AEG-101-Class-11 and 12 : DESIGNING SOIL CONSERVATION STRUCTURES – CONTOUR BUND AND GRADED BUND

Introductory Soil and Water Conservation Engineering

II Semester 3^rd Feb to 30^th June 2020, 2019-20

Teacher Information

Professor	Email	Phone
Dr. K. C. Shashidhar	shashidhar.kumbar@gmail.com	9448103268

Class-11 and 12 – Reference Material

DESIGNING SOIL CONSERVATION STRUCTURES – CONTOUR BUND AND GRADED BUND

Soil conservation is the preservation of soil from deterioration and loss by using it within its capabilities and applying the conservation practices needed for its protection and improvement.

Contour bund (Narrow based terracing): Bunds constructed along contours or with permissible deviations from contours are called contour bund.

Function

The long slope is intercepted into a series of short ones Each contour bund act as a barrier to flow of water and impounds a greater part of it against it for increasing soil moisture.

Soils suitable

All types of relatively permeable soils (alluvial, red, laterite, brown soil, shallow and medium black soils) except the clayey or deep black soil.

Where we can adopt

For rolling (Slopes less than about 6%) and flatter lands

Scanty or erratic rainfall conditions

Design procedure: For planning contour bunding for any area, the planners need information on:

• How far these bunds should be installed?

• What should be the deviation freedom to go higher and lower than the contour bund elevation for getting better alignment in undulating part of the land?

• What should be the cross section of the bund?

i) Spacing of Contour Bund

Find out the vertical interval by using Cox’s formula. Cox’s formula is given by:

VI = (X S + Y) 0.3

Where, X – rainfall factor

Y – Infiltration and crop cover factor

S – Slope percentage

VI- Vertical interval (m).

Values of X, the rainfall factor

Rainfall	Value of X	Actual rainfall (cm)
Scanty	0.8	64
Moderate	0.6	64-90
Heavy	0.4	More than 90

Values of Y, the infiltration and crop cover factor

Intake	Crop cover during erosive period of rains	Value of Y
Below average	Low coverage	1.0
Average or above	Good coverage	2.0
One of the above factors favorable and the other unfavorable		1.5

ii) Horizontal interval

The horizontal distance between two corresponding adjacent bunds is called Horizontal Interval (HI). This can be worked out by using the following formula:

HI = (VI X 100) / S

Where, HI = Horizontal Interval (m)

VI = Vertical Interval (m)

S = Slope (percentage)

iii) Limits of deviation

Vertical spacing can be increased by 10% or by 15 cm to provide better location, alignment or to avoid any obstacle. In case a small strip is left in between the last bund and field boundary, the vertical fall may be distributed in all the bunds, rather than in only last bund.

iv) Cross sectional area (CSA)

To work out CSA first ‘d’ should be determined.

a) Calculate the runoff in cm using rainfall and infiltration.

Runoff (cm) = Rainfall (cm) – Infiltration (cm)

b) Storage area required: A1 = (Runoff,cm) (HI) / 100

c) Calculate the storage capacity of the bund

In the above fig. Assume the depth as ‘d’ and by using side slope (Z1 given in the table) calculate the storage area of triangle a1 in terms of d. The side slope of the bund depends upon the angle of the repose of the soil. To calculate the area of triangle a2, calculate the water storage distances by using water storage distance = (d x 100)/S equation where S is percentage slope, then calculate the area in terms of d.

Add a1 and a2 to obtain storage capacity of bund (A2).

The side slope of the bund for different type of soil

Type of soil Side slope Horizontal : Vertical (Z₁: 1)

Clayey soil 1:1

Loamy soil 1.5:1

Sandy soil 2:1

c) Add 20% of d as free board to d

H = ( d + 20d ) /100

After calculating d and H using side slope (Z1) and seepage line slope (Z2) calculate the base width and top width.

Seepage line slope

Type of soil	Slope of seepage line Horizontal to Vertical Z2: 1
Clayey soil	3:1
Sandy loam soil	5:1
Sandy soil	6:1

Based on above seepage line and side slope, the base width of bund of a particular soil is:

Base width (B.W) =Z₁ d + Z₂d

Top width of bund (TW) is calculated from base width and height of the bund (H) and side slope:

T.W = B.W – (Z₁H + Z₁H)

The cross section of the bund (CSA) CSA = (( T.W + B.W ) / 2) x H

Total earth work is calculated by multiplying CSA and total length of the bound.

Cost estimation of bund= Total volume of earth work X rate of earth work

Classroom Video

Video by : DR. Skashidhar K. C.

Design of Contour Bund

Side Slope

Problem:

Calculate the storage area requied and height of bund in medium soils having an average slope of 2%. The maximum expected rainfall during the 10 year recurrence interval is 15cm. The infiltration capacity of soil of the area is such that 40%of the rainfall is absorbed in the field. The horizontal interval between the bund is 60m. (For this situation take seepage line slope as 1:4 and side slope as 1:1.5)

Solution

Maximum runoff to be stored against the bund = Rainfall-Infiltration = 15 - ( 15 x 40 ) / 100 = 9cm

Storage area required A= (Runoff in cm x Horizontal interval between bunds ) / 100 = ( 9 x 60 ) / 100 = 5.4 Sq.m

For easy working draw

a₁ + a₂ = A = 5.4 Sq.m.

a₁ = ¹/₂  x 1.5d x d = 0.75d²

a_{2 =} ¹/₂ x 50d x d = 25d²

A = a₁+a_{2 =} 0.75d² + 25d² = 25.75d²

A = 5.4 Sq.m.

25.75d² = 5.4 Sq.m.

d = 0.457m  = 45 cm

Free board 20% of d i.e.9cm

H = 45 + 9 = 54cm

Bottom width = dxZ1+dxZ2 = 0.45x1.5+0.45x4 = 2.45m

Top width = BW - ( H x 1.5 + H x 1.5 ) = 2.45 - ( 0.54 x 1.5 + 0.54 x 1.5 ) = 0.83m = 83cm

Classroom Videos on Problems:

Contour Bunds on Problems:

Surplusage Arrangement

In order to protect the counter bund from breaching and standing crop damage, masonry outlet structure which can drain away excess water are constructed.

Graded bund: Graded bunds are laid along a predetermined longitudinal grade instead of along contour. These terraces act primarily as drainage channels for inducing and regulating the excess runoff water and draining the same with a mild and non-erosive velocity.

Design procedure:

Design of graded bund involves:

1.Selection of the vertical interval

2.Provision of suitable grade

3.Cross-section for the bund and channel

i)Vertical Interval: The vertical interval and horizontal spacing at which the graded bunds are to be formed, will be the same as that of contour bunds.

ii)Grade: Graded bunds may have either uniform or variable grade. In the uniform graded bund, the slope remains constant throughout its entire length. Uniform graded bunds are most suitable where drainage is a problem and where the terraces are short. For the uniform graded bunds, a grade from 0.1% to maximum of 0.4% is usually satisfactory, depending upon the soil conditions and the length of the terrace. Generally, the steeper grades are selected for impervious soils and long terraces.

The use of a variable grade often permits better alignments of the terrace, making it easier to fit the terrace to the field. The grade usually varies from a minimum at the upper portion to a maximum at the outlet end. For variable graded bunds the grade in the channel may vary from 0 to 0.5% depending on the length.

Channel grades for a variable graded bund

Length of terrace(m)	Grade in per cent
	Lower 1/4	Second 1/4	Third 1/4	Upper 1/4
30-120	0.3	0.3	0.2	0.2
150-240	0.4	0.3	0.2	0.2
270-360	0.5	0.4	0.3	0.2
More than 400	0.5	0.4	0.3	0.2

i)Cross – section:

A) Estimate the peak rate of runoff to be carried by the bund channel by the Rational method (Q = 0.0278 CIA)

Rational Method

Q = 0.0278 CIA

where,

Q .. Peak runoff in cubic m/sec.

C .. Runoff co-efficient

I .. Maximum average rate of rainfall, centimeters per hour over the entire area which may occur during the time of concentration.

A .. Watershed/catchment area, hectare

Runoff co-efficient:It should be selected from the table given below, using soil type, slope and vegetative cover information.

Values of C in the rational formula

Vegetative cover & slope	Soil texture
	Sandy loam	Clay and silt loam	Silty clay
I. Wood land
0-5% slope	0.10	0.30	0.40
5-10% slope	0.25	0.35	0.60
10-30% slope	0.30	0.50	0.60
II. Pasture land
0-5% slope	0.10	0.30	0.40
5-10% slope	0.16	0.36	0.55
10-30% slope	0.22	0.42	0.60
III. Cultivated land
0-5% slope	0.30	0.50	0.60
5-10% slope	0.40	0.60	0.70
10-30% slope	0.52	0.72	0.82

‘I’ Calculation

Using time of concentration and one hour rainfall intensities ‘I’ should be determined with the help of a Figure-1. Figure gives a graph for converting the one-hour rainfall intensities to intensities at other durations

.

Figure depicting relation of one-hour rainfall intensities to intensities at other durations

T_c Calculation of time of concentration.

To estimate the time of concentration, use the following empirical formula:

T_c = 0.0195 L ^0.77 S ^-0.385

T_c = Time of concentration, minutes

L = The maximum length of flow meters

S = Average slope of the area meter/s

Maximum length of the flow

L = HI + bund length

Average slope of the area

Average slope should be worked out by considering the field general slope and slope given to the bund.

Slope percentage m	Distance (m)	Drop for the distance (m)
n	m	n/100 x m
	∑m	∑(n/100 x m)

S=[∑(n/100 x m)]/ ∑m

One hour rainfall intensities, should be collected from nearest IMD center.

“A” Calculation: 

Consider the area in between two consequent bunds:

Area in hectare = ( HI X length of bund ) / 10000

B) Calculation of cross- sectional area, available for carrying runoff water.

· Select side slope
· Assume depth - d
· Since water stands in a triangular area against the bund, calculate the area of triangle by using assumed depth .

Classroom Videos

Design of Graded Bund

C) Estimation of velocity of runoff water flowing behind the bund.

By using Manning’s formula mean velocity can be estimated:

Mean velocity m/sec= V = (R ^2/3 x S ^1/2)/n

n = Constant = 0.04, R= Hydraulic radius in m

S = Slope(highest slope provided to channel in m/m)

R = A / P

A = Cross sectional area of channel in sq.m.

P = Wetted perimeter m

P = S₁ + S₂

S₁ and S₂ can be determined by using AB² = BC² = AC² relation

D) After calculating velocity, this velocity should be compared with safe velocity for that particular soil. If the calculated velocity is less than the safe velocity then the design is a safe design, otherwise, assume different depth and repeat the process until the calculated velocity is less than safe velocity.

Safe velocity for different types of soils

Type of soil	Velocity
Loamy sand	0.45
Sandy loam	0.60
Loam	0.75
Clay loam	0.90
Clay	1.00

E) Calculate the discharge capacity of the channel:

Q = A x V

Where, Q = Discharge in Cu m/sec, A = Cross sectional area of channel Sq.m.,V = Velocity m/sec (Velocity obtained by using Mannings formula)

F) Compare the discharge capacity of the channel and peak runoff rate.

If the discharge capacity of the channel is more than the peak runoff rate, it means channel has sufficient capacity otherwise assuming different depth repeat the entire process.

G) Calculate the cross section of bund using depths, side slope and seepage line slope as described in the contour bund design.

Classroom Videos

Graded Bunds on Problems: