The relationship between tidal fluctuation and the elevation of the various components in a pond system is very important. To determine this the tidal elevation must be measured at the pond site, preferably at the location of the main gate. If possible the measurements should be made during the time when the lowest critical tides of the year occur. The time of year when the lowest critical tides occur can be obtained from the tide tables. If measurements cannot be taken during the lowest critical tide of the year, they should be taken during the lowest and highest tides of the month. When measuring tide, the following steps should be followed:
From a tide table select the days with the lowest tides, taking note of the O datum or mean lower low water (MLLW).
Drive a semi-permanent stake in front of the area where the main gate will be constructed to mark the lowest point of the tide. The point at which the water level was lowest is marked on the stake. This is then correlated to O datum by use of the tide table, and the O datum level is also marked on the stake. This serves as the base line for determination of all elevations in the pond system (Denila, 1976).
All low depressions should be filled in before dike construction is started. When the area is crossed by creeks or rivers, the portion of the dike running across these should be constructed first.
A puddle trench is essential to prevent water seepage under the dike. The earth should be packed as it is replaced into the trench. The dimensions of the trench should be 0.5 to 1 m deep and 0.5 to 1 m wide depending on the size of the dike.
The following slopes are recommended for dikes built with good clay soil.
2:1 when dike height is above 4.26 m and exposed to wave action
1:1 when dike height is less than 4.26 m and the tidal range is greater than 1 m
1:2 when the tidal range is 1 m or less, and the dike height is less than 1 m
The crown should be no less than 0.5 m. The actual width depends upon the activities which will be performed during culture operations.
The main dike surrounding the farm should be 0.5 cm above the highest tide or flood level recorded in the locality.
During construction 15 to 20 percent excess height should be allowed for shrinkage due to settling.
Construction should be in stages. First, build the dike to ⅓ of its final height all the way around the pond. Then build the height to ⅔, and finally to its full height. This allows the base to consolidate so it can support the weight of the top portion.
A berm built on the inside of the dikes should be slightly above the water line. This will minimize the effects of wave action on the dikes. A berm is also an advantage when it is necessary to dig out and replace soil to repair damage caused by crab holes.
To calculate the cross-sectional area of a dike to determine the amount of soil needed, the following formula can be used:
The area of the cross section is then multiplied by the length of dike to get the amount of soil required.
Growth of cover on the completed dike should be encouraged to prevent erosion (see Section 8.2.2).
Mangrove or other branch placed at the water's edge will retard erosion of the dikes. Mangrove can be planted at the water's edge.
In large ponds small submerged dikes can be built 10–15 m from the dikes during construction. Wind waves will break up on these submerged dikes and the water control dikes will not be damaged (Figure 15).
In areas where burrowing organisms are a known problem, damage caused by their burrowing can be prevented by incorporating a bamboo screen or plastic film in the puddle trench during construction.
The same materials can be placed in a cut made in the berm to repair damage. To stop leaks, slaked lime can also be added to a cut made in the berm. Easily applied as a powder it sets up hard on contact with water.
Canals which are to be used for harvesting should be 30 cm below the level of the pond bottom.
The width of the canals depends on the amount of water they must carry. The following factors must be taken into account:
The volume of water which will be held in the ponds.
The time requirements for filling or draining the ponds.
The amount of rainfall which must be carried off in a given period of time.
Elevation of the canal bottom in relation to tide. For instance, tidal ponds cannot be drained during high tide, but a pond built at a higher elevation and filled by pumping may be drained during any tidal phase.
Other uses. This might include transportation, harvesting milkfish, holding broodstock, etc.
There are numerous types and sizes of water control gates, and construction can be of many kinds of materials. There are, however, certain requirements that all gates at a shrimp pond should meet.
A gate should first of all be of adequate capacity for the amount of water required to be taken in or drained.
It should be constructed so that water can be taken in and discharged at the bottom.
It must have provision for draining surface water from the pond.
The bottom of the gate must be at an elevation which permits all the water to be drained.
It should be water-tight.
It must have slots or grooves for the placement of screens on the outside to keep trash undesirable species out of the pond, and on the inside to prevent shrimp from leaving the pond.
It should have a place to install a net for harvesting during draining.
It should be durable.
It should be easy to operate. If there are closure boards, all should be interchangeable.
Gates should be made of locally available products.
Anti-seep boards or collars will prevent lateral seepage and resultant washouts. A rubber liner of automobile inner tube attached to the closure boards helps to make a good water-tight seal. Winches can be used to remove boards. This allows the use of heavier, one or two-piece boards. In gates designed for use in ponds with shallow water such as for lab-lab culture, the side boards can be placed inside the support bracing. This allows the boards to be replaced easily when they decay. Gates to be used in ponds with deeper water, such as for plankton culture, should have the side boards placed outside the support bracing. This is necessary because the greater pressure pushes the side boards inward and if the boards were inside the bracing, they would become loosened and water leaks would occur.
Construction of gates has to be supervised closely, especially with concrete gates. Otherwise, the sides might slope and different length boards would be needed for each level. Also in multilane gates, the widths might vary and the boards would not be interchangeable.
Concrete gates, if not properly constructed, would be better made of wood. If workmanship is poor, the gates might not hold water. If construction is faulty or the design is inadequate, repairs will be costly and there may still be no guarantee for safety. The following four basic considerations should be taken into account when constructing gates.
That the foundation is adequate.
That there is adequate reinforcement against side pressure from the dike and water.
That the concrete is properly mixed and cured.
That measures are taken to prevent under-cutting by seepage of water along the sides and bottom of the gate.
To make a strong foundation, bamboo stakes are driven into the ground as far as they will go. The stakes should be 30 cm apart. The stakes are cut off, leaving a sufficient length to penetrate into the concrete slab. To prevent under-cutting, boards 5 × 15 cm and 1.8 m long should be driven across the place where the gate slab will lay. There should be a row of boards directly under the centre of the gate, below the wall and apron and under each end.
If poured concrete is used, the mixture of cement to sand to gravel should be 1:2.5:5 (Class B) for the wall and 1:2:4 (Class A) for the flooring. If concrete hollow blocks are used, the mixture should be one part cement to seven parts sand. The amount of water added should be 22.2 liters per 45 kg bag of cement. Spacing of steel reinforcing bars should be 40 cm, centre to centre. Bars of 1.2 cm (½ inch) diameter are used for vertical reinforcement and 1 cm (3/8 inch) bars for horizontal. Both hollow blocks and poured concrete walls should be at least 15 cm thick. The proportion of cement to sand should be 1:3 in mortar for finishing. Mortar must not be applied more than 0.6 cm (¼ inch) thick. The concrete should be cured (hydrated) for 21 days. This is done by covering the concrete with sacks and keeping the sacks moist for the whole 21-day period (Denila, 1976).
In areas with acid sulfate subsoils, special procedures are sometimes advisable in order to ensure pond fertility and prevent mortalities due to low pH. The economics has to be calculated for every farm to determine if such procedures are advisable. Some of the things which can be done are:
Excavate only enough soil from internal canals to construct the dikes and leave as much topsoil undisturbed as possible to serve as the pond bottom. In some cases, pumps might be required to fill this type of pond.
Scrap off the topsoil and set it aside. Then after the pond has been excavated, replace the topsoil over the pond bottom to cover the poor subsoil.
If the top layer of good topsoil is thick enough, alternate strips can be excavated twice as deep as necessary and then good soil from the unexcavated portion is placed in the deep portions to level the pond bottom.
Excavate the pond by sections. Excavate alternate 10 or 20 m wide strips within the pond and leave the strips between undisturbed. The pond is used for culture for several years, and after the excavated portions have aged, the remaining strips are excavated.
If the subsoil is not very acid, but has high potential acidity, a method of construction can be used in which the soil is kept moist so it will not be oxidized and become highly acid.
If acid soil, or potentially acid soil, has to be used for dike construction, the dikes bordering small nursery ponds can be surfaced with good topsoil. This is advantageous because acid runoff during rains has a greater effect in small ponds since the ratio of pond area to dike area is more critical.
If the dikes are to be constructed of acid soil, the nursery ponds should not be located next to large main dikes. This is recommended, because runoff is much greater from the large dikes than small ones, and as a result, more acid will be washed from them during rains.
If dikes are constructed of acid soils, the pond system should be designed to ensure that there is a minimum amount of seepage through the dikes into the ponds. This can be accomplished by having the pond water inside the pond higher than water outside the pond. Drainage canals should be constructed around the pond to make sure water does not stand there.
A berm can be constructed near the water's edge to catch acid runoff during rains and prevent it from washing into the ponds.
Every man should always have his own specific job or area in which to work. This is because when the job is done in a group, if one is lazy the others will also follow. If one goes to the toilet two times a day, the others will do the same. If work is done individually, others cannot complain that two or three of them are earning better. Even if work is done in a group, time records should be kept to find out the number of working hours for each person (Denila, 1976).
The best way to get the maximum output from pond workers is to promote competition among them. Give rewards for the highest earner of the week, and for the job or year. It is important that accurate records of work accomplished are kept and that the awarding of prizes is based on the records.
Great care should be taken in selecting the foreman for the success of the job will depend on him.