There is no standard design for a shrimp rearing pond. Present day farming practices still heavily rely on the experience of individual farmers, financial capability and the environmental condition prevailing at the site. A shrimp pond from the engineering view point is essential an improved finfish culture pond.
The improved structural design is largely due to the behavior of the Penaeid shrimps. P monodon is a benthic animal and it has a habit of gathering along tank walls. Any design to increase the wall surface eg. adding substrates or an elevated earthen platform extending along the edges of a pond, can promote high stocking densities.
An ideal shrimp farm is a complex establishment consisting of: (a) various size ponds for nursery and grow-out, (b) water control structures including embankments, supply and drainage canals and sluice gates, and (c) support facilities such as roads, bridges, living quarters, workshops and warehouses, etc.
Careful layout of the described facilities and appropriate structural design in relation to the physical features of the area ensure smooth and effective operational management.
Rectangular or square pond are appropriate for shrimp culture. The longest axis of a pond should be parallel to the prevailing wind direction. This facilitates water movement generated by wind action thereby increasing dissolved oxygen in the water and minimizing water temperature fluctuations in summer or warmer months.
The breadth of a pond depends largely on the purpose and the operational system employed. The following are the various sizes recommended:
|Nursery pond||500 to 1,000 m2|
|Grow-out pond||- intensive||0.25 to 1.0 ha|
|- semi-intensive||0.5 to 2.0 ha|
|- extensive||1.0 to 10 ha|
The rearing pond must have a minimum depth of 1.0 meter. Most traditional brackishwater ponds for shrimp farming are relatively shallow. To satisfy depth requirement, a ditch is constructed along the dike or a central canal between two opposite sides of the pond. The average depth is 1.0 – 1.2 meters (Fig. 6) and depth of the platform is 30–60 cm.
Such pond design with peripheral ditches and central platform affords several benefits:
Pond bottom should be as even as possible; free from projecting rocks and tree stumps. The bottom must have a gradual slope from the inlet gate towards the drainage gate. The suggested ratio of the slope is 1:500.
Dikes do not only serve as boundaries to indicate pond size and shape but also function to hold water within the pond as well as protecting other farm facilities from flood. Diking materials must preferably be tested for load bearing capabilities and compactibility. In some cases where the quality of the soil is inferior for diking, other materials, viz: concrete or clay must be used as core materials to be placed at the pond bottom (Fig. 7). Design and construction of embankment must be based on sound engineering principles and economic feasibility. A typical design of perimeter dikes facing the sea or a river is shown in Fig. 8.
Fig. 6. Layout of earthen pond with peripheral canal.
5.2.1 Height of dike
Coastal soil used as diking material usually shrinks initially. As such, the height of perimeter dike should have a free board of 0.6–0.7 meter above the desired water depth. Free board allowance is determined from the occurence and frequency of flood levels over a period from 5–15 years at the farm site.
To compute for the height of dike, the following formula could be used:
|H||=||height of designed dike|
|Hw||=||highest high water level from past record|
|G||=||ground level over mean sea level|
|FB||=||height of free board|
To give a concrete example, let us assume that a proposed shrimp farm has a ground elevation of 1.0 meter above mean sea level and normal high tide of 2 meters. Previous records indicate that the highest tide occuring every 10 years is 2.8 meters. The rate of soil shrinkage after the embankment have been consolidated is 20% and the estimated free board allowed is 0.60 meter. Height of dike is then calculated from the formula:
H = 3 meters
The slope of perimeter dike is maintained at an average ratio of 2:1 to 3:1. Very often, external slopes are made at a ratio of 2.5:1 to 3:1. Dikes with steep slopes are always subjected to erosion and require higher maintenance cost (Fig. 9). Slope of a dike also highly depends on soil quality. For good clay soil, the recommended slopes are:
1:2 when dike height is above 4.26 m and exposed to wave action;
1:1 when dike is less than 4.26 and the tidal range is greater than 2 meters;
2:1 when the tidal range is 1.0 m or less and the dike height is less than 1.0 meter.
The crown of the dike between ponds should be 1–5 meters. It would be advantageous if fast growing grass species are planted on the dikes to control soil erosion.
Fig. 7. Cross section of dike
Fig. 8. Sample design of perimeter dike
Fig 9. Typical slope of dike.
Not all shrimp farms are located close to the coast or estuaries. For those that are located far away from the water sources, it is necessary to construct supply and drainage canals.
Conceptually, a shrimp pond must possess separate canals for drainage and supply and for avoiding probable contamination of the water supply. Both supply and drainage canals would likewise serve as water level control in the pond and as temporary holding areas for shrimps. It is important that the siting of the canal systems takes advantage of the natural waterways within the proposed site.
Dimensions of supply and drainage canals are calculated by using the following equation:
Q = AV
|Q||=||volume of water discharge|
|A||=||cross-sectional area of the canal|
|V||=||velocity of water flow|
V value can be calculated by the following formula:
V = R⅔ × S½ × 1/n
|R||=||depth of water flow|
|S||=||canal bed gradient|
|n||=||coefficient of roughness (0.02)|
Assume that R = 1.25 m
|then||V||=||[1.25]⅔ × [1/5000]½ × 1/0.02|
Assume that the pond is 50 ha with an average depth of 1.0 meter and that 10 hours is required to drain the water completely; then
volume of water discharge/second
= 13.89 m3/sec
from the equation Q = AV
= 16.94 m2
The width of the canal can then be calculated from the equation:
A = R (b + 2R)
Slope of the canal is 1:2, R = 1.25
16.9 = 1.25 (b + 2 × 1.25)
= 11.02 m
Therefore, for a pond farm covering 50 hectares, width of the canal at the bedline should be 11.0 meters.
When designing a sluice gate, it is essential to consider tidal fluctuations and gravity in order to ensure effective control of the inflow and outflow of water within a given period of time.
The water gates are classified according to function as main (primary) gate or secondary gate. Main gates are strategically situated at the perimeter and are usually constructed of reinforced concrete. These are the main structure controlling the quantity of water for distribution to the shrimp farm.
Irrespective of the material to be used to construct the water gates (eg., wood, reinforced concrete, ferrocement), the following requirements should be met (Fig. 10, 11):
A standard gate design consists of tide stern wing, side walls and bed structure. The side walls are often designed in accordance with the slope of the earthen dike. Grooves for slabs are usually set at the inner side of the gate (Fig. 12).
The size of the gate is based on the total water requirement of a pond. Water intake volume is calculated using the equation:
Q = CA [2g(H - h)]½
|Q||=||rate of flow (m/sec)|
|C||=||cross section of the flux (calculated by multiplying the width of gate opening and its depth)|
|A||=||coefficient of discharge (0.61)|
|g||=||gravitational constant (9.8 m/sec2)|
Fig. 10. A typical wooden gate.
Fig. 11. A reinforced concrete gate
|H||=||tidal level of the river or sea|
|w||=||water level in the canal or pond|
A more detailed discussion on this topic is adequately covered by the works of Katch (1980) and dela Cruz (1983) (See Section 13).