MORPHOMETRIC STUDY OF RIVER BASINS
River characteristics are reasonably understood by the morphometric analysis of that particular river basin. The morphometric studies on river basins were first introduced by Horton in 1932 and the study of geometry of landform and the various morphometric parameters are discussed below
Drainage basin, drainage divide, and drainage pattern -The entire area of a river basin whose runoff drains into the river basin is considered as a hydrologic unit and is called a drainage basin (Pradeep Kumar, 1999). The boundary line along a topographic ridge separating two adjacent drainage basins is called drainage divide (Upendran, 1999). Drainage pattern or drainage arrangement refers to the particular plan or design, which individual stream courses collectively form, and is influenced by factors like initial slope, inequalities in rock hardness, structural control, recent geologic and geomorphic history of the drainage basin (Leopold et al, 1964).
Stream order- First order streams are those that do not have any tributary. The smallest recognizable channels (streams) are called first order and these channels normally flow during wet weather (Chow et al, 1988). A second order stream forms when two first order streams join and a third order when two second order streams are joined and so on (Strahler, 1964). Where a channel of lower order joins a channel of higher order, channel downstream retains higher of the two orders and order of river basin is order of the stream draining its outlet, highest stream order in the basin (Chow et al, 1988).
Stream Number- The order wise total number of stream segments is known as the stream number.
Stream length- Horton’s law of stream lengths states that mean lengths of stream segments of each of the successive orders of a basin tend to approximate a direct geometric sequence in which the first order term is the average length of segments of the first order (Horton, 1945). The longer stream length indicates slower the appearance of flood and larger surface flow.
Mean stream length (Lu)- The mean stream length (Lu) is a dimensional property, revealing the characteristic size of the components of a drainage network and it contributes basin surface (Strahler, 1964). Mean stream length, Lu = å L / Nu, where åL is the total length of the stream of particular order and Nu is the number of stream segment of that order. In general Lu increases as the order of segment increases.
Drainage density (Dd)- Drainage density (Dd) is the total length of the streams in a given drainage basin divided by the area of drainage basin (Horton, 1932, 1945). Dd = åL / A, where åL is the total length of the stream and A is the area of drainage basin. Difference in Dd is due to rock type variation, run off intensity, rainfall variation, soil type, relief, initial resistivity of the terrain to erosion infiltration rate and the total drainage area of the basin.
Stream frequency (Df)- The stream frequency is defined as the number of stream segments per unit area (Horton, 1932, 1945). Stream frequency (Df) is determined by dividing the total number of streams in a basin by area of drainage basin. Thus Df=åN/A, where N is number of stream segments and A denotes drainage area.
Stream length ratio (RL)- Stream length ratio is the ratio of mean length of streams of one order to that of the next lower order, which tends to be constant throughout the successive orders of a watershed (Horton, 1945). The stream length ratio RL is given by the equation, RL = L u / Lu-1; where Lu is mean stream length of order u and Lu-1 is the mean stream length of next lower order. The larger RL values indicate more lower order sources for the next higher order streams and lower values indicate the limited length of lower order drainages to serve as hydrological sources (Kumaraswamy and Sivagnanam, 1998) and low values of RL indicate the lesser number of lower order Hortonian streams.
Relief ratio (Rh)- The elevation between highest and lowest points in a basin is called basin relief. Relief ratio indicates the overall steepness of drainage basin and is an indication of intensity of degradation processes operating on slopes of the basin and is the ratio between the total relief of the basin and its longest dimension parallel to the principal drainage line. Rh = H / Lh; where H is the total relief and Lh is the basin length.
Elongation ratio (E)- Elongation ratio is defined as the ratio of the diameter of a circle having the same area as the basin and maximum basin length (Schumm, 1956). It is a measure of shape of river basin and the value ranges between 0.6 and 1(Chow, 1964). Lower the E value, more elongated shape for the basins (Srivastava, 1978). Value ranges from 0.6 to 0.8 are regions of high relief. By analyzing elongation ratio, we can predict shape of basins. Basins with elongation ratio values above 0.9 are circular in shape, between 0.8 and 0.9 are oval shaped, 0.7 and 0.8 are less elongated and below 0.7 are elongated. A circular basin is more efficient in the discharge of runoff than an elongated basin and is significant in flood forecasting (Singh and Singh, 1997) besides concentration time is less in circular basins (Upendran et al, 1998).
Bifurcation ratio (Rb)- It is defined as the number of segments in an order to the number of segments in the next higher order (Horton, 1945). It is represented by the equation Rb = Nu / Nu+1; where Rb is the bifurcation ratio, Nu is the number of segments in an order u and Nu+1 is the number of segments in the next higher order. The bifurcation ratio ranges between 3 and 5.0 for watersheds in which geometrical structures do not distort drainage pattern (Chow et al, 1988). Theoretical minimum value of 2.0 is rarely approached under natural conditions. It is a dimensionless ratio, as drainage systems in homogeneous materials tend to display geometrical similarity, the ratio shows only a small variation from region to region. If within a net, bifurcation ratios are equal, it is called Hotron’s net.
Form factor (F)- It is the ratio of basin area to the square of basin length (Horton, 1932) and is calculated by F = A / Lb2; where A is drainage area and Lb is length of the river basin. Highly elongated basins with F value of zero and circular basin with one. Basins with high F values have high peak flows for shorter duration where as elongated basins with low F value will have a flatter peak flow for longer duration and flood flows of elongated basins are easier to manage (Nautiyal, 1994).
Circularity ratio (C)- It is the ratio of area of river basin to area of a circle having the same perimeter as basin (Miller, 1953). Like form factor, it is also a dimensionless ratio to express outline of drainage basin (Strahler, 1964) and C values are influenced by length and number of streams, geological structures, land use / land cover, climate, relief and slope of the basin (Singh and Singh, 1997).
Sinuosity index (S)- It is the ratio of channel length and river valley length (Muller, 1968). Sinuosity index reveals the topographic and hydraulic conditions of streamlines.
Drainage basin, drainage divide, and drainage pattern -The entire area of a river basin whose runoff drains into the river basin is considered as a hydrologic unit and is called a drainage basin (Pradeep Kumar, 1999). The boundary line along a topographic ridge separating two adjacent drainage basins is called drainage divide (Upendran, 1999). Drainage pattern or drainage arrangement refers to the particular plan or design, which individual stream courses collectively form, and is influenced by factors like initial slope, inequalities in rock hardness, structural control, recent geologic and geomorphic history of the drainage basin (Leopold et al, 1964).
Stream order- First order streams are those that do not have any tributary. The smallest recognizable channels (streams) are called first order and these channels normally flow during wet weather (Chow et al, 1988). A second order stream forms when two first order streams join and a third order when two second order streams are joined and so on (Strahler, 1964). Where a channel of lower order joins a channel of higher order, channel downstream retains higher of the two orders and order of river basin is order of the stream draining its outlet, highest stream order in the basin (Chow et al, 1988).
Stream Number- The order wise total number of stream segments is known as the stream number.
Stream length- Horton’s law of stream lengths states that mean lengths of stream segments of each of the successive orders of a basin tend to approximate a direct geometric sequence in which the first order term is the average length of segments of the first order (Horton, 1945). The longer stream length indicates slower the appearance of flood and larger surface flow.
Mean stream length (Lu)- The mean stream length (Lu) is a dimensional property, revealing the characteristic size of the components of a drainage network and it contributes basin surface (Strahler, 1964). Mean stream length, Lu = å L / Nu, where åL is the total length of the stream of particular order and Nu is the number of stream segment of that order. In general Lu increases as the order of segment increases.
Drainage density (Dd)- Drainage density (Dd) is the total length of the streams in a given drainage basin divided by the area of drainage basin (Horton, 1932, 1945). Dd = åL / A, where åL is the total length of the stream and A is the area of drainage basin. Difference in Dd is due to rock type variation, run off intensity, rainfall variation, soil type, relief, initial resistivity of the terrain to erosion infiltration rate and the total drainage area of the basin.
Stream frequency (Df)- The stream frequency is defined as the number of stream segments per unit area (Horton, 1932, 1945). Stream frequency (Df) is determined by dividing the total number of streams in a basin by area of drainage basin. Thus Df=åN/A, where N is number of stream segments and A denotes drainage area.
Stream length ratio (RL)- Stream length ratio is the ratio of mean length of streams of one order to that of the next lower order, which tends to be constant throughout the successive orders of a watershed (Horton, 1945). The stream length ratio RL is given by the equation, RL = L u / Lu-1; where Lu is mean stream length of order u and Lu-1 is the mean stream length of next lower order. The larger RL values indicate more lower order sources for the next higher order streams and lower values indicate the limited length of lower order drainages to serve as hydrological sources (Kumaraswamy and Sivagnanam, 1998) and low values of RL indicate the lesser number of lower order Hortonian streams.
Relief ratio (Rh)- The elevation between highest and lowest points in a basin is called basin relief. Relief ratio indicates the overall steepness of drainage basin and is an indication of intensity of degradation processes operating on slopes of the basin and is the ratio between the total relief of the basin and its longest dimension parallel to the principal drainage line. Rh = H / Lh; where H is the total relief and Lh is the basin length.
Elongation ratio (E)- Elongation ratio is defined as the ratio of the diameter of a circle having the same area as the basin and maximum basin length (Schumm, 1956). It is a measure of shape of river basin and the value ranges between 0.6 and 1(Chow, 1964). Lower the E value, more elongated shape for the basins (Srivastava, 1978). Value ranges from 0.6 to 0.8 are regions of high relief. By analyzing elongation ratio, we can predict shape of basins. Basins with elongation ratio values above 0.9 are circular in shape, between 0.8 and 0.9 are oval shaped, 0.7 and 0.8 are less elongated and below 0.7 are elongated. A circular basin is more efficient in the discharge of runoff than an elongated basin and is significant in flood forecasting (Singh and Singh, 1997) besides concentration time is less in circular basins (Upendran et al, 1998).
Bifurcation ratio (Rb)- It is defined as the number of segments in an order to the number of segments in the next higher order (Horton, 1945). It is represented by the equation Rb = Nu / Nu+1; where Rb is the bifurcation ratio, Nu is the number of segments in an order u and Nu+1 is the number of segments in the next higher order. The bifurcation ratio ranges between 3 and 5.0 for watersheds in which geometrical structures do not distort drainage pattern (Chow et al, 1988). Theoretical minimum value of 2.0 is rarely approached under natural conditions. It is a dimensionless ratio, as drainage systems in homogeneous materials tend to display geometrical similarity, the ratio shows only a small variation from region to region. If within a net, bifurcation ratios are equal, it is called Hotron’s net.
Form factor (F)- It is the ratio of basin area to the square of basin length (Horton, 1932) and is calculated by F = A / Lb2; where A is drainage area and Lb is length of the river basin. Highly elongated basins with F value of zero and circular basin with one. Basins with high F values have high peak flows for shorter duration where as elongated basins with low F value will have a flatter peak flow for longer duration and flood flows of elongated basins are easier to manage (Nautiyal, 1994).
Circularity ratio (C)- It is the ratio of area of river basin to area of a circle having the same perimeter as basin (Miller, 1953). Like form factor, it is also a dimensionless ratio to express outline of drainage basin (Strahler, 1964) and C values are influenced by length and number of streams, geological structures, land use / land cover, climate, relief and slope of the basin (Singh and Singh, 1997).
Sinuosity index (S)- It is the ratio of channel length and river valley length (Muller, 1968). Sinuosity index reveals the topographic and hydraulic conditions of streamlines.
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