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CHAPTER 4

Topographic Analysis

4.1 Digital Elevation Model (DEM) in topographic analysis

The term digital elevation model or DEM is frequently used to refer to any raster

representation of continuous elevation of a topographic surface with a common

datum. Burrough (1986) has defined DEM as any digital representation of continuous

variation of relief over place. With increased popularity of GIS technology and

availability of DEMs the potential of using DEMs in studies of surface process has

been widely recognized (Wharton, 1994). DEM has been utilized as one of the core

databases in many GIS application practices. DEM not only provides the description

about three-dimensional surface and data foundation for impressive three-dimensional

visualisation of geographical data, but also sets the foundation for deriving other

surface morphological parameters such as slope, aspect, curvature, slope profile and

catchment areas. Among all the morphological parameters, slope and aspect have

been arguably the most frequently utilised in GIS applications.

DEMs are data files that contain elevation of a terrain over a specified area, usually at

a fixed grid interval over the surface of earth. The individual between each of grid

points will always be referenced to some geographical co-ordinate system. DEM is

used for extracting the terrain information and determining the terrain attributes such

as elevation, slope aspect etc to delineate drainage networks and watershed

boundaries. New methods and algorithms have been developed to automate the

procedure to terrain characterization (Hogg et al., 1993; Guth 1995; Desmet and

Govers, 1996). It also helps to geological and geomorphological mapping In addition;

DEMs have been incorporated in distributed hydrologic models (Garrote and Bras,

1995).

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Shuttle Radar Topography Mission (SRTM) is lunched on 11th February, 2000 and

available in public domain is a useful elevation model data source for regional

landscape analysis even with its coarse spatial resolution (pixel size ~90m). SRTM 3-

arc second DEM is the result of a collaborative effort by the National Aeronautics and

Space Administration (NASA), the National Imagery and Mapping Agency (NIMA),

the German space agency and Italian space agency (van Zyl 2001, Rabus et al. 2003;

Foni and Seal 2004).

In the present study, the SRTM data for whole Jia Bharali River was clipped (Figure,

4.1) and brought in to GIS environment for further analysis maintaining same datum

and projection (WGS-1984, UTM Zone-46) as in the satellite data. The SRTM DEM

has been downloaded from the Global Land Cover Facility (GLCF) Web site of the

Maryland State University.

Figure 4.1: SRTM DEM of the Jia Bharali River Catchment

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From the SRTM DEM an elevation map is prepared in Arc-Info by reclassifying the

DEM into eleven zones of elevation differences (Figure, 4.3). The maximum

elevation is more than 6000 in the Indo-Tibet boarder while the minimum elevation is

less than 100m. The minimum elevation of the area is less than 68m near the

confluence of Jia Bharali with Brahamaputra River. More than 83% of the total area

lies above the 500m elevation value and almost 17% of total area lies within the 500m

contour. Thus the upper part of the 500m elevation divided into seven divisions with

an interval of 1000m. Below the 500m upto the 200m demarcated as one zone and

after 200m three divisions is made with 50m elevation interval upto 100. (Table 4.1)

Table 4.1: Table showing the distribution of area within different elevation class

Figure 4.2: Bar diagram showing area distribution within different elevation

Classes

Elevation (m) Area (km2) Area % <100 876.5 7.8100-150 317.4 2.8150-200 127.1 1.1200-500 527.0 4.7500-1000 1304.5 11.61000-2000 3380.3 30.02000-3000 2872.7 25.53000-4000 1167.0 10.44000-5000 603.6 5.45000-6000 94.8 0.8>6000 9.6 0.1

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Areas are computed from the attributed values under different elevation zones shows

that about 7.8% of the total area lies within the 100m elevation. This area mostly

represents the low lying alluvial plain, wetlands, active channels and the flood plain

of Brahmaputra, Jia Bharalai and Mara Bharali. Areas falls within the 100-150m

elevation are mostly composed of older alluvium, river terrace occupying 2.8% of the

total area. Within 150-500 m elevation interval occupying the 5.8% of the total area

comes under the piedmont zone and the alluvial fan deposits and Siwaliks in the

foothill. In the elevation range of 500-1000m, 11.6% of area is occupied by the

Siwaliks and Gondwana and some parts of structural hills with low elevation along

the Kameng River. ~30% of the total area within 1000-2000m elevation interval lies

in the highly dissected structural hills. 25.47% of areas lie within 2000-3000m

elevation interval. Areas above 3000m elevation lie along the NW boundary of the

catchment. Elevation with more than 6000m elevation, with areas less than 1% of the

total area lies along the India-Tibet boarder.

Figure 4.3 Classified elevation map of the Jia Bharali River Catchment based on

SRTM DEM

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4.2 Slope

The two main derivatives of a continuous surface are slope and aspect. Slope is the

rate of change in gradient of a surface and can be defined by a plane tangent to a

topographic surface (Burrough, 1986). Analytically slope gradient is defined as the

maximum rate of change in altitude (tan θ), aspect as the compass direction of this

maximum rate of change. The angles being measured in a vertical plane along the

direction in which the change of gradient is highest. And this direction, along which

the gradient is maximum, is the aspect at that point. Thus at any given point of surface

comprises of two components namely gradient i.e., slope and the direction, i.e., aspect

(Evans, 1980). Singh (1998) defines slope as angular inclination of terrain between

hill tops (crest) and valley bottoms, resulting from the combination of many causative

factors like geological structure, climate, vegetation cover, drainage, drainage texture

and frequency, dissection index, relative relief etc. The slope ranges between 0o-90o,

while aspect ranges between 0o-360o. Slope gradient can be express in percent or in

degrees.

Slope map (Figure 4.5) of Jia Bharali River Catchment is created from the SRTM

DEM using ArcGIS 9.1. The slope map is classified into 6 divisions as shown in the

table 4.2. It is observed from the table that 13% area has slope less than 5o and ~ 60%

of the area has a slope >20o. The alluvial plains show comparatively low slope (<5o),

gentle in nature. It is clearly observed that the piedmont area, alluvial fan and terraces

have a moderate slope (5o-10o). Interestingly < 2% of the total area has a slope of >

45o. An area of 33.79% of total area has a slope of 20o-30o (moderate gradient), which

is the dominant slope in the area.

Table 4.2: Slope classes and their area coverage

Slope Angle Area (km2) Area %

<5° 1466.6 13.0

5°-10° 573.9 5.1

10°-20° 2365.7 21.0

20°-30° 3811.3 33.8

30°-45° 2919.3 25.9

>45° 143.9 1.3

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Figure 4.4: Bar and pie-diagram showing areas under different slope classes

Figure 4.5 Classified slope map of the Jia Bharali river catchment (Source: SRTM

DEM, pixel size~90m)

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4.3 Aspect

Aspect map is created using the same SRTM DEM of the using ArcGIS 9.3. The Map

is classified into eight divisions at 45o interval viz., NNE (0o-45o), ENE (45o-90o),

ESE (90o-135o), SSE (135o-180o), SSW (180o-225o), WSW (225o-270o), WNW (270o-

315o) and NNW (315o-360o) (Figure 4.6).

The aspect of the basin is shown by the pie diagram (Figure 4.6) which shows that

there is a uniform distribution of slope in all directions. Among these eight group

most of slope faces are directed towards SSE (14.41%), followed by SSW (13.91%),

ESE (13.81%), WSW (11.91%), NNE (11.75%), NNW (11.65%), ENE (11.38%) and

WNW (11.18%).

Table 4.3: Showing the distribution of slope in different direction

Figure 4.6 Pie-diagram showing equal distribution of aspects

Direction Area (km2) Area %

NNE (0o-45o) 1325.82 11.75ENE (45o-90o) 1284.06 11.38ESE (90o-135o) 1557.69 13.81SSE (135o-180o) 1625.05 14.41SSW (180o-225o) 1569.24 13.91WSW (225o-270o) 1343.59 11.91WNW (270o-315o) 1261.42 11.18NNW (315o-360o) 1313.65 11.65

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Figure 4.7 Classified aspect map of the Jia Bharali river catchment (Source:

SRTM DEM, pixel size ~90m)

4.4 Shaded relief:

Shaded relief is a raster surface that provides an orthogonal view of the DEM. Shaded

relief surface is created by illuminating values of each cells in a DEM in relation

with neighboring cells when illuminate from a point light source. Shaded relief

depends on mainly the elevation of the area, direction of light and the inclination of

the light source. The shadow created from these three factors, clearly represent the

topographic nature of the area. A vertical exaggeration factor allows the perception of

relief in the resulting image. Shaded relief of the Jia Bharali River catchment has

been generated using with input parameter of 270o azimuth angle and 45oelevation of

the sun (Figure 4.8). To get clear perception of the topography, the process repeated

interactively with the changing the direction and inclination of light source.

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Figure 4.8: Shaded relief of the Jia Bharali river catchment.

The fine texture in the shaded relief map indicates smooth topography in the southern

part of the basin i.e., alluvial plain of Jia Bharali and Brahmaputra. While the

prominent geomorphic units are the highly and moderately dissected hills in the

northern, Arunachal Himalayan part. Various topographic features can be clearly

visible and clearly identifiable. Major fault and lineaments are identifiable. The

erosional dissection more prominent that help to analysis regional deformation. The

trends lineament and other structural features with diversified orientation can be

extracted. The major structural trends is mainly controlled by major fault system i.e.,

MCT, MBT and in the foothill HFT. From the shaded relief two major trends NW-SE

and NE-SW are prominent.

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4.5 Topographic profiles

Profiles are the representation of the gradient of slope along a line. It is the best way

to representing the topographic surface in a vertical plan along the line. From SRTM

DEM profiles in any line and in any direction can be easily generated. Nine profile

(Figure 4.9) created from the Dem of the basin are to assess topographic

characteristics that exist in the area. Topographic profiles are useful in visualizing the

slope along a particular trend. Among the nine profiles are created for the basin, five

in N-S direction and other two in the E-W direction. Two profiles generated

diagonally in NE-SW and NW-SE direction.

Figure 4.9: Map showing the topographic Profile Location on Geology Map (GSI, 1973) and overlapping with DEM of the study area.

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Table 4.4: Details of topographic profile sections

Profile Starting Point End Point

Latitude Longitude Latitude Longitude

AB 26o 36’ 06.43” 92o 51’ 02.45” 27° 58’ 29.97” 92° 44’ 47.30”

AC 26° 43’ 40.66” 92° 50’ 16.40” 27° 22’ 51.29” 92° 47’ 29.17”

EF 26o 49’ 03.08” 92o 37’ 15.72” 27° 15’ 11.95” 93° 07’ 22.43”

GH 26° 52’ 13.12” 92° 56’ 21.86” 27° 21’ 35. 16” 92° 00’ 22.30”

IJ 27° 03’ 14.72” 92 27’ 11.86” 27° 39’ 17.29” 92° 23’ 04.69”

KL 27° 27’ 11.91” 92° 01’ 30.27” 27° 29’ 17.79” 93° 18’ 57.33”

MN 26° 56’ 46.11” 93° 02’ 40.57” 27°22’ 22.42” 92° 59’ 58.67”

OP 27° 07’ 00.21” 93° 11’ 15.87” 27° 49’ 06.68” 93° 07’ 48.99”

RS 26° 57’ 09.80” 92° 36’ 38.62” 26° 57’ 54.31” 93° 05’ 22.53”

Profile AB, AC, IJ, OP, MN are drawn from south to north of the basin. All these

profile shows the elevation decreases from south to north. In these profiles the

position of the major thrust clearly identifiable. The AB profile shows that the

elevation is increase from south to north. AC profile is made in large scale along the

AB profile where position of the thrust shown. In all the profile the position of rives

are well visible and it seem that there is control of structural setup of the area. From

the IJ, OP and KL profile section it is seen that the river in the western part is deep

incised than the river in the hills of eastern. This may be a tectonic upliftment in the

western side. The western side drainage are of young stage. From hypsometric

analysis it is seen that the western part drainage are of more tectonically younger.

From the Jia Bharali basin asymmetry (66.3) it is also seen that the left side of the

basin is tilted down. From the lineament map (Figure 6.20) there is two structural

lineament NNE-SSW fault and NW-SE lineament (Duarah et. al., 2011; Das, 2004).

This NNE-SSW fault divides the basin centrally and it accumulates the stress, which

may cause the numerous earthquakes in the central part of the basin (Seismic Atlas of

NE). East of this fault, the fan-shaped structure with smooth mountain front convex

toward south.

Profile EF and GH are made diagonally in the middle part of the basin. From these

profile it is clearly observable the different geomorphic surface. The alluvial plain,

piedmont zone, fluvial terrace, dissected older alluvium and the Siwaliks hill. From

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KL, EF and GH profile it is observed that the hills, north of MBT, the hills of the

western side are highly dissected than the hills of eastern side, which may be due to

horse-tail geometry of the fault system (Duarah et. al., 2004). RS profile is made

along the foothills. In this profile the different geomorphic surface, Rangapara

surface, fluvial terrace near Naduar and Seijosa are prominent. The older alluvium

and the piedmont zone have a dissected surface. The alluvial fan topography is well

exposed in the profile.

Figure 4.10: Topographic profile along AB, across the basin in N-S direction shows

the gradual increase in elevation from south to north.

Figure 4.11: Topographic profile along AC, across the basin in N-S direction in the

same line with AB. Kameng is flowing through MBT

Figure 4.12: Topographic profile along IJ, across the basin in N-S direction in the

western part of the Kameng River

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Figure 4.13: Topographic profile along OP in N-S direction in the eastern part of

the Kameng River. Valleys represent the position of different stream.

Figure 4.14: Topographic profile along MN in N-S direction in the western part of

the Kameng River across Lesser Himalayan and Sub Himalayan part. The profile represents the high dissection in the area comparing to the profile OP. The height and distance ratio shows more incision in MN profile

Figure 4.15: Topographic profile along EF in diagonally the basin. Alluvial plain,

HFT, Siwalik Hills, Gondwana is well representing. The Jia Bharali is following through the piedmont zone and Siwalik through a structural valley.

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Figure 4.16: Topographic profile along GH diagonal to the basin in NW-SE

direction, represents the high elevation in the western part and high dissection in between MBT and MCT

Figure 4.17: Topographic profile along KL in E-W direction represents the high

elevation in the western part compare to the eastern part. Kameng is flowing with a lower elevation

.

Figure 4.18: Topographic profile along RS in E-W direction in the foothill region. The profile represents the Older Alluvium and alluvial fan deposits of the piedmont zone