IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002 
distributed between elevation of 800-2400m with maximum 
occurrence between 1200-2000m (57%). About 50% of the area 
occupied by Nagthat quartzites are found between 1600-2000m, 
30% is between 1200-1600m and rest is equally distributed 
between 800-1200m and 2000-2400m. Metabasics are found 
between 800-1600m. 
5.2.4 Landuse-landcover Vs Elevation: About 57% of the 
area under elevation of 800-1600m is occupied by agriculture 
lands. Between 1600-2400m majority of area is covered by 
forests in which open forests dominate. Between 2400-2800m 
dense forests cover about 76.5% of the total area. 
5.2.5 Landuse-Landcover Vs Slope: Majority of agricultural 
lands is developed on slopes between 15°-35° (88%). Forests 
are distributed mainly over 15°- 45° (dense, 93%, open, 94%, 
degraded, 96%). Forest blanks are dominant between 25°- 45° 
(86%) whereas the scrubland are mainly developed over 25°- 
35° (65%). Barren lands occur mostly in 35°-55° (73%). Built 
up land shows more or less equal development over slopes of 
less than 5° to 35°. 
5.2.6 Slope Vs Elevation: The majority of slopes (43%) 
of 25°-35° are confined to 800-1200m whereas 15°-25° 
slopes are mainly distributed between 800-1200m and 
1200-1600m. The slopes of less than 15° are under 
elevation of 1200m. 
5.3 Incidence of landslides 
Landslide is a major land degradational process in the area. A 
total of 92 active and 69 old landslides were identified in the 
area. Majorities of active landslides are located along the 
drainage channels and roads (Figure 4) indicating that the 
dynamic processes of fluvial action and anthropogenic 
activities related to road and infra- structure development are 
mainly responsible for landsliding in the region. 
The incidence of landslides in an area is governed by a number 
of terrain parameters. In order to access the contribution of each 
parameter in inducing landslides around Tehri dam, the 
thematic map pertaining to old and active landslides is utilized 
for calculation of their incidence in terms of their area and 
frequency with respect to different terrain units. The details of 
each are described below. 
5.3.1 Landslide Vs Lithology: Over the lithounits, about 77% 
of the total number of active slides occur over Chandpur 
phyllites and 16% over Pratapnagar quartzites. Old slides also 
show similar distribution with maximum number over 
Chandpur phyllites (72%), 10% over Pratapnagar quartzites and 
nearly equal distribution over Nagthat and Bhelunta limestones. 
In term of area of landslides, about 71% of active and old slides 
occur over the Chandpur phyllites. 
5.3.2 Landslide Vs Landuse: On comparing the landslide 
occurrence over various landuse categories it is noticed that 
landslides are maximum in number over agricultural lands. In 
the agriculture lands, 35 active and 40 old landslides were 
delineated. There are 30 active and 11 old slides over the 
scrubland, 18 active and 9 old slides over degraded forests and 
9 of each category over open forests. About 73% of the area, 
occupied by old slides, occur over the agricultural lands. The 
presence of large number of slides over agricultural lands can 
be attributed to their location along the major river valleys and 
weathered rocks under the agricultural lands. 
652 
5.3.3 Landslide Vs Slope: On comparing landslide occurrence 
over different slope classes, it is found that majority of active 
(78) and old slides (65) are occurring over 15° 45° slopes, out 
of which 38 active and 40 old slides are located between 25?- 
35°. There are 79 landslides on the northern aspect (including 
29 old slides). The area occupied by active and old landslides is 
1.6 sq. km and 8.2 sq. km respectively. It is noticed that about 
46 active and 8 old landslides will be submerged in the 
reservoir. 
5.3.4 Landslide Vs Elevation: About 7996 of the active 
landslide area lie under elevation of 1200m. Majority of old 
slides are confined between 800-1400m (53%). 
6. RESULTS AND DISCUSSION 
The area statistics of lithological units show that the majority of 
area around Tehri dam is occupied by Chandpur phyllites 
(57%) and Pratapnagar quartzites (26.4%). About 91.5 % of the 
areas, which will be submerged in the reservoir, are constituted 
by Chandpur phyllites, which may leads to heavy siitation due 
to fragile nature of phyllitic rocks. The various types of 
landuse-landcover around Tehri dam mainly pertains to 
agricultural land (47.2%) and forests (42.4%). The agricultural 
lands, which will be submerged in the reservoir, constitute 52.6 
% of the reservoir area whereas about 13.8 % include forest 
areas. The built up land at old Tehri town and the highly fertile 
land of river terraces along the Bhagirathi and Bhilangna rivers 
at Tehri, Seansu and Chinyalisaur etc. will also be submerged. 
The majority of slopes around Tehri dam are moderately steep 
(25?-35?, 56.496). About 42.4 % of the area that will be 
submerged in the reservoir are under 25°-35° slopes indicating 
increase in landslide incidence after submergence especially in 
potential landslide zones. The spatial relationship between 
different terrain parameters through cross tabulation in GIS 
environment indicates that forests are dominant over quartzites 
and limestones whereas over the phyllites and slates, 
agricultural lands occupy majority of their area. The Chandpur 
phyllites (slaty) are maximum over slopes of 25°-35° (54%) 
whereas Pratapnagar quartzites are always found over the 
slopes greater than 15°. About 60% of the Chandpur phyllites 
(slaty) occur within 800-1200m. Pratapnagar quartzites are 
having maximum occurrence between 1200-2000m (57%). 
About 57% of the area under elevation of 800-1600m is 
occupied by agriculture lands. Between 2400-2800m dense 
forests cover about 76.5% of the total area. Majority of 
agricultural lands is developed on slopes between 15°-35° 
(88%). Forests are distributed mainly over 15°-45° (dense, 93%, 
open, 94%, degraded, 96%). The majority of slopes (43%) of 
25°-35° are confined to 800-1200m. A total of 92 active and 69 
old landslides were identified in the area. About 77% of the 
total number of active slides occur over Chandpur phyllites and 
16% over Pratapnagar quartzites. Landslides are maximum in 
number over agricultural lands (35 active and 40 old 
landslides). Majority of active (78) and old slide (65) are 
occurring over 15° 45° slopes. About 79% of the active 
landslide area lies under elevation of 1200m. Majority of old 
slides are confined between 800-1400m (53%). 
The Tehri dam is under construction in a seismically and 
tectonically active zone of the Central Himalayas, which have 
experienced Uttarkashi earthquake in 1991 and Chamoli 
earthquake in 1999 of intensity M-6.6 and M-6.8 respectively 
(Valdiya 2001). The Chandpur phyllites, which are highly 
sheared in the vicinity of active North Almora Thrust, cover