Chapter 5

CHAPTER FIVE
Soil Erosion Control Measures
Mengistu Zantet (MSc.)
Lecturer @ Hydraulic and Water Resources Engineering department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
P.O.Box: 260
Tepi, Ethiopia
7/15/2021 1
Soil and Water Conservation
Engineering
mengistu.zantet@gmail.com .
lecturer@ Hydraulic and water
resources Engineering Department

Outline
5.1.BiologicalMeasures/Agrono
mic practices
5.1.1 . Mulching
5.1.2. Crop management
practices
5.1.3. Soil Management
practices
5.2. Mechanical/Engineering
Measures
5.2.1.Terraces
5.2.2. Bunds
5.2.3. Vegetated waterways
5.2.4. Gully control measures
7/15/2021
Engineering
resources Engineering Department 2

5.1 Biological Measures/Agronomic practices
Conservation measures the preference is always
given to this methods due to:
 It is less expensive
 Reduce the rain drop impact, increase infiltration
rate, reduce runoff volume and decrease the
velocity of runoff and wind.
 It is easier to fit them into existing farming
7/15/2021
Engineering

Cont.…
 Agronomical measures are referred to the
practices of growing vegetation on mild
sloppy lands to cover them and to control soil
and water losses.
 The role of agronomical measures in
achieving soil and water conservation has
immense importance, much more than
engineering measures
7/15/2021
Engineering

The practices included under agronomic
measures are
Contour cultivation
Strip cropping
Tillage practices
Mixed cropping/inter planting
7/15/2021
Engineering

Contour Cultivation:
 is tillage and planting of crops across the land
slope along the contour lines rather than up and
down hill or parallel to field boundaries.
 Contour cultivation in humid and moist sub
humid regions is mainly to reduce soil erosion.
 It is used in semiarid and drier portions of sub
humid regions primarily to increase soil moisture
by reducing runoff losses
7/15/2021
Engineering

Contour Cultivation:
7/15/2021 7

Strip Cropping
it is the practice of growing alternate strips of
different crops in the field across the land slope.
The strips are so arranged that the strip crops
should always be separated by strips of close-
growing and erosion resistance crops.
Strip cropping is more intensive practice for
conserving the rainwater than contouring, but it
does not involve greater effect on soil erosion
7/15/2021
Engineering

Tillage practices
7/15/2021 10

Mixed cropping/inter planting
7/15/2021 11

The four types of strip cropping are:
Contour strip cropping
Field strip cropping
Buffer strip cropping
Wind strip cropping
7/15/2021
Engineering

5.2. Mechanical/Engineering Measures
 Mechanical or engineering measures for
protection of soil and water loss are all the
methods which involve earth moving
 They are constructed by manipulating the
surface topography.
 The agronomic measures combined with good
soil management practices provide better
influence on the detachment and transportation
of soil particles in the process of soil erosion
7/15/2021
Engineering

The mechanical measures are not much preferred
than agronomic measures because
 Many mechanical works are costly to install and require
regular maintenance.
 Structures like, terrace and bunds create problems for
agricultural operation.
 At shallow soil depth, the terrace construction exposes
the bed rock or less fertile sub-soil and therefore results
in low crop yield.
 There is a risk by severe storm with return periods of 20
7/15/2021
Engineering

The principles of water erosion control measures
are the same wherever serious water erosion
occurs.
These principles are:
Reduce rain drop impact on the soil
Reduce runoff volume and velocity
Increase the soil’s resistance to erosion
7/15/2021
Engineering

Prevention and Control Measures for Water Erosion
1) Sheet and Splash Erosion
 prevented by maintaining plant cover (preventing splash
erosion) and maximizing infiltration of ponded water
through the maintenance of soil structure and organic
matter
2) Rill Erosion :
Reducing Flow Velocity (settle suspended particles):
Flow velocity can be reduced by either reducing the flow
volume or roughening the soil surface.
7/15/2021
Engineering

3) Tunnel Erosion
 Tunnel erosion is particularly difficult and
expensive to control and not always
successful.
 Combinations of mechanical, chemical and
vegetative measures are usually required to
control or prevent tunnel erosion
7/15/2021
Engineering

 Permanent structures are built for long-term
erosion control and are established for a long-
term use.
 permanent measures include terraces, drop
structures, spillways, culverts, gabions,
ripraps, and ditches
 temporary measures include contour bunds,
sand bags, silt fences, surface mats, and log
7/15/2021
Engineering

•The choice of mechanical
measures depends on the severity
of erosion, soil type, topography,
and climate
7/15/2021
Engineering

5.2.1. Terraces
 Terracing is an engineering soil and water
conservation practice used to control soil and
water loss in sloping areas.
 Terracing involves construction of
embankments, ridges, or channels or land
leveling in steps across the land slope.
 In terrace systems the effective length of land
slope and slope steepness are reduced to large
7/15/2021
Engineering

Types of Terraces
1) Graded/diversion terraces: These terraces are constructed to intercept
the overland flow and channel it across the slope to a suitable outlet, i.e.
grassed water way etc. built at a slight down slope grade from contour.
2) Retention terraces: These terraces are used where conservation of
surface water by storing it on hill side is required. They are also termed
as level terraces. The permeable soil with the land slope less than 4.5%
are suitable for retention type terraces. In areas of low rainfall they are
constructed for the purpose of water conservation. They may also be
constructed where rainfall is high and the infiltration rate is also high
allowing all rainfall to enter to the soil.
3) Bench terraces: They are platform like constructions along the contours
of the sloping land. They are generally constructed on lands 6 to 33%
7/15/2021
Engineering

Graded/diversion terraces
7/15/2021 23

Retention terraces
7/15/2021 24

Terrace Design
The determination of a terrace components depend
up on:
The soil characteristic of the area
Topography of the area
Climate of the area
Type of terrace
Agricultural practices (tillage practices)
7/15/2021
Engineering

Designing a terrace involves
Proper selection of the terrace type
Determination of proper spacing (considering
the farmable cross-section)
Determination of terrace cross-section
Terrace spacing: The spacing of terraces is
expressed as the vertical distance between the
channels of successive terraces
7/15/2021
Engineering

The factors which affect terrace spacing
are
 climate,
 soil,
 topography,
 type of terrace and
 the tillage practice.
•But the important ones are climate, topography
and the soil type
7/15/2021
Engineering

estimate the terrace spacing.
U.S Soil Conservation Service devised a formula for
estimating the best vertical interval
• VI = XS +Y…………………………………………………5.1
• Where X = rainfall factor, dimensionless
• S = average land slope, %
• Y = Soil infiltration and vegetation cover factor, dimensionless
• VI = vertical interval, m
• Y = 0.3, 0.6, 0.9, or 1.2 with the low value for highly erodible soils
with no surface residues and the high value for erosion resistant soils with
conservation tillage and good crop cover.
• X = ranges from 0.12 to 0.24 with low value for highly erosive rainfall
and high value for less erosive rainfall.
• Horizontal interval ……………………………5.2
• Where S = average land slope, %
• The spacing computed by the above formula can be modified as much as
25% to allow for soil, climate and tillage conditions.
7/15/2021
Engineering

Terrace Length
 Size and shape of the field, outlet
possibilities, rate of runoff and channel size
are factors that influence the terrace length.
 The length of a terrace should be decided so
as to avoid the erosive velocity and large
cross-section of the channel
7/15/2021
Engineering

Critical Slope Length
 The slope length of a field at which the
overland flow becomes erosive is called
critical slope length.
 Provided the effective slope length below the
critical slope length, serious erosion will not
take place
7/15/2021
Engineering

7/15/2021
Engineering
Consider overland flow rate from the field per unit width to be Q

cos
)
( L
i
R
Q 

And again from manning equation,
2
1
3
2
1
S
R
n
V 
width
unit
for
perimeter
wetted
P
depth
flow
r
where
r
A
But
S
R
n
A
AV
Q
,
1
,
1 2
1
3
2






2
1
3
5
2
1
3
2
1
1
s
r
n
s
r
r
n
Q 


Then equating the two equations and solving for L we get:
 
  

cos
tan 2
1
3
5
i
R
n
r
L

 ………………………………………………………………6.3

The computation of terrace spacing can be
accomplished using the following steps.
1) Determine the maximum depth of the productive top soil
2) Find out maximum admissible depth of cut for the land slope
of the field and crop to be
3) grown based on the existence of maximum depth of
productive soil range
4) After determining the depth of cut, find out the width of
terrace using the equation
7/15/2021
Engineering
100
SW
VI  (For level bench terrace)
 
 
bench
the
of
slope
outward
or
inward
s
where
inward
sloping
for
s
S
W
VI
outward
sloping
for
s
S
W
VI

















100
100
Inward or outward slope of the bench: They depend on the soil type and average rainfall of the
area.

The size of the shoulder bund:
 It depends upon the type of the bench terrace.
 For the terrace sloping inward the size of the shoulder bund
is kept nominal (minimum possible) while for sloping
outward and level top terraces the shoulder bund comprises
larger section for holding runoff.
 The terrace section and soil characteristics affect the size
(angle of repose of soil and permeability). On most soils for
outward sloping terraces,
Top width of shoulder bund = 30 cm
Height of shoulder bund = 45 cm
7/15/2021
Engineering

5.2.2. Bunds
 It is a type of engineering soil and water
conservation measure which helps to control
soil erosion and retain rainfall water from
runoff.
 They are simple soil embankment structures
constructed across land slope by less soil
movement than for bench terracing
7/15/2021
Engineering

Cont..
 The practice is, generally, suitable for lands
having 2-10% slope ranges.
 However, it can also be practiced on areas
that have slope greater than 10% but with
closer spacing, which may require high cost of
construction and on area that have lesser
slope than 2% with wider spacing.
7/15/2021
Engineering

Classification of bunding systems
1) contour bunding
2) graded bunding
1) Contour bunding: it is construction of bunds that
pass through equal elevation. The method can be
adapted on all types of soils but not for deep black
clay soils. The practice is suitable for areas, which
receive annual rainfall less than 600mm.
7/15/2021
Engineering

Cont..
It is not technically feasible on land slopes
greater than 6%
Contour bunding system is sub divided in to
following sub-groups
a) Narrow based contour bunds, creates
obstruction for crossing farm implements
b) Broad based contour bunds
7/15/2021
Engineering

2) Graded bunding
 In this bunding system, some grade is provided to the channel
behind the bund (0.2 to 0.3%). Graded bunding is used in areas
that have average annual rainfall greater than 700mm. However, it
can also be used on areas of lesser average annual rainfall if the soil
is of heavy texture (clayey).
 The functions of graded bunds are:
 to reduce soil erosion
 To dispose surplus rain water safely to a suitable outlet, the system
may require grassed waterway.
 Graded bunding is not recommended on land slopes less than 2%
or greater than 8%
7/15/2021
Engineering

Types of graded bunds
a) Narrow based graded bunds
• - provides obstruction to crossing farm implements
b) Broad based graded bunds
• - Does not provide obstruction to crossing of farm
implements; the entire area can be put under
cultivation. Therefore, the original cross -section of
the bunds does not remain unchanged, resulting in
the requirement of frequent maintenance.
7/15/2021
Engineering

Some of considerations for bund
construction:
1) Economy:
2)Rainfall characteristics and soil
type:
3) Land submergence:
4)Seepage rate:
7/15/2021
Engineering

Design specifications for contour bunds
1) Spacing:
• In contour bunding, the bund needs to check the surface runoff at the
point where its flow attains erosive velocity, and should meet the
requirements of agricultural operation.
• Vertical interval between two bunds (VI): Different empirical formulae
have been suggested to estimate the vertical interval between two bunds.
a) For areas of heavier rainfall
Where VI = vertical interval in (cm) ; S = land slope in percent
b) For areas of low rain fall.
7/15/2021
Engineering

Cont.…
 But the above equations were developed by considering only land
slope & rainfall amount; other factors such as infiltration rate, surface
cover, etc. were not considered. Incorporating the effects of the
remaining factors, COX developed more reasonable relation which is
expressed as:
 VI = 0.3 (XS+Y)
 Where VI = Vertical interval in (m),
 S = Land slope in percent
 X = rainfall factor,
 Y = infiltration rate and crop cover factor
7/15/2021
Engineering

Con…
Table: 5.3. Values of rainfall factor (X)
Rain fall distribution Average Annual Rainfall (mm) X
Scanty
Moderate
Heavy
<640
640-900
>900
0.8
0.6
0.4
Table: 5.4. Values of infiltration rate & crop cover factor (Y)
Intake rate crop cover during erosive period of rain Y
Low ( below average) Poor 1
Medium to high Good 2
Low Good 1.5
Medium to high Poor 1.5
7/15/2021
Engineering
Horizontal interval (HI): 100
S
VI
HI 

2) Size of the bund
• It is determined by the height, top width, bottom
width, and side slope of the bund
a) Height of the bund
• It is determined on the basis of the amount of
water to be intercepted. The determination
procedure can be illustrated as follows:
7/15/2021
Engineering

Cont..
7/15/2021
Engineering
Consider two triangles ABC and CDE in the above diagram to be similar. Then
VI
h
HI
W

The amount of water checked behind the bund per unit width (V) can be expressed as:
wh
V
2
1
 h
VI
h
HI
V .
2
1

 .
VI
h
Hl
V
2
.
2
1


Let the depth of excess rainfall that has to be retained by the bund to be ‘b’ units deep, and then
the volume of runoff water that has to be checked by the bund, V, is
V = b. HI
If we equate the two equations for V, we get
HI
b
VI
h
HI
.
.
2
1 2
 VI
b
h .
2
2


VI
b
h .
2
 Or
S
HI
b
h .
.
2

To get the practical value of the bund height, we need to add for freeboard 25% of theoretical
height.

b) Side slope
The side slopes of the bund are dependent on the angle of repose of the fill material.
Recommended values of side slopes of bund section for different types of soil are given in the
tablebelow.
Table5.5Recommendedvaluesofsideslopesofbund
Soiltype Sideslope(H:V)
Loam 1.5:1
Clay 2:1
Sand 2.5:1
For broad based contour bunds the side slope may be recommended from 4:1 to 5:1. Its height
mayrangefrom30–50cm.
Topandbottomwidthsaredecidedonthebasisofthepermeabilityofthesoilfill
7/15/2021
Engineering

3) Earth work:
 The earth work of the bunding system includes
the sum of earth works of the main bund, side
bunds and lateral bunds formed in the field.
 For calculation, the sum of earth works of side
and lateral bunds is considered to be 30% of the
earth work of the main bund.
 The earth work of any bund is determined by
multiplying cross-sectional area and the total
7/15/2021
Engineering

4) Area lost due to contour bunding
 It is calculated by multiplying the length of the contour
bund by its base width.
Cross-Section of Graded Bunds
 The cross-section of a graded bund should have sufficient
carrying capacity and the velocity of flow in the channel
must not cause scouring of the channel bed. On the basis
of this point the cross-section of graded bund can be
determined by using manning equation.
•
7/15/2021
Engineering

5.2.3. Vegetated/grassed waterways
 Runoff that may be concentrated by the natural topography
or by graded bunds, terraces, or other human works must
flow in a controlled manner that will not result in gully
formation.
 This can be achieved by practicing either of the following
activities.
a) Reducing the peak flow rates of runoff by full utilization of
field protection practices earlier, or
b) Providing a stable channel that can handle the peak flow
7/15/2021
Engineering

Vegetated/grassed waterways
7/15/2021 54

Design of Vegetated Waterways:
The design parameter estimation techniques are
described below.
1)Waterway capacity
2) Velocity of runoff flow
3)Shape of Vegetated Waterway Cross-section
4) Vegetation type
5)The gradient of waterway
6) Roughness coefficient
7/15/2021
Engineering

The general procedure that has to be followed in
designing a vegetated waterway is the following:
Step 1: Determine the peak runoff rate from the area Q
CIA
Step 2: Find out the value of permissible flow velocity for
vegetated waterway based on soil type and type of
vegetation
Step 3: compute the cross-sectional area of the waterway
handle the peak runoff rate and permissible flow velocity
Q = VA; A = Q/V Keep in view that the runoff rate
7/15/2021
Engineering

Cont..
Step 4: After determining the cross-sectional area of the waterway, compute its
various dimensions (depth, bottom width, side slope) to suit the area of cross-section
section obtained.
Step 5: Calculate the hydraulic radius and decide the value of manning roughness
coefficient R = A/P
Step 6: compute the grade (s) of the waterway using manning formula
Step 7: Check the elevation of the outlet computed using the grade and the elevation
of the field outlet. They should coincide. The grade computed can be rounded or
or otherwise.
Step 8: Using the rounded value of the waterway grade compute the flow velocity at
a section. It has to be lower or equal to the permissible velocity. Otherwise,
7/15/2021
Engineering

5.2.4. Gully control measures
The causes that activate gully formation are:
 Making the land surface to be without vegetation by over grazing
and other biotic pressure, clearing for cultivation, firing,
deforestation
 Improper construction of water channels, roads, rail lines, cattle
trails, etc.
 adoption of faulty tillage practice
 Not smoothening of rills, small channels or depressions present on
the ground surface.
7/15/2021
Engineering

Gully Development
Gullies are developed by the following processes,
which may be activated either singly or in
combination
Scouring of the bottom
Gully head erosion
Sliding or mass movement of soils from the gully
banks, due to seepage, freezing & thawing
7/15/2021
Engineering

Stages of gully development
Stage 1: initiation stage
Channel erosion & deepening of the gully takes place.
Stage 2: development stage:
Width & depth are enlarged due to runoff from up stream.
The gully depth reaches up to c – horizon
Stage 3: healing stage
vegetation's start to grow in the channel
There will not be appreciable erosion.
Stage 4: final stage (stabilization)
The gully has been fully stabilized. There will be no further development of gully unless the
healing process is disturbed.
7/15/2021
Engineering

Classification of Gully:
Classification on the basis of shape
a) U -Shape gullies
b) V – Shape gullies
Classification based on the state of the gully
a) Small gullies
b) Medium gullies
c) Large gullies
7/15/2021
Engineering

Gully Control:
Principles of gully control
1) Determine the cause of gully and take counter measure as early as possible
2) Restore the original hydraulic balance or create new condition.
That is, either the flood has to be reduced to its original volume or a new
channel has to be provided to accommodate the increased flood. Therefore, for
controlling gully erosion, the following activities are very important:
A) Improving the drainage area of the gully.
B) Stabilization of gully
I) Stabilization of the gully head
II) Stabilization of the gully side and bed
7/15/2021
Engineering

Structures for gully control:
Temporary structures
1) Wire bolsters
2) Netting dams
3) Brushwood dams
4) Log dams
5) Brick weirs
• Permanent Structures:
1) Silt trap dams
2) Regulating dams
3) Gully-head dams
4) Gabions
5) e) Drops & chutes
7/15/2021
Engineering

Design of Permanent Structures
• The design of permanent gully control structures is
completed under the following three design steps:
1) Hydrologic design
Estimation of design runoff rate and flood volume
2) Hydraulic design and
determining the dimension of different
components of the structure, on the basis of
expected maximum runoff rate,
3) Structural design:
determination of strength and stability of
7/15/2021
Engineering

7/15/2021
Engineering

the end
7/15/2021
Engineering

Chapter 5

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie Chapter 5

Ähnlich wie Chapter 5 (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

Chapter 5