Lecture notes
on
Soil conservation Engineering
and
Watershed management
for
Bsc - 3rd Year Students
Kathmandu Foretry College
( KAFCOL)
(For the use of Kathmandu Forestry College Students only)
1
Course Tittle : Soil Conservation Engineering and Watershed Management
Unit 1. Lands Degradation Problems in Nepal
1.1
1.2
Introduction to land degradation and its consequences
Water Erosion
Erosion is a process of detaching soil particles from the land surface of one
place and their transportation and deposition to another place.
Three Processes of Erosion :
1. Detachment
Process depends upon type of soil, OM, moisture, nature of
detaching agents (energy).
2. Transportation
Process depends upon size, density and shape of detached materials
and velocity of the transporting agent.
3. Deposition
Soil that is eroded from the original location is always deposited
somewhere else. This may be close to its place of origin position, it
may be the longest distance down to the sea or at any point between
the place of origin to the sea.Process depends upon soil particles and
velocity of the agent.
Example:
-- Coarse sand particles in eroded soil move the shortest
distance and deposit first.
-- Fine sand and silt deposit next as run-off water slows
down.
-- Some very fine silts settle out only in standing water.
-- Very fine clay and colloidal humus will not settle out even
in standing water but stay suspended in the water
indefinitely
2
1.2.1 Types of water erosion
 Geological (Natural/ normal)
Geological ( Natural/ normal ) erosion are caused by :
action of water, geology, wind, temperature, gravity,
glaciers, earth quakes.
Examples are : Naturally wearing away of hills and
mountains: sculptured hills/ mountains, canyons/gorge ,
stream channels, deltas etc.
 Man-made ( Accelerated )
Man-made ( Accelerated erosion are caused by : human or
anthropogenic activities. Change in land use, destruction
of natural cover and soil conditions are main elements
responsible for accelerated erosion.
Agents responsible for Soil Erosion are : Water, Wind and Gavity
Types of Erosion by Agents (Water induced ) :
Main types of erosion
1. Rain Drop / Splash Erosion. Rain drops falling into ground cause
splash, detach, carry water soil particles with small film of water into
air in horizontal/vertical directions. The splash soil particles may move
as much as 2 ft. into the air and simultaneously more horizontally as
much as 5 ft on level surface. On level surface, the splashed soil
particles scatter all over the land surface. The smashed up soil particles
disperse and cause surface sealing and reduces infiltration capacity of
land surface. Besides, the impact of kinetic energy of raindrops on land
surface compacts the soil and further help reduce infiltration capacity
of soil. These activities of raindrops increase the runoff, which starts
moving down to the slope due to gravitational force causing surface
erosion.
The bigger the size of rain drops, the greater the kinetic energy and
greater will be the rain drop erosion. Direction and strength of wind
velocity deflect the rain drops and reduces their kinetic energy and
there by reduces rain drop erosion.
3
(Figure : Splash of soil particles by raindrop erosion on a level and
sloping surface)
2. Sheet Erosion ( Inter-rill erosion). When the rate of rainfall exceeds
the rate of infiltration of water into the soil, water starts to flow over
sloping surface carrying detached particles in a form of thin sheet or
layer. They carry organic matter and fertile top soil. Such a removal of
more or less uniform thin layer or sheet of soil from a given area of
land surface by the action of rain drop is called sheet erosion or interrill erosion. The appearance of light color soil on the land surface,
after the removal of thin layer of dark color soil rich in organic matter,
is the sign of sheet erosion after rain fall events. The sheet erosion
cause loss of organic matter, expose light color sub-soil and eventually
cause loss in productivity of surface soil.
The sheet erosion occurs in smooth and uniform sloping surface and it
removes only the top layer of soil. It is a thin surface flow rather than
runoff. Sheet erosion is a function of depth and velocity of run-off for a
given size, shape and aggregates of soil particles.
Sheet erosion is better termed as "inter-rill" erosion which means
detached soil particles by raindrop are moved and transported by thin
surface flow of water, the capacity of which to sheet erosion is
increased by raindrop impact turbulence.
3. Rill Erosion. As discussed earlier, sheet erosion occurs mainly on the
smooth and uniform sloping land. When the land surface is irregular,
the irregularity of land surface force the flowing water to flow through
a certain well defined direction, which then form small channels called
rills. So rill erosion is a removal of soil by water from small but well
defined channel. The size of rills vary from minute channels to a size
4
that is easily observed. Rills can easily be broken by tilling operations.
Detachment and transportation of rill erosion are always greater than
sheet erosion. In rill erosion, the detachment of soil particles are from
energy of flowing water and not by the raindrop impact as in the case
of sheet erosion. Detachment of soil particles by flowing water in rill
erosion (D) varies with square of velocity of flowing water ( V2 )and
ability of flowing water to transport soil particles (T) varies with fifth
power to the velocity of flowing water (V5). If continues, can extend
up to sub-soil.
D ∞ V 2 and T ∞V 5
4. Gully Erosion (Channel erosion). Gully erosion is an advanced form of
rill erosion. It is a channel erosion that cuts so deeply into the soil that
the ground cannot be smoothed out by ordinary operations. It often
follows sheet and rill erosion. If rill erosion is unchecked and allow
water to move continuously in the channel developed by rill, then
gully erosion occurs. Gully erosion can be of V- shaped, if the soil
strata at the bottom of gully is strong and gully slopes are weak to
erosion and U-shaped gullies are formed, if the soil strata at the slopes
of gully are strong and bottom are weak to erosion. Rate and extent of
gully erosion is related with amount and velocity of run-off water.
( Figure : gully erosion at a gully head)
5
Stages of water erosion:
1st. stage = rain drop erosion
2nd. stage = sheet erosion
3rd. stage = rill erosion
4th. erosion = gully erosion
Dimension of gully erosion :
Rill erosion
1 ft. deep
Small gully
> 1 -- .3 ft. deep
Medium gully
> 3 – 13 ft. deep
Large gully
> 13 -- 30 ft. deep
Ravines
> 30 ft. deep
Figure : Different form of erosions
6
5. Stream bank Erosion: This is erosion caused by scoring and cutting of
river bank due to flooding of river.
Some other types of erosion :
-- Wave erosion
-- Rock erosion
-- Waterfall erosion
-- Tunnel erosion
-- Pothole erosion
-- Snow slide
1.2.2 Agents active in water erosion
Agents active in water erosion are : Rainfall intensity and pattern, soil
structure and soil erodibility, type and density of vegetation, slope factor,
slope length factor, slope gradient factor, cropping management factor
and erosion control practice factor
Factors Affecting Water Erosion are :
1. Vegetation : vegetation plays important roles in water erosion.
It has great importance in controlling erosion because of the
following reasons :
i)
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.
Lands without vegetation will be vulnerable to
raindrop, sheet and gully erosions.
ii)
Vegetation improves the soil structures by adding
organic matter into the soil. High organic matter
content soil will be more pores and, which in turn helps
increase the infiltration and water holding capacity of
soil. Further, the humus layer itself acts as a sponges
and absorbs moisture and allows it to enter into the
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soil. Soil having low organic matter and humus layer are
susceptible to erosions because of increased runoff.
iii)
In the vegetative areas, the root network, the channels
of root decay, animal burrows help dissipated runoff
water.
iv)
The root systems below the soil surface binds and
aggregates the soil through the mechanical actions and
prevent from erosion and landslide.
v)
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
2. Soil : Soil particles and their sizes are important factors for soil
erosion. The detachability and transportability of soil in the
erosion process increase or decrease based on kind and size of
soil particles. In fact, detachability of flowing water or runoff
decrease with increase of size of soil particles and
transportability of flowing water or runoff increases with
decreases of soil particle size. For example, the clay particles
difficult to detach than sand but easier to transport. Soil with
large stable particles such as sand grains or iron cemented soil
particles are difficult to detached and transported, which
seldom erode.
Infiltration capacity of soil play important role in soil erosion.
Infiltration which is indirectly affected by permeability of
different soil horizon is factor governing runoff and then to
erosion. Soil erosion by water occurs when there is runoff or
overland flow. When rainfall intensity exceeds the infiltration
capacity of soil then runoff or overland flow occurs, which
causes erosion. If the infiltration capacity of soil is higher than
the intensity of rainfall, then the runoff or overland flow will
be lower and less erosion occurs.
3. Climate : In climate, the rainfall and teh temperature play
important role in erosion. There is a direct relationship
between the amount of rainfall and erosion. Rainfall intensity
8
influences both the rate and volume of runoff and then to
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.
4. Physiography : Slope steepness is one of the important factors
in 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, where surface detention of water is
low and breaking of rain drop energy cannot form in 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 ( erosion ) and slope length,
which states that erosion ( E ) is approximately equal to the
square root of the slope length (L1/2 ) . Soil loss varies with
square root of slope length.
Slopes facing south gets more sun directly than the slopes of
other aspects. Because of high radiation and temperature in
southern aspects, soils of southern aspects are lower in
9
organic matter than those facing in northern slopes. Because
of the low organic matter content in soil and sparse vegetation
cover in southern aspects, the southern aspects are more
susceptible to erosion than other aspects.
Activities of water, which are responsible for erosion
1.
2.
3.
4.
Water-flow
Lubrication of slope layers
Saturation of soil
Weathering and dissolving of minerals
1. Water-flow = Physical force of water-flow (run-off) causes flow-erosion (
sheet erosion, rill erosion, gully erosion ). Flow-erosion is proportional to
water depth ( channel ) and slope gradient and inversely proportional to
size or strength of the eroded materials. Flow erosion starts when flow
energy surpasses slope resistance and increase with increasing run-off. The
bigger the water flow, the bigger the flow- erosion.
2.Water- lubrication = It causes slide- erosion on slopes. Water seepage
and infiltration reduces friction between soil and rock or between two
rocks layers. The bigger the seepage/ infiltration, the more likely the
occurrence of slides. Seepage/ infiltration of rail increases more with
duration than intensity. Heavy rains of long duration produce more slides
than short rain of very high intensity. Yield of underground sources like
springs also increases more with high duration of rain than intensity also
helps increasing slides. Heavy permeable strata on top almost impermeable
ones provide for maximum lubrication ones provide for maximum
lubrication of the slide plane and are very susceptible to sliding.
3.Water- saturation = Water- saturation of slopes of deep soil leads to the
reduction of angle of repose and liquification of the soil and to slope failure.
Highly permeable layers form "pressure tanks" on top of soil layers of little
permeability. The soil layers near the surface of little permeability gradually
saturates and loose strength while the weight of the pressure tanks
10
increases. When the soil layers near the little permeability strata` becomes
weak to support the increasing weight of permeable layers slope failure
occurs.
4.Weathering = It converts rock into gravel or soil, reduces slope strength
and supports erosion of any form. Dissolving of easily soluble minerals
leads to the formation of under-ground cavities and later by their breaking
in leads to erosion
1.2.3 Water Erosion Prediction Equation (universal soil loss equation,
USLE )
The universal soil loss equation ( USLE ) is :
A = R K LS C P where as,
A = Soil loss per unit area, it is an estimate of soil loss from sheet and rill
erosion from rain fall events. It does not include the estimation from
stream bank erosion, landslides, gully erosion, snowmelt erosion.
R = rainfall factor, K = soil erodibility factor, L = slope length factor, S =
slope gradient factor, C = cropping management factor, P = erosion control
practice factor.
R (rainfall factor) = This is determined from the product of the kinetic
energy of an individual storm (E) times the maximum 30-minute intensity
for the storm (I), that is EI. Summation of all storms in a year is the annual
value of the R factor. E is calculated according to:
E = 916 + 331 (log I) foot-tons/acre/inch of rain and I in inches/hour, this in
metric unit is E = 210.1 + 89( log I) joules/ sq.m/cm of rain and I in
cm./hour.
K (Erodibility factor) is determined from a nomograph as illustrated in the
figure.
11
Erodibility factor is the susceptibility of soil to erosion. So it is a function of
soil containing percent of silt, fine sand, organic matter, soil structure, soil
permeability.
Figure : Nomograph to determine K factor
Slope Length Factor ( L ) = It refers to overland flow or run-off from where
it originates to and where deposition begins. Maximum slope lengths are
seldom longer than 600ft. or shorter than 15 to 20 ft. However, it requires
judgments and on-site inspection.
The Slope Length Factor, L = ( ^ / 72.6 ) m ( feet)
L = ( ^ / 22.1 ) m ( meter )
12
where as, ^ = field slope length (feet, meter)
m = 0.3 for very long slopes gradient less than 0.5 %
= 0.6 for very long slopes gradient more than 10 %
= 0.5 applicable in most cases ( average value)
Slope gradient factor (S) = It is a ratio of soil loss from a given slope
steepness to that from a 9% slope under the same condition . It will be
evaluated as: ( 0.43 + 0.30 s + 0.043 s 2 ) / soil loss from 9 % slope under the
same condition, where as s = slope gradient (%) .
Cropping Management Factor (C) = It is a ratio of soil loss from the
condition of interest to that from tilled continuous fallow . This is an
integration of several factors that affects erosion, vegetation ( plant
canopy), binding effects of plants roots, soil surface, cropping patterns,
plant residue, land use/ land management etc. C factor describes the total
effects of vegetation, plant residue, soil surface and land management on
soil erosion The value of C factor is not constant over the year. Therefore,
the value of C factor need to be determined experimentally.
Erosion Control Practice Factor (P) = It is a ratio of the soil loss with the
control measures ( Ex : contouring, strip cropping, terracing, etc. ) to that
with farming up and down the slope. The value for P are usually based on
judgment and experience obtained from non-crop situation.
Modified equation is :
A = R K L S VM, where as,
VM = the vegetation management factor which is the ratio of soil loss from
land manage under specified condition to that from the fallow condition on
which the K factor is evaluated.
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1.3 Wind Erosion
1.3.1 Factors causing wind erosion
Wind acts on the soil surface the same way as flowing water. Gusty winds
are able to dislodge small soil particles, lift them upward and carry them
away.
Wind can move larger soil particles by making them jump along the ground.
The jumping particles also apply energy to the soil surface each time they
hit the ground and dislodge other particles so that they too can be moved
by the wind. This process is called saltation.
The largest soil particles that can be moved by wind are about 1 mm. Very
fine clay and silt particles ( less than 0.02 mm ) are lifted into the air and
carried away as wind blown dust. Sand dunes are severe stage of wind
erosion.
The erosive power of wind, as that of water, increases exponentially with
velocity, but is not affected by gravity. The erosive power of wind in wet
soil condition and vegetation covered soil is slow.
Slope inclination is not a factor in wind erosion, except where slopping
terrain may form barriers to wind. However, the length of unobstructed
terrain over which the wind flows is important in allowing the wind to gain
momentum and increase erosive power. Winds with velocity less than
about 12 to 19 km/ hr. at one meter above the ground not quite often
dislodge and put into motion sand size particles.
Potential average annual soil loss (tons / acre / year) is a function of
I = soil erodibility
K = soil roughness factor
C = Average wind velocity and surface soil moisture
L = field length
V = vegetative cover
14
Symbolically, the average annual soil loss due to wind, E =ƒ( I K C L V
)tons/acre/yr.
Wind erodibility Soil Group:
Soil
Group
Predominate Soil classes
Dry soil
aggregates (
%)
Erodibility
T/ A/ Year
1
very fine, fine, medium sand/ dune
sand
3
220
2
loamy very fine, fine, medium sands
10
134
3
fine, medium, coarse sandy loams
25
86
4
clay, silty clay
25
86
5
loams, sandy clay loams, sandy clays
40
56
6
silt loams, clay loams
45
47
7
silty clay loams, silts
50
38
Responsible factors for wind erosion :
1. Nature of wind ( velocity, direction etc.)
2. Types and density of vegetation and ground cover.
3. Topographic features ( terrain, flat, undulating, rolling and
continuous )
4. Nature of land surface ( plain, degree of roughness, land cover by
plant )
5. Characteristics of soil ( physical, chemical, OM, moisture content etc.)
6. Biotic factors ( land use, over used, over grazing , plant residue etc.)
15
1.3.2 Soil movement due to wind erosion :
Efflation : Very fine particles carried off by wind
Extrusion : Large soil particles carried of by rolling and sliding by wind.
Effluxion : Soil particles of intermediate size carried off by the bouncing
action of wind.
Detrusion : Wearing away of rock and land projection by soil particles
carried in suspension, this is a result of efflation .
Abrasion : Wearing away of rock and land projection by larger soil particles
moving as bouncing, rolling and sliding which is the result of extrusion and
effluxsion
1.3.3 Control of Wind Erosion :
1.
2.
3.
4.
5.
6.
7.
Avoid removal of vegetation
Maintain shelterbelts / windbreaks
Maintain plant residue / stubble on land
Plant shrubs / grasses
Access the spacing of shrubs and trees to reduce wind velocity
Maintain good balance between grass, shrubs and trees
Avoid locating LS watering facilities on erodible soils
UNIT 2. Design and Construction of Gully Control structures and Terraces
2.1 Introduction to mechanical control measures
This is non- vegetative and apply engineering works
Why mechanical control measures ? :
 If the condition of eroded area do not permit establishment of
vegetation.
 If no scope of quick cover of vegetation.
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 If establishment of eroded area takes long time by vegetation
measures .
Important mechanical measures are :
 Rip-rapping : Loose stones are used for the construction of riprapping. Stone size should be large enough to resist the force of
water. Minimum size of stones be 0.5 m. ( diameter) for bottom /
toe of the channel and 0.3m. for the protection of the upper part
of the channel
17
 Retaining wall : These are structures to protect the gully and
torrent banks against lateral erosion and undercutting. Concrete (
porous/ non- porous), gabion and loose stones are used for the
construction. If loose stones are to be constructed large-sized
stones must be used.
 Checkdam : Most important device to control torrent and gully
erosions is the check dam. This is most common and effective
mechanical means of stabilizing active gully erosion.Structures
built across stream channels or gullies to stop the lateral and
horizontal gully/ channel erosion and to stabilize and stabilizing
them by retaining bed load and other debris.
Its purpose is to reduce the gradient and break the velocity of
flow/ runoff.
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Check dam guide the water flow/ runoff from a higher point to
lower point without causing erosion at the gully bed and banks. In
general, there are two types of check dam.
1. Non- porous with no weep holes
2. Porous dams which release part of the water flow to
reduce hydro-static pressure
Concrete (porous/ non-porous), gabion, loose stone, brush wood
or wooden check dams are used depending upon site condition,
objectives of gully control, availability of construction materials
and cost effectiveness.
Mostly used in series to meet the following purposes :
a) to raise the bed level up to a height where safe support is
provided for the slopes
b) to reduce the gradient of the channel / increase resistance of
the channel bed
c) to reduce the water depth by widening the gully/ channel bed
 Spur and Embankment: These mechanical measures are used to
control torrent/stream/river bank erosions or bank cutting by
diverting flowing water
2.2.1 Check dams
2.1.1.1 Types of check dams
1. Concrete check dam : This is built, where higher structures are
required. weep holes are provided to release hydrostatic pressure.
19
2. Gabion check dam: This is constructed from gabion boxes with
appopriate wire mesh. Usually 10 and 8 gauge GI wire of mesh size
10 by 10 cm. or 15 by 15 cm. size of the stone which is used in gabion
should be large enough than size of the mesh. The maximum hieght
of the gabion check dam should not be more than 5 m. for mountain
gullies and torrents.
20
Figure : gabion check dam
3. Loose stone check dam: Big loose stones are used in this type of
check dam. The quality, shape, size of the stones used affect the
success of the check dam.
4.
Brushwood check dam: Logs, branches bamboos are used in this
type of check dam. They are piled across the cross-section of
gullies. Series of such structures can be installed to plough the gully
erosion.
21
Figure : Brushwood check dam
22
Figure : Plan and layout of check dam
2.1.1.2 Design of check dams
Before designing a checkdam, it is necessary to calculate the maximum
runoff discharge that should passes through the spillway of the
checdam.The runoff discharge Qmax can be calculated from the following
formulas :
23
a) Formula to calculate Qmax
i ) General formula :
Q max = V *A, where as
Q max = maximum discharge of the gully catchment at the proposed
check dam point (m3/sec or cumecs)
A= cross-section area of gully bed (m2)
V= velocity of flowing water ( m/sec), which can be obtained from
ii) Manning,s formula
Manning,s formula = V = ( R 2/3 *S 1/2)/n, where as V = velocity of flow
R = Hydraulic radius in meters = Area/wetted perimeter (surface in
contact with water)
S = Water surface slope = ratio of vertical drop to the length of the
stream
R = roughness coefficient ( obtain from table for major rivers,
floodplain, excavated channel at different land condition)
iii ) Rational formula :
Q max = CIA/360, where as
Q max = maximum discharge from catchments area of gully (m3/sec)
C = catchments characteristics constant which varies from 0.4 to 0.95
depending upon types of land
I = intensity of rainfall (mm/hr) for the designed return period and for
a duration equal to the time of concentration of the watershed.
A = watershed area (ha)
The time of concentration (Tc) is defined as the longest time taken
for water to travel by overland flow from any point in the watershed.
It is a storm duration that produce max. rate of runoff or the storm
duration which will correspond with the max rate of runoff to the
outlet. The time of concentration (Tc) can be calculated by Kirpich
method, which is :
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Tc = 0.0195 L 0 . 77 × S
0 . .385
, where as
Tc = time of concentration (min)
L = max. length of flow (m)
S = watershed gradient or the difference in elevation between the outlet
and the most remote point divided by the length L (m)
After the calculation of Q max , the spillway or notch of the check dam
need to be designed, using the following formula :
i. Spillway formula :
Q max = CLH 3/2 where as ,
C = constant, L = length of spillway (m) and
H = height of spillway (m)
Here, since the gully width or torrent width is known, the length of spillway
need to be assumed. Then substituting the known value in the spillway
formula, the height of checkdam can be determined. After calculating the
actual spillway height, certain free board usually 5 to 30 cm. need to be
added in the actual height of checkdam.
Example : ( Numerical )
Runoff ( Discharge ) calculation by using Rational Formula :
The rational formula is : Qmax = CIA/ 360
Given : L = 1.2 Km. ( max. length of flow )
A = 2.1 Km2 (Area of watershed)
S = 15 m ( watershed gradient or difference in elevation
between the outlet and the most remote point divided by L)
C = 0.03 ( catchments characteristics constants )
Calculate the time of concentration, intensity of rainfall and Runoff
discharge for a return period of 20 years.
Here, Time of Concentration ( T ) = 0.0195 L 0. 7 7 × S 0. 385 = X ( m)
25
Intensity of Rainfall for a return period of 20 years,
Therefore, I x = a/( b + T ) = 2850 / (10 + X ) mm/hr = Y mm/hr
0.03 ×Y × 2.1
Q max =
_______________
cumecs ( m3/sec )
360
The values of intensity constants "a" and "b" are either given or supplied in
a table :
Table : Intensity Constants
_______________________
a
b
________________________
1 year
830
5
2 year
1400
7
5 years
2100
9
10 years
2590
10
20 years
2850
10
50 years etc.
3220
11
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2.1.1.2 Design of a Check dams :
After calculating the Qmax, calculate the design of spillway of a check dam
as follows :
Qmax = CLH 3/2,
Here, Qmax is calculated from the given example above, C is given, assume L
and caculate H. After the calculation of H, place the value in the equation.
Manupulate the value of L and H until the Qmax (calculated) comes to equal
or more than given Qmax ( discharge) to pass through the spillway.
c) Spacing of check dam :
i ) Spacing :
In most of the gully slopes, the spacing of check dam is arranged in such a
way that the top of the lower check dam should be at level with the bottom
of the upper check dam. By this spacing method, many check dams are
required in steep gully slopes. Therefore, a compensation gradient is
required to minimize the number of check dams and then to the cost
involved. Compensation gradient is the slope between the top of the lower
check dam and the bottom of the upper one. Compensation gradient of
about 3 to 5 percent is provided between two check dams.
Spacing of
height of check dam (h) * 100
the check dam D = -------------------------------------% slope of gully (So) - % compensation
gradient (Sc)
D = spacing between two successive check dam ( horizontal distance)
h = Height of check dam ( up to notch)
So = slope of the gully or existing slope of bed in %
Sc = Compensation gradient or stabilizing slope of bed in%
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Figure : gully slope, checkdam and compensation gradient
ii) Number of check dam
The number of check dam can be calculated from the following formula
(Hiller, 1997) :
The number of check dams is calculated as follows :
Number of check dams = A ÷ H or A/H. If compensation gradient is used,
then the number of check dams = (A-B) ÷ H or ( A-B)/H, where as
28
A= the total vertical distance between the first and the last check dam in
that particular portion of the gully or torrent.
B = the total vertical distance calculated according to compensation
gradient for that particular portion of the gully or torrent.
H = average height of the check dams.
d ) Base width of check dam : The base width of the check dam can be
calculated by using following formula:
i ) Kronfellner - Kraus formula : Here the base width of the check dam is
calculated if the check dam has a height of less than 8m. The base width
is :
D1 = 0.35 ( H + U ), where as D1 = base width (m).
H = difference between the crest of the spillway and the base of the
check dam (m).
U = depth of the spillway (m).
If the height of the check dam is between 2 to 6m, then the Hoffmann
formula may be used to calculate the base width.
ii ) Hoffmann formula :
D1 = 0. 462 H, where as D1 = base width (m).
H = difference between the crest of the spillway and the base of the
check dam (m).
e) Top width of the check dam ( Crest thickness) : Following empirical formula
can be used to calculate the top width of the check dam or crest thickness, if
the height of a check dam is between 6m to 7m.
D2 = 1 + H/10, where as D2 = top width or crest thickness of the check dam
(m)
H = height of the check dam including foundation (difference between the
water level upstream and water level downstream of the check dam
(m).
29
f) Scouring depth: The depth of foundation of the check dam must be deeper than
the bottom of the scouring zone below the check dam. Therefore, it is
necessary to find the scouring depth before deciding the foundation of the
check dam. To calculate the scouring depth of the check dam, following
formula can be used:
Schocklitsch's formula :
4.75 × h 0.2 × q 0..57
S (d 90) = ----------------------dm
S (d90) = scouring depth ( in meter) below the water
level
h = difference between the water level upstream and water level
downstream of the check dam (m) or water level difference ( in
meter) above and below thw check dam
q = specific runoff, which is the runoff that passes through a section
of 1m. of the spillway ( m /sec).
dm = diameter of soil particle in which 90 % soil particles is smaller
and 10 % soil particles bigger than 90 mm diameter size.
The modified version to calculate the scouring depth is
h
S = 0.79 ×
0. 343
×q
0. 686
_______________
d 90
0. .372
where as,
h = fall height of a check dam,
q = specific runoff
d 90 = 90 % soil/ bed material is smaller and 10 % soil/ bed material
bigger than the diameter of 90mm.
30
2.1.1.3 Stability analysis of check dams
There are three possibilities of failure of checkdam. They are :
i)
i)
Check dam may sliding forward
ii)
Check dam may overturning
iii)
Check dam may crushing at the base
To check against sliding forward
Check dam has many elements and various forces are acting on it .
P = lateral thrust
μ = coefficient of friction ( Table.........)
W= weight of check dam
w = specific weight of soil ( Table.........)
H = height of check dam
31
Y = specific weight of check dam material
A = top width of spillway section
B = bottom width of check dam
Ø = angle of internal friction of soil ( Table........)
Z = distance of intersection point of resultant of P and WU
The check dam may not slide forward if the lateral pressure (P) is less than
the product of weight of check dam and coefficient of friction (W×U) , i.e P
< W×U. The lateral thrust ( P ) can be calculated as follows:
P = (wH 2 /2 ) * ( 1-sin θ/1+sinθ),
W = Y{( A + B)/2 )} H
ii) To check against overturning
The resultant of P and WU should passed within 2/3 of the bottom widthor
base of the foundation of checkdam. That is Z < or = 2/3 B, where as Z =
distance of intersection point of resultant of P and WU, which can be
calculated as:
Z = (A2 + AB + B2) / 3 ( A + B ) + H/3 × P/W
iii ) To check against crushing
In order avoid crushing of a checkdam, the maximum compresive stress at
the base of a checkdam should be less than the allowable working stress for
acheckdam. In this case the maximum compressive stress Pmax. should be
less than the allowable working stress
Pallowable or Pmax < Pallowable
Pmax. can be calculated from the following relationship:
Pmax = W/B ( 1 + 6e/B ), where as e = eccentricity of loading.
e, the eccentricity of loading can be determined from the table or already
given
Pallowable for a given checkdam can be determined from the table or already
given
32
Numericals :
Example 1. Check the stability of a check dam against sliding and bearing
pressure based on the following information :
Coff. of friction = 0.60 = u
Total wt. of the check dam per unit length = 2.6 t = W
Total horizontal pressure per unit length = 2.15 t = P
Soil bearing capacity = 10 t/m2 = Pallowable
Density of stone masonry = sp. wt. of check dam = 2.15 t/m3 = Y
Density of silt = 1.14 t/m3 = e
Width of foundation = 3.5m = B
Is the designed dam section safe ? if not suggest safe alternate design.
Solution :
a. To check against sliding forward.
Formula : P < uW
uW = 0.60 * 2.6 = 1.56 t
Here P > uW, so design is not safe. Increase the value of W by increasing
size of the checkdam.
W = Y{( a + B)/2 )} H
b. To check the bearing pressure ( against cruising) :
P max < P allowable
Pallowable = 10 t/m2
P max = W/B (1 + 6e/B), where as e = eccentricity of loading.
= 2.6/3.5 (1 + 6e/B)
= 0.70 (1 + 6 *0.58/3.5) = 0.70 * 2.9 = 2.069
Here, P max < P allowable
So the design is safe for cruising
33
Example 2. calculate the depth of scour to be considered for the check dam
having 5mt long spill way with the total discharge of 6.25 m3/sec with 15
mm grain diameter with 2 m. water level difference above and below the
check dam
Solution :
Here, dm = grain diameter = 15mm,
h= 2m. = water level difference between the water level upstream and
water level downstream of the check dam
q = specific runoff, which is the runoff that passes through a section of
1m. of the spillway ( m /sec).
Therefore, q = 6.25/5 = 1.25 m3/sec
Apply Schocklitsch's formula :
4.75 × h 0.2 × q 0..57
S (d 90) = ----------------------dm
4.75 × 2 0.2 × 1.25 0..57
= -----------------------15
= 0.42 m
Therefore, the foundation of check dam should be > 0.42. We can add
some more for the further safety ( 20-30%)
Example 3. Find the spacing of check dam provided the height of 1m. on
the gully bed having 15 % slope in which 5 % compensation gradient is to
be maintained.
Solution :
Given h = 1 m
slope = 15%
compensation gradient = 5 %
34
height of check dam (h) * 100
Spacing of the check dam D = -------------------------------------% slope of gully (So) - % compensation gradient
(Sc)
1 * 100
= ----------- = 10 m
15 – 5
Example 4. Calculate the number of check dam in a 300 m. long sloping
gully having the vertical distance between the Ist and last check dam is 21
m. and 3% compensation gradient is to be mentioned after construction of
1.5 m height of check dams
Given :
Vertical distance between the two check dams = 21m
Length of gully = 300m
Height of check dam = 1.5m
Compensation graidient to be maintained = 3 %
Number of check dams = A ÷ H or A/H. If compensation gradient is used,
then the number of check dams = (A-B) ÷ H or (A-B)/H, where as
A= the total vertical distance between the first and the last check dam in
that particular portion of the gully or torrent.
B = the total vertical distance calculated according to compensation
gradient for that particular portion of the gully or torrent.
The compensation gradient has to be maintained as 3 % through out the
300 m long sloping terrace
therefore B = 300 * .03 = 9 m
H = average height of the checkdams.
Number of check dams = (A-B)/H=(21-9)/1.5=12/1.5 = 8
Other Numericals :
35
Given : L = 1.2 Km. ( max. length of flow )
A = 2.1 Km2 (Area of watershed)
S = 15 m ( watershed gradient or difference in elevation between the
outlet and the most remote point divided by L)
C = 0.03 ( catchments characteristics constants )
Calculate the time of concentration, intensity of rainfall and Runoff
discharge for a return period of 20 years.
Runoff ( Discharge ) calculation by using Rational Formula
The rational formula is : Qmax = CIA/ 360
Here,
Time of Concentration (T) = 0.0195 L0. 7 7× S -0. 385 = X (m)
Intensity of Rainfall for a return period of 20 years, I X =
a/(b+T)
= 2850 / (10 + X ) mm/hr
= Y mm/hr
0.03 ×Y × 2.1
Q max =
_______________
cumecs ( m3/sec )
360
The values of intensity constants "a" and "b" are either given or supplied in
a table :
36
Table : Intensity Constants
_______________________
a
b
________________________
1 year
830
5
2 year
1400
7
5 years
2100
9
10 years
2590
10
20 years
2850
10
50 years etc.
3220
11
37
2.2.2 Terraces
Terracing : Terraces are the oldest structural conservation measure
applied in sloping lands. They are essentially level or nearly level strips
running across the slope. These strips are supported by risers in down hill
sides and the whole system looks like a series of steps. Terracing is
practiced in many countries for thousand of years.
Objectives :
-- to reduce runoff and velocity of flowing water and thereby
minimizing erosion from the land and preventing heavy
sedimentation to the streams and low land below.
-- to conserve soil moisture and soil fertility, thus increasing farm
production.
-- to facilitate farming practices such as mechanization on steep slopes
and there by increasing the arable land.
-- to promote proper land use in hilly regions and to reclaim eroded
slopes
2.1.7.1 Types of Terraces :
There are several types of terraces with paticular objectives, functions,
climate and crop conditions. Commonly used terraces are :
a ) Level Bench Terraces: This is completely a level terraces i.e all
points of the surface is at the same height. The bench is
surrounded by a dyke of about 20 cm. height to retain water and
a notch can be cut in the dyke to flow water in the next terrace or
bench. Terraces to grow paddy belongs to this type and also
useful in low rainfall areas and highly permeable.
38
b) Outward Sloping Terraces : These terraces slope outwards. A
drain is usually dug either along the top line or at the toe of the
riser to collect runoff. This type of terraces are useful in semi-arid
areas where rainfall intensity is not high. The amount of
earthwork in this type is less than for the level terrace. Here,
runoff flows down the filled portion of riser of next terrace so
terraces are usually easily eroded.
c ) Conservation Bench Terraces : Level contour benches are
built at intervals on the slopes to catch and hold runoff water
from the land above. A terrace ridge is built on the lower side of
the bench to hold runoff water. This type of terraces are used in
arid or semi-arid countries.
d) Reverse Sloped Terraces: Here the terraces are built sloping
inwardly towards the toe. The runoff will first be collected at the
toe of the bench then gradually drained to a protected waterway.
The riser will be kept free from over flowing water and is
protected usually by grass cover. This type of terrace is suitable in
heavy rainfall regions and on soils with low permeability.
39
Benefits of Terraces :
a. protect soil erosion
b. minimize sediments and water pollution
c. reduce run-off water and flood damage
d. intensify land use
e. stimulate improved farming practices and increased
production
f. improve drainage and provide better sites for cultivation
40
g. facilitate mechanization of steep slopes
h. conserve moisture and maximize irrigation benefits
i. create arable land and enable a wider choice of crops
and cropping pattern
j. encourage established farming system and reduce
shifting cultivation and forest fires
k. create labor intensive programs and create local job
opportunities
l. beautify landscape and provide a better environment
for farming
Limitation for Terraces :
 can be built in any slope with deep soil
 cultivable parts become narrow as slopes become steep
 slopes need high height riser and risk of failure of riser becomes
high
 produce limited level land or area for cultivation
2.1.7.2 Design of Terraces:
The design includes : Width, Vertical Interval ( V I ), Lengths and
Grades, Riser and Reverse Height, Net Area, Cross-section and
Volume of Terraces
In designing the terraces, the Width of Terraces must first be
decided.
41
Figure : cross-section of terrace
42
Width :
The width of terraces are determined by many factors : soil depth,
slope, the crop, farming methods, tools to be used and farmers
choice and preference. The thumb rule is for manually cultivated
terraces, the range of terrace width is about 2mt. to 5mt and for
tractor cultivation, the range of terrace width is about 4mt. to 8mt.
depending upon soil depth and degree of slope.
Vertical Interval :
This is a elevation difference between two succeeding terraces,
which is determined by the slope and desired width of the terraces.
VI is important to determine since it gives the approx. height of
riser and basic information for calculating cross-section and volume
of soil to be cut and moved about.
V I = S x Wb / 100- S x U, where
S = average land slope in %
Wb = width of terrace ( need to be pre-determined )
U = slope of riser ( ratio of horizontal distance to vertical
rise)
Example : Terrace of 3mt. width to be built on average land slope
of 20 % with riser slope of 1:1. Find the vertical interval
V I = 20 x 3 / 100- 20 x 1 = 0.75 mt.
Terrace Length and Grades :
The length of a terrace is influenced by size, shape and degree of
fragmentation of land and permeability and erodibility of soil, the
cross-section of terrace and outlets.
43
The length of terrace should not be very long in one direction as
the velocity of flowing water may cause surface erosion. A
maximum of 100 mt. in one direction is considered effective.
The grade of terrace should vary from 0.1 % to 1% that is 1/10 to 1
mt. fall per 100 mt. running length.
Low rainfall and permeable site = grade < 0.5%
High rainfall and impermeable site = grade 1 %
In the case of longer terrace = variable grades should be used e.i 0.25
% for the first part from the top, 0.5% for the second part and 1
% for the last part towards the outlet
Terrace Riser and Reverse Height :
The height of the riser of the reverse type of terrace :
Hr = V I + RH, where as Hr = ht. of the riser
V I = vertical interval
RH = reverse ht ( 1/2 of depth of cut ).
The width of a riser, Wr = Hr x U (riser slope)
Width of terrace, Wt = Wr + Wb ( width of bench )
Linear length of terrace, L = 10,000/ Wt (per ha.)
Net area of terrace, A = L x Wb
Cross-section of Terrace, C = Wb x Hr/ 8
Volume to be cut and filled, V = L x C
44
2.1.7.4 Construction and layout procedure of protection work :
Lay-out :
Needs careful field observations of topography, slope, soil
depth,texture erosion, presence of rocks, vegetation, landuse,
future cropping plan. After the careful observations of above
mentioned factors decisions should be taken on the proper type of
terraces, their width and tools to be used. The vertical interval, the
height and width of riser can be obtained either using above
mentioned formula or or prepared specification table (attached ).
Before staking out the layout of terrace, it is necessary to decide
the site and waterway system. Wind breaks if necessary should also
be located. All of these should be integrated into the terracing plan
with sketches.
45
Figure : Construction consideration and layout of terraces
46
Surveying and Staking :
Survey and stakeout the central line of the terraces. This central
line stakes should be retained as a guide for the non-cut and and
non-fill line during construction. The area above this central line is
cut line and down is fill out area.
The second method is to survey and stake both the upper and
bottom lines as determined by width of the terrace. Make sure that
the space staked out between two lines will give the width of the
terrace needed.
General Guidelines :
1) Clear the sites for clear vision to carry-out survey.
2) Surveying and staking should start at the top and proceed
downward
3) Stake the graded contour lines
4) Stakes should be placed at every 5 to 10 mt. and at all points
where topography changes
5) All the staked lines, waterways, roads etc. should be leveled.
6) Construct when soil is not too dry or too wet.
7) Begin construction from the top of the slope and proceed
downwards
8) Begin the cut at the top and fill in the bottom
9) After each 15 cm. fill, the soil should be compacted
10) The edge of the terrace should be little higher than required to
take care of settling of soil.
47
2.1.7.3 Stability/Protection :
1) New terraces should be protected at its riser, outlets and waterways.
Should be carefully protected for at least 2 years.
2) After construction, the riser should be shaped and planted with
grasses. Sod forming or rhizome types of grass prefered than tall or
bunch types. Although some tall grass like Napier, Setaria can be
used but they need frequent cutting. and care. Carpet grass found
very useful in protecting riser
3) Terrace outlets (point where the runoff leaves the terrace and goes
into the waterways) needs to be protected by sodding, bricks, stones
4) Keep the toe-drain open, and do not permit any accumulation of
water in any part of the terrace
5) Allow all runoff to collect in the toe drain for safe disposal of water
into protected waterway. Any kind of obstruction in the toe drain
should be taken out.
6) Ploughing should be done carefully not to destroy the toe drains
and the reverse grades.
7) Do not allow runoff flow over the risers. If necessary dyke along the
edge of bench should be built.
8) Keep grasses growing well on the risers. Weeds and vines which kills
grasses should be cleared.
9) Any small damage or break of the riser should be repaired
immediately.
48
Numerical :
1. A 15% hilly land is proposed for construction of bench terrace.
Calculate the following parameter of bench terrace using 2.5 m
as vertical interval and 1:1 is batter slope. Here calculate the
following parameters :
a) Total width of the bench ( Wt)
b) Length/ha
c) Earth work
d) Area lost
Given : U = Slope of riser = 1:1
S = Average land slope = 15%
VI= Vertical interval =2.5 m
V I = (S x Wb ) / (100- S x U), where
S = average land slope in %
Wb = width of terrace ( need to be pre-determined )
U = slope of riser ( ratio of horizontal distance to vertical
rise)
Solution :
a) In a bench terrace,VI = Hr = 2.5
VI = (S x Wb) / (100 – S x U)
or (15 x Wb) / (100 – 15 x 1) = 2.5
Wb ( Width of bench) = 14.16
Wr ( Width of riser ) = Hr x U = 2.5 x 1 = 2.5 m
Total width of bench Wt = Wr + Wb = 2.5 + 14.16
= 16.66m
i. Length/ha ( terrace length/ha)
L = 10000/Wt = 10000/16.66 = 600 m/ha
49
ii. Earthwork (volume) : ( L x Wb x Hr)/8
= ( 600 x 14.16 x 2.5)/8
= 2655 m3
iii. Area lost due to slope = (S + 200) / (200/s + S/100)
= (15 + 200) / (200/15 + 15/100)
= 15.95 %
2. If the vertical interval of bench terrace is 1.5 m, land slope is
25% and riser slope is 0.50 Compute the width of the terrace
Solution : V.I = (S x Wb) / (100 – S x U)
1.5 = (25 x Wb) /(100 – 25 x 0.50)
Wb = 5.25
3. Calculate V.Iof terrace, if width of terrace =3m; land slope =
20%; riser slope=1:1
Solution : V.I = (S x Wb) / (100 – S x U)
= (20 x 3)/(100 – 20 x 1) = 0 .75m
2.2.3 Waterways
2.1.5.1 Types of waterways/drainage
Grass waterways : These are grass waterways in which a parabolic or
slightly trapezoidal shaped channel are made and planted with low
height rhizome type perennial grass. Grass turfing at the bottom and at
the sides of the channel will be more advantageous in vegetative
waterways. However, it has some limitations such as it can not be used
where the velocity of water flow is more than 2m / sec, it is not safe in
steep slopes of more than 10 degree, it needs drop structures if the
length is longer than 35m, it is not safe in the area where there is a
continuous flow of water, it can't be used until the grasses are well
established at the canal course.
50
Stone-lined waterways: In this case, flat stones or stones of 15-20 cm
diameter are pitched and keyed at the bottom and on sides of the
channel. This is used if the slopes are steep and volume of runoff is
large. In some cases, wire mesh filled with stones are also used.
Grass waterways with drop structure:
In this case series of drop structures or checkdams, as needed are
constructed at the course of channel to break the velocity of running
water. These waterways are made in steep slope, which exceeds the
prescribed slope limit of grass waterways as mentioned above. The
channel course between the drop structures are maintained as in grass
waterways. However, the drop structures should not be taller than 12m. and the slope between the apron of the upper structure and weir of
the structure immediately below should not be more than 3 % for the
sake of stability and safety. The drop structure can me made from
brushwood, loose stones, mesh wire or concrete.
Concrete waterways: A parabolic or V-shaped waterways can be
costructed at the middle of channel with grassed channel slopes in the
sites with steep slope, frequent rainfall, seepage sites.
Stepped grass waterways : This is a parabolic grass waterway which is
cut by number of steps or drop structures to control the channel flow
and to protect the riser and bottom of steps. This is commonly used in
steep slopes giving channel slope between 3 - 4 %.
51
2.1.5.2 Design of waterways/drainage (hydraulic channels ):
Design of hydraulic channels is applied in selecting a structure which will
pass the design flow through the cannels without excessive headwater
elevation. In designing the hydraulic channels, It is necessary to determine
the peak discharge for the area to be drained. The peak discharge or
maximum rate of runoff can be calculated by using Rational Formula i.e
Qmax = CIA/ 360 ( m3 /sec or cumecs ), where as
Qmax = maximum discharge or rate of runoff ( cumecs )
C = runoff coefficient
A = watershed area from where the runoff accumulates (
ha.)
I = intensity of rainfall(mm/hr) for designed frequency for
duration equal to time of concentration.
After determination of peak discharge, allowable velocity of flow for
vegetative or non-vegetative channels are fixed using the figures given in
the table:
52
Canal
Materials
Manning's(n)
coefficient
Fine sand,
colloids
0.020
0.45
0.75
0.45
Sandy
loam, noncolloids
0.020
0.53
0.75
0.60
Silt loam,
noncolloids
0.020
0.60
0.90
0.60
Alluvial
silts, noncolloids
0.020
0.60
1.05
0.60
Ordinary
firm loam
0.020
0.75
1.05
0.68
Stiff
clay,very
colloidal
0.025
1.13
1.50
0.90
Alluvial silt,
colloidal
0.025
1.13
1.50
0.90
Fine gravel
0.025
0.75
1.50
1.13
Coarse
0.025
1.20
1.80
1.90
Velocity ( m/s )
Clear
Water
with W
a
colloidal
t
silts
e
r
Waterwith sand
gravel/fragment
53
gravel, noncolloidals
Cobbles
and
shingles
0.035
1.80
1.80
1.50
Table: Maximum allowable velocities for non-vegetative channels having
different channel materials
After fixing the allowable velocity of flow, then calculate the cross-section
area of hydraulic channel to carry out the calculated maximum discharge or
runoff by using the formula :
Qmax = a × V or a = Qmax / V, where as
Qmax = estimated peak discharge or rate of runoff ( cumecs)
V = permissible velocity ( m/s ) and
a = cross-section area of channel ( m2 )
After calculating the cross-section area of channel using above mentioned
formula, then calculate manually the cross-section area of channel by
adjusting dimensions like debth and width of the channel in order to make
the manually calculated cross-section area of the channel equals to the
cross-section area as calculated from the formula. In other words,
Cross-section area from formula (a = Qmax / V) = calculated cross-section
area.
After determining the cross-section area or height and width of the
channel, now calculate the gradient or slope of the channel to be
maintained. This can be evaluated by using Manning's Formula, which is :
V = ( R 2/3 × S 1/2 ) / n or S = ( nV / R 2/3 ) 1/2, where as
S = gradient or slope of the channel
n = manning roughness coefficient( figure given in the
Table above )
V = allowable velocity of flow ( m/sec)
R = hydraulic radius (m), which is cross-section area
divided by wetted perimeter i.e a / p and wetted perimeter ( p) is the
length of line of inter section of the plane of the cross-section with the
54
wetted surface of the channel. In general, wetted perimeter can be
considered as width of the channel.
If the hydraulic properties of the channel such as width, depth and slope
are known, the Manning's formula can be used to determine Qmax (
cumecs), which is Qmax = 1/ n × a R 2/3 × S 1/2 .
In the case of bank full discharge of channel, the hydraulic properties
can be related with channel discharge as :
b ∞ Qmax × b 1/ 2
d ∞ Qmax × b 1/ 3
s ∞ 1/ Qmax × b 1/ 6
,where as b = width, d = depth and s = slope of a
channel
The calculated runoff (Qmax ) should be more than or equal to given
runoff ( Qmax ) to accept that the design of waterway or channel is
safe.
Design of storm water drain:
Storm water drains are waterways, drainage and watercourse in
which the runoff occurs and drains during the rainfall or storm . It is
important to guide the storm water drain to certain safe point in
order protect the land from erosion. The purpose of designing storm
water drains are :
-- to manage the surface runoff
-- to provide safe drainage to excess runoff that may developed
during rainfall
-- to divert the runoff from entering into gully, erosion or landslide
heads
-- to divert the runoff and dispose off into safe area or land surface
55
-- to prevent land from the formation of rill and gully erosion and
landslide
-- to harvest the runoff for domestic purpose.
The storm water drain can be managed by several means. The mostly
used techniques are constructing waterways or drainage or diversion
ditches. These techniques can be used at places depending upon
availability of construction materials, purposes and structures that can
be constructed. Before selecting the techniques and designing the
structures for the management of storm water drain, it is necessary to
know the hydrological elements of storm water such as quantity of peak
discharge, velocity of runoff, slope of land, volume of runoff, soil and
vegetation conditions and land-use. Waterways are classified into two :
Vegetative(Grass) and Non-vegetative or structural waterways.
Shapes and design of waterways :
Based on the site condition, objectives and availability of materials, the
shapes of water ways can be chosen. Normally, the shapes are:
-- Trapezoidal
-- Parabolic and
-- Triangular or V- shaped
Having calculated the Qmax , cross-section area , velocity of flow
and surface slope following the steps as described aboveve, the
side slope, bottom width and the depth of the channel can be
calculated for different shapes of waterways.
56
57
Numericals (Examples) :
1. To calculate whether or not the design of channel or waterway
is safe.
Always keep in mind that the calculated Qmax > or = Given Qmax
Given :
a) For a Trapezoidal cross-section
Given Qmax = 3.5 cumecs
Channel slope, S = 0.3% or 0.003
Coefficient factor, n = 0.045
Side slope, Z = 2
Height of channel/ waterway, d = 1 m.
58
Base breadth or width of channel/ waterway, b = 2
m.
Now, calculate Qmax = A ×V = Crosection area × Velocity of flow of water
A = b × d + Z d 2 = 2 × 1+ 2 × 1 2 = 4 m2
V = ( R 2/3 × S 1/2 ) / n, R = hydraulic radius and S = channel slope
R = Cross-section area of channel (A ) / wetted perimeter ( P)
P = b + 2 d ( 1+ Z 2 ) 1/2 = 2 + 2 ×1 ( 1 + 2 2 ) 1/2
= 6.47 m 2
Now, R = A / P = 4 / 6.47 = 0.62
V = ( 0.62 2/3 × 0.003 1/ 2 ) / 0.045 = 0.885 m / sec
Qmax ( calculated ) = A × V = 4 × 0.885 = 3.6 cumecs
Here, calculated Qmax > given Qmax , so design is safe and acceptable.
In this way, we can calculate the safety or acceptance or rejection of
channel cross-section of parabolic, triangular or V-shaped channels. If
there is rejection,
we can adjust either breadth (b) or height (h) of the channel crosssection and bring it to the acceptable level.
Important : a ) For trapezoidal cross-section: bottom breadth (width),
height (depth), side slope, canal gradient are provided
b ) For parabolic cross-section : top breadth ( width ), height (
depth ), channel gradient are provided.
c)
For triangular or v-shaped cross-section : top breadth (
width ), height (depth ), side slope, canal gradient are
provided.
59
2.2.4 Bunds
Bunds are structures constructed across the land slope to reduce
the length of slope and velocity of runoff in down hill side for
agricultural operation. Bunds are generally preferred to construct in
the slope between 2 -10 % slope and soil depth ranging from
shallow to medium.
Its main purpose are :



To reduce the velocity of runoff
To intercept the flowing water or runoff
To hold water and maintain moisture in the catchment for a
longer period
 To allow more water to infiltrate
 To reduce soil erosion
2.1.6.1 Types of Bunds:






Contour bunds : This is constructed along the contours, low
rainfall areas, soil depth > 20 cm, slope < 7 %, good infiltration
capacity, built in series to divide the length of the slope
Graded bunds : This is constructed with some longitudinal
slope, this is to be used for partly conservation of moisture and
safe disposal of excess water or runoff, suitable for high rainfall
areas and soil having less infiltration capacity, poor soil depth,
built in series to divide the length of the slope
Peripheral bunds : This is constructed to incircle the
boundaries of ares
Marginal bunds : This is constructed in the lowest part of the
catchment without any consideration of contour
Side bunds: this is constructed along the slope at the two sides
of contour bunds
Lateral bunds : bunds are constructed along the slopes in
between two side bunds to reduce the length of contour. This
reduces accumulation of runoff along one side
60
2.1.6.2 Design of bunds .
The design criteria of bunds include following parameters :
 Choice of bunds : Choice of bunds depends upon rainfall, soil
condition and types of out lets used. Contour bunds are used
in low rainfall areas and graded bunds are used in medium
rainfall areas with slope percentage between 0.2 – 0.3 %
 Spacing : The basic principle that decides the bund spacing are
: a) able to intercept the surface runoff before the runoff
attains erosive velocity. b) it should meet all requirement for
agriculture operation.
 Size of bunds
 Side slopes
 Alignment
 Land submergence
 Moisture conservation /dry period
 Cost
Design :
The general relationship between land slope % (S) and the vertical
interval between the two consecutive bunds are :
VI = (S /a ) + b, where a and b are constants depend upon soil and
rainfall characteristics. Add 25 % extra spacing of bunds if
conservation measures are applied and reduce 15 % spacing for
unfavorable condition of conservation measures.
Following Formula can also be used for the calculation of spacing
between two consecutive bunds :
a) Ramsar’s Formula :
For normal to moderate rainfall,
V I = 0.3 ( S/3 + 2) where S = Land slope (%) (
For heavy rainfall area the above equation is modified as :
61
V I = 30 ( S/3 + 60) and for low rainfall area the equation is V I = 30 ( S/2 +
60)
b) Cox’s Formula :
VI = ( X *S + Y) 0.3 where X = Rainfall factor, S= land slope in % and Y =
infiltration and crop cover factor. The value of X and Y can be received from
the following table :
Rainfall
Condition
X values
Average rainfall
(cm)
Y value
Scanty
0.8
64
1.0
Moderate
o.6
64-90
2.0
heavy
0.4
>90
1.5
Size and Height of bunds :
Bunds height depends on : Depth of water storing, highest flood level, free
board, soil infiltration and vertical interval
a ) Cross-section area of Bund=(base width + top
width)* Ht/2
B )Bund height = (24 hr. Rainfall storage * VI/50)1/2
62
Numerical : Calculate the VI of contour bunds on a 4.5 % land slope. The
rainfall is moderate with average infiltration and good coverage of land
with vegetation
Solution : a) Using Ramsar formula
V I = 0.3 ( S/3 + 2) where S = Land slope (%)
= 0.3 ( 4.5/3 + 2) = 1.05 m
Add 25% extra spacing because the ground cover is good
Therefore, VI = 1.05 + 0.26 = 1.31 m
b) Using Cox’s formula VI = ( X *S + Y) 0.3
= ( 0.6*4.5+2)0.3
= 1.41 m
Unit 3 Design and Construction of River Training Works
3.1 Spurs :
Spurs are structures/ obstructions constructed in rever banks to divert
water away from a specific location (eroding river bank ). Spurs are used to
deflect flow into a preferred path.
Spurs are constructed perpendicular to the river bank. Constructed in series
of three or more from a varieties of materials and requires toe protection (
apron) at their ends or noses to protect against scouring.
The purpose of these structures are to protect the river banks from the
direct impact of the water flow and to train the river by deflecting its flow.
These structures are constructed into the river banks with some extension
across the river flow.
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3.1.1 General Types :
a) Deflector Spurs
b) Retarder Spurs
a) Deflector Spurs : Impermeable, which function by diverting the flow
of water away from the bank.
b) Retarder Spurs : Permeable, which function by retarding flow velocity
at the bank and diverting away from the bank.
Specific Types :
a)
b)
c)
d)
e)
f)
g)
Bar spur
T- head spur
Sloping spur
Timber / bamboo crib spur
Round nose spur
Hockey spur
J- head spur
64
Main functions :
--- deflect flowing water/ channel
--- reduce velocity of flow near the stream bank
--- prevent erosion of bank
--- establish more desirable channel width
--- encourage sediment deposition due to reduced velocity and
increased protection of bank
--- halt meander at a bend
--- channelize wide, poorly defined streams into well defined channel
65
3.1.2 Criteria for Design:
--- spur length
--- spur orientation
--- spur permeability
--- spur height
--- bed and bank contact
--- spur spacing
Spur Length : Projected length of spur should be perpendicular to the
bank or flow direction. Normally, length of spur should be
within 20 % of the channel width for both deflector
(impermeable) and retarder ( permeable ) spurs. However,
field installation have shown that length of 3 to 30 % of
channel width have also been successful. Deflector spurs
length usually with less than 15% of channel length and
retarder spur up to 25 % of channel width are successful.
Spur Spacing : Normally 2 to 3 times the length of spurs. In fact, the
spacing of spurs is a function of spur length, spur angle to
river bank, permeability and degree of curvature of the bed.
The spur angle or flow expansion angle is an angle at which
flow expand towards the down stream bank of spur .Spur
angle or the flow expansion angle is a function of spur
permeability and the ratio of spur length to channel width.
As permeability increases, the flow expansion angle or spur
angle increases, similarly as length of spur increases the
expansion angle or spur angle increases. The expansion
angle for impermeable spurs is constant i.e 17 o. Therefore
the spacing of spurs can be determined by,
66
S = L cot Ø, where as
S = spacing of spurs, L = length of aspur, and Ø = expansion or
spur angle.
Spur Orientation: Spurs are normally placed at normal to the bank/ flow
of water. Spur angle < 90 degree ( acute angle ) oriented
towards upstream and > 90 degree oriented towards
downstream. Bank erosion is more severe if the spur is
oriented in the downstream. Spur orientation affects
spur spacing, degree of flow control, scouring depth at
the nose of spur.
Spur Permeability : Deflector spurs can be used on sharp bends to
divert water flow away from the bank. It can cause
erosion of stream bank at the spur root, if the height of
spurs are lower than the height of bank. Under submerged condition flow passes over the height of the
spurs generally perpendicular to the spurs. Retarder
spurs can be used where bends are mild and small
reduction in velocity are required.
Spur Height : Height of Deflector spurs should not exceeded the height
of river bank. If the flood height is > or = bank height,
deflector spur height should be equal to the bank height. If
the flood height is lower than the bank height, the height of
deflector spurs should be designed not to overtopping
occur at bank. For deflectors, the crest profile should slope
downward away from the bank line, which will avoid the
possibility of stream bank erosion and overtopping of flood
water.The crest profile of retarders is generally level.
67
Design Parameters of Spurs :
--- Design discharge
--- Flow velocity
--- Scour magnitude
--- Degradation and Aggradations
Design discharge:
Before designing a spur, it is necessary to calculate design discharge of
flood. The design discharge can be calculated using the following
formula:
Qmax = bA c (cumecs), where as
Qmax = Mean annual flood peak of daily discharge.
b and c are coefficient which can be fixed from the table
as given below.
A = Watershed area ( Km.2 ) below elevation of 3000m.
Table : Coefficients for Mean Annual Flood Peak Discharge
_______________________________________________________
____
Regions
Coefficients
b
c
___________________________________________________________
__
68
Karnali ------------------------------------------------1.27
0.864
Gandaki ----------------------------------------------- 2.39
0.826
Kosi
----------------------------------------------- 1.92
0.854
Southern rivers ---------------------------------------- 3.03
0.747
___________________________________________________________
__
River discharge can also be calculated from Manning's formula, which is
:
AR
2/3
×S
1/ 2
Qmax = ____________ , where as
n
Qmax = Mean annual flood peak of daily discharge
(cumecs)
A = Average cross-section area of river ( m2 ), this
should be taken corresponding to high water
mark. For a multi-channel or braided river, the
cross-section should be taken at bank full slope.
R = Hydraulic depth ( m)
S = River slope
n = Manning's roughness coefficient
Hydraulic depth is calculated by dividing average cross-srction area by
average width of the river i.e R = A / W, where W is average width of the
river. The Manning's roughness coefficient should be fixed as per the
width of a river from the table as given below :
Table : Manning's roughness coefficient ( n )
69
Major rivers ( width > 50m )
a. Straight, alluvial, sand
------ 0.020 to 0.040
b. Straight, gravel
------ 0.020 to 0.045
c. Irregular section
------ 0.035 to 0.100
Minor streams ( width < 30m )
a. Straight, regular section
------ 0.025 to 0.035
b. Winding, irregular
------ 0.035 to 0.060
Flood plains
a.
b.
c.
d.
e.
Pasture, short grass ------- 0.025 to 0.035
Pasture, high grass ------- 0.030 to 0.050
Cultivated, no crop ------ 0.020 to 0.040
Cultivated, field crops ------ 0.030 to 0.050
Light, scattered brush ----- 0.035 to 0.070
Medium to dense brush ----- 0.070 to 0.160
g. Tree land, stumps ---- 0.050 to 0.080
h. Heavy stand trees ---- 0.080 to 0.120
Excavated Channel
b. Earth, recently completed ----- 0.016 to 0.020
c. Earth with grass
----- 0.018 to 0.033
d. Rock, smooth
----- 0.025 to 0.040
e. Rock, jagged
----- 0.035 to 0.050
Surface Condition
a.
b.
c.
d.
e.
Smooth and impervious surface
---------- 0.02
Smooth and bare surface
---------- 0.10
Cultivated row crops moderate rough surface --- 0.20
Pasture or average grassed surface
---------- 0.40
Forest area with dense cover
---------- 0.80
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Flow Velocity :The flow velocity of a river is calculated from the
following formula:
Design Discharge
Average flow velocity ( Va ) =
____________________________________
Average cross-section area of river
If the river is approximately straight and as a thumb rule, the velocity
of flow that hits a spur on the river bank can be considered as, V1 =
Va, whereas V1 is velocity of flow that hits a spur and Va is average is
average flow velocity. If there is a sharp bend, then the velocity of
that hits a spur on the river bank can be considered as, V1 = 4/3 Va.
Scouring Magnitude : The scouring depth of a spur can be calculated
as,
S = Mean hydraulic depth ( d ) × scour or z factor ( Z ) for bank
full condition. Z factor can be assumed from the table as
given below.
Mean hydraulic depth = Cross-section area (A) Average
width (W) of a river at bank full condition
As a thumb rule, if the angle of flow to the bank of river is 30o, 45o, 90o,
the Z value can be taken as 2, 3 and 4 respectively.
Return Period : Return period of a flood does not mean that this flood
will only occur once in that return period. It is a probability
or chance that the flood can occur within that period. For
example, if the return period is 50 years, it does not mean
that the similar kind of flood will occur once in 50 years.
What it mean is there is probability of having a similar flood
1 in 50 or 2 % risk of occurrence every year. Similarly, if the
return period is 10 years, there is a probability of having a
similar flood 1 in 10 or 10 % risk of occurrence every year.
71
Relationship between return period, service life and risk of a structure
R = { 1 - ( 1 - 1 / T ) L }100 % where as,
R = Probability or risk that a flood occurs
T = Flood return period (yrs.)
L = Service life of structure
Degradation and Aggradation
Degradation : Bed falling ( Drop in average bed level ). It is also
downstream and upstream progressing degradation. It
occurs when bed materials load is reduced by a dam, when
water discharge increases from a diversion, bed materials
size reduces along a river.
Aggradations : Bed rising ( sediment deposition). It occurs due to
deposition of bed materials along the central portion of
river. Due to aggradations, outside banks are attack and
bank erosion occurs. It causes several channels in the river
bed and river is braided. It normally occurs in alluvial fans of
river.
Guidelines for Spur design and layout :
--- Spacing 2 to 3 times spur length.
--- Spur nose ( tips) should be well protected.
--- Spurs should be set out in the feild of three or more
--- Spurs should be tied in bank and normally be perpendicular to the
bank/flow
--- Spurs nose (tips) should be protected by apron.
3.2 Embankments :
Embankmens are the oldest and most widely used structures to check
bank erosion, minimize flooding on the alluvial fans. These structures
72
are constructed on the river banks parallel to the river
banks.Embankments are also levees or dykes or revetments. However,
these structures should be constructed along the whole eroded length
of the river bank with well keyed inthe bank.
These structures are generally made from concreate, gabions filled with
stones, earthern and stones, sand bags and logs. Hydraulic properties of
river and river characteristics such as:
River Discharge (Qmax)
Bankfull Discharge (Q b)
Width or cross-section of river (b)
Water depth (d )
Slope of river bed ( S ) and
Scouring depth are necessary to calculate for the design of the
embankment.
River discharge can be calculated from Manning's formula and other
parameters such bankfull discharge, width or cross-section of river,
water depth, hydraulic radius, river slope, scouring depth and Manning's
roughness coefficient can be calculated as explained earlier.
A free board of about 1mt. should be provided at the top of a
embankment to protect against over flow of river and an apron should
be provided with 1.5m.horizontal and 1.0 m. vertical and should be
larger than the scouring depth. However, the size of apron and free
board depends upon the design and type of embankment.
The river characteristics such as width ( b), depth (d), and slope of river
(S) are directly related with bankfull discharge. Their relationships are:
73
b ∞ Q b 1/2
d ∞ Q b 1/3
S ∞ 1 / Q b 1/6
Figure : Cross-section of embankment or Revetment
74
Figure : Construction of Embankment or Revetment
Figure : Different types of Embankment
75
3.3 Bolster
Bolsters are tubes of gabion wire or jute net filled with stones are placed
along the counter or laid in shallow trenches across the slopes, which act as
scour checks and reduce surface movement of debris and prevent the
down slope movement of debris while vegetation cover is established.
Common type bolsters are :
a) Wire
c) Hessian and
d) Jute bolsters
76
Figure : Construction of Bolster
Functions :
Tubes of 30 cm diameter are made from gabion wire and laid in shallow
trenches across the slope. They act to prevent the down slope movement
of debris and also prevent the surface scour and gullying. They are
constructed and laid down on most long exposed slopes where there is a
danger of scour or gullying on the slope
77
Materials :
a)
b)
c)
d)
Woven gabion panel ( 5*1m or 5*2m), 10 gauge wire
12 mm mild steel rod cut into 2m length
Digging and binding tools
Skilled and Unskilled labor
Unit 4 : Design and Construction of Water Harvesting Structures
Water Harvesting :
Rainwater harvesting is the accumulating and storing , utilizing of rainwater
for reuse, before it reaches the aquifer and loss due to overland flow. It
means capturing rain water where it falls or capturing the run off in the
houses, village or town The harvested rain can be used for drinking water,
irrigation, water for livestock etc. depending upon the level of pollution in
the water.
Water harvesting can be undertaken through a variety of ways




Capturing rain from rooftops
Capturing runoff from local catchments
Capturing seasonal flood waters from local streams
Conserving water through watershed management
These techniques can serve the following purposes:




Provide drinking water
Provide irrigation water
Increase groundwater recharge
Reduce storm water discharges, urban floods and overloading of
sewage treatment plants
In general, water harvesting is the activity of direct collection of rainwater.
The rainwater collected can be stored for direct use or can be recharged
into the groundwater. Rain is the first form of water that we know in the
hydrological cycle, hence is a primary source of water for us. Rivers, lakes
78
and groundwater are all secondary sources of water. In present times, we
depend entirely on such secondary sources of water. In the process, it is
forgotten that rain is the ultimate source that feeds all these secondary
sources and remain ignorant of its value. Water harvesting means to
understand the value of rain, and to make optimum use of the rainwater at
the place where it falls.
Theoretical assumption (quantity of water that can be harvested)
The total amount of water that is received in the form of rainfall over an
area is called the rainwater endowment of the area. Out of this, the
amount that can be effectively harvested is called the water harvesting
potential.
Water harvesting potential = Rainfall (mm) x Collection efficiency
The collection efficiency accounts for the fact that all the rainwater falling
over an area cannot be effectively harvested, because of evaporation,
spillage loss etc. Factors like runoff coefficient and the first-flush wastage
are taken into account when estimated the collection efficiency.
The following is an illustrative theoretical calculation that highlights the
enormous potential for water harvesting. The same procedure can be
applied to get the potential for any plot of land or rooftop area, using
rainfall data for that area..
Consider your own building with a flat terrace area of 100 sq m. Assume the
average annual rainfall in your area is approximately 600 mm (24 inches). In
simple terms, this means that if the terrace floor is assumed to be
impermeable, and all the rain that falls on it is retained without
evaporation, then, in one year, there will be rainwater on the terrace floor
to a height of 600 mm.
1. Area of plot = 100 sq. m. (120 square yards)
2. Height of the rainfall = 0.6 m (600 mm or 24 inches)
3. Volume of rainfall over the plot = Area of plot x height of rainfall
4. Assuming that only 60 per cent of the total rainfall is effectively
harvested
5. Volume of water harvested = 36,000 litres (60,000 litres x 0.6)
79
This volume is about twice the annual drinking water requirement of a 5member family. The average daily drinking water requirement per person is
10 litres.
Water harvesting can be done through three process :
Roof water harvesting
Runoff water harvesting and
Flood water harvesting
a. Rain water harvesting from roof
Figure 1 Rain water harvesting from roof
80
Figure 2 Collection process of rain water from roof
Figure 3 Different parts of collection system of rain water harvest from
roof
Catchment Area : zinc sheet roof or Concrete roof ( size depends on the size
of house
Gutter : Size depends. Normally 3”*3”or 4”* 4” size
Water delivery pipes : 4”* 4” size
Water filter tank : size depends
Water collection tank : size depends on water demand, household water
consumption/day, rainfall intensity, roof area ( catchment), Dry season
period
Water pump : hand pump or electric pump
81
b. Harvesting run off water (catchment pond):
Water availability for agriculture, home garden can be improved by
harvesting runoff water. Small scale catchment pond or impoundments or
reservoir are constructed to capture and store rainfall runoff within the
catchment area. Amount of runoff generated depends on the catchment
characteristics (area and size, type of vegetation, land use, soil,
topography, etc.) and rainfall and intensity pattern (amount, duration and
intensity)
Small-farm reservoirs sites in elevated or depressed areas (valley) where
irrigation is possible are suitable sites. Sites that are commonly owned
should be properly managed to ensure sharing among the intended
beneficiaries. Places with springs or flowing streams to ensure a year round
water supply are good sites for reservoirs. Topography that undulating or
rolling with slopes of 2 to 18 % is desirable. The size of catchment pond or
impoundments or reservoir depend upon the objectives of water
harvesting, financial resources and labor availability
82
Design consideration :
Following points should be considered in the design of water harvesting
ponds. They are ;
2)
3)
4)
Water is the main agent of erosion and landslides in the hills. Storage
of water in the pond could threat or cause damage in the certain
situation
If the water over flow the pond during summer, the overflow water
can cause scouring of the side of the pond and gully may form which
ultimately can lead to sudden burst of the pond.
Therefore, ensure that the feed drains in the pond are well protected
against erosion.
Design of water harvesting structure :
Design of ponds :
1 Calculate the desired storage capacity
2. Find the depth of water table
3. calculate the area of catchment
4. calculate amount of runoff at peak rate of rainfall
5. Observe the sub soil condition
6. Stability of side slope
7. Suitability of site
8. Safe discharge
9. Economics of construction
10. Provision of catch drain
Layout procedure :
The layout procedure of water harvesting pond depends on a) site of the
structures to be constructed and b) surveying and pegging of the water
harvesting structures
83
Consideration in the stability of water harvesting ponds :
1. Pond must be located to gain the maximum amount of water
storage for the designed size of the dam
2. The seepage from the pond should be checked or check the
soil strata of the bottom so that the seepage will not take
place in future
3. The dam and the bottom of the pond with core of clay soil is
desirable
4. The top width of the earthen dam should be at least 1m wide
and side slope at 1:2. This means 2 m high will be at least 1m
wide at the bottom.
5. Never built pond if there is adanger of slope failure or any
evidence of slope movement/failure
6. Do not built pond less than 50m upslope from a house
7. Core overflow area area should be well protected
8. Stone pittiching in the side slope of the pond if the slope is > 50
9. For the embankment to be constructed in the pond, use top
width , W = 1.105 H0.5 + 0.91, where W = top width and H =
height of dam. In general, up to 5m height of embankment a
minimum width of 2.5m is recommended
10. Always built emergency spillway in the pond using weir
formula Q = CLH3/2
Types of Ponds :
1. If the embankment type of ponds have to be built, consider the
design storage capacity on the basis of requirement and available
surface runoff.
2. Earthen embankment in the shape of semi circle can also be built
3. Trapezoidal bunds can also be used
4. Graded bunds can also be considered between the slope 0.5 -2.0 %
slope
5. Catchment storage tank of 100m2 –30,000m2 are also constructed
For long term storage, the most common runoff harvesting tank are :
6. Dugout ponds/conservation ponds/farm ponds : These ponds are
constructed by excavating the soil from ground surface. Here ground
water or surface runoff are collected
7. Embankment type reservoir; In this kind of pond a dam or
embankment around the valley or depression or a creek
84
Illustration of runoff harvesting ponds are
85
c.
Flood water harvesting : Flood water can be diverted from river and
store in a pond. Diversion structure, diversion channels are used to
divert the river flood. Structure such as spurs and diversion channels
are made at the bank of the river to divert the water into the ponds,
This water can be used during the water deficiency period.
Advantages of water harvesting:
 Excellent alternative source of water in water shortage areas
 Simple Construction - The construction of rainwater collection
systems is not complicated and most people can easily build their
own system with readily available materials.
86
 Easy to Construct and Maintenance - The installation, operation and
maintenance of a household rainwater collection system is controlled
by the individual no need to rely on water supply agencies.
 Good Water Quality - Rainwater is generally one of the better
sources of an alternate water supply when compared with other
sources of water that may be available.
 Convenience - Rainwater collection provides a convenient source of
water at the immediate place where it will be used or consumed.
 Systems are Flexible and Adaptable - Rainwater collection systems
can be adapted to suit most individual circumstances and to fit most
any household’s budget.
 Adaptation : Best way to adapt in the scarcity of water imposed by
climate change
 Improves food production
 Protects against drought
 Allows irrigation by gravity (no additional power cost required)
 Promotes soil and water conservation and ecological balance
Disadvantages of water harvesting :
 High Initial investment Costs - The main cost of a rainwater
collection system generally occurs during the initial construction
phase and no benefit is derived until the system is completed.
 Requires large number of labor
 High seepage and evaporation losses are possible
 Floating vegetation may infest reservoir
 Regular Maintenance - Regular maintenance, cleaning and repair
will be required for the operation of a successful rainwater collection
system.
 Vulnerable Water Quality - The quality of rainwater can be affected
by air pollution, insects, and dirt or organic matter. The type and kind
of construction materials used can also adversely affect water
quality.
 Water Supply is Climate Dependent - Droughts or long periods of
time with little or no rain can cause serious problems with your
supply of water.
 Storage Capacity Limits Supply - The supply of water from a
rainwater collection system is not only limited by the amount of
87
rainfall but also by the size of the collection area and your storage
facilities.
 Additional power cost require to pump the water
Unit 5 Bio-Enginering Techniques
Definition / Concept :
Various definitions and concepts of bio-engineering are being used. This is
besause to suit and specific objective for stabilizing and protecting
unstable/degraded slopes, landslides and erosion.
In general, the term bio-engineering is a technique where living vegetation
(plant) provides engineering functions to stabilize degraded slope. This can
be administered with living vegetation alone or in combination with nonliving plant materials or soft engineering to stabilize / protect degraded
slopes. In other word, bio-engineering practice can also be administered
through the integration of vegetative methods with soft/ normal
engineering practices in order to protect/stabilize degraded slopes, land
slides and erosion.
Terms like bio-engineering , vegetative engineering or vegetative structures
are commonly used by implementers / users.
Why Bio-Engineering ? Bio-Engineering technology emerged based on the
following reasons
--- engineering practices alone are not always the solution.
--- engineering practices are expensive.
--- needs high skills and tecnology.
--- not always affodable by users due to resource constraints.
--- living vegetation has many functions ( hydrological, ecological and
engineering ) where they grow.
--- living vegetation provides additional strength to the engineering
structures in integration.
88
Functions of Bio-Engineering Practices:
Vegetation/ plants used for bio-engineering work can be woody and nonwoody. Both woody and non-woody plants perform engineering,
hydrological and ecological functions.
a) Engineering functions :
Catch : Process for holding thin layer of moving soil particles /debris by
multi stemmed shrubs and bamboos.
Armour : Process for protecting soil surface and soil particles from
movement by providing protective cover.
Reinforce : Mechanism for providing strength to bind the soil particles
by densely rooted trees and grass.
Anchor : Process for firmly fixing the soil particles and debris by
anchoring action of deep / long and strong roots of trees and
shrubs.
Support : Mechanism for providing support to soil mass and rocks from
the mechanical action of root system of plants.
Therefore the engineering functions of vegetation are compatible with the
functions of engineering structures and are co-existence, because
engineering structure protects vegetation and vegetation protects
engineering structure. In some cases, vegetation replace engineering
structure.
Engineering structures for slope stabilization works are also designed to
meet the purposes of catch, armour, reinforce, anchor and support.
89
b) Hydrological function :
 Interception
 Leaf drip
 Stem flow
 Evaporation
 Transpiration
 Infiltration
 Soil water storage
 Overland flow
 Sub-surface flow
c) Ecological functions :
--- Improves harsh environment of degraded slopes into better
ecological condition.
--- Improves soil and moisture conditions and generates better
micro climate for the establishment and growth of plants
--- Encourage micro-organism and small animals and helps increase
the bio-diversity.
90
Diagram of Life span / Longevity of Bio-Engineering versus Engineering
Structures is given below:
Engineering structure
Time
Bio-engineering
Longevity
Various Forms of Bio-Engineering Techniques : Various techniques can be
used based on purpose, site conditions and availability of resources
a) Planting trees, shrubs and grasses : alone or in combination of two or
all. Develops dense network of roots in soil and dense canopy of plants
helps protect the slope from erosion. Contour line planting at regular
interval is general practice in Nepal
91
b) Planting Stumps / woody stems : Stumps / woody stems can be planted
along the contour lines to trap soil particles and debris falling down the
slopes. Sprouting stumps / stems are more preferred.
c) Seeding grass, trees and shrubs : Seeds can broadcasted manually to
cover large areas in short time and with low cost. This can be done in a
very steep unstable slopes and rocky areas where seedlings and
stumps/ woody stem can not be planted directly.
d) Contour Strip planting : Plants are planted in strips along the contour in
order to reduce erosion. Strips are left or strips interval are selected in
the slope and plants are planted along the contour. Strip intervals
depend upon the slope of the field. Cuttings or pot plants are planted.
Plant composition may include grass, fruit plants and trees. Only one or
two species of plants can be planted if the area is small. This is similar
to palisade method
92
e) Brush Layering: It is a method of planting branches of sprouting living
woody plants making small individual terraces in the slope along the
terraces. Small terraces of 15 to 100 cm wide are dug at the toe of the
slope with inward inclination of 10 % and the branches are placed
crosswise at the terraces and well covered with soil and pressed so that
the individual branches are completely embedded and covered by soil
thus encouraging root formation. The construction of brush layering
should start from bottom of the slope. The distance of individual
terrace should be less than 1.5 m to reduce possibilities of slope failure
93
f) Turfing/Sodding ; This is a method to turf or sod fresh cutting soil and
check erosion. To hold and mat the soil surface by roots, rhizome and
stolons of grass and other plants is sodding. Turf and sod consisting of
shallow rooting grass slab with soil, rhizome or stolon is placed on the
fresh cutting slope and cover by 1-5 cm of soil layer . The size of turf or
sod slab of 30 cm square or the size of 30cm * 60 cm are easy to
handle. The turf or sod should be kept moist and should be planted
when the soil surface is moist. The turfing/sodding should be done on
gentle slope less than 350 slope. Watering should be done after
plannting. Examples of turf grass are : Cynodon dactylon (dubo) ;
Trifolium species (clover)
g) Palisade : Woody cuttings are planted in lines across the slope or along
the counters. It forms a strong barrier and trap soil and derbies moving
down the slopes. Shrubs and trees suitable are : Adhatoda vasica ;
Ipomoea fistula ; Vitex negundo and other fodder trees are also
suitable. This can be used up to about 75 % slope in low rainfall where
94
conservation of moisture is needed.
h) Fascine : Also called contour wattling. It involves the bundling of
branches of sprouting types of live plants and laying them in shallow
trenches below the soil surface. After laying them in the trench, they
should well cover by soil. After trenching them well inside the trench
and cover by soil roots and shoots sprouts after the monsoon. They
form a strong line of vegetation and traps the soil and debris moving
from the upstream. This technique is suitable for the site having good
soil depth and soft cut slopes. This needs live branch cutting of one
meter length of sprouting type of species. Bundle of 5 to 6 branches
with minimum diameter one cm. are needed. Pegs of living or dead
plant should be inserted inside the surface to hold the bundles of
branches trenched in the soil. The recommended slope for this
technique is about 45 %. Species suitable for this purpose are Vitex
negundo ( simali), Pennisetum purpureum ( Napier)
i) Bamboos / Amliso planting : Rooted clum cutting, rhizomes, wild or
nursery plants of bamboos or amliso can be planted directly on slopes.
These plants perform the slope stabilization effectively once they are
established.
95
j) Wattling : Bundles of live branches with buds are put in the trench
along the contour and covered with thin layer of soil. When the
branches put out roots and shoots it forms a strong vegetative barriers
for holding soil particles moving down the slope. However, this
technique is expensive and suitable for gentle to moderate slopes.
k) Brush wood check dams : Brush wood check dams using bamboos and
woods are commonly used to stabilize gullies on slopes. Grass and
shrubs are planted down stream of check dam and on slopes os gullies.
l) Vegetated rip- rap : This is a method to strengthen slopes by a
combination of dry stone walling and planting vegetation or
broadcasting seeds in the gaps between the stones. Side slopes of
gullies and gullies bed are some time protected by constructing dry
stone walls and grass seeds are sown in gaps between the stones in
order to reinforce toe walls and gully beds. Plants to be planted in this
technique are either grass or small shrubs with strong root system.
96
Broadcasting or sowing seeds of Acaica pennata (Areri) and Butea
minor ( Bhujetro) are recommended. Cuttings of Vitex negundo
(Simali), Napier and Broom grasses are also recommended. Plants of
big trees are not recommended in this method.
m) Loose stone and Gabion check dams : Seedlings of trees, shrubs and
grasses are planted either seperately or in combination on gully heads,
side slopes, gully bed, in and around the structure to reinforce the
structure.
n) Brush wood embankment / spurs : Bamboos and woods are used to
construct embankment and spur to protect stream bank erosion. On
the back side of the structures, trees, shrubs, grass seedlings and
woody cuttings are planted to provide vigor and anchorage to the
structures. These techniques have been effective in the torrents and
small rivers of Churia and Terai.
o) Jute netting : Jute netting is another way to protect the slopes using
woven jute netting and planting grass slips or seedlings. Its main
functions are: protection of slopes, by allowing seeds to hold and
germinate; improvement of micro-climate on the slope surface by
97
holding moisture and increasing infiltration ; act as mulch for the
vegetation when the jute net is decay. It is useful on steep and hard
slopes where establishment of vegetation is difficult due to harsh
environment. Surface of the slope should be cleaned and smoothed,
cover the slope with net and peg the net at different points to support
the net. Netting should be done just before the rain starts and planting
of grass slips or cutting or planting or showing of seeds should be done
during rainy season. This method is very useful on steep and hard
slopes where establishment of plants is difficult due to steep and harsh
environment. Acaica pennata (Areri) and Butea minor ( Bhujetro) are
recommended. Cuttings of Vitex negundo (Simali), Napier and Broom
grasses are also recommended. Plants of big trees are not
recommended in this method. However, this method is expensive,
needs skills and cautious mind.
98
Application of Bio-Engineering :
Where to apply ? :
Rill erosion, shallow rock failure, shallow mass movement, small gully and
small river/ stream bank erosion
Erosion and landslides and Bio-engineering practice
Depth
Length
Upto 25cm.
upto 30cm
25- 50cm
,, ,,
> 50cm
,, ,,
Bio-engineering practice
vegetative. practice alone
vegetative and Soft engineering
soft to hard engineering + veg
Condition of Hazards
(erosion/ landslide)
Practice of Bio-engineering
a) very active
practice does not work
b) Active
practice can be used but high risk of
failure
c) dormant (but still active)
best condition to apply
d) In active ( dead )
practice not necessary
99
Limitations of Bio-engineering
-- cannot apply everywhere and every time.
-- it has some limitations on dimension of slope, nature of landslides
and erosion,
--- geo-morphological phenomenon,
--- configuration and degree of physical, climatic and environmental
stress
Plants suitable for engineering functions :
Catch : shrubs, bamboo (many stems)
Armour : grass carpets ( dense, fibrous roots )
Reinforce : densely- rooting grasses and trees
Anchor : deeply rooted trees ( long, strong roots )
Support : shrubs, large trees ( deep, dense root systems forming a soil
cylinder)
Importance of Different Types of Plants in Bio- engineering
Woody
Non-Woody
Trees
Shrubs
Bamboos Clumpig
grasses
Matting
grasses
Herbs
*
***
***
*
-
Engineerig
fonction
Catch
**
100
Armour
*
*
-
**
***
*
Reinforce
***
***
***
**
*
-
Anchor
***
**
-
*
-
-
Support
***
**
***
**
-
-
Intercept
**
**
***
***
**
**
Evaporates
***
**
***
*
*
-
Store
**
**
***
**
-
-
Leaf drip
***
**
*
-
-
-
Retard
*
**
**
**
***
*
Infiltrate
***
***
***
***
***
*
Improve
*
**
-
-
-
***
Pioner
-
*
-
**
*
***
Hydroligical
functions
Special
Functions
*** Excellent
**
Very Good
*
Good
-
Poor
101
Improved plants : Leguminous plants able to extract nitrogen from air and
converts into nitrogen compound, which increases the level of nitrates in
the soil and helps plant growth
Pioneer Plants : Plants which establish easily in harsh environment .
Produce better conditions to establish successive plants
Pioneer/ Colonizing versus Climax Species :
Pioneer/ Colonizing
Plants that first appear naturally
and can adopt in harsh
environment, they are robust and
can withstand in harsh condition.
Better to establish newly degraded
sites. Need sunlight and are short
lived .
Climex
Plants permanently appear after
series of successions. They need
better environment condition to
grow and their growth are slow.
They appear slowly after crossing
series of succesions. Normally
appears on already established
sites or improved environment.
Special adaptation of pioneer plants :
Condition of Site
Adaptation
Poor soil
Plants requiring Low nutrient
Dry soil
Drought resistant plants
Moving soil
Plants able to recover even after
disturbance
Poor germination
condition
Plants producing more seeds
102
Beneficial and adverse effects of vegetation:
Mechanical Effects
Effects
1. Stems and trunks trap materials that are moving down Good
the slope
2. Roots bind soil particles of the ground surface and
reduce their susceptibility to erosion.
Good
3. Roots penetrating through the soil cause it to resist
deformation
Good
4. Woody roots may open the rock joints due to
thickening as they grow
Bad
5. Root cylinder of trees holds up the slope above
through buttressing and arching
Good
6. Tap root or near vertical roots penetrate into the
firmer stratum below and pin down the overlying
materials
Good
Bad
Hydrological Effects
Effects
103
1. Leaves intercept raindrops before they hit the ground.
2. Water evaporates from the leaf surface
Good
Good
3. Water is stored in the canopy and stems
Good
4. Large or localized water droplets fall from the leaves
5. Surface runn-off is checked by steams and grass leaves
Bad
6. Stems and roots increase the roughness of the ground
surface and infiltration of soil
Good
7. Roots extract moisture from soil which is then
released to the atmosphere through transpiration
Site
dependent
Weather
dependent
effect
Contributions of Bio-engineering Plants :
Trees: Trees can permanently stabilize the soil horizon up to main rooting
zone (2 mt.). Soil srtucture and organic matter content Improves. Provide
shelter to the top soil, reduces KE of falling water, reduce peak run-off and
surface erosion, riverbed scouring decreases risk of mass wasting.
104
Disadvantage : Trees can not prevent natural landslides with deep sliding
zones. Trees can not prevent on-going sliding and slipping processes. It
takes 5-10 years until a dense rooting system developed Huge and heavy
trees increase the risk of sliding due to addiitional weight.
Bushes/ Shrubs : reduce splash erosion, top soil movement. faster react to
rill and gully erosions because of dense root system. Stabililize soil within
their rooting zone (1mt.). mechanical strees in sliding and steep sloping
areas.
Disadvantage : Can not help to stabilize land slides or slopes below the
main rooting zone (1mt).
Bamboos: help stabilize slopes / soil horizon within the reach of their main
rooting zone of 2m. Under normal condition, can reach up to 2/3 the ht. of
culms. Rooting systems extraordinary dense and more or less uniform. Best
retaining quality, able to survive in smoothly sliding zones. rooting system
extremely resistant to mechanical stress and force in sliding areas. Dense
culms prevents all kinds surface erosion however.
Disadvantage: It can not rapidly develop their soil conservation abilities
and takes 3 to 6 years to fully expand and grow.
Grasses: can permanently conserve the top soil within the reach their main
rooting zone of 25 cm. Protect surface soil, improves the micro-climate at
top soil. Establishment of grass layer withen one or maimum two rainy
seasons.
Disadvantage : Can not contribute to protect sliding due to shallow root
system.
Herbs/ Legumes : contribute by improving soil fertility by adding fixing
nitrogen dense rooting species. Extra-ordinary quality as pioneer plants.
Provide synergetic effects with other plants.
Disadvantage : Hardly build a dense potential vegetation cover alone.
105
Engineering Structures : to stabilize gully erosion, stream bank erosion,
land slides, stabilize steep slopes. These structures require high cost and
repair, no guarantee for permanent cure.
Biological or Vegetative measures : need time to establish, low cost , less
maintenance, use of local materials, once establish it guarantees for
permanent cure provided there is no natural havocs and abuse by people.
Biological or Vegetative Measures :
1. Agronomic Conservation Measures
2. Vegetative Conservation Measures
Agronomic Practices:
--- Crop Rotation ( maintain soil structure, less erodable than poorly
structured soil )
---- Cover Cropping ( used to prevent erosion, use permanent crops such
as orchard, horticulture crops, helps add OM through roots,
residues/ remnant of plants, when legumes are used adds nitrogen,
improves soil physical condition )
---- Mulching ( covers soil, prevent from splash erosion, affords
infiltration, regulate soil temperature, improves moisture and
reduced evaporation
---- Contour Farming ( practice of ploughing, planting and cultivating
land across the slope, acts as small terraces, holds water and
minimize erosion)
---- Minimum Tillage ( tilling only on seed bed instead of conventional
practices, minimizes erosion and tilling costs, manages soil for water
intake and reduces water and wind erosion)
106
---- Green Manuring ( practice of ploughing in a growing crop,
improves the condition of soil by humas formation and OM, if
legumes are used nitrogen adds to soil, also adds P, K and lime to
the top soil through decay of deep roots containing P, K and lime,
newly constructed terraces more advantages by adding fertility to
soil )
---- Composting/ Farm Manuring ( resuses from animals on the farm,
consists of solid and liquid in a ratio of 3:1, contains
N, P, K
---- Conservation Plantation : Harsh environment : sandy, steep slopes,
dry, eroded, rocky, water logged, flood plain, low moisture, poor
soil , poor productivity and other harsh conditions. Planting
materials : trees, shrubs, grasses, seeds that can stand in harsh
environment
.
Plantation : with or without soft structural works ( soil works) e.g
--- contour terracing
--- contour trenching
--- contour bunding
--- contour wattling
Forest and grazing land :
Forests : helps controlling erosion. Protects soil from water and wind
erosion. The vegetation on forest acts as shield against splash/ raindrop
impacts. The root system binds the soil and decreases the erosive power
of running water. The OM content of forest soil helps soil infiltration
capacity and reduces overland flow / runoff.
107
Forests helps control erosion in different ways.
Tree canopy intercepts the rainfall, which would have direct impact on
soil causing splash erosion. Tree increases the evapo-transpiration.
Under growth or ground vegetation acts as blanket to soil surface,
intercepts rain drops and protects soil from direct impact of rainfall.
Under growth or ground vegetation helps infiltration capacity of soil and
breaks water velocity. The root system binds the soil particles and
protects against running with water.
Lack of vegetation intensifies erosion through the structural weakness
and absence of armouring of the soil surface. Therefore, forests need to
be managed not only for productive point of view but also for protective
purposes. The forests of steep slopes, fragile lands, near river bank/
stream should strickly be managed under protection.
In Nepal, erosion in forest lands are mainly due to: heavy felling of trees,
over grazing of forests floor, forests fire, excessive removal of leaf
litters/ fodder /undergrowth, shifting cultivation and encroachment.
These factors cause degradation of forest and degraded forests are
vulnerable to erosion.
Control Measures : Structural, vegetative and both
Structural measures (Curative): Check dams, runoff diversion channel,
wattlings, fascines, conservation pond, retaining wall, terracing,
bunding, trenching, foot trail improvement.
Vegetative measures : conservation plantation ( trees, shrubs, grasses,
seedings ), forest management
Preventive measures : prevent over grazing and over felling of trees,
practice stall feeding, reduce unproductive cattle, prevent forest fire,
shifting cultivation and forest encroachment, avoid unsustainable use of
forests resources.
108
Grazing land : Grazing land problems responsible to erosion :
--- over stocking of animals
--- grazing beyond the carrying capacity of land
--- excessive trampling
--- year round grazing
--- lack of ground cover
Control measures :
Structural measures (Curative): Check dams, runoff diversion channel,
wattlings, conservation pond, retaining wall, terracing, bunding,
trenching, foot trail improvement.
Vegetative measures : palatable grass and shrub planting, seeding
improved varieties of grass, control undesirable/ unpalatable invader
plants.
Preventive measures : avoid overgrazing and over stocking, adopt
rotational/ diferred grazing, control fire, adopt stall feeding, fencing.
Criteria for vegetative measures :
--- Choice of species based on disturbed sites
--- Site characteristics (affects the choice of species): soil, slope, degree
of disturbances, moisture condition , existing vegetation etc.
( Species selection and planted should grow satisfactorily in a given site
condition)
--- Pioneering / colonizing species ( in badly degraded sites)
--- Mixture of grasses and legumes
--- Use of natural vegetation
--- Perinnial / multipurpose species
--- Plant rooting habit
--- Wide range of vegetation ( mixture of grass/ shrubs/ trees )
--- Grass followed by shrubs and tree seedlings and Heterogeneity of
species
109
General criteria for selection of plants :
--- ability to grow on poor site condition (exposed, water logged
degraded, dry, harsh environment)
--- fast growing/ high rate of biomass production
--- nitrogen fixing
--- coppicing ability
--- preferred by the local communities
For Gullies plugging :
--- Intensive plantation around gully heads / sides
--- Use vegetative gully plugging by planting agave and other plants in
dense lines across the gully bed.
--- Construct live checkdams
--- Where vegetation alone is not adequate, construct brush wood
checkdams or check dams using sand bags by intensive plantation.
--- In active and deep gullies use gabion/ loose stone check dams with
platation.
--- Plant grasses/ shrubs/ trees on the gully slopes depending on site
conditions
For Degraded slopes :
--- improve pits soil by adding compost/farm yard manure/humus rich
forest soil
--- established vegetation along the contours ( if slopes are too steep
use contour trenching, bunding, terracing, wattling etc.)
--- encourage natural regeneration.
110
Unit 6 Conservation Techniques Soil Conservation techniques used
by DSCWM)
6.1Gully and Landslide Treatments
Gully treatments are generally done to prevent further degradation of
the gully and its watershed through controlling runoff and erosion. Gully
control can be done either from vegetative or structural measures or
combination of both in the gully and its catchment. The activities in the
gully treatment includes :
 Gully head diversion ditches/drainage
 Gully head plugging by building structures like check dam
 Gully bank and its catchment revegetation, counter wattling and
turfing
 Gully bank slope correction
 Conservation ponds to store and divert excess runoff
 Appropriate landuse practice in the gully catchments
 Fencing the gully periphery and its catchment to prevent from
cattle grazing
Landslide treatment :
Landslide treatment refers to the vegetative and structural measures
applied in the landslide area and its catchment. These activities are
generally done to reduce soil erosion and mass movement from
landslide and reduce devastating effects on the downstream and
surroundings, where landslide occurs threatening the life and
property. The activities under the landslide treatment are:
 Construction of diversion channels around the landslide area
to drain water or to stop water to enter into the landslide
 Structural erosion control measures such as construction of
retaining wall/breast wall and check dam
 Landslide stabilization through bio-engineering practices
 Conservation ponds to store and divert excess runoff
 Appropriate land use practice in the gully catchments
111
 Fencing the gully periphery and its catchment to prevent from
cattle grazing
6.2 Slope stabilization
Slope stabilization refers to the vegetative and structural measures
applied to stabilize degraded slope and reduce erosion. This activity
include :
 Tree and grass planting with necessary conservation measures using
bioengineering techniques
 Erosion control measures such as micro-gully plugging, contour
wattling and structures such as check dam, retaining wall etc.
 Improvement of drainage system
 Silvi-pasture management
 Fencing the slope to check livestock grazing
6.3 Sream/River bank Erosion Control
Stream/River bank erosion control measures refers to prevent
stream / river bank erosion or bank cuttings through vegetative and
structural measures. Its main aim is to prevent bank erosion and
protect the land from stream/river cutting. The activities under the
Stream/River bank erosion control include :







Construction of embankment/revetment
Construction of spurs
Construction of water flow retarding structures
Channelization efforts to manage discharge
Flood plain stabilization through bio-engineering measures
Vegetative measures
Silvi-pastoral management in the catchment
112
6.4 Road Slope Erosion Control
The road slope erosion control measure refers to vegetative and
structural measures applied in the road slope to reduce erosion
and protect the road from erosion and landslides and to improve
the road for general traffic. It is similar to slope stabilization to
maintain stability of the slope. The activities under this programme
are :
 Trees and grass planting with necessary conservation
measures such as contour terracing, cotour trenching,
contour bunding
 Retaining/breast walls with planting bio-engineering plants
 Erosion control measures such as micro gully plugging,
contour wattling, and structure like check dam
 Improvement of roadside drainage system
 Fencing road slopes for livestock control
6.5 Cultivated, Forests Lands and Pasture Lands
Cultivated : Erosion from cultivated lands are due to :
 Improper land use: cultivation of steep slopes not suitable for
cultivation
 Improper cultivation practices: ploughing up and down hill
slopes etc.; continuous use of land for the same crop without
fallow or rotation
 Inadequate use of farm manure
 Compaction of soil through the excessive use
Erosion from cultivated land can be control through adapting :
 Agronomic practice : it includes mulching and crop
management. Crop management includes : high density
planting, multiple cropping and cover cropping
113
 Mulching : It is a method of spreading mulches above
the ground to keep the soil cool during warm and warm
during cold and moist. It also conserve moisture in the
soil and root zone of the plant. Mulching enhance the
rate of growth of plant by providing moisture to the
plant. Mulching materials are leaves, straw, sawdust and
other organic materials. Mulch reduce the rain drop
erosion, surface flow and increases infiltration. Mulching
should be done during dry season to conserve moisture
and reduce evaporation loss.
 High density planting : To cover the ground as quickly
and as densely possible
 Multiple cropping : it includes crop rotation and strip
cropping
 Soil management : includes conservation tillage such as
contour farming and minimum or no tillage. Contour
farming refers to all the field operations such as
ploughing, seeding, planting and other cultural practices
along the contour. Contour farming reduce the velocity
of runoff/overland flow, controls soil erosion and
conserve moisture
 Mechanical method : Mechanical method includes
tillage. The effect of tillage on the soil erosion is function
of aggregation, surface sealing, infiltration and resistant
to erosion. For soil conservation, the conservation tillage
should be adopted in which tillage should not be more
than necessary, till only when the soil moisture is in
favorable limit. Adopt minimum/no tillage practices
which is the preparation of seed bed with minimum
disturbances of soil
114
Forests land : Forests vegetation has played various roles in controlling
erosion. Causes of erosion in forest area :
 Heavy felling
 Heavy grazing
 Shifting cultivation
 Lack of mixed vegetation
Roles of forest in soil conservation:
Three roles of forests
 Canopy level : intercepts rainfall and increase evapo-transpiration
 Ground level : intercept the rain, increase evapo-transpiration, litters
at ground acts as sponge for water retention and reduce run-off
 Root level : Root system of vegetation helps to bind the soil and
increase the infiltration rate
Pasture lands :
Problems of erosion :
 Overgrazing
 Premature grazing
 Continuous grazing
 Trampling
Soil conservation measures :





Reduction of number of livestock
Pasture land improvement
Stall feeding
Fodder plant and grass planting in community land
Fodder and grazing management in community forests
115
Unit 7 Conservation Farming Techniques ( adopted by DSCWM)
7.1 Shelter Belts/Green Belts :
 This is a belt of trees, shrubs and grass maintained to protect soil from
wind erosion and conserve moisture to increase productivity of
cultivated land. It generally includes planting of trees, shrubs and grass
in rows mostly across general wind direction. Its main objective is to
reduce the wind velocity and thereby reduce the wind erosion and
conserve soil and moisture foe better production of agricultural land.
Tall wind resistant crops and normal crops are planted alternatively in
narrow strips perpendicular to the direction of prevailing winds.
This programme includes the following activities :
 Surveying and preparation of location/orientation of strips
 Planting of trees, shrubs, hedge and grass in combination as a
shelter belt in apredesigned pattern of spacing and height
 Protection of vegetation by fencing
 Seedling production
 Water erosion control measures in case of erosion prone area
7.2 Hedgerows:
These are simple erosion control practices on sloping land. In these
practices nitrogen fixing trees, shrubs, grass, fruit trees and other
crops are planted as hedge in a rows along the contour. Various
trees and crop species are established in the hedgerows to enhance
farm income and diversity. Trees, shrubs horticulture plants,
grasses are planted in the outer edges of strip. Food crops are
planted in the strip between the rows . These practices help slow
down the passage of rain water and trap soil to gradually form
natural terraces. These practices also improve soil fertility and crop
production. Contour hedgerows are indigenous practices, which are
adopted in many developing countries. These activities Include :
116
 Planting of fruit, grass, trees, grass in the strip of rows along
the contour
 Plantation of food crops between the rows
Advantages :





Reduces soil erosion
Improves soil fertility and soil moisture
Provides biomass for green leaf manure
Provides shading for young plants
Serves as a source of fodder, fuelwood and light construction
materials
 Improves soil structure and water infiltration
 Provides a source of mulch
Limitations :
 Loss of land for cultivation due to establishment of contour
hedgerow
 Hedge rows compete with food crops planted between the rows
 Hedgerow plants may be host to pests
 Retention of excess water may result in soil slippage on steep slopes
7.3 Minimum tillage/Zero tillage :
This is a technique to adopt minimum tilling practice in the farm
land. In this practice heavy equipments and full tilling operations
will not be carried out. Full tilling practices disturb and dislodge the
soil particles and structures and cause erosion. Simple farm
equipments such as hoes and digging sticks are used to prepare
land and plant food crops. This practice is common and effective in
controlling soil erosion particularly in highly erodible and sandy soil.
Advantages :
 Lessens the direct impacts of raindrops on bare soil, thus
minimizing soil erosion
 Minimizes degradation of soil structure
117
 Slows down the rate of mineralization, leading to more
sustained use of nutrient in the organic matter
 Requires less labor than full tillage
 Can be practiced on marginal soil/land that might not
otherwise be fisible to cultivate
Limitations :
 Inadequate seed bed preparation may lead to poor establishment
and low yield of crops
 Rooting volume may be restricted in soils.
7.8 Cover cropping
Cover crops are close growing crops planted mainly for protecting
soil between regular crops. The types of cover crops can be annual
or perennial legume crops (beans, cow peas, peas, rahar) and
grasses depending on actual needs of the farmers.
Cover cropping is a practice to protect soil from erosion and to
improve the condition of soil through green manuring. In cover
cropping usually short term crops (less than 2 years) planted in the
fields or under the trees during fallow period. Cover crops are also
inter-planted with grains crops such as maize or planted once in a
cropping cycle. Cover cropping is also practiced to suppress weeds
under the tree crop and supply as a forage to livestock. The practice
of cover cropping can also be used in fallow systems to improve soil
fertility and shorten the fallow period. Most of the cover crops
belong to the legume family such as : kudzu, pigeon peas, mung
bean, arahar, cow pea etc.
Cover crops
Cover crops protect the soil from wind and water erosion by covering it
and, because they form a mulch, they greatly reduce annual weeds in the
next growing season. They are frequently used to cover the soil over winter
either alive or as a dead, dense mat. They can also be used in summer,
118
especially when a crop fails because of adverse weather. Examples include
red and sweet clover, hay or pasture seedings, hairy vetch, winter cereals
and buckwheat. A volunteer crop seeded from harvest losses, can also be a
cover crop. On the Prairies, organic farmers use annual legumes as cover
crops for at least part of the season rather than leaving summer fallow
bare. Indian Head lentils and Sirius peas have been developed for this
purpose.
Advantages :







Improves soil fertility, physical and chemical properties of soil
Reduces soil erosion and water loss
Suppress weeds
Reduces need for fertilizer and herbicides
Provides human food and animal forage
Increases soil organic matter
Helps retain moisture in the soil and prevent soil from drying
Limitations :
 May compete for soil moisture and nutrients with the
perennial plants
 Involves additional farm labor and inputs
 May result in weed problems
 May be alternate host for pest
 Some cover crop species may contain chemicals which
inhibit subsequent crop growth
7.9 Mulching
Mulching is a soil and water conservation practice in which a
covering of cut grass, crop residues or other organic matters is
spread over the ground between rows of crops or around the
plants. This practice helps to retain soil moisture, prevents weed
growth and enhances the soil structure. It is commonly used in
areas subject to drought and weed infestation. The choice of mulch
119
depends on locally available materials. The optimum density of soil
cover ranges between 30% to 70%
Advantages :
 Intercepts the direct impacts of rain drop on bare soil and
reduces runoff and soil loss
 Suppresses weeds and reduces labor costs of weeding
 Increases soil organic matter
 Improves soil chemical and physical properties
 Increases the moisture holding capacity of soil
 Helps to regulate soil temperature
Limitations :





Possible habitats for pests and diseases
Not applicable in wet condition
Difficult to spread evenly on steep land
Lack of available materials suitable for mulching
Some grass species used as mulch can root and become a weed
problem
7.11 Compost manure
Compost is a type of organic fertilizer derived from the
decomposition of plant and animal waste. It is an excellent source
of plant nutrients. Composting is common in home gardens. There
are many ways to prepare compost manure depending upon
socioeconomic and biophysical factors. The use of compost is a
traditional soil fertility management practice through out the
developing countries. Composting involves the decomposition of
plant animal waste. The decomposition process involves bacteria,
beetles and earthworm. Moisture content, adequate supply of air
and temperature control are important parameters for quality
compost production.
120
Advantages :
 Generates nutrients for crop
 Generates heat and maintain temperature
 Maintains soil structure and increases soil infiltration
 Minimize soil pollution
Limitations :
 Compost making requires a large quantity of plant materials
or biomass
 Limited use in low land where severe weed infestation is a
problem
 Difficult to practiced on steep slopes
 High labor requirement to harvest, haul and distribute
7.10 Green manure
Green manure are plants that are shown specifically to improve soil
fertility. They are not harvested for food and not allow to flower.
A green manure cover crop, or plowdown crop, is any crop that is turned
into the soil to add organic matter, nitrogen or other nutrients. It is a
Plowing-under of a green crop or other fresh organic materials
Traditionally, green manure crops are sown and allowed to grow, either
until the land is needed again or until the plants have reached a certain
growth stage. At this point, they are cut down, dug in to the soil and are left
to decompose, releasing vital plant nutrients back into the soil which are
then used by the next crop.
But if it is not dig, then green manure crops can also be composted or used
as a mulching material instead.
There are many varieties of plants which are suitable for use as a green
manure crop and some of these are listed in the table below.
However, if there is not enough land left, to devote entirely to growing a
green manure crop, it is also possible to sow some green manure crops (e.g
121
white clover) on paths between beds. And crops, such as field beans, can
even be sown in between rows of vegetables in your raised bed system if
you are short of space. Mixtures of green manure plants can also be used.
For example: fieldbeans/mustard; or vetch/clover/rye.
When selecting the crops that you are going to grow, you should bear in
mind the following points:




Choose either a quick or a slow-growing crop - to fit in with the
time that the land will be left vacant.
The season of the year. (Not all varieties will survive the
winter.)
Whether you want your crop to fix nitrogen or not.
Your soil type and how much drainage it offers.
When is the Green Manure Crop Ready for Use?
On the whole it is better not to leave your green manure crop in the ground
for too long, as land occupied in this way can not be used for growing other
crops. Also, if green manure plants get too old, then they can become tough
and will take longer to decompose and be incorporated into the soil by soil
organisms. For most green manure crops, it is usually recommended that
they are cut and used before they flower.
How to Use Green Manure Crops
Usually, green manure crops are cut down and dug into the top 15-20 cm of
soil with a spade. But, veganic gardeners, or anyone else who wishes to
avoid digging the soil, can simply hoe off young plants (or chop down older
ones) and leave them on the soil surface as a mulch. If plants are chopped
down, then to prevent any regrowth of the stubble, cover the ground with a
light-excluding mulch (e.g. black polythene/newspaper) until you are sure
that the green manure crop is dead. If you are in a hurry to start replanting
the ground, then you can of course simply plant through the mulch. In any
case, you will need to allow several weeks before planting the next crop in
the mulched area, in order to give the mulch some time to decompose and
release its nutrients back into the soil. Alternatively, if you do not wish to
use your crop as a mulch, then you can compost it instead. Composting is in
fact a very good way of using up any crops which have been allowed to get
too old and tough!
122
The best time to cultivate the green manures is after most of the plants have
started to bloom or are close to heading, but before they go to seed. Waiting
too long allows the plants to become woody and will be slower to
decompose. Harvesting earlier is fine but the plants will not have reached
their maximum amount of stored nutrients and potential organic matter. Use
a spade, mower, or string trimmer to chop up the green manures, then either
mix them in with the top few inches of soil or rake them up and compost
them. If they are removed to be composted, remember that you are removing
soil nutrients temporarily and compost will need to be added before planting.
If the green manure is turned into the soil, wait until they have decomposed
before planting the next crop. This is usually one to three weeks depending
on the crop, the soil and the weather.
Suitable Green manure crops :
NAME
Alfalfa
*Winter
Field
Beans
Buckwhea
t
*Clover,
Alsike
*Clover,
crimson
*Clover,
Essex red
LATIN
NAME
WINTE
SOWIN
R
G TIME
HARDY
WILDLIFE
VALUE
GROWING
TIME
Medicago
sativa
Yes
Apr-July Bee
1-2 mths or a
few yrs
Vicia faba
Yes
Sept-Nov Bee plant
overwinter
No
MarchAug
up to 2-3 mths
Yes
Apr-Aug Bee plant
1-2 mths or a
few yrs
Possibly
MarchAug
2-3 mths
Yes
Apr-Aug Bee plant
1-2 mths or a
few yrs
Possibly
MarchAug
Butterfly
nectar
2-3 mths
Possibly
MarchJune
Bee plant
2-3 mths
Fagopyrum
esculentum
Trifolium
hybridum
Trifolium
incarnatum
Trifolium
pratense
Trigonella
Fenugreek foenum
graecum
Lupinus
*Lupin,
angustifoliu
bitter
s
Hoverfly
nectar
Bee plant
123
Mustard
Phacelia
Rye,
grazing
*Trefoil
*Tares,
winter
Sinapis alba Possibly
Phacelia
Yes
tanacetifolia
Secale
cereale
Medicago
lupulina
Yes
Yes
Vicia sativa Yes
MarchSept
MarchSept
Aug-Nov
MarchAug
MarchSept
None
2-8 wks
Bee plant
2
mths(summer)
, 5-6 mths
(winter)
Bee/caterpilla
r food
Bee/butterfly
nectar
Bee/butterfly
nectar
autumn-spring
up to a few yrs
2-3 mths or
overwinter
Advantages of Green Manure
1. They're cheap and easy to grow.
2. A packet of green manure seeds is easy to carry home - unlike a
large sack of animal manure!
3. They can increase soil fertility.
4. They improve soil structure and help prevent soil erosion.
5. They encourage efficient use of land. So why not grow a green
manure crop on your unused land this winter?
6. Most green manure crops are very attractive to wildlife.
7. Bare soil encourages weed growth, so green manure bare
ground to keep weeds in check.
8. By taking up nutrients from the soil, green manure crops
prevent them from being washed away when it rains.
9. Some green manure plants (legumes) are nitrogen fixers.
10.Green manuring increases the humus content of the soil.
Limitations :
• Seed cost and availability vary considerably.
• Seeding, maintenance and incorporation requires extra time, labor and fuel.
• Green manures may harbor pests and plant disease so a good rotation
should be followed.
124
• Some cover crops are difficult to turn under and may require repeated
tillage which will accelerate organic decomposition and soil erosion.
• While the green manure residue decomposes, there may be a short period
when nitrogen will be unavailable to the following crop.
• Residue of any sort can become allelopathic (exude toxic chemicals) to the
following crop and may interfere with seed germination.
• Living or winter-killed green manure can retard spring soil-warming by
acting as a mulch. This, in turn, can delay or retard growth of temperaturesensitive crops such as corn.
• Some green manure crops, such as oilradish and buckwheat will become a
weed in the succeeding crop if they are allowed to set seed.
7.11 Strip cropping
It is a conservation practice mainly to grow crops in systematic strips or
bands, which serve as barriers to water and wind erosion.
 Contour strip-two or more crops are grown along the contours in
alternative strips.
 Field strip cropping- The alternative strips are uniform widths across the
field and not necessarily curved to conform to the contour.
 Wind strip cropping- Tall wind resistant crops and normal crops are
planted alternatively in narrow strips perpendicular to the direction of
prevailing winds.
7.5 Relay cropping
Growing of another crop before harvesting the main crop. There is
no need of tillage operation. Example: Paddy-lentil
7.6 Multiple cropping
Growing of more than one crop on the same land in a year.
7.7 Mixed cropping
125
Growing of two or more crops together at the same time in same
land. Example: wheat + mustard, wheat + peas etc.
Unit 8. Erosion Process and Monitoring
8.1 Erodibility of Soil
Erodibility of soil 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 detachment and transporting agents. In
erodibility of soil, the properties of soil are most determinant and
important factors. Erodibility of soil varies with soil texture, aggregate
stability, shear strength, infiltration capacity and organic and chemical
content of soil. Erodibility of soil also depends in part on topography,
slope steepness, and level of disturbances imposed on soil by human
activities.
In the case of soil texture, soil having large particles are resistant to
detach and transport, since it requires greater raindrop kinetic energy
and greater force for transport. Similarly, fine particles of soil also
resistance to detachment because of their cohesiveness. The least
resistant particles are silts and fine sands. Thus soil with a high silt
and sand content are more erodible. By experiment, it was observed
that soils with 40-6-% silt content are most erodible and soils with 930% clay content are also found most susceptibility to erosion.
However, the range of different soil textures susceptible to erodibility
are found different in different studies.
In terms of aggregate of soil, for example, clay particles combine with
organic matter to form soil aggregate or clods is more stable to
erosion. Erodibility of soil also depends on types of clay materials
content on soil aggregates. Soil with high content of base minerals are
more stable to erosion as they contribute to chemical bonding of the
aggregates. The greater the proportion of soil aggregates, the more
resistant to erosion
126
In shear strength of soil, which is a measure of its cohisiveness and
resistance to shearing forces, the more shear strength of soil, the less
is erodibility, but it can cause mass movement of land.
The infiltration capacity is influenced by soil pore space or size, pore
stability and form of soil profile. Soil with stable aggregates maintain
their pore space better and is less erodible than the soils with swelling
clays and minerals because of having low infiltration capacity.
The organic and chemical contents of soil are important because of
their influences on aggregatee stability. It has been found that that
soils with 0-4% organic matter content can be considered erodible.
In the case of wind erosion, the erodibility of soil depends upon wet and
dry aggregate stability and moisture content of soil. Wet soil is less
erodible to wind than dry soil with low aggregates. Other elements of
erodibility of soil by wind erosion are same as the elements discussed
above in water erosion.
Soil Erodibility Factor ( K) : The soil erodibility factor (K) reflects the
susceptability of soil to erosion. The soil erodibility factor ( K) is a function
of soil's percent silt and very fine sand, percent sand, percent organic
matter, index of soil structure, index of soil permeability. K value can be
obtained if grain size distribution, organic content, soil structures and
permeability are known.
Parameters for Consideration of Soil Erodibility :
127
The parameters for the consideration of soil erodibility are :
Particle Size : Soil particle size is a major determinate of soil erodibility,
which is defined as the products of % silt and and very fine
sand % sand
Organic Matter : This is combination of organic matter contents in top soil
and subsoil. OM in the range of 0 - 4% is inversely related to
erodibility or succeptable to erosion
Soil Structures : Soil structures such as type of soil and size particles of
are important in determining erodibility. Structures are:
i.
ii.
iii.
iv.
Very fine grannular and very fine crumb ( <1mm)
Fine grannular and fine crumb (1-2mm)
Medium grannular, medium crumb (2-5mm)
Platy prismatic, columber, blocky and very coarse
grannular
Soil Permeability : (Surface and sub-surface)
1.
2.
3.
4.
5.
6.
Rapid to very rapid
Moderately rapid
Moderate
Moderately slow
Slow
Very slow
Nomograph need to be referred for determining the Soil Erodibility Factor
( K)
128
Some Factors Responsible for Soil Erodibility :
1) Slope Length and Gradient factor :
Erodibility also depends on slope steepness, slope length, velocity and
volume of surface runoff. As these factors increases, the erodibility of soil
increases. The relationship between erosion , slope steepness and slope
can be expressed by an equation :
Q ∞ tan m A x Ln , where as Q = erodibility per unit area, A = gradient
angle, L = slope length, m and n are factors, which values depends upon
interaction of other factors in the erosion - slope relationship. These
factors are : grain size of materials, rainfall, slope and its shape, surface
runoff, type of vegetation cover, processes of erosion.
Slope length also affects erodibilty. Longer the slope, more will be the
erosion. As slope increases, the land surface catch large amount of rain. If
the infiltration capacity of land surface is slower than the intensity of
129
rainfall then overland flow occurs. As slope gradient increases, the
detention of water on land surface becomes less, the velocity of flowing
water will be high, which in turn, will detached and transport more soil
causing erosion.
2 ) Vegetation :
The major role of vegetation is to intercept raindrops so that their kinetic
energy is dissipated by the plants. The effectiveness of vegetation cover
in reducing erosion depends upon the height and continuity of canopy,
the density of plant cover and the root density. Ground cover by
vegetation not only intercepts the rain but also dissipitates the energy of
flowing water and wind, imparts roughness to the flow and thereby
reduces the velocity Erosion rates varies cube or fifth power of velocity, V
3
or V 5. The effect of root network of the vegetation is in openning up the
soil, thereby enabling water to penetrate and increasing infiltration
capacity. Generally, forests are the most effective in reducing erosion
because of their canopy. A dense growth of grass may be also efficient as
forests. It has been noted that for adequate protection of erosion 70 % of
the ground surface must be covered by vegetation. Mechanical binding of
soil particles by the net work of root. Vegetation improves the soil
structures by adding organic matter. Humas layer acts as sponges and
absorbs enough water and moisture which helps water to enter into the
soil. Vegetation obstruct the velocity of flowing water and helps
dissipitate energy of running water and sometime hold the soil particles
flowing through the running water. However, the rain drops interceptd
and collected by the canopy may form larger drop which when drops on
the ground are more erosive
8.2 Erosivity of Rainfall :
Soil loss is closely related to rainfall partly through the raindrops and
contribution of rain to runoff. Effect of intensity of rainfall is cosidered to
130
be the most important factor. It has been found that average soil loss per
rain event increasaes with the intensity of the storm.
It also appears that erosion is related to two types of rain events, the
short intense rain where the infiltration capacity of the soil is exceeded
and the prolonged rain of low intensity which saturate the soil.
The most suitable expression of the erosivity of rainfall is an index based
on the kinetic energy (KE) of the rain. Thus the erosivity of a rain is a
functions of its intensity, duration and the mass, diameter and the
velocity of the rain drops. To compute erosivity requires an analysis of the
rain drop size and distribution of rain. Rain drop size characteristics vary
with the intensity of the rain, the medium drop diameter increases with
the increase in rainfall intensity. However, the medium drop size
decreases with increasing intensity presumably because greater
turbulence makes larger drop size unstable. Some studies have shown
that drop size and rainfall intensity is not always constant and both vary
for rain of same intensity but different origin.
Wischmier and Smith relationship is : KE = 13.32 + 9.78 log I, where I =
rainfall intensity (mm/hr) and KE = kinetic energy of strom ( J/m2/mm )
Hudson Equation is : KE = 29.8 - 127.5 / I
To compute the KE of the storm, a trace of the rainfall from automatically
recorded rain gauge is analyzed and the storm divided into small time
increments of uniform intensity. For each time period, knowing the
intensity of the rain, the KE of rain at that intensity is estimated from one
of the above equations and this multiplied by the amount rain received
gives the KE for that time period. The sum of the KE values for all time
periods gives the total KE of the storm.
Calculation of erosivity :
Time from
Rainfall
Intensity
KE
Total KE
131
start (min)
(mm)
( mm/hr) ( J/m2/mm)
( col. 2 × col. 4 )
0-14
1.52
6.08
8.83
13.42
15-29
14.22
56.88
27.56
391.90
30-44
26.16
104.64
28.58
747.65
45-59
31.50
126.00
28.79
906.89
60-74
8.38
33.52
26.00
217.88
75-89
0.25
1.00
------
-------
8.4 Erosion Monitoring :
(Justification/ Importance)
Erosion monitoring is a continuous process for studying the level of
erosion and runoff from a paticular land-use, where the maximum
sustained productivity is threatened by excessive soil loss or erosion. Such
study should aim at collecting information on erosion and runoff by
allocating an area into different plots having different land use.
Nepal has serious erosion problems in almost all ecological regions. It has
caused serious on and off site environmental, economic and social impacts.
The main causes of soil erosion in Nepal are due to fragile geology of
mountains, high erosivity of vaused by monsoon rain and unsustainable
132
human activities. Land degradation, gully formation, landslides, riverbank
erosion, floods are prevelant. Therefore, studies on soil erosion are of great
importance to understand such problems. In addition, the runoff and soil
loss behaviors are different in different land use type, condition and
treatments. Therefore the basic information on the runoff and soil loss
patterns under different land use types is needed for better watershed
management. Erosion monitoring entails how effectively the government
resources and efforts are being utilized to minimize erosion. Information
received from erosion monitoring can be taken as a basis for future
planning of sustainable soil conservation interventions. In other word,
erosion monitoring gives insight for better land use recommendations and
justify the implications of conservation activities.
8.4.1 Run-off Plot monitoring
Erosion monitoring is a technique to enclose a part of land and
monitoring the surface erosion and runoff from the enclosed area by
collecting the overland flow in tanks placed at the bottom of the
enclosure. This technique is known as runoff plot or erosion plot (EP)
monitoring, which has been used extensively all over the world to
measure runoff and soil erosion from small areas.
A number of such run-off plot monitoring works has been carried
out in the different parts of middle mountain physiographic zone
of Nepal by different organizations at different times. Such
organizations are Department of Soil Conservation and watershed
Management (DSCWM), National Agricultural Research Council (
NARC ) and International Centre for Integrated Mountain
Development (ICIMOD).
Objectives of EP or Run-off plots: Main objectives of erosion or
runoff plots are :
1. To estimate the run-off and soil loss under different
land-use practices
2.
To study the effects of conservation measures
133
Methodology : Run-off or erosion plot (EP) monitoring consists of
studying surface runoff and soil erosion with corresponding
rainfalls for the land type where the EP is located. Through the
size of EP varied from 48-100m2 , the lay-outs of the EPs were
almost the same that is it could be rectangular and elongated
towards the land slope. In most of the cases, EPs size were 100m2
in size, 20m along the slope and 5m across. EPs were bordened on
all sides with metal sheets buried in the ground and a gutter was
set up at the bottom to direct the runoff and its contents to the
collection tank system. The parameters studied in the EPs
technique are related with rainfall pattern, a meteorological
station was installed near the EP to record rainfall data.
For each rainfall day or event, runoff volume was measured from
the water collected in the tanks/drums. Sediment samples were
collected from the tanks containing runoff volume by agtating the
water in the tanks. A sample bottle of known volume, usually 500
or 1000ml. was used for this purpose. After the sample was
filtered, oven-dry and weighed, the amount of sediment in the
sample bottle was determined. Based on this figure, the actual
sediment in the tank and drum was calculated, and by summing
the quantities of sediments from all the tanks or drums, the total
soil loss from the particular event or day was determined.
Following information need to be collected before setting EP plots
at site :
-- Altitude of the site
-- Land use
-- Soil type
-- Soil Texture
-- Slope/ gradient of EP
134
-- Crop rotation / Cropping patterns
-- Treatments to be administered ( Each EP should have one
treatment )
-- Rainfall data by erecting rain gauge
Treatments : Treatments need to be pre-determined based on
the objectives of study. Example of treatments are :
In Kulekhani Burrow Pit ( 1985 - 1989), the treatments were :
3. Outward sloping terraces
4. Inward sloping terraces
5. Outward sloping terrace with cotour ridges
6. Hillside ditching ( hillside ditch at 8m. intervals)
In Tistung EP (1996-2000), the treatments were :
1. Farmers' practice without hedgerows
2. Farmers' practice with hedgerows of most prefered
species Alnus nepalensis
3. As treatment II but without nutrient inputs
4. Farmers' practice with hedge of second most preferred
species ( Indigofera sp.)
5. Farmers practice with hedgerows ( Alnus nepalensis )
and inclusion of fruit trees, vegetables and cash crops
for higher economic gain
The Tistung site represented outward sloping rainfed agricultural
terraces. The major crops grown in the treatments were potato,
maize and mustard
Examples :
Kulekhani Burrow-pits ( 1985-1989)
Site location and characteristics :
135
The EPs were located at an altitude of 1650m in Markhu village of
Kulekhani watershed, Markhu district. The plots were laid on the
former grazing land with very shallow and stony soil. The soil
texture was clay to clay loam with very low humus content. The
original gradient of the plot ranged from 14% to 22%. The major
cropping pattern was maize followed by mustard. The following
four treatments were applied in the EPs :
I. Outward sloping terrace
II. Inward sloping terrace
III. Outward sloping terrace with countour ridges
IV. Hillside ditching ( hillside ditch at 8m. intervals )
The rainfall, runoff percent and soil loss in the above treatments from 1985
to 1990 are presented in Tables 1 and 2. These figures and information on
EP have been obtained from Kulekhani Erosion Plot Annual Reports of
DSCWM.
Table : Rainfall and runoff percent for different treatments, 1985-1990,
Kulekhani.
Year
Rainfall (mm)
Runoff %
Treatment I
Treatment II
TreatmentIII
TreatmentIV
1985
1662
18.1
23.0
19.4
25.2
1986
1559
23.7
27.1
27.9
31.0
1987
1295
24.4
24.1
22.9
30.3
1988
1470
11.9
10.8
12.7
16.4
1989
1338
14.4
12.4
15.3
18.2
1990
1185
20.7
16.1
22.5
22.7
Average
1418
18.8
18.9
20.1
24.0
Table : Soil loss for different treatments, 1985- 1990, Kulekhani
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Year
Soil loss ( t / ha )
Treatment I
Treatment II
Treatment III
Treatment IV
1985
1.22
1.27
2.66
1.58
1986
2.58
2.37
3.98
3.68
1987
1.07
0.85
1.27
1.35
1988
1.58
1.15
1.45
2.01
1989
1.17
1.27
2.18
1.88
1990
1.49
1.18
1.91
1.94
Average
1.52
1.35
2.24
2.07
Applying the same procedures, techniques and principles as explained
above but with different treaments, various EP sites at Tistung (Makwanpur
district), Subbakuna (Surkhet district), Jhikukhola (Kavrepalanchowk),
Yarshakhola (Dolakha district), were established in collaboration of DSCWM
and ICIMOD.
Design consideration : It includes :
1. Plot design : Size of EPs varies, however DSCWM used 16m by 5m (
80m2 ) EP. EP are bordered with metal sheet extending 40-50cm.
above surface and 25 cm. buried in the ground.
2. Runoff Tank design : For collection of runoff and sediments that
produced by rainfall event, two metal tanks ( A and B ) are installed
for each EP. The maximum capacities of A tank was 3m3 and that of
B tank was 1.5m3. Tank A consists a divisor that allows about 1/4 of
the overflow into tank B. The effective capacity of two tanks per
treatment with two replications is 9m3 However, the size of tanks
should be designed in such a that they can accommodate
maximum 24 hr. rainfall of that area.
3. Replication design : EP plots can be divided into blocks. Selection of
replications for each treatment is determined by random selection
from each block. Although three replication for each treatments is
desirable, there were only two replications in each treatment.
137
Measurement Procedures : It includes :
1. Field measurement : The average water depth in the tank is
computed by taking water depth at four corners of the tank and
averaged them. The volume of the runoff water in the tank is
directly determined by taking reading from the Calibrating Chart.
The water depth is measured every morning at 9 AM. The amount
of rainfall is recorded by both a standard rain gauge and an
automatic rain gauge. Both the rain gauge should be placed near the
EP. Intensities of rainfall are calculated on the basis of the recorded
marking on the chart. Water samples for sediment analysis are to
collected in plastic bottles and be taken in the laboratory for
analysis.
2. Laboratory Procedures : About 500mm of water sample need to be
taken for filtration. Filter the water carefully and oven dry the
sediments to calculate its amount by the use of physical balance.
3. Calculation Procedures :
Total Runoff in litres = Volume of water tank A (litres) + Volume of
water outflow from tank A
Volume of water outflow from tank A (litres) = Reciprocal or ratio
water inflow from tank A to Tank B × volume ofwaterin TankB (
litres).
Sediment weight ( gm) / litre = Volume of water in Tank A (litres) × Average
sediment weight (gm)/ litre for tank A + Volume of water outflow from
Tank A ( litres) × average sediment weight (gm)/litre from tank B.
Precautions and Error ( Drawbacks) :
1. EP only provide information on surface soil loss.
2. Generalization of erosion and runoff of a paticular land use system
using the information produced from such a small scale plot level
studies can produce insignificant and meaningless results.
3. Extreme variation of biophysical conditions for a given land use type
often poses problems in interpreting the EP data.
138
4. Plot level studies of soil loss and runoff is limited within the enclose or
plot but not associated with upland runoff outside the boundary, which
have implications on soil loss and runoff on paticular land use or
treatment. Therefore, soil loss and runoff obtained from EPs might be
very low than from the whole landscape
5. Sediment sampling is taken by stirring water contents in the collection
tank in order to make all the contents in the tank homogenous. This is
only true if the sediment consists of clay, silt and very fine sand. The
coarser and heavy particles will be found on or near the bottom of the
tank. Sediment sampling may not content the samples of all kinds of
soil particles that brings by runoff.
6. EPs are located to represent the surrounding land, the soil of which tend
to be less disturbed as compared to the surrounding areas. So, soil loss
figures may be less compared to the actual situation in the given land
use.
7. Displacement of plot borders, occasional leakage from gutter, overland
flow from the tanks during extreme events of rainfall and keakage from
sediment collection bottles during transportation to laboratory could
also lead to some errors
8.4.2. Paired Catchment Studies :
Another way of erosion monitoring is paired catchment studies. In
run-off or erosion plot study, only a small area or plot are used to
monitor erosion of that particular land, which does not represent
the erosion problems in real field practices. Therefore, to reflect
the erosion status or problem to some extent to field reality,
paired catchment studies are being carried out. In this case, paired
catchments are taken, in which one is treated and another is
controlled. These pairs are studies with and without treatments or
one is left in natural system and another receive some treatments.
139
Paired catchments studies aims at monitoring the erosion in the
aspects of land-use, hydrology, micro-catchment, soil and water
conditions, socio-economic, agriculture and livestock.
Paired catchment study needs ( Components) : Paired catchments (
treated and natural ), hydrological and meteorological stations,
Stream gauging station.
Such paired catchments are located in Kulekhani and Phewa Tal
with differently treated and control environment. In the paired
catchment studies, the information such as areas, soil, gradient,
aspects, land-use, vegetation types and covered, runoff, rainfall are
taken. Based on these factors sediment and runoff are calculated.
8.4.3. Sedimentation Survey :
Sedimentation survey is another way erosion monitoring.
Sedimentation survey of Phewa Lake, Kulekhani Reservoir, Begnas
Lake and Rupa Lake are carried out. These sedimentation surveys
involved measurements of water and sediments in the lakes using
boat and echo-sounding instruments , which is a micro-processorcontrolled depth recorder. This is based on the fact that decrease
in water depth indicates deposition or sedimentation in the lakes
and increase in water depth indicates erosion in the lake bottom.
In Nepal, two methods are generally being used in sedimentary
survey. In Method I, sediment deposition and erosion at lake
bottom are calculated by multiplying the mean of the average
water depths of two cross-sections by the area of the lake surface
between these two cross-sections. In this method, benchmarks
and survey lines are fixed between the bench marks. The
measurements are taken in the fixed survey lines repeatedly. The
survey lines are marked at several places with known and fixed
intervals and these positions are recorded in the chart at the time
of echo sounding survey. The distance and water depth are
recorded manually from the sounding profile and processed to
140
estimate the average water depth using Microsoft Excel Spread
Sheet Software. If sounding profile is not clear, the water depth
can be measured manually using boat and rope tied with stone and
incorporated with the echo sounding graph for the analysis. In
Method II, A bathymatic map is prepared, water depths of two
contour lines are taken and averaged. This averaged water depth is
multiplied by the area of the lake surface between these two
contours using the bathymatic map for the calculations of
sediment deposition and erosion at lake bottom
The End
Additional informations
Water Climate
Water Climate : Water climate is a sum or integration of all
weather conditions including solar radiation, precipitation ( rainfall
and snow ), cloud, temperature, relative humidity and wind
pressure over a given period of time.
Solar energy : Of the above mentioned elements of water climate
solar rediation plays principle role in water climate, since sunshine
decides the solar energy that reach to the ground through which
the hydrological regime is affected.
141
Cloud cover : cloud cover is second important element of water
climate, since it entails the quantity and intensity of solar radiation
that reaches in the ground and reflects back to space. This also has
some implication in hydrological regime or cycle.
Temperature: This is impatacted and influenced by radiation, cloud
cover and humidity.
Wind and its pressure : wind represents the air movement where as
pressure represents weight of air in the atmosphere.
Humidity : This is the amount of water vapor in the atmosphere
Precipitation : This is amount of rainfall and snow that fall in the
ground.
All the above mentioned elements constitute water climate.
However, the amount and quantities of the elements that reaches
in the ground depends on latitude, longitude aspects wind direction
and barriers. Understanding water climate is important, since all its
elements affect the hydrological cycle and regime.
Climatic Zones of Nepal : ( MPFS, 1982)
142
Because of wide and varied topography of Nepal, a wide range of
climates have been identified. The followings are broad climatic
zones of Nepal :
Sub-Tropical Zone -- This is a hot monsoon zone below 2000m., in
which summer is hot and wet and winter mild and dry. The Terai,
Bhabar, Siwaliks and Inner Terai zones have this type of climate
Warm temperate Zone -- The lower middle mountains up to an
elevation of 2100m have this type of climate. The summer is warm
and wet and winter cool and dry.
Cool temperate Zone -- This type of climate prevails in the higher
middle mountains up to an elevation of 3300 m. The summer is
mild and wet and the winter is cool and dry.
Alpine Zone-- In the high mountains, up to an elevation of 4800 m,
the summer is cool and the winter is extremely frosty
Arctic or Tundra Zone-- This type of " arctic or tundra" climate
prevails in the high himal above the snow-line where there is
perpetual frost, snow and low precipitation.
143
Agro-Climatic Zones of Nepal :
2) Growing season of agricultural crop is based on temperature
and rainfall. Plants grow slow and growing season of crop are
short in areas having low temperature. Areas having high
temperature and low rainfall also affects the agricultural crops.
However, temperature plays vital role in the growth of
agricultural crops. Based on temperature, seven mean monthly
air temperature zones are recognized through which agroclimatic zones are classified.
-- temperature > 22 c
--
""
20- 22 c
--
""
16- 20 c
--
""
10-16c
--
""
4- 10c
--
""
<0c
There is a strong relationship between mean annual air
temperature and elevation. As elevation increases, mean annual
air temperature decreases. A regression model showing the
relationship between mean annual air temperature (T) and
elevation (E) has been developed. This relationship is:
T = 25.3822 - 0.0054 E
144
3) Agriculture needs various forms of energy, to produce food,
fodder, fiber and other agricultural crops. Out of which, solar
energy is a major part of energy inputs among all . These
energy needs are :
m. Solar energy : radiation and temperature
n. Human energy: irrigation fertilizers, mechanization
facilities,
o. Mineral sources : soil, water and air
p. Animal energy : OM, compost, animal waste, ploughing
Natural inputs such as : solar radiation, temperature, rainfall, snow,
relative humidity are equally important and play vital role in agriculture
production.
Physiographic Inputs : topography, aspects and slopes are equally
important.
Agro-climatic classification are generally based on the number of rainfall
months, mean monthly temperature and annual rainfall. Based on these
factors, agro-climatic zones are classified as follows :
145
Agro-climatic Zones :
1. Lower Sub-Tropical Monsoon Zone: the altitude is < 800m., mean
annual temp. is > 21c and rainfall is >1000mm
2. Upper Sub-tropical monsoon Zone : altitude varies from 800-1200m,
mean annual temp. is 19-21c and rainfall >1000mm
3. Warm temperate Monsoon Zone : altitude varies from 1200-1900m,
mean annual temp.is 15-19c and rainfall > 1000mm
4. Cool temperate Monsoon Zone : altitude varies from1900-2800m,
mean annual temp. is 10-15c and rainfall < or >500 mm.
5. Subalpine Monsoon Zone : altitude varies from 2800-4100 m. mean
annual temp.is 3-10c and rainfall < or >500 mm.
6. Alpine monsoon Zone : altitude varies from 4100 - 4700 m. mean
annual temp. is 0-3c and rainfall scattered
7. Aractic Zone : altitude > 4700 m. Mean annual temp. < 0c, rainfall is
scattered and snowfall.
146
Factors affecting micro-climate patterns
Micro-Climate :
This is a local climatic condition of a given pocket or specific area, which is
based on great variations of land use, land form and physiography. In
Nepal, there is a great variety in microclimate because of vast changes in
land-use, land form and physiography. It has been said that at each 100
m. elevation differences in mountain and hills in Nepal, there is a change
in micro-climate. Variation of florestic composition at altitudinal change
reflects the variation in micro climate. There are several factors affecting
micro climatic condition of a given area.
Factors affecting micro-climate patterns :
The most influential factors for creating variation of micro climate are
light / radiation ( temperature), humidity, wind and frost. Where as the
factors that create the micro climate are : air drainage, aspect, slope,
vegetation, soil etc.
Light/ Radiation : Southern aspect receive more light and radiation than
northern aspect. Reaching light and radiation also vary in east west faces
and ridges
147
Slope : Steeper the slope, more pronounce are the variation of light and
radiation, evapo-transpiration, soil moisture content and so on.
Air/wind : In deep valleys and shallow basins , the drainage of air/wind is
limited or poor and fluctuates . These areas are usually foggy and
temperature also tend to fall than the normal mountain slope. Local wind
in mountain region varies, which affects temperature in the areas and
there by affects the suitability of crops.
Vegetation : Vegetation patterns changes as micro climate changes and
vice-versa. Areas having vegetation and water source are generally cool
and humid than the area where vegetation and water source are absent.
Temperature, humidity also differs and makes the area different from
other.
Soil : Stand of vegetation and types differs according to the condition of
soil. As vegetation patterns changes, there will be change in micro
climate.
Frost/ hailstone : Occurrence and frequency of frost and hailstone is
different and random in different physiographic zones, districts, valleys
and bottoms which also affects micro climate.
NATURAL DISASTER.
148
Flood and Landslide
Flood :
By definition, a flood is " an overflow of lands used by man and
not normally covered by water ". What causes the overflow
obviously is more water than than the river's channel can carry.
However, nature of river channel itself is responsible for overflow of river channel. The main source of the excess water is
rainfall. In the case of snow-fed rivers, excessive snow-melt
could swell up the streams. Obviously, flood is basically the
result of rainfall runoff, which is too great to be supported by
the existing river channels. This uncontrolled high runoff
overtops the natural or artificial banks and starts spreading over
the flood plain causing damage and threatening the property in
the vicinity of the flood plain.
Flood can't be controlled but one can prevent damage likely to
be caused by the flood, by manipulation of the components of
land resources from where the flood originates.
Causes of flood : Man's interferences with the natural
ecosystem has crucially modified the existing hydrological
regime and the water balance. Interferences like uncontrolled
and destructive harvesting of timber and fuel wood, clearing
forests to give ways to shifting cultivation / settlement and over
grazing have greatly decreased the water holding capacity of
the land, the consequences of which is increased runoff. During
149
heavy rains, the runoff so produced is often not retained by the
land. It just moves down through many steep, barren, eroded
slopes and reaches down into small narrow streams and rivers.
These streams and rivers in the hills carrying sand, silt, water
surges, which erode their own banks, have a tendency to
deposit the sediments they carry on the main rivers beds
increasing the bed load and resulting in the spread and surge of
water over the flood plains. Virtually, it is a flat land where peak
runoff turns into floods, which suffers the greatest damage.
Flooding of the main rivers of Nepal are the clear examples of
having an immense volume of runoff and the bed load being
transported from its tributaries.
Effects of Floods :
Floods affect and damage property mainly through the process
of inundation in low-lying areas, backed by up-stream
destruction, river bank erosion and the shifting of river
channels. The surrounding areas of flood plains and valleys may
suffer heavily from floods, since the flood plains and valleys
created a long time ago by the rivers, have become the centers
of economic growth and are heavily populated and exploited for
agriculture.
However, the effect and the problem of flooding in developing
countries has largely been aggravated by the socio- economic
problems and environmental factors. Increased population has
left no alternatives except for people to occupy and migrate to
150
highly fragile lands. In developing countries, 50 to 80 % of the
population occupy fragile lands, and 10% of the world's
population live in the steep and fragile lands of the earth.
Cultural practices on these lands have destroyed and inhibited
the natural protective means, which would have prevented
flood erosion.
Loss and siltation of prime agricultural land, siltation and
destruction of lakes, reservoir and irrigation structures, water
pollution, and the destruction of settlements and
communications are the principal damage brought by floods.
The meandering and shifting problems of rivers may create
legal problems. Meandering and shifting of rivers causes
damages on one bank leaving the fertile alluvial land on the
opposite bank having different land rights. This problem may
cause serious disputes in the community, where the availability
of flat land is scarce.
Preventing Flood :
Flood is largely a result of abuse of land, natural resources and
hydrological function. To prevent primary damage of flood to
some extent, a three phase action program has to be carried
out simultaneously :
a ) Treatment of watershed ( land use management )
151
b) Treatment of upstream or torrents, and
c) Treatment of main rivers
a) Treatment of watershed :
It includes vegetative, simple mechanical measures
adjustments in existing land-use. Purposes of these
measures is to increase the rate of infiltration, decrease
runoff volume and velocity, conserve soil and water.
Watershed treatment is one of the most effective and
widely used measures in the control of runoff and
sedimentation, which cause floods in lower reaches. The
treatments include afforestation, contour farming,
vegetative waterways, management of forest, grass land,
agriculture, water resources and so on.
b) Up-stream treatment : This treatment is mainly
concentrated on the up-stream torrents, gullies and water
ways. Objectives of this treatment is to retard the runoff and
to stabilize the source of sediments that reaches in the down
stream. Structures used for retarding flood water ( runoff )
and minimizing sedimentation are check-dams, sediment
detention basins, catchment ponds, channel improvement
structures are commonly used.
c) Treatment of main rivers : This is treatments in which big
structures are usually built in the main rivers in order to
prevent the flood hazards. In this treatment emphasis is
given in to river training by stabilizing the river channel. The
objectives of this measures are:
152
--- to prevent the tendency of channel shift or changing
the river course.
--- to maintain the river channel capacity for the
transportation of sediment load and excess runoff.
--- to protect the bank cutting.
--- to protect the flood plain from flooding.
For channel stabilization, following river training works are usually
warrented :
--- river training work to prevent the river from mentaring and causing
bank erosion
--- river training work to to provide sufficient cross-section area fro easy
flow of sediments.
--- river training work to keep sufficient depth of water for navigation.
153
Following are engineering structures widely used for channel stabilization
or river training works:
--- Embankments ( Refer Unit 2. of lecture note)
--- Spurs ( Refer Unit.2. of lecture note)
--- Guide banks ( are structures usually constructed to
confine the river flow in a certain direction in order to
protect other structures, like bridge or a weir from
damage. this structure are built in banks in parallel to flow
).
--- Pitching on bank slopes ( this is done on the banks of the
river, parrellel to the river flow. Its purpose is to check the
erosion on banks due to direct impact by the river, and to
check the sliding of banks due to the scouring action of
the water. bank slopes can be pitched by laying loose
boulders, concreate blocks or even grass turfing ).
--- Cut-offs ( An artificial cutoff can be made in order to make
river flow straight from its mendering position and
reclaim the land. this is a very simple and temporary
measure of channel stabilization work and shortening of
the river channel ).
154
In addition to above mentioned measures, flood prevention
measures also includes the integration of following practices :
e) Flood Prevention : This is simply a watching, guarding and alerting
from excessive rain. If excessive rain is occuring since long
duration, preventive measures need to be applied in order make
safe from flood damage.
f) Flood Prediction : With the data and information of past
discharge and rainfall record and rise and fall of river level up
stream gauges, chances of occurring flood can be predicted, or
flood warning can be predicted. Flood prediction can help us to
take timely action for minimizing the flood damage. This is a flood
warning system, if there is such warning, quick communication to
the community should be done to make them alert from the likely
damage that can happen from the flood.
c) Flood Plane Zoning : Zoning of flood plane should be made in
order to minimize the risk from flood. Basically this is
categorized into three zones. They are :
Critical Zone : This is a zone of river channel, which gets
flooding in every monsoon. In such a zone, no human
settlement and any infrastructure should be planned.
155
Restrictive Zone : This zone is very close or adjoining area to
critical zone. This area may get flooding when there is a big
flood. Construction of infrastucture should be avoided except
some agriculture practices.
Warning Zone : This zone falls adjoining to restrictive Zone.
This zone will have a chance of flooding if there is largest and
unprecedented flood. Construction of infrastructures need
necessary precaution.
d) Physical interventions : Refer Preventing Flood as mentioned
above.
Flash flood :
Flash floods are events with very little time lapsing between the start of the
flood and peak discharge. They are often associated with short intervals
between storm incidence and arrival of the flood wave, but this is not
always the case. Floods of this type are particularly dangerous because of
the suddenness and speed with which they occur. Flash floods are more
common with isolated and localized intense rain fall originating from
thunderstorms.
Landslides :
The term landslide is used to denote the downward and outward
movements of slope forming materials along surface of separation. They
156
are mass movements of land caused by various factors such as heavy rain,
earthquake, or geological factors. Landslides are rather quick mass wasting
processes. Mass wasting or movement are generally : debris slides, rock
slides, debris flows and deep seated rotational slides. Plane rock slides
occur on steep slopes where bed rock is close to the surface. Debris and soil
slides occur on steep slopes that are deeply weather residual soils.
Landslide causes damage to cultivated land, settlements, roads and other
infrastructures.
Landslides can be classified in terms of two criteria:
-- Types of movement
-- Types of material
Types of movement : Rock falls, topples, slides ( debris, rock slides ) and
flows
Types of materials :
Bedrocks, soils
Rock falls : They are movement of masses of geologic materials detached
from slopes or cliffs. they occur by free fall, bouncing and rolling.
Depending upon the type of materials, they are referred as rock falls, soil
fall, debris fall earth fall etc.
Topples : Block of rock that falls, tilt or rotate forward on a pivot or hinge
point and then separates from the main mass and falling and rolling down
to the slopes.
157
Slides : Movement of soils or rocks along a distinct surface of rupture which
seperates the slide materials from more stable underlying materials. Two
major types are : rotational and translational slides.
Rotational : In this type, the surface of rupture is curved concavely
upwards and the slide movement is more or less rotational in an axis that is
parallel to the contour of the slope.
Translational : In this case, the mass moves out or down and outwards and
slides out on top of the original ground surface. Such a slide may progress
over great area. Slide material may range from loose unconsolidated soils
extensive slabs of rock.
Flows: Flows are many kinds such as : Creep, Debris, Debris avalanche,
Earth flow, Mud flow, Lahar.
Creep flow : This is steady downward movement of slope forming
materials. Creep is indicated by curved tree trunk, bent fences or retaining
wall, tilled poles or fences and small soil ripples.
Debris flow : This is a rapid mass movement in which loose soils, rocks and
organic matter combine with air and water to form a slurry that then flows
down slope. Debris flow areas are usually associated with steep gullies and
drainage basin.
158
Debris avalanche : This is a varieties of very rapid to extremely rapid debris
flow.
Earth flow : In this case, a bowl depression forms at the head of land slope.
The central area is narrow and usually becomes wider as it reaches the
valley floor. This type of flow generally ocurs in fine- grained materials or
clay- bearing rocks on moderate slopes and with saturated conditions.
Mud flow : This is an earth flow that contains of materials that is wet
enough to flow rapidly and that contains at least 50 % sand, silt and claysized particles.
Lahar : A lahar is a mud flow or debris that originates on the slope of
volcano. Lahar are usually by heavy rainfall eroding volcanic deposits,
sudden melting of snow and ice due to heat from volcanic events or by the
breakout of water from glaciers, crater lakes dammed by volcanic
eruptions.
Causes of Landslides :
Several factors like geology, geomorphology, hydrology, climate and
vegetation, which interact each other in a complex ways are responsible for
landslide or slope stability.
Geology : The composition, texture, physical and chemical content of rock
and soil as well as the shear strength of particles, permeability, structures,
159
weathering and other characteristics of rock and soil play important roles in
landslide. The solid constituents, mineral content and their distribution in
rocks and soil such as grain, size, distribution, shape, area, surface
characteristics, amount of cement, mechanical strength of particles, clay
minerals, inter particle bonds, presence of water and chemicals, nature of
bedding planes, joints, faults, folds, fractures and their orientation etc. are
important factors, which play vital role in landslides or slope stability.
Geomorphology : Steepness of slope, strength of slope forming materials,
relationship between slope and stability, presence or absence of former
landslides also is a basis to analyze scope of landslide.
Hydrology : Hydrology of a particular area is another factor in landslide and
slope stability. Amount and source of water, water movement, ground
water flow and pressure of water are equally important factors to be
considered in landslides and slope stability.
Climate : Climate such as temperature and precipitation are important
factors to be considered in landslides and slope stability. These two
elements vary from region to region, which in turn, makes variation in the
scale and the severity of landslides that may occur in different regions. In
temperate regions, with moderate rainfall and change in ground water flow
and water pressure may activate landslides. Similarly, in semi-arid regions
with heavy rainfall and in temperate and tropical regions, where monsoons
or cyclonic storm occurs may have debris landslide.
Vegetation : The mechanical action of root network for holding the rock
mass as well as its role for controlling direct water movement or
160
permeability and ground water movement in the land mass are imperative
in landslide.
Factors Supporting Landslide ( Triggering factors ) :
In addition to the factors responsible for landslides or slope stability as
mentioned earlier, there are some more factors, which backup landslide or
slope failure. If there is a abrupt or gradual variation or change in shearing
stress and strength of materials and water content in the geological mass,
landslide may occur. The elements that alters the shearing stress and
strength of materials are : vibration, absence of lateral support, weight,
water pressure and friction, weathering ( physical and chemical actions).
Vibration : Vibration from earthquake, blasting, traffic, thunder storm
weaken the shearing stress and strength of materials of geological mass
and may cause landslides.
Absence of lateral support : The removal of lateral support by erosions,
quarries, previous slope failure, constructions etc. also affect shearing
stress and strength of geological mass and help causing landslide.
161
Weight : Weight of rain, snow, hailstone, accumulation of debris, loose rock
materials, volcanic materials, waste piles, weight of physical infrastructures etc. may enhance the landslide.
Water pressure and friction : Water pressure in the rock pores and
frictional changes in the rock fractures due to water help change and affect
the shearing stress and strength of geological mass and may cause
landslides.
Weathering : The weathering of rock mass due to physical and chemical
actions also affect the shearing stress and strength of rock mass and help
enhance landslides.
Mitigation Measures :
Since landslide falls under one of the natural hazards, its mitigation
measures needs combination or integration of several interventions.
Curative/ rehabilitative, preventive measures and public awareness
programs need to be incorporated.
Curative/ Rehabilitative measures : These are measures, where immediate
actions are administered in landslide that are already occurred in order to
safe-guard the property and people from the further effect of landslide.
These measures need high level of investment and technology. Retaining
walls, breast walls, gully control ( check-dams), water diversion structures,
river training works and re-vegetation are some of the important curative
measures to control landslides.
162
Preventive measures : Preventive measures are those interventions that
need to be administered in an area where landslides are likely to occur or
there are high possibility to occur landslide in future. This measures are
applied before landslides occur. Preventive measures help people to be
alert from the effect of landslide damage that may occur in future.
Drainage improvement, terrace improvement, gully control, plantation of
trees and grasses, catchment ponds, trail improvement, slope stabilization,
water source protection, forest protection, grazing management, fodder
management, fruit tree plantation, agro-forestry, improved agriculture
practice are some of the preventive measures.
In both the programs, involvement or participation of local community is
imperative in order to internalize and develop a sense of felling in local
people that the programs are for their benefits. Once the people realize
about the benefits of the programs, they will be ready to participate and
share the cost of construction and maintenance of the program and
sustainability programs can be existed.
Public awareness programs: Local people may not be aware of landslide,
how they occur, how to recognize the landslide prone areas, what will be
their impacts in daily life, what precaution measures need to be taken for
prevention and rehabilitation, technical know-how, social and economic
implications. Therefore, it is very urgent to carry out awareness program in
the local community. Interactions programs in academic institutions, open
discussion and interactions in public gathering, field demonstration,
observation tours, newsletter, film strips, documentary, slide show,
slogans, early warning process and system, extension works, motivation
etc. are some of the effective public awareness programs. Public awareness
program also help mobilize and generate public participation.
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Other disasters ( fire, earthquake, volcanic eruption )
Fire : Fire as a natural disaster, has affected a substantial portion of
forests of the world. Although it is a disaster, fire has been a
powerful natural factor affecting forests, wildlife and rangeland.
Lowlands such as swamps, maeshes, prairies and semitropical
forests of high humidity also have been burned and their
vegetation markedly affected.
Fire plays several major roles in fire dependent ecosystem around the
world. Fire has always been a natural and extremely important
environmental factor since it has influence on species traits and life history
as well as ecosystem characteristics and processes such as carbon, nutrient,
water cycling, productivity succession and diversity.
Fire is irregular in frequency, intensity and burning pattern. These
characteristics are primarily controlled by climate, fuel accumulation and
flammability, soil site condition and topography. Fire frequency is greatest
in grasslands, grasslands usually burn every 2 to 3 years.
Influences of Fire : Fire influences,
q.
r.
s.
t.
Physical and chemical properties of site
Dry matter accumulation
Genetic adoptions of plant species
Species establishment, development, composition and
diversity
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u. Wildlife habitat and wildlife population
v. Presence and abundance of forest insects, parasites and
fungi
Causes of Fires :
w. Lightning
x. Meteorites
y. Volcanic eruption
z. Sparks from falling quartzite rocks
aa. Human activities
Kinds of Fires : Three kinds of fires are recognized according to the level at
which they burn. They are : Ground fires, Surface fires and Crown fires
Ground fires : Fires sweeping the forests floor may generate or called
ground fires. In ground fires, the thick accumulations of organic matter
burns, which overlies mineral soil. This is flameless and may kill most plants
with roots growing in organic matter. Ground fires burn slowly and usually
generate very high temperatures. Ground fires tend to serve as ignition
sources for surface fires.
Surface fires : This is a most common type of fire. It burns over the
forests floor and range consuming litter, humus and killing herbaceous
plants, shrubs and fauna. The greater the fuel accumulated on the surface,
the greater the mortality of plants and fauna. The amount of mortality
depends on the species, the age and rooting habits. For example, the young
pines succumb to a surface fire, where as older individual of the same
species survive due to thicker bark protecting the cambium layer from heat
damage. A shallow rooting plant will be more susceptibility to fire injury
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compared to that of deep rooting plants. However, surface fires tend to kill
young trees of all species and most of the trees of less fire resistant species
of all sizes (often just the above ground portion ). The pole-size to mature
trees of fire -resistant species survive light surface fires. Killing of plants by
surface fires are due to damage of cambium, root injury and scorching of
the crowns by hot gases rising above the flame.
Crown Fires : Surface fires fueled by accumulations of organic matter
and whipped by winds may scorch and ignite crowns of trees, thus
generating a crown fires. Crown fires travels from one crown to another
and kills most trees in its path. Conifers are most susceptible to crown fires
because of the high flammability of their foliage and occurrence in pure
stand than broad leafed species.
Fire is one of the management tools of vegetation and wildlife, however
wildfires are always harmful and detrimental to the environment than
controlled fire or burning.
Control of fires : The following techniques are used to control wildfires :
bb.
Fireline : a narrow
line of 2 to 10 ft. wide, from which all vegetation is
removed down to mineral soil by sterilization of the soil,
by yearly maintenance or by clearing just ahead of firing
out from the line.
cc. Firebreak : a fireline wider than 10 ft., frequently 20 to
30 ft. wide prepared each year ahead of the time it may
be needed for use in controlling a fire.
dd.
Fuelbreak : a strategically located block or strip on
which a cover of dense, heavy, or inflammable
vegetations has been changed to new vegetation of
lower fuel volume or inflammable are maintained to
control fires.
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ee. Fuel modification : fire control practices including
cleanup of fuel hazards, permanent fuel reduction on
limited areas or periodic fuel reduction on large areas.
Indigenous Technology and Knowledge
Please refer report on "Salient Indegenous Technology Practices for
Watershed Management in Nepal " by D. D. Kandel and M.P. Wagley.
(Copy available in Library)
Desertification
Please refer Policy and Programme Responses for Combating
Desetification by M.P. Wagley in " Combating Desertification - A national
report of the National Seminar on Desertification and Land Improvement
". (Copy available in Library)
167
L
RH
toe
edge
riser
Rs
Cut
Fill
Hr
V.I
Cut
Wb
Wr
Fill
Area loss
Wt
width of riser
original land slope
Cross-section of a Bench Terrace
V.I = Vertical Interval
Wb = Width of a bench
Wr = Width of a riser= Hr * U
V.I = (S* Wb)/ ( 100 – S*U)
Wt = Width of a terrace = Wb + Wr
Rh is zero in bench terrace, so V.I = Hr
U = Riser slope = Wr/Hr
L= length of bench
A = net area of bench = L* Wb
C = Cross-section of terrace = Wb * Hr/8
% bench in terrace = A/100
Volume to be cut = C*L
S = Land slope (%)
Rh = Reverse height = Wb * Rs
Rs = Reverse slope
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169