6.1 Slope and Aspect:
Slope is the most basic parameter for stability considerations, especially in mountains, as with increase in slope, shear stress of basal material increases in direct proportion (Lee et al. 2004). It is the first derivative of elevation with each pixel (pixel size 80 m × 80 m) denoting the angle of slope at a particular location.
Study of the slope aspects of the area reveal that most of the area have gentle (less than 15o) to moderately gentle (25o – 35o) slope. Few areas on the northern, north-western and western border of the town have steep slope i.e. 35o – 50o. The slopes immediately adjoining the Naini Lake on both sides and the southern part have steep slopes too. Escarpment having very steep slope i.e. more than 50o lies in the western part of the town (Figure 7). The slope aspect plays a great role in determining the type and magnitude of landslide in any area. The areas in Nainital town having steep slope and escarpment are likely to be mass wasted easily and are prone to slides and falls. Gentle slope areas on the other hand are more prone to slow creeping movements. They pose no major danger if vegetated. On the other hand the slope and aspect of the area affect the rate of insolation received by the place, type and intensity of precipitation, runoff and distribution of vegetation in and around the town. Many areas like Sher ka Danda, Naini peak, Golf course, Deopatha and Krishnapur also show steep slopes and the Balia Nala flowing roughly in south-easterly direction has carved out a deep valley due to intense shearing in the area.
On the context of this study involving a small area and highly unsuitable to analyze disparity of precipitation due to unavailability of spatial data, aspect is rendered one of the significant parameters in preparing landslide hazard zonation maps. Exposure to solar radiation, rainfall and discontinuities which are related parameters of aspect largely control the occurrence of landslides. Aspect degree is classified according to the aspect classes as flat, north, east, south and west so on and so forth based on directions, i.e. an aspect map generally refers to the direction to which a mountain slope faces. Nainital topography circumscribes the position of the lake in a basin and left slope having north to north east aspect while the right slope have south- south west – west aspects marking windward and leeward locations. The aspect map is a very important parameter to understand microclimatic effects or impact of solar radiation on local climate of the Nainital area and was derived from ASTER DEM.
The Aspect Map reveals that the area around the lake is flat. The steep slopes right to the lake are directed west and south-west, while that on the left of the lake trend north, northeast and east . Majority of the area in the northeast and southwest have south, southwest and west aspect (Figure 8). The rate of insolation, the amount of rainfall and the type of ground cover and tourism based land use depends on the aspect of the place much more than any other factor.
6.2 Relief
Terrain elevation is an important factor to identify other factor impacts of this landslide-prone area. The elevation map of Nainital MC depicts the relief characteristics with the help of contours drawn at interval of 20m. The average elevation is 2084 metres (6837 ft) above sea level. The highest point is the China or Naina Peak with an elevation of 2619 metres (8593 ft). The average elevation steeply increases from the extreme southeastern corner of the town till the extreme northwestern sector. The contours, especially in the northern, northeastern and western part of the town show a close spacing indicating steepness of the terrain. The presence of a ridge in the northern part and escarpment in the western part is clearly seen from the arrangement of contours. However the area adjoining the Lake has uniform relief (Figures 9a, b).
6.2.1 Relative Relief (RR) depicts the range of elevation of a particular place. High values of relative relief indicate a rugged terrain, whereas low values of relative relief indicate that the relief is almost featureless. The average RR is approximately 900 metres. Elevation is low (1935.5) around the Naini Lake but steeply rises towards the north and south west of the town. Naini peak with height of 2611.5 meters above msl is the highest point of the area while Sher ka Danda, Deopatha and Ayarpatha respectively measure 2402, 2435.1 and 2352 meters above msl. The area around the lake has moderately steep slope and steep slope in some places, which indicates that the RR is also high. The map shows that the area around the Naini Lake has a relative relief of 305m to 350m. The values rise to about 600m towards the north and southwestern part of the town. This indicates that the terrain is rugged due to the presence of rocky knobs, outcrops and escarpments (Figure 10).
6.2.2 Dissection Index (DI) is an indicator of the relationship between maximum relief and relative relief. It thus gives an idea of the nature and magnitude of degradation of absolute relief or the whole terrain. The value to DI varies from 0 which indicates no dissection and 1, which indicates high dissection. DI values in the town ranges between 0.16 to 0.36 (approx.), indicating low to moderate dissection. The region around the Naini Lake is least dissected. The northern part having high relief and the whole southern part of the town having steep slopes and escarpment is more dissected (Figure 11).
6.3 Lithology
Nainital Lake is covered by limestone or dolomitic limestone and north, eastern and south eastern parts are covered by slate and shale. There are a number of fault lines and small thrust lines in and around the city. A fault runs across and along the lake intersecting from N-S the entire city.
The oldest rocks exposed in the study area, thus, belong to the Almora Group. Three distinct lithological suits can easily be identified within the Almora Group. These are as follows:
- Crystallines and Schists
- Quartzite
- Granite and Augen gneisses
The rocks of the Almora group tectonically lie over the Ramgarh group along the SAT (South Almora thrust). Three distinct lithological sites can easily be identified within the Ramgarh Group also. These are:
- Profoundly mylonitized chlorite quartz porphyry
- White and purple sericitic medium grained quartzite interbedded with grey, green and silvery sericite schist and chocolate brown, limonitic phylite
- Carbonaceous and locally pyretic phyllites and slates altering with blue green banded marmorized limestone.(Figure 12a)
Lineament studies have been integrated with drawing the geological features and are supposed to help reveal generalities on areas of weakness in understanding the cause of landslides. In the Himalayas that landslide phenomena are aggravated and hastened by being close to regional geological lineaments (Pachauri and Pant 1992; Mathew et al. 2007; Ramli et al. 2010). The lineament density of the study area was analyzed by using the line density analyst extension of ArcView GIS.
The lineaments thus represent weak zones along which mass wastage is likely. It was encouraging to note that some of the lineaments correlate positively with the structural discontinuities even through the two does not exactly overlap each other. It is important to note that the main lineaments are observed to show a trend both parallel and transverse to the alignment of the Naini Lake. Most lineaments show NE-SW to NNE-SSW and NW-SE trend. A significant lineaments showing NNW-SSE trend is observed to traverse through Hanumangarhi area while another one trends parallel to Balianala. The lineament pattern of Nainital area shows a relative close collation with that of drainage line and the ridge patterns (Figure 12b). As for the analysis, the result in this study indicates that most of the landslides have been found to occur at 50-100 metres from lineaments especially along the lake and in the town’s south and north outskirts.
6.4 Drainage Density and Distance to Drainage
Drainage is one of the most important factors that control landslides as its densities denote the nature of the ruggedness of the area, its soil, geological properties and even condition of land use/ land cover.
The streams emerging from the hills on the plain below are strikingly divided into numerous branches and channels. The River Debka, Nihal, Kosi, Gola and Bhakra, all of which presents the same character over the foot hill plain. In the hilly region the main water parting may be marked distinctly, which runs from Nainital north-west ward, separating the south-westerly drainage from the north-easterly drainage which is carried by Jokh Nadi, Ghora Gad and Mawani Gadhera etc. The main river of the basin may be easily marked along the line from which the drainage is flowing into the Tal on one hand and also flowing outwards to the southeastern part of the city.
The Drainage Density (DD) map reveals that the DD is highest in the northern part of the town. Here the values range between 5.8 km/sq.km. to 6.9km/sq.km. The whole area in the northern and northeastern part of the town has high DD values ranging from 4.7 to 5.8 km./sq.km. This corresponds to the location of the Naini Lake. The values continuously decrease outward from the lake area towards the area having gentler slopes then again starts increasing towards the southwestern part of the town from the escarpment area downslope (Figure 13a). The drainage buffer map shows areas close to drainage lines (Figure 13b) and this has been collated further with land use map and landslide occurrence sites to reveal further that areas close to drainage are most vulnerable locations.
6.5 Ruggedness Index
Ruggedness Index (RI) is an indicator of the relationship between relief and drainage. The RI values around the Lake and main settlement area ranges between below 0.7 to 1.4. Like the DI values RI values also increase northwards and southwestwards. With respect to landslide susceptibility values, however, unlike other factors ruggedness values indicate an assigned subclass value of only 0.3 (Figure 14).
6.6 Landforms
The Topographic view of Nainital MC and its surroundings reveal that the area near the mall road appears to have a flat structure with slopes surrounding it on all sides. The stream Balia Nala dissects it from a north west to southeast direction. The area to the southwest appears like a table land having dense vegetation. The boundary of the town has a much higher elevation than the central part, which appears like a basin. Landslide scars are visible on the slopes along the town boundary and on the slopes surrounding the Lake (Figure 15).
The Stability Map of Nainital Town depicts that Ridge Crest lies on both sides of the Lake. The immediate right slope of the Lake is dominated by creep as the mass wasting process. Wasted debris are found on the northern part of the Lake. Both slopes of the ridge, especially the right slope is an active landslide zone. Landslide scars are present in the area bordering the Lake especially to the northeast, northwest and south of the Lake. Landslide areas are seen scattered throughout the town, especially the rock escarpment present along the northwestern boundary of the town. Other major landslide areas are present to the south and southeastern boundary and minor areas are found towards the northeastern boundary (Figure 16).
6.6.1 Land use and Land cover of Present Nainital
Residential and built-up area surrounds the Naini Lake, although there is absence of houses along the immediate left and right bank of the lake upto a distance of about 0.5 kilometre owing to the steep slopes. Settlements are mainly concentrated to the northern and southern part of the Lake and in the high and middle zone at an elevation varying between 1400 to 2000 meters and in the lower valley stretch at an elevation less than 1200 meters. Settlements are absent in the upper high zones at elevation above 2000meters. The transportation routes provide a good connectivity to the settlements. The network of roadways is concentrated in the settlement zone around the lake and runs towards the southwest and southeastern part of the town. A playground is located towards the north of the lake and other scattered areas near the lake. Shallow and dense vegetation is found on the steep slopes and escarpment area. The northwestern, western and southeastern part of the town have few scattered open spaces. Only a minor portion in the western part is put to agricultural use. The overall map shows distinct landuse zones existing in relation to relief and slope characteristics (Figure 17).
Land use / Land cover Change: The history of evolution of Nainital Town since 1841 to 1947 shows that the built up area was scarce till 1900 except a few government buildings present near the southern bank of Naini Lake. Between 1900 and 1947 urban built up area increased but growth occurred in a scattered manner along the northern bank of Naini Lake near Kumaun University, surrounding Sukha Tal. From 1947 to present day there has been growth of settlements mainly for tourism purposes, mainly concentrated along the north-western and south-eastern banks of Naini Lake surrounding the Municipal Office and Market area respectively, as well as along the eastern bank of the lake. There has been a rise of about 15.96% in the areal coverage of built up space from 1990 to 2010. There has been about 15.36 % decrease in vegetation cover from 1990 to 2010, although there has been slight increase in tree-cover through plantation near the southern banks of Naini Lake. Area under agricultural land use has decreased slightly by about 0.75% (Figures 18a, b, c, d). It can be seen that land use in different slope areas clearly show relation of land use with slope aspect in parts of Nainital. It is evident from the diagram that the intensity of agricultural land use is highest in south western part of Nainital and built up area is highest in north eastern part. This clearly indicates that the southern part of Nainital has comparatively gentler slope. Percentage of forested area is highest in the moderate and steep slope areas, whereas, open area is present in gentle slope areas. Proportion of built up area declines with increase in steepness but a considerable development has taken place on moderate slopes. The same trend is followed by agricultural land use.
6.6.2 Vegetation quality:
Vegetation quality based on Normalised Difference Vegetation Index (NDVI) has been done to show that much of Nainital has lost its original once present dense vegetation cover suited naturally to the terrain. Less than 40% area is presently under dense vegetation (0.32- 0.58) while lesser dense (0.24- 0.32) encircle these areas and are dissected in continuity by scattered trees. Much of the town centre surrounding the lake has lost vegetation and has scattered trees depicted by low NDVI values (0.049 – 0.24) (Figure 19).
6.6.2 Distance from Road
Nainital Town and its surrounding areas like Sukha Tal, Bhimtal, Saat Tal have high level of accessibility. There is good connectivity of the lake area with nearby towns like Haldwani, Kathgodam etc. Accessibility is highest in the whole region surrounding the Lake, even on the steep and moderately steep slopes on the right bank of the Lake. It continues to be moderately high on the northern and southern part of the lake to the southwest and southeast part of the town. But the southern and extreme northern and northeastern part of the town is least accessible. A study of land use type and relief in connection to accessibility reveals that in case of Nainital town both play an important role in case of accessibility. In fact some areas are inaccessible due to the presence of dense forest here. However, the road network concentration on steep slopes and rugged areas indicate greater danger from landslides which may lead to road closures and related problems.
Road density is highest in the area where Kumaun University and High Court is located and in the residential area surrounding Sukha Tal. Density of road is also high along the eastern bank of Naini Lake dotted with hotels and residences. The southern bank of Naini Lake with Market, Bus stand and Government offices also has high road density. The buffer map (Figure 20) shows the areas in proximity to road network and upon collation with landslide occurrences and other factors reveal that areas close to roads are more prone to landslides, for eg. The eastern bank road of Naini Lake.
7.0 Major Findings:
The main aim of this study is to analyse the landslide susceptibility of the Nainital Municipal Corporation area with the help of the Landslide Susceptibility Index and a Landslide Susceptibility Map prepared on the basis of the LSI. All the factors responsible for landslides in any area are studied in details with the help of maps. The assigned subclass values for each of the factors and their subclasses have been analysed and finally the following inferences have been drawn.
- The steep slopes (35o to 50o) and moderately steep slopes (25o to 35o), bordering the Lake have a subclass value to 7 and 5 respectively. Only a small part of escarpment to the southwest part of the town has a subclass value of 10. These areas are more probable to suffer due to landslides. Other areas having moderate and gentle slopes can be assigned subclass values of 2 and 1 respectively. These have a less probability of occurrence of landslides.
- The area surrounding the lake has a sub-class value of 0.6. The north and southwestern part has a sub-class value ranging between 1.4 and 2.2 indicating that they have a greater potential of being affected by landslides.
- Some areas have low DI values, in case of which the assigned sub-class value is 0.2. The values gradually increase in accordance with the changes in relative relief. The northern part has higher DI values of about 0.36. In this case the assigned sub-class value is 1.5 making it a high probability zone for landslides.
- The Nainital town is underlain by shale, siltstones and sandstones, dolomites and limestone and shale which has a subclass value of 4.5 each indicating weak lithology. Moreover the presence of lineaments, thrusts, nappe etc. increases the susceptibility of these areas to landslides.
- The northern part has a high subclass value of 1.5, as the DD here is more than 5.8 km/sq. km. In other parts of northern and northeastern Nainital, the assigned subclass value is 1.2. Thus it is clear that with respect to drainage density the probability of landslides is very high here.
- The built-up area in the high relief part itself is an indication of high probability slope instability. The settlements have been assigned a high sub-class value of 6; open spaces have been assigned a sub-class value of 4. The presence of these indicates the susceptibility of this area to landslides.
- As the road is present around the Lake area which also has steep slope the probability of landslide becomes high as here the assigned subclass value is 5.
- The presence of features like ridge crest, escarpment, landslide scar etc. is an indication of landslide susceptibility, with the assigned sub- class value being 0.4, 2 and 1.5.
7.1 Landslide Vulnerability State
The Landslide Vulnerability Index Map of the town (Figure 21) is used to identify the areas most vulnerable to the hazard. Except for small scattered areas in the northeast, east and south the area is moderate to highly vulnerable to slides. The values range between 97 and more than 156. More than 90% area of the town is vulnerable to landslides, of which more than 65% area has vulnerability values above 127. Major areas around the Naini Lake, the western, southwestern and extreme southeastern part of the town is highly vulnerable to landslides with values above 156. A small part of Hari Nagar and Raj Bhawan, which has least area affected by slides and few occurrences of such hazard, is identified to be least vulnerable. Other parts of Raj Bhawan, Tallital Bazar, Krishna Pur and Upper Mall have moderate vulnerability to landslides. The whole northern part of the town comprising large parts of Narayan Nagar, Sher ka Danda, Sukha Tal, Nainital Club, Ayarpatta, Aawaghar and snow view are highly vulnerable to slope failure. Western part of Sukha Tal, northern part of Naini Lake area, western part of Sher ka Danda, northern part of Upper Mall, eastern part of Mallital Bazar, southern part of Snow View and southern part of Narayan Nagar have the highest vulnerability to slides. This region is connected by transportation routes and settlements are mainly present along these arteries. Slope failures in this area would have disastrous consequences if suitable measures are not taken to check them.
After thorough analysis of the landslide locations and areas covered, the Landslide Susceptibility Zonation Maps prepared with the help of it the following observations can be made:
Landslide Susceptibility Index has been calculated based on the area of landslides in Nainital MC (Figure 22a). The LSI values range between less than 5 to more than 56. The southern part of the Naini Lake i.e. Tallital Bazar, Hari Nagar, Shri Krishna Pur, parts of Upper Mall and Narayan Nagar has very low LSI values and hence low area affected by landslides. The central part of the town comprising the area surrounding the above places has moderate LSI values, between 6 and 22. The northern part of the Lake, comprising parts of Sher Ka Danda, Sukha Tal, Naini Lake and Snow View and the southwestern section of Narayan Nagar, has LSI values ranging between 40 and 56. The extreme northern part comprising Awaghar, Mallital Bazar and Nainital Club has highest LSI values i.e. above 56. Thus it is seen that landslides affect a greater part of the town in the northern and southwestern sectors compared to other areas.
A second map has been prepared with the help of LSI values calculated based on the number of occurrence of landslides (Figure 22b). The value ranges between 201 and 1550. It is seen that with respect to number of slides the LSI value is lowest in the southeastern, central and western part of the town comprising areas under Raj Bhawan, Tallital Bazar, Narayan Nagar, Upper Mall and Hari Nagar. The LSI value is moderate, ranging between 651 and 1000 in parts of Sukha Tal, Ayarpatta, Naini Lake and Sher ka Danda area. The LSI is highest, i.e it ranges between 1101 and more than 1550 in the northern and northeastern part of the town comprising Nainital Club, Aawaghar, Snow View, Mallital Bazar and western part of Sher ka Danda. Thus, it roughly coincides with the region where the total area affected by slides is highest.