SING
INS.
> terrain
d using
aluation
low that
various
n forest
is under
| terrain
gar and
My over
and old
napping
r forest
—78?40'
area of
arhwal,
located
r of the
rivers
he dam
arkashi
arryout
ence to
evation
6) and
zed for
iducing
terrain
| (GIS)
spatial
Ised as
iber of
lcover,
Islides,
red on
ts and
IAPRS & SIS, Vol.34, Part 7, “Resource and Environmental Monitoring”, Hyderabad, India, 2002
available literature. These maps are transferred on mylar sheet
using 0.3 drafting pen. All the maps were marked with tics
(gradicules) taken from Survey of India toposheets, 53J/5, 6, 7.
9, 10, 11. These maps were scanned using AO scanner with 600
dots per inch (DPI). The scanned data available in rater format
for various thematic maps was then vectorised and converted to
Arc-Info coverages (polygon, line and point) using "generate"
module in the Arc/info GIS package. The coverages of different
thematic maps generated were edited and transformed to real
world coordinates for spatial analysis.
5. TERRAIN MAPPING AND SPATIAL ANALYSIS
Terrain mapping in mountainous terrain is always a challenge
as major part of the area is inaccessible. The regional coverage
of the study area acquired by satellite images provided pictorial
representation of geological units with imprints of drainage,
geomorphic landforms, landuse, erosional and tonal
characteristics. This helped in demarcation of different
lithological domains. The delineation of regional extent of
thrusts and faults on satellite image also helped in lithological
correlation in the area.
The various types of landuse — landcover present in the area
reflect the incident light differently causing each type to be
differentiated by a unique signature. Different types of
vegetation (forest, crop, scrub, plantation etc.) because of their
different structure, biochemical and biophysical properties were
identifiable on the F.C.C. image. Active and old landslides have
been demarcated on satellite images followed by ground
checks. Active landslides were identified in IRS-1C, FCC as
high reflection areas with light blue tones. Old landslides
generally occur within the confine of agricultural land and are
characterized by hummocky topography with old slide scars.
The GIS technique is very useful for spatial analysis involving
area statistics and cross tabulation of thematic data. The
accurately prepared GIS database gives high level of accuracy
in area calculation as compared to manual methods. The area
calculation pertaining to lithology, landuse-landcover and slope
was performed in order to know their spatial distribution
around Tehri dam. The arial extents of these units, which will
be submerged in the Tehri dam reservoir, were also calculated
to assess the impact of reservoir water in causing land
degradation.
5.1 Area Statistics of Thematic Database
5.1.1 Lithological units: The lithological units encountered in
the area are of Precambrian age and occur in the form of nappes
defined by thrusts at the base (Saklani, 1993). The rock
formation strikes in a NW-SE direction with a general NE dip.
The area is traversed by two major thrusts viz. North Almora
Thrust (NAT) and Garhwal Thrust (Figure 1). Effects of
shearing and crushing are discerned near the thrust contacts.
The area statistics of lithological units show that the majority of
area around Tehri dam is occupied by Chandpur phyllites
(57%) followed by Pratapnagar quartzites (26.4%), Bhelunta
limestones (8.63%), Nagthat quartzite (6%), Bhainga slates
(1.2%), Paturi quartzites (0.51%) and metabasics (0.17%).
About 91.5 % of the area, which will be submerged in the
reservoir, are constituted by Chandpur phyllites.
5.1.2 Landuse-landcover: The various types of landuse-
landcover around Tehri dam pertain to agricultural land
(47.2%), dense forest (7.1%), open forest (21.2%),
degraded forest (14.1%), forest blank (1.02%), scrub land
(6.6%) and barren land (0.83%). The water body covers
about 0.88% of the area whereas built up land accounts
for 1.39% (Figure 2). The agricultural lands which will be
submerged in the reservoir constitute 52.6 % of the
, reservoir area whereas about 13.8 % includes forest areas
651
and 16% scrub lands. 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.
5.1.3 Slope: The slope map of the area is prepared by analyzing
the contour spacing in the topographical sheets of Survey of
India on 1:50,000 scale (Figure 3). The various slope categories
around Tehri dam in terms of the slope angle and percentage
area covered is: very gentle sloping (slope angle <5°, area
covered 1.2%), gentle (5°-15°, 3%), moderate (15°-25°, 24%),
moderately steep (25°-35°, 56.4%), steep (35°-45°, 12.4%),
very steep (45?-55?, 2.196) and precipitous (>55°, 0.44%).
Slopes less than 15° are met within river terraces whereas
majority of slopes around Tehri dam is inclined at 25°-35°,
About 42.4 % of the area that will be submerged in the
reservoir is under 25°-35° slopes.
5.2 Cross Tabulation of Thematic database:
The cross tabulation of one thematic map with another
constitutes an important application of GIS. Different thematic
maps were used for cross tabulation to understand the spatial
relationship of one terrain parameter with respect to another.
The incidence of landslides over different terrain units was also
calculated to access the influence of each in inducing
landslides. The details of cross tabulation are described below.
5.2.1 Lithology Vs Landuse-landcover: It is noticed that
forests are dominant over quartzites and limestones whereas
over the phyllites and slates, agricultural lands occupy majority
of their area. Agricultural lands occupy about 60-7096 of the
area of Chandpur phyllites and Bhainga slates. Built-up lands
are present mainly over Chandpur slaty phyllites (9396). In the
Pratapnagar quartzites forests occupy about 70% of its area and
remaining is under agricultural lands. In the Nagthat quartzites
forests occupy 60% of its area. Bhelunta limestones show equal
distribution of agricultural lands and open forests. Over the
Paturi quartzites, 70% of its area is under open forests. The
metabasics show nearly equal distribution of agricultural lands
and forests.
Dense forests are dominant over Pratapnagar quartzites (48%)
followed by 39% over Chandpur phyllites. Degraded forests
mainly spread over Chandpur phyllites (48%) and Pratapnagar
quartzites (28.5%). Forest blanks are 39% over Bhelunta
limestones and 36% over Pratapnagar quartzites. Scrubland
occurs mainly over the Chandpur phyllites (84 %). About 83 %
of the barren land are located over Bhelunta limestones.
5.2.2 Lithology Vs Slope: The Chandpur phyllites (slaty)
occupy significant area of all the slope categories. They occupy
the entire area having slope of less than 5° and are dominant
over slopes of 25°-35° (54%). The Pratapnagar quartzites are
found over the slopes greater than 15° with maximum
occurrence over 25°-35° (57%) and the remaining area is
equally occupied by slopes of 15°-25° and 35°-45° (19%).
About 73% of the Nagthat quartzites occur between 25°-35°,
5.2.3 Lithology Vs Elevation: About 60% of the Chandpur
phyllites (slaty) occur within 800-1200m while schistose
phyllites are distributed in 800-1200m (44%) and 1600-2000m
(55%). Bhainga slates are found in equal proportions between
800-1200m and 1200-1600m. Pratapnagar quartzites are mainly
