DRAINAGE BASIN -SHAPE AND TEXTURE

MRS MAUSHAMI ASSISTANT PROFESSORP.G DEPT OF GEOGRAPHYM.U BODHGAYA.E - CONTENT

SEM-1stDate-05/05/2020

LEARNING OBJECTIVES;

BASIN SHAPE.LAW OF BASIN PERIMETER , BASIN

LENGTH AND BASIN AREA.BASIN TEXTURE.

BASIN SHAPE.

It generally refers to differentgeometrical forms of river basin.

Assessment of outline of thedrainage basin.

Basin shape is dependent on ;- sizeof the basin (its area), length of themaster stream of the basin and itsperimeter.

These factors are dependent on other variables such as absolute

relief, slopes ,geological structure and lithological characteristics.

. The ideal shape of the basin is usually of Pear -shaped (Horton 1941).

. On an average three sub categories three sub categories of basin have

been recognized.- a)circular b) elongated and C) indented.

. Several methods of computation of basin shape are as follows.

1) Horton’s Form factor

F= A/L2

F= form factor showing the elongation of the basin shape.

A= BASIN AREA.

L(max)= BASIN LENGTH.

.It represents the dimensionless ratio of the area of the drainage basin to the square of its maximum length L,

.The form factor is equal to unity when the basin shape is a square, and decreases according to the extent of elongation. For a basin whose shape approximates a circle, the ratio is higher than unity.

Use of the factor;

.This factor indicates the flow intensity of a basin of a definedarea (Horton, 1945). This factor has been considerably used inconnection with maximum flood-discharge formulas..The sub-basin with high form factors have high peak flows ofshorter duration, whereas elongated sub-basin with low formfactor 0.25 indicating them to be slightly elongated in shapeand lower peak flows for longer duration. Flood flows ofelongated basins are easier to manage than those of thecircular basin (Nautiyal, 1994).

BASIN SAHPE AND FLOOD DISCHARGE.

Source - Google image

2) stoddart’s Elipticity ratio

E=ꙤL2 / 4 * A

E= ELIPTICITY RATIO

Ꙥ= 3.14

A= BASIN AREA

L= BASIN LENGTH.

It varies from 1 to 0.

V.C .MILLER’S CIRCULARITY INDEX (1953)

introduced the circularity ratio RC, which represents the quotientbetween the area At of a basin and the area Ac of a circle whosecircumference is equal to the basin perimeter :

RC = At/Ac ,( RC= 4Ꙥ A/ p2) ,p= perimeter of the basin.

To establish this ratio, the area and perimeter of a drainage basinmust thus be measured, the ratio is equal to unity when the basinshape is a perfect circle, decreasing to 0.785 when the basin is asquare, and continues to decrease to the extent to which the basinbecomes elongated. The circularity ratio can be derived directlyfrom the formula.i.e 0= (a line ) to 1 ( circle)

S.A. Schumm’s (1956) Elongation ratio (R).

given by the quotient of the diameter Dc of a circle of area equal to that of basin and maximum basin length Lb measured parallel to the axis of main stream.

R= DIAMETER OF CIRCLE WITH SAME AREA AS BASIN

BASIN LENGTH (maxm)

R varies from 0 (highly elongated shape) to unity 1 (circle).

LEMNISCATE’S RATIO.

Starting from Horton’s observation that the “ideal” form of a drainage basin developed on initially sloping ground closely resembles a “pear’ or “drop” shape.

Chorley et al. (1957) and subsequently Morisawa (1958) and Strahler (1964) noted that there is a close resemblance between such shapes and lemniscate curves.

Thus, the polar form of the lemniscate equation is chosen to express the variation of basin shape:

p = l cos p ø

where p is the radius from the outlet to the rim, ø the angle between the baseline and the radius under consideration, and l the greatest diameter, analogous to the drainsge-basin length, which becomes equal to p when ø is zero .

Regression equation.

y= a + bx (1)

Where ‘b’ is the co-efficient of the Regression equation, which can be calculated from the following formula –

By plotting the values of ‘x’, ‘a’ and ‘b’ in the regression equation (1), we get the value of ‘y’ for corresponding stream number and stream length .

Law of Basin perimeter ,Basin length,and Basin area.

Basin area ,basin perimeter , channel length are important variables which determine the shape, size, and genetic aspect of drainage basin.

Basin perimeter can be directly coorelated with the square roots of basin area and increase and decrease in the former indicates the increase or decrease in the latter. Both the variables shows the strong positive correlation as coefficient of correlation r= 0.99= 1 ,similarly coefficient of correlation between basin perimeter and channel length is quite significant.

DRAINAGE TEXTURE AND ITS IMPLICATIONS

It is the relative spacing of drainage lines.

G.H. Smith (1950),Horton (1945) defined drainage texture on the basis of Drainage frequency and Drainge density.

Drainage texture depends on the underlying lithology, infiltration capacity and relief aspect of the terrain.

Smith (1950) has classified drainage texture into 5 different textures i.e., very coarse (<2), coarse (2 to 4), moderate (4 to 6), fine (6 to 8) and very fine (>8).

More is the texture more will be dissectionand leads more erosion.

Badland topography-fine drainagetexture. Impermeable clays and shales,sparse vegetation and scanty rainfall areresponsible for fine drainage texture.

Sand and gravel outwash plains- coarsedrainage texture.

Q- Find out the correlation coefficient between the two variables stream order and stream number.

STREAM ORDER STREAM NUMBER

1 109

2 56

3 25

4 1

References/ suggested readings;

Thornbury,W.D.,(1960):”PrinciplesofGeomorphology”,CBS publishers and distributors pvt ltd.

A. N. Strahler, “Quantitative geomorphology of drainage basin and channel networks,” in Handbook of Applied Hydrology, V. T. Chow, Ed., McGraw Hill, New York, NY, USA, 1964.

Singh, savindra ,(2018): “Geomorphology”, Prayagpustak bhawan ,Allahabad