Soil and Water Conservation
Part-2
- Ø Erosion
Due to Water
Erosion of soil by water is caused by its two forms:
liquid as the flowing water, and solid as the glaciers.
The impact of rainfall causes splash erosion. Runoff
water causes scraping and transport of soil particles leading to sheet, rill
and gully erosion. Water waves cause erosion of bank sides of reservoirs, lakes
and oceans. The subsurface runoff causes soil erosion in the form of pipe erosion,
which is also called tunnel erosion. The glacial erosion causes heavy
landsides. In India, glacial erosions are mainly confined to Himalayan regions.
- Ø
Factors Affecting Water Erosion
Water erosion is due to dispersive and transporting
power of the water; as in case of water erosion first soil particles are
detached from the soil surface by the raindrop force and then transported with
surface runoff. The
factors influencing the water erosion are discussed below.
- Climatic Factors: Climate includes rainfall,
temperature and wind
- Soil Characteristics: Soil characteristics include
texture, structure, organic matter content and permeability
- Vegetation Cover
- Topographic Effect: land slope, length of slope and
shape of slope
Ø
Types of Water Erosion
1.) Splash Erosion: It is also known as raindrop erosion, because it is caused by the impact of raindrops on exposed soil surface. The process of raindrop erosion can be described as: when raindrop strikes on open soil surface it forms a crater. This is accomplished by forming a blast which bounces the water and soil up and returns back around the crater. The soil may be splashed into the air up to a height of 50 to 75 cm depending upon the size of rain drops.
- 2) Sheet
Erosion: Sheet
erosion may be defined as more or less uniform removal of soil in the form of a
thin layer or in “sheet” form by the flowing water form a given width of
sloping land. It is an inconspicuous type of soil erosion because the total
amount of soil removed during any storm is usually small. In the sheet erosion
two basic erosion processes are involved. First process is the one in which
soil particles are detached from the soil surface by falling of raindrop and in
the second one the detached soil particles are transported away by surface
runoff from the original place.
- 3) Rill
Erosion: This type
of water erosion is formed in the cultivated fields where the land surface is
almost irregular. As the rain starts, the water tends to accumulate in the
surface depressions and begins to flow following least resistance path. During
movement of water large amount of soil particles are eroded from the sides and
bottom of the flow path, which are mixed in the flowing water. This surface
flow containing soil particles in suspension form moves ahead and forms micro
channels and rills
- Ø Agronomical
Measures of Water Erosion Control
Soil conservation is a preservation technique, in
which deterioration of soil and its losses are eliminated or minimized by using
it within its capabilities and applying conservation techniques for protection
as well as improvement of soil. Widely
used agronomic measures for water erosion control are listed below.
- ·
Contour Cropping
Contour Cropping is a conservation farming method that
is used on slopes to control soil losses due to water erosion. Contour cropping
involves planting crops across the slope instead of up and down the slope. Use
of contour cropping protects the valuable top soil by reducing the velocity of
runoff water and inducing more infiltration.
- ·
Strip Cropping
Strip cropping is the practice of growing strip of
crops having poor potential for erosion control, such as root crop (intertilled
crops), cereals, etc., alternated with strips of crops having good potentials
for erosion control, such as fodder crops, grasses, etc., which are close
growing crops. Strip cropping is a more intensive farming practice than contour
farming. The farming practices that are included in this type of farming are
contour strip farming, cover cropping, farming with conservation tillage and suitable
crop rotation. Strip
cropping is laid out by using the following three methods:
1) Contour strip cropping
2) Field strip cropping
3) Buffer strip cropping
- ·
Mulching
Mulches are used to minimize rain splash, reduce evaporation,
control weeds, reduce temperature of soil in hot climates, and moderate the
temperature to a level conducive to microbial activity. Mulches help in
breaking the energy of raindrops, prevent splash and dissipation of soil
structure, obstruct the flow of runoff to reduce their velocity and prevent sheet
and rill erosion.
Types
of mulching material: To protect the land from erosion different types of materials
are used as listed below.
1.
Cut grasses or foliage
2.
Straw materials
3.
Wood chips
4.
Saw dusts
5.
Papers
6.
Stones
7.
Glass wools
8.
Metal foils
9.
Cellophanes
10.
Plastics
- Ø Terraces
for Water Erosion Control
One of the most effective actions that can take to
mitigate the problem of an eroding slope is to break up the rate of water
decent by constructing terraces. The terraces for water erosion control consist
of some mechanism to protect land surface as well as to reduce the erosive velocity
of runoff water.
Terraces and their Design
A Terrace is an earth-embankment, constructed across the
slope, to control runoff and minimize soil erosion. A terrace acts as an
intercept to land slope, and divides the sloping land surface into strips. In
limited widths of strips, the slope length naturally available for runoff is
reduced. It has been found that soil loss is proportional to the square root of
the length of slope; i.e. by shortening the length of run, soil erosion is
reduced. The soil eroded by the runoff scour and the raindrop splash flows down
the slope, and gets blocked up by terraces. The classification of the terraces is given below:
- · Bench Terracing
The original bench terrace system consists of a series
of flat shelf-like areas that convert a steep slope of 20 to 30 percent to a
series of level, or nearly level benches. In other words, bench terracing
consists of construction of series of platforms along contours cut into hill
slope in a step like formation.
Design of Bench Terraces
For the designing of the bench terraces for a
particular tract the average rainfall, the soil type, soil depth and the
average slope of the area should be known. In addition the purpose for which
the terraces are to be constructed should also be known. The design of bench
terraces consists of determining the (1) type of the bench terrace, (2) terrace
spacing or the depth of the cut, (3) terrace width, and (4) terrace cross
section.
Terrace spacing is generally expressed as the vertical
interval between two terraces. The vertical interval (D) is dependent upon the
depth of the cut.
The width of the bench terraces (W) should be as per
the requirement (purpose) for which the terraces are to be put after
construction. Once the
width of the terrace is decided, the depth of cut required can be calculated
using the following formulae.
Case 1: When the
terrace cuts are vertical
S is the land slope in percent; D/2 is the depth of
cut and W is the width of terrace.
- Ø
Bunding Methods for Water Erosion
Control
Bunding is a mechanical method for control of soil erosion.
When agronomical measures alone are not sufficient, such and other mechanical
measures should be adopted.
Bunds (Contour Bunds,
Graded Bunds) and their Design
Bund is an engineering measure of soil conservation,
used for creating obstruction across the path of surface runoff to reduce the
velocity of flowing water. It retains the running off water in the watershed
and thus to helps to control soil erosion. Bunds are simply embankment like
structures, constructed across the land slope. Different types of bunds are used
for erosion control and moisture conservation in the watersheds. When the bunds
are constructed along the contours with some minor deviation to adapt to
practical situation, they are known as contour bunds. If the bunds are
constructed with some slope, they are known as graded bunds. No farming is done
on bunds expects at some places, where some types of stabilization grasses are
planted to protect the bund. The choice of the types of bund is dependent on
land slope, rainfall, soil type and the purpose of the bund in the area.
Contour Bunds
Contour bunds are laid out in those areas which have
less rainfall and permeable soils. The major requirements in such areas are
prevention of soil erosion and conservation of rain water in the soil for crop
use. To maximize the conservation of rainwater in the soil, no longitudinal
slope is provided to the field strip. In such a system of bunding, the bunds
are designed to be laid out on contours with minor adjustments, wherever
necessary.
The main functions of
contour bunds are:
1. It reduces the length of slope which in turn
reduces the soil erosion.
2. The water is impounded for some time and gets
recharged into the soil which helps in crop cultivation.
The limitations of
contour bunds are:
1. The contour bunds are suitable for those areas,
which receive the annual rainfall less than 600 mm
2. It is not suitable for clayey soils
3. Contour bunding is not suitable on the land slopes
greater than 6%.
Graded bunds are laid out in areas where the land is
susceptible to water erosion, the soil is less permeable and the area has water
logging problems. A graded bund system is designed to dispose of excess runoff
safely form agricultural fields. A graded bund is laid out with a longitudinal
slope gradient leading to outlet. The gradient can be either uniform or
variable. The uniformly-graded bunds are suitable for areas where the bunds
need shorter lengths and the runoff is low. The variable-graded bunds are required
where bunds need longer lengths, owing to which the cumulative runoff increases
towards the outlets. In these types of bunds, variations in the grade are
provided at different sections of the bund to keep the runoff velocity within
the desired limits so as not to cause any soil erosion.
The limitations of
the system are:
- Due to crossing of farm implements, the bunds are disturbed and some soil is lost.
- Proper maintenance is required at regular interval.
Design Specification
of Bunds
The following parameters should be considered for bund
design:
1. 1.Type of Bund: The type of bund (contour or graded bund) to be constructed depends upon the rainfall and soil condition. Contour bunds are preferred for construction in areas receiving annual rainfall less than 600 mm and where soil moisture is a limiting factor for crop production. Graded bunds are recommended in heavy and medium rainfall areas. The grade to be provided to the bund may vary from 0.2% to 0.3%.
2. 2.Spacing of the Bunds: The basic principles to be adopted for deciding the spacing of bunds are: (1) the seepage zone below the upper bund should meet the saturation zone of the lower bund; (2) the bunds should check the water at a point where the water attains erosive velocity and (3) the bund should not cause inconvenience to the agricultural operations.
For
determining the spacing of the bunds the following formula is used:
where,
V.I.
= vertical interval between consecutive bunds,
S =
land slope (percent) and
a
and b = constants, depend upon the soil and rainfall characteristics of the
area.
The
above equation is area specific. It can be modified for areas with different
rainfall amounts.
1.
For
the areas of heavy rainfall:
3. Size of the Bund: The size of bund includes its
height, top width, side slopes and bottom width. The height of bunds mainly
depends upon the slope of the land, spacing of the bunds and the maximum
intensity of rainfall expected in the area. Once the height of the bund is determined,
other dimensions of the bund viz., base width, top width and side slopes are determined
using the information on the nature of the soil. Depending on the amount of water
to be intercepted, the height of the bund can be calculated as given below
(Fig.)
Let X = height of the bund, L = distance between
bunds, V = vertical interval between bunds, and W = width of water spread.
Considering 1m length of the bund, amount of water stored = ½ WX
Substituting for W from Eqn., amount of water stored
4. Length of Bund: The length of bund is determined by calculating the horizontal interval of the bund formed. The length of bund per hectare area of land is given as:
L= 10000/H.I
= (10000*S)/(VI*100)
= 100(S/VI)
5. Earth Work: The earth work of bunding system
includes the sum of earthwork made in main bunds, side bunds and lateral bunds
formed in the field. The earthwork of any bund is obtained by multiplying the
cross-sectional area to its total length. The total earthwork can be given by
the following equation.
Et = Em + Es + El
El = earthwork of lateral bunds,
Em = cross-sectional area * total length of bund =
(100S/VI)*cross-sectional area.
Therefore, total Et = Em + Es + Et
= (100S/VI + 30S/VI)* cross-sectional area
= 130S/VI * cross-sectional area
Et = 1.3 * (100S/VI) * cross-sectional area of bund
Drop
Spillway
The drop structure is a weir structure, is limited to
a maximum drop of 3 m and it is not a favorable structure where temporary
spillway storage is desired to obtain a large reduction in the discharge at or
d/s from the structure.
It is a weir structure, in which flow passes through the weir opening, fall or drops on an approximately level apron or stilling basic and then passes into the downstream channel. Its use is limited to a maximum drop of 3 m. It is mainly used at the gully bed to create a control point. Several such drop structures are constructed across the gully width throughout the length at fixed intervals. The series of such structures develop a continuous break to flow of water, causing deposition of sediments and thus filling the gully section. Sometimes, the drop structures are also used at the gully head to pass the flow safely and controlling the gully head. The different components of drop structures are shown Fig.
Components and
Functions
1. Head wall: It acts as a front wall against
runoff flow in the drop spillway. It is constructed across the gully width. A
notch of suitable size is also made at the top in the headwall for easy water
conveyance. Rectangular notch is most commonly used. The size of the notch
should be sufficient to allow the water very safely.
2. Head Wall
Extension: It is
the extended portion of head wall into the gully sides. It permits stable fill
and prevents piping (due to seepage) around the structure. Its main function is
to provide structural strength against sliding of the structure and also to
prevent the flow of water from the sides of the drop spillway.
3. Wing Walls: These are constructed at the rear
end of the structure with some inclination, usually at 45o from the vertical.
These walls are extended up to the gully sides and perform the function of
preventing the flow backward into the space left between gully wall and side
wall of the structure. They provide stability to the fill and protect the gully
banks and surface.
4. Cut-off Walls: These are constructed to provide
structural strength against sliding of the structure. They increase frictional
resistance of the structure which opposes the force causing the slide. In other
words, cut-off walls act as a key for the structure; prevent piping under the
structure besides reducing uplift and sliding.
5. Toe Walls: Prevent undercutting of apron.
6. Side Walls: These are constructed in the side
along the gully walls. They guide the water and protect the fill against
erosion. The function of the side walls is to prevent splashing of water over
the gully banks and also to confine the water flow within the apron.
7. End Sills: These are the elevated portion of rear
end of the apron. Its main function is to obstruct the water from directly
moving into the channel below.
They also raise the tail water level to create hydraulic
jump and to dissipate the energy of the flowing water.
8. Longitudinal
Sills: These are
constructed in the apron section. They are constructed lengthwise parallel to
the side walls. The sills are useful to make the apron stable.
9. Apron: It is one of the main downstream
components of the straight drop spillway as it receives the gully flow with
high velocity and changes the flow regime so as to minimize the soil erosion on
the downstream channel. It includes several elevated blocks to make the apron surface
rough. This feature of apron is responsible for dissipating the maximum kinetic
energy of falling water by creating hydraulic jump. As a result the velocity of
outflow water is significantly reduced.
Uses of Drop Spillway
1.
To
control gradient in either natural or constructed channels, To control
tail-water at the outlet of a spillway or conduit.
2.
To
serve as a reservoir spillway where the total drop is relatively low.
3.
To
serve as inlet and outlet structures for tile drainage system in conjunction
with gradient control.
4.
To
use as grade stabilization in lower reaches of waterways and outlets.
5.
To
use as erosion control, to protect the roads, buildings etc.
6.
Straight
drop spillway as an outlet in tile drainage system and for releasing the
irrigation water into the field in irrigation system.
7.
In
the reservoir, for letting out the water through low height drop spillway of
less than 3 m,
8.
For
controlling irrigation in the water distribution system and
9.
As
an outlet for disposing surface water from large areas, especially with
drainage ditches.
Drop
Inlet Spillway
A drop inlet or shaft spillway is one in which the
water enters through a horizontally positioned circular or rectangular box type
riser or inlet and flows to some type of outlet protection through a circular
(horizontal or near horizontal) conduit. The drop inlet spillway is ideally
suited to conditions when there is need to control the downstream channel flow
by providing a temporary storage upstream of the structure. It consists of an
earthen dam and a pipe spillway. The dam provides the temporary storage of
runoff from the contributing watershed while the spillway permits the design
discharge to pass downstream. It is adapted where drop is > 3.0 m. The drop
inlet structure consists of the inlet, conduit and the outlet. Where the inlet
is funnel shaped, this type of structure is often called as Morning glory or
Glory hole spillway.
Advantages
·
Drop
inlet structures are used in gullies towards the downstream part to create storage
of water.
·
These
structures not only help in protecting gullies but also create water storage.
·
The
stored water could be useful for irrigation or other farm use purposes.
·
A
large number of drop inlet structures will have a retarding effect on
downstream flows. A reduction in the sediment load could also occur.
·
An
earthen embankment helps in storing the water and the drop inlet essentially
lets out the excess water safely.
·
These
are frequently used for headwater flood control and as outlets for farm ponds and
reservoirs, silt detention reservoirs and settling basins.
·
They
are suitable as gully control structures for the stabilization and control of advancing
gully heads when the gully is more than 3 m deep.
·
They
are relatively simple to build.
Design of Drop Inlet
Spillways
The design consists of hydrologic and hydraulic
designs.
Hydrologic Design
The hydrologic design of the drop inlet structure
consists of knowing both the peak rate of runoff expected and also the inflow
hydrograph. The latter is needed as temporary storage of water is created in
case of these structures. The outflow will not be same as the inflow like in
drop or chute spillways.
Hydraulic Design
To understand the hydraulic design of the structure,
the different types of flow that occur in the conduit are to be considered. The
flow through the structure could be controlled first by the inlet and latter by
the conduit. A typical discharge characteristic curve is shown in Fig.
To calculate the inflow capacity of straight drop inlet
spillway, the following weir formula is used.
Where,
Q = peak discharge rate to be handled by the structures (m3/s); g =
acceleration due to gravity (9.81 m/s2); H = head over the crest (m); Cd =
coefficient of the discharge (0.6). A free board is also added to H. Generally
15 to 30 cm of free board is added to the calculated value of H.
Chute Spillway
Chute (open channel or trough) spillway is a spillway
whose discharge is conveyed from the upper reach of the channel or a reservoir
to the downstream channel level through an open channel placed along a dam,
abutment (supporting wall), or through a saddle. Chute structures are useful
for gully head control and they could be used for drops upto 5 to 6 m. Chute
spillways are constructed at the gully head to convey the discharge from
upstream area of gully into the gully through a concrete or masonry open
channel, when drop height exceeds the economic limit of drop structures. Chute
spillway has more advantage than a drop spillway, when a large runoff volume is
required to be discharged from the area. Flow in a chute spillway is at
super-critical velocities.
The chute spillway consists of the following three
design components
1. Inlet
or Entrance Channel:
The most common type of inlets used in chute spillways are the straight inlet,
box type inlet and sometimes side channel inlet also. The box type inlet is
generally used in a situation when straight type inlet is not sufficient to carry
the runoff at the desired drop.
2. Channel
Section or Conduit:
In chute spillway, the rectangular type conduits are mostly common. The side
walls of conduit confine the flow rate and discharge distribution. The top edge
of side walls is constructed in such a way that it may be flushed with the
embankment slope. The vertical curve section is continued through the channel
in such a manner so that it conveys and guides the discharge to the lower elevation
without erosion.
3. Outlet: The outlet dissipates the energy
of the flowing water and provides non erosive velocity downstream. Straight
apron type outlets are most commonly used in small gully control structures.
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