Soil and Water Conservation, Part-2

 

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.

Fig. Splash erosion.

 

  1. 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.


                                                                  Fig. Sheet erosion.

 

  1. 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


                                                                     Fig.Rill erosion.

 4.)      Gully Erosion: Rills are small in size and can be leveled by tillage operations. When rills get larger in size and shape due to prolonged occurrence of flow through them and cannot be removed by tillage operation, these are called gullies. Large gullies and their network are called ravines. It is the advanced and last stage of water erosion. In other words it is the advanced stage of rill erosion. If the rills that are formed in the field are overlooked by the farmers, then they tend to increase in their size and shape with the occurrence of further rainfall. Some of the major causes of gully erosion are: steepness of land slope, soil texture, rainfall intensity, land mismanagement, biotic interference with natural vegetation, incorrect agricultural practices, etc.

                                                                Fig. Gully erosion.

 5.)     Stream Bank Erosion: Stream bank erosion is defined as the removal of stream bank soil by water either flowing over the sides of the stream or scouring from there. The stream bank erosion due to stream flow in the form of scouring and undercutting of the soil below the water surface caused by wave action is a continuous process in perennial streams. Stream bank erosion is mainly aggravated due to removal of vegetation, over grazing or cultivation on the area close to stream banks.

                                                        Fig.Stream bank erosion.

  • 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.

                                                             Fig.Contour cropping.

 

  • ·        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

                                                              Fig. 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.

                                                             Fig. Mulching of cropped field.

 

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.


                                      Fig. Bench terrace and its different components.

 

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


Case 2: When the batter slope is 1:1





S is the land slope in percent; D/2 is the depth of cut and W is the width of terrace.

 Case 3: When the batter slope is ½: 1

 

  • 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

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:




For the areas having low 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.


                              Fig. Basic diagram for deriving the height of bund.

 




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

 where, Et = total earthwork, Em = earthwork of main bunds, Es = earthwork of side bunds,

El = earthwork of lateral bunds,

Em = cross-sectional area * total length of bund = (100S/VI)*cross-sectional area.

 Therefore, Es + El = ((100S/VI) * 30/100) * 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.



Fig. Drop spillway.


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.

Fig. Drop inlet spillway and its components.

 

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.

Fig. Discharge characteristic curve of a drop inlet spillway.

 

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.


Components of Chute Spillway

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.

Components of Chute Spillway.

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