CATCHMENT CHARACTERISTICS
4.2 CATCHMENT CHARACTERISTICS
The entire area of a river basin whose surface runoff (due to a storm) drains into the river in the basin is considered as a hydrologic unit and is called drainage basin, watershed or catchment area of the river flowing (Fig. 4.3). The boundary line, along a topographic ridge, separating
Water shed, or Water shed, or drainage basin, or drainage basin, or
A=L b ´ W
boundary 2 =r p or divide
Basin
catchment area b catchment area
Axial length Axial length
width width
Discharge site
Q (cumec) Remote
Main stream
Axial Axial
fringe of catchment
Equivalent circular area 2
A=r p Circumference = 2 p A
Fig. 4.3 Drainage basin characteristics
HYDROLOGY
two adjacent drainage basins is called drainage divide. The single point or location at which all surface drainage from a basin comes together or concentrates as outflow from the basin in the stream channel is called concentration point or measuring point, since the stream outflow is usually measured at this point. The time required for the rain falling at the most distant point in a drainage area (i.e., on the fringe of the catchment) to reach the concentration point is called the concentration time. This is a very significant variable since only such storms of duration greater than the time of concentration will be able to produce runoff from the entire catchment area and cause high intensity floods.
The characteristics of the drainage net may be physically described by: (i) the number of streams
(ii) the length of streams
(iii) stream density
(iv) drainage density
The stream density of a drainage basin is expressed as the number of streams per square kilometre.
stream density, D
where
N s = number of streams
A = area of the basin
Drainage density is expressed as the total length of all stream channels (perennial and intermittent) per unit area of the basin and serves as an index of the areal channel develop- ment of the basin
Drainage density, D d =
where L s = total length of all stream channels in the basin. Drainage density varies inversely as the length of overland flow and indicates the drain-
age effeciency of the basin. A high value indicates a well-developed network and torrential runoff causing intense floods while a low value indicates moderate runoff and high permeabil- ity of the terrain.
total fall of the longest water course Average stream slope = length of the longest water course
Horton has suggested a method of determining the slope of large drainage areas, i.e., the area is subdivided into a number of square grids of equal size. The number of contours crossed by each subdividing line is counted and the lengths of the grid lines are scaled. Then the slope of the basin is given by
15 .( CI N ) c
where S = slope of the basin CI = contour interval N c = number of contours crossed by all the subdividing lines Σ L = total length of the subdividing lines The boundary line along a topographic ridge, separating two adjacent drainage basins
is called the drainage divide. The line of the ground water table from which the water table
RUNOFF
slopes downward away from the line on both sides, is called the ground water divide. The shape of a drainage basin can generally be expressed by:
(i) form factor (ii) compactness coefficient
Form factor,
A=W b .L b
where W b = axial width of basin L b = axial length of basin, i.e., the distance from the measuring point (MP) to the most
remote point on the basin.
Compactness coefficient, C c =
2π A
where P b = perimeter of the basin
2 πA = circumference of circular area, which equals the area of the basin. If R is the radius of an equivalent circular area,
A = πR 2 ,
Circumference of the equivalent circular area = 2πR = 2π =2 πA
The compactness coefficient is independent of the the size of the catchment and is de- pendent only on the slope.
A fan-shaped catchment produces greater flood intensity since all the tributaries are nearly of the same length and hence the time of concentration is nearly the same and is less, whereas in the fern-shaped catchments, the time of concentration is more and the discharge is distributed over a long period (Fig. 4.4).
Remote fringe of catchment
ies utar Tr ib
stream Main
Less concentration time causes intense floods
More concentration
time Q
(a) Fan shaped (b) Fern (leaf) shaped
Fig. 4.4 Fan-and fern-shaped catchments
HYDROLOGY
Schumm S.A. (1956) used an ‘elongation ratio (E r )’, defined as the ratio of the diameter of a circle of the same area as the basin to the maximum basin length; the values range from
0.4 to 1.0. Miller V.C. (1953) used a dimensionless ‘circularity ratio (C r )’, defined as the ratio of the
basin area to the area of a circle having the same perimeter as the basin; the values range from
0.2 to 0.8. The drainage basin characteristics influence the time lag of the unit hydrograph and
peak flow (Taylor and Scwartz, 1952). Example 4.1 The contour map of a basin is subdivided into a number of square grids of equal
size by drawing horizontal and vertical lines as shown in Fig. P4.1. The contour interval is
25 m.
Vert. grid
Horz. grid lines
Contour intersections
Basin boundary Contour lines
Fig. P4.1 Horton’s grid for basin slope
The number of contour intersections by vertical lines is 75 and by horizontal lines 126. The total length of the vertical grid segments (after multiplying by the scale) is 53260 m and of the horizontal grid segments 55250 m. Determine the mean slope of the basin .
Solution Slope in the vertical direction
N c ×.. CI 75 × 25
= Σ = 0.0352 m/m
Slope in the horizontal direction
N c ×.. CI 126 × 25
= Σ = 0.0570 m/m
X 55250
∴ Mean slope of the basin
= 0.0461 m/m or 4.61%
Also, from the Hortons equation,
15 .(.) CIN c 1.5 × 25 (75 + 126)
= Σ = 0.0695 or 6.95%
A basin has an area of 26560 km 2 , perimeter 965 km and length of the thalweg 230 km. Determine: (i) form factor, (ii) compactness coefficient, (iii) elongation ratio, and (iv) circularity ratio.
Example 4.2
Solution (i) Form factor, F f = 2 =
b 230
An inverted factor will give 2 (ii) Compactness Coefficient C c
Radius R of an equivalent circular area is given by
26560 = πR 2 ∴
R = 91.9 km
(iii) Elongation ratio E r =
b 230
(iv) Circularity ratio C r Radius R′ of a circle of an equivalent perimeter as the basin is given by
2πR′ = 965
R ′ = 153.5 km