Human settlement and infrastructure in mountainous regions are constantly exposed to the risk of natural hazards like, rock avalanches, and debris flows. People in these regions are always challenged to find a balance between potential landslide hazards and spatial development, which makes it a complex scientific problem. The estimation of the flow parameters such as landslide velocity, volume, and depth is important for disaster prevention and mitigation (Crozier and Glade, 2012; Fuchs et al., 2008; Guzzetti, 2000). The estimation of these parameters can be carried out using experimental, numerical, and mathematical or analytical solutions. Massive landslides are usually characterized by high speed, large volume, and massive scale of destruction. It is important to understand and estimate their velocity for mitigation measures, design and construction of protective infrastructures, and population settlement (Christen et al., 2010; Kattel et al., 2018; Pudasaini et al., 2008). Moreover, the impact force, which can induce a chain disaster in terms of airblast also strongly related to the front velocity of the landslides (Evans et al., 2009).
The movement of landslides usually depends upon qualitative and quantitative parameters. Qualitative parameters such as the path materials (Hungr and Evans, 2004; Voight and Sousa, 1994), i.e., glacial ice (De Blasio, 2014; Schneider et al., 2010; Sosio et al., 2012), snow (Boultbee et al., 2006; Deline et al., 2011), sediments (Hungr and Evans, 2004; Sassa and Wang, 2007), bedrock (Whittall et al., 2017), etc. influence the mobility, other than path materials phenomena like, fragmentation, frictional heating, and the presence of water also greatly influence the mobility of landslides. There can be numerous quantitative parameters that contribute to the velocity of landslides such as slope/release angle, particle size, particle density, volume, friction coefficients, energy transfer, etc. Over the years scientists have used various parameters to develop models to understand the dynamics of gravitational mass movements.
Previously, scientists have developed models to predict the velocity of landslides, ranging from empirical to semi-empirical to physical-based models. One of the earliest efforts to predict the velocity can be traced back to the Heim (1932) model and formulation of Heim’s ratio H/L, where H is the elevation difference and L is the horizontal distance. Heim’s ratio described the conservation of energy from potential to kinetic and the effect of frictional force. Heim’s ratio can be interpreted, as the ratio of H/l decreases the volume increases and so does the velocity. Similarly, Scheller (1970) empirically plotted tan (α) against the log of volume obtained a curved correlation, and concluded that the velocity of slide increases as their volume increase, whereas the frictional coefficient decreases as the velocity increases either by exponential square or by hyperbola. Based on these assumptions he tried to establish a correlation but at that time the calculations became quite complex and the formula could not be written in his final closed form as there were a lot of parameters involved. So, there was a need to establish a more simplified closed-form relation to predict the velocity and reach of the landslide. So, Scheidegger (1973) extended the work, and based on his empirical observations developed a logarithmic model that relates friction to the volume of landslides, as shown in Eq. 1.
$${log}\left(f\right)=a{log}\left(V\right)+b$$
1
Where f is the friction factor, V is the volume (m3), a is the empirical constant and b is the intercept. It can be interpreted that the friction coefficient has a direct effect on the volume, runout distance, and velocity of the landslide. Later another empirical relation developed by Scheidegger suggests a direct relationship between velocity and geometric parameters such as drop height and runout distance, Eq. 2.
$${v}_{max}={10}^{{log}\left(H\right)-{log}\left(L\right)+C}$$
2
Whereas Vmax is the maximum velocity, H is the vertical drop (m), L is the horizontal runout distance (m), and C is the empirical constant. Another energy-based model which is also attributed to Scheidegger (Eq. 3) suggests that there exists a relationship between maximum velocity as a function of gravitational acceleration (\(g\)), effective friction(\(\mu )\), height (H), and length (L).
$$v=\sqrt{2g\left(H-\mu L\right)}$$
3
Later, Hungr et al. (1984) developed a simplified dynamic model by refining a simple energy-based model and incorporating frictional and drag forces in landslide dynamics. The proposed model suggests the velocity of landslide reduces due to internal friction and external forces. Voellmy-Salm developed a semi-empirical model that combines empirical observations with theoretical constructs to cater to varying slopes and friction conditions. Similarly, McDougall and Hungr (2005) developed a model to determine the velocity of a landslide by combining velocity-squared drag and frictional forces. The proposed model emphasizes the role of material heterogeneity and terrain roughness. Iverson (1997), proposed a depth-averaged model by incorporating the depth of sliding mass and momentum conservations. Recently, the field of mass movements has seen an increasing trend of numerical modeling for understanding the physical characteristics and dynamics (Cascini et al., 2014; Cuomo et al., 2016; Gardezi et al., 2023, 2022, 2021; McDougall and Hungr, 2005; Mergili et al., 2020) The models discussed till now use various parameters to predict the velocity of gravitational mass movements, but there is no single model that incorporates all the governing quantitative parameters and predicts the velocity of landslides more accurately. Though the numerical models can predict the velocity more accurately, they are quite time-consuming and require a lot of expertise, and computation power.
Since the experimental and numerical models are time-consuming, laborious, and require expertise to evaluate the landslide dynamics, to overcome this problem there is a need for models that require less time, effort, and expertise such as artificial intelligence (AI) based prediction models for a swift guesstimate of landslide characteristics and dynamics. Previously the prediction models used regression analysis but now the use of artificial intelligence (AI) models is encouraged. Many scientists have used AI-based models such as ANN and other models to predict landslide susceptibility worldwide (Chang et al., 2023; Kainthura and Sharma, 2022; Lv et al., 2022; Merghadi et al., 2020; Wang et al., 2019; Y. Wang et al., 2020; Zhang et al., 2022; Z. Zhao et al., 2022). Despite the usage of AI in susceptibility mapping it is hard to find it in predicting the landslide dynamics.
Other than neural network-based models there has been some latest development in genetic programming (GP) based models, i.e., genetic algorithms (GAs) to identify the optimized solution. Genetic programming (GP) was first introduced by (Cramer, 1985) and was later modified and improved by (Koza, 1994). The most advanced and sophisticated models among the existing genetic programming techniques are multi-expression programming (MEP) and genetic expression programming (GEP). Both MEP and GEP are gene-type programming techniques that produce tree-like models. The models behave like living organisms as they can learn and adapt by modifying their size, shape, and composition (Usama et al., 2023). The MEP adopts a demonstrative method for programming and uses linear chromosomes that can determine the best mutation probability, population size, and evolutionary model. MEP is a cutting-edge methodology and advanced form of GP and can produce precise results even when the complexity of the target is not known.
In this study, we have made an effort to predict the frontal velocity of landslides/rock avalanches using physical and machine learning models. The physical modeling was carried out using Discrete Element Models (DEM) whereas the prediction modeling was carried out using multi-expression programming (MEP). The most important input governing parameters that control the velocity of the landslide in DEM, i.e., the density of rock, slope angle, volume, coefficient of restitution, coefficient of sliding friction, and coefficient of rolling friction, were determined from the literature. The density of the slide was kept constant to an average value obtained from 20 rock avalanche cases. Moreover, we have also determined the percentage effect of input parameters on the velocity of landslides which itself is a novel idea.
Once the input parameters were determined it was important to know the range of the parameters being used. In this study, the range of the input parameters was selected from the previous rock avalanche cases around the world especially from China, Table 1, and experimental studies performed to understand the behavior of landslides (Pudasaini et al., 2008; Qing-zhao et al., 2019; Scheidl et al., 2015).
Table 1
Indicates the rock avalanche (RA) cases from where the range of the parameters was taken.
Rock Avalanche | Density (Kg/m3) | Slope angle | Friction coeff. | Reference |
Yigong RA | 2400 | 17 | 0.08 | (Yin and Xing, 2012) |
Nayong RA | 2380 | 20 | 0.58 | (Luo et al., 2020) |
Fuquan RA | 2100 | 23 | 0.1 | (Xing et al., 2016b) |
Gualing, Guizhou RA | 2350 | 16 | 0.1 | (Yin et al., 2011) |
Zhaotong, Touzhai Valley RA | 2675 | 15 | 0.12 | (Xing et al., 2016a) |
Wenjia valley RA | 2000 | 27 | 0.09 | (Zhuang et al., 2019) |
Niumen valley RA | 2650 | 18 | 0.19 | (Xing et al., 2015) |
Nyexon RA | 2600 | 35 | 0.18 | (Cui et al., 2022b) |
Tagarama RA | 2860 | 16 | 0.17 | (Y. F. Wang et al., 2020) |
Luanshibao RA | 2800 | 12 | - | (Wang et al., 2018) |
Sedongpu RA | 2000 | 12 | 0.27 | (C. Zhao et al., 2022) |
Tangjia valley RA | 2300 | 18 | 0.12 | (Li et al., 2017) |
Ganqiuchi RA | 2680 | 21 | - | (Zhou et al., 2020) |
Lymek RA | 2390 | 35 | 0.17 | (Shi et al., 2023) |
Luchedu RA | 2370 | 16 | - | (Cui et al., 2022a) |
Ultar RA | 2500 | 35.8 | 0.25 | (Gardezi et al., 2023) |
Attabad RA | 2100 | 25 | 0.13 | (Gardezi et al., 2021) |
Langtang valley RA | 2600 | 37 | 0.2 | (Zhuang et al., 2023) |
Turnoff creek RA | 2600 | 15.5 | 0.2 | (An et al., 2020) |