MORPHOMETRIC ANAYSIS OF THE MAJOR VALLEY SYSTEMS AROUND BENGALURU

The three major Valley systems of Bengaluru namely Vrishabhavathi Valley, Hebbal Valley and Kormangala-Challaghatta Valley houses many lakes and play a very important role in its hydrological processes. The morphometric analysis helps us to learn about the characteristics of the underlying rock type, pervious nature of soil, slope gradients, runoff behavior and water retention potential within the Valley systems. Morphometric analysis was carried out for Linear, areal and relief aspects. The Survey of India topographical maps and Digital Elevation Model data were used to prepare the base map and the drainage maps with the help of GIS software. The Strahler system of stream ranking was adopted. Among the three Valleys, Vrishabhavathi or a flat surface as a whole. The value of the relative ratio for Vrishabhavathi Valley is 0.008, Hebbal Valley is 0.003 and Kormangala-Challaghatta Valley is 0.004. The results show that the Slope in the Valley regions vary from nearly level to gentle slope or very gentle slope. The relative relief value of Vrishabhavathi Valley is 0.002, Hebbal Valley is 0.003 and Kormangala-Challaghatta Valley is 0.001. The results show that the Slope in the Valley regions vary from nearly level to gentle slope or very gentle slope. (In few points within the Valley we can notice moderate slope and steep slope). The values of the ruggedness number computed for Vrishabhavathi Valley is 0.528, Hebbal Valley is 0.112 and Kormangala-Challaghatta Valley is 0.140. Low values of ruggedness number indicate low relief and coarse drainage density. cover pattern, which affects the infiltration process and the watershed response time between the precipitation and discharge. It measures how well or poorly a watershed or basin is drained by the stream channels. The V-Valley exhibits moderate to fine drainage texture and the other two Valleys coarse drainage texture. Low Values indicate permeable to semi-permeable subsoil material and low relief. The drainage pattern formed within the Valley systems is dendritic, characterized in irregular branching of tributary streams in all the directions. The dendritic drainage is an indication of lack of structural control. The dendritic drainage pattern is formed in areas with flat and uniform bedrock type(that has not been folded). This drainage pattern indicates that the soil is semi-pervious in nature. The relief aspects of the three Valleys exhibit low reliefs indicating almost a pediplain or a flat surface as a whole. The results show that the Slope in the Valley regions vary from nearly level to gentle slope or very gentle slope. (with few points moderate to steep slope). Relief and drainage density contribute to the ruggedness number. Low values of ruggedness number indicate low relief and coarse drainage density. The morphometric analysis helps us to learn about the characteristics of the underlying rock type, the pervious nature of soil, the slope gradients, the runoff behavior and the water retention potential within the Valley systems. This helps in designing a sustainable management plan for the conservation of three major Valley systems and ensure sustainable soil and water usage within the Valley systems.


ISSN: 2320-5407
Int. J. Adv. Res. 9(07), 991-1002 992 between various attributes of streams. It also helps to understand the relationship of various aspects within a drainage basin and compare various basins developed under different topographical and geo-morphological regimes. The Linear aspects include stream order, stream length, bifurcation ratio etc. Areal aspects include area, perimeter, watershed shape factor, drainage density, drainage texture, stream frequency etc. Relief aspects include watershed relief, relief ratio, relative relief, ruggedness number etc. Remote sensing and Geographic information systems are powerful systems which are very useful for carrying out the morphometric analysis.

Materials and Methods:-Preparation of the Base Map
The road networks, railway lines, water bodies, stream networks were digitized using the Survey of India (SOI) Topographical map of 1:50,000 scale using ArcGIS Software. Satellite data was used to update the data in the preparation of the base map.

Morphometric analysis
Morphometric analysis was carried out to analyze the physical characteristics, hydrological processes and drainage patterns in the three Valley systems. Linear, areal and relief aspects were considered for the analysis.

Linear aspects
Linear aspects include the measurements of linear features of drainage such as stream order, stream length, stream length ratio etc and are discussed below.

Stream order (N u )
The drainage map was first prepared using SOI Topographical map and SRTM (Digital elevation model) satellite data. Stream ordering is very important for the drainage basin analysis and systematic mapping of any river system/streams. Strahler system of stream ranking [22] was followed in the present study and ArcGIS Spatial Analyst Hydrology tool was used for stream ordering. Strahler system of stream ordering is a "top down" approach where stream segments are ordered numerically from stream's headwater to a desirable point downstream called an outlet. In this system, the ordering begins with the tributaries at the stream's headwaters being assigned as one(smallest, outermost, unbranched tributaries). When the two-first order streams joins, a second-order stream is formed. When two second-order stream segments meet, a third-order stream is formed and so forth. The higher order stream is always associated with the high discharge, The stream segment of the highest order called the trunk stream/principal stream. It is the one through which all the discharge of sediments and water passes.

Bifurcation ratio( R b )
Bifurcation ratio is defined as the ratio of number of segments of a given order (N u ) to the number of streams of the next higher order (N u+1 ) by Schumm [18]. According to Horton, the bifurcation ratio varies from a minimum of 2 in "flat or rolling drainage basins" to 3 to 4 in "mountains or highly dissected drainage basins". Bifurcation ratio is a prime parameter in the drainage basin analysis of the Valley systems because it helps to link the hydrological regime of the watershed under topological and climatic conditions. It helps in interpreting the shape of the catchment and its runoff behavior. The following expression was used to calculate the Bifurcation ratio Where, R b = Bifurcation ratio; N u = number of stream segments of u or N (u+1) = number of streams of the (u+1) order.

Stream length(Lu)
Stream length is the total length of streams in the given catchment area or any watershed. The length of the stream is an indication of the contributing area of the catchment. The streams of various order were counted and their lengths measured using ArcGIS software. The total length of stream segments decreases with increasing order of the streams. Deviation from this general behavior indicates that the terrain is characterized by moderately steep slopes and/or high relief, underlain by varying lithology and probable uplift across the basin (Singh and Singh 1997) [19]. Steep well drained areas have numerous small tributaries.

Mean Stream length( u L )
The mean stream length is calculated by dividing the total stream length of order u(L u ) by the number of streams(N u ). The unit of mean stream length is represented in Kms. It indicates the characteristic size of the components of a drainage network and its contributing basin surfaces. Topography of the catchment has influence on the mean stream length. The following equation was used to calculate the mean length of the channel.
Where u L is the Mean length of channel of order u in Km Lu is the total stream length of order u i Nu is the total number of stream segments of order u.

Stream length ratio ( u L )
It is defined as the ratio of the stream length of any stream order to the next lower order of the stream segments (Horton, 1945). Horton also states that the stream length ratio will remain constant throughout the successive orders of the stream and the variation is mainly due to the slope as well as topographical changes in the basin area. The equation used to calculate the stream length ratio is expressed as The shorter the length of the overland flow, the surface runoff is more faster. It is inversely related to the average slope of the channel.

Areal Aspects
Areal aspects of a watershed or basin is defined as the total area projected upon a horizontal plane contributing overland flow to the channel segment/streams of the given order and includes all tributaries of lower order. Areal aspects include area and perimeter of the drainage basin, drainage pattern, drainage density, stream frequency etc.

Area and Perimeter
The area and perimeter of the Valley basins were directly measured with the help of ArcGIS software using calculate geometry tool.

Drainage Pattern
A drainage system is the pattern formed by the rivers, streams and lakes in a particular fashion in the drainage basin. It is the planimetric arrangement of streams on the land surface forming a drainage system. The drainage pattern may reflect the bedrock type (hard rock or soft rock), the original slope and structure, landforms, modification of the earth surface(uplift depression, tilting etc), structural elements (faulting, folding, warping etc) and soil characteristics. It is also controlled by the topography of the land, structural and lithological aspects. The drainage pattern may be dendritic or deranged, parallel, rectangular, radial, trellised, annual, angular and centripetal.

Watershed shape factor
Watershed shape is the shape of projected surface on the horizontal plane of the watershed map. The shape of a watershed has a profound impact on the runoff and sediment transport process. The shape of the catchment also regulates the rate at which water enters the stream. The quantitative expression can be characterized in terms of form factor, compaction coefficient, circularity ratio, and elongation ratio. Form factor (R f ) Horton [7] defined form factor as the ratio of the area of the watershed to the square of the watershed length and is expressed as; Where, Rf = Form factor A = Area of the watershed in sq.km L =Length of the watershed in km

Compactness coefficient (Cc)
Compactness coefficient of a watershed is the ratio of perimeter of watershed to the circumference of circular area, which is equal to the area of that watershed. The Cc is independent of size of watershed and depends only on the slope. A circular watershed is most susceptible to runoff from a drainage stand point as it yields the shortest time of concentration before peak flow occurs in the watershed. Compactness coefficient is directly proportional to elongation of the basin and a perfectly circular basin has its Cc as unity. Circularity ratio is the ratio of the area of watershed to the area of circle having the same circumference as that of the perimeter of the watershed (Miller, 1953) [11]. The value of Circularity ratio ranges from 0.2 to 0.8. Lower Rc values indicate strongly elongated basin. It is expressed as Where, Rc = circularity ratio A = watershed area in sq. km P = Perimeter of watershed in km

Elongation ratio (Re)
The elongation ratio is defined as the ratio of diameter of a circle of the same area as the watershed to the maximum watershed length (Schumm,1956) [17]. It is expressed as Where, Re is the elongation ratio A is watershed area in sq. km L is length of the watershed in km The value of the elongation ratio ranges from 0.4 to 1, lesser the value, more is the elongation of the watershed. We can find many classifications and the one mentioned in the table 1 below is used in the present study.

Drainage density (D d )
Drainage density is defined as the ratio of total length of all streams to the area of the basin or watershed. It represents the closeness of the spacing of channels. It is expressed as km/Sq.km. It is one of the important indications of the linear scale of landform elements in stream eroded topography and it varies inversely with the length of the overland flow. In the areas of higher drainage density the infiltration is less and runoff is more. The wells in this region of low drainage density will have good water potential leading to higher specific capacity of wells. Thus, it indirectly indicates the groundwater potential. Drainage density is collectively influenced by climate, topography, soil infiltration capacity, vegetation, and geology. It is mathematically expressed as:  Res. 9(07), 991-1002 Drainage texture Drainage texture is the total number of stream segments of all the orders in a river basin/catchment to the perimeter of the basin. Unit of drainage texture is km-1.

Constant of channel maintenance (c)
Constant of channel maintenance is defined as the inverse of drainage density (Schumm, 1956) [17].

Relief Aspects
Relief aspects are the significant parameters which help to understand the denudation processes within the catchment and are also the indicator of the water flow direction.

Watershed relief (H)
Watershed relief is the difference in the elevation between the highest point of a watershed and the lowest point on the Valley floor or watershed. It can be either measured on contour map or satellite derived DEM or field measurements. It is expressed by the following equation H= Elevation of the highest point of watershed -Elevation at the watershed outlet

Relief ratio (R h )
Relief Ratio is the ratio of maximum watershed relief to the horizontal distance along the longest dimension of the basin parallel to the principal drainage line (Schumm, 1956) [17]. It measures the overall steepness of the drainage  Res. 9(07), 991-1002 basin and is an indicator of the intensity of erosion processes happening on the slopes of the basin. The high relief ratio indicates high relief and steep slope, where as the low value of relief ratio is mainly due to the resistant basement rocks in the basin and low degree of slope (Mahadevaswamy G. et. al) [9]. It is expressed by the following equation Where, H = Total catchment relief in k L = Maximum length of the catchment in km.

Relative relief (R r )
Relative relief is defined as the ratio of the maximum watershed/catchment relief to the perimeter of the watershed. It is expressed by the equation Where R r = Relative relief H = Maximum relief in km. P = Perimeter in km.

Ruggedness number (R n )
Ruggedness number is defined as the product of the watershed relief and drainage density (Strahler 1964) [22]. It combines steepness of the slope with its length. Larger values of the ruggedness is due to high relief and high drainage density. The lower value of ruggedness number implies that the area has intrinsic structural complexity and is less prone to erosion. It also indicates low relief and drainage density.
Where R n = Ruggedness number H = Watershed relief in km. D d = Drainage density in km/Sq.km.

Results and Discussion:-
The various morphometric parameters computed based on the above mentioned criteria are tabulated in the table 3 below.
The stream lengths, mean stream lengths and stream length ratios calculated for the three major Valley system are mentioned in the below table 5.

Conclusions:-
Morphometric analysis was carried out for linear aspects, areal aspects and relief aspects. Among the three Valleys, Vrishabhavathi Valley is larger in geographical area and perimeter compared to the other two Valley systems. Vrishabhavathi Valley basin has the sixth order stream as its highest stream order where as the other two Valleys have fifth order stream as their highest order. The mean bifurcation ratios stay within normal values indicating less structural disturbance. The watershed shape factor calculated in terms of form factor, compaction coefficient, circularity ratio and elongation ratio indicates that the Vrishabhavathi Valley is elongated in its shape, where as the K-C Valley and the Hebbal Valley are less elongated in their shape. Drainage density reflects land use and land cover pattern, which affects the infiltration process and the watershed response time between the precipitation and discharge. It measures how well or poorly a watershed or basin is drained by the stream channels. The V-Valley exhibits moderate to fine drainage texture and the other two Valleys coarse drainage texture. Low Values indicate permeable to semi-permeable subsoil material and low relief. The drainage pattern formed within the Valley systems is dendritic, characterized in irregular branching of tributary streams in all the directions. The dendritic drainage is an indication of lack of structural control. The dendritic drainage pattern is formed in areas with flat and uniform bedrock type(that has not been folded). This drainage pattern indicates that the soil is semi-pervious in nature. The relief aspects of the three Valleys exhibit low reliefs indicating almost a pediplain or a flat surface as a whole. The results show that the Slope in the Valley regions vary from nearly level to gentle slope or very gentle slope. (with few points moderate to steep slope). Relief and drainage density contribute to the ruggedness number. Low values of ruggedness number indicate low relief and coarse drainage density. The morphometric analysis helps us to learn about the characteristics of the underlying rock type, the pervious nature of soil, the slope gradients, the runoff behavior and the water retention potential within the Valley systems. This helps in designing a sustainable management plan for the conservation of three major Valley systems and ensure sustainable soil and water usage within the Valley systems.