In this article we will discuss about:- 1. Meaning of Farm Ponds 2. Types of Ponds 3. Site Selection for Farm Ponds 4. Capacity of the Pond 5. Spillway and Outlet Structures 6. Seepage Losses in Ponds 7. Construction of Farm Ponds 8. Water Quality Aspects in Farm Ponds 9. Protection and Maintenance of Farm Ponds.
Contents:
- Meaning of Farm Ponds
- Types of Ponds
- Site Selection for Farm Ponds
- Capacity of the Pond
- Spillway and Outlet Structures
- Seepage Losses in Ponds
- Construction of Farm Ponds
- Water Quality Aspects in Farm Ponds
- Protection and Maintenance of Farm Ponds
1. Meaning of Farm Ponds:
Farm ponds are small tanks or reservoirs constructed for the purpose of storing water essentially from surface runoff. Farm ponds are useful for irrigation, water supply for the cattle, fish production, etc.
A large number of ponds constructed in a catchment will have a retarding effect on the flood flows downstream. Farm ponds have a significant role in areas of rainfed agriculture. They are used for storing water during rainy season and using the same for irrigation subsequently.
The design and construction of farm ponds require a thorough knowledge of the site conditions and requirements. Some sites are ideally suited for locating the ponds and advantage of natural conditions should always be taken.
2. Types of Ponds:
Depending on the source of water and their location with respect to the land surface, farm ponds are grouped into four types.
These are –
(1) Dugout ponds or excavated ponds,
(2) Surface ponds or watershed ponds,
(3) Spring or creek fed ponds, and
(4) off-stream storage ponds.
Dugout ponds are excavated at the site and the soil obtained by excavation is formed as embankment around the pond. The pond could either be fed by surface runoff or groundwater wherever aquifers are available. In case of dugout ponds, if the stored water is to be used for irrigation, the water has to be pumped out. Surface water ponds are the most common type of farm ponds.
These are partly excavated and an embankment is constructed to retain the water. Generally a site which has a depression already is chosen for the pond construction. The pond is fed by surface runoff from its catchment area.
The earthen embankment, mechanical spillway and an emergency spillway are parts of the pond. It is desirable to take the water out of the pond through gravity outlet for irrigation. Selection of suitable site for this purpose is, therefore, important.
Spring or creeks fed ponds are those where a spring or a creek is the source of water supply to the pond. Construction of these ponds, therefore, depends upon the availability of natural springs or creeks. Off-stream storage ponds are constructed by the side of streams which flow only seasonally. The idea is to store the water obtained from the seasonal flow in the streams.
Suitable arrangements need to be made for conveying the water from the stream to the storage ponds. Another type of ponds, referred to as levee ponds are generally used in aquaculture. These types of ponds are formed by embankments and are usually of a regular shape (rectangles) and are of uniform depth. Levee ponds are filled by water from wells, storage reservoirs, streams, or estuaries.
Components of a Farm Pond:
The pond consists of the storage area, earthen dam, mechanical spillway and an emergency spillway. The mechanical spillway is used for letting out the excess water from the pond and also as an outlet for taking out the water for irrigation. The emergency spillway is to safeguard the earthen dam from overtopping when there are inflows higher than the designed values.
Design of Farm Ponds:
The design of farm ponds consists of-
(1) Selection of site,
(2) Determination of the capacity of the pond,
(3) Design of the embankment,
(4) Design of the mechanical spillway,
(5) Design of the emergency spillway, and
(6) Providing for seepage control from the bottom.
3. Site Selection for Farm Ponds:
Selection of suitable site for the pond is important as the cost of construction as well as the utility of the pond depend upon the site.
The site for the pond is to be selected keeping in view of the following considerations:
1. The site should be such that largest storage volume is available with the least amount of earth-fill. A narrow section of the valley with steep side slopes is preferable.
2. Large areas of shallow water should be avoided as these will cause excessive evaporation losses and also cause water weeds to grow.
3. The site should not cause excessive seepage losses.
4. The pond should be located as near as possible to the area where the water will be used. When the water is to be used for irrigation, gravity flow to the areas to be irrigated is preferable.
4. Capacity of the Pond:
The capacity of the pond is determined from a contour survey of the site at which the pond is to be located. From the contour plan of the site, the capacity is calculated for different stages using the trapezoidal or Simpson’s rule (Simpson’s rule gives more accurate values than the trapezoidal formula).
For this purpose, the area enclosed by each contour is measured using a planimeter. According to the trapezoidal rule, the volume V between two contours at an interval H and having areas A1 and A2 is given by –
This formula is also known as the prismoidal rule. For using this equation, the number of contours should be odd, i.e., the number of intervals considered should be even.
Example 1:
Calculate the capacity of a pond having the area enclosed by different contours as follows:
Solution:
Trapezoidal formula has been used to calculate the volume increments given in this table.
A depth-capacity curve of the reservoir for different elevations is plotted from the depth-capacity data calculated as in Example 1 (Fig. 27.2 a and b). The depth-capacity curve is useful in deciding a suitable height of the embankment such that the required capacity is available. The contour plan of the proposed reservoir site is useful in determining the water-spread for a given depth of water.
5. Spillway and Outlet Structures:
Spillways are provided in farm ponds to dispose the excess water and also to a serve as a controlled outlet for use of the stored water. In dugout ponds there will not be any spillway but the inflow is suitably regulated.
A mechanical spillway is provided in the embankment type pond to let out water from the storage in a regulated manner. To protect the embankment from overtopping due to unexpected inflows into the storage, an emergency spillway is provided.
The emergency spillway is located on one end of the embankment. The bottom elevation of the emergency spillway should be the maximum flood level expected for the selected frequency of runoff into the pond.
The spillway and the outlet channel should be well protected by vegetation and should lead well below the dam into the waterway. The outlet should be protected against scouring by vegetation or stone pitching.
An emergency spillway will not be necessary if a surplus weir is provided as the mechanical spillway for the farm pond as the surplus weir will be able to discharge the excess runoff.
Permanent structures are constructed to serve as mechanical spillways for farm ponds. The drop spillway and the drop-inlet are the two structures which are commonly used. The drop spillway can handle higher discharges than the drop-inlet.
However, the drop-inlet provides a better control over the water stored in the pond. The design and construction of these structures are same as discussed earlier under gully control.
Some minor modifications in these structures are done when they are used in farm ponds. The drop structure constructed in the embankment of the pond is referred to as surplus weir. A row of vertical pillars on the crest is sometimes constructed. When storage of additional water is desired, soil is filled between these pillars so as to block the water.
The drop inlet spillway is also constructed as a simple pipe culvert. Provision of a control valve or a sliding head-gate is provided in order to regulate the outflow of water.
In case of ponds constructed for the aquaculture purpose, outlet structures are needed for draining out the water as required.
Example 2:
The peak rate of runoff expected from the catchment area of a tank is 4 cu.m per second. Assuming no temporary storage, find the length of the surplus weir, if the depth of flow over the weir is not to exceed 0.75 m.
Solution:
Using Francis formula for discharge through broad crested weirs, we have
4 = 1.84 (L – 0.1 x 2 x 0.75) 0.753/2
L = 3.5 m
6. Seepage Losses in Ponds:
At some locations this may take considerable time and as such lining of the pond becomes necessary. If the soil is well graded at the pond bottom (about 70% sand, 20% clay and 10% silt), excavating up to 30 cm and re-compacting the soil will reduce the seepage losses. When proper soil types are not available at the site, other methods are to be used.
Reduction in seepage rates are achieved by mixing swelling clay materials, such as bentonite with the soil. In order to support the water column at least 10 to 15 per cent sand should be present in the soil.
Polythene sheets have been used as a lining material for ponds. The sheets are spread in the bottom and the sides are joined using a mildly hot ironing box. A layer of soil of about 15 cm thick above the sheet is necessary to keep the sheet in place and avoid external damage.
A permanent and effective lining material is the brick and cement masonry lining. It is costlier than the other forms of lining. Cheaper lining materials like soil cement, bitumen etc. have been tried but they are not durable.
7. Construction of Farm Ponds:
For the construction of the embankment the first step is to clear the site and remove all vegetation and roots. The proposed base area of the embankment is thoroughly stirred by ploughing so that a good bond is obtained with the material. The core trench is excavated and the material selected for filling the core trench is filled in at the optimum moisture content.
Optimum Moisture Content:
The optimum moisture content in case of embankment construction is defined as the moisture content which is too wet for good tilth but not wet enough to give moisture out under compaction. The moisture content at the time of compaction should be such that the maximum density for the compacted soil is obtained. Fig. 27.13 shows the relation between moisture content and density.
In order to determine this moisture content a standard test known as the Proctor density test is conducted. In this test, the soil is placed in a container and compacted in layers with a weight dropped from a certain height for a definite number of times. The moisture content and density are determined after each compaction. Water is added to the sample and the test is repeated. A curve shown in Fig. 27.13 is prepared from these tests.
Core:
The fill material of the core and the embankment should be compacted well. Compaction should achieve 85 to 100 per cent of maximum density.
Compaction is obtained by rolling at optimum moisture content. Two types of rollers are used for the purpose.
These are –
(1) Pneumatic Tyred Rollers, and
(2) Sheep foot rollers.
Pneumatic tyred rollers consist of a pair of axles, one in front of the other over which a number of tyres are mounted. Extra loads are added to the assembly. These type of rollers are suited for compacting soils of low plasticity.
Sheep foot rollers consist of a drum over which feet like attachments are welded. The feet extend to a distance of 15 to 25 cm and have a surface area ranging from 25 to 80 sq.cm. Different size of rollers is used depending upon the construction needs. Light rollers arc used for smaller embankments.
For compacting the earth fill, the fill material is placed in layers of 20 to 25 cm thick. The fill material should have a mild slope (20 : 1 to 40 : 1) away from the centre of the dam. Tests should be conducted to determine the amount of water needed and also the required number of passes by the compacting equipment in order to achieve the desired density.
After knowing the initial moisture content of the fill material, the required amount of water is sprinkled over the fill material before the compacting rollers are put into operation. If the fill material is already wet, disking and exposure will reduce moisture content to the desired levels.
The nature of the fill material needs to be considered for compacting them. Soils of high clay content need to be compacted at moisture levels slightly lower than the plastic limit. Clay gets excavated in chunks and the use of sheep foot rollers reduces the spaces between the chunks leaving the void ratio inside each chunk unaffected.
Spillway construction should be taken up when the embankment comes up to the bottom height of the structure. Special care needs to be taken for compacting materials around the spillway components.
The embankment should be trimmed to the designed slopes. Protection measures proposed like rip rap, vegetative planting etc. should be done immediately.
8. Water Quality Aspects in Farm Ponds:
The quality of water in the ponds depends on the quality of runoff entering the pond, properties of the soil at the site of the pond and any contaminants added to the pond due to human and animal activities.
Runoff could bring sediment into the pond. The sediment settling at the bottom can gradually, reduce the ponds capacity. If the sediment has fine particles the water in the pond can be turbid. Ponds constructed on acid sulphate soils can result in acidic water.
Thermal stratification resulting from temperature differences in the water occurs particularly in ponds or large lakes with greater than 2 m depth of water. Energy from sunlight heats the surface layers and the heat is transferred to the lower layers by mixing of water by wind effects. Warm water is lighter than cool water and wind induced circulation cannot mix them with deeper layers of cool water. This causes thermal stratification.
The layer relatively warm surface water is known as the epilimnion, and the bottom layer of cooler water is known as the hypolimnion. The temperature changes rapidly with increasing depth in the transition zone between the epilimnion and hypolimnion and this transition zone is known as the thermocline (Fig. 27.15).
Thermal stratification influences the photo- synthetic activity of plant species in the ponds and consequently the dissolved oxygen in the water. It also influences the chemical stratification.
For aqua-cultural purposes, these are undesirable and as such the water depth in the aqua-cultural ponds is kept around 2 m. Further details about aqua-cultural ponds are given in Yoo and Boyd (1993). It is necessary to understand the water quality aspects in ponds in relation to the use of the water.
9. Protection and Maintenance of Farm Ponds:
The farm pond needs to be protected from cattle trespass. This is especially important if lining materials have been used which are likely to be damaged by cattle. Vegetative plantings around the periphery of the pond area prevent rapid inflow of sediment into the pond.
The watershed area of the pond should also be protected from soil erosion with appropriate soil conservation measures. Regular maintenance of the pond and ancillary structures is essential.