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Torrent rainfall-induced large-scale karst limestone slope collapse at Khanh waterfall, Hoa Binh Province, Vietnam

Torrent rainfall-induced large-scale karst limestone slope collapse at Khanh waterfall, Hoa Binh... In recent years, many landslides have occurred in Vietnam, particularly in the Northern mountainous region during the rainy season from May to October. On the morning of October 12, 2017, the Khanh waterfall landslide in Khanh Village, Hoa Binh Province, Northern Vietnam occurred. The landslide killed eighteen people and destroyed five houses. Topographical and geological surveys were conducted around the area to determine its causes. The rainfall data and flow discharge were also analyzed. The results showed that this collapse was different from some previous ones collapsed due to the erosion at the toe of the slope. Khanh waterfall landslide occurred due to the increasing amount of water in cracks and caves in the limestone layer in the slope. The collapse process was simulated based on Coulomb mixture theory. The numerical simulation results show similarities with the actual collapse process. The results provide indicators for assessing the risk of such limestone waterfall landslides in the future. Keywords: Khanh waterfall landslide, Torrent rain, Limestone, Cave, Numerical simulation Introduction in Northern Vietnam (Fig.  1). In this study, we refer to Vietnam has a humid monsoon climate with a high aver- this landslide as the local name, Khanh waterfall land- age annual rainfall. Damaging landslides occur every year slide. This used to be a tourist destination with a beauti - during the rainy season. Studies on rainfall-triggered ful scenery of the waterfall. landslides in Vietnam such as in Binh Dinh Province The landslide caused a great damage killing 18 peo - (Duc 2013), Lao Cai Province (Bui et  al. 2017), Quang ple and destroying 5 households on the opposite hill of Ninh Province (Loi et  al. 2017; Nguyen et  al. 2020), and the Khanh waterfall. Before this disaster, Khanh Village Danang City (Quang et  al. 2018) have identified the tor - residents witnessed minor size slope collapses earlier in rential rain as the primary cause of landslides in Viet the rainy season in September. However, no significant Nam. Hoa Binh is the northern mountainous province landslides have previously been recorded at the Khanh most affected by landslides during the rainy season (Bui waterfall. As a result, landslide risk early warning and et  al. 2013). On October 12, 2017, around 01:00 local evacuations were not contemplated before the disaster. time, a landslide occurred at Khanh waterfall, Phu Cuong The sudden landslide moved a large amount of rock and Commune, Tan Lac District, Hoa Binh Province, Viet- soil downhill, across the river, and up the opposite hill for nam. The Khanh waterfall is located in a limestone area a total moving distance of about 400 m. There are many tourist destinations that feature beau - tiful and famous waterfalls in the world. Another well- *Correspondence: goto@yamanashi.ac.jp known waterfall in the limestone belts in Vietnam is Ban Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, Gioc waterfall located in Cao Bang province, Northern University of Yamanashi, Yamanashi, Japan Vietnam. In other countries, there are many scenic spots Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Do et al. Geoenvironmental Disasters (2022) 9:4 Page 2 of 20 Fig. 1 Location map of Khanh waterfall landslide in Hoa Binh province, Vietnam in karst terrain such as Tamul Falls in Mexico and Plitvice craton and Indochina craton during the late Paleozoic to Lakes Falls in Croatia. However, until now little is known early Mesozoic period. The limestones are distributed about landslides that cause a great loss of life at lime- along the northwest-southeast tectonic line (Da River stone waterfalls known as scenic spots. Hence, it is very fault) (Fig.  1). In recent years, many landslides due to important to understand the mechanism of the landslide heavy rainfall have occurred in the margin of this lime- occurred at the Khanh waterfall. stone zone (Tung et  al. 2016). The main geology of the Several previous studies on the collapse of waterfalls study area consists of shale and limestone from the Trias- focused on the transition process of rivers (Scheingross sic of Mesozoic (Fig. 2). and Lamb 2017). However, there were only a few studies Khanh waterfall is located on the right bank of the on the collapse of waterfalls as disasters such as Kegon Kem river that flows under the cliff. Behind it, steep ter - waterfall in Nikko, Japan (Hayakawa 2013), Niagara rain with unevenness peculiar to the limestone area at an waterfall between America and Canada (Hayakawa and altitude of 350  m to 1000  m spreads from northwest to Matsukura 2010). This study is focused on the clarifying southeast. On the other hand, the left bank of the River of the mechanism of the Khanh waterfall landslide. Such Cam, where shale is mainly distributed, is a relatively flat factors as topographical and geological background, rain- terrain with an altitude of 200  m to 250  m, and villages fall and groundwater are analyzed. The water quality and are formed along the national highway. the flow simulation of the moving soil and water masses Shale is distributed on the left bank of the Kem river, were also analyzed. Finally, as the expansion of cave in and limestone with shale on the right bank. The stratig - limestone areas in Vietnam is still developing (Khang raphy is N40–50°W, the dip is 70°–80°W. There is a small 1985), assessing the possibility of recurrence of similar fault on the slope of the Khanh waterfall that dips about landslides would be very meaningful. 45°E as shown in Fig. 18 later. The Kem river itself, which flows in the direction of the strike of stratum, may also Study area correspond to a fault. General geology Geotechnical investigation including the strength and The study area is located at the Song Ma suture zone permeability of each layer was not conducted in detail in (Hau et al. 2018) due to the collision of the South China this study. However, preliminary testing of the strength D o et al. Geoenvironmental Disasters (2022) 9:4 Page 3 of 20 Fig. 2 Geological map and schematic geological columns around Khanh waterfall (Modified from Department of Geology and Minerals of Vietnam 2004, 2005) of the rock with a hammer was carried out. Both shale age (Konecny et  al. 2017), the unconfined compressive and limestone have a stratified structure. These rocks are strength of the limestone layer is 69–204 MPa. soft with weathering at the surface of the outcrop. But The river is littered with limestone boulders as land - the fresh part is hard that it cannot be easily broken by slide moving masses as well as stalactite rubble rich in hammering. Based on the study on limestone at a similar cavities. They were probably formed by groundwater Do et al. Geoenvironmental Disasters (2022) 9:4 Page 4 of 20 in cracks and cavities in the limestone. It is considered branches of the fall named Khanh waterfall 1 and Khanh that the permeability of such shale and limestone itself waterfall 2 (Fig. 4). is low. However, the slope of these strata forming a lay- When the landslide occurred, only Khanh waterfall 1 ered structure is almost vertical from 70° to 80°. Moreo- collapsed. Khanh waterfall 2 did not collapse. Accord- ver, there were many cracks and cavities in the limestone ing to the survey of the upper stream network of Khanh layer. Hence, surface water easily penetrated into cracks waterfall, waterfall 1 and waterfall 2 have the same and cavities in the limestone layer. water source. However, the main stream flows towards waterfall 1, the secondary stream flows towards water - Features of the slope failure fall 2. Therefore, when the heavy rain occurred, more The Khanh waterfall landslide occurred around 01:00 water was directed to the waterfall 1 area due to its am local time on October 12, 2017 after 48 h of a heavy geological characteristics that resulted in the slope col- rainfall of 394.8 mm on October 10 and 11, 2017 (Fig. 3). lapse. The original dimensions of the Khanh waterfall The rainfall data was collected at Mai Chau district rain 1 were a height of approximately 120  m, a maximum gauge station, which is located 12 km from Khanh water- thickness of 50  m, and a width of 200  m. A part of the fall (Vietnam Meteorological and Hydrological Admin- landslide mass deposited at the Kem River forming a istration 2019). At the Khanh waterfall, there were two natural dam. Other parts of the mass crossed the Kem Fig. 3 Daily rainfall in October 2017 when the Khanh waterfall landslide occurred ( Vietnam Meteorological and Hydrological Administration 2019) Fig. 4 The overview of Khanh waterfall landslide (Photo taken by Tuoi Tre Media 2017) D o et al. Geoenvironmental Disasters (2022) 9:4 Page 5 of 20 River, passed over the top of the 30  m high hill on the Figure 6 shows photos of the Khanh waterfall landslide opposite riverbank, and reached the paddy field. The taken at different times before and after the collapse of deposit part at opposite hill killed 18 people living the waterfall. It shows the increase in the water around there (Fig. 5). the waterfall during a heavy rain in 2017. The slope in Fig. 5 Photos of Khanh waterfall along the line A-A’ of the landslide moving direction: a top view, b side view Do et al. Geoenvironmental Disasters (2022) 9:4 Page 6 of 20 considered as the factors contributing to the landslide. Moreover, a numerical model was created to simulate the landslide process. The affected area from the simulation results was compared with the one determined from the actual image. Field study Field surveys were conducted to understand the geol- ogy, topography and hydrogeology around Khanh water- fall landslide (Fig.  7). The range of movement and the amount of moving mass were estimated based on the actual geological evidence, information provided by the local residents, and the images taken by an Unmanned Aerial Vehicle (UAV). Figure  8 shows the topographic classification map and the distribution map of the landslides in the Khanh waterfall area. The soil layers between the collapsed and the non-collapsed areas were investigated to determine the position of the collapsed surface. On the right bank of Kem river 200 m downstream from Khanh waterfall land- slide, there were two areas P1 and P2 which were parallel concave terrains similar to the traces of past landslides. There was no movement at these points when the land - slide occurred at Khanh waterfall. Water quality analysis and geology investigation at P1 and P2 areas were con- ducted to compare these condition with Khanh waterfall. One water sample No. 012 (surface water) was col- Fig. 6 Front view photos of Khanh waterfall: a in dry season before lected from Khanh waterfall 1. Two water samples No. failure, b in rainy season 2017 reportedly days before the landslide 010 (surface water), 011 (spring water) were collected occurred, c after the landslide occurred in 2017 (Photo taken by Tuoi from Khanh waterfall 2. Two samples No. 005 and 008 Tre Media 2017; Vietnamnet 2017) (spring water) were collected from areas P1 and P2. In order to investigate the water quality characteristics, samples of spring water and swamp water around the the dry season before 2017 was introduced as a beauti- landslide area were collected to analyze in the laboratory. ful waterfall destination for tourism (Fig.  6a). Figure  6b Such parameters as the water temperature (T), pH, and shows the situation of the slope in the rainy season of electrical conductivity (EC) were measured in the field. 2017, days before the landslide occurred. A large amount of brown water flowing from the entire slope included the waterfall as well as water ejected from the middle of Rainfall and flow discharge calculation the slope. Figure  6c shows the landscape after the slope Figure  9 shows the position of Khanh waterfall landslide area around the waterfall mostly collapsed. The moving in 2017 and areas in close proximity where landslides mass containing boulders reached the opposite hill on occurred previously (Tan et  al. 2015) plotted on Google the left bank of the river. Earth, and a rainfall data graph from 1961 to 2018. In Fig. 9a, the location of Khanh waterfall, Mai Chau rainfall Methodology station and the previous nearby landslides areas in Pu Bin To study the mechanism of the Khanh waterfall land- commune and ung Th Khe commune, Mai Chau district, slide, field surveys in the area surrounding it were con - Hoa Binh province are shown. There were two landslides ducted. Rainfall data around this area was also collected in 1996 and 2007 in Pu Bin commune and one landslide for the study. To evaluate the drainage capacity upstream in Thung Khe commune in 2005. The exact locations, of the waterfall, a measurement of the upstream network sizes and types of these landslides were not specified. channel was conducted. The surface water and spring However, the time when the landslides occurred has water were collected to analyze the water source rela- been reported. Figure 9b shows the maximum 24 h, 48 h tionship between the collapse area and the neighboring rainfall data at Mai Chau rainfall station from 1961 to area. Topography, geology, rainfall, drainage capacity are 2018 with the time when Khanh waterfall landslide and D o et al. Geoenvironmental Disasters (2022) 9:4 Page 7 of 20 Fig. 7 Field survey area including the location of water sampling points around Khanh waterfall (KW ) landslide. Sample 012 (surface water) was collected from KW1. Samples 010 (surface water), 011 (spring water) were collected from KW2. Samples No. 005 and 008 (spring water) were collected from P1 and P2 areas Fig. 8 Topographic classification and landslide distribution map around Khanh waterfall previous nearby landslides occurred. It shows that the 2005. The data of the previous landslides and the current 48  h rainfall higher than 300  mm caused landslides that one at Khanh waterfall in the context of the rainfall data occurred in Pu Bin commune in 1996, 2007 and Khanh for 58 years indicates that torrent rainfall in 24 h and 48 h waterfall landslide in 2017. The 24 h rainfall higher than is an important factor causing landslides around Khanh 200  mm caused a landslide in Thung Khe commune in Do et al. Geoenvironmental Disasters (2022) 9:4 Page 8 of 20 Fig. 9 a Location of Khanh waterfall (2017) and previous nearby landslides in Pu Bin commune and Thung Khe commune, Mai Chau district, Hoa Binh province ( Tan et al. 2015); b data of maximum 24 h and 48 h rainfalls from 1961 to 2018 ( Vietnam Meteorological and Hydrological Administration 2019) waterfall. Rainfall in 48 h greater than 300 mm should be area was 21.17 km . The water was gathered behind the considered a warning landslides around this area. Khanh waterfall and flowing down to the Khanh waterfall In 2017, when the landslide occurred, there were large (Fig. 12). typhoons in October in Vietnam. The monthly rainfall The flood discharge that flowed down to the Khanh in October of 448.8 mm was 2.6 times the usual amount waterfall during heavy rainfall when the landslide of 170.4 mm (Fig. 10). On October 10 and 11, 2017, 48 h occurred was calculated using the following Rational before the landslide occurred, the rainfall was recorded at Method (Hicks et  al. 2009). This is a common method 394.8 mm. A maximum of 44.1 mm/h was recorded at 4 used to calculate flow discharge for rivers in mountain - am on October 11 (Fig. 11). This hourly rainfall value was ous areas where the watershed area is less than 100 k m used to calculate the flow discharge in the upstream area and there are no adjustment facilities along the river. of Khanh waterfall. The watershed behind the landslide D o et al. Geoenvironmental Disasters (2022) 9:4 Page 9 of 20 Fig. 10 Comparison of an average monthly precipitation and 2017 monthly precipitation in 1961–2018 ( Vietnam Meteorological and Hydrological Administration 2019) Fig. 11 Hourly rainfall on October 11 and October 12, 2017 when the Khanh waterfall landslide occurred ( Vietnam Meteorological and Hydrological Administration 2019) ′ ′ c l + (W cosθ − U − V sinθ)tanφ Q = .f .r.A (1) (2) FS = 3.6 W sinθ + V cosθ where Q is peak flow (m /s), f is discharge coefficient, r is where FS is factor of safety, c’ is the effective cohesion rainfall intensity (mm/h), and A is watershed (km ). of sliding joint, l is length of the sliding joint, W is the weight of sliding block, U is the uplift water pressure, V is the horizontal water pressure, θ is the angle of the slid- Slope stability analysis and numerical simulation ing surface, φ’ is the effective friction angle of the sliding of collapse process joint. To explain slope instability, the method from Hoek It is assumed that when heavy rain, the surface water and Bray (1974) is used. A simply modified theoreti - flowed into the void in the cracks and cavities in the cal models of plane slope failures in Khanh waterfall limestone layer. This amount of water increased the is drawn in Fig.  13. It is assumed that there are many water pressure that destroys the slope. The specific cal - cracks and caves in the limestone layer in Khanh water- culation of the factor of safety was not considered in fall slope. The water in cracks and caves is from the this study. However, to simulate the slope collapse due upper stream network channel of Khanh waterfall. The to a large amount of water in the limestone layer cor- factor of safety against the sliding block was calculated responding to the peak flow discharge, the simulation as follows: Do et al. Geoenvironmental Disasters (2022) 9:4 Page 10 of 20 Fig. 12 The upper stream network channel and watershed of Khanh waterfall landslide method to predict the behavior of grain-fluid flows Figure  14 shows the relationship between the slope due to slope collapse (Zhang et  al. 2004) is applied in angle of the source area (θ), runout distance (L), and this study. This method assumes that grain-fluid flows runout height (H) (Moriwaki 1987). The coefficient behave as mixtures of interaction of Newtonian fluids of friction is close to the the tangent of the slope angle and Coulomb solids. The equations that describe the (Scheidegger 1973). It is considered that the slope angle mixtures based on Coulomb mixture theory (Denlinger before and after collapse explains the internal friction and Iverson 2001). In order to exactly predict the range angle and basal friction angle. of sediment, the momentum equation was discretized Table 1 shows the physical properties of the flow simu - by the finite difference method with applied a stop lation. It is assumed that the resistance force on the fail- condition of grain-fluid flow, the third-order upwind ure surface during the flow depends only on the friction scheme, and the preserving mass conservation method at the contact surface between the landslide mass and to the numerical model (Zhang et al. 2004). D o et al. Geoenvironmental Disasters (2022) 9:4 Page 11 of 20 the basal rock. The tangent of the ratio between runout height (H) and runout distance (L) after collapse is equal to the basal friction angle. The tangent of the ratio of height of source slope (H’) and the length of source slope (L’) before collapse is equal to the internal friction angle. In the case of Khanh waterfall, the internal friction angle (φ ) and the basal friction angle (φ ) of grain-fluid int bed flows were 30 degrees from tanθ ≒0.55 and 15 degrees from H/L≒0.23 based on the geometric shape of slope before and after collapse (Table 2). Table 3 shows the calculation of the ratio of basal pore fluid pressure. The density of solid was referred from Fig. 13 Slope stability analysis of rock slope (modified from Hoek Konecny et al. (2017). Based on the field survey, the ratio and Bray 1974) of solid and void was determined as 0.6 and 0.4, respec- tively. The ratio of water and air in the void represents the different cases of pore water pressure. Since it was tor - rent rainfall at the time of the slope collapse, the ratio of water and air was determined as 0.4 and 0, respectively. Simulations of cases where the water ratio is less than 0.4 are also calculated. However, the simulation results of these cases differ from the real landslide aftermath. As a result, the ratio of basal pore fluid pressure (λ) was set to 0.5 corresponding to the peak flow discharge. Assuming that the basal pore fluid pressure (p ) was the same as bed the horizontal water pressure of almost the same height with a depth of grain-fluid flow (h). The general soil prop - erty values of solid density (ρ ), fluid water density (ρ ), s f the ratio of solid volume (υ ), and the ratio of fluid vol - ume (υ ) were used for analysis. The simulation model was created from a three-dimen - sional terrain model by SfM of aerial photos taken by the UAV and ALOS Global Digital Surface Model, ALOS World 3D 30  m (AW3D30) (Japan Aerospace Explora- tion Agency Earth Observation Research Center 2020) in Fig.  15. The position of the failure surface is determined based on the position of the slope surface before and after collapse. Figure  15a–c show the contour of slope Fig. 14 Correlation between H/L and slope tangent of source area surface before and after collapse which were determined tanθ (modified from Moriwaki 1987 and added Khanh waterfall from UAV photos and AW3D30. The three dimensional landslide by authors) where H is runout height, L is runout distance, H’ surface of the slope before and after collapse were simu- is height of source slope, L’ is length of source slope lated by GIS in Fig. 15d, e. The cross-section of the slope surface before and after collapse were determined in Fig.  16 from three dimensional surface. The failure sur - Results of analysis face was determined based on knowledge from the field Geology and water quality analysis results survey and the conservation of volume before and after The geology of the area consists mainly of Triassic shale collapse (red line in Fig. 16). and limestone of the Mesozoic. The strike of the geo - The collapse process was simulated in which the lime - logical layer is N40–50°W, and the dip is 70–80°W. The stone block at the upper part of a waterfall and the talus Khanh-waterfall section is a small fault that is inclined deposit at the bottom of the slope moved together. The 45°E. The course of the Kem River along the strike direc - moving mass crossed the river and moved about 400  m tion of the geological layer might correspond to that (Fig. 16). fault. The limestone is mainly distributed on the right riverbank in the steep terrain, while the shale is mainly Do et al. Geoenvironmental Disasters (2022) 9:4 Page 12 of 20 Table 1 Physical properties of the flow simulation Parameter Value Unit Time step Δt 0.005 s Mesh size Δx × Δy 5 × 5 m Internal friction angle before collapse (calculated in Table 2) φ 30 ° int Basal friction angle after collapse φ 15 ° bed (calculated in Table 2) Ratio of basal pore fluid pressure (calculated in Table 3) λ 0.5 – Gravity acceleration g 9.8 m/s distributed on the left bank. The difference between these analyzed by ion chromatography. The alkalinity (corre - terrains is likely due to the difference between the weath - sponding to HCO concentration) was determined by ering resistance characteristics of the distributed geology. acid titration using the Gran method (Gran 1952). Then, Figure  17 shows the side view and photos at different the dissolved silicic acid (silica SiO ) was analyzed by the points along line A–A’. At the top of Khanh waterfall, the molybdenum yellow absorption spectrophotometry. The limestone layer had many cracks and creeping toward result of water quality analysis are listed in Table 4. (Fig. 17A). At the middle of the slope, the sliding surface The water quality analysis results were organized as was found at the fault between the limestone layer and a stiff diagram (Fig.  21). G005, G008 collected from the shale layer (Fig. 17B). Limestone containing stalactite spring water has less silica and more calcium ions than that formed the waterfall collapsed along the boundary G010, G011, G012. It is highly possible that it was cul- with the shale, slipped together with the talus deposit at tivated in limestone. Silica-rich G010, G011, and G012 the bottom of the slope, and moved across the river to are greatly affected by weathering of non-limestone as the opposite hill. Along the line A–A’ along the landslide well as limestone (Fig.  22). These differences are con - moving direction, many limestones were found at the toe sistent with the different collection basins at Khanh of the slope and at the opposite hill (Fig.  17C, D). From waterfall landslide and areas P1 and P2. The surface the field survey at the site, the geological cross-section of and groundwater in areas P1 and P2 and the Khanh Khanh waterfall slope was drawn in Fig. 18. waterfall landslide area were considered to have differ - Downstream of Kem river, the geological cross-section ent water supply channels due to differences in water of P1 and P2 areas were drawn in Fig.  19. Both points quality. P1 and P2 had a concave topography which was divided by the ridgeline into the northeast-southwest direction. Rainfall and flow discharge calculation results They show a terraced topography with steps of various The characteristics of the upper stream network channel heights of 2  m to 5  m at the Kem River direction below of Khanh waterfall are listed in Table 5. From these char- the slope (Fig.  20a). A lot of spring water was observed acteristics, the peak flow discharge and the allowable flow in the whole area (Fig.  20b). A shale with a thickness discharge were calculated. Using the discharge coefficient of 30  cm to 2  m was projected on the slope at a nearly f is 0.7 which is general value in mountains, peak hour vertical angle in a plate shape and parallel to the slope rainfall data in Fig. 11 is 44.1 mm/h on October 11, 2017, (Fig. 20c). watershed around Khanh waterfall in Fig. 12 is 21.17 km , The protruding plate-shaped shale is arranged parallel the peak flow Q is calculated from Eq. (1). on the strike of the stratum, and the geology surround- 1 1 ing the shale is limestone. The shale layer sandwiched Q = .f .r.A = × 0.7 × 44.1 × 21.17 = 181.5(m /s) 3.6 3.6 between the limestone layers can be seen in the karst (3) topography left behind by the differential erosion. There From the above calculation results, the amount of water was a small-scale collapse topography with a width of collected around the waterfall during the peak rainfall on 20  m to 30  m that occurred recently at the end of the October 11 is about 181.5 m /s. While the allowable flow slope of area P1 (Fig. 20d). However, a main scarp in the is about 3.55 m /s, which is about 51 times smaller than entire area P1 and P2 was not recognized. peak flow. That causes the water to overflow at Khanh The water samples filtered through the 0.45  µm mem - waterfall. This is consistent with the situation around the brane filter were taken in a polyethylene container for waterfall before the landslide in Fig. 6b. analysis of major ions and dissolved silicic acid. Cations + + 2+ 2+ (Na, K, Ca, Mg ) were analyzed by atomic absorp- − − 2− tion spectroscopy. Anions (Cl, NO, SO ) were 3 4 D o et al. Geoenvironmental Disasters (2022) 9:4 Page 13 of 20 Table 2 Calculation of correlation between H/L and slope tangent of source area (tan θ) in the case of Khanh waterfall landslide as shown in Fig. 14 Characteristics of slope After collapse Before collapse Runout distance after collapse (before collapse) L (L’) (m) 520 220 Runout height after collapse (before collapse) H (H’) (m) 120 120 Correlation between H/L (H’/L’) H/L (H’/L’) 0.23 0.55 Slope tangent of source area θ (°) 13 29 Rounding up slope tangent of source area (friction angle) Rounding up θ (degrees) 15 30 Tangent of rounding up slope tangent of source area θ 0.27 0.58 Slope stability analysis and numerical simulation result October, and higher than the 58-year average maximum From the geological survey results in “Geology and monthly rainfall in August with a rainfall intensity of water quality analysis results” section, there were many 344.3 mm (Fig.  10). The rain was concentrated on Octo - cracks and caves in the limestone layer at Khanh water- ber 10 and October 11, 2017 with a rainfall intensity of fall. Torrent rainfall that caused water overflow at Khanh 394.8 mm in 48 h, just before the landslide at 1:00 am on waterfall was analyzed in   “Rainfall and flow discharge October 12, 2017 (Fig. 3). The torrent rainfall flowed into calculation results” section. The water level in cracks and the upper stream network channel and concentrated on caves increased the horizontal water pressure and uplift Khanh waterfall (Fig. 12). The extremely large amount of water pressure. According to the Eq.  (2), the increasing water exceeded 51 times of the drainage capacity of the of horizontal water pressure and uplift water pressure upper stream network channel (Table  5), causing a mas- reduced the factor of safety of the slope. sive overflow on the top of Khanh waterfall (Fig. 6b). The numerical simulation result of Khanh waterfall Overflow water on the top of Khanh waterfall flowed landslide is presented in Fig.  23. The simulation result into the cracks and underground caves of the lime- is shown with a 5-s from the initial condition (t = 0  s) to stone layer (Fig.  17A, B). The water in cracks and the end of moving (t = 30 s). The red line is a boundary of underground caves increased the uplift water force and the mass moving area estimated based on the informa- driving water force (Fig.  13). The increasing of desta - tion provided by local residents and by UAV photo. The bilizing forces caused the slope to fail. The collapsed horizontal force was increased and failure started from surface was the fault between the limestone layer and the upper part of the slope at points A and B. The talus the shale layer (Fig. 18). The limestone blocks at the top deposit part at point C moved together with the lime- of slope moved together with the talus deposit at the stone from points A and B to points D and A’. After 15 s, toe of slope. This mixture crossed the river and passed the moving mass crossed the river and the opposite hill over the opposite hill. The large and fast moving mass at point D. Finally, after 30  s, the mass was deposited destroyed five houses and killed eighteen people at at point A’ behind the hill. However, most of the mov- the opposite hill. The collapse process was simulated ing mass was deposited in front of the hill. Based on the based on Coulomb mixture theory (Denlinger and Iver- boundary of mass moving area estimated by the informa- son 2001). This method assumes that grain-fluid flows tion of local residents and by UAV photo, the simulation behave as mixtures limestone layer at the top and the result is similar to the actual phenomena. talus deposit at the toe of slope. The simulation results show that the range of moving mass is similar to the actual phenomena at Khanh waterfall landslide. Discussion Some previous research on landslides in waterfall The rainfall data in Fig.  9b suggests that the global areas indicated that they are often caused by the ero- weather patterns have been rapidly changing in recent sion at the toe of the slope (Hayakawa and Matsukura decades. One of such changes is an extremely large 2010; Hayakawa 2013; Scheingross and Lamb 2017). amount of rainfall in a short period of time that causes The collapse of Khanh waterfall was different to this landslides. This trend appears to be a global issue being phenomena. Khanh waterfall collapsed due to effects of triggered by climate change. In October 2017, the recored cracks and caves in the limestone layer of the slope. A rainfall was 448.8  mm. This rainfall intensity was more large amount of water from stream flowed into cracks than two times higher than the average annual rainfall in and caves in limestone caves. The water in cracks and Do et al. Geoenvironmental Disasters (2022) 9:4 Page 14 of 20 Table 3 Calculation the ratio of basal pore fluid pressure Density ρ kg/m Ratio of volume ν Weight kg (ρ.ν) Solid 2600* 0.6 1560.0 Water 1000 0.4 400.0 Air 1 0.0 0.0 Total weight of landslide mass 1960 Ratio of basal pore fluid pressure (λ) = Water weight/Total weight 0.5 * The density of solid was referred from Konecny et al. (2017) Fig. 15 Simulation model of Khanh waterfall landslide using SfM of aerial photos taken by the UAV and AW3D30 (Japan Aerospace Exploration Agency Earth Observation Research Center 2020). a Contour of slope after collapse from UAV and AW3D30, b Estimated contour of slope before collapse based on result of field survey, c Simply estimated contour of collapse surface, d Simulate the slope surface before collapse by GIS, e Simulate the slope surface after collapse by GIS Fig. 16 Cross-section of slope surface before and after failure D o et al. Geoenvironmental Disasters (2022) 9:4 Page 15 of 20 Fig. 17 Cracks, karsts and crystallized calcite in limestone along the line A–A’. Shale at failure surface at point B and at opposite hill at point A’ caves increased the uplift water force and driving force collapse similar to the phenomena that occured at Khanh causing slope failure. waterfall 1. Regarding the possibility of similar landslides around Another nearby areas are P1 and P2 areas on the right Khanh waterfall in the future, it is necessary to have bank downstream of Khanh waterfall. Since the topogra- a further investigation and assessment of the current phy of the P1 and P2 areas is similar to the old landslide underground karsts around Khanh waterfall. Especially a site, it was feared that landslides would continue to occur remaining part of Khanh waterfall, Khanh waterfall 2 in in the future. However, the survey results show that the Fig. 4, has a similar geology and same water supply chan- geology at areas P1 and P2 is different from the geology nels. There is a high possibility that Khanh 2 waterfall will at Khanh waterfall (Fig. 19). The geology at areas P1 and Do et al. Geoenvironmental Disasters (2022) 9:4 Page 16 of 20 Fig. 18 Geology along line A–A’ (center line A–A’ in Fig. 5) of Khanh waterfall landslide—Outcrop of shale and limestone at the right bank of the Kem River after collapse P2 is mainly limestone interspersed with a thin shale grain-fluid flows behave as mixtures limestone layer layer. In addition, the surface and groundwater in areas and the talus deposit. The simulation results proved P1 and P2 and the Khanh waterfall landslide area are the landslide process from collapse to flow that we considered to have different water supply channels due inferred. to differences in water quality. It suggests that areas P1 3. Due to the wide distribution of limestone, and the and P2 may have a different phenomena compared with development of karsts in it in many parts of the Khanh waterfall landslide. world, further studies on the development of karsts in waterfall areas are needed to avoid similar disas- Conclusion ters as in Khanh waterfall. In this study, we investigated the mechanism of a land- 4. This research did not focus on the climatological slide occurrence by focusing on the topographical and reasons of an abnormal torrent rain leading to the geological background, the relationship between rain- Khanh waterfall disaster. However, the data pre- fall and water flow conditions that caused the Khanh sented in this research show that it could be a recent waterfall landslide, and the flow mechanism of the pattern caused by the climate change and having moving mass. Furthermore, the possibility of recur- global implications. rence of such landslide damage was analyzed. The con - clusions of this study can be summarized as follows: 1. The collapse of Khanh waterfall was different from some previous collapses of waterfalls due to ero- sion at the toe of the slope. Due to the torrent rain continuing for 48  h, a large amount of water flow - ing from the upper stream network channel con- centrated in the Khanh waterfall area. It resulted in a large amount of groundwater flowed in the cracks and karsts in Khanh waterfall slope. The water in cracks and karsts increased the uplift water force and driving force causing slope failure. The increasing of destabilizing forces caused the slope to collapse. The sliding surface was the small fault between the lime- stone layer and the shale layer. 2. The collapse process was simulated based on Cou - lomb mixture theory. This method assumes that Fig. 19 Geology at P1 and P2 areas as shown in Fig. 7 D o et al. Geoenvironmental Disasters (2022) 9:4 Page 17 of 20 Fig. 20 Photos of a, b, c, d position in Fig. 18, a: terraced topography, b: spring water, c: thin shale layer, d: small collapse area near the river Fig. 21 Stiff diagrams Table 4 The results of the water quality analysis + + 2+ 2+ − − 2− Measurement point Measuring date T EC pH Na K Ca Mg alk Cl NO SO SiO 3 4 2 [℃] [mS/m] [mg/L] [mg/L] [mg/L] [mg/L] [meq/L] [mg/L] [mg/L] [mg/L] [mg/L] G005 Spring water 2019/9/10 11:33 25.8 33 7.50 0.8 0.1 66.8 1.98 3.42 0.3 0.7 3.6 7.3 G008 Spring water 2019/9/10 13:16 26.0 35 7.31 1.1 0.8 60.0 8.34 3.47 0.7 0.7 3.3 7.4 G010 Surface water 2019/9/10 14:04 25.5 28 8.03 1.3 1.1 45.6 8.43 2.85 0.5 2.2 4.9 10.5 G011 Spring water 2019/9/10 14:18 25.9 30 8.12 1.2 1.1 47.4 9.29 3.06 0.7 4.8 4.0 9.9 G012 Surface water 2019/9/10 14:33 25.5 28 8.09 1.3 1.0 43.0 8.53 2.8 0.4 3.4 3.9 10.8 Do et al. Geoenvironmental Disasters (2022) 9:4 Page 18 of 20 Fig. 22 Electrical conductivity (EC) and silica concentration (SiO ) Table 5 The characteristics of the upper stream network channel of Khanh waterfall Basic characteristics Unit Value Calculated equation Width of channel (B) m 3.00 Height of channel (H) m 2.00 Left slope of channel (1:m1) 0.50 Right slope of channel (1:m2) 0.50 Roughness coefficient (n) 0.10 Slope gradient (l) % 0.18 Peak flow (Q) m /s 181.5 α 0.80 Cross-sectional area (A) m 8.00 A = H{2B + (m1 + m2)H}/2 2 1/2 2 1/2 Wetted perimeter (P) m 7.47 P = B + H{(1 + m1 ) + (1 + m2 ) } Hydraulic radius (R) m 1.07 R = A/P 3 1/2 2/3 Allowable flow (Qmax) m /s 3.55 Qmax = I .R .A/n Peak flow/allowable flow 51 Q/ Qmax D o et al. Geoenvironmental Disasters (2022) 9:4 Page 19 of 20 Fig. 23 Simulation result of Khanh waterfall landslide with a 5-s interval from initial condition to the end of moving: a at initial condition, b at 5 s, c at 10 s, d at 15 s, e at 20 s, f at 25 s, g at the end 30 s Do et al. Geoenvironmental Disasters (2022) 9:4 Page 20 of 20 Acknowledgements Hayakawa YS, Matsukura Y (2010) Stability analysis of waterfall cliff face at Authors are grateful to Mr. Satoshi Suzuki of Okuyama Boring Co., Ltd, Japan, Niagara Falls: an implication to erosional mechanism of waterfall. Eng Mr. Doan Huy Loi, Mr. Le Hong Luong, Mr. Huynh Thanh Binh, and Mr. Phan Geol 116:178–183 Van Chuong of Institute of Transport Science and Technology, Vietnam, and Hicks B, Gray S, Ball JE (2009) A critical review of the urban rational method. In: Mr. Tran Song Manh for their support during this study. Proceedings of H2009, 32nd hydrology and water resources symposium, engineers Australia, ISBN 978-08258259461 Authors’ contributions Hoek E, Bray J (1974) Rock slope engineering. Institution of Mining and Metal- NHD, SG, SA, KTN, KH and OW visited the landslide site and conducted the lurgy, London field survey. NHD, SA and SG contributed to geology survey and slope stabil- Japan Aerospace Exploration Agency Earth Observation Research Center ity analysis. KTN, TM and KH contributed to mapping. OW, NHD, SG and SA (2020) ALOS Global Digital Surface Model "ALOS World 3D - 30m contributed to water quality analysis, rainfall and flow discharge calculation. (AW3D30)". https:// www. eorc. jaxa. jp/ ALOS/ en/ aw3d30/ index. htm/. KH and SA contributed to numerical simulation of collapse process. NHD, SG Accessed July 2020 and SA wrote the manuscript, edited and finalized the corrections. All authors Khang P (1985) The development of karst landscapes in Vietnam. Acta Geol read and approved the final manuscript. Pol 35(3–4):305–319 Konecny P, Hagi A, Plevova E, Vaculikova L, Murzyn T (2017) Characterization Funding of Limestone from Cement Plant at Berbera (Republic of Somaliland). The authors received no specific funding for this research work. Procedia Eng 191:43–50 Loi DH, Quang LH, Sassa K, Takara K, Dang K, Thanh NK, Tien PV (2017) The 28 Availability of data and materials July 2015 rapid landslide at Ha Long City, Quang Ninh. Vietnam Land- All data and materials are available from the corresponding author upon slides 14:1207–1215. https:// doi. org/ 10. 1007/ s10346- 017- 0814-y reasonable request. Moriwaki H (1987) A prediction of the runout distance of a debris. J Jpn Landslide Soc 24(2):10–16. https:// doi. org/ 10. 3313/ jls19 64. 24.2_ 10 (in Declaration Japanese with English abstract) Nguyen LC, Tien PV, Do TN (2020) Deep-seated rainfall-induced landslides on a Competing interests new expressway: a case study in Vietnam. Landslides 17:395–407. https:// The authors declare that they have no competing interests.doi. org/ 10. 1007/ s10346- 019- 01293-6 Quang LH, Loi DH, Sassa K, Takara K, Ochiai H, Dang K, Abe S, Asano S, Ha DN Author details (2018) Susceptibility assessment of the precursor stage of a landslide Civil Management and Engineering Major, Environmental and Social System threatening Haivan Railway Station. Vietnam Landslides 15:309–325. Science Course, Integrated Graduate School of Medicine, Engineering, https:// doi. org/ 10. 1007/ s10346- 017- 0870-3 and Agricultural Sciences, University of Yamanashi, Yamanashi, Japan. F acult y Scheidegger AE (1973) On the prediction of the reach and velocity of cata- of Engineering, Graduate Faculty of Interdisciplinary Research, University strophic landslides. Rock Mech 5:231–236 3 4 of Yamanashi, Yamanashi, Japan. Okuyama Boring Co., Ltd, Akita, Japan. I nsti- Scheingross JS, Lamb MP (2017) A Mechanistic model of waterfall plunge pool tute of Transport Science and Technology, Hanoi, Vietnam. Advantechnology erosion into bedrock. J Geophys Res Earth Surf, pp 2079–2104 Co., Ltd, Miyagi, Japan. Suimonkikaku LLC, Miyagi, Japan. Tan MT, Liem NV, Tuan DA, Tien NV (2015) Correlation analysis between land- slides and rainfall in Mai Chau, Hoa Binh. J Sci Vietnam Natl Univ Hanoi Received: 28 August 2021 Accepted: 24 January 2022 31(4):51–63 (in Vietnamese) Tung BD, Luong LH, Cuong PV, Thanh NK, Manh TS, Watanabe O, Fujii N, Abe S (2016) A preliminary study on the mechanism of landslide in limestone formation area in Son La. Northern Viet Nam. In: Proceedings of the 2016 international conference on sustainability in civil engineering, Geotechni- cal Engineering, pp 76–80 References Tuoi Tre Media (2017) Khanh waterfall before and after a serious landslide Bui DT, Pradhan B, Lofman O, Revhaug I, Dick OB (2013) Regional prediction of caused 18 people buried. YouTube https:// www. youtu be. com/ watch?v= landslide hazard using probability analysis of intense rainfall in the Hoa B6iU3 eYA7_Q. Accessed Dec 2019 (in Vietnamese) Binh province. Vietnam Nat Hazards 66:707–730. https:// doi. org/ 10. 1007/ Vietnam Meteorological and Hydrological Administration (2019) Accessed Dec s11069- 012- 0510-0 Bui DT, Tuan TA, Hoang ND, Thanh NQ, Nguyen DB, Liem NV, Pradhan B (2017) Vietnamnet (2017) Photos of 200m-waterfall before burying 18 people in Hoa Spatial prediction of rainfall-induced landslides for the Lao Cai area Binh. 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J Geophys Res 106(B1), 553–566 Department of Geology and Minerals of Viet Nam (2004) Geological and min- eral resources map of Viet Nam on 1:200.000 Ninh Binh F-48-XXXIV Publisher’s Note Department of Geology and Minerals of Viet Nam (2005) Geological and min- Springer Nature remains neutral with regard to jurisdictional claims in pub- eral resources map of Viet Nam on 1:200.000 Ha Noi F-48-XXVII lished maps and institutional affiliations. Duc DM (2013) Rainfall-triggered large landslides on 15 December 2005 in Van Canh District, Binh Dinh Province, Vietnam”. Landslides 10:219–230. https:// doi. org/ 10. 1007/ s10346- 012- 0362-4 Gran G (1952) Determination of the equivalence points in potentiometric titra- tions—part II. Analyst 77:661–671 Hau VT, Kim Y, Ngo T, Tran HT, Yi K (2018) Neoproterozoic deposition and Trias- sic metamorphism of metasedimentary rocks in the Nam Co Complex, Song Ma Suture Zone. NW Vietnam Geosci J 22(4):549–568 Hayakawa YS (2013) Stability analysis of cliff face around Kegon Falls in Nikko, Eastern Japan: an implication to its erosional mechanisms. Int J Geosci 4:8–16 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Geoenvironmental Disasters Springer Journals

Torrent rainfall-induced large-scale karst limestone slope collapse at Khanh waterfall, Hoa Binh Province, Vietnam

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Abstract

In recent years, many landslides have occurred in Vietnam, particularly in the Northern mountainous region during the rainy season from May to October. On the morning of October 12, 2017, the Khanh waterfall landslide in Khanh Village, Hoa Binh Province, Northern Vietnam occurred. The landslide killed eighteen people and destroyed five houses. Topographical and geological surveys were conducted around the area to determine its causes. The rainfall data and flow discharge were also analyzed. The results showed that this collapse was different from some previous ones collapsed due to the erosion at the toe of the slope. Khanh waterfall landslide occurred due to the increasing amount of water in cracks and caves in the limestone layer in the slope. The collapse process was simulated based on Coulomb mixture theory. The numerical simulation results show similarities with the actual collapse process. The results provide indicators for assessing the risk of such limestone waterfall landslides in the future. Keywords: Khanh waterfall landslide, Torrent rain, Limestone, Cave, Numerical simulation Introduction in Northern Vietnam (Fig.  1). In this study, we refer to Vietnam has a humid monsoon climate with a high aver- this landslide as the local name, Khanh waterfall land- age annual rainfall. Damaging landslides occur every year slide. This used to be a tourist destination with a beauti - during the rainy season. Studies on rainfall-triggered ful scenery of the waterfall. landslides in Vietnam such as in Binh Dinh Province The landslide caused a great damage killing 18 peo - (Duc 2013), Lao Cai Province (Bui et  al. 2017), Quang ple and destroying 5 households on the opposite hill of Ninh Province (Loi et  al. 2017; Nguyen et  al. 2020), and the Khanh waterfall. Before this disaster, Khanh Village Danang City (Quang et  al. 2018) have identified the tor - residents witnessed minor size slope collapses earlier in rential rain as the primary cause of landslides in Viet the rainy season in September. However, no significant Nam. Hoa Binh is the northern mountainous province landslides have previously been recorded at the Khanh most affected by landslides during the rainy season (Bui waterfall. As a result, landslide risk early warning and et  al. 2013). On October 12, 2017, around 01:00 local evacuations were not contemplated before the disaster. time, a landslide occurred at Khanh waterfall, Phu Cuong The sudden landslide moved a large amount of rock and Commune, Tan Lac District, Hoa Binh Province, Viet- soil downhill, across the river, and up the opposite hill for nam. The Khanh waterfall is located in a limestone area a total moving distance of about 400 m. There are many tourist destinations that feature beau - tiful and famous waterfalls in the world. Another well- *Correspondence: goto@yamanashi.ac.jp known waterfall in the limestone belts in Vietnam is Ban Faculty of Engineering, Graduate Faculty of Interdisciplinary Research, Gioc waterfall located in Cao Bang province, Northern University of Yamanashi, Yamanashi, Japan Vietnam. In other countries, there are many scenic spots Full list of author information is available at the end of the article © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/. Do et al. Geoenvironmental Disasters (2022) 9:4 Page 2 of 20 Fig. 1 Location map of Khanh waterfall landslide in Hoa Binh province, Vietnam in karst terrain such as Tamul Falls in Mexico and Plitvice craton and Indochina craton during the late Paleozoic to Lakes Falls in Croatia. However, until now little is known early Mesozoic period. The limestones are distributed about landslides that cause a great loss of life at lime- along the northwest-southeast tectonic line (Da River stone waterfalls known as scenic spots. Hence, it is very fault) (Fig.  1). In recent years, many landslides due to important to understand the mechanism of the landslide heavy rainfall have occurred in the margin of this lime- occurred at the Khanh waterfall. stone zone (Tung et  al. 2016). The main geology of the Several previous studies on the collapse of waterfalls study area consists of shale and limestone from the Trias- focused on the transition process of rivers (Scheingross sic of Mesozoic (Fig. 2). and Lamb 2017). However, there were only a few studies Khanh waterfall is located on the right bank of the on the collapse of waterfalls as disasters such as Kegon Kem river that flows under the cliff. Behind it, steep ter - waterfall in Nikko, Japan (Hayakawa 2013), Niagara rain with unevenness peculiar to the limestone area at an waterfall between America and Canada (Hayakawa and altitude of 350  m to 1000  m spreads from northwest to Matsukura 2010). This study is focused on the clarifying southeast. On the other hand, the left bank of the River of the mechanism of the Khanh waterfall landslide. Such Cam, where shale is mainly distributed, is a relatively flat factors as topographical and geological background, rain- terrain with an altitude of 200  m to 250  m, and villages fall and groundwater are analyzed. The water quality and are formed along the national highway. the flow simulation of the moving soil and water masses Shale is distributed on the left bank of the Kem river, were also analyzed. Finally, as the expansion of cave in and limestone with shale on the right bank. The stratig - limestone areas in Vietnam is still developing (Khang raphy is N40–50°W, the dip is 70°–80°W. There is a small 1985), assessing the possibility of recurrence of similar fault on the slope of the Khanh waterfall that dips about landslides would be very meaningful. 45°E as shown in Fig. 18 later. The Kem river itself, which flows in the direction of the strike of stratum, may also Study area correspond to a fault. General geology Geotechnical investigation including the strength and The study area is located at the Song Ma suture zone permeability of each layer was not conducted in detail in (Hau et al. 2018) due to the collision of the South China this study. However, preliminary testing of the strength D o et al. Geoenvironmental Disasters (2022) 9:4 Page 3 of 20 Fig. 2 Geological map and schematic geological columns around Khanh waterfall (Modified from Department of Geology and Minerals of Vietnam 2004, 2005) of the rock with a hammer was carried out. Both shale age (Konecny et  al. 2017), the unconfined compressive and limestone have a stratified structure. These rocks are strength of the limestone layer is 69–204 MPa. soft with weathering at the surface of the outcrop. But The river is littered with limestone boulders as land - the fresh part is hard that it cannot be easily broken by slide moving masses as well as stalactite rubble rich in hammering. Based on the study on limestone at a similar cavities. They were probably formed by groundwater Do et al. Geoenvironmental Disasters (2022) 9:4 Page 4 of 20 in cracks and cavities in the limestone. It is considered branches of the fall named Khanh waterfall 1 and Khanh that the permeability of such shale and limestone itself waterfall 2 (Fig. 4). is low. However, the slope of these strata forming a lay- When the landslide occurred, only Khanh waterfall 1 ered structure is almost vertical from 70° to 80°. Moreo- collapsed. Khanh waterfall 2 did not collapse. Accord- ver, there were many cracks and cavities in the limestone ing to the survey of the upper stream network of Khanh layer. Hence, surface water easily penetrated into cracks waterfall, waterfall 1 and waterfall 2 have the same and cavities in the limestone layer. water source. However, the main stream flows towards waterfall 1, the secondary stream flows towards water - Features of the slope failure fall 2. Therefore, when the heavy rain occurred, more The Khanh waterfall landslide occurred around 01:00 water was directed to the waterfall 1 area due to its am local time on October 12, 2017 after 48 h of a heavy geological characteristics that resulted in the slope col- rainfall of 394.8 mm on October 10 and 11, 2017 (Fig. 3). lapse. The original dimensions of the Khanh waterfall The rainfall data was collected at Mai Chau district rain 1 were a height of approximately 120  m, a maximum gauge station, which is located 12 km from Khanh water- thickness of 50  m, and a width of 200  m. A part of the fall (Vietnam Meteorological and Hydrological Admin- landslide mass deposited at the Kem River forming a istration 2019). At the Khanh waterfall, there were two natural dam. Other parts of the mass crossed the Kem Fig. 3 Daily rainfall in October 2017 when the Khanh waterfall landslide occurred ( Vietnam Meteorological and Hydrological Administration 2019) Fig. 4 The overview of Khanh waterfall landslide (Photo taken by Tuoi Tre Media 2017) D o et al. Geoenvironmental Disasters (2022) 9:4 Page 5 of 20 River, passed over the top of the 30  m high hill on the Figure 6 shows photos of the Khanh waterfall landslide opposite riverbank, and reached the paddy field. The taken at different times before and after the collapse of deposit part at opposite hill killed 18 people living the waterfall. It shows the increase in the water around there (Fig. 5). the waterfall during a heavy rain in 2017. The slope in Fig. 5 Photos of Khanh waterfall along the line A-A’ of the landslide moving direction: a top view, b side view Do et al. Geoenvironmental Disasters (2022) 9:4 Page 6 of 20 considered as the factors contributing to the landslide. Moreover, a numerical model was created to simulate the landslide process. The affected area from the simulation results was compared with the one determined from the actual image. Field study Field surveys were conducted to understand the geol- ogy, topography and hydrogeology around Khanh water- fall landslide (Fig.  7). The range of movement and the amount of moving mass were estimated based on the actual geological evidence, information provided by the local residents, and the images taken by an Unmanned Aerial Vehicle (UAV). Figure  8 shows the topographic classification map and the distribution map of the landslides in the Khanh waterfall area. The soil layers between the collapsed and the non-collapsed areas were investigated to determine the position of the collapsed surface. On the right bank of Kem river 200 m downstream from Khanh waterfall land- slide, there were two areas P1 and P2 which were parallel concave terrains similar to the traces of past landslides. There was no movement at these points when the land - slide occurred at Khanh waterfall. Water quality analysis and geology investigation at P1 and P2 areas were con- ducted to compare these condition with Khanh waterfall. One water sample No. 012 (surface water) was col- Fig. 6 Front view photos of Khanh waterfall: a in dry season before lected from Khanh waterfall 1. Two water samples No. failure, b in rainy season 2017 reportedly days before the landslide 010 (surface water), 011 (spring water) were collected occurred, c after the landslide occurred in 2017 (Photo taken by Tuoi from Khanh waterfall 2. Two samples No. 005 and 008 Tre Media 2017; Vietnamnet 2017) (spring water) were collected from areas P1 and P2. In order to investigate the water quality characteristics, samples of spring water and swamp water around the the dry season before 2017 was introduced as a beauti- landslide area were collected to analyze in the laboratory. ful waterfall destination for tourism (Fig.  6a). Figure  6b Such parameters as the water temperature (T), pH, and shows the situation of the slope in the rainy season of electrical conductivity (EC) were measured in the field. 2017, days before the landslide occurred. A large amount of brown water flowing from the entire slope included the waterfall as well as water ejected from the middle of Rainfall and flow discharge calculation the slope. Figure  6c shows the landscape after the slope Figure  9 shows the position of Khanh waterfall landslide area around the waterfall mostly collapsed. The moving in 2017 and areas in close proximity where landslides mass containing boulders reached the opposite hill on occurred previously (Tan et  al. 2015) plotted on Google the left bank of the river. Earth, and a rainfall data graph from 1961 to 2018. In Fig. 9a, the location of Khanh waterfall, Mai Chau rainfall Methodology station and the previous nearby landslides areas in Pu Bin To study the mechanism of the Khanh waterfall land- commune and ung Th Khe commune, Mai Chau district, slide, field surveys in the area surrounding it were con - Hoa Binh province are shown. There were two landslides ducted. Rainfall data around this area was also collected in 1996 and 2007 in Pu Bin commune and one landslide for the study. To evaluate the drainage capacity upstream in Thung Khe commune in 2005. The exact locations, of the waterfall, a measurement of the upstream network sizes and types of these landslides were not specified. channel was conducted. The surface water and spring However, the time when the landslides occurred has water were collected to analyze the water source rela- been reported. Figure 9b shows the maximum 24 h, 48 h tionship between the collapse area and the neighboring rainfall data at Mai Chau rainfall station from 1961 to area. Topography, geology, rainfall, drainage capacity are 2018 with the time when Khanh waterfall landslide and D o et al. Geoenvironmental Disasters (2022) 9:4 Page 7 of 20 Fig. 7 Field survey area including the location of water sampling points around Khanh waterfall (KW ) landslide. Sample 012 (surface water) was collected from KW1. Samples 010 (surface water), 011 (spring water) were collected from KW2. Samples No. 005 and 008 (spring water) were collected from P1 and P2 areas Fig. 8 Topographic classification and landslide distribution map around Khanh waterfall previous nearby landslides occurred. It shows that the 2005. The data of the previous landslides and the current 48  h rainfall higher than 300  mm caused landslides that one at Khanh waterfall in the context of the rainfall data occurred in Pu Bin commune in 1996, 2007 and Khanh for 58 years indicates that torrent rainfall in 24 h and 48 h waterfall landslide in 2017. The 24 h rainfall higher than is an important factor causing landslides around Khanh 200  mm caused a landslide in Thung Khe commune in Do et al. Geoenvironmental Disasters (2022) 9:4 Page 8 of 20 Fig. 9 a Location of Khanh waterfall (2017) and previous nearby landslides in Pu Bin commune and Thung Khe commune, Mai Chau district, Hoa Binh province ( Tan et al. 2015); b data of maximum 24 h and 48 h rainfalls from 1961 to 2018 ( Vietnam Meteorological and Hydrological Administration 2019) waterfall. Rainfall in 48 h greater than 300 mm should be area was 21.17 km . The water was gathered behind the considered a warning landslides around this area. Khanh waterfall and flowing down to the Khanh waterfall In 2017, when the landslide occurred, there were large (Fig. 12). typhoons in October in Vietnam. The monthly rainfall The flood discharge that flowed down to the Khanh in October of 448.8 mm was 2.6 times the usual amount waterfall during heavy rainfall when the landslide of 170.4 mm (Fig. 10). On October 10 and 11, 2017, 48 h occurred was calculated using the following Rational before the landslide occurred, the rainfall was recorded at Method (Hicks et  al. 2009). This is a common method 394.8 mm. A maximum of 44.1 mm/h was recorded at 4 used to calculate flow discharge for rivers in mountain - am on October 11 (Fig. 11). This hourly rainfall value was ous areas where the watershed area is less than 100 k m used to calculate the flow discharge in the upstream area and there are no adjustment facilities along the river. of Khanh waterfall. The watershed behind the landslide D o et al. Geoenvironmental Disasters (2022) 9:4 Page 9 of 20 Fig. 10 Comparison of an average monthly precipitation and 2017 monthly precipitation in 1961–2018 ( Vietnam Meteorological and Hydrological Administration 2019) Fig. 11 Hourly rainfall on October 11 and October 12, 2017 when the Khanh waterfall landslide occurred ( Vietnam Meteorological and Hydrological Administration 2019) ′ ′ c l + (W cosθ − U − V sinθ)tanφ Q = .f .r.A (1) (2) FS = 3.6 W sinθ + V cosθ where Q is peak flow (m /s), f is discharge coefficient, r is where FS is factor of safety, c’ is the effective cohesion rainfall intensity (mm/h), and A is watershed (km ). of sliding joint, l is length of the sliding joint, W is the weight of sliding block, U is the uplift water pressure, V is the horizontal water pressure, θ is the angle of the slid- Slope stability analysis and numerical simulation ing surface, φ’ is the effective friction angle of the sliding of collapse process joint. To explain slope instability, the method from Hoek It is assumed that when heavy rain, the surface water and Bray (1974) is used. A simply modified theoreti - flowed into the void in the cracks and cavities in the cal models of plane slope failures in Khanh waterfall limestone layer. This amount of water increased the is drawn in Fig.  13. It is assumed that there are many water pressure that destroys the slope. The specific cal - cracks and caves in the limestone layer in Khanh water- culation of the factor of safety was not considered in fall slope. The water in cracks and caves is from the this study. However, to simulate the slope collapse due upper stream network channel of Khanh waterfall. The to a large amount of water in the limestone layer cor- factor of safety against the sliding block was calculated responding to the peak flow discharge, the simulation as follows: Do et al. Geoenvironmental Disasters (2022) 9:4 Page 10 of 20 Fig. 12 The upper stream network channel and watershed of Khanh waterfall landslide method to predict the behavior of grain-fluid flows Figure  14 shows the relationship between the slope due to slope collapse (Zhang et  al. 2004) is applied in angle of the source area (θ), runout distance (L), and this study. This method assumes that grain-fluid flows runout height (H) (Moriwaki 1987). The coefficient behave as mixtures of interaction of Newtonian fluids of friction is close to the the tangent of the slope angle and Coulomb solids. The equations that describe the (Scheidegger 1973). It is considered that the slope angle mixtures based on Coulomb mixture theory (Denlinger before and after collapse explains the internal friction and Iverson 2001). In order to exactly predict the range angle and basal friction angle. of sediment, the momentum equation was discretized Table 1 shows the physical properties of the flow simu - by the finite difference method with applied a stop lation. It is assumed that the resistance force on the fail- condition of grain-fluid flow, the third-order upwind ure surface during the flow depends only on the friction scheme, and the preserving mass conservation method at the contact surface between the landslide mass and to the numerical model (Zhang et al. 2004). D o et al. Geoenvironmental Disasters (2022) 9:4 Page 11 of 20 the basal rock. The tangent of the ratio between runout height (H) and runout distance (L) after collapse is equal to the basal friction angle. The tangent of the ratio of height of source slope (H’) and the length of source slope (L’) before collapse is equal to the internal friction angle. In the case of Khanh waterfall, the internal friction angle (φ ) and the basal friction angle (φ ) of grain-fluid int bed flows were 30 degrees from tanθ ≒0.55 and 15 degrees from H/L≒0.23 based on the geometric shape of slope before and after collapse (Table 2). Table 3 shows the calculation of the ratio of basal pore fluid pressure. The density of solid was referred from Fig. 13 Slope stability analysis of rock slope (modified from Hoek Konecny et al. (2017). Based on the field survey, the ratio and Bray 1974) of solid and void was determined as 0.6 and 0.4, respec- tively. The ratio of water and air in the void represents the different cases of pore water pressure. Since it was tor - rent rainfall at the time of the slope collapse, the ratio of water and air was determined as 0.4 and 0, respectively. Simulations of cases where the water ratio is less than 0.4 are also calculated. However, the simulation results of these cases differ from the real landslide aftermath. As a result, the ratio of basal pore fluid pressure (λ) was set to 0.5 corresponding to the peak flow discharge. Assuming that the basal pore fluid pressure (p ) was the same as bed the horizontal water pressure of almost the same height with a depth of grain-fluid flow (h). The general soil prop - erty values of solid density (ρ ), fluid water density (ρ ), s f the ratio of solid volume (υ ), and the ratio of fluid vol - ume (υ ) were used for analysis. The simulation model was created from a three-dimen - sional terrain model by SfM of aerial photos taken by the UAV and ALOS Global Digital Surface Model, ALOS World 3D 30  m (AW3D30) (Japan Aerospace Explora- tion Agency Earth Observation Research Center 2020) in Fig.  15. The position of the failure surface is determined based on the position of the slope surface before and after collapse. Figure  15a–c show the contour of slope Fig. 14 Correlation between H/L and slope tangent of source area surface before and after collapse which were determined tanθ (modified from Moriwaki 1987 and added Khanh waterfall from UAV photos and AW3D30. The three dimensional landslide by authors) where H is runout height, L is runout distance, H’ surface of the slope before and after collapse were simu- is height of source slope, L’ is length of source slope lated by GIS in Fig. 15d, e. The cross-section of the slope surface before and after collapse were determined in Fig.  16 from three dimensional surface. The failure sur - Results of analysis face was determined based on knowledge from the field Geology and water quality analysis results survey and the conservation of volume before and after The geology of the area consists mainly of Triassic shale collapse (red line in Fig. 16). and limestone of the Mesozoic. The strike of the geo - The collapse process was simulated in which the lime - logical layer is N40–50°W, and the dip is 70–80°W. The stone block at the upper part of a waterfall and the talus Khanh-waterfall section is a small fault that is inclined deposit at the bottom of the slope moved together. The 45°E. The course of the Kem River along the strike direc - moving mass crossed the river and moved about 400  m tion of the geological layer might correspond to that (Fig. 16). fault. The limestone is mainly distributed on the right riverbank in the steep terrain, while the shale is mainly Do et al. Geoenvironmental Disasters (2022) 9:4 Page 12 of 20 Table 1 Physical properties of the flow simulation Parameter Value Unit Time step Δt 0.005 s Mesh size Δx × Δy 5 × 5 m Internal friction angle before collapse (calculated in Table 2) φ 30 ° int Basal friction angle after collapse φ 15 ° bed (calculated in Table 2) Ratio of basal pore fluid pressure (calculated in Table 3) λ 0.5 – Gravity acceleration g 9.8 m/s distributed on the left bank. The difference between these analyzed by ion chromatography. The alkalinity (corre - terrains is likely due to the difference between the weath - sponding to HCO concentration) was determined by ering resistance characteristics of the distributed geology. acid titration using the Gran method (Gran 1952). Then, Figure  17 shows the side view and photos at different the dissolved silicic acid (silica SiO ) was analyzed by the points along line A–A’. At the top of Khanh waterfall, the molybdenum yellow absorption spectrophotometry. The limestone layer had many cracks and creeping toward result of water quality analysis are listed in Table 4. (Fig. 17A). At the middle of the slope, the sliding surface The water quality analysis results were organized as was found at the fault between the limestone layer and a stiff diagram (Fig.  21). G005, G008 collected from the shale layer (Fig. 17B). Limestone containing stalactite spring water has less silica and more calcium ions than that formed the waterfall collapsed along the boundary G010, G011, G012. It is highly possible that it was cul- with the shale, slipped together with the talus deposit at tivated in limestone. Silica-rich G010, G011, and G012 the bottom of the slope, and moved across the river to are greatly affected by weathering of non-limestone as the opposite hill. Along the line A–A’ along the landslide well as limestone (Fig.  22). These differences are con - moving direction, many limestones were found at the toe sistent with the different collection basins at Khanh of the slope and at the opposite hill (Fig.  17C, D). From waterfall landslide and areas P1 and P2. The surface the field survey at the site, the geological cross-section of and groundwater in areas P1 and P2 and the Khanh Khanh waterfall slope was drawn in Fig. 18. waterfall landslide area were considered to have differ - Downstream of Kem river, the geological cross-section ent water supply channels due to differences in water of P1 and P2 areas were drawn in Fig.  19. Both points quality. P1 and P2 had a concave topography which was divided by the ridgeline into the northeast-southwest direction. Rainfall and flow discharge calculation results They show a terraced topography with steps of various The characteristics of the upper stream network channel heights of 2  m to 5  m at the Kem River direction below of Khanh waterfall are listed in Table 5. From these char- the slope (Fig.  20a). A lot of spring water was observed acteristics, the peak flow discharge and the allowable flow in the whole area (Fig.  20b). A shale with a thickness discharge were calculated. Using the discharge coefficient of 30  cm to 2  m was projected on the slope at a nearly f is 0.7 which is general value in mountains, peak hour vertical angle in a plate shape and parallel to the slope rainfall data in Fig. 11 is 44.1 mm/h on October 11, 2017, (Fig. 20c). watershed around Khanh waterfall in Fig. 12 is 21.17 km , The protruding plate-shaped shale is arranged parallel the peak flow Q is calculated from Eq. (1). on the strike of the stratum, and the geology surround- 1 1 ing the shale is limestone. The shale layer sandwiched Q = .f .r.A = × 0.7 × 44.1 × 21.17 = 181.5(m /s) 3.6 3.6 between the limestone layers can be seen in the karst (3) topography left behind by the differential erosion. There From the above calculation results, the amount of water was a small-scale collapse topography with a width of collected around the waterfall during the peak rainfall on 20  m to 30  m that occurred recently at the end of the October 11 is about 181.5 m /s. While the allowable flow slope of area P1 (Fig. 20d). However, a main scarp in the is about 3.55 m /s, which is about 51 times smaller than entire area P1 and P2 was not recognized. peak flow. That causes the water to overflow at Khanh The water samples filtered through the 0.45  µm mem - waterfall. This is consistent with the situation around the brane filter were taken in a polyethylene container for waterfall before the landslide in Fig. 6b. analysis of major ions and dissolved silicic acid. Cations + + 2+ 2+ (Na, K, Ca, Mg ) were analyzed by atomic absorp- − − 2− tion spectroscopy. Anions (Cl, NO, SO ) were 3 4 D o et al. Geoenvironmental Disasters (2022) 9:4 Page 13 of 20 Table 2 Calculation of correlation between H/L and slope tangent of source area (tan θ) in the case of Khanh waterfall landslide as shown in Fig. 14 Characteristics of slope After collapse Before collapse Runout distance after collapse (before collapse) L (L’) (m) 520 220 Runout height after collapse (before collapse) H (H’) (m) 120 120 Correlation between H/L (H’/L’) H/L (H’/L’) 0.23 0.55 Slope tangent of source area θ (°) 13 29 Rounding up slope tangent of source area (friction angle) Rounding up θ (degrees) 15 30 Tangent of rounding up slope tangent of source area θ 0.27 0.58 Slope stability analysis and numerical simulation result October, and higher than the 58-year average maximum From the geological survey results in “Geology and monthly rainfall in August with a rainfall intensity of water quality analysis results” section, there were many 344.3 mm (Fig.  10). The rain was concentrated on Octo - cracks and caves in the limestone layer at Khanh water- ber 10 and October 11, 2017 with a rainfall intensity of fall. Torrent rainfall that caused water overflow at Khanh 394.8 mm in 48 h, just before the landslide at 1:00 am on waterfall was analyzed in   “Rainfall and flow discharge October 12, 2017 (Fig. 3). The torrent rainfall flowed into calculation results” section. The water level in cracks and the upper stream network channel and concentrated on caves increased the horizontal water pressure and uplift Khanh waterfall (Fig. 12). The extremely large amount of water pressure. According to the Eq.  (2), the increasing water exceeded 51 times of the drainage capacity of the of horizontal water pressure and uplift water pressure upper stream network channel (Table  5), causing a mas- reduced the factor of safety of the slope. sive overflow on the top of Khanh waterfall (Fig. 6b). The numerical simulation result of Khanh waterfall Overflow water on the top of Khanh waterfall flowed landslide is presented in Fig.  23. The simulation result into the cracks and underground caves of the lime- is shown with a 5-s from the initial condition (t = 0  s) to stone layer (Fig.  17A, B). The water in cracks and the end of moving (t = 30 s). The red line is a boundary of underground caves increased the uplift water force and the mass moving area estimated based on the informa- driving water force (Fig.  13). The increasing of desta - tion provided by local residents and by UAV photo. The bilizing forces caused the slope to fail. The collapsed horizontal force was increased and failure started from surface was the fault between the limestone layer and the upper part of the slope at points A and B. The talus the shale layer (Fig. 18). The limestone blocks at the top deposit part at point C moved together with the lime- of slope moved together with the talus deposit at the stone from points A and B to points D and A’. After 15 s, toe of slope. This mixture crossed the river and passed the moving mass crossed the river and the opposite hill over the opposite hill. The large and fast moving mass at point D. Finally, after 30  s, the mass was deposited destroyed five houses and killed eighteen people at at point A’ behind the hill. However, most of the mov- the opposite hill. The collapse process was simulated ing mass was deposited in front of the hill. Based on the based on Coulomb mixture theory (Denlinger and Iver- boundary of mass moving area estimated by the informa- son 2001). This method assumes that grain-fluid flows tion of local residents and by UAV photo, the simulation behave as mixtures limestone layer at the top and the result is similar to the actual phenomena. talus deposit at the toe of slope. The simulation results show that the range of moving mass is similar to the actual phenomena at Khanh waterfall landslide. Discussion Some previous research on landslides in waterfall The rainfall data in Fig.  9b suggests that the global areas indicated that they are often caused by the ero- weather patterns have been rapidly changing in recent sion at the toe of the slope (Hayakawa and Matsukura decades. One of such changes is an extremely large 2010; Hayakawa 2013; Scheingross and Lamb 2017). amount of rainfall in a short period of time that causes The collapse of Khanh waterfall was different to this landslides. This trend appears to be a global issue being phenomena. Khanh waterfall collapsed due to effects of triggered by climate change. In October 2017, the recored cracks and caves in the limestone layer of the slope. A rainfall was 448.8  mm. This rainfall intensity was more large amount of water from stream flowed into cracks than two times higher than the average annual rainfall in and caves in limestone caves. The water in cracks and Do et al. Geoenvironmental Disasters (2022) 9:4 Page 14 of 20 Table 3 Calculation the ratio of basal pore fluid pressure Density ρ kg/m Ratio of volume ν Weight kg (ρ.ν) Solid 2600* 0.6 1560.0 Water 1000 0.4 400.0 Air 1 0.0 0.0 Total weight of landslide mass 1960 Ratio of basal pore fluid pressure (λ) = Water weight/Total weight 0.5 * The density of solid was referred from Konecny et al. (2017) Fig. 15 Simulation model of Khanh waterfall landslide using SfM of aerial photos taken by the UAV and AW3D30 (Japan Aerospace Exploration Agency Earth Observation Research Center 2020). a Contour of slope after collapse from UAV and AW3D30, b Estimated contour of slope before collapse based on result of field survey, c Simply estimated contour of collapse surface, d Simulate the slope surface before collapse by GIS, e Simulate the slope surface after collapse by GIS Fig. 16 Cross-section of slope surface before and after failure D o et al. Geoenvironmental Disasters (2022) 9:4 Page 15 of 20 Fig. 17 Cracks, karsts and crystallized calcite in limestone along the line A–A’. Shale at failure surface at point B and at opposite hill at point A’ caves increased the uplift water force and driving force collapse similar to the phenomena that occured at Khanh causing slope failure. waterfall 1. Regarding the possibility of similar landslides around Another nearby areas are P1 and P2 areas on the right Khanh waterfall in the future, it is necessary to have bank downstream of Khanh waterfall. Since the topogra- a further investigation and assessment of the current phy of the P1 and P2 areas is similar to the old landslide underground karsts around Khanh waterfall. Especially a site, it was feared that landslides would continue to occur remaining part of Khanh waterfall, Khanh waterfall 2 in in the future. However, the survey results show that the Fig. 4, has a similar geology and same water supply chan- geology at areas P1 and P2 is different from the geology nels. There is a high possibility that Khanh 2 waterfall will at Khanh waterfall (Fig. 19). The geology at areas P1 and Do et al. Geoenvironmental Disasters (2022) 9:4 Page 16 of 20 Fig. 18 Geology along line A–A’ (center line A–A’ in Fig. 5) of Khanh waterfall landslide—Outcrop of shale and limestone at the right bank of the Kem River after collapse P2 is mainly limestone interspersed with a thin shale grain-fluid flows behave as mixtures limestone layer layer. In addition, the surface and groundwater in areas and the talus deposit. The simulation results proved P1 and P2 and the Khanh waterfall landslide area are the landslide process from collapse to flow that we considered to have different water supply channels due inferred. to differences in water quality. It suggests that areas P1 3. Due to the wide distribution of limestone, and the and P2 may have a different phenomena compared with development of karsts in it in many parts of the Khanh waterfall landslide. world, further studies on the development of karsts in waterfall areas are needed to avoid similar disas- Conclusion ters as in Khanh waterfall. In this study, we investigated the mechanism of a land- 4. This research did not focus on the climatological slide occurrence by focusing on the topographical and reasons of an abnormal torrent rain leading to the geological background, the relationship between rain- Khanh waterfall disaster. However, the data pre- fall and water flow conditions that caused the Khanh sented in this research show that it could be a recent waterfall landslide, and the flow mechanism of the pattern caused by the climate change and having moving mass. Furthermore, the possibility of recur- global implications. rence of such landslide damage was analyzed. The con - clusions of this study can be summarized as follows: 1. The collapse of Khanh waterfall was different from some previous collapses of waterfalls due to ero- sion at the toe of the slope. Due to the torrent rain continuing for 48  h, a large amount of water flow - ing from the upper stream network channel con- centrated in the Khanh waterfall area. It resulted in a large amount of groundwater flowed in the cracks and karsts in Khanh waterfall slope. The water in cracks and karsts increased the uplift water force and driving force causing slope failure. The increasing of destabilizing forces caused the slope to collapse. The sliding surface was the small fault between the lime- stone layer and the shale layer. 2. The collapse process was simulated based on Cou - lomb mixture theory. This method assumes that Fig. 19 Geology at P1 and P2 areas as shown in Fig. 7 D o et al. Geoenvironmental Disasters (2022) 9:4 Page 17 of 20 Fig. 20 Photos of a, b, c, d position in Fig. 18, a: terraced topography, b: spring water, c: thin shale layer, d: small collapse area near the river Fig. 21 Stiff diagrams Table 4 The results of the water quality analysis + + 2+ 2+ − − 2− Measurement point Measuring date T EC pH Na K Ca Mg alk Cl NO SO SiO 3 4 2 [℃] [mS/m] [mg/L] [mg/L] [mg/L] [mg/L] [meq/L] [mg/L] [mg/L] [mg/L] [mg/L] G005 Spring water 2019/9/10 11:33 25.8 33 7.50 0.8 0.1 66.8 1.98 3.42 0.3 0.7 3.6 7.3 G008 Spring water 2019/9/10 13:16 26.0 35 7.31 1.1 0.8 60.0 8.34 3.47 0.7 0.7 3.3 7.4 G010 Surface water 2019/9/10 14:04 25.5 28 8.03 1.3 1.1 45.6 8.43 2.85 0.5 2.2 4.9 10.5 G011 Spring water 2019/9/10 14:18 25.9 30 8.12 1.2 1.1 47.4 9.29 3.06 0.7 4.8 4.0 9.9 G012 Surface water 2019/9/10 14:33 25.5 28 8.09 1.3 1.0 43.0 8.53 2.8 0.4 3.4 3.9 10.8 Do et al. Geoenvironmental Disasters (2022) 9:4 Page 18 of 20 Fig. 22 Electrical conductivity (EC) and silica concentration (SiO ) Table 5 The characteristics of the upper stream network channel of Khanh waterfall Basic characteristics Unit Value Calculated equation Width of channel (B) m 3.00 Height of channel (H) m 2.00 Left slope of channel (1:m1) 0.50 Right slope of channel (1:m2) 0.50 Roughness coefficient (n) 0.10 Slope gradient (l) % 0.18 Peak flow (Q) m /s 181.5 α 0.80 Cross-sectional area (A) m 8.00 A = H{2B + (m1 + m2)H}/2 2 1/2 2 1/2 Wetted perimeter (P) m 7.47 P = B + H{(1 + m1 ) + (1 + m2 ) } Hydraulic radius (R) m 1.07 R = A/P 3 1/2 2/3 Allowable flow (Qmax) m /s 3.55 Qmax = I .R .A/n Peak flow/allowable flow 51 Q/ Qmax D o et al. Geoenvironmental Disasters (2022) 9:4 Page 19 of 20 Fig. 23 Simulation result of Khanh waterfall landslide with a 5-s interval from initial condition to the end of moving: a at initial condition, b at 5 s, c at 10 s, d at 15 s, e at 20 s, f at 25 s, g at the end 30 s Do et al. Geoenvironmental Disasters (2022) 9:4 Page 20 of 20 Acknowledgements Hayakawa YS, Matsukura Y (2010) Stability analysis of waterfall cliff face at Authors are grateful to Mr. Satoshi Suzuki of Okuyama Boring Co., Ltd, Japan, Niagara Falls: an implication to erosional mechanism of waterfall. Eng Mr. Doan Huy Loi, Mr. Le Hong Luong, Mr. Huynh Thanh Binh, and Mr. Phan Geol 116:178–183 Van Chuong of Institute of Transport Science and Technology, Vietnam, and Hicks B, Gray S, Ball JE (2009) A critical review of the urban rational method. In: Mr. Tran Song Manh for their support during this study. Proceedings of H2009, 32nd hydrology and water resources symposium, engineers Australia, ISBN 978-08258259461 Authors’ contributions Hoek E, Bray J (1974) Rock slope engineering. Institution of Mining and Metal- NHD, SG, SA, KTN, KH and OW visited the landslide site and conducted the lurgy, London field survey. NHD, SA and SG contributed to geology survey and slope stabil- Japan Aerospace Exploration Agency Earth Observation Research Center ity analysis. KTN, TM and KH contributed to mapping. OW, NHD, SG and SA (2020) ALOS Global Digital Surface Model "ALOS World 3D - 30m contributed to water quality analysis, rainfall and flow discharge calculation. (AW3D30)". https:// www. eorc. jaxa. jp/ ALOS/ en/ aw3d30/ index. htm/. KH and SA contributed to numerical simulation of collapse process. NHD, SG Accessed July 2020 and SA wrote the manuscript, edited and finalized the corrections. All authors Khang P (1985) The development of karst landscapes in Vietnam. 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Journal

Geoenvironmental DisastersSpringer Journals

Published: Feb 21, 2022

Keywords: Khanh waterfall landslide; Torrent rain; Limestone; Cave; Numerical simulation

References