| AFRICAN REGIONAL AQUACULTURE CENTRE, PORT HARCOURT, NIGERIA | ARAC/87/WP/12(7) |
| CENTRE REGIONAL AFRICAIN D'AQUACULTURE, PORT HARCOURT, NIGERIA |
M. N. KUTTY AND G. DELINCE
African Regional Aquaculture Centre
Port Harcourt, Nigeria
Lectures presented at ARAC for
the Senior Aquaculturists course
UNITED NATIONS DEVELOPMENT PROGRAMME
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH
PROJECT RAF/82/009
JUNE, 1987
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The estimation of the quantity of water required in a farm and the ways and means to meet the needs are the essential factors to be considered in the choice of a site.
In the feasibility study one must take into account the requisite quality of water needed to fill the ponds and also the losses encountered in the ponds and during the transportation of the water from the supply to the ponds. These losses are due to seepage and evaporation. One must also take into account the requirement of water for filling the ponds initially, for refilling and partial replacement of water as needed and also the frequency of draining off. The need for water of the other infrastructures in the farm such as hatchery, concrete tanks, the laboratory etc, are also to be computed in the feasibility study according to the specific needs.
In other farming systems, where the water is used partially, re-used after purification, the quantity of water needed should be modified as needed. The sources of water for the farm can be - a river or a stream, a lake or a dam, or rain water collected in wells and ponds. It could also be pumped up water from underground sources. In any of the cases, one has to estimate the quantities of water available and compare them to the needs. This estimation has to take into account the eventual fluctuations due to the change of season. Then, the methods of bringing the water from the supply source to the ponds and other infrastructures have to be determined. This implies, on one hand, that one must determine if the level of the supply source is higher or lower than the level of the fish farm and this will determine if the water can be brought to the fish farm by gravity or by using a pump. On the other hand one must choose the means of transporting the water either by open channel, by covered channel or by pipes (the nature of the material, the diameter of pipes etc have to be determined). The choice of the means of transportation depend on the length of the envisaged channel or pipes and their costs and the availability of materials.
The requirements of water for a fish farm depend on the area of the ponds, their average depth and the frequency of draining and refilling and also the losses dues to seepage and evaporation.
Water requirement for a fish farm can be estimated according to the formula (Kovari, 1984):
QR = Annual requirement of water in m3 or l/see
VF = A × h = volume of the ponds to be filled (m3)
A = Average area under water of the ponds (m2)
h = Average depth of the ponds (m)
VRf = No × VF = volume of water to be replaced (m3)
No = Number of filling per annum.
Le = A × E = Loss by evaporation (m3)
E = Average evaporation per annum (m)
Ls = A × T × S = losses by seepage (m3)
S = Coefficient of seepage (m/d)
Le = Ac × 1,2 × E = losses encountered during transportation in the earthen channel (m3)
Ac = Area under water of the supply channel (m2)
VRa = Aeff × Ra = Rain water (m3)
Aeff = Total area of the ponds plus partial area of the dikes affected by the rain (m3)
Ra = Average rain fall per annum
T = Number of working days in the farm.
The volume of water in the pond is obtained by multiplying the area of the pond by the average depth. By adding the volume of water in each of the ponds the total volume of water required is obtained. For the feasibility study, specific size of the total water area in the farm also should be established. The losses in water by seepage are through earthen dikes and channel and through the pond bottom. The losses through the pond bed depend essentially on the nature of the soil (structure, porosity, permeability and moisture content) the hydraulic gradient and of the depth of the underground water. An acceptable soil of the pond bottom should have a permeability coefficient less than 5.10-6 m/s.
The seepage through the dikes can be estimated if the following factors are known: the coefficient of permeability of the dike (recommended to be less than 1.10-4 m/s); the effectif level of water in the pond; the width of the dike; the nature of the soil on which the dike is built. (permeable soil or impermeable). When the soil used for the construction of the dike is of homogenous material, the percentage of seepage will be calculated with Casagrande's formula:
1) When the dike is built on a permeable soil.
where q = the percentage of seepage by cm of length of the dike (cm2/h).
K = the coefficient of permeability of the dike (cm/h)
K1 = the coefficient of permeability of the soil on which the dike is built.
h = hight of the level of water, in cm
w = effective width of the dike in cm.
When calculated from the level of water in the pond:
w = L - 0,7C
where L = width of the dike at the bottom
c = h x a.
h = height of water in the pond.
a = slope of the dike, e.g. 1:2, where a is the second number.
II) When the soil is impermeable, the Casagrande formula becomes
In the same manner, a quantity of water is lost through the supply in earthen channels of the farm. A percentage of seepage of 1 to 2 cm/day is acceptable. If it exceeds this rate some measures to stop the excessive seepage should be taken.
The losses through evaporation depend on the climate of the region and in particular of the temperature, sun light, humidity of the wind and of the area of water exposed.
The determination of the losses by evaporation can be estimated by the percentage of evaporation recorded in any meteorological station with the help of the class A Pan- or with the Penman method. As the evaporation in a small tank (evaporation tank) is higher than the evaporation in nature, the percentages in the pan are too high and have to be reduced by a corrective factor. The corrective factor to be appied under tropical conditions is of 0.75. The value of evaporation obtained are then multiplied by 0.75 and by the area under water in view to get the total volume of water evaporated. Usually in the tropics, the figures of evaporation estimated are around 2.5 cm/day.
The quantity of rain water added to the ponds is calculated from the volume of rain water collected in a rain-gange multiplied by the area of the ponds and a part of the area of the dikes (about half). But one must be careful in the calculation of the area of the dikes because they absorb also a quantity of rain water. Sometimes the rains are not falling unformly everywhere and the quantity of rain water vary accordingly to each spot in the farm.
According to Kovari (1984), the need in water supply for a fish farm
can be
In most fish farms, the water used to fill the ponds come from a stream (brook) or from a small water body (lake or dam) located very near the farm. Depending on the flow in the stream, the supply to the farm may be adequate or in excess. To determine if the stream can supply the requisite quantities of water for the good operation of the farm, one has to determine the flow of the stream and its seasonal variations. To do that, several methods can be employed, and the choice of any of them depends on the dimension of the stream, the accuracy wanted and the equipment available. The flow is determined by measuring the volume of water flowing in a unit of time or by the speed of flow through a section. These two ways to measure the flow of the stream indicate the principles of measuring the flow: one could make an artificial dam on the stream and at the same time to arrange for a device (pipe or any channel of regular dimension) or an overflow (outlet) which allows the flowing of the water while measuring the quantity of water which flows in a minute or second, or one could measure the flow by taking two perpendicular sections of the area and the speed of the water flowing between them.
Fig. 7.1A. Seepage flow in dike of homogeneous material placed on shallow permeable foundation (see text for details)
Fig. 7.1B. The seepage line drawn in the dike with homogeneous material placed on impermeable foundation (see text for details)
The method of arranging for overflow is widely used for small and medium streams. The overflow covers completely the stream and forces the water to pass through a triangular or rectangular opening (see Fig.7.2) The flow is calculated by using Tables 7.I, 7.II, 7.III taking into account the charge of the flow, that is the difference in height between the level of water and the height of the top of the Notch and the width of the Notch. The nomogram of a rectangular notch is different from that of a triangular notch. The triangular notch is used for flows less than 114 l/S. While the rectangular notch is used for flows over 114 l/S (see Fig.7.2) (For more details on the construction another use of an overflow, ref: Coche A.G and H. Van der Val, (1986). Aquaculture training series No. 4, simple methods used in aquaculture, FAO, Rome).
To determine the flow by using the profile of the river (wet section) and the speed of the water, the depth of two perpendicular sections of the river at least 20 m apart from each other should be taken then the average area of the sections should be calculated. The speed of the current is measured with a current meter connected to a revolution counter which records the numbers of revolutions made in a given time and which can determine depending on the characteristics of the instrument, the speed of the tide. As this speed is different according to the depth and the proximity of the bank some depth measurements should be taken at several points along the section. An average current speed is then determined. The multiplication of the average cross section area by the average current speed gives the flow.
A lake or any natural water body or a dam presently is another possible supply of water to a fish farm. With an approximate first hand knowledge of the water body (supply) one can determine if the needs in water of the farm can be met. In cases where water is taken from a stream and if the flow drops considerably during some months in the year that the needs in water of the farm could not be met, then it's necessary to create a dam up-stream, for having a reserve of water for the months of scarcity. The dimension of the dam would depend on the volume of water to be stocked; the dam is placed where it is easier to block the valley above the site of the ponds so as to bring the water to the farm.
During the rainy season when the water swells, the volume of flow would increase and would create floods. In some regions, with a flat topography considerable areas of lands are flooded and covered by water. These temporary water bodies are very productive and traditionally they are used for fishing (e.g. Wledo's in Benin). They constitute a spawning place for several species of fishes. Where the depth is adequate a pond retaining water for many mouths can be made and can be managed as an aquaculture pond. This type of farming would however be extensive. It is difficult to evaluate the quantity of fishes already in the water brought by the floods and also the extent of predator. There is no control over the quantity of water and the harvest is done only when the water is low.
The underground water is also used as supplies to the ponds. In this category we have the water from the wells and the aquifers. The equiferous water in the swamps are not easily drained off. This makes their management difficult (harvesting of fish). Underground water has little or no oxygen and does not have any plankton. The water may contain dangerous gases such as methane or hydrogen sulfide. The chemical quality of the water depends on the nature of geological layers through which it has passed.
A spring is underground water rising at the ground and is fed by underground source and conseaquently its flow can fluctuate with changes of the level of the underground water. Considering the nature of the spring (temporary or permanent spring) and its flow, one can determine if the need of the farm can be met. A reservoir to store water is often built to overcome the scarcity of water during dry season. Spring water is taken as much as possible to the farm by gravity. Generally the quality of this water is good; but the water should be aerated.
Wells are sunk to get access to the underground water. In most of the cases, the water has to be pumped up. Only the water from the artesian well rises to the surface or to a certain height by hydrostatic pressure. This is caused by the geological morphology of the ground which keeps the water under pressure. The choice of the place of sinking the well, the depth, and the flow expected should be done by qualified staff. The water coming from the well should be aerated and is normally good for water supply to a hatchery because the water does not usually contain any contaminants.
Rain water is another possible supply to the ponds although it is difficult to depend on it solely. To determine the quantity of rain water one has to study closely the rainfall of the place where the farm will be sited and its seasonal variations. The quantity of rainfall would determine the amount of water which will flow into the sloping pond and influence the rise of the water in the collecting pond. The maximum of the water can be calculated with an empirical formula (see ADCP/REP/84/21 page 171 Kovari, 1984, for more details).
Fig. 7.2. Flow through a rectangular weir - general view on top and sectional view below (From Coche and Van der Wal, 1981)
Table 7.I
Water flow estimates using a triangular or V-notch weir
(H= Head in centimetres: F= Water flow in litres/second)
| H | F | H | F | H | F | H | F | H | F | H | F | H | F | H | F |
| - | 10.5 | 4.89 | 20.5 | 26.07 | 30.5 | 70.38 | 40.5 | 143.01 | 50.5 | 248.28 | 60.5 | 390.04 | 70.5 | 571.73 | |
| - | 11.0 | 5.50 | 21.0 | 27.69 | 31.0 | 73.30 | 41.0 | 147.46 | 51.0 | 254.48 | 61.0 | 398.15 | 71.0 | 581.92 | |
| - | 11.5 | 6.14 | 21.5 | 29.36 | 31.5 | 76.30 | 41.5 | 152.00 | 51.5 | 260.76 | 61.5 | 406.36 | 71.5 | 592.22 | |
| 2.0 | 0.08 | 12.0 | 6.83 | 22.0 | 31.10 | 32.0 | 79.36 | 42.0 | 156.62 | 52.0 | 267.13 | 62.0 | 414.67 | 72.0 | 602.63 |
| 2.5 | 0.14 | 12.5 | 7.57 | 22.5 | 32.90 | 32.5 | 82.50 | 42.5 | 161.32 | 52.5 | 273.60 | 62.5 | 423.08 | 72.5 | 613.15 |
| 3.0 | 0.21 | 13.0 | 8.35 | 23.0 | 34.76 | 33.0 | 85.70 | 43.0 | 166.11 | 53.0 | 280.16 | 63.0 | 431.59 | 73.0 | 623.77 |
| 3.5 | 0.31 | 13.5 | 9.17 | 23.5 | 36.68 | 33.5 | 88.99 | 43.5 | 170.98 | 53.5 | 286.82 | 63.5 | 440.20 | 73.5 | 634.51 |
| 4.0 | 0.44 | 14.0 | 10.05 | 24.0 | 38.66 | 34.0 | 92.35 | 44.0 | 175.93 | 54.0 | 293.57 | 64.0 | 448.92 | 74.0 | 645.36 |
| 4.5 | 0.59 | 14.5 | 10.97 | 24.5 | 40.70 | 34.5 | 95.78 | 44.5 | 180.98 | 54.5 | 300.41 | 64.5 | 457.74 | 74.5 | 656.31 |
| 5.0 | 0.77 | 15.0 | 11.94 | 25.0 | 42.81 | 35.0 | 99.29 | 45.0 | 186.10 | 55.0 | 307.35 | 65.0 | 466.66 | 75.0 | 667.38 |
| 5.5 | 0.97 | 15.5 | 12.96 | 25.5 | 44.99 | 35.5 | 102.87 | 45.5 | 191.32 | 55.5 | 314.38 | 65.5 | 475.69 | 75.5 | 678.56 |
| 6.0 | 1.21 | 16.0 | 14.03 | 26.0 | 47.22 | 36.0 | 106.53 | 46.0 | 196.61 | 56.0 | 321.51 | 66.0 | 484.82 | 76.0 | 689.85 |
| 6.5 | 1.48 | 16.5 | 15.15 | 26.5 | 49.53 | 36.5 | 110.27 | 46.5 | 202.00 | 56.5 | 328.73 | 66.5 | 494.05 | 76.5 | 701.25 |
| 7.0 | 1.78 | 17.0 | 16.32 | 27.0 | 51.90 | 37.0 | 114.08 | 47.0 | 207.47 | 57.0 | 336.05 | 67.0 | 503.39 | 77.0 | 712.77 |
| 7.5 | 2.11 | 17.5 | 17.55 | 27.5 | 54.33 | 37.5 | 117.98 | 47.5 | 213.04 | 57.5 | 343.47 | 67.5 | 512.84 | 77.5 | 724.39 |
| 8.0 | 2.48 | 18.0 | 18.83 | 28.0 | 56.83 | 38.0 | 121.95 | 48.0 | 218.69 | 58.0 | 350.99 | 68.0 | 522.39 | 78.0 | 736.13 |
| 8.5 | 2.89 | 18.5 | 20.17 | 28.5 | 59.41 | 38.5 | 126.00 | 48.5 | 224.43 | 58.5 | 358.60 | 68.5 | 532.04 | 78.5 | 747.99 |
| 9.0 | 3.33 | 19.0 | 21.56 | 29.0 | 62.05 | 39.0 | 130.13 | 49.0 | 230.26 | 59.0 | 366.31 | 69.0 | 541.81 | 79.0 | 759.96 |
| 9.5 | 3.81 | 19.5 | 23.00 | 29.5 | 64.76 | 39.5 | 134.34 | 49.5 | 236.17 | 59.5 | 374.12 | 69.5 | 551.67 | 79.5 | 772.04 |
| 10.0 | 4.33 | 20.0 | 24.51 | 30.0 | 67.53 | 40.0 | 138.63 | 50.0 | 242.18 | 60.0 | 382.03 | 70.0 | 561.65 | 80.0 | 784.23 |
Source: Coche & Van der Wal, 1981
Table 7.II
Water flow estimates using a rectangular weir
Weir crest length (cm,
(Source Coche & Van der Wal, 1981)
| Head (cm | 15 | 20 | 25 | 30 | 60 | 90 | 120 | 180 | Per 10 cm | Head (cm) |
| 4 | 2.09 | 2.83 | 3.56 | 4.30 | 8.71 | 13.13 | 17.55 | 26.38 | 1.45 | 4 |
| 6 | 3.73 | 5.08 | 6.44 | 7.79 | 15.90 | 24.01 | 32.13 | 48.35 | 2.65 | 6 |
| 8 | 5.58 | 7.66 | 9.74 | 11.82 | 24.31 | 36.80 | 49.30 | 74.28 | 4.05 | 8 |
| 10 | 7.56 | 10.47 | 13.38 | 16.29 | 33.75 | 51.20 | 68.66 | 103.57 | 5.65 | 10 |
| 12 | 9.64 | 13.46 | 17.29 | 21.11 | 44.06 | 67.00 | 89.95 | 135.84 | 7.50 | 12 |
| 14 | 11.76 | 16.58 | 21.40 | 26.22 | 55.13 | 84.05 | 112.96 | 170.79 | 9.30 | 14 |
| 16 | 13.90 | 19.78 | 25.67 | 31.56 | 66.89 | 102.22 | 147.54 | 208.20 | 11.30 | 16 |
| 18 | 23.04 | 30.07 | 37.10 | 79.25 | 121.41 | 163.56 | 247.87 | 12.50 | 18 | |
| 20 | 26.33 | 34.56 | 42.79 | 92.16 | 141.53 | 190.91 | 289.65 | 15.50 | 20 | |
| 22 | 39.11 | 48.61 | 105.57 | 162.53 | 219.49 | 333.41 | 18.30 | 22 | ||
| 24 | 43.70 | 54.52 | 119.42 | 184.32 | 249.22 | 379.03 | 20.75 | 24 | ||
| 26 | 48.30 | 60.50 | 133.68 | 206.86 | 280.04 | 426.40 | 23.30 | 26 | ||
| 28 | 66.52 | 148.30 | 230.09 | 311.88 | 475.45 | 26.00 | 28 | |||
| 30 | 72.56 | 163.27 | 253.97 | 344.67 | 536.08 | 28.70 | 30 | |||
| 32 | 178.53 | 278.45 | 378.37 | 578.22 | 31.55 | 32 | ||||
| 34 | 194.07 | 303.50 | 412.94 | 631.81 | 34.40 | 34 | ||||
| 36 | 209.85 | 329.08 | 448.31 | 686.78 | 37.40 | 36 | ||||
| 38 | 225.85 | 355.16 | 484.46 | 743.07 | 40.40 | 38 | ||||
| 40 | 242.05 | 381.70 | 521.35 | 800.64 | 43.50 | 40 | ||||
| 42 | 258.43 | 408.68 | 558.93 | 859.43 | - | 42 | ||||
| 44 | 274.96 | 436.07 | 597.17 | 919.39 | - | 44 | ||||
| 46 | 291.72 | 463.84 | 636.05 | 980.49 | - | 46 | ||||
| 48 | 308.40 | 491.97 | 675.54 | 1042.68 | - | 48 | ||||
| 50 | 325.27 | 520.43 | 715.59 | 1105.91 | - | 50 | ||||
| 52 | 342.22 | 549.21 | 756.19 | 1170.17 | - | 52 | ||||
| 54 | 359.23 | 578.27 | 797.32 | 1235.40 | - | 54 | ||||
| 56 | 376.29 | 607.61 | 838.94 | 1301.58 | - | 56 | ||||
| 58 | 393.37 | 637.20 | 881.03 | 1368.68 | - | 58 | ||||
| 60 | 410.47 | 667.02 | 923.57 | 1436.66 | - | 60 |
NOTE: The accuracy of the water flow values decreases when head values are
greater than one third of the crest length. Water values in this table
are divided into three sections, white, blue and grey. The values in
the white section are the most accurate. In the blue and grey sections,
the accuracy decreases as the head increases towards a value equal to
the crest length.
With full end contractions and sharp edges - Approximate water flow
for each additional 10 cm of weir crest (for crest lengths of 30 cm
or longer and for values in the unshaded upper part of the table only).
Table 7.III
Water flow estimates using a rectangular weir
(source: Coche & Van der Wal, 1981)
In the regions along the coast, the water is brought by the tide. The ebb and the flow of the water allows the filling or the drainage of the ponds. The water requirements are therefore easily satisfied.
The transportation of water from the supply to the ponds is through channels or pipes. The channels or gutters are either dug right into the ground or made in concrete. They are either open or covered channels. The pipes can be either concrete or in PVC, often PVC pipes are used. The choice of where to lay the pipes, the dimensions of the channel, (its form, width, capacity) and the diameter of the pipes, depend on the farm needs and of the available funds and resources. One must take also into account the laws of hydrodyramic to be applied while transporting the water in conduits (pipes). The main laws to be applied are the following: the law of conservation of mass. The law of energy conservation and loss of energy during transportation. The law of the conservation of mass states that in an closed system, the mass of a liquid passing through a section of a pipe must pass through any other section of the same pipe:
Q = VA = V1A1 = V2A2
where Q = The mass of liquid (fluid)
A = The section of the pipe (A1; A2)
V1, V2 = The speed of the fluid (see Fig.7.3)
The law of conservation of energy or Bernouille's law says that at any point in the pipe in which a liquid is passing through, the sum of energies of gravity, potential pressure energy and kinetics remain constant. The energy of gravity is the one due to the altitude of the point above a level of reference (effect of the gravity). The potential energy of pressure is equal to the pressure (P) divided by the volumic weight ( ) of the fluid and the kinetic energy is equal to the square of the speed of the fluid divided by twice the coefficient of acceration, g
This law is applicable only to ideal fluids. In reality, there is loss of energy due to internal friction in water and friction against the surface (lining) of the pipe. This energy is transformed into heat. These losses of energy are continuous all occuring along the length of the pipes or are localised at the joint when the cross sections of the pipe change or the direction changes. Practically, one has to take into account losses of energy due to the rouhness of the internal surface of the pipes and the kind of liquid flow (laminar and turbulent). Losses due to changes of cross sections and direction: these facts are taken into account to choose the types of pipes necessary to be laid from the source of water to the ponds. Practical calculations of channels and pipes are given under “Pond Construction”.
5. References
ADCP. 1984. Aquaculture Development Coordination Programme. Inland Aquaculture Engineering. Lectures presented at the ADCP inter-regional training course in Inland Aquaculture Engineering, Budapest, 6 June - 3 Sept. ADCP/REP/84/21, FAO, Rome. 591 pp.
Coche, A. G. and H. Van der Wal. 1981. Water for freshwater fish culture. FAO Training Series, 4., FAO, Rome, 111 pp
Kovari, J. 1984. Preparation of plans and cost estimates and tender documents. In: A.D.C.P. Inland Aquaculture Engineering ADCP/REP/84/21. P 127 – 203 FAO, Rome.
Fig. 7.3. Change of water velocity due to change in C. S. of pipe (see text for details)
