GEOMORPHOLOGY. Geomorphology deals with different landforms that define the. earth s surface, their origin, sequence of evolution, stage of

Transcription

1 GEOMORPHOLOGY 4.1 Introduction Geomorphology deals with different landforms that define the earth s surface, their origin, sequence of evolution, stage of development and also their future trend. Topography and stage of geornorphi.c evolution are important with respect to the development of aerially extensive weathered layers. Land forms with their naturally associated soil and vegetation cover give indirect clues to the occurrence of groundwater. Thus, mapping of various land forms and studying different physiognomic features are important steps in groundwater investigation. This section deals with th,e study of land forms and morphometric analysis of Sasive Halla Basin. 4.2 Physiography The area forms a part of Karnataka Plateau and slopes towards northwest. Northern part of the basin is occupied by the Malgitti-Gajendragad hill ranges which also forms the water divide. Maximum elevation of 800 m from mean sea level is noticed f in these hill ranges near Gajendragad. Major portion of the basin forming a pediplain ranging in elevation from 527 to 640 m. f 48

2 4.3 Drainage System The Sasive Halla Basin is a part of the' Malaprabha drainage system. The mainstream i.e, the Sasive Halla joins Malaprabha river at a place about 2 kms south of the village Cholachgud. It has two tributaries viz, The Chaul Halla and the Sasive Halla which join together near Tallihal. Chaul Halla and its small tributaries flow in NNW direction. Sasive Halla and its tributaries flow in W direction. Some of the tributaries of the Sasive Hal'la flow in NW direction also. The major direction of drainage in the basin is towards NW and i West and the streams are generally ephemeral except one or two streamlets near Kalakaleshwar which are fed by natural springs. The overall drainage pattern is dendritic, parallel to subparallel.and mostly controlled by joints and fractures. Certain directional trends in parts of the basin seem to be guided by structural conditions such as fault lines or major planes of joints and fractures in the gneissic rocks. These are indicated as lineaments in the Fracture pattern map (Fig.2.3). Major streams have a shallow, wide sandy courses. 4.4 Morphometric Analysis The most important element of fluvial system of a basin landscape is the stream channel system which develops in relation to / hydraulic, geomorphologic and erodibility characteristics of the basin. The influence of lithology and structure on drainage 49

3 network has been discussed by Moeen and Babu (1988), Sidhu et.al (1988), Mukhopadhyay (1973). Quantitative morphometric analysis of the drainage basins have also been carried out by Horton (1932, 1945), Morisawa (1972), Desai and Tigale (1980), Meera Agarwal (1972), Reddy and Reddy (1983), Anil Kumar and Pandey (1981) etc. Various characteristics of any channel network can be measured and grouped under the following broad heads : basin geometry, linear parameters and intensity of dissection. These morphometric parameters of Sasive Halla Basin have been measured and studied to understand their influence on the groundwater occurrence. The morphometric parameters in the present work have been derived from the topographical maps, aerial photographs, satellite imagery and field checks. The same are given in Table Stream Order Streams are graded to ascertain the dimension of the basin and to measure the position of the stream in the hierarchal network of streams. In the present case, the method evolved by Strahler (1952) has been used. The Sasive Halla Basin consists of 605 first order streams,142 second order streams, 31 third order,9 fourth order 3 fifth order and a sixth order stream. Entire discharge of water and sediment passes through the sixth order trunk stream i.e, the Sasive Halla (Fig.4.1). 50

4 4.4.2 Bifurcation Ratio It is defined as the ratio between the number of streams of a given order (Nu) and the number of streams of the next higher or^ier (Nu+1). At least two segments of the same order are required to produce a stream of next higher order. Hence, bifurcation ratio can not be less than two. This parameter indicates the propensity of streams to develop tributaries and frequency with which streams of a given order enter streams of next higher order. Variation in bifurcation ratio is due to different lithological units and structural control. Generally, well jointed and fissured basins have higher bifurcation ratio. In Sasive Halla basin it ranges from 3 to 4.29 with an average of Stream Length Mean stream length is the average of the lengths of all orders in a particular basin. Stream length of various orders and average length, help in the identification of terrain character. Factors associated with the stream length are slope, rainfall, porosity, permeability of the soil and grain size, structure of the underlying rock. In the red soil region of Sasive Halla Basin, streams irrespective of their order are generally longer except those along the steep slopes of the hills where they are shorter and follow fracture planes. While in the black soil plains only higher order streams are longer and the lower order streams are 51

5 shorter. Total and average length of the streams of different orders are determined and are given in the Table Area of the drainage basin Basin area is a dimensional parameter. Factors associated with it are length of stream, lithology, slope, stream frequency and drainage density. In the study area the lithology is almost homogeneous. Influence of stream frequency, nature of soil cover are exhibited by the areas of third order sub-basins. Conspicuously the average areas of the third order basins is higher (excluding those in the steep slopes) in the red soil o country underlain by granites (14.65 Km ) than the areas of third order basins in black cotton soil underlain by migmatitic ( ' 2 gneisses (10.65 Km ). This change in average areas is due to comparatively impermeable black soil and steep gradients aided by closely spaced joints in red soil. Thus, area of the drainage basin gives an idea regarding the recharge characteristics of the areas Drainage density Drainage density is also a dimensional parameter expressed as Kilometer per unit area. It is used in measuring the fineness of a basin topography. The factors influencing this are nature of rock, vegetative cover, slope, rainfall etc., The high drainage density is related to steep slopes and high average relief. In 52

6 the Sasive Halla basin average drainage density is 1.28 Kins. per unit area. Generally high drainage density is seen in black cotton soil area and in the areas of steep gradients. t Stream frequency Stream frequency, the number of channels per unit area is a measure of intensity of dissection'. Investigations reveal that high stream frequency is found in areas of non porous bedrock, relatively low average slope, less vegetative cover. In Sasive Halla Basin the average stream frequency is 1.2 channels per unit area. In general, stream frequency is high in the areas covered by black cotton soil where porosity and permeability are considerably low and the average slope is l'ess than 1. However, higher stream frequency is noticed in some parts in the red soil also where numerous streamlets along steep slope follow closely spaced joint planes Slope Analysis i Slope is an important morphometric factor because it controls the precipitation run off on the surface. In a hilly terrain where average slope is high, all the precipitated water forms run off quickly giving little scope for infiltration. While in the plain area where average slope is less, generally water spreads over large area and finds scope for infiltration.

7 Average slope of the Sasive Halla Basin has been determined by the Wentworth s (1930) method. Variation of average slope in the basin is shown in Fig.4.2. Based on the average slope determined by the above method the area is classified into four categories as follows. Slope Category- Average slope Level Slightly inclined Moderately inclined Steep i Less than More than Land forms The Sasive Halla Basin presents a denudational landscape.. These land forms have been identified on the basis of their geomorphic expression, relief, slope factor, surface cover with soil or vegetation. They were delineated on the basis of their image/ photo characteristics. Different denudational land forms that could be delineated are 1) Denudational hills 2) Dissected pediment 3) Bajada/ Buried pediment 4 ) Pediplain 5) Colluvium (Fig.4.3). Generally, bajada, pediplain and colluvium have average slope less than 3 and the dissected pediments have 3-5 average slope. Average slope of the denudational hills exceeds 5. Brief description of different land forms and their hydrogeologic importance are as follows:

8 4.5.1 Denudational Hills The geomorphic expression of the denudational hills is largelycontrolled by the lithologic variation. They are the remnants of the natural dynamic processes of denudation, weathering aided by fluvial action. These occur as mesa and buttes and linear ridges and inselbergs with little,debris at the foot slopes, Mesa and buttes The sandstones (cave temple Arenites) forming flat topped hills with steep escarpme.nts are identified as mesa and buttes (Plate. 15}.' They appear on the aerial photo as well as satellite imagery as distinct features with medium grey and greyish yellow tone with rough texture. Their well jointed nature together with low drainage density points to highly porous and permeable nature. Soil cover over the sandstones is negligible and hence supports no vegetation. However, small depressions covered by sandy debris near Gajendragad have supported minor cultivation (Plate.6). These behave as zones of recharge Ridges and inselbergs Granites in the study area form small linear ridges and mounds at one or two places near Nallur, Kalliganur but at all other places they underlie horizontal sandstones and form steep erosional slppes. The hills as well as Piedmont slopes are essentially 55

9 stony with very little soil cover and mostly barren of vegetation. These denudational hills are characterized by horizontal, curvilinear and vertical joints which accelerate the process of weathering and give rise to the typical semi-arid type I of tropical geomorphic forms such as domes, inselbergs and tors with steeper slopes along whi,ch sliding of the bigger boulders takes place. Occasional growth of low thorny bushes and shrubs marks the deep fractures indicating their recharge capacity Dissected pediment The term pediment is defined as rock cut surface at the foot of a mountain slope or face with or without a thin veneer of detritus formed normally under a semi-arid cycle of erosion. But away from the Piedmont junction the thickness of debris generally increases forming a bajada. Dissection of this pediment surface due to fluvial action by sheet wash results in dissected pediment almost devpid of the natural cover of soil. In Sasive Halla Basin dissected pediment formed by the above process occupies a narrow band next to the category of denudational hills and patches near Naregal and Itgi (Plate.16). Boundary of pediment zone is marked by the confluence of first order streams. The pediment is noticed over pink granites in the Northern part of the basin and migmatitic gneisses near Itgi and Naregal. It is characterized by undulating topography with bouldery, gravelly coarse grained, sandy residual soils (red) and scattered outcrops. The depth of weathering is highly variable ranging from 1 m to 5 m and intense along the fracture lines extending 56

10 upto 20 m. Sheet erosion and gullying are ' very active in this zone exposing underlying weathered mantle bedrock. At many places soil cover is almost absent Bajada/ buried pediment Generally away from the Piedmont junction due to the gentle slope, accumulation of debris increases over the pediment forming * buried pediment or bajada. Drainage density normally becomes less owing to the higher rates of infiltration through the more porous and permeable transported debris over the pediment. Hence depth of weatherings also increases in this zone. In Sasive Halla Basin this zone is identified by slightly darker tone, coarse texture and low drainage density, more or less flat topography (Plate.18). The higher order streams occupy fracture or joint planes. This zone is noticed as the extension of pediment all along the northern portion which is covered by red sandy soil. The thickness generally ranges from 1 to 2 m. Depth of weathering generally ranges from m and along the fractures/ joint planes it extends even upto 25 m. Most of the area in this zone is cultivated except at some places near Lakkalakatti and Gogeri intense gullying and headward erosion has caused development of bad lands. 57

11 4.5.6 PedjLplain Pediplain refers to the flat or gently sloping surface which is the end product of coalescence of several pediments. About twothird of the Sasive Halla Basin forms pediplain (Plate.18). These are identified as vast, highly matured undulating plains much dissected with fine dendritic drainage and fine texture. Average slope is less than 1. The pediplain spreads over the weathered migmatitic gneiss. These are covered with Kankary black cotton soil ranging in thickness from 1.5 m to 5 m. Zone of weathering is moderate to deep (15-25 m). At many places sheetwash, rill and gully erosion are quite common.the drainage over pediplain is fine dendritic and at places controlled by fractures. / Colluvium This geomorphic feature is clearly identified on the aerial photographs as infilled valleys and separated from one another by the other geomorphic units. Colluvium is predominantly a sheetwash material derived from the adjoining upland and deposited in the low lying shallow fluvial channel. In Sasive Halla Basin two such zones are noticed. One is near Kalakaleshwar and the other near Kalliganur. Two major fractures have been confirmed in these zones and deep weathering (more than 40 m. ) has been noticed in both the cases. Palm trees are the commonly noticed vegetation types in. these areas. The highly porous and 58

12 permeable nature of the deposited material which is aided by lithological structure i.e, fractures has been responsible for high groundwater potential in these areas.! 58

13 Tables MORPHOMETRIC PARAMETERS OP THE SASIVE HALLA DRAINAGE BASIN. 4-1 Basin Geometry Area of the basin (A6) * 653 Sq.Kms N1 *605 Length of the B asin (L6) * 37 Kms. N2 * «3 = = 3 3 N6 = 1 3 (EN)u* 790 Mean L1 * 558 L2 * 115 L4 * 47 L5 *24.5 L6 =22 (EL )u * 839 O.92 o L3 « Mean= 2.05 drainage D ensity (Dd.) 1.28 km/jcnr Drainage frequency(fu) * 1.20 stream s/ km2 Constant Channel maintenance = 0,7783 5* Texture Batio* = n Width of the Basin (W6 ) = 36 kms P erim eter of the ^ a sin (P6 ) *118.5 kms C irc u la rity Ratio (Be) * k b K» A rcsin K * 52 38' P erim eter c a lc u la te d (P ) * 95*24 kms E (k ) * Lamniscate R atio * L inear Param eters Number of B ifu rca- Total stream of tio n 1 ength the order fiatio of the u (lib) stream s of the order u in Kms Mean Stream styeam len g th le n g th Lu In te n sity of D isection

14 75 45' J 50' 15 DRAINAGE NETWORK IN SASIVE HALLA BASIN 40' 15 X<? ' 7^ 'Aez- 1st order steeams 2nd order streams 3rd order streams 4th order streams 5th order streams 6th order stream 75? 45' 76 0' i Fig. 4.1

15 Fig. 4.2 v

16 50' 15 75^45' 76 0' GEOMORPHOLOGICAL MAP OF SASIVE HALLA BASIN ' 15 45' 15 40' ~is Denudational hills 40' IS Dissected pediment Buried pediment/bajada Colluvium Pediplain 35' 15' 75? 45' 76 0' i Fig. A.3