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A geomorphic evaluation of the landslides around Dehradun and Mussoorie, Uttar Pradesh, India
G[OHORHO ELSEVIER
Geomorphology 15 (1996) 169-181
A geomorphic evaluation of the landslides around Dehradun and Mussoorie, Uttar Pradesh, India Sambhu V. Panikkar, V. Subramanyan Department of Earth Sciences, Indian Institute of Technology, Bombay 400 076, India
Received 25 November 1994; revised 12 May 1995; accepted 5 June 1995
Abstract The Mussoorie hills of northern India are characterized by a rugged topography, with hill ranges rising steeply from about 600 m above mean sea level to over 2300 m and deep cut valleys. The area comprises the Proterozoic-Cambrian rocks of the Krol belt which have been thrust over the Neogene sedimentary rocks (Siwalik Group) along the Main Boundary Thrust. Various geomorphological parameters have been assessed, in conjunction with the geological and anthropogenic aspects, to evaluate the occurrence of the different types of landslides in the area. The evidence of neotectonism was provided by the various geomorphic criteria. Seventy-five landslides were identified by remote sensing. These were studied in the context of geological aspects such as lithology, proximity to active faults and lineament density, geomorphological aspects such as landform, slope, lateral erosion by streams, drainage texture, spring sapping, elevation difference between adjacent valleys, altitude and relief and anthropogenic factors including landuse/land cover and distances from roads. The important causes were found to be lithology, proximity to the active faults (Main Boundary Thrust and Sairku fault), slope angle and aspect, lateral erosion by stream undercutting and deforestation due to human interference. The triggering factors include rainfall and seismicity. The preventive measures to be adopted to stabilise the slopes have also been described.
1. Introduction Landslides are phenomena that may interfere with human activities. The high energy environment in mountainous terrain is characterized by instability and variability (Gerrard, 1994). The Himalaya of northern India is particularly susceptible to the occurrence of slope instability phenomena because of the rugged nature o1! the terrain. The increased anthropogenic activitie:~ over the years have also contributed to the worsening of the situation. A geomorphological approach to landslide hazard assessment has been attempted here. The purpose of the study is an evaluation of the various geological, geomorphological and anthropogenic parameters that might have served as possible causes for the occurrence of land-
slides. The study area falls between the 30°15 ' and 30°30'N and 78°0 ' and 78°10'E, covering an area of about 445 km 2 in the Dehradun and Tehri-Garhwal districts of Uttar Pradesh, India (Fig. 1). The impact of landslides on the human population is significant, particularly because of the fact that Mussoorie is a prominent hill station.
2. Geologic setting The study area includes parts of the Lesser and Sub Himalayan zones. The Main Boundary Thrust (MBT) marks the northern boundary of the Sub Himalaya. The fluvial sequences of Neogene age
0169-555X/96/$15.00 ¢~ 1996 Elsevier Science B.V, All rights reserved SSDI 01 69-555X(95)001 2 I-2
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S.V. Panikkar, V. Subramanyan/Geomorphology 15 (1996) 169-181
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(Siwalik Group) form the Sub Himalayan belt. These are overlain by the Doon Gravels of Late Pleistocene age. The Lesser Himalaya, which is bounded by the MBT in the south comprises the ProterozoicCambrian shelf to shallow marine sequences. These include the Mandhali, Chandpur, Nagthat, Blaini, Krol and Tal Formations (Fig. 2). Several workers have dealt with the regional geological aspects of the Himalaya in general and the Lesser Himalaya, in particular. The earliest is that by Medlicott (1864). Auden (1934) has given a detailed account of the geology of the Krol belt, which still forms the primary reference for all work in this area. The other workers include Bhargava (1972), Rupke (1974), Valdiya (1980), Singh (1979), Joshi et al. (1989) and others. The tectonic aspects, particularly the neotectonism and seismicity, have been studied, among others, by Middlemiss (1910), Nakata (1975), Gupta (1992) and Valdiya (1993).
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3. Seismicity and neotectonics The Himalaya is considered to be a seismically active zone. The area falls in Zone IV of the five seismic zones of India (horizontal seismic coefficient of 0.05). The isoseismal line with a value of 8 fell in the area during the Kangra earthquake of 1905 which damaged the town of Kangra in Himachal Pradesh (Middlemiss, 1910; Auden, 1942). The main epicenter was in Kangra and a secondary epicenter near Mussoorie. More recently, there was a devastating earthquake of magnitude 7.1 in Uttarkashi, northeast of Mussoorie on the 20th October, 1991 (Gupta et al., 1994). The area is still tectonically active. The geomorphic evidence of neotectonic activity includes the diversion of streams by massive landslides, uplifted terraces, entrenched rivers and the presence of zones of active mass movement. The fast uplift (about 5 m m / y r ) has resulted in the "superimposition of a youthful topography on a mature relief" (Valdiya, 1993). The Song river to the east of Dehradun flows northwesterly before taking an abrupt right angled bend, cutting across the strike of the Lesser Himalayan formations and then flows southerly (Fig. 3). The Baldi Nadi joins the Song from the north resulting in a conspicuously straight river course.
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Fig. 1. Location map of the study area.
The abrupt increase in the elevation of the Lesser Himalayan formations to the east of these rivers and the presence of massive landslides and huge triangular fault scarps all along the river course suggest the possibility of an active fault (Sairku fault). In the Baldi Nadi section at Sahasradhara, brecciated argillaceous limestone occurs, which is totally shattered. The river course here is absolutely straight. Furthermore, there is an abundance of springs on both slopes. These suggest an intraformational fault. The presence of waterfalls (Fig. 3) implies knickpoints in the stream gradient suggesting rejuvenation. The Doon Valley and its surroundings have undergone slow uplift (1 m m / y r ) with respect to the alluvial plains in the south (Nakata, 1975). A distinctive feature of the rivers in the Doon Valley is their deeply entrenched nature. Valley cross profiles drawn across the various rivers indicate that the side elevations vary from 20 m to as high as 100 m. This is substantially high for the rivers that have reached the plains, where one would normally expect braided river channels. The tributaries to the Song river flowing from the west are suddenly incised above their junctions with the main stream. Three levels of
S.V. Panikkar, V. Subramanyan /Geomorphology 15 (1996) 169-181
171
The geomorphic aspects like the landforms and slopes were studied on the aerial photographs and in the field. A slope vector map, delineating units homogenous in terms of slope angle and direction, was prepared from the Survey of India topographical map on the scale of 1:50,000. These units were then regrouped into seven classes (Fig. 4) according to the classification scheme by Young and Young (1974). The SPOT (MLA) and IRS (LISS II) digital data were used to classify the area according to the landuse/land cover (Fig. 5). The geological linea-
terraces have been identified on the aerial photographs (Fig. 3). The deeply entrenched nature of the rivers is significant evidence for the uplift of the Doon Valley.
4. D a t a c o l l e c t i o n
A check list was ~tsed to collect the data, in the field, relating to the various geological, geomorphological and anthropogenic attributes (Table 1). 78"a0'
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172
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
ments were picked up on the SPOT images based on the straight segments in the river courses, linear ridges and straight boundaries between contrasting tones or colours. 216 lineaments were identified in all. The lineament density was then computed for the area. The landslides were identified on the aerial photographs. The SPOT (MLA) FCC of 1987 and SPOT
(PLA) imagery of 1989 on the scales of 1:50,000 and resolutions 20 m and 10 m respectively were used to update the landslide occurrences in the area. The landslides were identified by their bright tone, the landslide trails and the data on typical landslide "recess" on the photographs. They appear bluish on the FCC. However, only the larger slides could be identified on the remote sensing media, which were
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Fig. 3. Geomorphological map of the study area prepared from aerial photographs and field surveys.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
then cross-checked in the field. The smaller ones were studied in the field. A total of 75 landslides were identified (Fig. 3) and examined.
5. Geomorphology The area of study lies partly in the outermost fringe of the Lesser Himalaya of Garhwal and partly in the Sub Himalaya represented by the Siwalik Hills and Doon Valley. Tile area is characterised by a rugged topography, with the hill ranges rising from
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600 m above mean sea level to over 2300 m. Three broad geomorphic complexes can be identified - Structural hills, Residual hills and Plains. The formations in the Mussoorie Hills dip NE in the south-facing slopes and SW in the north-facing slopes, thus forming a syncline. The Lesser Himalaya is characterized by craggy, elevated hills and well-developed dipslopes. The ridge crests are sinuous because of the large number of amphitheatre valley heads. Triangular facets have developed in an elongated manner close together with three or four of them riding over one another. The usual landforms 78"10' N
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Fig. 4. Slope map prepared from the topographical map on the scale of 1:50,000. The seven classes have been delineated based on the classification scheme by Young and Young (1974).
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
174
that develop on inclined formations (homoclinal ridges) have been, however, modified in this area because of the complexity of the terrain in terms of the tectonic activity. The litho-morphological relations are such that the different rock types are indicated by topographic breaks in slope. These breaks can be identified on aerial photographs as well as in the field. The weaker rocks, namely the slates, form the gentler spur slopes, in contrast to the massive limestones and quartzites which form tough ridges. At places, however, the limestones are more weathered and form fiat-topped to rounded hills.
78"00'
The MBT can be traced by a distinct break of slope between the Sub Himalayan and Lesser Himalayan formations. The drainage pattern also indicates the structural control by way of numerous kinks. The most obvious indication of the Sairku fault is the Song river to the east of Dehradun where it has adapted itself to the fault by turning abruptly south from a westerly course. The slope varies from being almost level in the plains surrounding Dehradun to near vertical cliffs in the Lesser Himalayan formations (Fig. 4). The valley-head slopes are invariably steep, whereas the 7~I0'
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Fig. 5. Landuse/land cover map. The landslide distribution is also indicated. 65.17% of the landslides are in sparsely vegetated areas.
S.V. Panikkar, V. Subramanyan/Geomorphology 15 (1996) 169-181
valley-side slopes show the entire range from gentle to steep. The spur-end slopes, on the other hand are always gentle. The :~outh-facing slopes are gentler than the north-facing ones, which can be attributed to microclimatological factors (Thornbury, 1969). The relative relief of the area increases from almost zero to over 400 m. The area exhibits, a range of drainage patterns
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from dendritic through trellis, radial, parallel and deranged to barbed. The dominant pattern, however, is dendritic, which has developed mostly over the Lesser Himalayan formations. There are a number of springs in the area (Fig. 3). Most emanate from the middle slopes, though there are also a few from the basal slopes. The lineament map (Fig. 6) shows the dominant 70 ~O'N
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Fig. 6. Lineament map prepared from SPOT MLA and PLA images. The continuous lines indicate the faults cross-checked in the field. The other lineaments are marked by broken lines.
176
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
Table 1 Check list for field data collection Geologic Lithology Density of joints Joint openings Joint infillings Direction of major joints with respect to steepest slopes Strong beds over weak beds Degree of weathering Geomorphic Relief Slope angle Height difference between adjacent valleys Drainage density River gradient Slope undercutting
Anthropogenic Road cut depth Road cut position on slope Removal of vegetation Loading of upper valley side
trend to be NNE-SSW, which corresponds to the trends of the active Sairku fault and the MBT to the east of Dehradun. The next prominent direction of N E - S W represents the numerous straight segments in the entrenched rivers in the Doon Valley. The land use/land cover ( L U / L C ) categories in the area have been recognised on the remote sensing media, with field checking. The different L U / L C types delineated are cultivated land, sparse vegetation, forest, urban land, quarries, barren land and rivers (Fig. 5). The area-wise distribution of the different L U / L C types is given in Table 2. The cultivated lands are mostly in the plains or on the gentler spur slopes. The southern slopes of the Mussoorie Hills are more barren than the northern slopes. Significantly, about 15% of the study area comprise sparse vegetation and barren land on the steepest slopes ( > 30°). These are the potentially unstable zones.
6. Landslide evaluation There are a few major and several minor landslides in the area. Problem related to landslides
becomes acute after rains, when the roads get blocked by landslide debris. The Nalota Nala, Kalagarh Nala, Signikhala and Jabarkhet slides are a few major ones on the Dehradun-Mussoorie road. Besides these, there are about ten major slides to the east of Dehradun in the Song river section. Most of these slides are very large and on steep (18-30 °) to very steep (30-45 °) slopes. The landslides in the area can be essentially divided into two types: rock slides and debris slides based on the materials involved. River bank slumping has been observed locally in the plains, especially along the Nun Nadi and Tons river. Soil creep was observed at places, indicated by the inclination of the trees at an angle of about 5 ° from the vertical on slopes of 35 ° in weathered argillaceous limestone. The landslides in the area are either naturally occurring or owe their origin to human interference, in terms of quarries, buildings and road cuts.
6.1. Geological aspects The landslides are mostly in the Krol limestone and Nagthat quartzite (67.3%), although there are a few in the Chandpur slates. The percentage-wise distribution of the landslides in the different formations in the study area is given in Table 3. Rock slides occur mostly in the Nagthat quartzite, whereas debris slides occur in limestones and slates. The solution activity in limestones, in places, has resulted in the formation of solution caves at the base resulting in the removal of toe support. There are several debris slides in the argillaceous limestone along the road leading to Mussoorie. The presence of shale intercalations and the foliated nature of the limestone, with the closely spaced intersecting joints Table 2 Area-wise distribution of the different landuse/land cover categories Landuse/land cover category
Area (km 2)
Perc.
Cultivated land Sparse vegetation Forest Urban land Quarries Barren land River
127.36 144.27 82.70 53.33 3.68 16.76 13.13
28.86 32.70 18.74 12.09 0.83 3.80 2.98
SN. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181 Table 3 Percentage distribution of the landslides in the different formations Doon Gravels Siwalik Tal Krol Blaini Nagthat Chandpur Mandhali
4.7% 3.5% 6.3% 39.3% 9.2% 28.0% 9.0% 0.0%
have contributed to these slides. Calcite, occurring as infilling material along the joints, has been removed by solution resulting in the further opening up of these joints. The microfolding of the beds has resulted in axial plane cleavages, which have subsequently been widened to act as potential slip surfaces. The Nagthat quarlzites are highly jointed with several sets of closely spaced, open, planar and smooth joints that have served as slip surfaces. These joints, especially when dipping downslope at angles greater than the slope angle, are significant. The stratigraphic control also plays a role here, with the competent Nagthat quartzite overlying the weaker and highly-sheared Chandpur slate. The proximity to the MBT and the Sairku fault, which are the two active faults in the area is a significant aspect in the context of the occurrences of landslides. 49.5% of the landslides in the study area occur within a distance of 1.5 km from the active faults. There are landslides in the Chandpur slates close to the MBT. The slates here are highly sheared at the contact between the Lesser and Sub Himalayan formations raarked by the MBT. The best example of landslides associated with neotectonism is the Song river section (Sairku fault, Fig. 3). There are huge triangular fault scarps representing the truncated spur slopes. The rock types here are Nagthat quartzite and Chandpur slate.
6.2. Geomorphological aspects The occurrence of landslides in terms of the landforms was studied. The slides in the area are on the escarpment side, where joint planes inclined steeply downslope have been critical. The develop-
177
Table 4 Percent distribution of the landslides in the geomorphic complexes Structural Hills Residual Hills Plains
95.5% 3.3% 1.2%
ment of the escarpments itself has been controlled by these steeply dipping joints. In the case of the Song river section, all the landslides occur on the escarpment side on the fault scarps, with the formations (Nagthat quartzite and Chandpur slate) dipping into the hill. The occurrence of landslides on dipslopes, with the bedding planes serving as the slip surface, is not observed in this area because of the synclinal structure. The distribution of the landslides in the different geomorphic complexes is given in Table 4. The landslides in the residual hills are on moderate slope areas of Doon gravels and sandstone. In the plains, as mentioned earlier, the rivers are deeply incised resulting in high bank elevations. This has resulted in river bank slumping, especially where there is pronounced undercutting by the stream. The distribution of the landslides in the different slope categories was studied with reference to the rock types (Table 5). The stromatolitic limestones in the Krol Formation, exposed in Barlowganj form gentler slopes. This, coupled with the near absence of joints, makes it relatively free of landslides. The high percentage of the landslides in Krol Limestone on steep slopes is due to the artificially oversteepened slopes left after the quarrying of the massive limestone. The occurrence of natural landslides in massive limestone is rare because of the lesser de-
Table 5 Limiting angles for the occurence of landslides in the different formations Tal quartzite Argillaceous limestone Massive limestone Nagthat quartzite Blaini slate Chandpur slate Siwalik sandstone
> 45 18-30 30-45 > 30 18-30 30-45 > 30
(87.5%) (52.2%) (43.3%) (47.6%) (80.9%) (73.9%) (82.3%)
Figures in parentheses indicate the percentage of landslides in the different formations.
S.V. Panikkar, V. Subramanyan/Geomorphology
178
gree of jointing, the wide spacing, tight, curved, rough and unsystematic nature of the joints and the presence of dense forest cover, in spite of the fact that they form steep slopes. The slopes in Nagthat quartzite vary from steep to very steep. About 47.6% of the slides in Nagthat quartzite occur on slopes )30 ° . This includes the massive landslides in the near vertical fault scarps along the Song river. However, the fact that these quartzites are highly jointed and overlie the weak and sheared Chandpur Slate explains the presence of landslides in them on slopes ranging from 18-30 ° . The Chandpur Slates are highly sheared and generally form the gentler spur slopes. However, in areas where they are not severely sheared, they form steeper slopes ( > 30 °) with the associated occurrences of landslides. In the case of the Doon gravels, slumps occur on the moderate slopes of the river banks, which are high due to the entrenched nature of the rivers. A significant feature of the relationship between the slope aspect and the occurrences of landslides is that all the slides in the area are on south-facing slopes. Not a single slide could be identified on north-facing slopes. The possible explanation for this could be microclimatological factors. Furthermore, anthropogenic activities are concentrated on the south-facing slopes, i.e., the area between Dehradun and Mussoorie.
15 (1996) 169-181
The removal of toe support by the undercutting of streams is one of the important causes for the occurrences of landslides in the area. The geomorphic map (Fig. 3) shows the areas of undercutting delineated from the aerial photographs. A comparison between these areas and the landslide distribution would suffice to realise the significance of stream undercutting. In fact, 85.1% of the landslides occur within a distance of 200 m from the rivers. The drainage density values are moderate ( 2 - 4 k m / k m 2) in 86.5% of the landslide occurring areas. The role of springs in the initiation of landslides was studied in the field. In many of the smaller slides, the process of spring sapping leading to headward erosion was found to be critical. However, a comparison of the occurrence of springs and the major landslides (Fig. 3) reveals no significant association between the two, indicating that the springs have not had a crucial role in the initiation of the major landslides. Elevation difference between adjacent valleys is another potentially destabilising factor. The profile (Fig. 7) across the Song river valleys, for example shows a difference in elevation of about 120 m between the two rivers within a distance of 2.5 km. The intervening slope is, in fact one with several landslide occurrences. The depth to the water table is as low as 5 m immediately after the rains in July and August. However, within a couple of months, by November, all the rivers in the area become absolutely dry and the water table goes
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Fig. 7. Topographic profile across the Song river, east of Dehradun. Note the elevation difference of about 120 m between the two adjacent valleys within a distance of 2.5 km.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
down. The water table fluctuations in the Doon Valley range from 17 m to 33 m. This is due to the highly permeable na~Lure of the Doon Gravels forming the plains in the Doon Valley. The effect of this fluctuation, in the context of the occurrence of river bank slumping, is ~Lat the sudden fall in the water table results in an apparent recovery of weight of the materials that were saturated with water. This would amount to overloading of the materials. This can initiate slumping in the river banks, especially where the toe support has also been removed by lateral erosion due to strea~a undercutting. The altitude in the area, as mentioned earlier, varies from 600 m to 2300 m. 75% of the landslides occur in the range 800-1800 m. Only 5.5% of the landslides are observed in areas o f elevation greater than 1800 m because, the Tal quartzite, which occurs in this range of elevation is relatively free of landslides. Moreover, 45,4% of the area above 1800 m is forested. In the case of the relative relief, 79% of the slides are in areas of moderate values (100-300 m). This is because of the fact that 75% of the quarries and barren land and 66% of the sparsely vegetated areas occur in the moderate relief regions. Most of the landslides are concentrated in these three land u s e / land cover categorie:~. The lineaments irt the area have been studied and the proximity to active faults has already been discussed in the context of landslide occurrences. The lineament density values are moderate (0.5-1.0 k m / k m z) to high (1.0-1.5 k m / k m 2) in 73% of the landslide province.
6.3. Anthropogenic aspects The distribution of the landslides was studied with reference to the landuse/land cover categories. It was observed that 65.17% of the slides occur in areas of sparse vegetation, which form 32.7% of the study area. The percentage-wise distribution of the landslides in the different landuse/land cover groups is given in Table 6. The significance of the forest cover in the present context can be gauged from the fact that 91.39% of the slides occur in non-forested areas. The residual hills in the Doon Valley, for example form moderate slopes, but account for only
179
Table 6 Percentage distribution of the landslides in the different landuse/ land cover categories Sparse vegetation Barren land Quarries Forest Cultivated land Urban land
65.17% 11.15% 9.98% 8.61% 3.72% 1.17%
3% of the total landslides in the area because they are densely forested. There are several minor landslides all along the road from Dehradun to Mussoorie. However, some of the landslides like the Nalota Nala, Kalagarh Nala, Signikhala and Jabarkhet slides (Sastry et al., 1981) are major ones. These slides block roads with their debris, especially after the rains. The landslide distribution in terms of the distances from the roads in the area was studied. Significantly, it was found that only 25% of the major slides occur within a distance of 200 m from the roads and 51.9% of them occur at distances greater than 400 m. This implies that the presence of road cuts has not served as a dominant cause in the case of the larger landslides. However, the problem still remains in the case of the minor slides, as observed in the field.
7. Triggering factors Irrespective of the combination of factors present in an area, the triggering factors invariably are rainfall and/or seismicity. Most of the landslides in the area occur after heavy rains, which serve to saturate the materials thereby increasing their effective weight. Moreover, it can cause the weakening of materials like clay which swell up when moist. The mean annual rainfall in the area is 2875 mm (Sastry et al., 1981), most of which is concentrated in the months of July and August. In fact 40% of the rainfall occurs in the month of August alone. The other triggering factor is seismicity. The Himalaya, as mentioned earlier, is known to be a seismic zone. The ground motion during an earthquake can trigger off large scale landslides in the area.
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8. Preventive m e a s u r e s
Once the causes for the occurrences of landslides have been established, the preventive measures can be considered. Since deforestation has been noticed to be one of the important factors for the occurrence of slides, afforestation programmes should be initiated to stabilise the slopes. This should be particularly concentrated on the steep barren slopes in the Nagthat quartzite and Krol limestone, including the abandoned quarries. The quarrying in the area has now been banned, except in a few places like Cloud End, Mussoorie. The quarrying should be done in such a way that the natural slope is approximated at the end, instead of leaving behind oversteepened slopes. The toe erosion by stream undercutting is a dominant destabilising factor in most of the landslides in the area. This can be minimised or fully avoided by the construction of toe walls along the concave bank of the stream. The height of the toe wall should be maintained higher than the highest expected flood level in the stream (Sastry et al., 1981). Another way to prevent erosion is by constructing deflecting terms to divert the flow away from the bank. Construction of retaining walls along the slope would serve to stabilise the potentially unstable slopes.
9. Conclusion
The geological, geomorphological and anthropogenic parameters have been discussed in the context of their relationship to the occurrence of landslides. The significant conclusions have been elaborated here. 1. The two types of landslides in the area, rock slides and debris slides, are either naturally occurring ones or owe their origin to human interference. River bank slumping occurs locally in the Doon Valley. 2. The landslides are concentrated in the Nagthat quartzite and Krol limestone. 3. The proximity to the active faults (MBT and Sairku fault) is a significant aspect in the occurrences of landslides. 4. The joint planes which have controlled the development of an escarpment have served as potential slip surface. All the landslides occur on escarp-
ments with the formations dipping into the hill, suggesting that the bedding planes have not served as the slip surfaces. 5. The limiting angles for the occurrences of slides are > 45 ° for Tal quartzite, 18-30 ° for argillaceous limestone and Blaini slate and 30-45 ° for massive limestone, Nagthat quartzite and Chandpur slate. 6. All the landslides occur on south-facing slopes, which may be attributed to the microclimatology and the greater degree of anthropogenic activities here. 7. The removal of toe support by stream undercutting is the most frequently observed cause for sliding in the area. 85% of the landslides occur within a distance of 200 m from the streams. 8. The headward erosion by the process of spring sapping was found to be significant in the case of the smaller slides. However, this does not seem to serve as a major cause in the larger slides. 9. The elevation difference between adjacent valleys, in many cases, has rendered the intervening slope unstable. I0. Water table fluctuations are a significant factor in the initiation of river bank slumping. 11. 91% of the landslides occur in non-forested areas, indicating the effect of vegetation on the initiation of slope instability. 12. Road-cuts have served to cause minor landslides, although they have not had a dominant influence in the case of the major landslides. 13. Rainfall and seismicity form the dominant triggering factors for landslides in the area. 14. Preventive measures which should be taken include afforestation programs, constructions of toe walls and deflection terms to prevent lateral erosion and stream undercutting, construction of retaining walls to stabilise slopes and proper quarrying practice where the original slope is restored after quarrying.
References Auden, J.B., 1934. Geology of the Krol Belt. Rec. Geol. Surv. India, 67: 234-454. Auden, J.B., 1942. A geological investigation of tunnel alignment for the Jamuna hydro-electric scheme. Rec. Geol. Surv. India, 77: 1-50. Bhargava, O.N., 1972. A reinterpretation of the Krol Belt. Himalayan Geol., 2: 47-81.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181 Gerrard, J., 1994. The landslide hazard in the Himalaya: Geological control and human action. Geomorphology, 10(1/4): 221230. Gupta, G.D., 1992. Himalayan Seismicity. Geol. Soc. India Spec. Publ., 23. Gupta, R.P., Saraf, A.K., Saxena, P. and Chander, R., 1994. IRS detection of surface effects of the Uttarkashi earthquake of 20 October 1991, Himalaya. Int. J. Remote Sensing, 15(11): 2153-2156. Joshi, A., Mathur, V.K. and Bhat, D.K., 1989. Discovery of Redlichid trilobites from the Arenaceous member of the Tal Formation, Garhwal S)mcline, Lesser Himalaya, India. J. Geol. Soc. India, 33(6): 538--546. Medlicott, H.B., 1864. On the geological structure and the relation of the southern portion of the Himalayan ranges between the rivers Ganges and the Ravee. Mere. Geol. Surv. India, 3: 1-212. Middlemiss, C.S., 1910. The Kangra earthquake of 4th April, 1905. Mem. Geol. Sure. India, 38: 1-409. Nakata, T., 1975. On Quaternary tectonics around the Himalayas. Rep. Tohoku Univ., 7t~a Ser, (Geography), 25:111-116. Rupke, J., 1974. Stratigraphic and structural evolution of the Kumaun Lesser Himalaya. Sediment. Geol., 11: 81-265.
181
Sastry, G., Mathur, H.N. and Tejwani, K.G., 1981. Landslide control in north-western Outer Himalaya - - A case study. Bull. No. R8/D6, Central Soil and Water Conservation Research and Training Institute, Dehradun. Singh, I.B., 1979. Environment and age of the Tal Formation of Mussoorie and Nilkanth areas of Garhwal Himalaya. J. Geol. Soc. India, 20(5): 214-225. Thornbury, W.D., 1969. Principles of Geomorphology. 2nd ed. Wiley Eastern Limited, 112 pp. Valdiya, K.S., 1975. Lithology and age of the Tal Formation in Garhwal and implication on stratigraphic scheme of Krol Belt in Kumaun Himalaya. J. Geol. Soc. India, 16(2): 119-134. Valdiya, K.S., 1980. Geology of the Kumaun Lesser Himalaya. Wadia Institute of Himalayan Geology Publn., Dehradun, pp. 1-205. Valdiya, K.S. , 1993. Uplift and geomorphic rejuvenation of the Himalaya in the Quaternary period. Current Sci., 64(11/12): 873-885. Young, A. and Young, D.M., 1974. Slope Development. Macmillan Education.
Geomorphology 15 (1996) 169-181
A geomorphic evaluation of the landslides around Dehradun and Mussoorie, Uttar Pradesh, India Sambhu V. Panikkar, V. Subramanyan Department of Earth Sciences, Indian Institute of Technology, Bombay 400 076, India
Received 25 November 1994; revised 12 May 1995; accepted 5 June 1995
Abstract The Mussoorie hills of northern India are characterized by a rugged topography, with hill ranges rising steeply from about 600 m above mean sea level to over 2300 m and deep cut valleys. The area comprises the Proterozoic-Cambrian rocks of the Krol belt which have been thrust over the Neogene sedimentary rocks (Siwalik Group) along the Main Boundary Thrust. Various geomorphological parameters have been assessed, in conjunction with the geological and anthropogenic aspects, to evaluate the occurrence of the different types of landslides in the area. The evidence of neotectonism was provided by the various geomorphic criteria. Seventy-five landslides were identified by remote sensing. These were studied in the context of geological aspects such as lithology, proximity to active faults and lineament density, geomorphological aspects such as landform, slope, lateral erosion by streams, drainage texture, spring sapping, elevation difference between adjacent valleys, altitude and relief and anthropogenic factors including landuse/land cover and distances from roads. The important causes were found to be lithology, proximity to the active faults (Main Boundary Thrust and Sairku fault), slope angle and aspect, lateral erosion by stream undercutting and deforestation due to human interference. The triggering factors include rainfall and seismicity. The preventive measures to be adopted to stabilise the slopes have also been described.
1. Introduction Landslides are phenomena that may interfere with human activities. The high energy environment in mountainous terrain is characterized by instability and variability (Gerrard, 1994). The Himalaya of northern India is particularly susceptible to the occurrence of slope instability phenomena because of the rugged nature o1! the terrain. The increased anthropogenic activitie:~ over the years have also contributed to the worsening of the situation. A geomorphological approach to landslide hazard assessment has been attempted here. The purpose of the study is an evaluation of the various geological, geomorphological and anthropogenic parameters that might have served as possible causes for the occurrence of land-
slides. The study area falls between the 30°15 ' and 30°30'N and 78°0 ' and 78°10'E, covering an area of about 445 km 2 in the Dehradun and Tehri-Garhwal districts of Uttar Pradesh, India (Fig. 1). The impact of landslides on the human population is significant, particularly because of the fact that Mussoorie is a prominent hill station.
2. Geologic setting The study area includes parts of the Lesser and Sub Himalayan zones. The Main Boundary Thrust (MBT) marks the northern boundary of the Sub Himalaya. The fluvial sequences of Neogene age
0169-555X/96/$15.00 ¢~ 1996 Elsevier Science B.V, All rights reserved SSDI 01 69-555X(95)001 2 I-2
170
S.V. Panikkar, V. Subramanyan/Geomorphology 15 (1996) 169-181
~'1o'
(Siwalik Group) form the Sub Himalayan belt. These are overlain by the Doon Gravels of Late Pleistocene age. The Lesser Himalaya, which is bounded by the MBT in the south comprises the ProterozoicCambrian shelf to shallow marine sequences. These include the Mandhali, Chandpur, Nagthat, Blaini, Krol and Tal Formations (Fig. 2). Several workers have dealt with the regional geological aspects of the Himalaya in general and the Lesser Himalaya, in particular. The earliest is that by Medlicott (1864). Auden (1934) has given a detailed account of the geology of the Krol belt, which still forms the primary reference for all work in this area. The other workers include Bhargava (1972), Rupke (1974), Valdiya (1980), Singh (1979), Joshi et al. (1989) and others. The tectonic aspects, particularly the neotectonism and seismicity, have been studied, among others, by Middlemiss (1910), Nakata (1975), Gupta (1992) and Valdiya (1993).
(UTTARKASHI
/5-/
t
3. Seismicity and neotectonics The Himalaya is considered to be a seismically active zone. The area falls in Zone IV of the five seismic zones of India (horizontal seismic coefficient of 0.05). The isoseismal line with a value of 8 fell in the area during the Kangra earthquake of 1905 which damaged the town of Kangra in Himachal Pradesh (Middlemiss, 1910; Auden, 1942). The main epicenter was in Kangra and a secondary epicenter near Mussoorie. More recently, there was a devastating earthquake of magnitude 7.1 in Uttarkashi, northeast of Mussoorie on the 20th October, 1991 (Gupta et al., 1994). The area is still tectonically active. The geomorphic evidence of neotectonic activity includes the diversion of streams by massive landslides, uplifted terraces, entrenched rivers and the presence of zones of active mass movement. The fast uplift (about 5 m m / y r ) has resulted in the "superimposition of a youthful topography on a mature relief" (Valdiya, 1993). The Song river to the east of Dehradun flows northwesterly before taking an abrupt right angled bend, cutting across the strike of the Lesser Himalayan formations and then flows southerly (Fig. 3). The Baldi Nadi joins the Song from the north resulting in a conspicuously straight river course.
/ N
/
/
,,~o, ~ , / " h
. . . .
Fig. 1. Location map of the study area.
The abrupt increase in the elevation of the Lesser Himalayan formations to the east of these rivers and the presence of massive landslides and huge triangular fault scarps all along the river course suggest the possibility of an active fault (Sairku fault). In the Baldi Nadi section at Sahasradhara, brecciated argillaceous limestone occurs, which is totally shattered. The river course here is absolutely straight. Furthermore, there is an abundance of springs on both slopes. These suggest an intraformational fault. The presence of waterfalls (Fig. 3) implies knickpoints in the stream gradient suggesting rejuvenation. The Doon Valley and its surroundings have undergone slow uplift (1 m m / y r ) with respect to the alluvial plains in the south (Nakata, 1975). A distinctive feature of the rivers in the Doon Valley is their deeply entrenched nature. Valley cross profiles drawn across the various rivers indicate that the side elevations vary from 20 m to as high as 100 m. This is substantially high for the rivers that have reached the plains, where one would normally expect braided river channels. The tributaries to the Song river flowing from the west are suddenly incised above their junctions with the main stream. Three levels of
S.V. Panikkar, V. Subramanyan /Geomorphology 15 (1996) 169-181
171
The geomorphic aspects like the landforms and slopes were studied on the aerial photographs and in the field. A slope vector map, delineating units homogenous in terms of slope angle and direction, was prepared from the Survey of India topographical map on the scale of 1:50,000. These units were then regrouped into seven classes (Fig. 4) according to the classification scheme by Young and Young (1974). The SPOT (MLA) and IRS (LISS II) digital data were used to classify the area according to the landuse/land cover (Fig. 5). The geological linea-
terraces have been identified on the aerial photographs (Fig. 3). The deeply entrenched nature of the rivers is significant evidence for the uplift of the Doon Valley.
4. D a t a c o l l e c t i o n
A check list was ~tsed to collect the data, in the field, relating to the various geological, geomorphological and anthropogenic attributes (Table 1). 78"a0'
10'
?8 0
J, DOON GRAVEL [Late PLeistocene ) SIWALIK SANDSTONE (Pliocene ] TAL QUN~TZITE (EaSy Cambrian)
-.....',,
KROL LIMESTONE
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30 °
BLAIN! SLATE NAGTHAT O.UARTZITE
I,:".'.
'
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.':'2",
~
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...... " ~ ' . -
,
~: , ~ " . . ~
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."
•
o
2km
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o
DEHRAOUN
e 0
•
O~
• • ,45o •
•
,-t~.
.,,,,,._~, ...,%~ •
e%
Fig. 2. Geological map of the study area, modified after Rupke (1974) and Valdiya (1975) by incorporating data from the aerial photographs, satellite images and field checks.
172
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
ments were picked up on the SPOT images based on the straight segments in the river courses, linear ridges and straight boundaries between contrasting tones or colours. 216 lineaments were identified in all. The lineament density was then computed for the area. The landslides were identified on the aerial photographs. The SPOT (MLA) FCC of 1987 and SPOT
(PLA) imagery of 1989 on the scales of 1:50,000 and resolutions 20 m and 10 m respectively were used to update the landslide occurrences in the area. The landslides were identified by their bright tone, the landslide trails and the data on typical landslide "recess" on the photographs. They appear bluish on the FCC. However, only the larger slides could be identified on the remote sensing media, which were
78 ° O'
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STRUCTURAL HILLS RESIDUAL HILLS PLA/NS
~
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4,m
WATER FALL jJ
RILL EROSION
Fig. 3. Geomorphological map of the study area prepared from aerial photographs and field surveys.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
then cross-checked in the field. The smaller ones were studied in the field. A total of 75 landslides were identified (Fig. 3) and examined.
5. Geomorphology The area of study lies partly in the outermost fringe of the Lesser Himalaya of Garhwal and partly in the Sub Himalaya represented by the Siwalik Hills and Doon Valley. Tile area is characterised by a rugged topography, with the hill ranges rising from
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173
600 m above mean sea level to over 2300 m. Three broad geomorphic complexes can be identified - Structural hills, Residual hills and Plains. The formations in the Mussoorie Hills dip NE in the south-facing slopes and SW in the north-facing slopes, thus forming a syncline. The Lesser Himalaya is characterized by craggy, elevated hills and well-developed dipslopes. The ridge crests are sinuous because of the large number of amphitheatre valley heads. Triangular facets have developed in an elongated manner close together with three or four of them riding over one another. The usual landforms 78"10' N
-]
LEVEL (/-2" GENTLE 2'-5" MODERATE
I~
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MODERATELY 10"-18' STEEP STEEP 1B'-30" VERY STEEP
36-45" •
PRECIPITOUS :> 45"
0 ?8"OD'
2 Km
?8" 10'
Fig. 4. Slope map prepared from the topographical map on the scale of 1:50,000. The seven classes have been delineated based on the classification scheme by Young and Young (1974).
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
174
that develop on inclined formations (homoclinal ridges) have been, however, modified in this area because of the complexity of the terrain in terms of the tectonic activity. The litho-morphological relations are such that the different rock types are indicated by topographic breaks in slope. These breaks can be identified on aerial photographs as well as in the field. The weaker rocks, namely the slates, form the gentler spur slopes, in contrast to the massive limestones and quartzites which form tough ridges. At places, however, the limestones are more weathered and form fiat-topped to rounded hills.
78"00'
The MBT can be traced by a distinct break of slope between the Sub Himalayan and Lesser Himalayan formations. The drainage pattern also indicates the structural control by way of numerous kinks. The most obvious indication of the Sairku fault is the Song river to the east of Dehradun where it has adapted itself to the fault by turning abruptly south from a westerly course. The slope varies from being almost level in the plains surrounding Dehradun to near vertical cliffs in the Lesser Himalayan formations (Fig. 4). The valley-head slopes are invariably steep, whereas the 7~I0'
N
~-~
Barren land • Cultivated [and '.-o : :.~j-I Forest 41
Quarry
~ River Urban l a n d / ! . Sparse ..~ vegetation Landslide
0
78"00'
2Kin
~8"Io'
Fig. 5. Landuse/land cover map. The landslide distribution is also indicated. 65.17% of the landslides are in sparsely vegetated areas.
S.V. Panikkar, V. Subramanyan/Geomorphology 15 (1996) 169-181
valley-side slopes show the entire range from gentle to steep. The spur-end slopes, on the other hand are always gentle. The :~outh-facing slopes are gentler than the north-facing ones, which can be attributed to microclimatological factors (Thornbury, 1969). The relative relief of the area increases from almost zero to over 400 m. The area exhibits, a range of drainage patterns
~'oo'
175
from dendritic through trellis, radial, parallel and deranged to barbed. The dominant pattern, however, is dendritic, which has developed mostly over the Lesser Himalayan formations. There are a number of springs in the area (Fig. 3). Most emanate from the middle slopes, though there are also a few from the basal slopes. The lineament map (Fig. 6) shows the dominant 70 ~O'N
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Fig. 6. Lineament map prepared from SPOT MLA and PLA images. The continuous lines indicate the faults cross-checked in the field. The other lineaments are marked by broken lines.
176
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
Table 1 Check list for field data collection Geologic Lithology Density of joints Joint openings Joint infillings Direction of major joints with respect to steepest slopes Strong beds over weak beds Degree of weathering Geomorphic Relief Slope angle Height difference between adjacent valleys Drainage density River gradient Slope undercutting
Anthropogenic Road cut depth Road cut position on slope Removal of vegetation Loading of upper valley side
trend to be NNE-SSW, which corresponds to the trends of the active Sairku fault and the MBT to the east of Dehradun. The next prominent direction of N E - S W represents the numerous straight segments in the entrenched rivers in the Doon Valley. The land use/land cover ( L U / L C ) categories in the area have been recognised on the remote sensing media, with field checking. The different L U / L C types delineated are cultivated land, sparse vegetation, forest, urban land, quarries, barren land and rivers (Fig. 5). The area-wise distribution of the different L U / L C types is given in Table 2. The cultivated lands are mostly in the plains or on the gentler spur slopes. The southern slopes of the Mussoorie Hills are more barren than the northern slopes. Significantly, about 15% of the study area comprise sparse vegetation and barren land on the steepest slopes ( > 30°). These are the potentially unstable zones.
6. Landslide evaluation There are a few major and several minor landslides in the area. Problem related to landslides
becomes acute after rains, when the roads get blocked by landslide debris. The Nalota Nala, Kalagarh Nala, Signikhala and Jabarkhet slides are a few major ones on the Dehradun-Mussoorie road. Besides these, there are about ten major slides to the east of Dehradun in the Song river section. Most of these slides are very large and on steep (18-30 °) to very steep (30-45 °) slopes. The landslides in the area can be essentially divided into two types: rock slides and debris slides based on the materials involved. River bank slumping has been observed locally in the plains, especially along the Nun Nadi and Tons river. Soil creep was observed at places, indicated by the inclination of the trees at an angle of about 5 ° from the vertical on slopes of 35 ° in weathered argillaceous limestone. The landslides in the area are either naturally occurring or owe their origin to human interference, in terms of quarries, buildings and road cuts.
6.1. Geological aspects The landslides are mostly in the Krol limestone and Nagthat quartzite (67.3%), although there are a few in the Chandpur slates. The percentage-wise distribution of the landslides in the different formations in the study area is given in Table 3. Rock slides occur mostly in the Nagthat quartzite, whereas debris slides occur in limestones and slates. The solution activity in limestones, in places, has resulted in the formation of solution caves at the base resulting in the removal of toe support. There are several debris slides in the argillaceous limestone along the road leading to Mussoorie. The presence of shale intercalations and the foliated nature of the limestone, with the closely spaced intersecting joints Table 2 Area-wise distribution of the different landuse/land cover categories Landuse/land cover category
Area (km 2)
Perc.
Cultivated land Sparse vegetation Forest Urban land Quarries Barren land River
127.36 144.27 82.70 53.33 3.68 16.76 13.13
28.86 32.70 18.74 12.09 0.83 3.80 2.98
SN. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181 Table 3 Percentage distribution of the landslides in the different formations Doon Gravels Siwalik Tal Krol Blaini Nagthat Chandpur Mandhali
4.7% 3.5% 6.3% 39.3% 9.2% 28.0% 9.0% 0.0%
have contributed to these slides. Calcite, occurring as infilling material along the joints, has been removed by solution resulting in the further opening up of these joints. The microfolding of the beds has resulted in axial plane cleavages, which have subsequently been widened to act as potential slip surfaces. The Nagthat quarlzites are highly jointed with several sets of closely spaced, open, planar and smooth joints that have served as slip surfaces. These joints, especially when dipping downslope at angles greater than the slope angle, are significant. The stratigraphic control also plays a role here, with the competent Nagthat quartzite overlying the weaker and highly-sheared Chandpur slate. The proximity to the MBT and the Sairku fault, which are the two active faults in the area is a significant aspect in the context of the occurrences of landslides. 49.5% of the landslides in the study area occur within a distance of 1.5 km from the active faults. There are landslides in the Chandpur slates close to the MBT. The slates here are highly sheared at the contact between the Lesser and Sub Himalayan formations raarked by the MBT. The best example of landslides associated with neotectonism is the Song river section (Sairku fault, Fig. 3). There are huge triangular fault scarps representing the truncated spur slopes. The rock types here are Nagthat quartzite and Chandpur slate.
6.2. Geomorphological aspects The occurrence of landslides in terms of the landforms was studied. The slides in the area are on the escarpment side, where joint planes inclined steeply downslope have been critical. The develop-
177
Table 4 Percent distribution of the landslides in the geomorphic complexes Structural Hills Residual Hills Plains
95.5% 3.3% 1.2%
ment of the escarpments itself has been controlled by these steeply dipping joints. In the case of the Song river section, all the landslides occur on the escarpment side on the fault scarps, with the formations (Nagthat quartzite and Chandpur slate) dipping into the hill. The occurrence of landslides on dipslopes, with the bedding planes serving as the slip surface, is not observed in this area because of the synclinal structure. The distribution of the landslides in the different geomorphic complexes is given in Table 4. The landslides in the residual hills are on moderate slope areas of Doon gravels and sandstone. In the plains, as mentioned earlier, the rivers are deeply incised resulting in high bank elevations. This has resulted in river bank slumping, especially where there is pronounced undercutting by the stream. The distribution of the landslides in the different slope categories was studied with reference to the rock types (Table 5). The stromatolitic limestones in the Krol Formation, exposed in Barlowganj form gentler slopes. This, coupled with the near absence of joints, makes it relatively free of landslides. The high percentage of the landslides in Krol Limestone on steep slopes is due to the artificially oversteepened slopes left after the quarrying of the massive limestone. The occurrence of natural landslides in massive limestone is rare because of the lesser de-
Table 5 Limiting angles for the occurence of landslides in the different formations Tal quartzite Argillaceous limestone Massive limestone Nagthat quartzite Blaini slate Chandpur slate Siwalik sandstone
> 45 18-30 30-45 > 30 18-30 30-45 > 30
(87.5%) (52.2%) (43.3%) (47.6%) (80.9%) (73.9%) (82.3%)
Figures in parentheses indicate the percentage of landslides in the different formations.
S.V. Panikkar, V. Subramanyan/Geomorphology
178
gree of jointing, the wide spacing, tight, curved, rough and unsystematic nature of the joints and the presence of dense forest cover, in spite of the fact that they form steep slopes. The slopes in Nagthat quartzite vary from steep to very steep. About 47.6% of the slides in Nagthat quartzite occur on slopes )30 ° . This includes the massive landslides in the near vertical fault scarps along the Song river. However, the fact that these quartzites are highly jointed and overlie the weak and sheared Chandpur Slate explains the presence of landslides in them on slopes ranging from 18-30 ° . The Chandpur Slates are highly sheared and generally form the gentler spur slopes. However, in areas where they are not severely sheared, they form steeper slopes ( > 30 °) with the associated occurrences of landslides. In the case of the Doon gravels, slumps occur on the moderate slopes of the river banks, which are high due to the entrenched nature of the rivers. A significant feature of the relationship between the slope aspect and the occurrences of landslides is that all the slides in the area are on south-facing slopes. Not a single slide could be identified on north-facing slopes. The possible explanation for this could be microclimatological factors. Furthermore, anthropogenic activities are concentrated on the south-facing slopes, i.e., the area between Dehradun and Mussoorie.
15 (1996) 169-181
The removal of toe support by the undercutting of streams is one of the important causes for the occurrences of landslides in the area. The geomorphic map (Fig. 3) shows the areas of undercutting delineated from the aerial photographs. A comparison between these areas and the landslide distribution would suffice to realise the significance of stream undercutting. In fact, 85.1% of the landslides occur within a distance of 200 m from the rivers. The drainage density values are moderate ( 2 - 4 k m / k m 2) in 86.5% of the landslide occurring areas. The role of springs in the initiation of landslides was studied in the field. In many of the smaller slides, the process of spring sapping leading to headward erosion was found to be critical. However, a comparison of the occurrence of springs and the major landslides (Fig. 3) reveals no significant association between the two, indicating that the springs have not had a crucial role in the initiation of the major landslides. Elevation difference between adjacent valleys is another potentially destabilising factor. The profile (Fig. 7) across the Song river valleys, for example shows a difference in elevation of about 120 m between the two rivers within a distance of 2.5 km. The intervening slope is, in fact one with several landslide occurrences. The depth to the water table is as low as 5 m immediately after the rains in July and August. However, within a couple of months, by November, all the rivers in the area become absolutely dry and the water table goes
lO00m "-'-~20m 600 m
00000000
0 0 0 0 0
"~
Om SW
['~
0 DOON GRAVEL
KROL LIMESTONE ~ ~"lC
BLAINI SLATE
1
2 km ~
NE NAGTHAT 0.UARTZITE
HANDPUR SLATE
Fig. 7. Topographic profile across the Song river, east of Dehradun. Note the elevation difference of about 120 m between the two adjacent valleys within a distance of 2.5 km.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181
down. The water table fluctuations in the Doon Valley range from 17 m to 33 m. This is due to the highly permeable na~Lure of the Doon Gravels forming the plains in the Doon Valley. The effect of this fluctuation, in the context of the occurrence of river bank slumping, is ~Lat the sudden fall in the water table results in an apparent recovery of weight of the materials that were saturated with water. This would amount to overloading of the materials. This can initiate slumping in the river banks, especially where the toe support has also been removed by lateral erosion due to strea~a undercutting. The altitude in the area, as mentioned earlier, varies from 600 m to 2300 m. 75% of the landslides occur in the range 800-1800 m. Only 5.5% of the landslides are observed in areas o f elevation greater than 1800 m because, the Tal quartzite, which occurs in this range of elevation is relatively free of landslides. Moreover, 45,4% of the area above 1800 m is forested. In the case of the relative relief, 79% of the slides are in areas of moderate values (100-300 m). This is because of the fact that 75% of the quarries and barren land and 66% of the sparsely vegetated areas occur in the moderate relief regions. Most of the landslides are concentrated in these three land u s e / land cover categorie:~. The lineaments irt the area have been studied and the proximity to active faults has already been discussed in the context of landslide occurrences. The lineament density values are moderate (0.5-1.0 k m / k m z) to high (1.0-1.5 k m / k m 2) in 73% of the landslide province.
6.3. Anthropogenic aspects The distribution of the landslides was studied with reference to the landuse/land cover categories. It was observed that 65.17% of the slides occur in areas of sparse vegetation, which form 32.7% of the study area. The percentage-wise distribution of the landslides in the different landuse/land cover groups is given in Table 6. The significance of the forest cover in the present context can be gauged from the fact that 91.39% of the slides occur in non-forested areas. The residual hills in the Doon Valley, for example form moderate slopes, but account for only
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Table 6 Percentage distribution of the landslides in the different landuse/ land cover categories Sparse vegetation Barren land Quarries Forest Cultivated land Urban land
65.17% 11.15% 9.98% 8.61% 3.72% 1.17%
3% of the total landslides in the area because they are densely forested. There are several minor landslides all along the road from Dehradun to Mussoorie. However, some of the landslides like the Nalota Nala, Kalagarh Nala, Signikhala and Jabarkhet slides (Sastry et al., 1981) are major ones. These slides block roads with their debris, especially after the rains. The landslide distribution in terms of the distances from the roads in the area was studied. Significantly, it was found that only 25% of the major slides occur within a distance of 200 m from the roads and 51.9% of them occur at distances greater than 400 m. This implies that the presence of road cuts has not served as a dominant cause in the case of the larger landslides. However, the problem still remains in the case of the minor slides, as observed in the field.
7. Triggering factors Irrespective of the combination of factors present in an area, the triggering factors invariably are rainfall and/or seismicity. Most of the landslides in the area occur after heavy rains, which serve to saturate the materials thereby increasing their effective weight. Moreover, it can cause the weakening of materials like clay which swell up when moist. The mean annual rainfall in the area is 2875 mm (Sastry et al., 1981), most of which is concentrated in the months of July and August. In fact 40% of the rainfall occurs in the month of August alone. The other triggering factor is seismicity. The Himalaya, as mentioned earlier, is known to be a seismic zone. The ground motion during an earthquake can trigger off large scale landslides in the area.
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8. Preventive m e a s u r e s
Once the causes for the occurrences of landslides have been established, the preventive measures can be considered. Since deforestation has been noticed to be one of the important factors for the occurrence of slides, afforestation programmes should be initiated to stabilise the slopes. This should be particularly concentrated on the steep barren slopes in the Nagthat quartzite and Krol limestone, including the abandoned quarries. The quarrying in the area has now been banned, except in a few places like Cloud End, Mussoorie. The quarrying should be done in such a way that the natural slope is approximated at the end, instead of leaving behind oversteepened slopes. The toe erosion by stream undercutting is a dominant destabilising factor in most of the landslides in the area. This can be minimised or fully avoided by the construction of toe walls along the concave bank of the stream. The height of the toe wall should be maintained higher than the highest expected flood level in the stream (Sastry et al., 1981). Another way to prevent erosion is by constructing deflecting terms to divert the flow away from the bank. Construction of retaining walls along the slope would serve to stabilise the potentially unstable slopes.
9. Conclusion
The geological, geomorphological and anthropogenic parameters have been discussed in the context of their relationship to the occurrence of landslides. The significant conclusions have been elaborated here. 1. The two types of landslides in the area, rock slides and debris slides, are either naturally occurring ones or owe their origin to human interference. River bank slumping occurs locally in the Doon Valley. 2. The landslides are concentrated in the Nagthat quartzite and Krol limestone. 3. The proximity to the active faults (MBT and Sairku fault) is a significant aspect in the occurrences of landslides. 4. The joint planes which have controlled the development of an escarpment have served as potential slip surface. All the landslides occur on escarp-
ments with the formations dipping into the hill, suggesting that the bedding planes have not served as the slip surfaces. 5. The limiting angles for the occurrences of slides are > 45 ° for Tal quartzite, 18-30 ° for argillaceous limestone and Blaini slate and 30-45 ° for massive limestone, Nagthat quartzite and Chandpur slate. 6. All the landslides occur on south-facing slopes, which may be attributed to the microclimatology and the greater degree of anthropogenic activities here. 7. The removal of toe support by stream undercutting is the most frequently observed cause for sliding in the area. 85% of the landslides occur within a distance of 200 m from the streams. 8. The headward erosion by the process of spring sapping was found to be significant in the case of the smaller slides. However, this does not seem to serve as a major cause in the larger slides. 9. The elevation difference between adjacent valleys, in many cases, has rendered the intervening slope unstable. I0. Water table fluctuations are a significant factor in the initiation of river bank slumping. 11. 91% of the landslides occur in non-forested areas, indicating the effect of vegetation on the initiation of slope instability. 12. Road-cuts have served to cause minor landslides, although they have not had a dominant influence in the case of the major landslides. 13. Rainfall and seismicity form the dominant triggering factors for landslides in the area. 14. Preventive measures which should be taken include afforestation programs, constructions of toe walls and deflection terms to prevent lateral erosion and stream undercutting, construction of retaining walls to stabilise slopes and proper quarrying practice where the original slope is restored after quarrying.
References Auden, J.B., 1934. Geology of the Krol Belt. Rec. Geol. Surv. India, 67: 234-454. Auden, J.B., 1942. A geological investigation of tunnel alignment for the Jamuna hydro-electric scheme. Rec. Geol. Surv. India, 77: 1-50. Bhargava, O.N., 1972. A reinterpretation of the Krol Belt. Himalayan Geol., 2: 47-81.
S.V. Panikkar, V. Subramanyan / Geomorphology 15 (1996) 169-181 Gerrard, J., 1994. The landslide hazard in the Himalaya: Geological control and human action. Geomorphology, 10(1/4): 221230. Gupta, G.D., 1992. Himalayan Seismicity. Geol. Soc. India Spec. Publ., 23. Gupta, R.P., Saraf, A.K., Saxena, P. and Chander, R., 1994. IRS detection of surface effects of the Uttarkashi earthquake of 20 October 1991, Himalaya. Int. J. Remote Sensing, 15(11): 2153-2156. Joshi, A., Mathur, V.K. and Bhat, D.K., 1989. Discovery of Redlichid trilobites from the Arenaceous member of the Tal Formation, Garhwal S)mcline, Lesser Himalaya, India. J. Geol. Soc. India, 33(6): 538--546. Medlicott, H.B., 1864. On the geological structure and the relation of the southern portion of the Himalayan ranges between the rivers Ganges and the Ravee. Mere. Geol. Surv. India, 3: 1-212. Middlemiss, C.S., 1910. The Kangra earthquake of 4th April, 1905. Mem. Geol. Sure. India, 38: 1-409. Nakata, T., 1975. On Quaternary tectonics around the Himalayas. Rep. Tohoku Univ., 7t~a Ser, (Geography), 25:111-116. Rupke, J., 1974. Stratigraphic and structural evolution of the Kumaun Lesser Himalaya. Sediment. Geol., 11: 81-265.
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Sastry, G., Mathur, H.N. and Tejwani, K.G., 1981. Landslide control in north-western Outer Himalaya - - A case study. Bull. No. R8/D6, Central Soil and Water Conservation Research and Training Institute, Dehradun. Singh, I.B., 1979. Environment and age of the Tal Formation of Mussoorie and Nilkanth areas of Garhwal Himalaya. J. Geol. Soc. India, 20(5): 214-225. Thornbury, W.D., 1969. Principles of Geomorphology. 2nd ed. Wiley Eastern Limited, 112 pp. Valdiya, K.S., 1975. Lithology and age of the Tal Formation in Garhwal and implication on stratigraphic scheme of Krol Belt in Kumaun Himalaya. J. Geol. Soc. India, 16(2): 119-134. Valdiya, K.S., 1980. Geology of the Kumaun Lesser Himalaya. Wadia Institute of Himalayan Geology Publn., Dehradun, pp. 1-205. Valdiya, K.S. , 1993. Uplift and geomorphic rejuvenation of the Himalaya in the Quaternary period. Current Sci., 64(11/12): 873-885. Young, A. and Young, D.M., 1974. Slope Development. Macmillan Education.