Data for pond farm design
Since the majority of aquaculture installations at present are land-based pond farms, we may first consider the procedures for designing those. Despite the similarity of basic principles involved, it will be convenient to consider inland fresh-water pond farms and coastal brackish-water or salt-water pond farms sepa-rately, mainly because of the differences in operational details.
As already indicated, the investigations prior to farm design will depend on the extent of information collected during the preliminary feasibility studies. The meteorological data relating to mean monthly temperature, rainfall, evaporation, humidity, sunshine and wind speed and direction should already be available. A contour map (scale 1:25000 to 1:50000) of the area will be most useful in determining the catchment area of the site and its relative location.A soil or geological map, if available, would be useful in studying the subsoil at the site.
Detailed investigations may be necessary with regard to water sources, soil characteristics and topography of the site. Topographic maps, if available, are likely to be of a small scale, which would not allow all the relevant features to be reflected. Therefore a new or updated map will have to be prepared showing the nature of the ground relief and its characteristics, such as differences in elevation, location and measurements of boundaries or fences, physical facilities if any (such as buildings, roads, canals, bridges), etc. It will assist in determining the direction of water movement, location of water control structures and quantity of earthwork needed. There are a number of methods used for surveying the land, such as
b) plane tabling, cross-section method with transverse survey,
c) radiating lines method with transverse survey and
Among these, tachymetry is relatively rapid in field surveys and more versatile in that it can be used for surveying all types of areas. Methods like gridding and plane tabling are more suited for relatively flat land, and the others cited above are especially useful for hilly terrains (Kovary, 1984). For field surveying a temporary benchmark with a convenient datum should be established. The location of this benchmark should be marked on the contour map and all the elevations of embankments, canals, ponds, structures, buildings, etc. set out from it. The contour map, which should show any structures observed or measured on the land, should preferably be scaled at 1 : 1000 to 1 : 5000, with contour lines of 10–25 cm vertical spacing, so complete pond drainage can be designed and earthwork volume estimated with the required accuracy. If the proposed construction is an extension of an existing farm, the cross- and longitudinal-sections of the adjacent ponds, drains and channels should be obtained.
The soil characteristics of importance in site selection have been described. Based on the results of feasibility investigations, the extent of further soil samplings required will have to be determined.
One or two sample stations to each 2 to 5 ha of site should be adequate if the soil conditions are uniform. If not, more sampling stations will be needed. The minimum depth of a bore hole for soil sampling is suggested to be 2 m below the deepest intended excavation of the project area. For the building of special structures, such as large water towers, greater depths of boring, commensurate with the size of thestructures, will be needed. The soil tests should be to estimate
a) seepage loss,
b) under-seepage conditions and the hazard of piping failure,
c) stability of dikes constructed with the soil,
d) the degree of compaction needed,
e) the permissible flow velocity in the earthen supply channels and
f) the foundation requirements of the structures.
Soil on potential borrow sites within economical hauling distance should be studied to determine the nature of the soil available for building embankments. The embankments for the farm have to be built with cohesive soils that have adequate plasticity (generally designated by the plasticity index – a measure of the interaction between water and the cohesive plastic components present in the soil), as for example soil with a plasticity index above 15 per cent. Such soils should be checked for their susceptibility to long-term changes in permeability caused by atmospheric factors, such as the development of stable density or aggregation of particles. The losses that are likely to occur due to under-seepage and infiltration have to be determined using standard methods. To estimate long-term losses through seepage it is necessary to take into account the sediment content of the water supply, which along with decaying debris, pond wastes, algal growths, etc. would cause natural sealing or colmatation in the course of time.
While embankment stability can be determined by standard methods of soil mechanics, the assessment of the possible long-term performance of structures is more difficult. Due to their relatively small size and the practice of repeated draining and filling, there is the greater possibility of entire embankments of farms becoming desiccated, causing cracks to develop and entry of water into the embank-ment at times of rain or pond filling. The soil will then swell, but the extent of swelling at any particular point will depend not only on the swelling potential of the soil, but also the magnitude of the confining pressure of the surrounding, especially overlying, soil masses. Repeated drying and wetting, and thereforeshrinking and swelling, will produce a stable density distribution, with higher densities in the interior of the cross-section. Szilvassy (1984) describes the adverse effects of drying and rewetting fish ponds. The cracks formed by drying facilitate the entrance of water into the body of the embankment. The crack faces are saturated and the moisture penetrates into the interior by capillary action. The saturated parts become almost impervious to air and the air in the pores comes under the combined pressure of the capillary action and the hydrostatic pressure of underwater parts. This pressure on the confined air leads to spalling and subsequent sudden liquefaction of unprotected slopes. If water flows through the cracks, the liquefying soil will be scoured at a faster rate, resulting in the development of gully or tunnel erosion, which is often the cause of failure of small embankments.
Besides careful exploration of the surface layer of the area where the ponds and water supply canals are planned, the soils along the canal traces should be investigated also for their hydraulic properties to estimate slope inclinations and the allowable (non-scouring) velocity of flow in the canal. The sequence of soil strata down to the first impervious layer should be determined as accurately as possible. If the soil is impervious to at least 0.6m thick-ness below the designed deepest bottom level in the ponds or the drainage channels, no further exploration may be needed. In view of the difficulties in obtaining fully undisturbed soil samples for laboratory tests, field permeability studies are recommended in the vicinity of each exploratory borehole by the infiltration method.
The buildings and other structures on fish farm sites are generally small, and so the loads acting on the foundation are not likely to be large. In cases where these are to be built on newly filled sites, special care should be taken to avoid damage resulting from future soil subsidence. The standard sounding methods used by building engineers should be applied.
While there is no denying the importance of careful soil studies in planning aquaculture farms, it has also to be remembered that laboratory tests for design values of soil strength are costly. Even when done, the engineer has to use his judgement to decide whether to use itfor the type of constructions involved in a pond farm with low dikes and dams. Because of this, in countries like Hungary, aquaculture engineers use special practical guidelines based on local experience for the construction of dikes, levees and dams lower than 3 m in height and retaining less than 3 million m3 water (Szilvassy, 1984).
Special features of soils on coastal sites, especially mangroves, have already been discussed. The presence of large quantities of organic matter in the soil, particularly man-grove roots, is a special problem to be reckoned with in pond design and construction in coastal areas. There is a growing body of opinion in favour of leaving the pond beds undisturbed without any excavation and, depending on the flocculation and settling of sediments brought in with tidal water, to build up a thick top layer on the bed to reduce acid soil problems. In that case, soil to build the embankments has to be obtained from outside the pond limits. If a mechanical means of construction is planned, the necessary cohesive soil should be available within reach of drag-line excavators or similar equipment, working from the embankment base. If manual means of construction are the choice, it may be possible to cut soil into blocks and transport them on rafts or flat-bottom boats at high tides to the pond site. Besides the comparative costs, the construction time has also to be taken into account in making decisions.
The chemical properties of the water source for the farm and the sources of pollution, if any, would already have been studied during site selection. Very often, further information would be needed at the design stage on the quantity of water required. For a fish pond with an average depth of 1.5 m the amount of water required to fill it initially is 15000 m3/ha. Loss through seepage and evaporation varies considerably between areas. In an arid climate, the average loss during the growing season could amount to 1–2 cm/day or more. With proper management, the total minimum quantity required for filling and topping under such a situation is estimated to be between 35 000 and 60 000 m3/ha per year. The size of the farm should naturally depend on the quantity of water available during the period of operations. When the source of supplies is a stream, data will be needed on the stages and flow rates to be anticipated at the diversion point in the periods of pond filling and for compensation of water losses. It has been recommended that flow rates should be designed for 80 per cent probability.
In areas exposed to floods, data on design floods and discharges will be required. Water control agencies can generally provide values for probability of occurrence of the design flood, but in cases where such values are not available it has been suggested that 1 per cent probability of occurrence (that is, once in a hundred years) should be adopted as the design flood for the spillway of a dam. In the case of smaller dams with a design volume less than 1 million m3 and of ponds with a water area less than 20 ha built farther away from human settlements, where the dike failure would not cause other losses, a flood of 3 per cent probability may be adopted as the design flood. The runoff of the water catchment area of the site should also be calculated to determine the capacity of the farm reservoir or ponds. Data on the peak values of monthly evaporation and rainfall are necessary to estimate water demand.
Estimates of the annual volume of sediment entering the ponds would be necessary to determine desilting requirements; or in cases where it is planned to build up a top layer of silt, to estimate the time it will take for it to be accomplished. Again, where the water turbidity is undesirably high and separate sedimentation tanks are required to reduce it, this information is essential. One of the problems in ponds filled from natural bodies of water is the entry of extraneous fish and other organisms in the egg or larval stages with the water, even when the inlets are protected by small-meshed screens. Filtration of such water to remove pests and predators is extremely difficult and expensive. In special circumstances, when considered essential, sand or other filters may be designed according to the size and quantity of sediments.
The use of waste water, including sewage effluents, to irrigate and increase productivity of ponds is an age-old practice and fish culture is used now in many places as an efficient means of recycling organic wastes. Reference has already been made to the use of heated water effluents from power stations in temperate and cold climates. The main problem with the use of wastes is the possible development of anoxia in ponds, due to excessive organic loading and contamination with toxicants and heavy metals. The risk of transmitting bacterial and viral pathogens through the use of domestic wastes has received some attention. It has been shown that under conditions existing in fish ponds an actual reduction of pathogens occurs. Due to high photosynthetic rates, such ponds have high dissolved oxygen contents and high pH values, which increase the rate of disinfection of coliforms. Investigations have not yet found evidence of the transmission of any human bacterial diseases through fish. Even though fish do not suffer from enterobacterial infections, the possibility still remains that fish can harbour bacteria in their alimentary tract, tissues and mucus and hence serve as passive vectors of pathogens. Experimental studies made on artificially infected fish have shown that by holding them in clean water for an adequate period of time they can be cleansed of pathogenic vibrios. Depuration is often practised in waste-water aquaculture. So, if the use of waste water is planned, necessary facilities will have to be included in the farm design. Similarly, possible measures should be adopted to avoid incorporation of toxic substances. This can best be done at source. Detergents are often difficult to exclude from domestic and municipal wastes, but at least their concentration should be kept under permissible limits. The lethal limits of detergent for common carp is reported to be 10 ppm ABS (alkylbenzene sulphonate), but even sublethal concentrations can affect their growth (Hepher and Pruginin, 1981). The short duration of the grow-out period in aquaculture reduces the risk of accumulation of heavy metals from waste water, unless the concentration is very high. Experience so far seems to show that even when there is some accumulation, it is generally within accepted standards for safe use.
Public attitudes to eating products grown in waste water, particularly sewage effluents, can be a problem and solutions have to be found on the basis of socio-cultural ambience. These should include public education and productpromotion. In modern aquaculture, only pre-treated wastes are used. In some cases, the use of wastes is avoided in the final grow-out stages, and when there is possible exposure to waste water at that stage the product is depurated for an adequate period before marketing. These are some of the measures that could help in meeting consumer concerns.
For designing coastal pond farms the most important data needed are the seasonal variations in salinity of the available water and access to fresh water to reduce salinity when required. When the ponds have to be filled using tidal energy, detailed studies are needed to determine the stage/duration/frequency relationship necessary for engineering designs. Continuous data from the site for as long a period as possible will be necessary to verify calculated values from available tide tables and observations during feasibility studies. For designing proper water management in tide-fed ponds, it is necessary to determine the ground elevation, which actually approximates the tidal levels of mean lower high water or of mean high water at neap tide. If possible, the measurements should be made when the lowest critical tides of the year occur (which can be found from the tide tables). Alternatively, the measurements should be taken during the lowest and highest tides of the month. The days with the lowest tides should be selected, and the O datum or mean lower low water (MLLW) noted. A fifteen-day observation during the dry season for the mean high water and another fifteen-day observation at the height of the rainy season for mean low water, are considered sufficient to ascertain whether the pondsystem will be drainable during the rainy season and whether the desired depth can be maintained. Measurements may best be done in front of the area where the main gate of the farm is likely to be constructed. On the tide gauge, which can be a measuring stake driven into the ground, the point at which the water level was lowest should be marked. The O datum level, correlated with the lowest water level, should also be marked. This will serve as the base line for determination of all elevations in the farm system. A benchmark can be estab lished by running a level to a permanent marker near the site to be developed. The elevation of points within the area can be measured as reckoned from the datum plane. It is considered uneconomical to excavate more than 50 cm for pond construction. If this is needed it will be better to resort to pumping than depend on tides for water supply and drainage.
The essential data required for hatchery design would become available through some of the investigations mentioned earlier. Decisions as to whether a hatchery, together with nursery facilities, should be established in the same farm complex or in a different locality have to be made on the basis of the site conditions, water quality requirements, ease of operation, security, etc. In certain types of coastal aquaculture, for example shrimp culture, the need for unpolluted high salinity water for hatchery operation may make it necessary to site hatchery installations nearer to the sea, rather than in the brackish-water areas where the grow-out ponds may be located. Similarly for the giant fresh-water prawn (Macrobrachium rosen-bergii), which requires saline water for spawning and larval development, the hatchery may have to be situated away from the fresh-water pond farms used for grow-out. However, in some circumstances it may be more economical to transport the necessary salt water to the inland farm site than to maintain two separate units. In the case of salmonid culture, especially of the trout, the low temperatures required for spawning, hatching and larval development may make it necessary to establish the hatchery at high elevations with cold water, and grow-out farms at lower elevations with higher temperatures for faster growth. Smolt production for salmon in fresh-water installations may have to be done in different locations and the smolt transported and acclimatized for salt-water culture or for sea ranching.
The other input production facility that may be considered for inclusion in the farm design is feed. For this, as for the processing of farm products, the main requirements to be investi gated are suitable land for the necessary constructions, clean water supplies and electricity.
The availability of skilled and unskilled labour in the area is an important factor in deciding on construction which would require adequate maintenance and careful operation. In many developing countries, priority is given to aquaculture development because of its potential to generate employment, and so there is a definite preference for the use of manual labour in construction and day-to-day operation. At the same time, it will be necessary to achieve cost-effectiveness and profitability. So, it will be necessary to obtain comparative information on costs of construction and maintenance, using mechanical equipment against manual labour. Besides the actual costs, the time it takes to construct the farm and bring it under production by these two methods and its economic consequences should also be considered.