CEE 3262 SOIL AND WATER RESOURCES PROTECTION
COMPREHENSIVE QUESTIONS
Course EXERCISES CH-I. INTRODUCTION
COMPREHENSIVE QUESTIONS:
1.1 Write a brief note the history of soil erosion throughout the world.
1.2 Describe the various effects of soil erosion
1.3 Explain the effects of soil erosion on public health hazard
1.4. Explain the effects of soil erosion on siltation of rivers.
Course EXERCISES CH-II. DEFINITIONS, CAUSES AND EFFECTS OF SOIL EROSION
COMPREHENSIVE QUESTIONS
Q1. DEFINITION, FORMS AND TYPES OF SOIL EROSION
QUESTIONS
1. Define the followings:
(1) Agriculture soil
(2) Soil erosion
(3) Water Erosion
(4) Accelerated erosion
(5) Mass movement
(6) Tillage translocation erosion
(7) Splash erosion
(8) Sheet erosion (Interrill erosion)
(9) Rill erosion
(10)
Gully erosion
(11)
Riverbank erosion or Stream bank erosion
Q2. CAUSES OF SOIL EROSION
QUESTIONS
 Explain how rainfall intensity, runoff and temperature are the main causes of soil erosion.
 Explain how slope gradient and slope length influence soil erosion
 Explain how vegetation is controlling soil erosion.
Q3. PROCESSES AND MECHANICS OF EROSION
QUESTIONS
 List and describe the processes of water erosion.
 Give and describe the 3 processes of water erosion.
Chap 5. FACTORS INFLUENCING WATER SOIL EROSION
 List and Describe the various Factors Influencing Soil Erosion.
 Define rainfall erosivity and explain the various factors affecting it.
Q4. EFFECT OF SOIL EROSION
QUESTIONS
 List and describe the effects of soil erosion.
 Land degradation
(1) Define land degradation and explain at list nine main causes of land degradation.
(2) List and describe some practices for controlling or measures to prevent land
degradation.
 Landslide
(1) List and explain the classification of land slide according to types of movement
(2) Explain the classification of landslide according to types of materials.
(3) Write and explain the natural causes of landslide.
(4) Write and explain the human causes of landslide.
(5) List the various factors that influencing the landslide (Triggering Factors).
(6) List and describe the effects of landslide.
 List and explain the Effects of Soil erosion on different scenarios of land use and land cover.
Q5. Other EXERCISES
Course EXERCISES CH-III. SOIL EROSION MEASUREMENT
QUESTIONS
 Explain the erosion plot for predicting the soil loss.
 Explain the erosion pins for predicting the soil loss.
 Explain the universal soil loss equation for predicting the soil loss.
4
USLE, MUSLE and RUSLE equations
(i)
What do you understand by USLE, MUSLE and RUSLE equations?
(ii)
Define and give respective units of each of USLE parameters
(iii) State the assumptions of USLE, MUSLE and RUSLE equations
(iv)
State the limitations of USLE, MUSLE and RUSLE equations
(v)
Determine the soil loss from a certain hill by using USLE equation knowing the
following data:
• Mean annual rainfall is 916.44 mm
• Soil texture is Sandy loam, organic content = 2% and soil erodibility = 0.24
• Slope = 20%, slope length = 45m and slope length factor = 5
• Land use is agriculture: crop management factor = 0.128 and P-factor = 0.92
Course EXERCISES CH-IV. SOIL EROSION AND SEDIMENT CONTROL METHODS
A. COMPREHENSIVE QUESTIONS





Write and explain the main approach to soil conservation.
List and explain the mechanical methods of erosion control.
Describe the mulching measure checklist.
Describe structural conservation practices.
Describe the structural conservation practices checklist.
B. COURSE EXERCISES FOR SOIL EROSION CONTROL MEASURES
4.1 Grassed waterways
4.2 Bench terraces
4.3 Contour bunds
4.4 Check dams
4.5 Splash erosion
Q1. Grassed waterways (20marks)
(a) Calculate the peak runoff rate expected to occur once in 10-years for design of a grassed
waterway to dispose from the watershed area of 35 ha. The other details are given as under:
1. Time of concentration of the watershed is 30 minutes
2. Runoff coefficient of the watershed for given characteristics is 0.40
3. The maximum rainfall recorded during 10-years duration is 6.00 cm in a storm duration of 30minutes.
(b) Using the peak runoff, determine the flow depth of a trapezoidal grassed waterway with
following details:
1. Side slope
= 4H:1V
2. Bottom width
=5m
3. Slope of waterway = 2%
4. Manning roughness coefficient: n = 0.04
4. Assume V = 1.50m/s
(c)Using the peak runoff, design a most economical trapezoidal grassed waterway with the following
data:
1. Side slope
= 4H:1V
2. Slope of waterway = 2%
3. Manning roughness coefficient: n = 0.04
4. Assume V = 1.50m/s
Q2. Bench terraces (20marks)
A 15% hilly land is proposed for constructing the bench terrace using 2.5 m as a vertical interval and
1:1 as a batter slope. Determine:
(i) The width of this bench terrace
(ii) Its length per hectare
(iii)The earthwork per hectare
(iv) The percentage of area lost.
Q3. Contour bunds (20marks)
Calculate the total length and earthwork of contour bund per hectare, which is constructed on 5%
land slope. The bund’s spacing was maintained as 25 m. The specification of bund is given as under:
• Top width = 50 cm
• Bottom width = 125 cm
• Height = 100 cm
• The lateral and side bunds are also formed in field.
Q4. Check dams (20%)
Before designing a check dam, it is necessary to calculate the maximum runoff discharge that should
pass through the spillway of the check dam. The following data are given:
(1) Watershed area : A = 1.85Km2
(2) Catchment slope : S = 0.004
(3) Maximum length of flow : L = 1.15 Km
(4) Runoff coefficient : K = 0.35
a. What do you understand by check dam?
b. Estimate the time of concentration of the basin
c. Estimate the average intensity of rainfall
d. Estimate the 25 years flood if rainfall is given by:
Where i(mm/hr); Tr(years) and t(min).
Q5. Splash erosion (20%)
a. Raindrops of 2.0 mm diameter are falling on an erodible soil surface. Compute the impact
energy of each drop. Assuming standard atmospheric conditions of 20oC temperature and
101.3 KPa air pressure.
b. Calculate the amount of soil eroded by splashing from a plot of 1.5 x 1.8 m2 size. The other details
are as follows:
g = 9.81 m/s2
γ = 1.5 t/m3
i = 2.8 mm/min
t = 30min
Vc = 5.49 m/s
dk = 3.5mm
ddk = 0.2 mm
sin α = 0.1
Where, q D = quantity of soil eroded by splashing (t/ha)
γ= density of soil saturated with water (t/m3)
i= rainfall intensity (mm/min)
Vc = terminal velocity of raindrop (m/s)
t= duration of rainfall (min)
dk = mean diameter of raindrops (mm)
ddk = diameter of rain drops (mm) of the critical size below splash erosion does not occur.
Vcd = critical velocity of falling rain drops, below which there is no appreciable soil damage.
Use the following equation:
0.13γ. i Vc2
dk . Vc
qD =
.t (
− 1) 4 sin α
2. g
ddk . Vcd
ANSWERS:
CEE 3262 SOIL AND WATER RESOURCES PROTECTION
Course EXERCISES CH-I. INTRODUCTION
COMPREHENSIVE QUESTIONS:
1.1 Write a brief note the history of soil erosion throughout the world.
The ‘erosion’ word was derived from Latin word ‘erodere’ which meaning is to eat away or to
excavate. The erosion word was firstly used in geology for describing the term hollow, created by
water (Penck, 1894). In addition to the erosion, several other terms were also introduced to express
geomorphological process caused by water and wind. There are corrosion, abrasion, denundation etc.
The history of soil erosion is an integral part of the agriculture. All over the world, wherever human
being started the agricultural operations, there exists the problem of soil erosion in some extent.
Spectacular examples of soil erosion are also found due to excessive grazing of grasses by cattle, the
natural vegetation on hilly land are lost, which resulted into severe erosion problem. Among all the
rivers, most of them lead to the formation of vast network of gully systems by destructing the
agricultural lands. Etc…
1.2 Describe the various effects of soil erosion
1. Siltation of irrigation channels and reservoirs
Silting of channels or reservoirs is another severe problem caused by soil erosion, as it is the main
source to yield the sediment from the watershed area and transporting them into the channels/
reservoirs. In siltation process when runoff, containing silt particles flows into the stream, the silts are
settled down over the channel bed and thus, reduces the effective capacity of channels.
2. Money Spent on Purification Work
Especially in rainy season, this problem of water pollution becomes very much which is impossible to
purify it.
The purification work of polluted water requires some sophisticated equipment. Thus, it involves
expenditure of extra money for purchasing the same. Indirectly, it can be said that, for purifying the
water, a huge amount of fund have to be invested
3. Silting of Rivers
Silting process of river is the same as irrigation channels. Silting of the river causes a severe problem.
In one direction, silt deposition on river bed causes the disease and public health hazards by polluting
the water, while in other direction it poses the navigation problem. Etc…
1.3 Explain the effects of soil erosion on public health hazard
Disease and public health hazard are also started with the erosion of the land surface and pollution of
water subsequently. Especially in monsoon season, this problem of water pollution becomes very
much which is impossible to purify it. The pollution of water of any place is the source of eradication
of Malaria disease. For example in the New Mexico city, the Malaria has become a great challenge to
the medical professions due to rapid silting of the Rio-Grande river. From investigation it has been
observed, that the river bed is rising at the rate of 2.5 cm/year and water is stagnated over several
adjacent alluvial plains at many places. These ponded water cannot be drained because most of these
points are lower in elevation than the river bed. Thus, stagnated water enhances the malaria problem
among the population, existing near by the areas.
1.4. Explain the effects of soil erosion on siltation of rivers.
Silting process of river is the same as irrigation channels. Silting of the river causes a severe problem.
In one direction, silt deposition on river bed causes the disease and public health hazards by polluting
the water, while in other direction it poses the navigation problem.
CEE 3262 SOIL AND WATER RESOURCES PROTECTION
Course EXERCISES CH-II. DEFINITIONS, CAUSES AND EFFECTS OF SOIL EROSION
COMPREHENSIVE QUESTIONS
Q1. DEFINITION, FORMS AND TYPES OF SOIL EROSION
QUESTIONS
2. Define the followings:
(12)
Agriculture soil
(13)
Soil erosion
(14)
Water Erosion
(15)
Accelerated erosion
(16)
Mass movement
(17)
Tillage translocation erosion
(18)
Splash erosion
(19)
Sheet erosion (Interrill erosion)
(20)
Rill erosion
(21)
Gully erosion
(22)
Riverbank erosion or Stream bank erosion
Q2. CAUSES OF SOIL EROSION
QUESTIONS
 Explain how rainfall intensity, runoff and temperature are the main causes of soil erosion.
 Explain how slope gradient and slope length influence soil erosion
 Explain how vegetation is controlling soil erosion.
Q3. PROCESSES AND MECHANICS OF EROSION
QUESTIONS
 List and describe the processes of water erosion.
 Give and describe the 3 processes of water erosion.
Chap 5. FACTORS INFLUENCING WATER SOIL EROSION
 List and Describe the various Factors Influencing Soil Erosion.
 Define rainfall erosivity and explain the various factors affecting it.
Q4. EFFECT OF SOIL EROSION
QUESTIONS
 List and describe the effects of soil erosion.
 Land degradation
(3) Define land degradation and explain at list nine main causes of land degradation.
(4) List and describe some practices for controlling or measures to prevent land
degradation.
 Landslide
(7) List and explain the classification of land slide according to types of movement
(8) Explain the classification of landslide according to types of materials.
(9) Write and explain the natural causes of landslide.
(10)
Write and explain the human causes of landslide.
(11)
List the various factors that influencing the landslide (Triggering Factors).
(12)
List and describe the effects of landslide.
 List and explain the Effects of Soil erosion on different scenarios of land use and land cover.
Q5. Other EXERCISES
ANSWERS:
Q1.DEFINITION, FORMS AND TYPES OF SOIL EROSION
Q1. Define the followings:
(1) Agriculture soil
Ans:
Agricultural Soil is the top layer of the earth’s surface that contains most of the soil’s nutrients,
organic matter, and pesticides (copper, 1997).
(2) Soil erosion
Ans:
Soil erosion is the result of detachment, transport and deposition process of soil (Panizza, 1996) . It
is a hazard traditionally associated with agriculture in tropical and semi-arid and it influences the
productivity and sustainability of agriculture in long terms (Morgan & Pimental et al., 2005)
(3) Water Erosion
Ans:
Water erosion is the wearing away of the soil surface by water from rain, runoff, snowmelt, and
irrigation […..]. Rainwater in the form of runoff is the main driver of water erosion. It refers to the
movement of soil organic and inorganic particles along the soil surface with flowing water and
deposition of the eroded materials at lower landscape positions and in aquatic ecosystems. The eroded
material can either form a new soil or simply fill lakes, reservoirs, and streams.
(4) Accelerated erosion
Ans:
Soil erosion becomes a major concern when the rate of erosion exceeds a certain threshold level and
becomes rapid, known as accelerated erosion. This type of erosion is triggered by anthropogenic
causes such as deforestation, slash-and burn agriculture, intensive plowing, intensive and
uncontrolled grazing, and biomass (Lal, 2008).
(5) Mass movement
Ans:
Soil mass movement occurs on slopes where forces promoting failure become large compared with
the resistance of soil to failure. Landslides, debris avalanches, slumps and earth flows, creep, and
debris torrents are examples of this movement (Brooks, 2013)
(6) Tillage translocation erosion
Ans:
Tillage translocation erosion is the redistribution of soil within a landscape caused directly by tillage.
For a range of landscapes, it has been shown that translocation erosion is a potential contributor to
the total soil erosion on cultivated fields (Govers, 1999).
(7) Splash erosion
Ans:
Splash erosion is caused by the bombardment of soil surface by impacting raindrops. Processes of
splash erosion involve raindrop impact, splash of soil particles, and formation of craters (Ghadiri,
2004). Raindrops striking the soil surface develop a raindrop-soil particle momentum before
releasing their energy in the form of splash (Lal, 2008)
(8) Sheet erosion (Interrill erosion)
Ans:
As soon as it starts, runoff promptly develops diminute rills, and that portion of runoff that flows
between rills is called sheet or Interrill erosion (Fig. 2.1). This type of erosion is mostly due to
shallow flow. Some particles are carried away in runoff flowing in a thin sheet and some concentrate
in small rills. Interrill is the most common type of soil erosion. Splash and interrill erosion make up
about 70% of total soil erosion and occur simultaneously although splash erosion dominates during
the initial process [..]. Interrill erosion is a function of particle detachment, rainfall intensity, and
field slope.
(9) Rill erosion
Ans:
It refers to the soil erosion that occurs in small channels or rills. Runoff water that concentrates in
small channels erodes soil at faster rates than Interrill erosion. The force of flow and the soil
particles creeping along the rill bed enlarge rills. Rill erosion is the second most common pathway of
soil erosion. The rills are easily obliterated by tillage operations but can cause large soil erosion
especially under intensive rains. Rill erosion is a function of soil erodibility, runoff transport
capacity, and hydraulic shear of water flow. Soil erosion occurs mostly through the simultaneous
action of interrill and rill erosion in accord with the steady-state sediment equation (Foster, 1982)
(10)
Ans:
Gully erosion
Gullies can arise from the progressive development of rills. The rills usually cuts so deeply that
normal farming methods can no longer be routinely employed to mitigate their development. Gully
formation depends on the supply of large quantities of runoff water of sufficient energy to detach
and transport the soil. The break in vegetation cover provides the locus for gully erosion to start.
Gully erosion creates either V- or U-shaped channels. As gullies grow, more sediment is transported
(Lal, 2008).
(11)
Ans:
Riverbank erosion or Stream bank erosion
It refers to the collapse of banks along streams, creeks, and rivers due to the erosive power of runoff
from uplands fields. Intensive cultivation, grazing, and traffic along streams accelerate stream bank
erosion. Planting grasses (native and tall grass species) and trees, establishing engineering structures
(e.g. gabions and stones revetement,…), mulching stream borders with rocks and woody materials,
geotextile fencing and diverting runoff are measures to control stream bank erosion (Lal, 2008).
Q2.CAUSES OF SOIL EROSION
(1)
Explain how rainfall intensity, runoff and temperature are the main causes of soil erosion.
Ans:
The rainfall and temperature play important role in erosion:
➢ There is a direct relationship between the amount of rainfall and erosion.
➢ Rainfall intensity influences both the rate and volume of runoff and then the scale of erosion.
➢ Temperature affects climatic type, which governs the types of crop grown and the amount of
ground cover that exists.
➢ Temperature is important in producing desired level of ground cover to protect soil from
erosion.
➢ In highlands, maintenance of desired level of ground cover is difficult because of low
temperature and short growing season of plants. In such areas, the intense rain can cause
severe erosion.
➢ Similarly, the soils of the arid regions are low in organic matter because warm temperature
have resulted in more rapid decomposition of organic matter. This lower organic matter
content in soil makes the soil more susceptible to erosion during intense rains (Wagley,
2009).
(2)
Explain how slope gradient and slope length influence soil erosion
Slope steepness is one of the important factors influencing soil erosion. Greater the slope
more is the erosion. Slope steepness influences erosion in several ways:
➢ The increased velocity of runoff water in steep slopes allows more soil to be
detached and transported especially where surface detention of water is low and
breaking down of rain drop energy is not done on steep slopes. Therefore, steep
slopes are susceptible to erosion.
➢ The slope length is also an important factor affecting soil erosion. If the slope is
longer, a
large quantity of rain will fall on it and if the rate of rainfall exceeds the rate of
infiltration, there will be large accumulation of water at the base. Therefore longer
the
slope length more is the erosion. There is a relationship between soil loss and
slope length. It states that “The amount of soil loss (A) varies with the square root
of slope length (L) (Wagley, 2009): A ~√𝐿
(3)
Ans:
Explain how vegetation is controlling soil erosion.
Vegetation: vegetation plays important role in water erosion. It has great importance in controlling
erosion because of the following reasons :
➢ Vegetation intercepts raindrop and hence reduces its energy and impact on land surface.
Raindrops that could have reached in the soil would be quickly taken by leaf litter. Lands
without vegetation will be vulnerable to raindrop, sheet and gully erosions.
➢ Vegetation improves the soil structures by adding organic matter into the soil. High organic
matter content soil will be more pores. The humus layer will act as a sponges which will
absorb and allow moisture to enter into the soil. This will in turn help to increase the
infiltration and water holding capacity of soil. Soil having low organic matter and humus
layer are susceptible to erosions because of increased runoff.
➢ In the vegetative areas, the root network, the channels of root decay, animal burrows help to
dissipate runoff water.
➢ The root systems below the soil surface binds and aggregates the soil through the mechanical
actions and prevent from erosion and landslide.
➢ High surface friction due to leaf litter and increased roughness of the ground in vegetative
land tends to spread out the runoff laterally and thus dissipates its energy (Wagley, 2009).
Q3.PROCESSES AND MECHANICS OF EROSION
Q1. List and describe the processes of water erosion.
Ans:
Erosion is a process of detaching soil particles from the land surface of one place and their
transportation and deposition to another place. Understanding the mechanisms and magnitude of
water erosion is vital to manage and develop erosion control practices (Lal, 2008).
Q2. Give and describe the 3 processes of water erosion
Ans:
Three Processes of Soil Erosion:
1. Detachment
This process depends upon the type of soil, OM, moisture, nature of detaching agent (water for this
case) (Wagley, 2009).
2. Transportation
This process depends upon size, density and shape of detached materials and velocity of the
transporting agent (Wagley, 2009).
3. Deposition
The Soil which is eroded from the original location is always deposited somewhere else. This may
be close to its place of origin position or it may be deposited at the longest distance down to the sea
or at any other point between the place of origin and the sea (Wagley, 2009).
Q4. FACTORS INFLUENCING WATER SOIL EROSION
Q1. List and Describe the various Factors Influencing Soil Erosion
1.
Rainfall
Rainfall initiates the process of erosion by provoking soil detachment and transport directly by
raindrop splash or through the contribution of rain to runoff. There is a direct relationship between
the amount of rainfall and erosion. Rainfall intensity influences both the rate and volume of runoff
and then to scale of erosion. (Kinnel, 2010).
2.
Soil
The likelihood that detachment and entanglement (embarrassing situation) of soil particles occur
depends not only on rainfall characteristics but also on the structural characteristics of the soil. Soil
erodibility is defined by (Morgan R. , 2005)as the resistance of soil to both detachment and
transport. It is dependent on parameters such as soil texture, aggregate stability, shear
strength and infiltration capacity (Wischmeier W. M., 1969).
3.
Topography
Evidently, mountainous regions are more prone to soil erosion. A higher slope gradient creates a
higher flow velocity which causes more detachment and transport of soil particles (Fox, 1999). In
general, soil loss increases exponentially with slope steepness for tropical soils (El-Swaify, p. 1987).
4.
Vegetation
Vegetation has several properties making it a useful tool for reducing soil erosion:
➢ First of all, a direct mechanical protection of the soil surface is provided by the canopy and
litter covers that intercept rainfall. This reduces the kinetic energy of water that reaches the
soil, causing a lower detachment of soil particles (Bochet, 1998). (Vegetation intercepts
raindrop reduce its energy and impact on land surface. Raindrops that could have reached in
the soil would be quickly taken by leaf litter).
➢ Plant roots have a mechanical effect on soil strength: by penetrating the soil mass, roots
reinforce the soil and increase soil shear strength (De Baets, 2006).
➢ Lands without vegetation will be vulnerable to raindrop, sheet, rill and gully erosions).
➢ Vegetation improves the soil structures by adding organic matter into the soil. High organic
matter content soil will be more pores (Further, the humus layer itself acts as a sponges and
absorbs moisture and allows it to enter into the soil). This process in turn helps increase the
infiltration and water holding capacity of soil.
➢ Soil having low organic matter and humus layer are susceptible to erosions because of
increased runoff.
5.
Management
Even in ancient times, farmers discovered that shaping lands in certain ways, such as
contourplanting and terracing, was necessary for sustained agricultural production (El-Swaify,
1982)Management techniques can work in two ways: human enforced mismanagement is a major
cause of erosion (Oldeman, 1993). Soil erosion control measures implemented can take various
forms and have variable effectiveness (König, 1992)
6.
Human activities
The human activities can cause the erosion, combination of two or more elements, erosion will
increase significantly; those are: Disturbance of existing of drainage flow, Removal of Native
vegetation, Construction of improperly designed erosion control facilities, construction of
impermeable of surfaces, Grading, construction or other disturbance of slopes over 10%,
Disturbance of highly erosive soil, Off- road use of vehicles, Roads and Restrictions of floodplains
(Donald J. funk, 2005).
Q2. Define rainfall erosivity and explain the various factors affecting it.
Ans:
Definition of Rainfall Erosivity
It refers to the intrinsic capacity of rainfall to cause soil erosion. (Water erosion would not occur if
all rains were non-erosive). Since this is hardly the case, knowledge of rainfall erosivity is essential
to understanding erosional processes, estimating soil erosion rates, and designing erosion control
practices. Properties affecting erosivity are: amount, intensity, terminal velocity, drop size, and
drop size distribution of rain (H. Blanco, 2008).
Table below indicates the factors affecting rainfall erosivity (H. Blanco, 2008).
Amount
• More rain
results
in more erosion
although this
correlation
depends on
rainfall
intensity.
Intensity
• Intensity is the
amount of rain
per
unit of time (mm
h−1).
•
Intensity is
normally <70 mm
h−1 in temperate
Terminal velocity
A raindrop
accelerates its
velocity until the
air resistance
equals the
gravitational
force, and then it
falls at that
constant velocity,
Drop size
Size of
raindrops
can range
between 0.25
and
8 mm in
diameter,
but those
between
•
•
Amount of rain
is
a function of
duration and
intensity of
rain.
Measurement
of
the amount of
rain is
influenced by
the type,
distribution,
and
installation
protocol of the
rain gauges.
regions, but it can
be as high as
150 mm h−1 in
tropical regions.
•
Intense storms
are
often of short
duration.
•
Intensity is
directly
correlated
with erosion.
•
The more intense
the rain, the
greater is the soil
erosion.
also known as
terminal velocity.
Raindrops can
strike the soil at a
speed as high as
35 km h−1 and
displace soil
particles as far as
2 m in horizontal
and 1 m in vertical
direction.
Terminal velocity
increases with
increase in
raindrop size.
2 and 5 mm are
common.
In intense
storms,
raindrops can
be
as large as 8
mm.
While drop size
increase with
increase in rain
intensity, it may
decrease when
intensities
exceed
100 mm h−
Q5. EFFECT OF SOIL EROSION
Q1. List and describe the effects of soil erosion
Ans:
A. Loss of topsoil: Obviously, this is the biggest effect of soil erosion. Because topsoil is so fertile, if
it is removed, this can cause serious harm to farmer’s crops or the ability to effectively work their land
(Rinkesh, 2009).
B. Soil compaction: When soil under the topsoil becomes compacted and stiff, it reduces the ability
for water to infiltrate these deeper levels, keeping runoff at greater levels, which increases the risk of
more serious erosion (Rinkesh, 2009).
C. Reduced organic and fertile matter: As removing topsoil that is heavy with organic matter will
reduce the ability for the land to regenerate new flora or crops.
D. Poor drainage: Sometimes too much compaction with sand can lead to an effective crust that seals
in the surface layer, making it even harder for water to pass through to deeper layers.
E. Issues with plant reproduction: When soil is eroded in an active cropland, wind in particular
makes lighter soil properties such as new seeds and seedlings to be buried or destroyed.
F. Soil acidity levels: When the structure of the soil becomes compromised, and organic matter is
greatly reduced, there is a higher chance of increased soil acidity, which will significantly impact the
ability for plants and crops to grow.
G. Long term erosion: Unfortunately, if an area is prone to erosion or has a history of it, it becomes
even harder to protect it in the future.
H. Water pollution: A major problem with runoff from soils particularly those used for agricultural
processes is that there is a greater likelihood that sediment and contamination like the use of fertilizer
or pesticide. This can have significant damage on fish and water quality.
Q2.Land degradation
Q2.1 Define land degradation and explain at list nine main causes of land degradation
Ans:
➢ Land degradation is a process in which the value of the biophysical environment is affected
by a combination of human-induced processes acting upon the land. It is viewed as any
change or disturbance to the land perceived to be deleterious or undesirable (Conacher &
Conacher, 1995).
➢ Nine Main Causes of Land Degradation are as follows:
1. Deforestation:
Forests play an important role in maintaining fertility of soil by shedding their leaves which
contain many nutrients. Forests are also helpful in binding up of soil particles with the help of
roots of vegetation. Therefore, cutting о forests will affect the soil adversely.
2. Excessive Use of Fertilizers and Pesticides:
Fertilizers are indispensable for increasing food production but their excessive use has occasioned
much concern as a possible environmental threat. Excessive use of fertilizers is causing an
imbalance in the quantity of certain nutrients in the soil. This imbalance adversely affects the
vegetation.
The word pesticides includes any form of chemical used for the control of unwanted herbaceous plants
(herbicides), woody plants (arboncides), insects (insecticides) or any chemical that has biocidal
activity affecting rodents, arachnids or any other population. After Second World War the use of
pesticides increased tremendously.
Although their success in controlling pests on a short-term basis cannot be denied, but their long-term
effectiveness in controlling pests or their overall effects on ecosystems (including human health) and
environment has to be seriously questioned on two major grounds.
These are:
(a) Increasing concentration of pesticides residues as they move up the food chain; and
(b) Rapid evolution of new breeds of pests that are immune to the pesticides applied.
Moreover, excessive use of these pesticides, results in an increase in the level of resistance by certain
pests and it may kill some useful species like earthworm which are very helpful in maintaining soil
fertility. Thus, the use of pesticides leads to decline in the fertility status of soil.
3. Overgrazing:
Increase in livestock population results in overexploitation of pastures. Due to this, grass and other
types of vegetation are unable to survive and grow in the area, and lack of vegetation cover leads to
soil erosion. Millions of people in Africa and Asia raise animals on pastures and rangelands that have
low carrying capacity because of poor quality or unreliable rainfall Pastoralists and their rangelands
are threatened by overgrazing.
Pastoral associations in West Africa have tried with mixed success to improve the productivity of
common held livestock pastures. The Aga Khan Rural Support Programme has been successful in
improving management of common grazing lands.
4. Salination:
Increase in the concentration of soluble salts in the soil is called salination. India has about six million
hectares of saline land.
The origin of saline soil depends on the following factors:
A. Quality of Irrigation Water:
The ground water of arid regions are generally saline in nature. The irrigation water may be itself rich
in soluble water and add to salinity of soils.
B. Excess Use of Fertilizers:
Excess use of alkaline fertilizers like sodium nitrate, basic slag, etc. may develop alkalinity in soils.
C. Capillary Action:
Salts from the lower layers move up by capillary action during summer season and are deposited on
the surface of the soil.
D. Poor Drainage of Soil:
Salts dissolved In Irrigation water accumulate on the soil surface due to inadequate drainage,
especially during flood.
E. Salts Blown by Wind:
In arid region near the sea, lot of salt is blown by wind and gets deposited on the lands.
5. Water-logging:
Excessive irrigation and improper drainage facility in the fields cause rise in the ground water level.
This ground water mixes with surface water used for irrigation and creates a situation called waterlogging. Ground water brings the salts of soil in dissolved state up to the surface where they form a
layer or sheet of salt after evaporation. The term salinity is used for such a situation.
6. Desertification:
Desertification is a widespread process of land degradation in arid, semi- arid, and dry sub-humid
areas resulting from various factors, including climatic variations and human activities. The UNO
Conference on Desertification (1977) has defined desertification as the “diminution or destruction of
the biological potential of land, and can lead ultimately to desert like conditions.”
The major causes of desertification are mismanagement of forests, overgrazing, mining and
quarrying. Dr. H. Dregne has listed desertification processes as follows:
(a) Degradation of vegetative cover;
(b) Water erosion;
(c) Wind erosion;
(d) Salinization;
(e) Reduction in soil organic matter; and
(f) Excess of toxic substances.
This problem varies from overgrazing in rangeland, to water and wind erosion in rainfed croplands,
and to salinization in irrigated lands. In dry-lands, however, the most serious land-degradation problem
is water and wind erosion.
Desertification and land degradation can contribute to local warming by reducing plant cover and
increasing soil exposure, which changes the energy balance of an area. Deserts, semi-arid lands, and
dry woodlands also constitute a large potential source of carbon emissions into the atmosphere.
Changes in climate, in turn, can intensify desertification and land degradation. These processes are
aggravated by variations in weather, and climate change can increase such variability. If climate
change continues unabated, the potential increase in the frequency and intensity of droughts would
reinforce the variability of dry-land ecosystems. The increasing rate of desertification will be a threat
to food security.
7. Soil erosion:
Accelerated soil erosion by water and wind is the major land degradation process and this is a
consequence of changed relationship between environmental factors which occur as a result of human
interventions. Adverse changes in physical, chemical or biological characteristics of the soil result in
reduced fertility and soil erosion. Other kinds of land degradation are as water-logging, chemical
contamination, acidification, salinity and alkalinity etc.
Land degradation results from the combined effects of processes such as loss of biological diversity
and vegetative cover, soil loss nutrient imbalance, decline in soil organic matter and decrease of
infiltration and water retention capacity. Soil erosion means the removal of top fertile layer of the soil.
Soil erosion by wind and water is the most common and extensive.
(i) Wind Erosion:
At places where there is no vegetation and soil is sandy, strong winds blow the loose and coarse soil
particles and dust to long distances. The depletion of forests lead to loosening of soil particles due to
lack of roots and moisture in soil. These loosened particles are more prone to soil erosion by winds.
(ii) Water Erosion:
Deforestation, overgrazing and mining, all are equally responsible for an increase in the rate of erosion
by water. Water erosion is caused either by water in motion or by the beating action of rain drops.
Water during heavy rains may remove the thin soil cover over large areas more or less uniformly.
It is called sheet erosion. If the erosion continues unchecked, numerous finger-shaped grooves may
develop all over the area as a result of the silt-laden run off. This is called rill erosion. Gully erosion
is an advanced stage of rill erosion because the unattended rills begin to attain the form of gullies,
increasing their width, depth and length.
Soil erosion due to water is the most serious land degradation problem in India. It causes land
degradation through huge loss of top fertile soil along with plant nutrients through runoff water. It
reduces the depth of soil where it takes place, depletes the ground water table, limits the moisture
storage capacity and feeding zones of the crops, deteriorates the soil organic matter, destroys soil
structure and impairs fertility due to nutrient losses.
Many factors contribute to water-logging. These include inadequate drainage, improved balance in the
use of ground and surface water, planning crops not suited to specific soils. Water-logging is most the
serious problem in Haryana, Punjab, West Bengal, Andhra Pradesh and Maharashtra.
8. Wasteland:
Wastelands are the lands which are economically unproductive, ecologically unsuitable and subject to
environmental deterioration. Estimates show that wastelands in India form about half of our country.
Wastelands are of two types:
(a) Culturable; and
(b) Unculturable.
The culturable wastelands include ravinous land, waterlogged land, marsh and saline lands, forest
land, degraded land, strip land, mining and industrial wastelands. On the other hand, unculturable
wastelands include barren rocky areas, steep slopes, snow-capped mountains and glaciers.
9. Landslides:
The sudden movement of the soil and the weathered rock material down the slope due to the force of
gravity is called a landslide. Lad-slides are common in mountainous regions especially those which
are situated along the river banks or near the coastline.
The flow of water continuously goes on doing the eroding work which results in landslides sooner or
later. Especially when the rivers are in flood they greatly add to landslides. In India, landslides are
common in the mountainous regions of the north and north-eastern parts. Human induced activities
are also responsible for landslides.
They are:
(a) Deforestation in hilly areas;
(b) Excessive mining in hilly areas;
(c) Construction of dams;
(d) Infrastructure; and
(e) Means of transport, especially construction of roads.
Q2.2 List and describe some practices for controlling or measures to prevent land degradation.
Ans:
Following are some practices for controlling land degradation (Mosayeb.H, 2013):
1. Strip farming:
It is & practice in which cultivated crops are sown in alternative strips to prevent water movement.
2. Crop Rotation:
It is one of the agricultural practice in which different crops are grown in same area following a
rotation system which helps in replenishment of the soil.
3. Ridge and Furrow Formation:
Soil erosion is one of the factors responsible for lad degradation. It can be prevented by formation of
ridge and furrow during irrigation which lessens run off.
4. Construction of Dams:
This usually checks or reduces the velocity of run off so that soil support vegetation.
5. Contour Farming:
This type of farming is usually practiced across the hill side and is useful in collecting and diverting
the run off to avoid erosion.
6. Gardening
Planting vegetation and grass can stop heavy rains from damaging our land and it protects the topsoil
from being washed away.
7. Afforestation and Reforestation
Planting trees can prevent flooding which causes soil movement and erosion.
8. Conservation Tillage
Avoid stripping a whole field uncovered because it can cause harmful effect on the soil. Leaving a
few vegetation or grass can keep the soil at bay.
9. Constructing Wind Breakers
Fences, bushes, hedges and trees can prevent gusty winds from damaging our soil.
10. Using Fertilizers
Applying fertilizers and compost can make the soil healthy and more resistant to soil erosion
Q3 . Landslide
Q3.1 List and explain the classification of land slide according to types of movement
Ans:
The five kinematically distinct types of movement are described in the
sequence:
1. fall,
2. topple,
3. slide,
4. spread,
5. flow.
Combining the two terms gives classifications such as:
Rock fall, Rock topple, Debris slide, Debris flow, Earth
slide, Earth spread etc.
1.Falls are abrupt movements of masses of geologic materials, such as rocks and
boulders, that become detached from steep slopes or cliffs. Separation occurs along
discontinuities such as fractures, joints, and bedding planes, and movement occurs by
free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical
weathering, and the presence of interstitial water.
2.TOPPLES:
Toppling failures are distinguished by the forward rotation of a unit or units about some
pivotal point, below or low in the unit, under the actions of gravity and forces exerted by
adjacent units or by fluids in cracks.
1. SLIDES:
Although many types of mass movements are included in the general term “landslide,” the
more restrictive use of the term refers only to mass movements, where there is a distinct zone
of weakness that separates the slide material from more stable underlying material. The two
major types of slides are rotational slides and translational
slides.
A. Rotational slide:
This is a slide in which the surface of rupture is curved concavely upward and the slide
movement is roughly rotational about an axis that is parallel to the ground surface and
transverse across the slide).
B. Translational slide:
In this type of slide, the landslide mass moves along a roughly planar surface with little
rotation or backward tilting.
C. A block slide is a translational slide in which the moving mass consists of a single unit or
a few closely related units that move downslope as a relatively coherent mass.
A translational landslide that occurred in 2001 in the Beatton River Valley, British
Columbia, Canada. (Photograph by Réjean Couture, Canada Geological Survey.)
2. LATERAL SPREADS:
Lateral spreads are distinctive because they usually occur on very gentle slopes or flat terrain.
The dominant mode of movement is lateral extension accompanied by shear or tensile
fractures. The failure is caused by liquefaction, the process whereby saturated, loose,
cohesion less sediments (usually sands and silts) are transformed from a solid into a liquefied
state.
Sunset Lake, WASHINGTON, USA, 2001(triggering factor – earthquake)
Photo: S. Kramer
VARNES´ CLASSIFICATION OF SLOPE MOVEMENTS
5. FLOWS:
There are five basic categories of flows that differ from one another in fundamental ways.
a. Debris flow: A debris flow is a form of rapid mass movement in which a combination of
loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope.
Debris flows include <50% fines. Debris flows are commonly caused by intense surfacewater flow, due to heavy precipitation or rapid snowmelt, that erodes and mobilizes loose soil
or rock on steep slopes. Debris flows also commonly mobilize from other types of landslides
that occur on steep slopes, are nearly saturated, and consist of a large proportion of silt- and
sand-sized material.
Debris-flow source areas are often associated with steep gullies, and debris-flow deposits are
usually indicated by the presence of debris fans at the mouths of gullies. Fires that denude
slopes of vegetation intensify the susceptibility of slopes to debris flows.
VARNES´ CLASSIFICATION OF SLOPE MOVEMENTS
b. Debris avalanche: This is a variety of very rapid to extremely rapid debris flow.
c. Earth flow: Earth flows have a characteristic “hourglass” shape. The slope material
liquefies and runs out, forming a bowl or depression at the head. The flow itself is elongate
and usually occurs in fine-grained materials or clay-bearing rocks on moderate slopes and
under saturated conditions. However, dry flows of granular material are also possible.
d. Mud flow:
A mudflow is an earth flow consisting of material that is wet enough to flow rapidly and that
contains at least 50 percent sand-, silt-, and clay-sized particles. In some instances, for
example in many newspaper reports, mudflows and debris flows are commonly referred to as
“mud slides.”
VARNES´ CLASSIFICATION OF SLOPE MOVEMENTS
e. Creep:
Creep is the imperceptibly slow, steady, downward movement of slope forming soil or rock.
Movement is caused by shear stress sufficient to produce permanent deformation, but too
small to produce shear failure. There are generally three types of creep:
(1) seasonal, where movement is within the depth of soil affected by seasonal changes in soil
moisture and soil temperature;
(2) continuous, where shear stress continuously exceeds the strength of the material;
(3) progressive, where slopes are reaching the point of failure as other types of mass
movements. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted
poles or fences, and small soil ripples or ridges.
Q3.2 Explain the classification of landslide according to types of materials
Ans:
The material types used by the various schemes are Rock, Earth,
Soil, Mud and Debris, being classified as follows:
1. Rock: is “a hard or firm mass that was intact and in its natural place before the
initiation of movement”.
2. Soil: is “an aggregate of solid particles, generally of minerals and rocks, that
either was transported or was formed by the weathering of rock in place. Gases or liquids filling
the pores of the soil form part of the soil”.
3. Earth: “describes material in which 80% or more of the particles are smaller than 2mm, the
upper limit of sand sized particles”.
4. Mud: “describes material in which 80% or more of the particles are smaller than 0.06mm, the
upper limit of silt sized particles”.
5. Debris: “contains a significant proportion of coarse material; 20% to 80% of the particles are
larger than 2mm, and the remainder are less than 2mm”.
Q3.3 Write and explain the natural causes of landslide.
Ans:
the natural causes of landslides are:
1. Climate
Long-term climatic changes can significantly impact soil stability. A general reduction
in precipitation leads to lowering of water table and reduction in overall weight of soil mass, reduced
solution of materials and less powerful freeze-thaw activity. A significant upsurge in precipitation or
ground saturation would dramatically increase the level of ground water. When sloped areas are
completely saturated with water, landslides can occur. If there is absence of mechanical root support,
the soils start to run off.
2. Earthquakes
Seismic activities have, for a long time, contributed to landslides across the globe. Any moment
tectonic plates move, the soil covering them also moves along. When earthquakes strike areas with
steep slopes, on numerous occasion, the soil slips leading to landslides. In addition, ashen debris
flows instigated by earthquakes could also cause mass soil movement.
3. Weathering
Weathering is the natural procedure of rock deterioration that leads to weak, landslide-susceptive
materials. Weathering is brought about by the chemical action of water, air, plants and bacteria.
When the rocks are weak enough, they slip away causing landslides.
4. Erosion
Erosion caused by sporadic running water such as streams, rivers, wind, currents, ice and waves
wipes out latent and lateral slope support enabling landslides to occur easily.
5. Volcanoes
Volcanic eruptions can trigger landslides. If an eruption occurs in a wet condition, the soil will start
to move downhill instigating a landslide. Stratovolcano is a typical example of volcano responsible
for most landslides across the globe.
6. Forest fires
Forest fires instigate soil erosion and bring about floods, which might lead to landslides
7. Gravity
Steeper slopes coupled with gravitational force can trigger a massive landslide.
Q3.3 Write and explain the human causes of landslide.
Ans:
1. Mining
Mining activities that utilize blasting techniques contribute mightily to landslides. Vibrations
emanating from the blasts can weaken soils in other areas susceptible to landslides. The weakening
of soil means a landslide can occur anytime.
2. Clear cutting
Clear cutting is a technique of timber harvesting that eliminates all old trees from the area. This
technique is dangerous since it decimates the existing mechanical root structure of the area.
Q3.4 List the various factors that influencing the landslide (Triggering Factors).
Ans:
1.
Geological Factors
2.
Soil Engineering, Chemical, and Mineralogical Factors
3.
Geomorphic Factors
4.
Hydrologic Factors
5.
Vegetation Influences
6.
Seismicity
7.
Volcanic Activity
Q3.5 List and describe the effects of landslide.
Ans:
1. Lead to economic decline
Landslides have been verified to result in destruction of property. If the landslide is significant, it
could drain the economy of the region or country. After a landslide, the area affected normally
undergoes rehabilitation. This rehabilitation involves massive capital outlay. For example, the 1983
landslide at Utah in the United States resulted in rehabilitation cost of about $500 million. The
annual loss as a result of landslides in U.S. stands at an estimated $1.5 billion.
2. Decimation of infrastructure
The force flow of mud, debris, and rocks as a result of a landslide can cause serious damage to
property. Infrastructure such as roads, railways, leisure destinations, buildings and communication
systems can be decimated by a single landslide.
3. Loss of life
Communities living at the foot of hills and mountains are at a greater risk of death by landslides. A
substantial landslide carries along huge rocks, heavy debris and heavy soil with it. This kind of
landslide has the capacity to kills lots of people on impact. For instance, Landslides in the UK that
happened a few years ago caused rotation of debris that destroyed a school and killed over 144
people including 116 school children aged between 7 and 10 years. In a separate event, NBC News
reported a death toll of 21 people in the March 22, 2014, landslide in Oso, Washington.
4. Affects beauty of landscapes
The erosion left behind by landslides leaves behind rugged landscapes that are unsightly. The pile of
soil, rock and debris downhill can cover land utilized by the community for agricultural or social
purposes.
5. Impacts river ecosystems
The soil, debris, and rock sliding downhill can find way into rivers and block their natural flow.
Many river habitats like fish can die due to interference of natural flow of water. Communities
depending on the river water for household activities and irrigation will suffer if flow of water is
blocked.
Q4. List and explain the Effects of Soil erosion on different scenarios of land use and land cover.
(1)
Land Cover
1. Forests
Soil erosion in forests generally follows a disturbance such as road construction, a
logging operation, or fire. In undisturbed forests, erosion is most often due to epochal
events associated with fire cycles, landslides, and geologic gully incision.
Ground cover by forest litter, duff, and organic material is the most important
component of the forest environment for protecting the mineral soil from erosion. Forest
litter provides most of the nutrients needed for sustainable forestry.
2. Wetlands (Jim Ritter, 2012)
Soil erosion reduces cropland productivity and contributes to the pollution of adjacent
watercourses, wetlands and lakes. Soil erosion can be a slow process that continues
relatively unnoticed or can occur at an alarming rate, causing serious loss of topsoil
3. Impervious surface (Chithra S.V. 1, 2015) (Arnold, 1996)
1. Replenishment of groundwater through infiltration is important for maintaining a
base flow in streams throughout the summer. Impervious surfaces prevent much
of the rainfall from replenishing the groundwater and this can cause the water
table (the level of groundwater) to drop. During dry to normal conditions, a low
water table may cause streams to dry up so they can no longer support fish and
other aquatic species.
2. During storm events, the high velocity and high volume of runoff from
impervious surfaces can overcome the capacity of streams. This can cause stream
banks to "blow out" and erode the sides and bottom of the channel. Aquatic
habitat is lost and the resulting erosion carries sediment (sand, silt, mud)
downstream.
3. Excessive sediment from erosion can smother salmon spawning beds and bury
the habitat of bottom-dwelling plants and animals.
4. Toxic chemicals are often present on impervious surfaces, and are carried
directly into streams, wetlands and the ocean. For example: oils and gasoline are
leaked from vehicles; heavy metals are deposited from the atmosphere in
industrial areas; pesticides and fertilizers are washed out onto streets and
sidewalks.
4. Open water bodies (Ashraf, 2017)
It has led to increased pollution and sedimentation in streams and rivers, clogging these
waterways and causing declines in fish and other species. Due to soil erosion open water bodies
changes:
➢ Color
➢ Odor
➢ Increasing of sediments
5. Biodiversity (Fauna and Flora) (TEAM, 2016)
Due to soil erosion, reduce the fauna and flora of the region.
A. Longer ecosystem recovery time
When soil erosion has occurred, it can take much longer for an ecosystem to recover than if the soil
wasn’t so severely disturbed. Rebuilding healthy soil is estimated to take around 1,000 years to form
one inch of soil when left up to nature’s processes alone
If an ecosystem has been very degraded due to soil erosion, it will take a very long time for its
biodiversity to recover, if it even recovers in that area at all.
B. Loss of topsoil and soil fertility, reduced native plant species
With a loss of topsoil, there is also a loss of soil fertility, as well as a loss of the optimal soil
conditions for many native plants to grow.
C. Increased risk of flooding
Soil that has been heavily eroded does not tend to hold onto water very well. This can lead to an
increased risk of flooding, which can in turn lead to even more erosion of remaining soil, negatively
impacting the biodiversity of an ecosystem.
Flooded soils
D. Negative impacts on aquatic ecosystems and species
Soil erosion can lead to the soil running off into waterways, which increases the sedimentation of the
water, and makes aquatic ecosystems inhospitable for those organisms that require clearer waters for
their habitat. Such negative impacts to aquatic ecosystems can also ultimately impact any species
that prey on those aquatic species that are intolerant to turbid waters, potentially impacting the whole
food chain of entire ecosystems.
In addition to the increased sedimentation of waterways, the pollution of water bodies can occur
when the soil that is eroded is carrying any pollutants such as agriculture chemicals. These pollutants
can negatively impact aquatic ecosystems, as well as any organisms that rely on aquatic organisms
for food.
E. Increased risk of desertification
As soil is eroded, fewer plant species are able to live in the depleted soil that remains, and can
ultimately lead to increase in desertification.
Q5. Other EXERCISES
Q1(5marks: 15min). Define the followings:
(i) Soil erosion
Ans:
Soil erosion may be defined as ‘detachment, transportation and deposition of soil
particles from one place to another under influence of wind, water or gravity forces”
(ii) Tillage erosion
Ans:
Tillage erosion or translocation erosion is the redistribution of soil within a landscape
caused directly by tillage. For a range of landscapes, it has been shown that translocation
erosion is a potential contributor to the total soil erosion on cultivated fields (Govers,
1999)
(iii) Organic erosion:
Ans:
The soil erosion caused by living organism is called as called as organic erosion. This type of soil
erosion is fairly common despite being little known. In broad sense, it forms a part of the total
destructive geological activities. It is seldom regarded as an erosion phenomena.
Organic erosion is again divided into two forms (Kettner, 1950-1954) as:
(a) Phytogenic erosion and
(b) Zoogenic erosion
Phytogenic erosion is the destruction of soil, caused by plant’s root system. Sometimes, it is also
referred as root erosion. In this type of soil erosion, weathering process replaces the soil loss in the
field, caused by outside forces and harvesting (i.e. removal) of plant materials.
In zoogenic erosion, the soil particles are removed by the animals, particularly when they move from
one place to another either in search of foods or excavating for shelters. During excavation of
holes/shelters, a considerable amount of soil in loose condition is deposited around the hole which is
subsequently carried away by the action of wind or water. This type of soil erosion is commonly
observed around dam sites. In these localities, this erosion is likely to be intensified during
occurrence of floods.
(iv) Anthropogenic erosion:
Ans:
Anthropogenic erosion is a type of erosion associated with the activities of human beings which are
responsible to cause soil destruction, indirectly. The man’s indirect activities such as destruction of
natural vegetation like deforestation, shifting cultivation, cultivation of crops without soil protecting
measures, exposing the bare soil increasing and concentrating runoff and changing the quality of soil
(i.e. decreasing the humus content, impairing soil structure, reducing the level of nutrients,
diminishing the fertility etc.) are taken into account as source to cause soil erosion.
(v) Geologic erosion
Ans:
Geologic erosion is a natural (i.e normal) process of weathering that generally occurs at low rates in
all soils as a part of the natural soil-forming processes. It occurs over long geologic time horizons
and is not influenced by human activity (Lal H. B., 2008).
In its broadest sense it is a normal process, which represents the erosion of soil in its natural
condition without the influence of human being. Geologic erosion is sometimes also known as
natural or normal erosion. This type of erosion contributes to the formation of soils and their
distribution on the earth surface. This erosion is said to be in equilibrium with the soil forming
processes. The geologic erosion is a longtime eroding process. The various topographical features
such as existence of stream channels, valleys etc. are the results of geologic erosion.
Q2. (7marks: 30min). Describe the following geologic actions generated by a flowing water over
the land surface by which soil erosion takes place:
(i) Detachment
Ans:
This process depends upon the type of soil, OM, moisture, nature of detaching agent (water for this
case) (Wagley, 2009).
(ii) Hydraulic action.
Ans:
The hydraulic action can be explained as: when water runs over the soil surface, it compresses
the soil. As a result, the air present in soil voids exerts a pressure on the soil particles, which
leads to the soil detachment (Suresh, 2000). The pressure exerted by the voids air is called as
hydraulic pressure. The soil particles so detached from their places are scoured by the running
water. The hydraulic action is more effective especially when soil is in loose state condition.
(iii)Abrasion
Ans:
In this geologic action, the soil particles mixed in the running water, create an abrasive power in
water by which the capacity of flowing water to scour the soil particle gets increased. Due to
this effect, greater soil particles are eroded by flowing water. The river bank erosion and erosion
from bottom of the valley are the result of abrasion action of running water.
(iv) Attrition
Ans:
This action includes the mechanical breakdown of loads running with the moving water due to
collision of particles with each other. It can be expressed in other way that, big size rock
fragments, boulders and pebbles present in the moving water of streams or rivers are broken due
to striking with each other. The broken particles moving with water generate abrasion effect on
the bottom and banks of the water course. This effect pronounces the water erosion.
(v) Solution
Ans:
This process is associated with the chemical action between running water, soil and rocks. This
type of happening is observed in those areas, where existing rocks and soil are easily dissolved
in the water. Actually in this action, the soil and rock materials are dissolved in the running
water due to chemical action and are carried away by the water flow.
(vi) Transportation
Ans:
It is the process under which soil particles which are dissolved or suspended in the running
water are carried from one place to another.
(vii)
Deposition
Ans:
The soil which is eroded from the original location is always deposited somewhere else. This may be
close to its place of origin position or it may be deposited at the longest distance down in or at any
other point between the place of origin and the water course or in the reservoir or in the lake or in the
sea. (Wagley, 2009).
Q3. (14marks: 20min)
(i)
Explain briefly the effect of slope gradient on splash erosion (2marks)
Ans:
The soil detachment is independent of land slope, but soil splash is affected by slope steepness and
the direction of rainfall vector in relation to the slope direction. A greater amount of soil material is
splashed towards slope because of gravity force, compared to up slope. It reveals that, soil can move
down slope even if there is no runoff. However, by a windless rain falling on a flat soil surface, the
net movement of soil from the area is zero. In brief, if all factors being the same, the net splash
increases in slope steepness. The amount of soil carried down slope, mainly depends on slope
steepness, drop size, wind velocity and surface conditions. (Bryan, 1969) has reported that, increase
in proportion of material landing down slope from the raindrop impact varies with increase in slope
angle. He also developed following regression equation relating the down slope splash and slope
angle, as:
𝐷 = 0.079 + 0.028𝑆 + 0.0007𝑆 2
(𝑅 2 = .63). In which, D is the down slope splash and S is the slope angle.
(ii)
Explain the effect of the depth of overland flow on splash erosion (2marks)
Ans:
The depth of overland flow affects both to the ability of raindrop to cause soil detachment and
splash. The raindrop impact on runoff flow increases the detachment significantly, because
impacting raindrops create turbulence. A turbulent flow always increases the detachment and
spattering of soil particles. (Spatter = to drop small drops of liquids). The falling raindrops transform
the hydraulic patterns in sheet flow and generate local turbulence and retard the flow velocity. The
drop diameter, overland flow and soil detachment have close interaction with each other. (Mutchler
& Young, 1975) observed the maximum detachment of soil particles, when depth of overland flow
was between 1/3 rd and 1/5 th of drop diameter. (Park, Mitchekk, & Bubenzer, 1982) reported that,
splash increases slightly with increase in depth of overland flow to a critical water depth,
approximately equal to one drop diameter; splash then decreases sharply as water depth increases.
(Mutchler & Larson, Splash amounts from water drop impact on a smooth surface, 1971) Found that,
splash produced by raindrop impact varies with raindrop diameter (D) and depth of overland flow
(d). They conducted that, soil splash increased from 0 for d/D = 0 to a maximum of d/D = 0.14 and
d/D = 0.20 for D values of 0.559 and 0.296 cm, respectively, when water depth was equal to or more
than 3 D. Furthermore, in spite to have the effect on soil splash, the impacting raindrops also have
pronounced effect on transport capacity of overland flow. In this direction, (Walker, Kinnel, &
Green, 1978) Observed that, overland flow bleated by raindrops contains about 5 times greater soil
load to that overland flow, not impacted by raindrops for the same flow rate. The size of soil
particles, detached is also influenced by the same flow rate. The size of soil particles, detached is
also influenced by the overland flow. (Ellison, Studies on Raindrop Erosion, 1944) Reported that,
the pebbles were partially submerged in overland flow.
(iii)
Explain the effect of the canopy cover on splash erosion (3marks)
Ans:
The splash erosion is caused by impacting of raindrops on exposed soil surface. To reduce it, the
crop canopy play a key role, by intercepting the raindrops above before reaching them on soil
surface. Also, canopy intercepts the wind pulses and thus decreases the kinetic energy of raindrop.
The kinetic energy at the top of canopy is maximum. The ratio of kinetic energy of drop which
reached the soil to the maximum kinetic energy at the top of canopy, is called as relative kinetic
energy, can be calculated by using the following equation (Lal, 1990)
𝐸𝑐 = 1 −
𝐸𝑡 −𝐸ℎ
𝐸𝑡
𝐹𝑐
Where,
𝐸𝑐 = relative energy under the tree
𝐸𝑡 = kinetic energy per unit area of rainfall at a terminal velocity
𝐸ℎ = kinetic energy per unit area of rainfall dropping from the canopy height h
𝐹𝑐 = fraction of canopy cover.
The vegetative over close to the ground surface breaks the raindrop impact and dissipates its
kinetic energy more effectively than the tall canopies, because in taller canopies case the impacting
velocity is equal to the free falling terminal velocity of raindrops. Although they also intercept and
dissipate the energy of rain drops, but their relative kinetic energy is greater, which is observed very
less in case of closer canopies. (Finkel, 1986) have reported the terminal velocity of the raindrops,
falling from different fall heights, given as under:
Drop diameter
(mm)
Fall heights
(m)
Terminal velocity
(m/s)
1
2.2
4.03
2
5.5
6.49
3
7.2
8.06
4
7.8
8.83
5
7.6
9.09
6
7.2
9.18
From above values of terminal velocity for different fall heights, it is observed that, as fall height
increases the terminal velocity also increases. There is also a relationship between terminal velocity
and kinetic energy of rain drops
1
𝐾𝐸 = 2 𝑚𝑣 2 .
At the greater terminal velocity, the K.E. is always greater. And if K.E. of striking raindrop is
greater, then there would be more soil erosion; this is the reason that the closer canopies are more
effective to control soil erosion than the taller tree’s canopies.
Apart from canopy height, the density of canopy cover or vegetative cover also plays a significant
role to control the splash erosion. Greater the percentage of ground cover, more will be the erosion
control. The ground cover varies with crop stages, which affects the soil erosion, accordingly. The
research findings have revealed that, a 100% ground cover provides perfect erosion control even on
a steep slope of 35%. (Aina, lal, & Taylar, 1976) observed a significant correlation between
percentage ground cover and soil erosion. The soil erosion decreased exponentially with an increase
in vegetal cover. For different cropping the regression equation between percentage ground cover
and soil loss, is shown in table 3.6
Table 3.6. Relationship between vegetal cover and soil erosion (Aina, lal, & Taylar, 1976)
Cropping
Regression equation
Correlation coefficient (r)
Soybean- soybean
𝑌 = 5.38 𝑒 −0.04 𝑥
0.63
Pigeon pea- pigeon pea
𝑌 = 3.27 𝑒 −0.01 𝑥
0.94
Maize – cassava (mixed
𝑌 = 2.20 𝑒 −0.01 𝑥
0.84
cropping)
𝑌 = 2.17 𝑒 −0.01 𝑥
0.90
Cassava (monoculture)
(iv)
List various characteristics (at least 5) of raindrop. (5marks)
Ans:
(1) The raindrop size,
(2) shape,
(3) mass,
(4) velocity,
(5) size distribution and
(6) Its direction of fall
(v)
Fill with increases or decreases:
As soil particles size increases the detachability increases (2marks)
Or
As soil particles size decreases the detachability decreases (2marks)
Q4. (20marks: 20min) List various (at least 20) soil properties which affect soil detachability:
Ans:
(1) soil type
(2) soil mineralogy (soil minerals)
(3) Clay content
(4) Soil structure
(5) Soil texture
(6) Soil humus
(7) Soil nutrients
(8) Soil aggressiveness
(9) Soil pH
(10) Soil moisture
(11) Soil porosity
(12) Soil colour
(13) Soil salts
(14) Soil depth
(15) Soil compaction
(16) Soil cementation
(17) Soil strength/aggregates stability/ structural stability
(18) Soil chemistry
(19) Soil organic matter
(20) Biologic composition of soil (ex: earthworm activity)
(21) Organic fertilizer
(22) The entrapped air in soil voids
(23) Soil permeability
Q5. (5marks:10min). Give various (at least 5) indicators of sheet erosion in an agriculture field.
Ans:
(1) Change of soil colour
(2) Expansion of depressions size
(3) Presence of coarse particles on the soil surface.
(4) Enlargement of rills
(5) Exposure of unproductive subsoil
--------------------------------
CEE 3262 SOIL AND WATER RESOURCES PROTECTION
Course EXERCISES CH-III. SOIL EROSION MEASUREMENT
QUESTIONS
 Explain the erosion plot for predicting the soil loss.
 Explain the erosion pins for predicting the soil loss.
 Explain the universal soil loss equation for predicting the soil loss.
4
USLE, MUSLE and RUSLE equations
(vi)
What do you understand by USLE, MUSLE and RUSLE equations?
(vii) Define and give respective units of each of USLE parameters
(viii) State the assumptions of USLE, MUSLE and RUSLE equations
(ix)
State the limitations of USLE, MUSLE and RUSLE equations
(x)
Determine the soil loss from a certain hill by using USLE equation knowing the
following data:
• Mean annual rainfall is 916.44 mm
• Soil texture is Sandy loam, organic content = 2% and soil erodibility = 0.24
• Slope = 20%, slope length = 45m and slope length factor = 5
• Land use is agriculture: crop management factor = 0.128 and P-factor = 0.92
ANSWERS:
Q1. Explain the erosion plot for predicting the soil loss.
Ans:
A widely used method of quantifying surface erosion is to measure the
amount of soil that washes from bounded plots. In installing these plots,
collecting troughs are sunk along the width of the bottom of the plots with
walls of plastic, sheet metal, plywood, or concrete are inserted into the soil
surface to form the boundaries of the plot (Mutchler et al. 1994, Morgan
1995, Brooks et al. 2013). The collecting trough empties into a tank or other
container in which both the entrained soil particles and surface runoff are
collected. These containers are sometimes designed with recording
instruments so that the rates of flow can be measured. The total volume of
soil particles and water is measured after a rainstorm has occurred in other
cases.
Plots vary in size from micro-plots of 1 to 2 square meters to the standard
plot of approximately 2 meters by 22 meters used in applying the
universal soil loss equation. Micro-plots are 11 Soil Erosion and Sediment
Production on Watershed Landscapes: Processes and Control less
expensive and more practical than the use of rainfall simulators for
multiple comparisons of vegetation, soils, and land-use activities.
However, larger plots can provide more realistic estimates of erosion
because they better represent the cumulative effect of increasing volume
and velocity of surface runoff downslope. Plots larger than the standard
plot for the universal soil loss equation can yield large volumes of surface
runoff and soil particles that are difficult to store. Devices that split or
sample a portion of total water and sediment flow are preferred in these
cases.
Q2. Explain the erosion pins for predicting the soil loss.
Ans:
The insertion of pins into the soil can be used to estimate soil losses and
deposition that occur along the hill slopes of a watershed. Commonly, a
pin consisting of a long metal nail with a washer welded to the top of the
nail is inserted into the soil and the distance between the head of the nail
and the washer is measured. Soil erosion is measured by the distance
from the cap of the pin to the soil surface while deposition is measured by
the accumulation of soil on the top of the pin. The pins are re-set to be
flush with the soil surface after the measurements are taken to facilitate
subsequent measurements. A benchmark should be established in close
proximity to the stakes as a point of reference and stakes should be clearly
marked so that original stakes can be accurately relocated on subsequent
surveys (Chevesich, 2013).
Q3. Explain the universal soil loss equation for predicting the soil loss.
Ans:
Soil conservationists around the world use the Universal Soil Loss Equation
to estimate soil erosion rates by water. The equation provides an estimate
of the Soil Loss Rate in Tones/hectare/year. This estimate can be used for
soil conservation planning (Wischmeier W. H., 1965).
The Universal Soil Loss Equation is: A = KR (LS) CP
1. The soil erodibility factor (K)
The soil erodibility factor (K) indicates the susceptibility of soil to erosion. It
is expressed as the soil loss per unit of area per unit of R for a unit plot
(Wischmeier W. H., 1971). From the soil classification and organic content
of soil, we got the value K.
2. Rainfall factor (R)
The R value in the equation takes climatic conditions into consideration.
The rainfall factor can vary from year to year, so an average over a number
of years is usually used (Chevesich, 2013)
3. Length/Slope Factor (LS)
The length and slope factors vary according to the size and shape of
different fields. The standard factor is calculated based on a standard
length of 22 m and a 9 percent slope (copper, 1997).
4. Crop management Factor (C)
The cropping-management factor can vary according to farming practices.
This value includes the effects of cover, crop sequence, productivity
level, length of growing season, tillage practices, residue management,
and the expected time distribution of erosive rainstorms (copper, 1997).
5. Supporting practice
Another variable that can be altered is the conservation practice factor.
This is the ratio of soil loss for a given practice compared to simple
up and down the slope farming. Contouring is one practice which
involves field operations such as plowing, planting, cultivating, and
harvesting approximately on the contour. The P values obtained using
contouring varies according to the slope of the field (copper, 1997).
Q4. USLE equation
(i)
What do you understand by USLE equation?
• Universal soil loss equation
• USLE equation : (A) = K*R*(LS)*C*P
• USLE is one of the methods used to estimate the soil loss rate in ton/ha/year
(ii)
Define and give respective units of each of USLE parameters
Ans:
•
•
•
•
Soil erodibility (K factor): is defined as the resistance of soil to erosion. In other terms, it is
termed as resistance of soil to both detachment and transport against the detaching and
transporting agents.
(unit : tons/ha)
Rainfall erosivity (R factor): It refers to the intrinsic capacity of rainfall to cause soil
erosion. Water erosion would not occur if all rains were non-erosive. Since this is hardly the
case, knowledge of rainfall erosivity is essential to understand erosional processes, to
estimate soil erosion rates and to designing erosion control practices. Properties affecting
erosivity are: amount, intensity, terminal velocity, drop size, and drop size distribution of
rain.
(unit : MJ/ha/hr/yr or MJ/ha)
Slope length factor (SL factor): it is a single index which expresses the ratio of soil loss
under a given slope steepness and slope length to the soil loss from the standard condition of
5o slope, 22m long, for which LS = 1.0
(unit: Dimensionless)
Crop management factor (C factor): The cropping-management factor can vary according
to farming practices. This value includes the effects of cover, crop sequence, productivity
•
level, length of growing season, tillage practices, residue management, and the expected time
distribution of erosive rainstorms. For example, the approximate C value for a rotation with
corn-corn-oatsmeadow is 0.18 if good management is used.
(unit: Dimensionless)
Erosion-control practice factor: This is the ratio of soil loss for a given practice compared
to simple up and down the slope farming. Contouring is one practice which involves field
operations such as plowing, planting, cultivating, and harvesting approximately on the
contour. The P values obtained using contouring varies according to the slope of the field.
(Cooper, 1997.)
(unit: Dimensionless)
(iii)
Determine the soil loss of a certain hill by using USLE equation knowing the following
data:
• Mean annual rainfall is 916.44 mm
• Soil texture is Sandy loam, organic content=2% and soil erodibility = 0.24
• Slope = 20%, slope length = 45m and slope length factor = 5
• Land use is agriculture: crop management factor = 0.128 and P-factor = 0.92
Ans:
USLE: (A) = K*R*(LS)*C*P
R= 79+0.363P
P= 916.44 mm
R= 79 + 0.363*916.44 =411.67 MJ/ha/hr/yr
K= 0.24, LS= 5, C= 0.128, P= 0.92
A = 0.24*411.67*5*0.128*0.92= 58.17 tons/ha/yr
CEE 3262 SOIL AND WATER RESOURCES PROTECTION
Course EXERCISES CH-IV. SOIL EROSION AND SEDIMENT CONTROL METHODS
A. COMPREHENSIVE QUESTIONS
 Write and explain the main approach to soil conservation.
 List and explain the mechanical methods of erosion control.
 Describe the mulching measure checklist.
 Describe structural conservation practices.
 Describe the structural conservation practices checklist.
B. COURSE EXERCISES FOR SOIL EROSION CONTROL MEASURES
4.1 Grassed waterways
4.2 Bench terraces
4.3 Contour bunds
4.4 Check dams
4.5 Splash erosion
ANSWERS
A. COMPREHENSIVE QUESTIONS
Q1. Write and explain the main approach to soil conservation
Ans:
A. Cultivated lands
A risk of erosion exists on cultivated land from the time trees, bushes and grasses are removed.
Erosion is increased by attempting to farm slopes that are too steep, cultivating up-and-down hill.
Least protection of the soil is afforded by crops grown in rows, tall tree crops and low-growing crops
with large leaves. As a result, crops like maize, rubber, oil palm, grape vines, cassava and sugar beet
can all give moderate to serious erosion problems. Small grain cereals, such as wheat and barley,
afford better protection provided they are planted at sufficient density (Morgan R. P., 2005)
B. Grazing lands
Erosion problems arise when the protective cover of rangeland vegetation is removed through
overgrazing. Erosion control depends largely on the use of agronomic measures (Morgan R. P.,
2005). The Turkana in Kenya the Cattle are grazed in the lowlands immediately after the wet season,
taking advantage of the annual flush of grass, and then moved to slightly wetter hilly areas during
the dry season (Barrow, 1989)
C. Forest lands
Forests provide excellent protection of the topsoil against erosion. They maintain high rates of
evapo-transpiration, interception and infiltration and therefore generate only small quantities of
runoff. A low runoff rate produces low erosion rates. Increases in erosion occur where the land is
permanently or, in the case of shifting agriculture, temporarily cleared for agriculture and for
firewood (Morgan R. P., 2005). In Sudan, Colombia, Ethiopia, Nigeria and Indonesia, fuel wood
accounts for 80 per cent or more of annual timber removals. Afforestation schemes that include
rapid-growing tree species that can be cropped for firewood are therefore an important feature of
erosion-control strategies (Thorsteinsson, 1971)
D. Urban areas
Urban development frequently results in intensive erosion. The exposure of bare soil during the
construction phase results in higher volumes of peak runoff, shorter times to peak flow, higher and
more frequent flood flows and rapid increases in erosion by overland flow. Erosion control in the
final phase when urban development is complete requires rapid establishment of plant cover and
permanent use of purpose-designed waterways and embankment stabilizing structures (Morgan R.
P., 2005)
E. Road banks
Poor disposal of runoff is an important cause of erosion associated with roads. Runoff upslope of the
road should be collected in roadside drains or ditches, which are then led into culverts under the
road, usually discharging into existing valleys. It is common (Nyssen, 2002)
Q2. List and explain the mechanical methods of erosion control
1. Contour bunds
Contour bunds are earth banks, 1.5–2 m wide, thrown across the slope to act as a barrier to runoff, to
form a water storage area on their upslope side and to break up a slope into segments shorter in
length than is required to generate overland flow. They are suitable for slopes of 1–7° and are
frequently used on smallholdings in the tropics where they form permanent buffers in a strip
cropping system, being planted with grasses or trees (Hurni, 1984)
2. Terraces
Terraces are earth embankments constructed across the slope to intercept surface runoff, convey it to
a stable outlet at a non-erosive velocity and shorten slope length. They thus perform similar
functions to contour bunds. They differ from them by being designed to more stringent
specifications. Terraces can be classified into three main types: diversion, retention and bench.
Diversion terrace: The primary aim of diversion terraces is to intercept runoff and channel it across
the slope to a suitable outlet. Diversion terraces are not suitable for agricultural use on ground slopes
greater than 7° (Marshall, 1983).
1. Figure 1 shows the Diversion terrace (Morgan R. P., 2005)
Retention terraces are used where it is necessary to conserve water by storing it on the hillside.
They are therefore ungraded or level and generally designed with the capacity to store the runoff
volume expected with a ten-year return period without overtopping. These terraces are normally
recommended only for permeable soils on slopes of less than 4.5°
2. Figure 2 shows the retention terrace (Morgan R. P., 2005)
Bench terraces consist of a series of alternating shelves and risers and are employed where steep
slopes, up to 30°, need to be cultivated. The riser is vulnerable to erosion and should be protected by
a vegetation cover or faced with stones or concrete. Unprotected risers can be the source of most of
the erosion in terraced systems (Critchley, 1995).
3. Figure 3 shows the bench terrace (Morgan R. P., 2005)
3. Waterways
The purpose of waterways in a conservation system is to convey runoff at a non-erosive velocity to a
suitable disposal point. Three types of waterway can be incorporated in a complete surface water
disposal system: diversion channels, terrace channels and grass waterways.
Diversions are placed upslope of areas of farmland to intercept water running off the slope above
and divert it across the slope to a grass waterway. Terrace channels collect runoff from the interterrace areas and also convey it across the slope to a grass waterway. Grass waterways are therefore
designed to transport down slope the runoff from these sources to empty into the natural river system
(Morgan R. P., 2005).
4. Roof Runoff and Cisterns
Drainage from roofs and other surfaces can be collected and directed through gutters and pipes to
cisterns, tanks or small ponds and reservoirs. This water can be used for irrigation or other purposes
if of adequate quality. These storage facilities should be designed properly to ensure that hazards do
not exist. Pond sand reservoirs often require engineering design to ensure that failure does not occur
(Donald J. funk, 2005).
5. Catch basin or Siltation Pond
Siltation ponds function to detain runoff and trap sediment before allowing water to pass
downstream. Ponds can be excavated or formed by a combination of dam and excavation. In any
case, the outlet from the pond should be protected from erosion (Donald J. funk, 2005).
6. Velocity or Energy Dissipater
7. A velocity dissipater is used to slow movement of water, and by doing so, reduce its
capacity to carry soil. Velocity dissipaters are commonly used at the end of a down
drain. A velocity dissipater can be anything from a concrete box with a spillway to a
pile of concrete rubble and boulders. It should provide permanent protection (Donald
J. funk, 2005).
8.
Vegetative Measures
Maintaining a vegetative cover on the soil surface protects against the energy of rainfall impact and,
therefore, reduces the surface erosion. The plants also increase the roughness of the soil surface that
increases the torturousity of the flow path and reduces the velocity (energy) of surface runoff. Soil
erodibility is also reduced by the occurrence of a network of plant roots that enhance soil strength
and improve soil structure through the addition of organic matter (Chevesich, 2013).
9. Mulching measure
Mulching is important for effective water control and microclimate creation, Purpose: To provide
immediate protection to exposed soils during the period of short construction delays, or over winter
months through the application of plant residues, or other suitable materials, to exposed soil areas.
Mulches also enhance plant establishment by conserving moisture and moderating soil temperatures.
Mulch helps hold fertilizer, seed, and topsoil in place in the presence of wind, rain, and runoff and
maintains moisture near the soil surface. In addition to stabilizing soils, mulching can reduce the
speed of storm water runoff over an area (Franklin, 2003).
Q3. Describe the mulching measure checklist
Ans:
No Mulching
Purposes
Pictures
checklist
1
Buffer zone
❖ To
Reduce
storm
runoff
velocity
❖ Flood protection
❖ Protect channel banks from
scour and erosion (Franklin,
2003).
2
Coastal dune
4. Figure 4 buffer zone measure
❖ To stabilize soil on dunes
stabilization
allowing them to become
(with
more resistant to wind and
vegetation)
waves. (Benton Ruzowicz,
2014)
5. Figure
5
coastal
dune
stabilization
3
Disturbed
Using plant residues or other suitable
area
materials on the soil surface to
stabilization
reduce runoff and erosion, conserve
with mulching moisture, prevent soil compaction
only
and modify soil temperature (Benton
Ruzowicz, 2014).
6. Figure
6
Stabilization
disturbed area with mulching.
4
Temporary
A temporary vegetative cover with
seeding
fast growing seeding for disturbed
area (Benton Ruzowicz, 2014).
of
Purpose: To improve aesthetics and
to improve infiltration and aeration.
5
Permanent
A permanent vegetative cover such
vegetation
as: trees, shrubs, vines, grasses are
7. Figure 7 Temporary area
stabilization
planted for the purposes: To protect
the soil surface from erosion, and to
reduce damage from sediment and
8. Figure
8
Permanent
runoff to down-stream areas
vegetation
area
stabilization
(Benton Ruzowicz, 2014)
6
Sodding
A permanent vegetative cover using
sods on highly erodible or critically
eroded lands (Benton Ruzowicz,
2014). Purpose: Establish immediate
ground cover, Reduce runoff and
9. Figure
erosion; improve aesthetics and land
stabilization (with sodding)
9
Disturbed
area
value, and Reduce dust and
sediments.
Q4. Describe structural conservation practices.
Ans:
In some instances, vegetative cover and mulches alone will not provide sufficient protection from
the erosive forces of water. In such cases, alternate structural practices can be used to curb erosion
and sedimentation during land-disturbing activities. These practices should be planned and employed
in a practicable combination with vegetative and mulching measures (Benton Ruzowicz, 2014)
Q5. Describe the structural conservation practices checklist.
Ans:
No Structural
conservation
Description and Purposes
Pictures
practices
checklist
1
Check Dam
To minimize the erosion rate by
reducing the velocity of the storm
water in areas of concentrated flow
(Benton Ruzowicz, 2014)
10. Figure 10: Stone check dam
2
Channel
In certain instances on selected
stabilization
development, it will be found that
11. Figure
existing channels will not be adequate
Channel stabilization
11
shows
the
to convey desired discharges. New
channels may be required to eliminate
flooding. (Benton Ruzowicz, 2014)
3
Construction
A stone-stabilized pad located at any
exit
point where vehicular traffic will be
leaving a site onto a public right-of
way, street, roadway, or parking area.
Its purpose is to reduce or eliminate
transportation of soil from the
construction area onto public rights-of 12. Figure 12: Crushed stone
way (Benton Ruzowicz, 2014).
4
Stream
A temporary channel diverts a stream
Diversion
around a construction site to protect
Channel
the stream bed from erosion and allow
Construction Exit
working in the dry area. This is used
where the linear projects such as
roads that frequently cross and impact
live streams and create a potential for
excessive sediment loss by both the
disturbance of the approach areas.
13. Figure
13:
Stream
Diversion Channel (Perspective
View) (Benton Ruzowicz, 2014)
5
Diversion
An earth channel with a compacted
supporting ridge on the lower side,
14. Figure
constructed above, across, or below a
Diversion measures
14
shows
the
slope. The purpose of this practice is
to reduce slope lengths, break-up
concentrations of runoff and move
water to stable outlets at non-erosive
velocities.
6
Temporary
A flexible conduit of heavy-duty
down drain
plastic or other material used as a
structure
temporary structure to convey
concentrations of storm water down
the face of a cut or fill slope. They are
removed once the permanent water
7
disposal system is installed (Benton
15. Figure 15 Temporary down
Ruzowicz, 2014)
drain structure
Permanent
A paved chute, pipe or a sectional
down drain
conduit of prefabricated material
structure
designed to safely conduct surface
runoff from the top to the bottom of a
slope. Down drain structures are to be
used where concentrated water will
cause excessive erosion of cut and fill
slopes (Benton Ruzowicz, 2014).
16. Figure
Permanent
structure
8
Filter ring
A temporary stone barrier used in
conjunction with other sediment
control measures and constructed to
reduce flow velocities and filter
sediment. A filter ring can be installed
16
shows
down
the
drain
at or a around devices such as inlet
17. Figure 17 shows the Filter
sediment traps, temporary down drain
ring
inlets (Benton Ruzowicz, 2014).
9
Sediments
A temporary structure constructed of
barrier
silt fences, straw or hay bales, brush,
logs and poles, gravel or other
filtering materials. They are installed
to prevent sediment from leaving the
site or from entering natural drainage
ways or storm drainage systems
(Benton Ruzowicz, 2014).
18. Figure
18
shows
the
shows
the
Sediment barrier
10 Temporary
Sediment basin
A basin created by an embankment or
dam containing a principal spillway
pipe and an emergency spillway.
They are used to trap sediment
contained in runoff water. Sediment
basins serve only during the
construction phase and are removed
from the site when the disturbed area
19. Figure
has been permanently stabilized
Temporary sediment basin
(Benton Ruzowicz, 2014).
11 Erosion control
fences
These are simple low wire netting and
jute geotextile fences with a thick
mulch layer that can slow and trap
runoff water and become a productive
vegetated belt across degraded veld or
stabilize small dongas and drainages
(Stroebel, 2011).
20. Figure 20
19
B. COURSE EXERCISES FOR SOIL EROSION CONTROL MEASURES
4.1 Grassed waterways
4.2 Bench terraces
4.3 Contour bunds
4.4 Check dams
4.5 Splash erosion
ANSWERS:
Q1. Grassed waterways (20marks)
(a) Calculate the peak runoff rate expected to occur once in 10-years for design of a grassed
waterway to dispose from the watershed area of 35 ha. The other details are given as under:
1. Time of concentration of the watershed is 30 minutes
2. Runoff coefficient of the watershed for given characteristics is 0.40
3. The maximum rainfall recorded during 10-years duration is 6.00 cm in a storm duration of 30minutes.
(b) Using the peak runoff, determine the flow depth of a trapezoidal grassed waterway with
following details:
1. Side slope
= 4H:1V
2. Bottom width
=5m
3. Slope of waterway = 2%
4. Manning roughness coefficient: n = 0.04
4. Assume V = 1.50m/s
(c)Using the peak runoff, design a most economical trapezoidal grassed waterway with the following
data:
1. Side slope
= 4H:1V
2. Slope of waterway = 2%
3. Manning roughness coefficient: n = 0.04
4. Assume V = 1.50m/s
Ans to this exercice as ASS: ….
Q2. Bench terraces (20marks)
A 15% hilly land is proposed for constructing the bench terrace using 2.5 m as a vertical interval and
1:1 as a batter slope. Determine:
(v) The width of this bench terrace
(vi) Its length per hectare
(vii)
The earthwork per hectare
(viii)
The percentage of area lost.
Ans to Q2
1. Width of bench terrace, W =
D(100−S)
S
2.5(100−15)
=
2. Length of bench terrace/ha =
10,000
15
W+VI
10,000
= 14m
=14+2.5 = 606.06 m
3. Earthwork/ha
1
2.5
= 2 ∗ 7 ∗ 2 ∗ 606.06 m3
= 2651.15 m3
4. Area lost (%)
𝐒 + 𝟐𝟎𝟎
AL = 𝟐𝟎𝟎
𝐒
=
+
𝐒
𝟏𝟎𝟎
𝟏𝟓 + 𝟐𝟎𝟎
𝟐𝟎𝟎 𝟏𝟓
+
𝟏𝟓 𝟏𝟎𝟎
= 215/(0.15+13.33)
= 15.95 %
Q3. Contour bunds (20marks)
Calculate the total length and earthwork of contour bund per hectare, which is constructed on 5%
land slope. The bund’s spacing was maintained as 25 m. The specification of bund is given as under:
• Top width = 50 cm
• Bottom width = 125 cm
• Height = 100 cm
• The lateral and side bunds are also formed in field.
Ans to Q3:
(i)
Computation of the length of the main contour bund per hectare:
10,000
L=
HI
=
10,000
25
= 400m
(ii)
Length of side and lateral bunds = 400 x 30/100 = 120 m
(iii)
Total length of the contour bund per hectare = 400 + 120 = 520 m
(iv)
Earthwork of the main contour bund per hectare:
0.50 + 1.25
Em =
x 1.0 x 400
2
= 350 m3
(v)
Earthwork of side and lateral bunds:
0.50 + 1.25
= 120 x
x1
2
= 105 m3
(vi)
Total earthwork of the contour bund per hectare :
Et = Em + (Es + El )
= 350 + 105
= 455 m3
Q4. Check dams (20%)
Before designing a check dam, it is necessary to calculate the maximum runoff discharge that should
pass through the spillway of the check dam. The following data are given:
(5) Watershed area : A = 1.85Km2
(6) Catchment slope : S = 0.004
(7) Maximum length of flow : L = 1.15 Km
(8) Runoff coefficient : K = 0.35
e. What do you understand by check dam?
f. Estimate the time of concentration of the basin
g. Estimate the average intensity of rainfall
h. Estimate the 25 years flood if rainfall is given by:
Where i(mm/hr); Tr(years) and t(min).
Ans to Q4: Before designing a check dam, it is necessary to calculate the maximum runoff discharge
that should pass through the spillway of the check dam. The following data are given:
• Watershed area : A = 1.85Km2
• Catchment slope : S = 0.004
• Maximum length of flow : L = 1.15 Km
• Runoff coefficient : K = 0.35
(i) What do you understand by check dam?
Ans: Check dam = a small sometime temporary dam constructed across a drainage ditch or a
waterway to prevent erosion by reducing the velocity of the flowing water
(ii) Estimate the time of concentration of the basin.
Ans: t c = 0.0195 L0.77 S −0.385; where L = 1150m, S = 0.004 and t c (min)
Thus t c = 37.2 min
(iii)Estimate the average intensity of rainfall.
Ans: i =
1000 Tr 0.2
(t+20)0.7
; where i(mm/hr); Tr = 25years and t = tc = 37.2min.
Thus i = 11.205 cm/hr
(iv) Estimate the 25 years flood for this rainfall:
Ans: Qp =
KiA
36
; where K = 0.35; i = 11.205 cm/hr; A = 185 ha and Qp (m3/s)
Thus Qp = 20.153 m3/s
Q5. Splash erosion (20%)
c. Raindrops of 2.0 mm diameter are falling on an erodible soil surface. Compute the impact
energy of each drop. Assuming standard atmospheric conditions of 20oC temperature and
101.3 KPa air pressure.
Ans to Q5a:
(i)
The raindrops strike the soil with their kinetic energies. The K.E. is given by:
1
KE = 2 mV
2
(ii)
In which, V stands for terminal velocity of falling drops, which is given as
4 x 9.81 x 0.002 998
Vt = √
3 x 0.517
( 1.2 − 1) m/s
= 𝟔. 𝟒𝟖 𝐦/𝐬
π
(iii)
Mass of raindrop = Volume x Density = (6 ) x (0.002)3 x 998 = 𝟒. 𝟏𝟖 𝐱 𝟏𝟎−𝟔 𝐤𝐠
(iv)
KE =
1
2
x 4.18 x 10−6 x (6.48)2 = 𝟖. 𝟕𝟕 𝐱 𝟏𝟎−𝟓 𝐤𝐠. 𝐦𝟐 𝐒 −𝟐
d. Calculate the amount of soil eroded by splashing from a plot of 1.5 x 1.8 m2 size. The other details
are as follows:
g = 9.81 m/s2
γ = 1.5 t/m3
i = 2.8 mm/min
t = 30min
Vc = 5.49 m/s
dk = 3.5mm
ddk = 0.2 mm
sin α = 0.1
Where, q D = quantity of soil eroded by splashing (t/ha)
γ= density of soil saturated with water (t/m3)
i= rainfall intensity (mm/min)
Vc = terminal velocity of raindrop (m/s)
t= duration of rainfall (min)
dk = mean diameter of raindrops (mm)
ddk = diameter of rain drops (mm) of the critical size below splash erosion does not occur.
Vcd = critical velocity of falling rain drops, below which there is no appreciable soil damage.
Use the following equation:
0.13γ. i Vc2
dk . Vc
qD =
.t (
− 1) 4 sin α
2. g
ddk . Vcd
Ans to Q5b:
(i)
Computation of critical velocity of falling raindrops below which there is no appreciable soil
damage, using the following formula, assuming M = 0.3 and C = 0.05 kg/cm.
0.7MC
Vcd = √
=√
ρ
0.7 x 0.3 x 0.05
1.2 x 10−6
= 5.49 m/s
(ρ = 1.2 x 10−6 kg. S 2 /cm4 )
(ii)
Quantity of soil eroded by splashing, using following formula,
qD =
=
0.13γ.i V2c
2. g
dk .Vc
. t (d
dk .Vcd
− 1) 4 sin α
0.13 x 1.5 x 0.00004667 x (5.49)2
2 x 9.81
0.0035 x 5.49
x1800 x (0.0002 x 9.81 − 1) x 4 x 1
= 0.958 ton/ha
γ= density of soil saturated with water (t/m3)
i= rainfall intensity (mm/min)
Vc = terminal velocity of raindrop (m/s)
t= duration of rainfall (min)
dk = mean diameter of raindrops (mm)
ddk = diameter of rain drops (mm) of the critical size below splash erosion does not occur.
Vcd = critical velocity of falling rain drops, below which there is no appreciable soil damage.