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In this article we will discuss about:- 1. Meaning of Farm Ponds 2. Types of Farm Pond 3. Components 4. Seepage through Embankment 5. Emergency Spillway 6. Seepage Loss 7. Protection 8. Sanitation.
Contents:
- Meaning of Farm Ponds
- Types of Farm Pond
- Components of Farm Pond
- Seepage through Embankment of Ponds
- Emergency Spillway of Farm Ponds
- Seepage Loss from Pond
- Protection of Farm Pond
- Sanitation of Farm Ponds
1. Meaning of Farm Ponds:
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Farm ponds are small tank or reservoir like constructions, are constructed for the purpose of storing the surface runoff, generated from the catchment area. The farm ponds are the water harvesting structures, solve several purposes of farm needs such as supply of water for irrigation, cattle feed, fish production etc.
Farm ponds also play a key role in flood control by constructing them in large numbers in the area. In addition, the farm ponds are also used for storing the monsoon water, which is used for irrigation of crops, and several other purposes, according to the need. A farm pond also has significant role in rainfed farming.
2. Types of Farm Pond
:
In broad sense, the farm ponds are divided in following two general categories:
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1. Embankment type, and
2. Excavated or dug out type
1. Embankment Type:
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Embankment type farm ponds are generally constructed across the stream or water course. Such ponds consist of an earthen embankment, which dimensions are fixed on the basis of volume of water to be stored, mainly. These farm ponds are usually constructed in that area where land slope ranges from gentle to moderately steep; and also where stream valleys are sufficiently depressed to permit a maximum storage volume with least amount of earthwork.
2. Excavated or Dug Out Type:
Dug out type farm ponds are constructed by excavating the soil from the ground, relatively in flat areas. The depth of pond is decided on the basis of its desired capacity, which is obtained almost by excavation. The use of this type of pond is suitable, particularly where a small supply of water is required.
Apart from above two types, the farm ponds are also of two more types, i.e. the spring or creck fed and off-stream storage pond, depending on the sources of water available for feeding them.
The spring or creck fed ponds are generally found is hilly areas, where natural crecks or springs are available to supply the water. Since, the source of water supply to these ponds is the creck or spring, therefore, they are named as spring or creck fed pond.
Off-stream storage ponds are constructed in the side of ephemeral streams, in which water flows seasonally. This seasonal water is stored in these ponds. This type of ponds are provided with a suitable arrangement for safe conveyance of flow into the pond and supply of water to d/s channels.
To design a dugout pond, the rainfall, runoff, evaporation and seepage data are essentially required, but these are generally not available for most of the farm sites. If these are available, then cost of an extensive hydrologic investigation is not required. However, based on few main factors affecting the pond design, a general guide line has been developed.
‘Seepage’ is counted as one of the main factors for pond design, e.g. in the areas of high gypsum the seepage loss is much more, causing construction of farm pond impractical. Although, everywhere there is seepage due to ponding of water but it should be under favorable limit.
Normally, it is advised to allow the seepage loss at the rate of 1m depth of water per year, when estimating minimum storage. The depth of farm pond also varies as per climatic condition. Typical values of average pond depth are given in Table 17.1.
A general approach to design a dugout farm pond is given under the following steps:
1. Collect the basic data of the area concern, i.e.
(a) Rainfall, evaporation, temperature etc.
(b) Topography
(b) Livestock etc.
2. Estimate the required storage capacity of pond. It is determined based on the livestock. Normally, the bigger live stocks such as horse and cattle require about 1 acre-ft/year per 75 heads and small livestock like sheep, hogs require about 1 acre-ft/year for per 750 heads.
The volume of water likely to lost through evaporation and seepage, plus about 10% more to adjust some unforeseen water losses from the pond, is also added to determine pond storage capacity.
3. Determine the catchment area of the farm pond to satisfy the required storage. It is performed by use of topographic map of the area.
4. Select a desirable pond depth. It is obtained from the Table 17.1.
5. Locate the possible sites of pond with their catchment areas. The pond sites are identified by using topographic map or aerial photographs of the locality.
6. Make field investigations of the sites, selected. Normally, the following considerations are made to assess a most desirable one.
(i) Should be easily assessable.
(ii) Should require a short dam below the pond area with steep sides.
(iii) It should require less removal of trees.
(iv) The site should have suitable foundation conditions for the dam.
(v) It should not have excessive seepage through the foundation.
(vi) The site should have suitable materials for dam construction in nearby areas.
(vii) There should be a natural waterway to use as emergency spillway.
Based on above requisites, the site which proves better in all respects, is selected for pond construction.
7. Determine the dimension of the pond and dam for the selected site, to ensure necessary storage capacity and depth of water.
8. Compute the maximum depth of farm pond, including additional depth for sedimentation and excessive seepage.
9. Fix the crest of emergency spillway, which is usually fixed at the height of maximum pond level.
10. Determine the size of spillway. To avoid prolong use of emergency spillway, the spillway should be large enough to carry the flow. It is predicted by computing the difference in elevation between the spillway crest and the maximum pond surface, which should be about 60 cm during the spillway flow. The discharge of spillway is calculated, usually for 25-years return period, by using any method.
Lastly, the bottom width is estimated for the obtained discharge and elevation difference between spillway crest and maximum pond surface. The side slope is fixed as per bottom width, usually 1.5:1 is used as side slope of the emergency spillway. If large bottom width is not practical for the site, then it is accommodated by constructing two small spillways, separately.
3. Components of Farm Pond
:
A farm pond essentially consists of following components:
1. Pondage or storage area
2. Earthen embankment
3. Mechanical and emergency spillways.
The mechanical spillway is used for letting out the stored water from the pond. In other words, M. spillway acts as an outlet for safe disposal of water from the storage area of the pond. On the other hand, the emergency spillway is used as safeguard for earthen dam against overtopping, when volume of inflow becomes greater than the pondage capacity of the farm pond.
4. Seepage through Embankment
of Ponds:
The success or failure of water harvesting structures depends very much on the storage capacity of pond, to hold against various losses. The loss of water from the pond mainly occurs due to seepage phenomena from the section of embankment. It should be controlled not only to remove the water loss, but also to ensure the safety of the embankment. If water is filled in the pond, then a seepage or phreatic line is established in the dam section. The phreatic line is the line below which there is a positive hydrostatic pressure and negative pressure, above it.
In a given embankment section, it is very important to predict the position of seepage line to satisfy the following points:
1. To ensure that the seepage line does not intersect the downstream face of the dam so that the causes of softening or sloughing at the toe can be checked.
2. To determine the position of divide line between the wet and dry soil zones in dam section for stability analysis.
3. To obtain the top boundary line for sketching the flow-net under seepage analysis.
In general, it has been found that the phreatic line is mostly affected by the following points:
1. The permeability of fill materials and foundation condition of the earthen embankment.
2. The type and design of the core wall or cut-off used in the embankment section.
3. Drainage provision at the down-stream face of the dam.
Furthermore, the position of seepage line also depends on the geometrical cross-section of the dam; not only on the permeability of the fill material as long as the section is homogeneous. A less pervious material takes relatively longer time to cause the seepage line at steady state condition, than a pervious material; however, the final position of the seepage line in both the cases is almost the same.
5. Emergency Spillway
of Farm Ponds:
The main function of emergency spillway is to protect the embankment from overtopping due to unexpected increase of inflow into the pond storage. The emergency spillway should be located at one end of the embankment. It should also be kept in mind that the bottom of the emergency spillway should be fixed at the maximum expected water level for the selected frequency of runoff, used for design of the pond.
The dimensions of the emergency spillway are determined on the basis of the runoff rate to be disposed through it. If the peak flow rate for the design of emergency-spillway is known, then the dimension of E. spillway can be predicted by using the weir formula, given as under –
Q = CLHm … (17.7)
Where,
Q = discharge rate, m3/s
C = coefficient of discharge. The value of C varies with the types of weir, used. The values of C for different types of weir are given in Table 17.2.
H = head at the crest, m
L = length of crest, m
m = exponent.
Its value depends on the shape of the weir’s opening, given in Table 17.2.
The side slopes of the emergency spillway 2:1 is recommended. The depth of flow over the spillway should not be more than 30 cm. The spillway and its outlet should be fully protected against soil erosion caused by water flow, either by establishing the vegetations or by stone pitching. However, the emergency spillway may not be necessary, if a surplus weir has already been installed in the farm pond, which is sufficient to discharge the excess water.
6. Seepage Loss from Pond
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The seepage loss from the pond could be considerable if proper measures to control that, are not adopted. It depends on the properties of the sub-surface soil of the pond site. However, the rate of seepage loss gets reduced with time due to silt deposition at the bottom, brought by the runoff water, but at some locations this may take considerable time. Under such conditions the lining of pond becomes necessary.
If the soil is well graded at the bottom of the pond then seepage loss can be reduced by excavating the soil up to 30 cm depth and re-compacting it. If the proper soil types are not available at the site, then other materials can also be used for controlling the seepage loss, e.g. the seepage loss can be reduced by mixing the clay materials (bentonite) with the soil.
The polythene sheets (400 micron thickness) are also used as a lining material for controlling the seepage loss from the ponds. The sheets are spread at the bottom and sides of the pond. The ends are joined using a mildly hot ironing machine. About 15 cm thick soil above the sheet is also kept to keep the sheet in proper place and also check the damage. The permanent and most effective lining materials are the brick, cement and masonry. But these are costly than the other lining materials, hence they are not used as common material, everywhere.
7. Protection of Farm Pond
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A farm pond should be protected against erosion, wave action, livestock and other sources of damage. Ponds without such protections have short life and high cost of maintenance.
In general, the farm ponds are protected against following damaging points:
1. Protection against Erosion:
In general, the exposed surfaces of the dam, spillways, berm areas and other surfaces are badly damaged due to soil erosion. These can be checked by establishing a good cover of sod-forming grasses. The planting of grasses should be done based on the local guidelines.
2. Protection against Wave Action:
Time to time there should also be used the measures for protection of earth dam against wave action.
Following measures are generally used for protecting the earthen embankment against wave action:
(i) Berm:
The protection of earthen embankment can be done by providing a berm of 2.5 to 3.0 m in width at normal pond level. In addition, the face of dam above the berm should also be protected by establishing the vegetations.
(ii) Boom:
A boom consist of one or two rows of logs, chained and securely anchored at the toe end of the dam- The logs should be tied by end to end. There should be enough slack in the line so that the boom may be adjusted according to the fluctuation of water level in the pond. Double row logs should also be framed together to act as one unit.
The boom should be so placed that it can float at the distance of about 1.8 m towards upstream from the face of the dam, for best result. Boom provides high degree of protection and relatively less expensive. This method is technically feasible, especially for those areas where timbers are readily available in nearby area. Riprap.
3. Riprap:
The rock ripraps are generally preferred in that condition, when water level in the pond is expected to fluctuate very widely, or where a high degree of protection is required. Riprap should be extended from the top to the bottom of the dam towards upstream face. At the bottom side, it should be done at least 1 m below the lowest expected level of the water in the pond.
The rip-rap may be done either by machine or by hand in the thickness of at least 30 cm. Stones to be used should be durable and large in size, so that they could not get displaced by wave action of pond’s water. In addition, when it is not possible to do the riprap up to the toe of upstream face, then a berm of suitable width can be provided on the upstream face for supporting the riprap.
4. Other Methods:
The other methods for protecting the earthen embankment of farm pond include increasing the top width of the dam, flattering the front slope and applying a layer of coarse sand and gravel on 1:1 slope. These methods are generally suitable in the arid zone areas, where rocks or timbers are not readily available.
5. Fencing:
The complete fencing of embankment type pond is generally recommended for that area, where livestock’s are allowed for grazing in adjacent areas of the farm pond. The fencing provides protection and also helps in maintaining the vegetative cover over the dam section.
8. Sanitation of Farm Ponds:
It is very essential to keep the pond’s water as clean and unpolluted as possible. Unnecessary tramping of livestock should not be permitted. In addition, the drainage from barn lots, feed yards, bedding grounds, gutters septic tanks or other sources of contamination should also be kept away from the farm pond. This is especially important, when the water storage is used for fish farming, wild life development etc.
The farm pond should be stocked with top-feeding fish, especially in that areas where surface water encourages the breeding of mosquitoes.
Sometimes, algae and other forms of plant lives are also grown in the pond, which may be objectionable because they cause disagreeable tastes or odours, encourage the bacterial growth and make the pond’s water unsuitable for use.