Chapter 3

CHAPTER THREE
EROSIVITY AND ERODIBILITY
Mengistu Zantet (MSc in Hydraulic Engineering )
Lecturer @ Hydraulic and Water Resources Engineering department
Mizan Tepi university
Email: mengistu.zantet@gmail.com
P.O.Box: 260
Tepi, Ethiopia
mengistu.zantet@gmail.com .
lecturer@ Hydraulic and water
resources Engineering Department
Mizan Tepi University

Outline
3. 1 Erosivity
3.2. Estimation of erosivity from rainfall
3.2.1. EI30 Index method
3.2.2. KE > 25 Index method
3.2.3. Application of an index of erosivity
3.3. Erodibility
3.4. Relationship between Erosivity and Erodibility

INTRODUCTION
• Raindrop or splash erosion is the
interaction of two basic components, rain
and soil.
• Generally, the quantity of erosion yield is
dependent upon the ability of rain, to
detach the soil particles (i.e. erosivity)
and at the same time the susceptibility of
soil to withstand against the raindrop (i.e.
erodibility).
• Thus, the soil erosion is the function of

Raindrop

3.1 Erosivity
 Erosivity of rainfall is the potential ability of rain
to cause erosion.
 It is the function of the physical characteristic of
rainfall.
 When rainfall erosivity exceeds the soil erodibility,
there causes soil erosion.
 The knowledge of rainfall erosivity is essential to
understanding erosional processes, estimating soil
erosion rates, and designing erosion control

Erosivity

Factors affecting the erosivity of rainfall
1)Rainfall intensity
2)Amount of rainfall
3)Terminal velocity
4)Drop size Distribution
5)Direction of slope

3.2 Estimation of Erosivity from Rainfall Data
Note:
 The factors which are most closely related in soil
erosion by rain are amount and intensity of
rainfall.
 A number of correlations have been established
in localized conditions. However, the large
variations in rainfall, both temporal and spatial,
mean that this approach can only yield

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This is b/c the large variations in
rainfall characteristics as well as soil
properties.
Despite considerable unexplained
variations, the factor most closely
related to erosion is the kinetic energy
(KE) of the rain.
From previous through studies, the best
estimator of soil loss was found to be a
compound parameter, the product of
kinetic energy of the storm and the

The following two methods are widely used for
computing the erosivity of rainfall
EI30 Index method and
KE>25 Index method

3.2.1 EI30 Index Method
 This method was introduced by Wischemeir
 mainly based on the fact that, the product of kinetic
energy of the storm and 30–minute maximum rainfall
intensity, gives a best estimation of soil loss.
 EI30 is the greatest average intensity experienced in
any 30–minute period during a storm and it is
computed from recording rain gauge charts by
locating the greatest amount of rain which falls in any
30–minute period, and then doubling this amount to

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• Rainfall Hyetograph
30-
minute
R
Depth
Methods of obtaining the 30-minute intensity
from recording rain gauge chart
Time

Relationship between KE & Rainfall intensity.
Different researches come up with various
relations of these parameters while
investigating in soil erosion.
The findings of some of them is listed.

3.2.2. KE > 25 Index Method
One, developed by the present author in
Africa, (Hudson) is based on the fact
that little erosion takes place at low
intensities.
This method is based on the concept
that erosion takes place only at
threshold value of rainfall intensity.

Thus this method takes care only those rainfall
intensities, which are greater than 25mm/h that
is why its name became as KE >25 index method.
 For both methods multiplication of rainfall
amounts in each class of intensity to the
computed kinetic energy values and addition of
all these values are together to get total EK of
storm.
 The obtained EK is again multiplied by the
maximum 30-minute rainfall intensity to
determine the rainfall erosivity value

Application of an index of erosivity
TWO advantages:
1) to improve the design of
conservation works
2) helps to increase our knowledge and
understanding of erosion in
research

3.3. Erodibility
 Erodibility is the reciprocal of soils’
resistance to erosion.
 It is the vulnerability or susceptibility of
the soil to get erosion.
 It is the function of physical characteristic
of soil (texture, structure, organic matter,
land use pattern etc.) and land
management practices used

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Therefore, the effect of physical
properties of the soil should always
be evaluated more precisely to
determine the soil erodibility than
the land management practices.
 Bouyoucos (1935) suggested that
the soil erodibility depends on the
mechanical composition of soil such
as sand silt and clay as:

Erodibility can change with time
and management.
Its change with time is
determined by the seasonal
variation in temperature, soil
moisture and land management
practices.

soil erodibility depends on several
factors
 mechanical structure,
 moisture content,
 soil density,
 capillary water balance,
 plasticity,
 soil swelling capacity and dispersion ratio

3.4. Soil Properties Affecting Erodibility
Soil texture,
soil structure
macro porosity,
aggregate properties
 organic matter content,
hydraulic properties, and wettability

3.4 Relationship between Erosivity and Erodibility
EROSIVITY
 is the function of rainfall
properties and to this
extent it is dependent on
the soil..
 measurement of erosivity
is not possible without
erodibility of the soil.
ERODIBILITY
is the function of
physical properties of the
soil and land management
practices; it is independent
on the rainfall.
 the erodibility
measurement cannot be

Activity 3.1
Explain briefly the Relationship between Erosivity
and Erodibility ???

the end

Chapter 3

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