Chapter 4

CHAPTER FOUR
SOIL EROSION
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
4.1.Principles of soil erosion
4.2. Factors affecting soil erosion.
4.3. Soil erosion problems
4.4. Types of soil erosion
4.4.1.Water Erosion
4.4.1.1. Mechanism of water
erosion
Types/forms of water erosion
4.4.1.3. Water erosion and sedimentation
4.4.1.4. Factors influencing water erosion
4.4.1.5. Estimating soil loss
4.4.2.Wind Erosion
4.4.2.1. Mechanism of wind erosion
4.4.2.2. Factors influencing wind erosion
4.4.2.3. Wind erosion control measures
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resources Engineering Department 2

What is soil ??
Soil is the upper most weathered and
disintegrated layer of the earth’s crust
which is composed of minerals and several
organic substances.
 In general, the depth of soil varies from
place to place.
However, the top 30cm soil depth is very
useful for human being and wild life.
This top layer is continuously exposed to
the actions of soil erosion.
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The two main active forces which
always tend to dislodge the top soil
layer and to transport them from one
place to another for soil erosion are .
1) wind and
2) water
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Cont.…
Soil erosion has three phase phenomena
Detachment,
 transportation and
deposition
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4.2 Factors Affecting Soil Erosion
grouped under the following three heads.
1) Energy,
It includes, the potential ability of rainfall, runoff and wind to
cause erosion
2) Resistance and
It referred to that group of factors, which affect the soil erodibility
e.g. chemical and mechanical properties of soil
3) Protection.
This group of factors focuses on the plant cover.
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4.3. Soil Erosion Problems
Disease and public health hazard
(water and air pollution)
Damages on engineering
structures
Silting of irrigation channels and
reservoirs
Loss of crops
Frequent Floods
Soil loss
Loss of plant nutrients
Change in soil texture
Soil structure
deterioration
Field dissection
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4.4. Types of soil Erosion
In broad sense, the erosion process can be classified
into two types
1) geologic erosion and
2) accelerated erosion
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1). Geologic erosion
 refers to the formation and loss of soil simultaneously which
maintain the balance between formation and various losses
 represents the erosion of soil in its natural condition without the
influence of human being.
 It is sometimes known as natural or normal erosion.
 This erosion is said to be in equilibrium with the soil forming
processes.
 The geologic erosion is long time eroding process. The various
topographical features such as existing of stream channels; valleys
etc. are the results of geologic erosion.
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GOLOGICAL EROSION
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2) Accelerated Erosion.
 Accelerated erosion is an excess of geologic
erosion.
 It is activated by natural and man’s activities
which have brought about changes in natural
cover and soil conditions.
 the accelerated erosion takes place by the action
of water, wind, gravity and glaciers
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ACCLERAED EROSION
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Accelerated erosion can further be sub-
classified
1) Water Erosion, and
2) Wind erosion.
Water erosion is the wearing away of the
soil surface by water from rain, runoff,
snowmelt, and irrigation.
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1) Mechanism of water erosion
Soil Detachment
Hydraulic action
Abrasion
Attrition
Solution
Transportation and
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Types/forms of water erosion
The main types of soil erosion are:
splash,
sheet,
in Terrill,
rill,
gully,
stream bank, and
tunnel erosion
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1) Splash Erosion or raindrop erosion
The basic factors involving the rate of raindrop
erosion are:
Climate, largely rainfall and temperature
Soil, its inherent resistance to dispersion and its
infiltration rate,
Topography i.e. steepness and length of the
slope, and
Plant cover.
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2. Sheet erosion
 is removal of a fairly uniform layer of soil from
the land surface by the action of rainfall and
runoff.
 In this, soil is removed from the surface in a thin
layer or sheet of relatively uniform thickness.
 This type of erosion is extremely harmful to the
land.
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3 IN Terrill Erosion
As soon as rain starts, runoff promptly
develops dominate rills, and that portion of
runoff that flows between rills is called
sheet or in Terrill erosion.
This type of erosion is mostly due to shallow
flow
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4 Rill Erosion
 It refers to the soil erosion that occurs in small
channels or rills.
 Rill erosion occurs due to concentrated rather
than shallow flow.
 Rill erosion is the second most common pathway
of soil erosion.
 Rill erosion is a function of soil erodibility, runoff
transport capacity, and hydraulic shear of water
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5. Gully Erosion
 Gully erosion creates either V- or U-shaped
channels.
 The gullies are linear incision channels of at
least 0.3m width and 0.3m depth.
 Gullies are primarily formed by concentrated
runoff converging in lower points of the field.
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Concentrated runoff forms gullies (Courtesy USDA-NRCS).
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Four stages of gully development are generally
recognized:
Stage 1. Formation stage: It comprises of channel erosion
by a downward scour of the top soil.
Stage 2. Development stage: It consists of upstream
movement of the gully head and enlargement of the gully in
width and depth.
Stage 3. Healing stage: During healing stage vegetation
begins to grow in the gully.
Stage 4. Stabilization stage: Stabilization takes place when
gully reaches a stable gradient
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• Typical gulley erosion (on cultivated wheat
farm)

6. Tunnel Erosion
Tunnel erosion, also known as pipe erosion,
is the underground soil erosion and is common
in arid and semiarid lands.
Runoff in channels, natural cracks, and
animal burrows initiates tunnels by infiltrating
into and moving thorough dispersible subsoil
layers
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7. Stream-bank Erosion
•Stream-bank or channel erosion is the scouring
of material from the sides and bottom of a
stream or water channel and cutting of banks by
running water, and due to the erosive power of
runoff from uplands fields.
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STRAM –BANK EROSION
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4.4.1.4 Factors influencing water erosion
Water erosion is the removal of soil from the land’s
surface by water in motion
Climate (C)
Topography (T)
Vegetation (V)
Soil (S)
These factors may be summarized as follows:
E= f (C, T, V, S) ,Where, E = the rate of erosion, C, T,
V and S stands for climate, topography, vegetation
and soils, respectively
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Activity 4.2
1) Discuss the different types of
water erosion that can suit to
methods of control measures
explain both temporary and
permanent control structures of
gully erosion???
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4.4.2 Wind Erosion
 Wind erosion is the process of detachment, transportation, and
deposition of soil material by the action of wind.
 It occurs almost in all parts of the world and is a cause of serious
soil deterioration.
conditions that favor the occurrence of wind erosion. These are:
Loose and finely divided dry soil.
Steady and strong prevailing wind at all levels, from the upper air to
the ground level
Smooth and bare soil surface.
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4.4.2.1 Mechanics of Wind Erosion
The occurrence of wind erosion could be described
under following three different phases.
These are:
Initiation of soil movement
Transportation of soil particles and
Deposition of the soil particles
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4.4.2.2 Factors that affect wind erosion
Climatic factors
 Soil factor
 Vegetation covers
Wind Velocity distribution with height
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4.4.2.3 Wind Erosion Control Measures
The principal methods of reducing surface wind
velocity are
A) vegetative measures and
 Cultivated crops
 Shrubs & trees (Wind break, Shelterbelt)
 Wind strip cropping
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Shrubs & trees (Wind break,
Shelterbelt)
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Wind strip cropping
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B) Tillage practices
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4.4 Estimating Soil Loss
4.4.1 Prediction of Soil Loss by Water Erosion
Various trials have been so far made for
estimating soil loss and predicting it. But the
Universal Soil Loss Equation (USLE) has got
pronounced application, due to the
incorporation of well known parameters
facilitating for soil erosion and loss.

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Components of USLE
• Erosion is a function of:
• Erosivity and Erodibility
RAINFALL
Energy
R
Physical charix Management
K
Land
management
Crop
manageme
P
LS C
A

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Procedure for Using the USLE
Determine the R factor.
Based on the soil texture determine the K value. If
there is more than one soil type in a field and the
soil textures are not very different, then use the soil
type that represents the majority of the field. Repeat
for other soil types as necessary.
Divide the field into sections of uniform slope
gradient and length. Assign an LS value to each
section.
Choose the crop type factor and tillage method
factor for the crop to be grown. Multiply these 2
factors together to obtain the C factor.
Select the P factor based on the support practice

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R - factor
The rainfall erosivity factor is
usually determined from: EI30 or
K.E.> 25 indices.
More preferable for tropical
regions is K.E.> 25.
Therefore, K.E.> 25 = R. in an
appropriate Unit.
Care should be taken when
computing the indices since it is an
Emperical formula.

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Convenient units of conversion should be
adopted while calculating for EI30 or KE>25.
To estimate soil loss erosivity index due to a
particular storm these methods can be used;
and to estimate weekly, monthly, annual soil
losses, summation must be taken in to
account.


 


m
k
n
j
k
I
E
n
R
1
30
1 100
1
*
]
)
)(
(
[
1

7/15/2021 51
units
• EI30 = K.E. X I30 • K.E.= 916 + 331 log I (Foot –
tons/acre-inch).
• K.E.= 916 + 331 log I (Foot –
tons/acre) after multiplied by
rainfall amount.
• I30 = inch/hr.
• EI30 = Foot – tons-inch/acre-hr.

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o R = average annual rainfall erosivity in hundreds of ft-ton -
in * (acre-h-Yr)-1
o and the division by 100 is made for
convenience of expressing the units.
o E = total kinetic energy
o I30 = maximum 30-min rainfall intensity
o J = index of number of storms in each year
o n = number of years used to obtain average R
o m = number of storms in each year and
o R = average annual rainfall erosivity

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But for design purposes, the rainfall
which is historically serious in the
area is considered and the intensity
of such is obtained. The EI30 of the
same can also be considered and
thus, the required design for

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But for design purposes, the rainfall
which is historically serious in the
area is considered and the intensity of
such is obtained. The EI30 of the
same can also be considered and
thus, the required design for

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The K factor
 Is a soil erodibility factor.
 It is the soil loss per hectare per erosivity index from a field of
9% slope and 22.1m slope length. **
 determined by considering the soil loss from continuously
cultivated fallow land with out the influence of crop cover or
management.
 Various local Emperical equations are available, but largely in
error for the other areas.
 Monographs are used to compute the k factor.

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• **K = Ao
• S * (∑ EI) Where, K = Soil
erodibility factor
• Ao = Observed soil loss
• S = Slope factor
• Applicable for soils containing less than
70% silt and very fine sand.
4
.
4
.....
).........
3
(
5
.
2
)
2
(
25
.
3
)
12
)(
10
(
1
.
2
100 4
14
.
1





 
c
b
a
M
K

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The Topographic factors, LS
LS = ( / 22.13) m (65.41 sin2 ß + 4.58 sin ß +
0.065)
Where; LS = topographic factor, dimensionless
 lambda = slope length, m
 ß = the angle of slope
 m = variable that depends on steepness of land
slope its value as stated at HO.
It is dimensionless factor



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The C Factor
•The c factor a function of sub-factors of:
•C = Cplu x Ccc x Csc x Csr x Csm
•Where: Cplu= prior to land use factor
• Ccc = canopy cover factor
•Csc = Surface cover factor
•Csr = Surface Roughness factor
•Csm = Antecedent moisture

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The p factor
It may be defined as the ratio of soil
loss for a given conservation practice to
the soil loss obtained from field with
plants rows up/ down the slope.
The conservation practice consists of
mainly contouring, terracing, and strip
cropping, in which contouring appears
to be most effective practice on medium
slope ranging from 2 to 7 percent.

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• The conservation practice factor, P, can be
found from the following equation
.
t
s
c p
x
p
x
p
p 
Where; Pc = contouring factor
Ps = Strip cropping factor
Pt = Terrace sedimentation factor

Application of Universal Soil Loss Equation (USLE
It predicts the soil loss;
It helps in selection of the agricultural practices
and
It provides recommendations on crop practice to be
used
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Limitations of USLE
•It is empirical: The USLE is very empirical. Mathematically it does not
illustrate the actual erosion process.
•It predicts average annual soil loss: This equation computes less value
than the measured, especially when rainfall occurs at high rate.
•It does not compute gully erosion: The USLE is employed for assessing the
sheet and rill erosion and not used for predicting the gully erosion. The
gully erosion caused by concentrated water flow is not counted by the
equation and yet can produce greater volume of eroded soil.
•It does not compute sediment deposition: This equation estimates only soil
loss but not the soil deposition.
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4.5.2 The wind erosion prediction equation
A wind erosion prediction equation was developed using wind tunnels and
field studies in the US and Canada
The present wind erosion prediction equation is: E = f (I, K, C, L, V)
•Where E = the estimated average annual soil loss (Mg/ha – year),
• I = the soil erodibility index (Mg/ha –yr),
•K = the ridge roughness factor,
•C = Climate factor,
•L = Unsheltered length of eroding field in meters,
•V = Vegetation cover factor
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the end
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