyanzi landslide.

To estimate the worse impulse waves, the kinematic parameters used for Hongyanzi landslide are also used in calculating the waves generated by zone C though the elevation of trailing edge of zone C is lower than that of Hongyanzi landslide and the underwater sliding distance should be shorter than that of Hongyanzi landslide. The input parameters and output results of TOPICS module are shown in Table 2. The potential impulse waves generated by zone C are estimated with GEOWAVE, which calculates 8000 time steps and simulates 1600 s using the same computational grid applied for Hongyanzi landslide.
Table 2

Tsunami source parameters table of zone C

Input

Output

Submerged landslide volume (m3)

70,000

Landslide deceleration (m/s2)

−0.07

Landslide width at shore (m)

35

Wave length (m)

96.3

Impact velocity (m/s)

2.5

Impact Froude number

0.25

Typical final depth (m)

10

Peak amplitude (m)

1.87

Run-out length under water (m)

40

Trough u velocity (m/s)

−2.42

Run-out time under water (s)

36

Trough v velocity (m/s)

2.03

Figure 15 demonstrates the distribution of maximum wave height and run-up obtained by numerical simulation. As seen from the figure, the zone C is likely to generate impulse waves of height up to 2.2 m and run-up to 2.0 m. Waves with 2∼3 m height cover two sites including the disturbed zone where the slope slides into water and the gully at Tourist Dock. Waves with 1∼2 m height covers six sites including the protruded platform at Maple Hotel, three gullies from Tourist Dock to Yushan Dock, Longtan gully ridge and Caiziba Trough at upstream Hongyanzi slope. Thus, the waterway is dominated by waves with 1 m less, and only a very few isolated places fall within the wave zones with above 1 m height. So the worst-case scenario of impulse waves generated by the 7 × 104 m3 mass is that the waves would probably influence some isolated shoreline from Maritime Dock in the county town to Longtan gully but in a limited way; and Daning River waterway would be slightly affected.
Fig. 15

Prediction on potential hazards of impulse waves generated by deformation mass adjacent to Hongyanzi landslide

Seen from the calculated process of impulse waves, the biggest waves at impact coast along the river way are the first or the second wave. Especially, the first wave has the biggest impact on the coast of the county. Seen from the predicted impacting time, the waves in the river way will mostly be lower than 1 m after about 5 min. Thalweg of Daning River is near the west of the river way. Seen from the wave height along C-C’ section of the thalweg, no wave is over 1 m along the thalweg (Fig. 16). This height is similar to that of waves produced by a normal big ship passing by. So it seems that big ships can pass by along the thalweg.
Fig. 16

The maximum wave height of C-C’ section of the thalweg, the location of C and C’ can be found in Fig. 15

However, when a landslide is in a serious deformation phase, a red pre-warning of landslide was set, and then, shipping will being forbidden in the reservoir as this is deemed to threaten severely the safety of voyages until the red pre-warning of the landslide is canceled. It ignores the calculation and analysis of impulse waves.

Discussion on risk management of navigation based on landslide-generated impulse waves

The waterway along Yangtze River has been closed for several times upon warning or occurrence of landslide since the water level reached 175 m in 2008. Nowadays, this inland waterway restriction is made based on the pre-warning of landslides. For instance, since October 2010, when Qingshi landslide was in serious deformation, which is located at the left bank of Shennu Stream, a branch of Yangtze River, the pre-warning grade of Qingshi landslide was set to red, and then, it turned to the warning of navigation, which led to the closure of the whole Shennu Stream branch for more than 1 year (Wang et al. 2015a). If this method is used to make a decision on the navigation for the zone C, the waterway will be closed until the pre-warning grade of zone C turns to yellow or blue. However, the method has at least three disadvantages: (1) it is assumed that all coastal landslides will generate disastrous impulse waves, which, in fact, do not always happen even after failure. (2) If landslides bring impulse waves, the risk range of such waves is relatively limited, and control is not necessary along the entire waterway. For example, on November 23, 2008, when a collapse happened in Gongjiafang, the highest wave produced in the river way was about 31.8 m, and the range was about 8 km where the waves went higher than 1 m. (3) If waterway control is initiated, closure is not the only option. It is especially important when waves are about 1 m. As the anti-wave capacity of some ships is over 1 m, when the waves are about 1 m, they can pass through carefully.

In this potential impulse wave disposal, the scale of potential waves is used to judge the risk of the waterway. Using the potential scale of impulse waves to judge whether the waterway is safe or not is the general rule of ocean shipping management, and it may be introduced to use in the risk management of inland waterway. It is mainly the tsunami induced by landslide that influences the safety of remote waterways, instead of the landslide itself. Therefore, the risk management of waterways based on impulse waves should be brought in to avoid equaling the warning grade of landslide point to the warning grade of big area of river way, which will cause the traffic pressure and economic loss. According to the classification of water course hazard alert levels specified in the Emergency Plan for Storm Surge, Sea Wave, Tsunami, and Sea Ice Disasters published by the State Oceanic Administration of P. R. C., the red alert zone is delineated for wave height of above 3 m, orange alert zone for wave height of 2–3 m, yellow alert zone for wave height of 1–2 m and blue alert zone for wave height of below 1 m. Warning grade of inland waterways may also be made like this grade.

Taking Hongyanzi landslide as an example, according to the division, the risk range of impulse waves of Hongyanzi landslide covers roughly the three oval areas in Fig. 17. The black oval area is the risk area of the sailing route while the two blue ovals are risk areas of boat docking. Red warnings are in the three ovals, which indicate that shipping or docking is very dangerous. The risk duration of this wave is about half an hour after the landslide. The potential wave risk range of Zone C is in the two oval areas in Fig. 18. The black oval is the risk area of the sailing route and the blue oval is the risk area of boat docking. The risk area of the sailing route mainly shows yellow warnings and orange warnings, which indicate that passing through is sub-dangerous and dangerous, respectively. In the risk area of boat docking, there are mainly yellow warnings, which means that docking in this area is sub-dangerous. Some ships should be allowed to go between the risk area of the sailing route and the risk area of boat docking through specific passages.
Fig. 17

Tsunami risk map based on the impulse wave generated by Hongyanzi landslide in June 24, 2015. Blue color region can be set to blue pre-warning, and yellow color region can be set to yellow pre-warning, so as orange region and red region

Fig. 18

Potential tsunami risk map based on the impulse wave generated by zone C landslide. Blue color region can be set to blue pre-warning, and yellow color region can be set to yellow pre-warning

According to the above analysis, only a part of Daning River needs to be restricted, but not the whole waterway. Based on this technique, the local government has taken the following measures: (1) To restrict navigation in Daning River reach before emergency engineering project started in zone C, only large ships to sail through the west channel were allowed and small boats were prohibited to sail. No warning was triggered for Yangtze River channel. (2) To share the emergency monitoring data of zone C with competent maritime authorities, monitoring data were used to judge whether the navigation of large ships may be restricted to ensure the safety of sailing ships. (3) Emergency response works in the deformation mass should carry out as soon as possible. With these measures, the traffic pressure along the river will be substantially reduced, especially for the route into Daning River or the Small Three Gorges, and economic losses will also decrease.

Conclusions and suggestions

As revealed by the emergency investigation, Hongyanzi landslide was a bedding landslide with a volume of 23 × 104 m3. Rainfall, watercourse flooding, and reservoir drawdown may be the triggers contributing to the small-scale bank failures detected on the foot of Hongyanzi slope during June 21–23. The collapses of the foot make the soil mass lose anti-sliding force, making Hongyanzi soil mass slide as a whole along the boundary of the intensely weathered zone on June 24.

The numerical simulation results indicate that Hongyanzi landslide-generated 6.3-m-high impulse waves in the watercourse; the maximum run-up of 6.0 m was observed near Yushan Dock in the county seat; and the course of waterway affected by waves with a height of 1 m above is approximately 4 km long. The propagation process of impulse waves was greatly influenced by the watercourse geomorphy; thanks to the narrows, Longmen Gorge in Daning River was slightly affected by the impulse waves, while the waves was undulated for long time in the waterway from Maple Hotel to Longtan gully. The waves moved forward rapidly in the form of alongshore currents along the straight shoreline from Maritime Dock to Xiaping Village downstream. All the above findings accord well with the observed conditions, and the survey data are in good agreement with the calculated results.

Additionally, the worst-case scenario of potential impulse waves to be generated by the 7 × 104 m3 deformation mass aside Hongyanzi landslide is predicted based on the same numerical model. The analysis results show that the maximum height and run-up of the potential impulse waves were 2.2 and 2.0 m, respectively, and only a few isolated sites in the waterway fell within the orange and yellow alert zones. These analysis results provide a technical basis for the navigation restriction management in Daning River; it pushes the safety risk management of the navigation based on the impulse wave generated by landslide.

Hongyanzi landslide was another incident of landslide-generated impulse waves in the Three Gorges Reservoir following Gongjiafang landslide in 2008. It reminds that hazards associated with landslide-generated impulse waves should be considered seriously in the geohazard early warning in the reservoir and river once more. Meanwhile, it is of great necessity to perform special surveys on the deformed slopes in the reservoir bank which is prone to generate impulse wave hazards, and the risk grading of waterway safety based on impulse waves should be introduced. It is advised that it should put more efforts in performing surveys and studies on the potential hazards associated with landslide-generated impulse waves in the Three Gorges Reservoir and other reservoirs around the world, so as to avert tragedy.

Notes

Acknowledgments

This work was supported by the National Natural Science Foundation of China (ID 41372321) and the China Geological Survey (ID 1212011014027). The authors would like to thank Ma Fei from the Office of Three Gorges Geohazard Management of Chongqi City and Huo Zhitao from the Command of Geohazard Prevention of Three Gorges Reservoir; they are of great help in the emergency investigation and analysis. Finally, we want to thank Dr. Philip Watts who helped us register for GEOWAVE and helped us much in using GEOWAVE.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2016

Authors and Affiliations

  1. 1.Wuhan Centre of China Geological SurveyWuhanChina
  2. 2.China Institute for Geo-Environment MonitoringBeijingChina
  3. 3.Chongqing Institute of geological disaster prevention and controlChongqingChina

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