Relief and Slope Analysis

Transcription

1 Relief and Slope Analysis Chapter-III A preliminary step to study the geomorphological features or morphometric study of any area is analysis of its relief feature. The variation of earth surface or a part of it becomes focus of a geomorphic study of landforms. The meaning of term relief may be defined as the difference in elevation of any part of the earth's surface or relative vertical inequality of land surface. Relative relief is among the techniques, those are effectively capable of presenting three dimensional relief characteristics with the help of twodimensional maps, without considering sea level. Relief and other geomorphic elements are also based in differences in elevation. These morphometric elements are absolute relief, relative relief, slope and dissection index, etc. who, help to classify morpho units of terrain. However, in this chapter, elements, that are directly related with relief have been discussed. These elements are known to help in analysis of any landform and also in categorizing landforms into various morphounits and their relation with geology and hydrology of the area. The morphometric elements are relative relief, absolute relief, slope, and dissection index and drainage characteristics. These elements of morphometry are valuable in dividing the landforms into various morphounits and their relation with geology of a particular area, climatic and hydrological conditions as well as classification and sub classification of landforms i.e. absolute relief, slope, relief profile etc.. So, relief analysis becomes essential for studying a given area. Relief also indicates the past geological events occurred in the particular area. In the present study, elements those directly related with relief including relative relief are discussed. The surface of study area has resulted in fragments or remnatal surfaces mainly caused by faulted action of erosion. At a glance, different organic phases of study area such as denudation process have also effected relief features. The whole processes of relief depended upon the nature of relief i.e. young, mature or old, geological condition, climatic condition, rainfall, temperature, humidity, evaporation slope aspect, etc. As far as the relief is a product of a number of factors, therefore a definite class value of relative relief is not suitable for a particular nature of relief. Relief is commonly regarded as the range in altitude (Smith, 1935). This definition is logically right and acceptable. Relief analysis of any terrain may be done with the help of morphometry. The absolute 42

2 relief and relative relief, slope and drainage with quantitative characters and dissection index are the main elements of morphometry which helps in the analysis of relief of any terrain (Thakur, 1991). On the basis of above mentioned facts, the present chapter 'Relief Analysis' deals with the absolute relief, altitudinal zone, area height relationship, relative relief, relief profile, slope analysis and dissection index. Analysis of relief in the study area is based on relief method of analysis adopted by Singh (1974), as he has used it for morphometrical analysis of three terrains in the sub continent- Almora region of the Himalayan, Vindhyan Plateau and Rajmahal upland. He has also suggested that, definite scale for different morphometric attributes like absolute relief, relative relief, dissection index, slope, drainage texture and frequency, on a regional scales which are helpful in explanation of the characteristics of a terrain. Absolute Relief Absolute relief means the maximum height of any region. The absolute relief may be analyses with help of two components; (i) Inter-altitudinal zones discrepancies (based on contour map (Fig.5) and (ii) discrepancies of regional distribution (based on relief profile drawn for the study area (Fig.13). In other words absolute relief of a region may be analyses in two ways. First with the help of contour map divided in zones and second by regional distribution discrepancies based on different profiles drawn for the study area, has been divided into grid units of one sq. km. Distribution of absolute relief The absolute relief of Dhundsir Gad showed that it increase from valley bottom to top of the mountain ranges. In Dhundsir Gad basin, altitude increases rapidly from south to northwards. Dhundprayag (Confluence of Dhundsir Gad and Alaknanda River) near Kirtinagar located in the south extreme part of the study area possessed the lowest elevation of 520m whereas the Gaddikhal peak, in the extreme north of the basin, is credited with the highest elevation of 2360m. Thus the average relative relief of the basin is 1780m. This explanation revels that, the total basin area comprises of the mountain terrain. Resani Top (1981m), Athani Dhar (2262m), Chauri Khal (1810m) Silkakhal (1606m), and Maninath Ka Danda (1680) are main mountain range and water divider of the study area those having maximum height. Athani Dhar (1729m.) and Nagraj Dhar (1640m) located in the middlemost part of the watershed is the main water divider between (Dhundsir Gad and Taula Gad), (Nagailagair Gad and Taula Gad) respectively. The other major ridges and water divides are Kot Dhar, Maninath Ka Danda etc. 43

3 Altitudinal Zone The height of Dhundsir Gad varies from 520m (at the confluence) to 2360m (at source). The altitude increases rapidly from south to northwards along the valley and along the valley sides. Different type of landforms found in this altitudinal variation within a short distance of 17.5 km.. The rise in the surface is 1840m relative relief. The study basin is divided into five-altitudinal zones of 400m contour interval. Table 3.1 presents the detailed description of altitudinal zone of Dhundsir Gad. Table 3.1 Variations in the altitudinal zonation of Dhundsir Gad Height (m) Area (km 2 ) % Cumulative % < > Total Table 3.1 shows that, the maximum area (33.6%) of the basin falls in the height group of m followed by m i.e. 26.9%. the minimum percentage of this area falls under the height group of blow 800m covering 6.95 area of the total basin. Similarly maximum height group above 2000 meter covers only 9.7% area (Fig.6). The detailed distribution of area is explained blow: (i) Below 800m: The Lowest height group covers a very small area 3.5 km 2 near the confluence. Dhundprayag, Dang & Sema localities are come under this group. In this part the shape of the valley is very narrow and flow in a narrow gorge. Both the sides of the gorge and V shaped valley subtropical bushes and scrubs are grown. Near Dang village the valley opened and cultivated land is found on the terraces. (ii) Between mL: This zone constitutes 11.6 km 2 area of the study area. It covers the lower valley bottoms of the middle valley and mid slopes ridges in the lower valley. The river gorge is the major characteristics features of the zone. The main localities are Maninath Ka Danda, Zirkoti, Kandi, Phayalgaon and Sirsed. The confluence of 44

4 Nagailagair Gad and Taula Gad and third order stream can be seen in this zone. This zone exhibits the true characteristics of moderate relief, interlocking spurs, convex slope, low water divides etc (iii) Between m: As already cleared that maximum 16.9 km 2 area comes under this zone. The main localities which falls in this zone are Silkakhal ridge, Kaproli Dhar, Nagraj Dhar, Athani Dhar and mid slope in which 16 villages are located (Rankandiyal, Sirsed, Sarkana, Koti, Durha, etc.). It is the most inhabited and cultivated zone. The major part of the 3 rd and 4 th order sub-basin comes under this zone. Secondary water divides landslides on scrapes and cliffs slopes in quartzite rocks are the prominent geomorprains ogical features of this altitudinal zone. (iv) Between m: This zone is confined to upper valley, the main localities are Dharkot, Margaon, and Sayalsaur and water divides of Dagar Gad and Dhundsir Gad. It is the major source zone of streams mostly upper part of this zone is under degraded temperate vegetation mainly pine is dominant species. This zone is highly degraded by biomass collection and over grazing. Occasional snowfall occurs during winter season.. (v) Above 2000m: This zone covers very little area 4.9 km 2 but it has a great importance. It is the major water divides of Dhundsir Gad and (Badiyar Gad in the East, Nalichami Gad in the north and Dagar Gad in the west). The main high peaks of the zone are Gaddikhal (2360) and Resani Top (1980). There are number of saddles which are locally known as Khals. Major Khals are Gaddikhal, Silkakhal, Chauri Khal and Tarpana Khal. The slope of this zone is not much higher in comparison to lower altitudinal zones. The conical and rounded hill tops are the prominent feature of the zone. Relative Relief The term relative relief means the actual variation of height i.e. different between maximum height and minimum height in per grid. Relative relief is one of the techniques which is effectively capable of presenting the relief characteristics with out considering sea level (Singh, 1992). Relief is not strictly a function of an elevation above sea level, yet there is a relation between altitudinal and local relief (Glock 1937). Portch (1911) gave the first idea about relative relief and subsequently Krebs (1922), Schrepfer and Kallner (1930), Johnson (1933), and Smith (1935) followed it. First time a scientific and systematic study of relative relief was done by Smith (1935). These has been frequent applications of relative concept since the time of Smith (1935) and its effect over the general land use patterns has also been to identify for which the land forms region have to demonstrate 45

5 (proved) much meaningful. Further improvement in relative relief analysis was done by Nir (1957) using a relief classification by suggesting the ratio between relative relief and absolute relief as a measure of 'dynamic potential' of the area. The study directly has been done by geomorphologists in different title viz. relative relief, relative altitudes, topographical relief, amplitude of available relief, drainage relief and local relief etc. Distribution of relative relief The present area under study has been covered by the grids of 1 km 2, which has been selected as the smallest unit of the region. For the purpose of relief analysis the variation in elevation between the heights and the lowest points within the grids has been computed for each grid. The highest value of relative relief in the study is maximum 2360m and the minimum value is 1840m, so relative relief is calculated 1840m. The grid shows the spatial relative relief height. The map (Fig.7) of relative relief of study area portrays the real distribution of such variations which has been classified into five groups ranging from below 200m to 600m and above at a class interval of 100m. The basin consists mainly of mountainous terrain with an elevation ranging from 520m to 2360m above sea level. Along the course of Dhundsir Gad several groups and terraces are visible as a water divider and is the source of Dhundsir Gad which is situated at the height of up to 2360m. For a more comprehensive and detailed study of the relative relief, the values plotted in the grids have been categorized into five groups ranging from 200m to 600m. Accuracy of grid frequencies under various relative relief groups has been made an objective of comprehensive comparative analysis. Distribution of total relative area on study area has been categorized into four major classes on the basis of quantitative nomenclature as suggested by Singh (1967). The percentage of a real distribution has also been calculated for the study area in Table 3.2. Table-3.2 Distribution of relative relief in Dhundsir Gad Relative Relief (m) Area (km 2 ) % Cumulative % Relative Relief < V. Low Low Moderate High Very High Total

6 The Table 3.2 reveals very clearly that the maximum percentage of relative relief frequencies is 46.8% occurs under the high relative relief group of m. The group of below 200m has only 2.8 % of the total grid area while in above 500m 21.4 % of total area occupies. The distribution of relative relief shown in Figure 7. It reveals clearly that more than 400m relative relief is common in the basin area. Where the peaks attain altitude over 2300m the main sea level and the valley attain 520m minimum elevation. In the northern hill part maximum concentration of higher values of relative relief is found. The distribution of relative relief is follows: (i) Low relative relief: Relative map reveals the regional discrepancy of relative relief in the study area. Low relative relief (0-300m) covers only 6.2 Km 2 or 12.3 % of total study area. It occupies the lower valley slopes of the Dhundsir Gad and most of the area of Sema valley, near Phayalgaon valley, Dang, and Shivalaya in the study area. (ii) Moderate relative relief : Moderate relative relief ( m) characterizes 9.8 Km 2.or 19.5 % of the study area. It includes the ridges of Sirsed, Silkakhal, Kandi, Margaon, Dharkot, Thapali, Nagraj Dhar, Kaproli Dhar, which have gentle valley side slopes. (iii) High relative relief: The high relative category (400m-500m) covers an area of 23.6 km 2 or 46.9 %. This category mainly occurs in middle part of the basin where intense agricultural activities are in practice. The main localities lies in this category are Dharkot, Parkot, Zirkoti, Rankandiyal, Semgarh, Khola, and Phayalgaon. (iv) Very high relative relief: Very high relative relief marks a maximum of 10.9 Km 2.or 21.4 % of the basin area. This category occurs in the upper reaches of almost all the northern part of the basin. Large tracks of high relative relief are found in the area of the source zone of the Dhundsir Gad i.e. Resani Top, Maninath Range, Gaddikhal, Athani Dhar, and northern part of the Rampur Village. Mainly this category of relative relief is found in scarp and very high slope zones. Slope Analysis Slope is an area of land that makes a definite angle to horizontal landscape. In geomorphology landscape is made up of slope units. The slope may be defined as the vertical inclination between the hill top and valley bottom, stands with the horizontal line and expressed generally in the degrees. Strahler (1964) expressed "the inclination or gradients of the surface of a basin in terms of maximum valley side slope, measured at 47

7 interval along the valley walls in the steepest part of the contour orthogonal running from divides to adjacent steam channels" Since the classical leadership of Davis (1924) concept of fluvial cycle of erosion an attempt of genetic or evolutionary sequence of land form development in term of structure processes and stage. Penk (1953) emphasis to work on slope, forms and angles and process through waxing and waning developments leading the stage of pediplanation and the recent quantitative approaches and methods of morphometric evolution of an area. Slope analysis method also suggested by geomorphologists of various shades and styles like Kesseli (1946), Russel (1949), Strahler (1956), Schumm (1956), Durray (1963), Young (1971), Hammond (1964), have contribute a great deal in physiographic investigation and research directed towards the evolution of earth's surface. The slope is a result of endogentic and exogenitic powers which works relatively. The indogentic processes produce change in the elevation and orientation of slope elements. The exogentic process, which originate on the earth's surface tend to reduce the landscape to base level. It is developed from the crest of hill to their drainage bottom.the slope of the terrain is the result of three processes operative on and under the surface of earth. The instance on the inventorying of areas, altitude, volume, slope profiles, texture of the concerned landforms through morphometric measurement includes the evolution, process and stage of development. Also has been intensively studied by agriculturalists, civil engineers and by soil conservation. However, in most case lithology, structure, geology, history and climate considerably complicated the interpretation of slopes. The grater part of the drainage system but also a determining factor of runoff velocity and slope provide water and sediments to streams. Therefore, hill slope is an important component of the complex landscape that forms a drainage basin (Chorley, 1958). The study of slope form and evolution may be under ideal conditions be a simple process, and this is the situation in rapidly evolving bed loads. However, in most cases lithology, structure, geologic history and climatic considerably complicate the interpretation of slopes (Pande, 1991). Slope analysis and its categorization has been the subject matter of geomorphologists. Historically, the research on slope study and analysis may be grouped into two phases. Initial phase is dominant by interpretation of different aspects of hill slope and valley side slope development on the basis of field observations. Modern phase is dominated quantitative analysis of slopes based on data derived from topographical maps and aerials photographs, measurement of slope angles in the field and 48

8 instrumentation of process (weathering, mass wasting and movement and erosion) acting on the hill slopes. The significant contribution in slope study have been made by Wentworth (1930), Raize and Henry (1937), Horton (1945), Strahler (1956), Singh and Singh (1979) and Young (1963). The action of water is one of the most ubiquitious processes on the hills slopes, water flow and amount of the rainfall. Such type of factors determine the slope category, many observations have been made on the subject of geological control of slope forms. Generally the problem is that rock structure acts as a combination of other factors to influence complex landscape. In the Dhundsir Gad area rock type influence the slope angle directly and through its control on the nature of superficial deposits. The layered sedimentary rocks of alternating resistance marked relief effects. Table: 3.3 Distribution of slope category in Dhundsir Gad Slope (m/km) Area in Km 2 % Cumulative % Remark Less than Moderately Gentle Moderate Moderate Steep Steep Above Very Steep Total For the present study Henery and Raize method has been adopted for slope analysis. Table 3.3 shows a clear picture of distribution pattern of frequencies according to the slope categories of the area. The slope values categorized from 0-800m/km these five categories of slope were further condensed into five broad categories, i.e. moderately gentle, moderate, moderate steep, steep and very steep slope. This classification reveals clearly the pattern of slope distribution in the area under study (Fig.8). (i) Moderately gentle slope (below-200m): The first category of low slope has a small coverage of 5.3 Km 2.or 10.5% of total area. This category is found over the Dang, Phayalgaon and Parkot area. A small patch of this category is also found in the Nagalagair, and Sirsed. The patches of the moderately gentle slopes scattered in the study basin are observed in the flat valleys resulting due to accelerated rate of erosion. The much higher 49

9 strip of this category in the extreme northern and patch in north eastern part where the saddle (Khal) such as such as Gaddikhal, Silkakhal, and Chauri Khal are developed. (ii) Moderate slope (200m-400m) This Category of slope occupies 8.5 Km 2. or 16.8% of the total area. This category found in the lower part of the Dang and upper part of the Silkakhal. This area is under of the dense forest. This slope category is associated with its own gradational factor active in the study area. Softer rocks i.e. ferruginous quartzite, phyllite mainly of Chandpur group, limestone exhibits in the northern part (Parkot village), eastern part (Silkakhal) of the area, which are easily eroded by the denudational processes, mainly the action of running water. (iii) Moderate steep slope ( m): This category of slope covers maximum part of the Dhundsir Gad area 23.2 Km 2.or 45.9% of the total area. This category is found in northeast part of Sirsed, lower part of the Kafana, upper part of the Margaon, Koti, and Shili. (iv) Steep slope (600m to 800m): Occurrence of steep slope category is visualized almost all over the study area having a small patch. It continent 11.3 Km 2.or 22.3 % of total area. The areal coverage of this category lies in N.A.T. area and along the river both sides. The slope morphology of steep slope category has mostly been affected by the frequency occurring landslides and other tectonic disturbance which are still prevalent in the entire area especially over the ridges and branches spurs of these ridges. A detailed relative relief analysis of these steep slopes localities revels that these area exhibits high to very high relative relief. Also other factor such as drainage density and confluence density etc. are responsible for the steep slope in the study area. (v) Very steep slope (above 800m) %: This category of slope occupies 2.2 Km 2 or 4.5% of Dhundsir Gad stressed in upper part of Semgarh, Shivalaya and on the bottom of Sil village. Although this slope category is of localized character like steep slope category, yet the factor and processes responsible for the development of steep slopes are also responsible for the development of very steep slope category. In conclusion the channel of the foot slope exerts considareable amount in influence on the hillside forms, where the stream will be tend to adjust its gradient. The several gully erosion characterizes the study area with ravenous land occurring on both the flanks of river. The term slope is used throughout the science of Geomorphology. Slope an attempt of inclined relief expresses the inclination of the relief from the horizontal or local base 50

10 levels in a unit and is sown often in degree. Pal (1986) attempted qualitatively analysis of slope angle of slope angle along Alaknanda river valley. Dutt (1983) also used the slope profiling and slope angles techniques for measuring slope in the Bino Basin of lesser Himalaya. Through the techniques of is (i) slope angling and (ii) slope profiling and morphological mapping a great deal of quantitative information is collected numerical angles. Measurement of slope angle Slope angle measured in degree for the analysis here to provide an accurate and comprehensive two dimensional representation of the three dimensional from of the ground surface (Pal, 1986). Generally slope is expressed in degree which shows maximum inclination down a hill side. The present analysis of slope angle mapping is based on 1 :50,000 topographical map. In deciding the location of individual slope angles contour map with 40 meters interval is the basic tool of slope map. The selection of points was taken in such a very that the line of slope was representing of the features i.e. spur, cliff, terrace, ridge and water divides etc. Slope angle map is the multipurpose which provides detailed information of each micro unit of the area. Slope angles are calculated by the following formula: Tan R = VI / HE Where VI is vertical contour interval between two points and HE is the horizontal equivalent of some points on the topographic map. Thus slope angles were determined in the Dhundsir Gad. Slope angle classification Slope angles are classification on the basis of geomorphological parameters. Although the continuous variables of slope angle are arbitrary but it delineates the micro units of landform. For the hydrological purpose the slope angles are best determiner of sediment load transported by a stream. Taking all these in consideration the slope angles are divided into five categories (Table-3.4). 51

11 Table-3.4 Classification of slope angling in Dhundsir Gad Angle class (0 o ) Frequency of slope angle Percentage (%) Cumulative (%) Remarks < Gentle slope Moderate slope Moderately steep slope Steep slope > Very steep slope Total Table 3.4 reveals that maximum frequency of slope angles in moderately steep category (22 O -31 O ) which is 34.81% of the total angles followed by moderate slope i.e.31.11%. Minimum slope angles (4.44%) lie in the category of very steep slope (>45 o ) followed by gentle slope (<12).The steep slope zone covers 25.93% of slope angles in the study area. The table summarizes that maximum area of the study watershed is under moderate to steep slope zone which indicates the mountainous terrain. The distribution of different slope angles classes are explain below. (i) Gentle slope (<-12 O ) The frequency under gentle slope category comprises of minimum numbers (3) of slope angles which is 3.70 % of total area. This slope category comprises the maximum gentle slope of the study area on planner surfaces, saddles, ridges, water divides, colluvial fans and river terraces (Fig.9). The lower valley bottoms where fluvial terraces ranging from few meters to 160m. in width have been marked in developed in the area. These terraces are irrigated through Guls and small canals. (ii) Moderate slope (12 o 22 o ): The area of moderate slope measures 42 or 31.11no. of total slope angle. This slope type is associated with the middle valley slopes of the Dharpayankoti, Dang and Rankandiyal villages. The areas of moderate slopes are characterized by the foliated quartzite rocks. The moderate slope area which occurs under agricultural and forests is sparsely distributed, resultantly, the severe soil erosion occurs in the region. The geo-catastrophic factors becomes more active on account of an excess of human interferences like deforestation, overgrazing, over-ploughing, overflowing, which 52

12 brings sudden changes in the slope to cause landslides, rock falls, soil creeping and mud flowing in the region. (iii) Moderately steep slope (22 o 31 o ): The frequency which occurrs under the moderately steep slope category comprises of first higher frequency, 47 slope angles which is 31.11% of total slope angles of study area. It has been marked that the surface of this slope category is moderately dissected, rugged and uneven. Low hills, spurs are the major landforms of this category which are approaching the position of gradational. The angles of moderately slope category are scattered as commonly observed in the flat valley. Anthropogenic activity like deforestation, overgrazing which becomes sudden changes in the slope to cause rock fall, land sliding, and soil creeping in the region. (iv) Steep slope (31 o 45 o ) The steep slope category occupies number of angles 35 or 25.93% of and second higher of slope frequency near to moderately steep slope category. The category comprises the maximum area between Khola village to Durha village situated in the western part of the study area. It is notable that this category is mainly occupies south-east slope aspect and is much affected by frequent tectonic disturbance responsible for steep slope in the Dhundsir Gad. The general gradient of the terrain in this category is much affected by the frequent landslides and tectonic disturbances which are still common in the area and are responsible for the development of steep slope in the area. (v) Very steep slope (> 45 o ) : The land under very steep slope category is very less in comparison to other slope category. This classes of slope angles have 6 slope angles i.e. 4.44% of total slope angles. Mostly the angles of very steep slope category stretched in the scarp edges of water divide adjoined to steep slope category. Cliffs and free faces are commonly marked in the adjoining of these areas. Cliffs and free faces are commonly marked in the adjoining of very steep slope category. The landform is being severely degraded by rill and gulley erosion through first and second order stream. Cliffs are generally observed as necked without any vegetal cover except little bushes, shrubs etc. Slope Aspect Slope aspects map show the relative position of slope facets in respect to direction of sun angle. Figure10 shows that the master stream of the area is flow from north to south direction between two hill ranges extending from north to south. On this basis only two aspects can be identified into study area. i.e. eastern and western. But due to the alignment of ridges and tributary valley within the watershed, these two aspects further classified into sub aspects. The entire basin is divided into eight slope aspects. The N, and NW aspects 53

13 are wet and moist while the E and SE aspects are much dryer. The NE and SW aspects are moderately wet and moderately dry according to the altitude, rock type and natural vegetation. Slope aspects of the area effect the denudational processes of the hills. Landslides, slumping and other mass wasting activities are very common on the northwestern aspect of the ridges. The colluvial fans on the south-eastern slopes are more stabilized and inhabited. Most of the villages are located in the south eastern aspects of the region, because they need to sun radiations in the high altitude (above-1500). No one settlement is located in the N, NW aspects. The direction of the vegetation and forest growth are also good indicator of slope aspects. In the high altitudes above 1500m the growth and density of oak and associate species are well in the northern and western aspects while the pine is well dominated in the south - eastern aspects in region. It is remarkably noted that the northern aspects are steeper than the southern aspects. It is not in the study area but also all over the Himalaya. The north western aspects are good for biodiversity. There is close relationship of slope aspects, landforms types and natural vegetation in the study area. Dissection Index The dissection index, which is the ratio between relative relief and absolute relief, gives better understanding of the landscape. According to Nir (1957) as criterion of relief energy, the concept of relative relief altitude is not entirely satisfactory. Equal relative altitude are not always of equal importance, since their absolute altitudes may differ. The picture gained from relative altitudes only is static, for it fails to take into account the vertical distance from the erosion base, i.e. the dynamic potential of the area studied. On this basis, he suggested the necessity of describing the relief in terms of the ratio between the two variables (absolute relief and relative relief). It can be obtained by the following formula: Dissection Index (D.I) = Relative Relief (R.R.) Absolute Relief (A.R.) The values of dissection index vary from 0 (complete absence of dissection) to 1 (vertical cliff at sea level). Thus, it is the index of the degree to which dissection has advanced. In other words, it express the relationship between the vertical distance of relief from the erosion level and relative relief (Jha, 1996). Thus the dissection index, for the study are calculated varies from 0.07 to It has been classified into three categories: 54

14 (i) Very low dissection index (< 0.20), (ii) Low dissection index ( ), (iii) Moderate dissection index (> 0.35) Distribution of dissection index Table 3.5 shows that the highest percentage (31.30) of the area lies under the category of low dissection index. Very low percentage 9.23 and is under very high and moderate respectively where percentage areas occupies moderately high dissection index and 19.21% is under very low category. The detailed description and distribution (Fig.11) of above categories are explained below: Table-3.5 Distribution of dissection index Dissection Index Area (km 2 ) Area (%) Remark Less than Very Low Low Moderate Moderately High Above High Total (i) Very low dissection index: The area of very low dissection index measures 9.70 km 2 or 19.21% of the study area. It occurs mostly in the north-eastern part i.e. Thapali, Dharkot, Parkot, Sayalsaur and Margaon village of the Dhundsir Gad and includes northern part of Silkahal. This category comprises the area on the floor of V shaped valley such as Dhankur Gad and Ulari Gad and saddles (Khal). (ii) Low dissection index: Low dissection index characterized the maximum part about 15.81km 2 or 31.30% area. It associates with areas of gently sloping topography and resistant rocks like quartzite, occurring in the Durha Khad, Athani Khad, Fatori Danda, Dona Khal, Kot Dhar, Goni Khal and Sirsed village. (iii) Moderate dissection index: Moderate dissection index accounts for 7.65 km 2 or 15.15% of the study area. It is associated with the middle valley slopes of the Achani Top, 55

15 Khola khad, Shivalaya, Semgargh. Other patches are occurs in the lower valley ridges such as Takwani Dhar and Sema village. (iv) Moderately high dissection index: The second largest area occurs in this category covering the area km 2 or 25.11% associated with southern part and middle part (the confluence of Dhundsir Gad and Chauni Gad). Other small patches also occurrs in the Mulana, Dhirkana. (v) High dissection index: High dissection index is associated with the rugged and broken hilly terrains forming divides and the steeply slopping areas of the study watershed. The percentage of under this category is very low (4.66 or 9.23%). The main localities under this category are Dang, Sema Sil, Khola, Kailpeer Parvat and northern part of Maninath Ka Danda. Relief Profiles Also become necessary to discuss the different relief profile (Woldridge and Morgan, 1946). For the identification of any erosion surface in the region, different techniques have been applied viz. serial profiles, projected profile, composite profile, superimposed profiles and altimetric frequency graph for the analysis of relief. The discussion of the general nature of regional discrepancy with the help of profiles is necessary because of the limitations of contour map. The contour map does not indicate complete picture of the terrain, whereas the various profiles helped to clear the sharpness of the absolute relief. Subsequently an extensive fieldwork has done to collect field evidence. This techniques has applied by Bauling (1940) in recognizing the different erosional forms caused by the change of the sea level. The serial, superimposed, composite and projected profiles have been drawn along the east-west lines, which present a clear picture and also a panoramic view of the existing landscape of the entire area drawn at equal distances. Fluvial and thrusted processes dominate the Dhundsir Gad, fluvial landforms like gorge, meanders, ridges (Dhar) open 'v' shaped valley, river terraces, like bar deposits etc. are well marked in the area. The longitudinal and traverses profiles of the Dhundsir Gad have been drawn to know the form of base level. The characteristics of the terrain, i.e. variations of slope, relief, ruggedness, declivity of landscape, sharpness of altitude and valley bottoms are reflected in it. Investigation on the profile of the river Dhundsir Gad is as follows. (i) Serial profile: There are five sets of serial profiles are drawn in different localities. Before taking the cross section some general topographical features i.e. valley, spurs, 56

16 confluences, ridges, (water divides) etc. are taken for consideration. Serial profiles indicate the true shape of terrain configuration. Cross section -1 show that the upper valley part of the study catchment is quite wide rather than lowest part (Cross section -5 Fig.12). It shows that the upper part of the basin is wide due to the litho-stratigraphic changes and erosional processes. In the lower part valley incision is much higher due to the deep base level (Alaknanda River). In between two serial profiles, three other cross section are also drawn. Cross section 3 and 2 are shows that the river flows in a very narrow gorge. In the middle part river passes through the quartzite rock which is resistant to erode quickly. Cross section 4 and 5 shows that the upper part of the profile is wide than the bottom because of the phyllitic terrain. Phyllitic terrain is weathered non-resistant surface which is easy to erode for erosional processes. Finally it can be concluded that the river is formed mature topography and flow in early mature stage of development. (ii) Superimposed profile: Superimposed profile was drawn to represent approximately the true character of the landforms. It provide the panoramic view of the high peak and deep valleys (Fig.13, A). All the profile drawn in the Dhundsir Gad reveals the erosional process by perennial stream and the slope of the Dhundsir Gad. It shows that the Resani Top, Gaddikhal, Athani Dhar, Silkakhal Dhar ridges in the highest range of Dhundsir Gad. The successive cross profiles from low heights to higher are clearly visible in the superimposed profile. (iii) Projected profile: Projected profiles have been drawn to relieve the obscurity. These features which are not observed by higher intervening topographic expressions are accepted to portray a panoramic view, the profile shows the undulating terrain's visible from valley mouth (Fig.13,B) (iv) Composite profile: The composite profile depicts only ruggedness of the skyline of the basin (Fig.13, C). It represents the surface view in the summit levels from the infinite distance. The composite profile of the Dhundsir Gad exhibits the ruggedness of the sky line and the sharp topped summit at the higher altitude. Longitudinal Profile A careful study of the long profile of any river reveals a variety of forms, which can be described under at best three major categories of the general shape, erosional features and depositional forms, which can be described under at least three major categories of the general shape erosional features and depositional forms (Faniran & Jeje, 1983). The characteristics features of the general shape of long profile of river include concavity and 57

17 convexity graded and upgraded forms. The erosional features of includes waterfalls, rapids, knick points, portholes, polished bed rocks, eroded banks, and slips of slope and gorges. Beside this the major depositional features includes channel bars, flood plains, alluvial fans and terraces. Basically the best description of the shape of the river long profile is its slope or gradient, which is graphical representation of the ratio of the falls of the channel to its length over a given reach (Faniran & Jeje, 1983). The channel bed configuration can clearly visible in longitudinal profile. Thus the average gradient of the present study river is about 0.07 or 134/km (Fig.14). It is a typical drainage pattern which can be seen in the Dhundsir Gad long profile. In the most cases of the mountain region the stream channel becomes concave. But in this case of Dhundsir Gad it is not seen in the present river. The length of the Dhundsir Gad is about 17.5 km from source to mouth but the gradient is various from source to mouth. In general a long profile is forms a concave save but their shape is not smooth in every stages upper, middle and lower. The lower course of the river is almost parallel to the base level. While upper course is seems to be concave. The middle course of the river is smooth. There are several possible explanation of such type of anomalies including (1. tectonic upliftment 2. lithological and structural control 3. nature of discharge 4. amount and caliver of load 5.topographical relief 6.length, depth, width and gradient of the river.) Profile Shape For the better understanding of the Dhundsir Gad long profile, it can be divided into three zones- 1. The upper valley profile course is steeper towards the sky. The average gradient of the river is 0.18 or m/km. There are much variations in the relief features in the stream which show that the upper part of the river is completely under youth stage. It shows that the erosional capacity of the stream is quite high which proposed that a graded channel is that where the load of a given degree of communions as grade as the stream capable of carrying. So that the entire energy of the descending water is consumed in the transportation of the water and its load. The sky zone of the river is under Sandraorthoquartzite and Upper Deoban- shale Quartzite of rocks. 2. From Shivalaya to Zirkoti the river flows in a almost state course (Fig.14) and the curve of the profile is smooth. Although there are number of waterfalls (below-2meter), knick points, rapids and gorges along the channel but they are not appeared in 1/50,000 scale. The average gradient of the middle part is 0.18 or m/km The basic reason of such 58

18 smooth profile curve is the river flows throughout quartzite resistant terrain. The river flows in a deep valley. Some transverse profile are also drawn along the longitudinal profile to know the valley shape in the different stage of Dhundsir Gad (Fig.14). In the middle part of the river is passing through a very narrow gorge. 3. The lower part of the long profile curve is almost parallel to the base line. There are also number of knick points, falls and rapids which shows the episodic upliftment of the Himalaya. This part is under phyllite and non-resistant rocks. The vertical profile show that the river flows in a V shaped valley. The upper margin of the valley is widened by weathering process on non-resistant rocks. The lower valley bottom walls are narrow and steeper. The average gradient this part of the river is 0.16 or 115m/km. It revels that the river almost is reached in late maturity stage of the geomorphic development. As already stated that the base of the river is Alaknanda river which course is deeper and wider. The figure14 shows in the lower part near it s confluence the gorge 39m deep and 70-m wide is developed. The lower portion of the gorge is only 35m wide under a foot path bridge. After the formation of main channel course along a main liner feature the later generation of transverse fault was developed which displaced the previous one. The axis of the fault runs in east-west direction. The fault appears to have helped to sift in stream channel towards the west. After entering the fault line, the channel runs straight towards east superimposed on fault alignment. Thus a bog-back feature is formed here. But towards the north of river, there is a sudden change in the steepness of profile. This area is effected by N.A.T. the river flows along the thrust. The profile also revels that the river is quite narrow and flat with gentle steep slope. The over all longitudinal profile curve of the Dhundsir Gad shows that the river flow in a graded profile curve. ***** 59