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In this article we will discuss about how to design and construct farm ponds for water conservation.
Requisites for Pond Design:
The main requisites for design and construction of farm ponds are out lined as under:
i. Adequate Water Source:
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This is an important requisite for pond design and construction. Without availability of water source in nearby area, the construction of farm pond is not possible. For feeding the farm pond there must be water source, which may be the spring, canal water, overland drainage, ground water, stream flow or the diverted water flow.
ii. Proper Catchment Area:
In case, when water source for pond feeding is the rainwater, then for an efficient design of farm pond the consideration on proper catchment area/drainage area is one of the important requisites. Proper catchment enables the pond to remain for longer duration with adequate capacity along with sufficient water storage. The soil of catchment area should be of good runoff producing potential, and least erodible.
iii. Sub-Soil Properties:
The sub-soil properties of the construction site also determine the suitability of pond construction. If sub-soil contains impermeable strata, then the loss of stored water due to deep percolation gets checked to a large extent; and water Storability of pond is enhanced.
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In order to feed the farm pond, timely, at fast rate, the water supply source must be adequate. Also, the pond should contain relatively constant water level round the year. In case of farm pond for fish farming, if water supply rate is very high, then there is possibility of overflow of water from the pond storage, which results into flushing of nutrients, and also allowing the fish to escape. A small size stream is always good source of water for ponds, provided that,
i. The flow is sufficient to fill the pond and maintain the water level in optimum limit.
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ii. Stream is not subject to flood occurrence.
iii. The watershed is well vegetated, and
iv. The stream carries least amount of silt load, especially during flood occurrence.
If streams are used as the water supply source, then site of pond construction should be close to the stream, so that the stream water could be easily fed to the pond. Also, at the entry point of stream water into the pond, there must be provided an inlet pipe equipped with screen to filter the foreign materials (silts or vegetations).
Another important water sources for farm ponds are the surface run-off, springs, wells and ground water. The ground water is usually of best quality to support aquatic lives. In case of well water, if there is excessive carbon dioxide or nitrogen, then it must be aerated.
Few ground waters may contain excess minerals, which are harmful to fish and other aquatic lives. In this condition, the water quality should be analyzed before pond feeding, so that there would not be any harmful effect on aquatic lives.
Selection of Farm Pond Site:
The selection of suitable site for construction of pond, the preliminary studies of different possible sites are carried out. Each site should be studied separately. A site which proves most practical and economical, is selected for construction of pond.
However, some important features of farm pond site are given below:
1. From economic point of view, a pond should be located at that site where sufficient storage volume can be obtained with least amount of earthwork. This condition generally gets satisfy at the site, where valley is narrow and side slopes are relatively steep.
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Such sites have greater water depth as compared to the exposed surface area, which is very conducive to reduce the evaporation loss. However, such sites must be checked carefully for adverse geologic conditions of the area.
2. If ponds are constructed for the purpose of livestock, then they should be constructed at such a point, from where the transportation distance of water will not be more than one-quarter mile in rough areas.
3. The pond site should be such that, from where the water can be conveyed for various uses, such as for irrigation or fire protection, very easily.
4. For the ponds to be used for fishing or other forms of recreation, the site should be readily accessible by transportation facilities.
5. The pond site should be such that, the drainage from farmsteads, feeding lots, corrals, sewage lines, mines dumps and other similar area should not be there. However, if it is not practically possible, then drainage line from the site should be diverted.
6. The site near to the dam should not be selected for pond construction, because if there is breaching of dam, then there is possibility of loss of human lives, injury to the persons or livestock, damaging of residences or industrial buildings, rail roads or highways or interruptions in services of public concerns.
If there is only one site is available in the area, then one or more of above points, should be studied very carefully.
7. The site, where low hanging H.T. power lines are present in the area, should be avoided for pond construction because they can create the problem for use of farm pond.
8. For selecting the pond site, it should also be ensured that there is no buried pipe lines or cables at the site. Otherwise, they might be damaged by the excavating machine, which can also cause injury to the operator of the machine. However, if use of such site is essential, then land owners must be contacted before starting the construction work.
Design of Farm Pond:
Design of farm pond consists of following parameters for determining:
1. Site selection
2. Pond capacity
3. Design of embankment
4. Design of mechanical spillway
5. Design of emergency spillway, and
6. Arrangements for seepage control from storage area of the pond.
Pond Capacity:
The ponds capacity depends on catchment size, volume of water requirement, and soil characteristics of the catchment, mainly. The catchment size directly affects the quantity of available water for storage. The volume of water requirement may be for crop cultivation, live stocks or domestic needs. The soil characteristics are concerned, only their erosion potential is considered. For high erodible soils in catchment, comparatively greater siltation storage is provided in the pond storage.
The capacity of farm pond is determined with the help of contour map of the watershed, contributing the runoff to the pond. From the contour plan of the site, the capacity of pond is computed for different stages, using area estimating formula. Generally, Trapezoidal and Simpson’s formulae are used for the purpose. The Simpson formula gives more accurate estimate than the Trapezoidal formula.
For determining the of pond’s capacity, the area enclosed by each contour line is measured with the help of plainimeter and by using Trapezoidal or Simpson rule the pond’s capacity is determined. According to the Trapezoidal rule, the volume (V) formed by two successive contour lines spaced at distance (H) with the area A1, A2, is given by –
The Simpson rule is also known as Prismodial rule. For using Simpson rule, the contour lines should be in odd numbers or the number of contour intervals should be in even number.
A depth-capacity curve of the pond is also prepared for different stages of water level, using the depth-capacity data. This curve helps in deciding the suitable embankment height with respect to the capacity of the pond. Fig. 17.2 presents the depth-capacity curve of the pond.
Data Required:
For design and construction of earthen embankment in farm pond, the following data are required:
1. Hydrologic Data:
(i) Stream flow (volume, rate).
(ii) Water requirement including various losses such as conveyance, seepage, percolation, evaporation, etc.
(iii) In coming flow data including design inflow and outflow to be expected.
(iv) Data on sediment flow rate.
(v) Ground water-table.
2. Climatic Data:
(i) Rainfall and storm intensity.
(ii) Temperature.
(iii) Evaporation rate.
(iv) Maximum, minimum and mean temperature.
(v) Sunshine hour.
(vi) Relative humidity.
(vii) Prevailing wind direction.
(viii) Wind velocity.
3. Geologic Data:
(i) Types of aquifer.
(ii) Exposed gravel.
(iii) Geological cross-sections where necessary.
4. Data for Dams:
(i) Dam Site:
(a) Topo map of the dam site.
(b) Horizontal and vertical controls, preferably as a triangulation system.
(c) Co-ordinate grid system.
(d) Location of rock outcrops and apparent geological features,
(e) Location of existing work at the site.
(ii) Tail Water Data:
(a) Stage-discharge relationship of the stream.
(b) Stream size.
(c) Tail and back water curves for the given high water marks.
The embankment design suitable to farm pond, consists of following parameters for determining: Foundation. Earthen embankments are constructed for a wide range of foundation conditions.
Overall, the foundation should be such that, it can provide a stable support to the dam and can also create a resistance to the flow (seepage) of water through the dam section. For this purpose, the sub-soil properties of the proposed site must be studied, carefully.
Good foundation materials should always be used, as they provide stability and imperviousness both to the foundation. The mixture of coarse and fine textured soils like gravel-sand-clay mixture, gravel-sand-silt mixture, sand-clay mixture and sand-silt mixture are counted as good foundation materials. The coarse textured materials, i.e. the gravel and sand even provide a good support to the foundation but they are highly pervious to water.
If such materials exist at the site, then precautions should be strictly followed to prevent the seepage flow below the embankment. However, for an embankment height of 5 m, the nature of foundation materials is not important; but for higher embankment if the materials are not suitable for construction, then base width should be increased so that the load per unit area is decreased.
Core Wall:
To check the seepage flow from the foundation of the earthen dam, a core wall of impervious material is provided. The construction of core wall should be started from a certain depth, below the ground surface up to the height of maximum expected water level in the pond. If there exists impervious layer in the soil at the construction site, then core wall should be keyed to the impervious layer for providing complete sealing of seepage flow.
The core wall should be placed at the centre of the embankment. For construction of core wall a trench is dug along the centre line of the purposed dam site, at least 60 cm deep into the sub-soil. The minimum width of core trench is kept about 120 cm at the bottom with the side slope 1:1. The core trench dug so, is filled in layers using impervious materials. The filled material is puddled and compacted for better result, and it is done up to 50 to 60 cm above the highest expected water level in the farm pond.
After construction of core wall, the earth fill materials for dam construction are placed in thin layers of about 15 to 20 cm, outside the core wall. Each layer is thoroughly compacted. The filling soil should be reasonably moist to have better compaction.
The fill material for clam construction should be taken from the purposed site, if possible. The ideal soil for construction of earthen embankment is that, which contains about equal proportion of clay, silt and sand. However, if there is different types of soil materials at the site, then the best material should be placed in the core trench or at the centre of the dam for constructing the core wall.
Cross-Section:
Cross-section of earthen embankment depends on the foundation and fill materials available at the construction site. The materials used for embankment construction should be fine and impervious in nature to the water.
The following parameters are considered for determining the cross-section of earthen dam:
(a) Side Slope:
The side slopes of earthen dam are dependent on the dam height, nature of foundation materials as well as the fill materials used for construction. The side slope which is commonly used for average quality fill material is 3:1 and 2:1 towards upstream and downstream sides, respectively for a dam height of 15 m. In case of relatively coarse fill material that cannot be compacted very well, a flatter slope, i.e. 3:1 and 4:1 towards u/s and d/s, respectively are provided.
Similarly, in case of well graded soil that can be compacted very well, the side slope 2:1 is provided on both the sides of the dam. For embankment’s height greater than 10 m, a berm of suitable width should be provided on down-stream side of the dam. The width of berm should be from 1 to 3 m with a mild inward slope for drainage facility.
The upstream side slope of embankment should be protected either by using riprap or rock pavement, to prevent the erosion from wave action. The down-stream side slope should also have a good vegetative cover to provide the protection against soil erosion caused by raindrop impact etc.
(b) Height:
The height of dam is determined after fixing the position of spillway in the dam section. The dam should be extended up to the height ranging from 60 to 100 cm above the spillway. For determining the net height of dam an adequate allowance for settlement and free board is also provided.
A newly constructed earthen dam tends to settle down after construction. The height of settlement depends on the method of construction and materials used. It may vary from 20 to 25% when construction is done by drag line method. It could be very small, when the fill materials are thoroughly compacted. However, the following formula can also be used for computing the height of settlement in case of earthen embankment –
Where,
S = height of settlement, meters
e1 = void ratio before consolidation
e2 = void ratio after consolidation
Y = depth of soil being consolidated, meters.
The free board ranging from 60 to 75 cm is added to the height of dam, after settlement. In bigger size earthen dams, the determination of exact value of free board, is done on the basis of engineering principles.
(c) Top Width:
The top width of dam varies. It mainly depends on the height of dam and mode of use of its top surface. However, the following formula can be used for determining the top width of earthen embankment.
Where,
W = top width of the dam, meter
H = maximum height of the dam, meter.
Normally, up to 5 m height of embankment the minimum top width 2.5 meters is recommended. If the top of the dam is used as road, then width can be up to 5 meters or more.
Earthwork:
For computing the volume of earth work involved under dam construction, the end area method is commonly used, which states that the volume between two successive cross sections is equal to the average of their end areas, multiplied by the distance between them. This is expressed as –
Where,
V = volume of earthwork, cu.m
A1 & A2 = end areas of the respective cross-sections, sq. m.
L = distance between the two cross-sections, m
Design of Mechanical Spillway:
One of the important point in dam construction is to provide a suitable means for disposing the water from the pond. Most commonly, the spillways are used for disposing the water from water storage bodies. The kind of spillway to be used, depends on the size of watershed contributing the runoff to the pond.
A pond having watershed area less than 4 hectares, the vegetative spillway can be suitably used for the purpose. A combination of vegetative and mechanical spillway should be preferred, if the area of watershed is from 4 to 12 hectares. Especially, when watershed area is more than 12 hectares, then the selection, design and location of spillway should be done, carefully.
The kind of spillway to be used in the dam also depends on the watershed characteristics and the site conditions. If the drop height is less than 4 meter and there is less possibility of silt deposition, then drop spillway may be used. The rectangular weir type drop inlet structures are also preferred, as they are less susceptible to get clog as compared to the others.
If the drop height exceeds 4 meter and there is chance of silt accumulation, then drop inlet type spillways are mostly preferred; it should have a box type inlet or an arched type inlet. Arched inlets are more preferred as they provide additional advantage of arch strength in case of masonry structures.
The outlet of the spillway should be so designed and constructed that the kinetic energy gained by flowing water, must be dissipated by maximum amount and or converted into potential energy before the flow is discharged into the stream. The stilling basin or rock riprap is provided for this purpose at the outlet end.
The spillway should be constructed on a firm foundation and with the help of durable materials, for providing long life. The drop structure used in embankment of farm pond is also referred by the name of surplus weir. The dimension of mechanical spillway is shown in Fig. 17.3.
Construction of Farm Pond:
The characteristics of contributing watershed should be studied and analysed before construction of the farm pond. The surface profile of the construction site should also be surveyed for estimating the earthwork and design of spillways etc.
The contour map of the construction site is also prepared to determine the pond’s capacity. Prior to start construction work, the location of core wall, earth fill for embankment, mechanical and emergency spillways, height of dam etc. are fixed and indicated by the stakes.
The area to be used as base of the dam is also cleaned from the trees, bushes, boulders, root stumps etc. whatever present on the surface and stock piled out of the way which can be used to cover the back side of dam to provide better soil for grass seeding. The entire width of the dam area should be disked or ploughed, before starting to place the soil materials. While placing the earth materials, a berm towards d/s side should also be formed to check the sliding action.
The moisture content of earth fill plays a great role in construction of stable earthen embankment. Generally, ‘optimum moisture content’ is preferred for the purpose.
Optimum Moisture Content refers to that moisture content at which the soil is sufficient wet for good tilth, but not so enough to cause the moisture out from the soil due to compaction. The moisture content of the till material at the time of compaction should be such that, the maximum density of the soil can be obtained after compaction. A greater density makes the dam section more stable and impervious to seepage.
Construction of earthen embankment should be performed by the construction of core wall first, just by filling the impervious earth material in centre and compacting them properly. The compaction should be continued for the time when 85 to 100 percent density has been developed there. This is obtained by rolling the compacting machine at optimum moisture content.
For compaction, the earth fill materials should be placed in the layer of 20 to 25 cm thick at the site. It should have a mild slope, away from the centre of the dam. At the same time, a test should also be conducted for determining the amount of water needed for spreading over the fill materials and number of passes of roller for compacting the soil, to get the desired density.
It is obtained by knowing the value of initial soil moisture content. If the initial moisture content of the fill material is less than the required amount of water is sprinkled and then compacting roller is put into operation. In case, when fill material is already sufficient wet, then disking of fill materials should be done to reduce the moisture content by evaporation loss.
The nature of fill material should also be determined as it plays an important role in compaction. From study it has been observed that the soils having high clay content should be compacted at the moisture level, slightly lower than the plastic limit to have better compaction.
The construction of spillway should be started, when the embankment has been reached to the level at which spillway is to be installed. Special care also needs to be taken for compacting the soil around the components of spillway.
Finally, the embankment is trimmed to get the designed slopes.
Construction of Dugout Farm Pond:
The dugout farm ponds are found suitable, where topography does not permit the construction of embankment. These ponds are constructed on the principle to expose a minimum water surface area in proportion to their volume. This feature of pond proves beneficial, particularly where evaporation loss is high and water is very scare.
Site Selection:
Prior to go for excavation work to construct the pond, it is very essential to carry out some observations on its suitability regarding different constraints. Most commonly, the factors such as watershed characteristics, topography and soil type, silting possibilities etc. are counted for site selection.
In regard to watershed characteristics, the following features are mainly observed:
1. Watershed must be capable to generate sufficient runoff to fill the dugout pond.
2. If there low spots are available in the watershed, they should be utilized for construction, as these spots involve less earthwork.
3. The site should be such that, form where the stored water can be utilized at low cost and very easily.
The yield of sediments from the watershed and their reaching to the pond is also one of the main factors, taken into consideration for pond construction. The areas having high sediment yield are not suitable for pond’s construction. However, if these areas are essential to use, then they should be kept under vegetative cover to reduce the charging of sediments from the soil surface.
The soil type is also one of the main factors for deciding the site of pond’s construction.
In this regard, the following points are considered to characterize the suitability of site for dugout pond:
1. The soil permeability should be low, as this property significantly reduce the loss of water through deep-percolation and seepage phenomena.
2. The soil should also not be very tough, because it creates the problem in cutting of the soil.
In the state of Karnataka the dugout farm ponds are constructed in deep-black soil. The objective is to reduce the water loss caused by seepage and deep percolation actions. In light and medium soils, the ponds are also constructed on low land spots with the provision of lined surface.
Apart from above features counted for site selection of pond construction, that site would also be more beneficial, where water-table is expected to rise within few meters from the ground surface because in this condition the fluctuation of water depth in the pond is easily adjusted. In addition the ponds constructed in such locations are also being able to supply the water throughout the year.
It includes following components to plan for construction:
i. Shape:
The excavated ponds can be constructed to any shape, as desired, but rectangular and trapezoidal shapes are assumed to be most convenient.
ii. Size:
It is governed on the factors such as area of watershed contributing the runoff to the pond, rainfall amount, soil type and expected runoff entering the pond. The size of dugout pond should also be such that the extent of area that to be put under pond, earth work involved and the amount of money to be spent should be within appropriate limit.
iii. Length and Width:
These two are not ordinarily limited; depend on the type and size of earth moving machines to be used for construction. A dug out farm pond of any length and width can be easily constructed, provided that the sufficient land is available at the site. Generally, the length and width of pond are kept in the multiples of width of machine.
iv. Side Slope:
The side slope is kept to suit the angle of repose of the soil material. It should not exceed the angle of repose. However, in most of the cases the side slope is normally used as 1:1.
v. Disposal of Excavated Material:
The excavated materials should not be dumped here and there, but they should be fully utilized, either in form of berm or bund construction around the pond. The use of excavated materials in such form, is useful for providing the stability to the side slope and also to retain the materials against washing due to rainfall.
Construction Procedure:
To proceed the construction of dugout pond, the pond site and waste areas are first cleaned from the vegetations. And then the excavation and soil placement areas are demarcated. Thereafter excavation is started.
Normally, for ease of excavation the step method is followed, in which a step like cross-section representing rectangular shape of bottom is followed for excavation. After excavation the provision of spillway at the entrance of runoff into the farm pond is also provided. Normally, a masonry chute spillway is used.
The following formula can be used for determining the volume of excavation under construction of farm pond. This formula is referred as Prismodial formula.
Where,
V = volume of excavation (m3)
A = area of excavation at the ground surface (m2)
B = area of excavation at mid-depth point (m2)
C = area of excavation at the bottom of the pond (m2)
D = average depth of pond (m).