In generally landslide occurred in relatively high elevated area. This research attempts to explore the land characteristics of previously occurred landslide area and produce a LSM according to the result. Findings show that the major surface geology that responsible for landslide occurrence is Dihing and Dupitila Formation Undivided and Boka Bil Formation. 80% landslide occurred in his two geological formations where Boka Bil formation alone is responsible for 47% landslide (Table 4 and Fig. 3). Distance from fault line has major role in landslide occurrence and 93% of the landslide occurred within a distance of 11000 meter from fault line where 33% within a distance of 5500 meter (Table 4 and Fig. 4). Twelve out of fifteen landslide event occurred where slope angle is between 3.71 and 16.85 degree and nine of them occurred where slope is below 12 degree (Fig. 6.B and Tables 3 and 4). Fourteen landslides occurred where normalized difference clay index is ranged medium (1.33–1.44) to high (1.44–1.52) (Fig. 5.C and Tables 3 and 4). This indicates huge saturated clay content flows according to low slope that can be regarded as mudflow. Low to medium bare soil index and medium to high normalized difference vegetation index shows similar result (Fig. 5.A and 5.B and Tables 3 and 4). Though aspect plays major role in weathering material for occurring landslide, in this area it is oriented according to water body (Kaptai Lake and Karnafuli river). In Rangamati, slope aspects that have similar orientation with fault line have most correlation with landslide occurrence. So that north, northeast, south and southwest slope aspect has major correlation with landslide occurrence.
Table 3
Results of various indices in the land slide occurrence site
Location | Year | NDVI | BSI | NDCI | Slope | Aspect |
Kaptai-Chondroghona main road | 1968 | 0.41–0.48 | -0.19 - -0.14 | 1.44–1.52 | 12.57–16.85 | SW |
Raozan-Ghagra-Rangamati main road | 1970 | 0.27–0.41 | -0.19 - -0.14 | 1.44–1.52 | 21.13–25.71 | NW |
Naniarchar | 2018 | 0.48–0.65 | -0.39 - -0.22 | 1.44–1.52 | 12.57–16.85 | NNW |
KPM | 2019 | 0.27–0.41 | -0.19 - -0.14 | 1.33–1.44 | 25.71–31.13 | NE |
Chandraghona | 2019 | 0.41–0.48 | -0.19 - -0.14 | 1.33–1.44 | 8.28–12.57 | SW |
Raikhali union | 2019 | 0.41–0.48 | -0.22 - -0.19 | 1.44–1.52 | 16.85–21.14 | NE |
Burighat | 2018 | 0.41–0.48 | 0.22 − 0.19 | 1.44–1.52 | 3.71–8.28 | S |
Vedvedi | 2017 | 0.27–0.41 | -0.19 - -0.14 | 1.44–1.52 | 8.28–12.57 | NE |
Reserve Bazar | 2008,2017,2018 | 0.27–0.41 | -0.19 - -0.14 | 1.33–1.44 | 3.71–8.28 | N |
Juraichhari | 2017,2018 | 0.27–0.41 | -0.19 - -0.14 | 1.33–1.44 | 8.28–12.57 | N |
Bilaichhari | 2017 | 0.41–0.48 | -0.22 - -0.19 | 1.52–1.80 | 8.28–12.57 | W |
Kawkhali | 2017 | 0.27–0.41 | -0.19 - -0.14 | 1.33–1.44 | 12.57–16.85 | SW |
(Source: Calculated by Author, 2020) |
Surface Geology
Soils of Rangamati hills are Brown Hill soils occupy gentle to very steep slopes which have been developed over consolidated or unconsolidated rocks. Generally the subsoils are yellow to strong brown, friable, porous, sandy loam to sandy or silty clay loam, very strongly to extremely acidic (FAO 1986, 1988; UNDP 1986; Brammer 1996). Chattagram Hill Tracts region is dominated by two major Geological Group-Formations: Surma Group and Tipam Group. Surma Group has two Formations: Bhuban Formation (Sandstone dominates this formation) and Boka Bil Formation (generally shale and siltstone dominate the lower and the upper parts while in the middle part the sandstone percentage is much higher). Tipam Group is dominated by Tipam Sandstone Formation and the Girujan Clay (Chowdhury, Khan and Uddin 2015). The geological formation of hills in those areas, although not very young, is still in the process of degradation and reformation through weathering or other natural processes. Chandraghona and Kaptai area contains sedimentary rocks comprising dominantly of sandstone, shale and siltstone (Rahman, Mohiuddin and Hassan 1985). The detailed surface geology of Rangamati District is shows eight geological subclass (Figure 3).
Result shows that about 45% landslide occurred in Boka Bil Formation, about 33% in Dihing and Dupitila Formation Undivided and about 13% in Tipam Sandstone areas. Dihing formation consists of medium-grained, occasionally pebbly sandstone and clayey sandstone with interbeds of mottled clay and has poorly consolidated rocks in most part. Dupi Tila Formation consists of fine to medium grained pebbly and cross-bedded sandstone with subordinate claystone and siltstone (Chowdhury, Khan and Uddin 2015). So it is evident that most of the landslide materials in Rangamati district are consist of clayey soil that has partly mixture of unconsolidated rock and sandstone. Due to the constituting material this kind of landslide can be defined as mudflow as we see this needs very low slope for triggering the landslide.
Distance from Fault Line
In Rangamati District about 93% landslides occurred within 11000 meters (11 Km) from the fault line (Table 4 and Figure 4). This indicates that like all over the world fault line plays a major role in occurring landslides in Rangamati District. Fault lines of Rangamati District are orientated southwest and north east direction (Figure 3). Due to this orientation land slide occurrence have shows strong correlation with the slope that have north, northeast, south and southwest aspect (Table 3 and 4 and Figure 6-A).
NDVI
State of vegetation can be a controlling factor for landslides (Tehranil and Hüsken 2019). This index output varies between −1.0 and 1.0, mostly representing greenness, where any negative values are mainly generated from clouds, water, and snow, and values near zero are mainly generated from rock and bare soil. Very low values of NDVI (0.1 and below) correspond to barren areas of rock or sand. Moderate values (0.2 to 0.3) represent shrub and grassland, while high values (0.6 to 0.8) indicate temperate and tropical rainforests (Ahmed et al. 2013). The NDVI of Rangamati District indicates that the landslide event occurred where the NDVI value is below 0.5 (Table 3 and Figure 5.A). It means that the landslide occurred area is covered by shrub or grass land or very little large trees.
BSI
Rapid urbanization and human development activities such as, building and road construction through deforestation and excavation of hill slopes have increased landslide in densely populated cities located in mountainous areas (Galli and Guzzeti 2007; Schuster and Highland 2007). The results of Bare soil shows positive DNs for built-up and bare areas, whereas the rest of land cover types had a value of zero or below (Bhatti and Tripathi 2014). Analysis reveals that maximum landslide events were occurred where the BSI value is medium (between -0.19 and -0.14) or high (between -0.22 and -0.19) (Table 3 and Figure 5.B). These areas are covered with scattered vegetation or small tree. In the landslide event area soil is bare or there is little vegetation and has built up area.
NDCI
Generally, the hills consist of unconsolidated sedimentary rocks such as sandstone, siltstone, shale and conglomerate. Soil of the Chattagram Hill Tracts are consists of alluvial, silty clay which is vulnerable to landslides (Islam 2018). At most areas, the soil consists of different layers of sand and clay/shale. Silty caly basically found in hill slope. Hill slope becomes unstable due to increased gravity load caused by heavy rainfall and other natural or anthropogenic actions on the slope (Alam et al. 2019) which makes it susceptible to massive soil erosion (Khan 2008). NDCI results reveal that most of the landslide occurred where medium clay content present on the surface. In this area the NDCI value ranges from 1.33-1.52 (Table 3 and Figure 5.C), which means landslide event occurred in the area where soil is composed with higher silt and clay content that can be easily saturated by rainwater. During heavy rainfall this saturated soil increases load and makes the mud eligible for flow. This mud load, by the pull of gravity, flows downward though it has not very steep slope (Figure 6.B and Table 3).
Slope Angle
Slope is the steepness of a surface, which is one of the indicators of stability of slopes (Clerici et al. 2006). Landslide occurred due to the downhill and outward movement of slope-forming materials under the influence of gravity (USGS 2004). Debris flows often begin as slides on steep hill slopes which transport material (Geertsema and Highland 2011). In Rangamati district almost all the event occurred where slope is below 16.85 degree (Table 3 and Figure 6.B). Excessive rainfall in Rangamati had saturated the soil resulting in increase of soil mass. The bedding of the soil formation is almost horizontal. Alternate layer of sand and very thin film of cements clay/shale layer. Rainwater seeps through the layers and creep effect reduces the bond between the sand and clay/shale layers (Alam et al. 2019). So behind the manmade cause rainfall and some geological causes also play role in triggering landslide in Rangamati. In Rangamati weak or sensitive weathered material composed with clay responsible for slide event that can be defined as mud flow in geomorphological terms.
Slope Aspect
Aspect is the orientation of slope, measured clockwise in degrees from 0 to 360, where 0, 90, 180 and 270 are north-facing, east-facing, south-facing, and west-facing, respectively. Aspect affects the exposure to sun-light, precipitation and wind thus affecting indirectly and other factors that contribute to landslides such as vegetation and, soil moisture and soil thickness (Clerici et al. 2006). In the Chattagram Hill Tracts the Upper Tertiary sandy-argillaceous sediments have been folded into a series of long submeridional (NNW-SSE) anticlines and synclines represented in the surface topography by elongated hill ranges and intervening valleys (Chowdhury 2015). Aspect of studied landlside events are SW, NW, NW, NE, SW, NE, S, NE, N, W, SW (Table 3 and Figure 6.B) which are highly expose to sun so that weathering occurred in this site rapidly. Rainfall also plays role in weathering and this site has get substantial rainfall during rainy season that loosen the soil and make it susceptible to landslide. The resulted aspects are also facing towards water body such as Kaptai Lake and Karnafuli River.
Landslide Susceptible Area
Occurrence of landslide in an area depends largely on complex interaction among a large number of factors (Dai et al. 2002). So the most exact method to landslide susceptibility is through field surveys (Catani et al. 2005; Nadim et al. 2006). But satellite remote sensing data can be a great information source for landslide susceptibility analysis. Data derived above are factors of landslide occurrence. By using frequency ratio method frequency ratio, relative frequency and predictability of each factor are identified. Frequency ratio indicates the correlation of landslide with the corresponding class. Results show that low slope has greater correlation with landslide occurrence (Figure 7 and Table 4). Northeast and southwest facing slopes also show same correlation (Figure 7 and Table 4). Greater correlation found for high to very high NDCI (Figure 7 and Table 4). Two major geological formations and low distance from fault has also grater correlation with landslide occurrence.
Table 4
The correlation between occurring landslides and each conditioning factor using bi-variate model (frequency ratio method)
Factor | Class | CP | % CP | LO | % LO | FR | RF | PR |
Geology | Tipam Sandstone | 695700 | 11.05 | 2 | 13.33 | 1.21 | 0.10 | 2.53 |
Dihing and Dupitila Formation Undivided | 299865 | 4.76 | 5 | 33.33 | 7.00 | 0.60 |
Boka Bil Formation | 2703472 | 42.93 | 7 | 46.67 | 1.09 | 0.09 |
Dupitila Formation (Pleistocene and Pleicene) | 173701 | 2.76 | 1 | 6.67 | 2.42 | 0.21 |
Valley Alluvium and Colluvium | 41656 | 0.66 | 0 | 0.00 | 0.00 | 0.00 |
Girujan Clay (Pleistocene and Pleicene) | 50227 | 0.80 | 0 | 0.00 | 0.00 | 0.00 |
Bhuban Formation | 1593601 | 25.31 | 0 | 0.00 | 0.00 | 0.00 |
Kaptai Lake | 738950 | 11.73 | 0 | 0.00 | 0.00 | 0.00 |
Distance from Fault Line (meter) | 0–5,486.91 | 4279837 | 70.09 | 5 | 33.33 | 0.48 | 0.11 | 1.93 |
5,486.91–10,973.82 | 1800142 | 29.48 | 9 | 60.00 | 2.04 | 0.45 |
10,973.82–16,460.73 | 206715 | 3.39 | 1 | 6.67 | 1.97 | 0.44 |
16,460.73–21,947.64 | 82330 | 1.35 | 0 | 0.00 | 0.00 | 0.00 |
21,947.64–27,434.56 | 34927 | 0.57 | 0 | 0.00 | 0.00 | 0.00 |
27,434.56–32,921.47 | 11903 | 0.19 | 0 | 0.00 | 0.00 | 0.00 |
Slope | 0–3.71 | 1283406 | 19.85 | 0 | 0.00 | 0.00 | 0.00 | 1 |
3.71–8.28 | 1439114 | 22.25 | 4 | 26.67 | 1.20 | 0.16 |
8.28–12.57 | 1205855 | 18.65 | 5 | 33.33 | 1.79 | 0.24 |
12.57–16.85 | 991836 | 15.34 | 3 | 20.00 | 1.30 | 0.17 |
16.85–21.14 | 702293 | 10.86 | 1 | 6.67 | 0.61 | 0.08 |
21.14–25.71 | 451482 | 6.98 | 1 | 6.67 | 0.95 | 0.13 |
25.71–31.13 | 250512 | 3.87 | 1 | 6.67 | 1.72 | 0.23 |
31.13–38.84 | 112549 | 1.74 | 0 | 0.00 | 0.00 | 0.00 |
38.84–72.83 | 29540 | 0.46 | 0 | 0.00 | 0.00 | 0.00 |
Aspect | Flat | 365135 | 5.65 | 0 | 0.00 | 0.00 | 0.00 | 1.30 |
North | 347585 | 5.38 | 3 | 20.00 | 3.72 | 0.31 |
Northeast | 768910 | 11.89 | 3 | 20.00 | 1.68 | 0.14 |
East | 849812 | 13.14 | 0 | 0.00 | 0.00 | 0.00 |
Southeast | 723460 | 11.19 | 0 | 0.00 | 0.00 | 0.00 |
South | 712398 | 11.02 | 1 | 6.67 | 0.61 | 0.05 |
Southwest | 866314 | 13.40 | 3 | 20.00 | 1.49 | 0.12 |
West | 862677 | 13.34 | 1 | 6.67 | 0.50 | 0.04 |
Northwest | 669773 | 10.36 | 2 | 13.33 | 1.29 | 0.11 |
North | 300523 | 4.65 | 2 | 13.33 | 2.87 | 0.24 |
BSI | -0.39 - -0.22 | 1743836 | 26.97 | 1 | 6.67 | 0.25 | 0.05 | 3.66 |
-0.22 - -0.19 | 2840443 | 43.92 | 3 | 20.00 | 0.46 | 0.09 |
-0.19 - -0.14 | 1070828 | 16.56 | 11 | 73.33 | 4.43 | 0.86 |
-0.14 - -0.08 | 727687 | 11.25 | 0 | 0.00 | 0.00 | 0.00 |
-0.08–0.14 | 83793 | 1.30 | 0 | 0.00 | 0.00 | 0.00 |
NDVI | -0.29–0.07 | 546938 | 8.46 | 0 | 0.00 | 0.00 | 0.00 | 3.51 |
0.07–0.27 | 241769 | 3.74 | 0 | 0.00 | 0.00 | 0.00 |
0.27–0.41 | 764844 | 11.83 | 9 | 60.00 | 5.07 | 0.83 |
0.41–0.48 | 2441392 | 37.75 | 5 | 33.33 | 0.88 | 0.14 |
0.48–0.65 | 2471644 | 38.22 | 1 | 6.67 | 0.17 | 0.03 |
NDCI | 0.534–1.14 | 599926 | 9.28 | 0 | 0.00 | 0.00 | 0.00 | 2.78 |
1.14–1.33 | 415078 | 6.42 | 0 | 0.00 | 0.00 | 0.00 |
1.33–1.44 | 1354756 | 20.95 | 8 | 53.33 | 2.55 | 0.66 |
1.44–1.52 | 2373567 | 36.71 | 6 | 40.00 | 1.09 | 0.28 |
1.52–1.80 | 1723260 | 26.65 | 1 | 6.67 | 0.25 | 0.06 |
(Legend: CP = Class Pixel, LO = Landslide Occurrence, FR = Frequency Ratio, RF = Relative Frequency and PR = Probability Ratio) |
According to this result weighted map of landslide susceptility of Rangamati district shows medium to high landslide susceptible areas are located near Kaptai Lake and Karnafuli river and in the slope of hills (Fig. 7). These areas are adjacent to fault line and have similar orientation with the slope aspect and have a low slope. There are many areas that are moderately susceptible to landslide during heavy rainfall.