2. Materials and Methods
2.1. Study Area
Tamakoshi Rural Municipality is located in Dolakha district of Bagmati Province, central-eastern Nepal, between 27°28′27″–27°36′47″ N and 86°04′22″–86°11′11″ E. The municipality lies within the Lesser Himalayan geological sequence and is located approximately 170 km northeast of Kathmandu. It is accessible through the Lamosangu–Ramechhap Highway and connected internally by rural roads. Elevation ranges from approximately 580 to 2240 m above sea level. The terrain is characterized by steep slopes, ridges, spurs, saddles, deeply incised valleys, alluvial terraces and colluvial deposits. The Tamakoshi River is the major drainage system, supported by several seasonal tributaries forming a dendritic drainage pattern. River undercutting, slope excavation and poor drainage management are major contributors to slope instability in the area.
The municipality is dominated by agricultural land and forest. Cultivated areas are mainly located in the central and southeastern parts, while forested areas are common in the northern and southwestern parts. Landslides occur in agricultural land, forests, grasslands, road-cut sections and river valleys, indicating the combined effect of natural and anthropogenic factors. The study area experiences tropical to temperate climatic conditions depending on elevation. Rainfall is mainly concentrated during the monsoon season, and annual rainfall ranges from about 1600 mm to 2000 mm. The location of the study area is shown in
Figure 1.
Figure 1. Location map of Tamakoshi Rural Municipality.
2.2. Research Design and Methodological Workflow
The study followed a GIS-based comparative framework for landslide susceptibility and relative risk assessment. The workflow included landslide inventory preparation, spatial data collection, conditioning-factor preparation, susceptibility modelling, validation, exposure-vulnerability assessment and relative risk zonation.
First, landslide polygons were identified using Google Earth imagery, satellite image interpretation and field verification. The inventory was divided into training and validation datasets. Ten conditioning factors were prepared and converted into a common raster format. Class-wise landslide and non-landslide pixel statistics were calculated by overlaying the training inventory with each conditioning-factor layer. These statistics were used to derive Frequency Ratio (FR), Shannon Entropy (SE) and Statistical Information Index (SII) weights. The resulting susceptibility maps were validated using Receiver Operating Characteristic-Area Under the Curve (ROC-AUC) analysis. Finally, the best-performing susceptibility output was combined with an AHP-based exposure-vulnerability index to generate the relative landslide risk zonation map. The methodological workflow is presented in
Figure 2.
Figure 2. Methodological workflow for inventory preparation, susceptibility modelling, validation, exposure-vulnerability assessment and relative risk zonation.
2.3. Data Sources and Spatial Preprocessing
Both primary and secondary datasets were used in this study. Primary data included field observations, GPS-based checking of selected landslide locations and documentation of landslide characteristics. Secondary data included Digital Elevation Model (DEM), land use/land cover, geological data, soil type, road network, river network, settlement and population-related datasets. All spatial layers were projected to WGS 1984 UTM Zone 45N. Vector data were converted to raster format and all raster layers were aligned to a common 30 m × 30 m grid, spatial extent and cell origin. Resampling and alignment were completed before overlay analysis to ensure cell-by-cell compatibility.
Table 1. Data sources used in this assessment.
Data layer | Source | Scale or resolution |
Landslide inventory | Google Earth imagery, satellite image interpretation and field survey | Polygon inventory |
DEM | USGS Earth Explorer | 30 m |
Land use and land cover | ICIMOD portal | 30 m |
Geological data | Department of Mines and Geology, Nepal | 1:50,000 |
Landsat-8 imagery | USGS Earth Explorer | 30 m |
Road network | Google Earth imagery and GIS digitization | Vector data |
River network | Humanitarian Data Exchange and hydrological sources | Vector data |
Soil type | FAO land resources database | Thematic data |
Settlement data | Google Earth imagery and field verification | Vector data |
Population distribution | Available population dataset | Raster or administrative data |
2.4. Landslide Inventory Preparation
A landslide inventory is the fundamental input for data-driven landslide susceptibility modelling because it records the spatial distribution of previous slope failures
| [9] | Dhital MR (2015) Geology of the Nepal Himalaya: Regional perspective of the classic collided orogen. Springer, Cham, 498 pp. https://doi.org/10.1007/978-3-319-02496-7 |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
[9, 15]
. In this study, landslides were identified using Google Earth imagery, satellite image interpretation and field verification. Diagnostic features such as fresh scars, exposed soil or rock, disrupted drainage, displaced road sections, accumulation zones and surface-texture changes were used for landslide identification.
A total of 121 landslide locations were mapped. These included rockfalls, debris slides, shallow translational slides, creep movements and active or reactivated slope failures. Field verification confirmed several instability indicators, including crown cracks, tension cracks, seepage, slope bulging, road deformation and damage to agricultural land and retaining structures. The landslide inventory was divided into training and validation datasets. The training dataset consisted of 85 landslide locations, representing 70% of the total inventory, and contained 938 landslide raster cells. The remaining 36 landslide locations, representing 30% of the inventory, were used for independent validation. The landslide inventory map is shown in
Figure 3.
Figure 3. Landslide inventory of Tamakoshi Rural Municipality prepared from image interpretation and field verification.
2.5. Landslide-Conditioning Factors
Ten landslide-conditioning factors were selected based on the geomorphic setting, field evidence, data availability and previous landslide susceptibility studies
| [14] | Lee S, Talib JA (2005) Probabilistic landslide susceptibility and factor effect analysis. Environmental Geology 47: 982–990.
https://doi.org/10.1007/s00254-005-1228-z |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
[14, 15]
. These factors include slope, aspect, curvature, elevation, Topographic Wetness Index (TWI), geology/lithology, soil type, land use/land cover, distance from roads and distance from rivers.
Table 2. Landslide conditioning factors used in the study.
Factor group | Conditioning factor | Data basis | Relevance to landslide occurrence |
Topographical | Slope | DEM | Controls shear stress, runoff and slope stability |
Topographical | Aspect | DEM | Influences moisture, solar radiation and rainfall exposure |
Topographical | Curvature | DEM | Controls runoff convergence and slope morphology |
Topographical | Elevation | DEM | Represents relief, terrain and climatic variation |
Hydrological | TWI | DEM-derived index | Represents potential soil moisture accumulation |
Geological | Geology/lithology | Geological map | Represents rock type, strength and weathering |
Environmental | Soil type | Soil database | Represents soil material and infiltration condition |
Environmental | Land use and land cover | ICIMOD land cover data | Represents vegetation, cultivation and disturbance |
Anthropogenic | Distance from road | Road network | Represents slope cutting and drainage disturbance |
Geomorphic | Distance from river | River network | Represents toe erosion and river undercutting |
The factor maps used in the susceptibility modelling are shown in
Figures 4–13.
Figure 4. Geological map of the study area.
Figure 5. Soil type map of the study area.
Figure 6. Slope map of the study area.
Figure 7. Elevation map of the study area.
Figure 8. Aspect map of the study area.
Figure 9. Curvature map of the study area.
Figure 10. Topographic Wetness Index map of the study area.
Figure 11. Land use and land cover map of the study area.
Figure 12. Distance from road map of the study area.
Figure 13. Distance from river map of the study area.
2.6. Preparation and Classification of Conditioning Factors
All conditioning factors were prepared in a raster-based GIS environment using the same projection, spatial extent, cell size and raster alignment. Slope, aspect, curvature, elevation and TWI were derived from the DEM. Distance rasters were generated from the road and river networks using Euclidean distance analysis. Geology, soil type and land use/land cover were converted into categorical raster layers. Continuous variables were divided into classes based on terrain characteristics and the original classification used in the analysis. Categorical variables retained their thematic classes. Each classified factor layer was overlaid with the training landslide inventory to calculate total class pixels and landslide pixels. These values were used to compute model-specific weights.
The TWI was calculated as:
where Aₛ is the specific upslope contributing area per unit contour length, ln is natural logarithm and β is the local slope angle expressed in radians. Higher TWI values represent greater potential for moisture accumulation, whereas lower values generally represent steeper or better-drained terrain
| [20] | Roodposhti MS, Aryal J, Shahabi H, Safarrad T (2016) Fuzzy Shannon entropy: A hybrid GIS-based landslide susceptibility mapping method. Entropy 18(10): 343.
https://doi.org/10.3390/e18100343 |
[20]
. For the mathematical notation used below, i denotes a class within a conditioning factor, j denotes a conditioning factor, m
j is the number of classes within factor j, and n is the total number of conditioning factors. In this study, n = 10.
2.7. Frequency Ratio Model
The frequency ratio (FR) model is a bivariate statistical method that measures the spatial association between historical landslides and individual classes of conditioning factors. It assumes that future landslides are more likely to occur under terrain and environmental conditions similar to those associated with previous landslides
| [11] | Fell R, Corominas J, Bonnard C, Cascini L, Leroi E, Savage WZ (2008) Guidelines for landslide susceptibility, hazard and risk zoning for land-use planning. Engineering Geology 102(3–4): 85–98. https://doi.org/10.1016/j.enggeo.2008.03.022 |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
[11, 15]
.
The FR value for class i of factor j was calculated as:
FRij= (Lij/ L) / (Aij/ A)(2)
where FRij is the frequency ratio of class i of factor j, Lij is the number of training-landslide cells within that class, L is the total number of training-landslide cells, Aij is the total number of study-area cells within that class, and A is the total number of valid raster cells in the study area. FRij > 1 indicates a positive association, FRij = 1 indicates an average association, and FRij < 1 indicates a comparatively weak association. Each conditioning-factor raster was reclassified using its calculated class-specific FR values. The FR-based landslide susceptibility index was then calculated using pixel-wise summation:
where LSI_FR is the frequency ratio-based landslide susceptibility index and FRij is the weight of the class occupied by the corresponding raster cell in factor j. Higher LSI_FR values indicate greater relative landslide susceptibility.
2.8. Shannon Entropy Model
The Shannon entropy (SE) model applies information theory to measure the degree of disorder, uncertainty and information content associated with the classes of each conditioning factor
| [12] | Guzzetti F, Carrara A, Cardinali M, Reichenbach P (1999) Landslide hazard evaluation: A review of current techniques and their application in a multi-scale study, central Italy. Geomorphology 31(1–4): 181–216.
https://doi.org/10.1016/S0169-555X(99)00078-1 |
| [13] | Jenks GF (1967) The data model concept in statistical mapping. International Yearbook of Cartography 7: 186–190. |
| [21] | Saaty TL (1977) A scaling method for priorities in hierarchical structures. Journal of Mathematical Psychology 15(3): 234–281. https://doi.org/10.1016/0022-2496(77)90033-5 |
[12, 13, 21]
. Factors showing greater contrast among class-level associations with landslides provide more information and consequently receive greater weights.
First, the FR values of the classes within each factor were normalized as:
where Pij is the normalized probability of class i within factor j and mj is the number of classes in factor j. For every factor, Σi=1mʲ Pij = 1.
The entropy value of factor j was calculated as:
Ej= −kjΣi=1mʲ[Pijln(Pij)](5)
where Ej is the entropy of factor j and kj is a scaling constant defined as:
The scaling constant constrains Ej to the interval from 0 to 1. Where Pij = 0, the term Pij ln(Pij) was treated as 0 according to its limiting value.
The information coefficient or degree of divergence was calculated as:
A factor with a larger Ij value shows greater differentiation among its classes and therefore provides more information regarding landslide occurrence. The normalized weight of factor j was calculated as:
where Wj is the normalized Shannon entropy weight and Σj=1ⁿ Wj = 1. The final SE-based landslide susceptibility index was calculated as:
LSI_SE = Σj=1ⁿ (Wj× FRij)(9)
where LSI_SE is the Shannon entropy-based susceptibility index. Higher LSI_SE values indicate greater relative susceptibility.
2.9. Statistical Information Index Model
The statistical information index (SII), also known as the information-value method, estimates the contribution of each factor class by comparing its landslide density with the overall landslide density of the study area
| [14] | Lee S, Talib JA (2005) Probabilistic landslide susceptibility and factor effect analysis. Environmental Geology 47: 982–990.
https://doi.org/10.1007/s00254-005-1228-z |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
| [22] | Saaty TL (1980) The analytic hierarchy process: Planning, priority setting, resource allocation. McGraw-Hill, New York, 287 pp. |
[14, 15, 22]
.
The SII weight for class i of factor j was calculated as:
Wij= ln[(Lij/ Aij) / (L / A)](10)
where Wij is the information weight of class i of factor j, Lij/Aij is the landslide density within that class, and L/A is the overall landslide density of the study area. Positive values indicate above-average landslide density, negative values indicate below-average density, and values close to 0 indicate density approximately equal to the map average. The SII-based landslide susceptibility index was calculated through pixel-wise summation of the relevant class weights:
where LSI_SII is the statistical information index-based susceptibility index. Higher values indicate stronger combined association with landslide occurrence. When a class contained no training-landslide cell, L
ij = 0 and the logarithmic expression was undefined. Following the procedure adopted in the original analysis, such a class was assigned the minimum finite SII weight calculated among the remaining classes of the same conditioning factor. This rule was applied consistently to all zero-landslide classes before raster summation
| [22] | Saaty TL (1980) The analytic hierarchy process: Planning, priority setting, resource allocation. McGraw-Hill, New York, 287 pp. |
[22]
.
2.10. Classification of Susceptibility Maps
The continuous susceptibility indices generated by FR, SE and SII were independently reclassified into five relative susceptibility classes: very low, low, moderate, high and very high. The Natural Breaks (Jenks) method was used for classification
| [23] | Saha AK, Gupta RP, Sarkar I, Arora MK, Csaplovics E (2005) An approach for GIS-based statistical landslide susceptibility zonation with a case study in the Himalayas. Landslides 2: 61–69. |
[23]
. It identifies break points that minimize within-class variation and maximize between-class variation. Because FR, SE and SII produced different index ranges and distributions, Jenks break values were calculated separately for each model. Therefore, identically named classes across the three models represent relative positions within each model rather than identical numerical thresholds. The resulting maps are presented in
Figures 14-16.
2.11. Model Validation Using ROC-AUC
The discriminatory performance of the susceptibility models was evaluated using receiver operating characteristic curves and the area under the ROC curve. The training subset was used to calculate success-rate AUC, whereas the independent validation subset was used to calculate prediction-rate AUC
. For threshold-based ROC analysis, landslide cells were treated as positive observations and non-landslide cells used in the ROC procedure were treated as negative observations. At each susceptibility-index threshold, the true-positive rate or sensitivity was calculated as:
TPR = Sensitivity = TP / (TP + FN)(12)
where TP is the number of landslide cells correctly classified above the threshold and FN is the number of landslide cells incorrectly classified below the threshold. The true-negative rate or specificity was calculated as:
TNR = Specificity = TN / (TN + FP)(13)
where TN is the number of non-landslide cells correctly classified below the threshold and FP is the number of non-landslide cells incorrectly classified above the threshold. The false-positive rate was calculated as:
FPR = FP / (FP + TN) = 1 − Specificity(14)
The ROC curve was constructed by plotting TPR against FPR over the range of susceptibility thresholds. Conceptually, the AUC is expressed as:
where u represents the false-positive rate. An AUC of 0.5 indicates discrimination equivalent to random ranking, whereas values increasingly above 0.5 indicate progressively better separation. The calculated success AUC values were 0.645 for FR, 0.634 for SE and 0.654 for SII. The corresponding prediction AUC values were 0.623, 0.613 and 0.628. These results indicate modest discriminatory performance, with SII marginally outperforming FR and SE. The ROC curves are presented in
Figures 17-19.
2.12. Exposure-Vulnerability Assessment
The available datasets represented settlements, population concentration, roads and public-service locations but did not contain detailed household-level social vulnerability, building fragility, income, age, disability or coping-capacity information. The derived layer was therefore interpreted as an exposure-vulnerability index rather than a complete social-vulnerability assessment. Six indicators were included: settlement proximity, population density, hospital proximity, school proximity, road proximity and temple proximity. Distance-based layers were reclassified to a common ordinal scale in which locations closer to mapped settlements and facilities received higher exposure scores. Higher population-density classes also received higher scores. The analytical hierarchy process was used to derive relative indicator weights through pairwise comparison using Saaty’s 1–9 importance scale
| [26] | Soeters R, van Westen CJ (1996) Slope instability recognition, analysis and zonation. In: Turner AK, Schuster RL (Eds) Landslides: Investigation and mitigation. Transportation Research Board Special Report 247. National Academy Press, Washington DC, pp 129–177. |
| [27] | Upreti BN, Dhital MR (1996) Landslide studies and management in Nepal. International Centre for Integrated Mountain Development, Kathmandu, 87 pp. |
[26, 27]
. The pairwise comparison matrix was represented as A = [a
kₗ], where a
kₗ is the relative importance of indicator k compared with indicator l, a
kk = 1 and aₗ
k = 1/a
kₗ. The priority vector was derived from the principal eigenvector:
where A is the pairwise comparison matrix, v is its principal eigenvector and λmax is the maximum eigenvalue. The normalized indicator weights were calculated as:
where wk is the normalized weight of indicator k, p is the number of indicators and Σk=1p wk = 1. Matrix consistency was evaluated using:
CI = (λmax − p) / (p − 1)(18)
where CI is the consistency index and RI is the random consistency index. For six indicators, RI = 1.25
| [27] | Upreti BN, Dhital MR (1996) Landslide studies and management in Nepal. International Centre for Integrated Mountain Development, Kathmandu, 87 pp. |
[27]
. The calculated CR was 0.0898, below the commonly adopted threshold of 0.10, indicating an adequately consistent comparison matrix.
Table 3. AHP weights for exposure-vulnerability indicators.
Vulnerability indicator | Weight (%) | Interpretation |
Settlement proximity | 36.03 | Direct exposure of people and houses |
Population density | 26.44 | Concentration of exposed population |
Hospital proximity | 15.58 | Access to health service during disaster |
School proximity | 10.73 | Exposure of educational institutions |
Road proximity | 6.98 | Access and road-related exposure |
Temple proximity | 4.24 | Exposure of cultural and social sites |
The exposure-vulnerability index was calculated using weighted linear combination:
where EV is the exposure-vulnerability index, w
k is the normalized AHP weight of indicator k, x
k is the reclassified score of that indicator at a raster cell, and p = 6. The resulting continuous EV raster was classified into very low, low, moderate, high and very high classes using the Natural Breaks (Jenks) method. The map is presented in
Figure 20.
2.13. Relative Landslide Risk Zonation
The SII susceptibility output was selected for final integration because it produced the highest prediction AUC, although its improvement over FR and SE was small. Because the susceptibility model does not include temporal probability, landslide magnitude or expected loss, the final output should be interpreted as a relative landslide risk map rather than an absolute probabilistic risk map because it integrates landslide susceptibility with exposure-vulnerability indicators and does not explicitly incorporate temporal probability or expected loss
| [7] | Dahal RK, Hasegawa S (2008) Representative rainfall thresholds for landslides in the Nepal Himalaya. Geomorphology 100(3–4): 429–443.
https://doi.org/10.1016/j.geomorph.2008.01.014 |
| [11] | Fell R, Corominas J, Bonnard C, Cascini L, Leroi E, Savage WZ (2008) Guidelines for landslide susceptibility, hazard and risk zoning for land-use planning. Engineering Geology 102(3–4): 85–98. https://doi.org/10.1016/j.enggeo.2008.03.022 |
| [29] | van Westen CJ, Castellanos E, Kuriakose SL (2008) Spatial data for landslide susceptibility, hazard, and vulnerability assessment: An overview. Engineering Geology 102(3–4): 112–131. https://doi.org/10.1016/j.enggeo.2008.03.010 |
[7, 11, 29]
. The SII raster was used as a spatial hazard proxy and integrated with the exposure-vulnerability raster.
Before integration, the SII susceptibility index was normalized using min–max scaling:
Sn= (S − Smin) / (Smax− Smin)(21)
where Sn is the normalized susceptibility value, S is the original SII susceptibility value, and Smin and Smax are the minimum and maximum values of the SII raster. The exposure-vulnerability index was similarly normalized as:
EVn= (EV − EVmin) / (EVmax− EVmin)(22)
where EVn is the normalized exposure-vulnerability value, EV is the original index value, and EVmin and EVmax are its minimum and maximum values. Both normalized rasters ranged from 0 to 1. The relative landslide risk index was calculated using a multiplicative model:
where Rrel is the relative landslide risk index. A high value indicates the spatial coincidence of high susceptibility and high exposure-vulnerability, whereas a low value indicates low susceptibility, low exposure-vulnerability or both. The continuous raster was classified into five classes using Natural Breaks (Jenks). The output represents relative spatial variation within the municipality and should not be interpreted as annual landslide probability, expected casualties or monetary loss. The map is presented in
Figure 21.
Google Earth and satellite imagery were used for inventory interpretation. ArcGIS was used for projection, DEM processing, rasterization, resampling, distance analysis, overlay, tabulate-area calculations, raster algebra and map production. Microsoft Excel was used for class-wise statistical calculation, AHP processing and tabulation. Reproducibility was supported by applying a common projection, cell size, spatial extent and alignment to all layers. The same training inventory and class definitions were used when calculating weights for the three models. Intermediate class counts, weights and model outputs should be retained as supplementary data or deposited in a repository when the manuscript is submitted.
3. Results
3.1. Landslide Inventory
A total of 121 landslide locations were identified. The inventory comprised rockfall, debris slide, shallow translational slide, creep and other active or reactivated failures. Field evidence included cracks, seepage, slope bulging, displaced road sections and damage to agricultural land and retaining structures. The training subset contained 85 locations and 938 landslide cells, while the independent validation subset contained 36 locations. Many mapped failures were associated with steep slopes, road cuts, fractured rock, colluvial deposits, river valleys and poorly managed drainage. Representative field-verified examples included the Majhi Gau rockfall and landslide, Tallo Majhi Gau landslide, Narayani Ma. Bhi. instability and Yekle Bari creep.
3.2. Relationship Between Landslides and Conditioning Factors
Slope displayed a positive association with landslide occurrence, and the class above 45° had a high FR value. Landslides were concentrated within the mid-elevation interval of approximately 1351–1680 m, where steep terrain, cultivated slopes, settlements and roads commonly overlap. Aspect influenced distribution, although failures occurred across several orientations. Concave and planar terrain contained comparatively high landslide concentrations, consistent with runoff and moisture convergence in concave positions. River proximity showed a clear relationship: many landslides occurred within 250 m of a river, reflecting toe erosion and valley-side undercutting. Road proximity was similarly important, with many failures located within 100 m of roads, indicating the effects of excavation, steepened cut slopes and altered drainage. Grassland showed a high class-level association in the FR analysis, while forest and cultivated land contained many mapped landslides because of their broad spatial extent and overlap with steep terrain. TWI displayed a variable relationship, indicating that moisture accumulation must be interpreted together with slope, soil and drainage conditions.
3.3. Landslide Susceptibility Maps
All three models produced five susceptibility classes. The FR model classified the largest area as moderate susceptibility. The SE model produced a broader distribution of high and very high classes, whereas SII produced more spatially focused high-susceptibility zones. The three outputs are presented in
Figures 14-16.
Figure 14. Landslide susceptibility map produced using the frequency ratio model.
Figure 15. Landslide susceptibility map produced using the Shannon entropy model.
Figure 16. Landslide susceptibility map produced using the statistical information index model.
Table 4. Susceptibility-class area distribution.
Susceptibility class | FR area (%) | SE area (%) | SII area (%) |
Very low | 5.71 | 7.49 | 5.02 |
Low | 26.40 | 10.74 | 15.37 |
Moderate | 40.99 | 27.00 | 50.05 |
High | 22.17 | 36.35 | 27.29 |
Very high | 4.73 | 18.42 | 2.27 |
High and very high susceptibility together accounted for 26.90% in FR, 54.77% in SE and 29.56% in SII. The SE model therefore generated the most precautionary spatial classification, while SII concentrated its highest classes into smaller areas.
3.4. Model Validation
SII produced the highest success and prediction AUC values, followed by FR and SE. The differences among prediction AUC values were small: SII exceeded FR by 0.005 and SE by 0.015. SII therefore marginally outperformed the other models rather than demonstrating substantially superior predictive capability. The ROC curves are presented in
Figures 17-19.
Table 5. ROC-AUC validation results.
Model | Success AUC | Prediction AUC | Performance interpretation |
Frequency ratio | 0.645 | 0.623 | Modest discrimination |
Shannon entropy | 0.634 | 0.613 | Modest discrimination |
Statistical information index | 0.654 | 0.628 | Highest, but still modest |
Figure 17. ROC-AUC curve of the frequency ratio model.
Figure 18. ROC-AUC curve of the Shannon entropy model.
Figure 19. ROC-AUC curve of the statistical information index model.
3.5. Exposure–Vulnerability Pattern
Settlement proximity received the highest AHP weight, followed by population density, hospital proximity, school proximity, road proximity and temple proximity. The CR of 0.0898 indicated an internally consistent comparison matrix. High and very high exposure-vulnerability zones were concentrated around Atmara, Sungure, Dumsi Bhanjhyang, Gahateri and Gobintar. Kiranetar, Gairigaun, Bhatere and parts of the central municipality were mainly within moderate or low classes. Northern and extreme southern areas generally had low to very low index values. The spatial pattern is shown in
Figure 20.
Figure 20. AHP-based exposure-vulnerability index for Tamakoshi Rural Municipality.
3.6. Relative Risk Zonation
The relative risk map identifies locations where high SII susceptibility overlaps with high exposure-vulnerability. High and very high classes were concentrated mainly in the southern and north-western parts of wards 1, 3 and 5, including areas near Sungure, Archale, Biruwa, Atmara and Kiranetar. Moderate classes surrounded many of the higher-risk zones and represented transitional terrain. Low and very low classes were concentrated mainly in north-eastern, south-western and central parts of wards 4, 6 and 7, including Lamatar, Raamata, Gahateri, Himalpani, Sajhgaira and Gairigaon. The final map is presented in
Figure 21.
Figure 21. Relative landslide risk zonation derived from normalized SII susceptibility and exposure-vulnerability indices.
Table 6. Relative risk-class distribution.
Risk class | Approximate area coverage (%) | Main spatial occurrence |
High to very high | 15 | Wards 1, 3 and 5, especially near Sungure, Archale, Biruwa, Atmara and Kiranetar |
Moderate | 21 | Transitional areas surrounding high-risk zones |
Low to very low | 63 | Mainly wards 4, 6 and 7 and relatively stable central, north-eastern and south-western areas |
4. Discussion
4.1. Controls of Landslide Occurrence
The results show that landslide occurrence in Tamakoshi Rural Municipality is mainly controlled by the combined influence of of slope, elevation, aspect, curvature, lithology, land use, river proximity, road proximity and local hydrological conditions. The concentration of landslides on moderate to steep slopes confirms the importance of terrain steepness in slope instability. Although the slope class above 45° showed a strong class-level association, most landslide pixels were recorded in the 25°–35° class because this class covers a larger portion of the study area and commonly overlaps with roads, agricultural land and settlements. This indicates that slope does not act alone; landslide occurrence is the result of interaction among terrain condition, material property, drainage and human disturbance
| [5] | Bhandari BP, Dhakal S, Tsou CY (2024) Assessing the prediction accuracy of frequency ratio, weight of evidence, Shannon entropy, and information value methods for landslide susceptibility in the Siwalik Hills of Nepal. Sustainability 16(5): 2092.
https://doi.org/10.3390/su16052092 |
| [6] | Chung CJF, Fabbri AG (2003) Validation of spatial prediction models for landslide hazard mapping. Natural Hazards 30(3): 451–472.
https://doi.org/10.1023/B:NHAZ.0000007172.62651.2b |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
[5, 6, 15]
.
Elevation also influenced landslide distribution. The highest concentration of landslides occurred within the 1351–1680 m elevation range, where weathered slopes, agricultural terraces, road sections and settlement areas commonly overlap. Aspect showed higher landslide concentration on south, southwest, west and northwest-facing slopes. This pattern may be related to rainfall exposure, solar radiation, moisture variation and weathering intensity. Similarly, curvature influenced landslide occurrence because concave and planar slopes can support runoff concentration, water retention and translational movement.
River proximity showed a clear relationship with landslide occurrence. Many landslides were concentrated within 250 m of rivers and streams, indicating the role of toe erosion, bank cutting and slope undercutting. Road proximity was also important, with many landslides located within 100 m of road corridors. Road construction in hilly terrain often disturbs natural slope geometry, removes toe support and changes drainage pathways, thereby increasing the likelihood of slope failure during rainfall events
| [5] | Bhandari BP, Dhakal S, Tsou CY (2024) Assessing the prediction accuracy of frequency ratio, weight of evidence, Shannon entropy, and information value methods for landslide susceptibility in the Siwalik Hills of Nepal. Sustainability 16(5): 2092.
https://doi.org/10.3390/su16052092 |
| [6] | Chung CJF, Fabbri AG (2003) Validation of spatial prediction models for landslide hazard mapping. Natural Hazards 30(3): 451–472.
https://doi.org/10.1023/B:NHAZ.0000007172.62651.2b |
[5, 6]
.
The land use and land cover result should be interpreted together with slope and geology. Forest and cultivated land contained many landslides because they cover large parts of the municipality and often occur on steep terrain. Although vegetation generally helps stabilize shallow soil, forest cover may not prevent failures where slopes are controlled by fractured rock, deep weathering, river erosion or poor drainage. Cultivated terraces may also become unstable where drainage is not properly managed.
4.2. Comparison of FR, SE and SII Models
The three susceptibility models produced different spatial patterns because they use different weighting procedures. The Frequency Ratio (FR) model is simple and directly represents the relationship between landslide occurrence and each conditioning-factor class. Because of its transparency and ease of interpretation, the FR model has been widely applied and compared with other statistical and machine-learning approaches in landslide susceptibility studies
| [5] | Bhandari BP, Dhakal S, Tsou CY (2024) Assessing the prediction accuracy of frequency ratio, weight of evidence, Shannon entropy, and information value methods for landslide susceptibility in the Siwalik Hills of Nepal. Sustainability 16(5): 2092.
https://doi.org/10.3390/su16052092 |
| [18] | Regmi AD, Devkota KC, Yoshida K, Pradhan B, Pourghasemi HR, Kumamoto T, Akgun A (2014) Application of frequency ratio, statistical index, and weights-of-evidence models and their comparison in landslide susceptibility mapping in Central Nepal Himalaya. Arabian Journal of Geosciences 7(2): 725–742. https://doi.org/10.1007/s12517-012-0807-z |
| [30] | Yilmaz I (2009) Landslide susceptibility mapping using frequency ratio, logistic regression, artificial neural networks and their comparison: A case study from Kat landslides, Tokat, Turkey. Computers and Geosciences 35(6): 1125–1138.
https://doi.org/10.1016/j.cageo.2008.08.007 |
[5, 18, 30]
. The Shannon Entropy (SE) model produced the largest high and very high susceptibility area, indicating a more precautionary classification. The Statistical Information Index (SII) model produced the highest AUC values and a more focused distribution of high-susceptibility zones. SII assigns both positive and negative information values, which helps distinguish classes with above-average and below-average landslide density
| [14] | Lee S, Talib JA (2005) Probabilistic landslide susceptibility and factor effect analysis. Environmental Geology 47: 982–990.
https://doi.org/10.1007/s00254-005-1228-z |
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
| [22] | Saaty TL (1980) The analytic hierarchy process: Planning, priority setting, resource allocation. McGraw-Hill, New York, 287 pp. |
[14, 15, 22]
. The validation results showed that SII performed slightly better than FR and SE. The success and prediction AUC values were 0.654 and 0.628 for SII, 0.645 and 0.623 for FR, and 0.634 and 0.613 for SE, respectively. However, the differences among the three models were small. Therefore, SII can be considered the most suitable model for this study area, but not overwhelmingly superior. The modest AUC values indicate that landslide occurrence in the study area is controlled by complex local conditions that are not fully captured by bivariate statistical models.
The results are consistent with previous landslide susceptibility studies in Nepal and similar Himalayan terrain, which show that bivariate statistical models can provide useful susceptibility maps in data-limited areas
| [15] | Ministry of Home Affairs (2019) Nepal Disaster Report 2019. Government of Nepal, Kathmandu. |
| [16] | Pathak D (2016) Knowledge-based landslide susceptibility mapping in the Himalayas. Geoenvironmental Disasters 3: 8.
https://doi.org/10.1186/s40677-016-0042-0 |
| [17] | Pourghasemi HR, Mohammady M, Pradhan B (2012) Landslide susceptibility mapping using index of entropy and conditional probability models in GIS: Safarood Basin, Iran. Catena 97: 71–84. https://doi.org/10.1016/j.catena.2012.05.005 |
| [18] | Regmi AD, Devkota KC, Yoshida K, Pradhan B, Pourghasemi HR, Kumamoto T, Akgun A (2014) Application of frequency ratio, statistical index, and weights-of-evidence models and their comparison in landslide susceptibility mapping in Central Nepal Himalaya. Arabian Journal of Geosciences 7(2): 725–742. https://doi.org/10.1007/s12517-012-0807-z |
| [19] | Reichenbach P, Rossi M, Malamud BD, Mihir M, Guzzetti F (2018) A review of statistically based landslide susceptibility models. Earth-Science Reviews 180: 60–91.
https://doi.org/10.1016/j.earscirev.2018.03.001 |
[15-19]
. However, the relative performance of each model varies from place to place depending on landslide inventory quality, conditioning-factor selection, factor classification, spatial resolution and geomorphic setting. This confirms the need for site-specific model comparison before selecting a final susceptibility map.
4.3. Relative Risk Zonation and Planning Implications
The relative risk zonation prepared in this study provides useful information for local planning and disaster risk reduction. High and very high-risk zones cover about 15% of the municipality and are mainly concentrated in wards 1, 3 and 5, particularly around Sungure, Archale, Biruwa, Atmara and Kiranetar. These areas should be prioritized for detailed field investigation, slope monitoring, road drainage improvement and landslide mitigation planning. Moderate-risk zones should also be considered carefully during infrastructure development because future road expansion or drainage alteration may increase instability. Low and very low risk zones are relatively more stable within the municipal context, especially in parts of wards 4, 6 and 7. However, these areas should not be considered completely free from landslide hazard. The generated map should be used as a regional planning and prioritization tool rather than a replacement for detailed geotechnical investigation before construction.
4.4. Limitations
The study has some limitations. The landslide inventory may not include all old, small or vegetation-covered landslides. The use of 30 m resolution datasets may limit the detection of small slope failures and narrow road-cut landslides. Rainfall intensity, antecedent moisture, soil depth and detailed geotechnical parameters were not included due to data limitations. Similarly, the vulnerability assessment was based on available exposure indicators and did not include detailed household-level socio-economic vulnerability. Therefore, the final output should be interpreted as a relative landslide risk zonation map rather than an absolute probabilistic risk map. Despite these limitations, the study provides a transparent and reproducible GIS-based approach for comparing FR, SE and SII models and integrating susceptibility with exposure-vulnerability information. The results can support municipal-level land-use planning, road management, field prioritization and disaster risk reduction in Tamakoshi Rural Municipality.