Title of the course : Watershed Hydrology Class : B. Tech. II year, II semester, 2019-20 Title of the Topic : Geomorphology of Watershed Name of the College : College of Agricultural Engineering Teacher : Dr M. L. Sahu. Associate Professor, SWE Mailed to : [email protected]

Watershed: There may be several definitions of watershed. Few are 1. Watershed is an area which contribute runoff to a single outlet. 2. it may be defined as an area surrounded by a ridge line. 3. It may also be defined as the area which drain its excess rainfall to a . 4. The area which drain its runoff to any Stream or is called the watershed area of that stream or river. The unit of watershed is the unit of land area. It may be few cm2 to several Km2. Watershed may be designated as tinny, micro, macro or milli watershed. Imagine a class room which is having a single door. The students sitting in front, middle or rear chairs of this classroom will go out through this door only after finish of their lectures. The walls of this classroom may be considered as a ridge line & door as outlet. The area of this classroom may be considered as area of watershed. Figure 1 & 2 show the pictures of classroom having single door. Figure 3 & 4 show the pictures of watersheds having an outlet & ridge line or divide. Basin, drainage area & catchment are the synonyms of watershed.

Fig. 1: Single door’s classroom Fig. 2: Single door’s classroom Fig. 3& 4: Watershed with ridge line & single outlet

Geomorphology of Watershed:

Watershed geomorphology refers to the study of the characteristics, configuration and evolution of land forms and properties. It comprises of the characteristics of land surface as well as the characteristics of the channels within the watershed/basin boundary. These properties of watersheds significantly affect the characteristics of runoff and other hydrological processes. The geomorphological attributes of watersheds often provide valuable insight into their hydrological behaviour and can be used to develop design hydrologic models.The quantification of these geomorphologic properties of watershedare important for estimating the watershed hydrologic processes.

The morphological Characteristics of watershed may Broadly be classified as A. Linear aspects (one dimensional) B. Aerial aspects (two dimensional) C. Relief aspects (three dimensional)

Linear aspects:

Linear aspects of watershed is concerned with the and its network. In general, these are one dimensional property. The important characteristics are watershed length& width, stream order, stream number, bifurcation ratio &stream length ratio. Areal aspects:

Areal aspects of watershed include the description of areal elements such as watershed area, Watershed slope. watershed shape, watershed perimeter, , stream frequency, is reflected by some parameters such as shape factor,form factor, elongation ratio, circulatory ratio and compactness coefficient. In general, these are two dimensional properties.

Relief aspects:

Relief aspect is related to elevation difference between reference points in watershed. In general, these are three dimensional properties. Few important characteristics are watershed relief, relative relief, slope, law of stream slope & ruggedness number.

1. Linear aspects:Linear aspects are one dimensional properties of watershed.

1.1 Watershed Length Lb&Width Lw: Watershed length has been defined in several ways however the most appropriate may be as the longest straight length parallel to main stream of watershed and watershed width may be measurement perpendicular to the direction of watershed length. In the given figure straight line 3 is the most appropriate watershed length.

Fig. 5. Diagram Defining Basin Length. (Source: Zavoianu, 2011) 1.2 Watershed Perimeter (Pp):

Watershed perimeter is the outer boundary of the watershed that enclosed its area. It is measured along the divides between watersheds. It is the indicator of watershed size and shape.

1.3 Stream Order:

There are two systems of stream order. First Strahler stream order (Fig. 2.) and second Shreve stream order (Fig.3.)

Fig. 6. Strahler stream order system.

Fig. 7. Shreve stream order system.

The Strahler (1952) system of stream order is more popular due to its simplicity. In this system smallest, un branched fingertip streams are designated as first order stream. Two first order streams join and form larger second order stream. Two second order streams join and form further larger third order stream and so on. Small streams entering a higher order stream do not change its order number. 1.4 Stream Number Nu:

The number of segments of a particular order is counted and expressed as number of that particular order. If we consider the Fig. 6. then the number of first order stream N1is 6, the number of second order stream N2 is 2 and the number of third order stream N3 is 1 only. It is related to bifurcation ratio.

1.5 Bifurcation Ratio (Rb):

The bifurcation ratio expresses the branching pattern of the stream network and is a measure of drainage density. The numerical value is in general more than 1.

It is defined as the ratio of number of streams of a particular order (Nu) to number of streams of next higher order (Nu+i).

Rb = Nu / Nu+1

1.6 Stream Length Lu:

Stream length is an important morphological property and is used to determine drainage density. It is the length of stream / channel of each order. The total length of any particular stream order is the length of that stream order. The mean length of stream of each higher order increases in geometric sequence. The mean length of first order stream is shorter than second order stream. Similarly, the mean length of second order stream is shorter than third order stream and so on.

1.7 Stream Length Ratio (RL)

It is defined as the ratio of mean stream length (Lu) of a particular stream order to mean stream length of the next lower order (Lu-1).

Thus RL = Lu / Lu-1 2. Areal aspects: Areal aspects are two dimensional properties of watershed.

2.1 Watershed Area (Aw):

Drainage area, basin area and catchment area are the synonyms of watershed area. It is the area surrounded by the ridge line / divide of that watershed. It can be expressed in m2, hectares or Km2. It is an important morphological feature as the amount of runoff is influenced by it. Watershed area is having two components stream area and inter basin area (Fig. 8).

Fig. 8. Stream areas & inter basin areas

The stream area discharges its runoff to stream order number 1 but inter basin area its runoff directly to the stream order higher than 1.

2.2 Watershed Shape:

Watershed may have several shapes. Broadly we may consider fan shaped (circular) and fern shaped (elliptical) watershed. Shape is closely related to contribution of runoff to outlet. In fan shaped watershed the runoff from various parts accumulate to outlet at almost same time thus magnitude of peak runoff is high. In fern shaped watershed the runoff from various parts reach gradually to outlet thus magnitude of runoff is lower as compare to fan shaped watershed. The watershed shape is reflected by number of parameters like form factor, shape factor, circulatory ratio, elongation ratio and compactness coefficient.

2.3 Form Factor (Rf):

It is defined as the ratio between watershed area (Aw)and the square of watershed length (Lw). It is a dimensionless number and will always be less than 1. For a perfectly fan shaped (circular) watershed the numerical value of form factor will be 0.786. Fern shaped watershed has value smaller than 0.786. Smaller numerical value of form factor indicates the more elongation of watershed.

2 Form Factor Rf= Aw / (Lw)

2.4 Shape Factor

It is defined as the ratio between watershed area (Aw) and the square of main flow path. It is a dimensionless number and will always be less than 1.

2.5 Elongation Ratio (Re): Elongation ratio is defined as the ratio of diameter of a circle of the same area as the watershed to the maximum watershed length. The numerical value varies from 0 (in highly elongated shape) to 1 (in circular shape). These values can be grouped as,

Elongation ratio Shape of watershed < 0.7 Elongated 0.8-0.7 Less elongated 0.9-0.8 Oval > 0.9 Circular

2.6 Circularity Ratio (Rc) Circulatory ratio is defined as the ratio of watershed area to the area of the circle having the same perimeter as the watershed perimeter. The numeric value mayvary in between 0 (in line)and1 (in a circle). In general most of the watersheds have values from 0.2 to 0.8.

2.7 Compactness Coefficient Compactness coefficient is defined as the ratio of the watershed perimeter to the circumference of equivalent circular area.

2.8 Stream Frequency (Fs): Stream frequency is defined as the number of stream segments per unit area

2.9 Drainage Density (Dd): Drainage density is defined as total length of all streams per unit area of watershed.

3. Relief aspects: Relief aspects are three dimensional properties of watershed.

3.1 Watershed Slope (Sw): It is very important property as it affects the velocity, momentum of runoff and potential of watershed. It also affects the ground water recharge. It is the rate of elevation difference along the principal flow path. It is calculated as the elevation difference between the two end points of the main flow path divided by its length.

Sw = hf / Lf Where Sw is the slope of watershed in m/m, hf is the elevation difference between upper and lower points of main flow path in m, and Lf is the length of main flow path in m.

3.2 Watershed Relief (H): Watershed relief is the elevation difference between highest and lowest points of floor.

3.3 Relief Ratio (Rh): Relief ratio is defined as the ratio between watershed relief and the longest dimension of the watershed parallel to the main flow path. High numeric value of relief ratio indicates the steep slope and vice-versa.

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