Chapter -IV
Geomorphic Indices of Active Tectonics
4.1 Introduction
Morphometry is defined as quantitative measurement of landscape shape. At the
simplest level, landforms can be characterized in terms of their size, elevation and
slope. Quantitative measurements allow geomorphologists objectively to compare
different landforms and to calculate less straightforward parameters (geomorphic
indices) that may be useful for identifying a particular characteristic of an area (its
level of tectonic activity).
Some geomorphic indices have been developed as basic reconnaissance tools to
identify areas experiencing rapid tectonic deformation. This information is used for
planning research to obtain detailed information about active tectonics. Other indices
were developed to quantify description of landscape. Geomorphic indices are
particularly useful in tectonic studies because they can be used for rapid evaluation of
large areas and the necessary data often can be obtained easily from topographic maps
and aerial photographs. Some of the geomorphic indices most useful in studies of
active tectonics such as Hypsometric integral by Strahler (1952), Asymmetry Factor
by Cox (1994), Stream length-gradient index developed by Hack (1973), Mountain
front sinuosity developed by Bull and Mc Fadden (1977), Ratio of valley floor width
to valley height etc. (Keller and Pinter 2002). Mountain front, valley, sinuosity of the
channels is surface features that construct the arid to semiarid landscape and exists at
large or small scales. To understand the way landforms evolve, it is essential to study
the underlying geology. In general, landform development implies deep structures of
the earth; therefore there is always a strong relationship between landscape and the
geologic environment (Keller and Pinter 2002). Morphotectonics has been considered
as a tool to determine the intensity of tectonic activity in the tectonically active areas
(Wells, Bullard et al. 1988; Merritts and Vincent 1989; Rhea 1993).
4.2 Remote sensing and GIS uses in geomorphology:
In recent years, the advancements in computer technologies and digital data
acquisition/processing has led to the improvement of the knowledge of geomorphic
processes and the development of the use of predictive models and quantitative
measurements to analyze, monitor, and understand landform changes. This
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Chapter -IV
advancement has allowed geographers, geologists, and geomorphologists to explore
human/land interaction utilizing modelling and systems analysis in their
geomorphological studies that relied on sophisticated hardware and software tools
(Ata 2008).
Earth-observing satellites, airborne sensor systems and aerial and space photography
have nearly complete coverage of the Earth’s surface that provides images of different
formats and various scales. This permits not only interpretation of landscape
evolution, but rather offers the opportunity to integrate observation of a variety of
processes over a large region. Geomorphic analysis from space has the advantage of
allowing the use of quantitative methods for both data gathering and information
extraction. Thus, satellite images are becoming useful and necessary in
geomorphology, especially in obtaining quantitative measurements and performing
geomorphic analyses (Hayden 1986).
Geographic Information Systems (GIS) have enhanced the applicability of geologic
mapping when integrated with data obtained by remote sensing using a wide range of
formats and scales. In addition, advancement in image analysis provides geologists
opportunity to enhance, manipulate, and combine digital remotely-sensed data with
several types of geographic information that in turn increases the amount of extracted
information related to topographic and geologic features (Horsby and Harris 1992).
Digital enhancement of satellite images yields much information about image
features. GIS techniques enable the integration and analysis of multi spatial and nonspatial data that have the same georeferencing scheme. Therefore, the integration of
GIS and remotely sensed data could be more informative and results would be more
applicable to image interpretation (Ehlers 1992; Horsby and Harris 1992; Saraf and
Choudhury 1998). Within the context of GIS, surface geomorphology is most
commonly represented in Digital Elevation Models (DEMs) especially when
quantitative measurement using geomorphometry is necessary. DEMs are generally
defined as a regular two dimensional array of heights sampled above some datum that
describes a surface (Wood 1996).
Below is a brief description of the most common geomorphic indices used in active
tectonic studies. The description includes the index definition/explanation,
mathematical formula, and tectonic geomorphological application.
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Chapter -IV
4.3 Geomorphic Indices:
4.3.1. Mountain front sinuosity (S mf )
Mountain front sinuosity is a widely used reconnaissance tool to identify areas on the
basis of the tectonic activity. S mf index reflects a balance between the tendency of
streams and slope processes to produce an irregular (sinuous) mountain front and
vertical active tectonics that tends to produce a prominent straight front (Bull and
Fadden 1977). A mountain front generated by faulting is a zone on which fluvial
system can adjust themselves to local base-level processes (Shaoping and Guizhi
1999). Such relative adjustment can be examined by the morphometric analysis which
includes several useful geomorphological indices and Smf index is one of them.
Bull and Mc Fadden in 1977 proposed the degree of tectonic activity and erosional
modifications of tectonic structures can be measured by this index. Mountain front
sinuosity (S mf ) is the ratio of the total length of the mountain front as measured along
the prominent break in slope along the foot of a mountain and the straight line length
of the mountain front. And it is expressed by the following formula-
S mf = L mf /L s
Where, Smf is the Mountain front sinuosity index, Lmf is the total length of the
mountain front and Ls is the straight line length of the mountain front.
Studies analysing the mountain front sinuosity from different regions, such as the SW
USA (Bull and Fadden 1977; Rockwell, Keller et al. 1985; Costa 1987; Wells,
Bullard et al. 1988; Rhea 1993; Silva, Goy et al. 2003) suggest that mountain fronts
with low values of Smf (<1.6) are categorized as active fronts (Malik and Mohanty
2007).
Thus, mountain fronts associated with active uplift are relatively straight, but if the
rate of uplift is reduced or ceases, erosional processes will begin to form a sinuous
fronts that become more irregular with time. Lower values of the mountain front
sinuosity indicates straight mountain front while higher values shows that front is
irregular.
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Chapter -IV
Mapping of mountain front and its sinuosity
The S mf index is particularly attractive because it can be quickly and easily measured
from aerial photographs, satellite or other high altitude imagery or topographic maps.
Values of S mf depend on image scale, small scale topographic maps (1:250,000)
produce only a rough estimate to mountain front sinuosity (Keller and Pinter 2002).
Mountain fronts can be measured manually from the high resolution satellite images
and topographic maps. For measurement of mountain front sinuosity in the ramganga
river basin, digital elevation models (DEMs) generated by stereo image bands 3N and
3B of ASTER were used. 3-D surfaces using DEMs were generated in Global Mapper
(version 11). The mountain fronts were traced by supervised navigation of the 3-d
surfaces. For this purpose, Selection of the mountain front is also a main task.
Ramganga river basin has been divided into 26 sub basins and in each sub basin many
fronts has been traced and measured, depending on the area of the basin. After
calculating of smf in each sub basin, the mean of the values calculated and used as
representative value of that basin. Smf index is unitless (Bull and Fadden 1977)
(Rockwell, Keller et al. 1985; Keller and Pinter 2002). Mountain front can be
calculated as one major front divided into segments of many fronts of same length or
several continuous fronts of various lengths (Bull 1984; Wells, Bullard et al. 1988;
Silva, Goy et al. 2003). In this study, later approach is adopted because this method
was more suitable for the present research. And with this method we achieve better
results about the tectonics of the area.
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Chapter -IV
Figure 4.1 (a).Calculating mountain front sinuosity (Smf) index (Modified from
Keller and Pinter 2002, figure 4.14, p. 137).
Figure 4.1 (b): Showing total length of mountain front (Lmf) and straight line length
(Ls) of the mountain front.
4.3.2. Drainage basin asymmetry
The geometry of stream networks can be described in several ways, both qualitatively
and quantitatively. Different drainage pattern reflects different structural control as
well as their tectonic settings also. If drainage pattern develops in the presence of
active tectonic deformation, the network often has a distinct pattern and geometry.
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Chapter -IV
Drainage basin asymmetry depicts that in which direction river migrate and how
much a basin is asymmetric. In other words we can say that we calculate tectonic
tilting of the basin and their direction and how much tilting is taking place in
comparison to other basin. Here we are discussing two parameters for deciphering
asymmetry of the basin.
To decipher the possible pattern of ground tilting in the study area we used two
quantitative morphometric parameters namely, Asymmetric factor and Transverse
topographic symmetry (T-vector).
(a) Asymmetry factor (AF):
The asymmetry factor was developed to detect tectonic tilting transverse to flow at
drainage basin or larger scale. The asymmetry factor is defined as
AF = 100 (Ar/At)
Where, Ar is the area of the basin to the right (facing downstream) of the trunk
stream. And At is the total area of the drainage basin. For most of the stream networks
that formed and continue to flow in stable setting, AF should equal about 50. The AF
is sensitive to tilting perpendicular to the trend of the trunk stream. Values of AF
significantly greater or less than 50 may suggest tilt.
So with the help of this simple formula we can easily calculate the tilting of the basin.
If AF is 50, it suggest stable setting and there is no tilt in the basin and if AF is more
or less 50, may suggest tilt and it indicates that basin is tectonically active and still
uplifting (Keller and Pinter 2002).
Any drainage basin with a flowing trunk stream that was subjected to a tectonic
rotation will most likely have an effect on the tributaries lengths. Assuming the
tectonic activity caused a left dipping to the drainage basin, the tributaries to the left
of the main stream will be shorter compared to the ones to the right side of the stream
with an asymmetry factor greater than 50, and vice versa (Hare and Gardner 1985;
Keller and Pinter 2002), as shown in following figure (fig 3).
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Chapter -IV
Figure 4.2: Block diagram shows the effect of an asymmetry factor with a left side
tilt on tributaries lengths (From Keller and Pinter 2002, figure 4.3, p. 125).
Procedure for calculating AF in GIS environment:
To calculate the Asymmetry factor (AF) in the Ramganga basin we found out the
general trend of the main stream of the basin. First, we took the main stream of the
basin and plotted the number of points depending on the length of the stream. After
this we assigned the latitude and longitude of those points and created a scatter plot in
Arc GIS and calculate the general trend of the stream using this scatter plot. After this
process we draw an arrow perpendicular on the general trend of the stream. Direction
of that arrow is showing the direction of tilting of the river channel of that basin.
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Chapter -IV
Figure 4.3: (a) Solid line showing the trend of the river channel
52
Chapter -IV
Figure 4.3: (b) Arrow showing the direction of the migration of the river channel.
53
Chapter -IV
(b) Transverse topographic symmetry factor (T-vector)
Another quantitative index to evaluate basin asymmetry is the transverse topographic
symmetry following the basic technique presented in (Cox 1994; Cox, Arsdale et al.
2001). This index is calculated with the formulaT= Da/Dd
Where,
Da= the distance from the midline of the drainage basin to the midline of the active
meander belt.
Dd= the distance from the basin midline to the basin divide.
Values of T=0 indicates perfectly symmetric basin as asymmetry increases; T
increases and approaches a value of 1.
Migration of the channel from the midline of the basin is an indication of the ground
tilting in the direction of migration. Thus T is a vector which has magnitude from 0 to
1 and direction. Values of T are calculated for different segments of the valley and
indicate preferred migration of stream perpendicular to the drainage-basin axis.
Procedure for plotting midline of the basin (Temme 2010):
Plotting of midline is followed by number of steps as1. Find lowest cell of watershed means outlet of the basin.
2. Find the cell from the same watershed that is furthest from this lowest cell.
3. Draw a straight line between the lowest and furthest cell (called stepline).
4. In case where this straight line is not insight the watershed (imagine a bananashaped watershed), the line is pushed sideways to cover the edge of the watershed
(called sidestepline).
5. Draw a perpendicular to the stepline in both directions from sidestepline to find the
watershed boundaries (these lines are called looklines).
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Chapter -IV
6. Finally halfway points are calculated from the pair of distances and the lines
formed by the joining of the mid points are called the midline of the basin.
Procedure of measuring Da and DdDa is the distance from the stream channel to the midline of its drainage basin
(measured perpendicular to a straight line segment fit to the channel) and Dd is the
distance from the basin margin (divide) to the midline of the basin (Tsodoulos,
Koukouvelas et al. ; Salvany 2004)
Figure 4.4: (a) Photograph Showing Da and Dd of the basin.
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Chapter -IV
Figure 4.4: (b) Arrow showing direction of migration of the channel from the midline
of the basin.
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Chapter -IV
Calculation of T-vector:
In order to calculate the T- index, Ramganga river watershed is divided into 26 sub
watersheds. And in each sub watersheds T index is calculated for each segment of the
river channel in which it migrates from the mid of the basin and it represents as a two
dimensional vector. The length of the vector is equivalent to the ratio Da/Dd, and its
direction is perpendicular to the segment of the stream. Statistical analysis of the
calculated vectors was used to estimate the most prominent direction of stream
migration, in a set of vectors, the most dominant direction can be found by calculating
the resultant vector (Davis 2002). The direction of the resultant vector is the mean
direction of all the calculated vectors. The length of the resultant vector divided by the
number of the calculated vectors gives the mean resultant length (R), which is a
measure of dispersion (Davis 2002). The mean resultant vector length ranges from 0
to 1. Values of the mean resultant length near 1 indicate small dispersion, while
values near 0 indicate that vectors are widely dispersed.
4.3.3. Hypsometric integral (HI)
In tectonic geomorphology, morphometric analysis is the key tool to evaluate the area
on the basis of the tectonic activity. The distribution of the elevations within a region
provides information on the balance between external processes (which tend to lower
the landscape) and internal processes (which tend to create relief). One of the most
useful parameter that describe and analyze the distribution of elevations in an area is
Hypsometry (Pena, Azanon et al. 2009).
Hypsometric integral, a dimensionless parameter, is proposed by Strahler in 1952.
The advantage of the HI is that we calculate and compare different basins of different
areas irrespective of scale. According to Strahler, Hypsometric analysis (or areaaltitude analysis) is the study of the distribution of horizontal cross-sectional area
of a landmass with respect to elevation. Classically, hypsometric analysis has been
used to differentiate between erosional landforms at different stages during their
evolution (Strahler 1952; Schumm 1956). HI thus helps in explaining the erosion that
had taken place in the watershed during the geological time scale due to hydrologic
processes and land degradation factors (P.Bishop, ShroderJr. et al. 2003). The HI is
also an indication of the cycle of erosion(Strahler 1952). The cycle of erosion is
defined as the total time required for reduction of a land topological unit to the base
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Chapter -IV
level that is the lowest level. The entire period or the cycle of erosion can be divided
into three stages viz. monadnock (old) stage, in which watershed is fully stabilized
characterized by a landscape near base level with subdued relief; equilibrium stage or
mature stage, where many geographic processes operate in approximate equilibrium
and inequilibrium or young stage, in which the watershed is highly susceptible to
erosion (Strahler 1952; Sarangi, Bhattacharya et al. 2001). which is characterized by
deep incision and rugged relief. Strahler (1952) found that the HI was inversely
correlated with total relief, slope steepness, drainage density and channel gradients.
HI is expressed as a percentage, and is indicator of the remnant of the present volume
as compared to the original volume of the basin (Ritter, Kochel et al. 2002).
HI is calculated by the following formula-
HI = mean elevation – minimum elevation / maximum elevation – minimum
elevation
Value of HI ranges from 0 to 1.High hypsometric integral values indicate that most of
the topography is high relative to the mean representing a youthful topography stage.
Intermediate to low hypsometric integral values represent more evenly dissected
drainage basins, indicating a mature stage of development (Strahler 1952; Mayer
1990; Keller and Pinter 2002).
How to calculate Hypsometric Integral?:
Calculating the hypsometric integral (HI) is achieve by deriving the maximum and
minimum elevation directly from a topographic map. The mean elevation is calculated
by obtaining the mean of at least 50 elevation values in the basin using point sampling
on a grid (Pike and Wilson 1971; Keller and Pinter 2002) It can also be evaluated
directly from the digital elevation model (DEM) of the basin (Pike and Wilson 1971;
Keller and Pinter 2002; Luo 2002; Luo and Howard 2005).
Method adopting for extraction of HI values in the Ramganga river basin:
In Ramganga river basin HI values were obtained automatically from ASTER DEM
of 30m resolution which was processed in Arc GIS 9.3 environment. For detailed
study of the HI in the Ramganga river basin, we crop the DEM data for individual 26
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Chapter -IV
sub basins. For this, in Global Mapper 11, we put the ASTER digital elevation model
of the Ramganga river basin and overlay the shape file of 26 sub basins. With the help
of the digitizer tool, select the shape file of the each sub basin and export DEM with
the function of the “Export raster and elevation data” and save them in desired
location. After this process, open the DEM data of each sub basin in the Arc GIS 9.3
environment and go to the properties of the opened DEM file and found the maximum
elevation, minimum elevation and mean elevation and put these values in the
mathematical formula of the hypsometric integral.
Keller and Pinter (2002) described the values of HI on the basis of the erosional status
of the basinIf, HI ≤ 0.3 (old stage), it means watershed is fully stabilized.
0.3 ≤ HI ≤ 0.6 (equilibrium or mature stage) indicate watershed is susceptible to
erosion.
HI ≥ 0.6 (inequilibrium or young stage) indicate watershed is highly susceptible to
erosion.
Their susceptibility of erosion is also an indicator of the tectonic conditions of the
basin. Basin with high susceptibility of erosion is tectonically active and old stage of
the basin is less active. Strahler (1952) found that the hypsometric integral was
inversely correlated with total relief, slope steepness, drainage density and channel
gradients.
So, to decipher the tectonic status in 26 sub basins of Ramganga, we have categorized
the values as0-0.3 (less active)
0.3-0.45 (active)
0.45-0.6 (highly active).
4.3.4. Channel sinuosity (S)
Channel sinuosity is a significant quantitative index for interpreting the significance
of streams in the evolution of landscapes and beneficial for Geo morphologists,
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Chapter -IV
Hydrologists and Geologists. Sinuosity deals with the pattern of channel of a drainage
basin(Pareta and Pareta 2011). (Muller 1968)defined channel sinuosity as it is the
ratio of channel length and river valley length. In practice no river follows straight
course from source to mouth. Sinuosity of the river helps in understanding the role of
the tectonics. The index value of 1 indicates straight river course. Values between 1.0
and 1.5 indicate sinuous river whereas channel sinuosity more than 1.5 represents
meandering course (Wolman and P.Miller 1964; Bhatt, Chopra et al. 2007; Rawat,
Tiwari et al. 2011)
To find out the channel sinuosity index we measure the total length of the river
channel and divide it by the straight line length of that river channel. And it is
calculated asS= SL/VL
Where, S is the channel sinuosity, SL is the stream length and VL is the valley length.
Sinuosity of the river channel depends on many factors such as underlying rock type,
structures present on that region, climate, vegetation, hydrological factor, deposition
of the sediments in river course, time etc.
Sinuosity or meandering usually occurs on those regions where relief is very low.
Those river channels which flow on the mountainous region and on steeper slopes
follow straight course.
In Ramganga river basin values of channel sinuosity ranges from 1.1 to 1.7 means it
indicates sinuosity of the river channel. Sinuosity of the river channel varied from one
sub basin to the other. Sinusity of the river channel ranged 1.7 in two sub basins
namely Bansbagad (sub basin 3) and Bhakuna (sub basin 10). In this case, high
sinuosity is not due to the absence of steeper slope and high elevation but because of
the structures present in underlying rock types. These two sub basins are present in
the close association of the Main Central Thrust, Bhujpatri Gad fault and Darun fault.
To assess the relative tectonic activity in the Ramganga river basin values are
classified as1.1 – 1.2 (tectonically more active)
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Chapter -IV
1.2 – 1.4 (less active)
1.4 – 1.7 (inactive settings)
4.3.5. The ratio of valley floor width to valley height (Vf)
The valley floor width to valley height ratio (Vf) is another index to assess the area on
the basis of the tectonic activity. This index reflects the differences between the Vshaped valleys down cutting in response to active uplift, where the stream is governed
by the influence of a base level fall at some point downstream that indicates a
relatively high tectonic activity, and the U-shaped broad-floored valleys with
principally lateral erosion into the adjacent hill slopes in response to relative base
level stability or tectonic quiescence that signifies a relatively low tectonic activity.
Therefore, this index uses one vertical and one horizontal dimension at a given point
along the stream in the erosional system. The ratio of valley floor width to valley
height is defined as:
Where, Vfw is the width of the valley floor, Esc is the elevation of the valley floor or
stream channel, and Eld and Erd are the elevations of the left and right valley divides
respectively.
Similar to the Smf index, lower values of the Vf index indicate relatively active
mountain fronts and reflect deep valleys with active incision related to uplift, whereas
higher Vf
index values are associated with relatively moderate to less active
mountain fronts that represent low uplift rates (Bull 1977a, 1978, (Bull and Fadden
1977; Burbank and Anderson 2001) (Rockwell, Keller et al. 1985; Wells, Bullard et
al. 1988; Keller and Pinter 2002; Silva, Goy et al. 2003)
Theoretically U-shaped valleys are indicative of the less tectonic activity and Vshaped valleys, as a response to uplift, are associated with high tectonic activity (Bull
and Fadden 1977; Burbank and Anderson 2001; Keller and Pinter 2002).
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Chapter -IV
Figure 4.5: Calculating valley floor width to height ratio (Keller and Pinter 2002,
figure 4.15, p. 139.
Digitizing valley profiles and measuring elevations and valleys’ widths:
The digitization of valley profiles will serve the purpose of calculating the four
unknown elevations of Eld, Erd, Esc and Vfw values in order to calculate the Vf
values of each individual valley. Towards this, all the valley profiles will be converted
into three dimensional profiles by using the 3D path profile/line of sight tool and
viewed in the global mapper to measure the elevation values of each valley
individually. Valley profile is oriented at the right angle to the valley of the basin. For
example, if the valley is oriented in the north-south direction, as a result, the valley
profile will be in the east-west direction. Therefore, during the calculation of the Vf
values, east end elevation of the valley will represent the left valley elevation of the
valley divide (Eld) while the west end will represent the right valley elevation divide
(Erd). Width of the valley floor (Vfw) and elevation of the valley floor (Esc) is
calculated directly from the reading of the 3D profile of the basin.
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Chapter -IV
Fig 4.6 Map showing the valley profile of a basin (Generated in Global mapper).
4.3.6. Stream length- gradient index (SL)
The Stream Length-Gradient Index (SL) is calculated along a river and used to
evaluate the erosional resistance of the available rocks and relative intensity of active
tectonics. The SL index has sensitivity to channel slope changes, which makes it a
good evaluation tool for the relationship between potential tectonic activity, rock
resistance, topography, and length of the stream (Hack 1973; Azor, Keller et al. 2002;
Keller and Pinter 2002). The stream length-gradient index (or SL index) proposed by
Hack (1973) calculated for a particular reach of interest and defined asSL = (ΔH/ΔL) / L
Where SL is stream length gradient, ∆H/∆L is the channel slope or gradient of
particular reach, ∆H is the change in elevation of the reach and ∆L is the length of the
reach and L is the total length from midpoint of the reach of interest upstream to the
highest point on the channel.
The stream length-gradient index (SL) correlates to the total stream power, available
at particular reach of channel is an important hydrologic variable, because it is related
to ability of a stream to erode its bed and transport sediment.
Total available stream power is the product of the slope of the water surface and
discharge. Slope of water surface generally measured by its slope of the channel bed.
The SL index is sensitive to change in slope and this sensitivity allows the evaluation
of relationship among the possible tectonic activity, rock resistance and topography.
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Index value will be significantly lower in softer strata; rock types are shale, siltstone
and carbonate rocks and increases, where the stream crosses the relatively hard rocks.
In landscape evolution, the adjustment of stream profiles to rock resistance is assumed
fairly quickly. Therefore, the SL index is used to identify recent tectonic activity by
identifying anomalously high index values on a particular type. For example, an area
of high index values on softer rock may indicate recent tectonic activity. Anomalously
low values of the index also may represent tectonic activity, along linear valleys
produced by strike slip faulting, indicates valleys is crushed by fault movement.
The SL index over a region can be computed from small-scale topographic maps. The
index could also be computed from analyses of elevation data stored in computer
systems. Therefore, in theory, large regions may be evaluated quickly, although
interpretation of the index will remain crude because it may be difficult to separate
effects of rock resistance from active tectonics. Nevertheless, the SL index is a
valuable reconnaissance tool useful in isolating smaller areas for detailed work.
Figure 4.7: Diagram shows the process of calculating the Stream Length-Gradient
Index (SL) for a given creek (Keller and Pinter 2002, figure, 4.6, p. 128).
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4.3.7. Basin elongation ratio (R e )
(Schumn 1956) defined the basin elongation ratio as the ratio of the diameter of a
circle of the same area as the basin to the maximum basin length. Elongation ratio
indicates how the shape of the basin deviates from a circle and this index tells about
the shape of the basin. The varying slopes of watershed can be classified with the help
of the index of elongation ratio (Pareta and Pareta 2011). According to (Suresh 2000)
this ratio runs between 0.6 and 1.0 over a wide variety of climatic and geologic types.
The variation of the elongated shape of the basins is due to the guiding effect of
thrusting and faulting in the basin. Values close to 1.0 are typical of regions of low
relief whereas values in the range 0.6-0.8 are usually associated with high relief and
steep ground slope (Strahler 1964). High Re values indicate that the areas are having
high infiltration capacity and low runoff (Sreedevi, Owais et al. 2009). And is
derived by following formula-
Re = (2√A: √π)/L
Where, Re is basin elongation ratio; A is the area of the basin; L is the length of the
basin.
A value of basin elongation ratio on the basis of the shape of the basin is described by
different researchers (Pareta and Pareta 2011; Rawat, Tiwari et al. 2011; Sethupathi,
Narasimhan et al. 2011) and grouped as Circular (0.9 to 1.0),
Oval (0.8 to 0.9),
Less elongated (0.7 to 0.8) and
Elongated (<0.7)
Values of basin elongation ratio in 26 sub basins ranged from 0.56 to 0.9. Lower
values of the basin elongation ratio indicates the elongated shape of the basin while
higher values indicates the oval to circular or near circular shape of the basin.
Elongated to highly elongated basins are less flood prone areas whereas circular to
near circular basins are highly susceptible to the flood.
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Minimum value (0.56) found in Namik (sub basin 1) and maximum value 0.9 found in
Sirtoli (sub basin 14) respectively.
Values near to 1.0 are typical of regions of very
low relief (Strahler
1964)
with
circular in shape and are efficient in the discharge of runoff than elongated basin
because concentration time is less in circular basins. Thus Re values help in flood
forecasting. The elongation ratio and shape of basin are given below.
Table 4.1 Showing the shape of the Basin on the Basis of Basin Elongation Ratio.
Elongation ratio
Shape of the basin
Circular
0.9 to 1.0
Oval
0.8 to 0.9
Less elongated
0.7 to 0.8
Elongated
< 0.7
The basin elongation ratio (Re) proposed by Bull and Mc Fadden (1977) is one of the
proxy indicators of recent tectonic activity (Cuong and Zuchiwicz, 2001). And in
terms of tectonic activity, they categorized the values of basin elongation ratio asRe < 0.50, Tectonically Active
Re = 0.50-0.75, Slightly Active
Re > 0.75, Inactive Setting
66