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FISHPOND ENGINEERING: A TECHNICAL MANUAL FOR SMALL-AND MEDIUM-SCALE COASTAL FISH FARMS IN SOUTHEAST ASIA

by

C.R. dela Cruz1

CHAPTER 1INTRODUCTION

1. BACKGROUND

Coastal pond aquaculture is best developed in southeast Asia and has existed in this region for a long time. It is an important source of food and has provided income and employment to thousands of people in the region. Based on a 1978 regional compilation2, it is estimated that there are over 400 000 hectares of developed coastal fishponds in the region, providing close to 300 000 metric tons of fish and crustaceans. Valued at equivalent to over US$350 million (Table 1.1). It is also estimated that there are over 70 000 coastal fish farm units in the region involving the employment of close to 30 000 workers (Fig. 1.1).

1.1 Status

Some 413 484 hectares of coastal fishponds are located mainly in the Philippines. Indonesia, Thailand, Vietman, Malaysia, Taiwan (China). Hong Kong and Singapore. They are used for raising finfish, mainly milkfish such as in Indonesia, Philippines and Taiwan or penaid shrimps such as in Thailand, Singapore and Malaysia. At present, polyculture of milkfish and penacid shrimps and stocking of sea-bass are fast developing.

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Fig.1.1 Areas in Southeast Asia where coastal fish farming is practiced

1. Indonesia2. Singapore

3. Malaysia

4. Thailand

5. Cambodia

6. Vietnam

7. Philippines

8. Hongkong

9. Taiwan

1 Consultant (Aquaculture Engineering). South China Sea Fisheries Development and Coordinating Programme. Manila, Philippines.2 SEAFDEC. Fisheres Statistical Bulletin for the South China Sea Area. 1978 (1980).

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Table 1Status and potential of brackishwater (coastal) pond aquaculture in Southeast Asia1

  Area (ha)Production

(mt)Value (1 000 US$)

Potential areas for development (1 000

ha)1

China (Taiwan) 18 665 50 317 75 981 -

Hong Kong 1 975 1 385 4 527 -

Indonesia 171 544 87 995 109 592 3 618

Kampuchea3 - - - 50

Malaysia4 - - - 652

Philippines 176 231 118 682 130 062 220

Singapore2 107 34 41 3

Thailand 24 962 10 049 18 392 312

Vietman3 20 000 10 000 12 000 600

TOTAL 413 484 278 462 350 595 5 455

1 Based mainly on data from Fishery Statistical Bulletin for the South China Sea Area. 1978. SEAFDEC (1980).2 Calculated from SS2.45 kg.3 Data on Kampuchea and Vietnam are based on information from government obtained by SCSP staff during travel to these countries.4 Based on total area of unexploited mangroves in Peninsular Malaysia and in Sabah and Sarawak states.5 Based on SCS 80 WP 94 (Revised).

Development of this industry at the beginning can be attributed primarily on private sector initiative. Based on very crude facilities and simple rural experience, the production has been generally low, at present about 670 kg/ha/year on the average in the whole region, although ranges from as low as 300 kg/ha to as high as 2 000 kg/ha (e.g., Taiwan) occur. The value of production from this industry is evaluated at over US$350 million per year, which can be a big boost to the economy of the region (Table 1). Prices has been relatively low for milkfish (average US$1/kg) but rather high for penaeid shrimps (US$3-8 kg) and also good for seabass (US$3-5/kg).

1.2 Potentials

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The potentials for the further development of this industry in the region is high. There are still large acreage of mangrove swamps and tidal mudflats that can be suitable sites for development into fishponds, estimated at about 5½ million hectares in the Southeast Asian countries (Table 1). Of course, it has now been realized that for ecological balance and for rational conservation of the fishery resources, some part of these mangrove areas should be reserved. But if some 10 to 30 percent of the existing swamps can be developed, such a ratio being deemed feasible and affords proper ecological considerations, the area that can be developed in the region can be about 0.5 to 1.6 million hectares.

Also the average production of about 670 kg/ha can easily be increased to double this amount with better engineering and the use of improved technology of management in the existing areas. This can increase the production capacity to over 550 000 metric tons per year. The additional pond areas to be developed can likewise contribute about 600 000 to 1.9 million metric tons per year or an overall production of 1 to 2 million metric tons per year in this region. Likewise, the value of production can considerably be increased.

1.3 Major engineering problems of coastal fishponds

Poor or inadequate engineering of coastal fishponds is one of the major causes of low production and/or failure. It can be said that 30 to 50 percent are inadequately constructed while close to 100 percent can take further engineering improvements. Such engineering deficiencies can be classified into three categories, viz. (i) problems brought about by climatic and hydrological factors: (ii) problems due to environmental factors; and (iii) engineering specific problems.

1.3.1 Problems due to climate and hydrology

The type of rainfall, occurrence of typhoons, and prevailing tidal charcteristics in the fishpond location can influence the nature of construction of fishponds in such area. Where rains are strong and severe and where typhoons are frequent, the fishpond structures need to be bigger and more firm. Likewise, areas with high tidal ranges (average daily range of 3 m or more) will require bigger dikes and sturdy water control structures, whereas areas where the tidal fluctuation is small (one meter or less daily range), the dikes can be smaller and water gates need not be massive. Areas prone to earthquakes and tidal waves should likewise make some extra provision for these occurrences.

Jenis curah hujan, terjadinya angin topan dan karakteristik pasang surut yang ada di lokasi tambak dapat mempengaruhi konstruksi alam dari tambak di area tersebut. Ketika curah hujan sangat besar dan badai berlangsung, dibutuhkan struktur tambak yang besar dan tanggul besar yang kuat sebagai pengontrol air, sedangkan are dimana fluktuasi pasang surutnya kecil (1 meter atau kurang per hari), tanggulnya dapat lebih kecil dan pintu airnya tidak butuh yang besar. Daerah yang rawan gempa dan fluktuasi ombak juga harus dibuat beberapa ketentuan tambahan untuk kejadian ini.

1.3.2 Environmental influences

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The engineering of coastal fishponds can be affected by various environmental influences. These include such factors as the nature of the soil, vegetation, elevation of site, topographic characteristics, availability of freshwater supply and occurrence of pollution. If the site has porous type soil (sandy or peaty), bigger dikes need to be provided. In some cases, better clayey soil for diking may have to be brought from outside. Well vegetated areas especially with big-size trees will require bigger construction effort. Elevation of the site based on the tidal datum will determine whether excavation or filling will be required, while those with uneven topography will need more work in levelling the area.

Masalah teknik dari tambak pesisir dapat dipengaruhi oleh pengaruh lingkungan yang beragam. Ini termasuk faktor – faktor alam dari tanah, tumbuhan, elevasi, topografi, ketersediaan air tawar dan polusi. Jika lokasi ini mempunyai tipe tanah yang porous (berpasir atau gambut), dibutuhkan tanggul yang lebih besar. Dalam beberapa kasus, tanah yang bagus untuk tanggul mungkin harus dibawa dari tempat lain. Vegetasi yang baik khususnya dengan pohon yang besar akan membutuhkan konstruksi yang lebih besar. Elevasi di lokasi berdasarkan pada data pasang surut akan menentukan penggalian atau pengisian yang dibutuhkan, ketika topgrafi tersebut tidak pas akan membutuhkan pekerjaan yang lebih banyak di area tersebut

It is better to have some source of freshwater supply for coastal fishponds so that the brackishwater salinity which is usually more suitable for growing food organisms as well as the cultured species can be maintained. If however this is not available, the fishpond should be so engineered so that the periodic occurrence of freshwater such as from rains can be taken advantage of.

Freshwater supply from the tidal river or stream is usually the cheapest source of freshwater as this can be taken in by gravity. However, this may not be available so that other sources have to be determined and tapped. The seasonal rains can be another source, although this can be seasonal and not very reliable. Underground water is another source of this, if available. Sometimes the pond bottom is low enough in relation to the water table so that underground water can seep in naturally to the ponds. Piped water through wells of varying depths is good, if this is available. All the above sources of freshwater will need engineering structures so that the required water can be put into use. Pumps, either to draw in or drain out excess water, may be found necessary and helpful.

Occurrence of pollution is a difficult problem in coastal fishpond areas and should be avoided if this was noted before the farm is established. However, if this condition should happen after the fishpond has been constructed, additional structures may need to be installed to minimize the effects of this adverse factor.

1.3.3 Engineering specific problems

These are the site specific problems that are encountered during actual construction or after the construction of the fishpond. For instance, after the fishpond has been constructed, there is a need to shift the kind of management from the traditional extensive method to the modular progression method or to the stock manipulation method: this will require a renovation of the

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layout of the fishpond system. Again, if the fishpond is to shift from milkfish monoculture to milkfish shrimp polyculture or to shrimp monoculture, some definite pond modifications have to be made for such a shift.

During the construction, it sometimes occur that there is excess soil that needs to be disposed of properly, or there may be lack of soil that can be adequate for the needed diking or filling work. These have to be solved through engineering means.

Many engineering problems occur with regard to the water control structures. These have to be properly designed and well constructed and located in appropriate places in relation to the entire fishpond system. These structures are usually expensive to put up and once made they are very difficult to change. It is noted however, that some progress have been attaind in better designs and in the method of constructing these water control structures. More lasting materials like fiberglass, ferrocement, etc., especially if these can be prefabricated may lessen the inherent costs encountered with these fishpond structures.

Correcting water leakages and seepages in finished fishponds often present many problems. Even if these have to be dealt with on a case to case basis, there is need for aquaculture engineers to develop and improve the technology involving these very frequent problems in coastal fishponds.

CHAPTER 2SELECTION OF FISH FARM SITE

2. EVALUATION AND SELECTION OF SITE

2.1 Criteria used

2.1.1 Water supply

Adequate supply of good quality of fresh and salt-water must be available year round in the site. Good quality water suitable for fish culture is rich in oxygen, nutrients and free from pollutants.

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Freshwater is important for mixing with sea water to maintain salinity level especially in the dry season when evaporation is rapid. Freshwater is also necessary for the daily use of the workers in the fish farm.

For freshwater, the reliability of supply or flow rate from the source can be evaluated by knowing the: (1) required rate of flow, Q (2) required depth for the pond, h (3) prescribed time, t of filling a given area, A of pond with water, and (4) total losses during filling time arising from the evaporation and seepage or leakage from water delivery canals and fishpond itself. The required rate of flow is determined from the formula:

 

Where Q = required rate of flow from the source (m3/sec)

t = time required to fill the pond (sec)

h= equivalent depth of water to be added to the pond for maintaining salinity (m)

A = pond area (m2)

The available rate of flow from the source (ground-water, spring, irrigation canal, river, creek, etc.) is compared to the required rate of flow by the farm. The available flow must be equal or greater than the required flow.

In brackishwater areas, knowledge of tidal characteristics in the site is very important in determining its suitability for fishponds. The height of the tide and its range determine the sufficiency of water, height of dikes, elevation of pond bottom and water gates, size of gate opening, construction cost and others. Sites near the source of pollutants that would pollute water supply such as mines, factories, food processing plants, oil rigs and densely populated areas should be avoided.

2.1.2 Tidal range and ground elevation

The depth of water in pond to be maintained is determined by the height of incoming tide and height or elevation of pond bottom based on zero tidal datum. Whenever possible, the available tidal range must be able to fill the ponds by gravity to the specified depths. In relation to tide ground elevation, this depth should allow the most economical construction (least cut and fill) of pond which would have an ideal pond elevation. The elevation of pond bottom is considered ideal if it enables draining of the pond almost any day of the year and flood it with seawater to the desired depth within five days or less during the critical spring tides. The critical spring tides usually occur in the Philippines during the months of February, March and April (Denila, 1976).

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Figure 2.1 serves as a guide in determining the suitability of fishpond site in relation to tidal conditions in the area.

Fig. 2.1 Suitability of proposed fishpond site based on tidal ranges and ground elevation under Philippine condition with tide range of (-) 0.6 to 2.2 m or 2.8 m

(After Jamandre and Robanal, 1975)

The tidal characteristics and effect of the magnitude of tidal range in fish farm management in the Southeast Asian region is described in Table 2.1. A typical example in relating the tidal characteristics with the ground elevation of the site is shown in the Ban Merbok estuary, Malaysia (Fig. 2.2).

The desired elevation for a pond bottom of a milkfish farm appears to be at least 20 cm from the zero datum (MLLW) or at an elevation when at least 50 cm depth of water can be maintained in the pond during ordinary tides.

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Fig. 2.2 Tidal characteristics of Ban Merbok estuary, Kedah state, Malaysia in relation to existing ground elevation of an area

(After Hechanova and Tiensongrusmee, 1980)

Table 2.1Tidal characteristics and suitabilities for aquaculture in some areas of the South China Sea

region (after Jamandre and Rabanal, 1978)

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LocalityHighest

recorded tide (m)

Lowest recorded tide (m)

Absolute annual

range (m)

Normal daily fluctuation

low/high (range) (m)

Remarks

INDONESIA

Jambi River, Jambi, Sumatra

3.7 0.5 3.2 1.4/3.4(2.0)

Tidal characteristics favourable for proper fish farm management

Musi River, Palembang South Sumatra

3.8 0.1 3.7 0.9/2.7(1.8) -do-

Tanjung Pandan, Belitung Island, South Sumatra

2.7 0.0 2.7 0.9/1.8(0.9)

Tidal fluctuation slightly narrow for proper fish farm management

Tanjung Priok, Jakarta, West Java

1.1 0.1 1.0 0.3/0.7(0.4)Tidal fluctuation too narrow: unfavourable

Samarinda, West Java

2.7 0.0 2.7 0.7/2.2(1.5)

Tidal characteristics favourable for proper fish farm management

Little Kapuas River, Pontianak, West Kalimantan

1.8 0.0 1.8 0.5/1.3(0.8)

Tidal fluctuation slightly narrow for proper fish farm management

Barito River, Banjarmasin South Kalimantan

2.8 0.1 2.7 0.8/2.2(1.4)

Tidal fluctuation favourable for proper fish farm management

Balikpapan East Kalimantan

2.9 0.1 2.8 0.5/2.3(1.8) -do-

Ujung Pandang,

1.3 0.1 1.2 0.3/1.0(0.7) Tidal fluctuation narrow for proper

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South Sulawesi

fish farm management

Menado, North Sulawesi

2.4 0.0 2.4 0.6/1.7(1.1)

Tidal fluctuation slightly narrow tidal range for proper fish farm management

Jayapura, Irian Jaya

1.1 (-)0.1 1.2 0.3/1.0(0.7)

Tidal range narrow for proper fish farm management

Manokwari, Irian Jaya

1.9 0.0 1.9 0.4/1.6(1.2)Fair tidal range for proper fish farm management

Sorong, Irian Jaya

1.9 (-)0.1 2.0 0.3/1.4(1.1)

Slightly narrow tidal range for proper fish farm management

Aiduna, Irian Jaya

2.6 0.0 2.6 0.4/2.1(1.7)

Tidal characteristics favourable for proper fish farm management

Merauke, Irian Jaya

5.3 0.2 5.1 1.3/4.3(3.0)

Tidal fluctuations too high for proper fish farm management

MALAYSIA

Penang, Penang

2.8 0.2 2.6 0.7/2.2(1.5)

Tidal fluctuation favourable for proper fish farm management

Kelang, Selangor

5.4 (-)0.1 5.5 1.2/4.1(2.9)

Tidal fluctuation high for proper fish farm management

Kuala Batu, Pabat, Johore

3.2 (-)0.2 3.4 0.7/2.6(1.9)

Tidal fluctuation favourable for proper fish farm management

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Kuantan, Pahang

3.5 (-)0.2 3.7 0.6/2.7(2.1) -do-

Kuala Trengganu, Trengganu

2.7 (-)0.4 3.1 0.3/1.8(1.5) -do-

Kuching, Sarawak

6.0 0.5 5.5 1.4/5.0(3.6)

Tidal fluctuation too high for proper fish farm management

Miri, Sarawak 2.1 0. 2.1 0.5/1.6(1.1)

Tidal fluctuation slightly narrow for proper fish farm management

Sandakan, Sabah

2.7 (-)0.1 2.8 0.4/1.9(1.5)

Tidal fluctuation favourable for proper fish farm management

PHILIPPINES

San Fernando La Union

1.04 (-).21 1.25 (-).03/.61(0.64)

Tidal fluctuation too narrow for proper fishpond management

Manila City 1.46 (-).34 1.8 0.14/1.05(0.91)

Tidal fluctuation slightly narrow for proper fishpond management

Legaspi City 1.83 (-).40 2.23 (-)0.09/1.4(1.49)

Tidal fluctuation favourable for proper fishpond management

Cebu City 1.98 (-).40 2.38 (-).03/1.49(1.52) -do-

Davao City 1.98 (-).49 2.47 (-).03/1.77(1.80) -do-

Jolo, Sulu 1.19 (-).12 1.31 (-)0.03/.98(1.01)

Tidal fluctuation slightly narrow for proper fishpond management

SINGAPORE

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Singapore 3.30 (-).30 3.60 0.6/2.7(2.1)

Tidal fluctuation favourable for proper fishpond management

THAILAND

Bangkok Bar 4.2 0.1 4.1 1.1/3.2(1.1)

Tidal fluctuation slightly low for proper fishpond management

Laem Sing, Chantaburi

2.50 0.50 2.00 1.2/1.9(0.7)

Low tidal fluctuation; low for proper fishpond management

Ko Nu, Songkhla

1.30 0.20 1.10 0.5/1.0(0.5) -do-

Ko Taphao, Phuket

3.7 0.5 3.2 1.2/3.0(1.8)

Tidal fluctuation favourable for proper fish farm management

Areas reached only by the high spring tides should not be selected as it is expensive to move large quantities of soil during excavation. There is also an added problem of disposing excess soil material. While constructing higher and wider dikes may solve the problem, this would result to occupying more space in the pond compartment and hence, less area intended for fish production. Low areas on the other hand will require much higher and wider dikes, thus soil is moved at far distances further increasing construction cost. Excessive construction cost for larger dikes is also true in areas where tides can reach as high as 3.5 m such as in East Java, Indonesia (Djajadiredja and Daulay, 1982).

2.1.3 Soil characteristics

Many soil characteristics, especially those related to texture, determine its suitability for fishpond purposes. Soil texture refers to the relative proportion of sand, silt and clay content of the soil. Table 2.2 below shows the different soil classification based from the U.S. Department of Agriculture Classification System.

Table 2.2Texture and textural name of the three main types of soil

Common name Texture Basic soil textural class name

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Sandy soils

Coarse Sandy; sandy loam

Moderately coarseSandy loam; fineSandy loam

Loamy soils

Medium Very fine sandy loam

Moderately fineLoam, silty loamSilt

Clayey soils Fine

Sandy clay; siltyclay; clay; clayloam; sandy clayloam; silty clayloam

Areas suitable for fish production must possess properties which allow for the economical construction of dikes, efficient growth of fish food, extended water holding and load carrying capacity and favourable chemical properties.

(a) Desirable soil texture for ponds. Soils belonging to the following textural classification are desirable for fishpond development: clay, clay loam, silty clay loam, silty loam, loam and sandy clay loam (Dureza, 1982).

Clayey soils are preferable because they are superior material for diking and holding water. They have good compaction characteristics and low permeability. A very simple general rule can be followed: As a clay content of the soil decreases, its suitability for fishpond construction also decreases. This is illustrated in Table 2.3

Loamy soils are also recommended. They have good organic matter content which favour the culture and growth of natural fish food.

(b) Determination of soil texture. Soil texture can be determined by various methods ranging from the sophisticated mechanical and laboratory methods to the simple practical and field methods. The following sections outline some of these methods.

(i) Mechanical method

The amount of each soil separate (size fraction of sand, silt and clay) in a soil mixture determine its texture. The popular Bouyocous mechanical test is a reliable method of determining the amount of each soil separate in the soil through laboratory tests. The test results are then compared with a soil triangle (Fig.2.3) to determine the textural name.

(ii) Field identification

There are three practical field identification methods to determine soil texture. These are the feel method using a modified soil triangle, feel method (alternative) and ball method.

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Table 2.3Relationship of soil classes and suitability for dike material (after BFAR-UNDP/FAO,

1981)

Class Permeability Compressibility Compaction Suitability

Clay Impervious Medium Fair to good Excellent

Sandy clay Impervious Low Good Good

Loamy Semi-pervious High Fair to very poor Fair

to impervious High

Silty Semi-pervious Medium Good to very poor Poor

Sandy to impervious to high

Peaty Pervious High Good Poor

Peaty   Negligible   Very poor

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Fig. 2.3 Texture triangle showing the percentages of sand, silt and clay in the textural classes. The intersection of the dotted lines shows that the soil with 55 percent clay, 32 percent silt and

13 percent sand has a clay texture (After Dureza, 1982)

The use of feel method requires considerable practice in order to attain accurate interpretations. Textural determination by feel involves the rubbing of a moist soil between the thumb and index finger. The ability of the soil mixture to form good, medium or poor or no ribbons determine the corresponding texture as indicated in a modified soil triangle shown in Figs. 2.4 and 2.5

Textural approximation by feel (alternative method) is also done by pressing a moist soil between the thumb and forefinger. The corresponding texture is then compared with the U.S. Soil Survey definitions of soil for various classes (Table 2.4)

The ball method consist of forming and squeezing a ball of moist soil in the hand. The stability of the balled soil mixture to hold its shape when released or touched determines its texture. Table 2.4 is a helpful guide in approximating soil texture for both the feel and ball methods.

VIEW OF SAMPLE AFTER ROLLING

DESCRIPTION OF TEXTURE

No roll, sand, loamy sand

Beginning of a roll, sandy loam

The roll is continuous, but breaks when ring is formed, loam and silt loam

The roll is continuous, but the ring cracks; clay loam, sandy clay loam, silty clay loam

The roll is continuous; the ring is also complete; silty clay; clay and sandy clay

Fig. 2.4 View of samples of rolling, description and texture of soil by touch and feel (After Singh, 1982)

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Fig. 2.5 Modified textural triangle for determining soil texture by the feel method (After Dureza, 1982)

Table 2.4Definition of soil for various textural classification

Textural class

Definition

Sand

Sand is loose and single-grained. The individual grains can readily be seen or felt. If squeezed in the hand when dry, it will fall apart when the pressure is released. If squeezed when moist, it will form a cast, but will crumble when touched; will not form ribbon between thumb and finger.

Sandy loam

This soil contains much sand but which has enough silt and clay to make it somewhat coherent. The individual sand grains can be readily seen and felt. If squeezed when dry, it will form a cast which will readily fall apart, but if squeezed when moist, a cast can be formed that will bear careful handling without breaking. Sands and sandy loams are classed as course, medium, fine or very fine, depending on the proportion of the different sized sand particles that are present.

Loam

Has a relatively even mixture of the different grades of sand, silt, and clay. It is mellow with a some-what gritty feel, yet fairly smooth and slightly plastic. If squeezed when dry, it will form a cast that will bear careful handling, while the cast formed by squeezing the moist soil can be handled quite freely without breaking.

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Silt loam

Has a moderate amount of fine grades of sand and only a small amount of clay, over half of the particles being of the size called silt. When dry, it may appear quite cloddy but the lumps can be readily broken and when pulverized it feels soft, smooth, and floury. When wet, the soil readily runs together and puddles. Either dry or moist, it will form casts that can be freely handled without breaking; it will not form a ribbon if squeezed between the thumb and finger when moist but will given a broken appearance.

Clay loam

This is a fine textured soil, the characteristics of which are midway between the characteristics of the silt loam and the clay loam. If moisture conditions are ideal, it is possible to form a ribbon with it when squeezed between the thumb and finger.

Silty clay loam

A silty clay loam is a fine textured soil which breaks into clods and lumps that are hard to break with the squeeze of the hand when the clods are dry. When the moist soil is pinched between thumb and finger it will form a cast that will bear much handling. When kneaded in the hand, it does not crumble readily but tends to work into a heavy, plastic, compact mass.

Clay

A clay is a fine textured soil usually forms very hard lumps or clods when dry and is quite plastic and is usually sticky when wet. When the moist soil is pinched out between the thumb and fingers, it will form a long, flexible ribbon. Some fine clays very high in colloids are friable and lack plasticity in all conditions of mixture.

(c) Soil investigation procedures and equipment. Both physical and chemical properties of the soil must be investigated and considered in making the final decision on the suitability of a site for fishpond development. The procedure consists of taking soil samples properly from the site from which measurements of some parameters that describe the characteristic of the soil useful in engineering are obtained. There are standard methods of tests which are specifically applied to determine certain soil properties.

(i) Soil sampling

As standard practice, several samples should be obtained from pre-determined and scattered locations within a given site through borings. It is better to have more number of borings in well-planned locations to cover the whole site.

For relatively shallow boring, soil samples are obtained by means of augers (Fig.2.6). The auger is turned vertically into the wet soil by hand and withdrawn after reaching a short distance. The soil sticking to tha auger is collected and labeled properly. The auger is driven again into the soil deeper and again withdrawn to get the soil. The process is repeated until the soil samples are collected from selected intervals of soil depth. Soil samples should at least be drawn from 10 random locations per hectare.

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The preliminary examination and classification of the soil texture may be done at the site. The samples from each selected depth interval is packed in a sealed plastic bag for further laboratory test. The bags should be marked with information on the date of boring, location, water table elevation and boundary or thickness of each soil layer. Rough sketches on the location and depth of borings from each site should be kept.

Fig. 2.6 A typical soil auger

Soil auger can also be fitted with sampling spoons for obtaining a tube size sample or “spoon sample” for undisturbed samples needed for permeability and compressibility tests (Hechanova, 1982). The device consists of a pipe with inside diameter ranging from 3.5 to 10 cm and split lengthwise. The operation of this device is similar to that of a soil auger. The total depth of sampling varies depending on the type of investigation being carried out. As a rule of thumb, the depth to be investigated should reach the hard soil (impermeable) layer. If the hard soil is so deep, depth of investigation should reach at least three meters.

(ii) Measurements to be done from the soil

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The most important factors to be determined from the soil are, for physical properties — textural class, load bearing capacity, and permeability; for chemical properties — soil pH and presence of acid-forming substance such as pyrites and sulfides.

The texture is the most important physical property of the soil in fishpond engineering. Knowing the texture, much can already be said on the characteristics of the soil. Soil texture gives information on its ability to resist the flow of water through it (seepage), attainable compaction, and load bearing capacity.

Load bearing capacity refers to the capability of the soil to carry, heavy loads. This information guides designers in determining the type of foundation for structures (gates and dikes) and the amount of pilings needed up to a certain point. Brackishwater pond site usually has low bearing capacity of foundation. In addition to driving piles to strengthen foundation, structures should be of lightweight construction. It has been experienced that these structures should not exert a contact pressure of more than 150 g/cm2 or 1.5 tons/m2 (Tang, 1976).

The possible seepage flow at the site or loss of water in the pond by passing through dikes and pond bottom is calculated by determining the permeability of the soil and the nature of foundation. There are extensive procedures in literatures in determining permeability of soil in the laboratory or in-situ.

Soil pH provides an initial and immediate measure of the chemical nature of the site. The soil reaction whether basic or acidic has direct relationship to fish production. The pH scale varies from 1 to 14 units and the desirable range of soil as well as water pH for aquaculture is 6.5 to 8.5. Lower than 6 is too acidic for most fish species and other aquatic animals and greater than 9 is likewise infavourable for growth. Greater values than 9 approach polluted condition.

Measurement of pH may be done on-site or in the laboratory by using pH meters or for less accurate values, litmus paper. Extensive determination of pH is done down to 1 m deep, at 0.25 m interval, especially in problematic acid sulfate soils.

Incidentally, a large hectarage of coastal areas tend to have acidic pH. It is noted that approximately 5 million ha of coastal area in the South and Southeast Asia are known to be potential or actual acid sulfate soils (Poernomo and Singh, 1982). Acid sulfate soils are associated with the accumulation of sulfides and pyrites after undergoing biological and chemical processes.

Sulfides are compounds that produce acidity upon oxidation. The resulting acidic soil condition in turn facilitate the release of aluminum and iron to levels that may be toxic to pond biota including the cultured species. Pyrite is a mineral which is fixed and accumulated by the reduction of abundant sulfate from seawater. The usual pH range of acid sulfate soils is 3 to 6.5 (Poernomo and Singh, 1982). Further discussion on this kind of soil is given in Appendix C.

2.1.4 Topography of the site

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Topography refers to the “lay of the land” or the changes in the surface elevations of the ground whether flat, rolling or sloping, undulating, and hilly. Fishpond design, layout and specifications are made largely in accordance with the land topography.

A suitable site for fishpond has a topography that can be converted into a pond economically. The cost of construction can be greatly reduced if the surface features of the land are used to advantage.

Flat coastal swamplands whose elevation are within the range of ideal pond bottom elevation are preferred for brackishwater pond culture. In such areas, excavation or filling are at a minimum, if any. Sites with rolling topography or those with elevations varying from lower than MLLW (00 tidal datum) to 4–5 m high must be avoided as these will be too costly to level (de los Santos, 1980).

Denila (1980) describes four zones in the coastal edge as probable sites for fishpond (Fig. 2.7).

(a) Zone A. Marginal lands along edges usually planted to lowland rice but generally unproductive due to salt water seepage can be converted into fishpond by lowering the elevation through excavation. These are usually productive as fishponds because of good soil quality. The cost of excavation can make these areas relatively more expensive to develop due to their high elevation.

(b) Zone B. The elevation is generally high with occasional earth mounds along the edges but can be reached by tides. High dikes not necessary but development cost may be expensive if a big portion of the area needs excavation.

(c) Zone C. This area is within the ideal range of pond bottom elevation, thus excavation cost is less. However, extreme acidity may occur because of the presence of vegetation that contributes to acidity.

(d) Zone D. The elevation is low or just a little higher than the 00 datum (MLLW). It is very exposed to wave action which may require expensive big dikes and wave protection structures. No acidity problem, hence, may be ideal for shrimp culture.

2.1.5 Type and density of vegetation

Coastal fishponds in the tropical and sub-tropical zones are constructed on tidal lands, river estuaries, bays and sheltered coasts. The vegetation present in these areas are varied depending on the land elevation and soil type. However, mangrove trees pose serious concern to fishpond development because of their extensive rooting system.

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Fig. 2.7 The four zones in typical swamps as probable sites for fishpond (After Denila, 1980)

(a) Vegetation uses and related benefits. Vegetation provides several beneficial uses not only in fish culture but also for other purposes. In Thailand, Malaysia, Bangladesh, and the Philippines, mangrove trees are utilized as timber for construction, furniture, charcoal sources and other uses. They have also been found to be useful in big offshore shrimp fisheries not only in these areas but also in Indonesia.

In addition to playing an important role in the maintenance of various forms of biological life, mangrove forests also serve as silt collectors promoting soil accretion, absorber of wave impact and buffer of storm surge levels (Menasveta, 1982).

(b) Effects of soil properties. The type and density of vegetation is also an important criterion in fishpond site selection. Knowledge of their composition and abundance can provide information for an easy and practical evaluation of the physical and chemical properties of soil.

A practical guide in evaluating some physical and chemical properties of soil based on type of vegetation is presented in Table 2.5 (Menasveta, 1982; Adisukresno, 1982; Poernomo and Singh, 1982).

Table 2.5Physical and chemical characteristics of soil in relation to type of vegetation found

Description of soil property

Type of vegetation/species

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Physical  

Elevated areas Avicennia

Low areasRhizophora, Melaleuca, Phoenix, certain shrubs and ferns

Sandy soils Nipa palm (Nypa fruticans), grasses

Peaty soils Nypa, Melaleuca

High organic content Rhizophora

Chemical  

Potentially acidic Nypa, Rhizophora, Melaleuca

Less acidic Avicennia

Mangroves with Avicennia usually indicate productive soil. Mangroves with Rhizophora, Bruguiera and Sonneratia are sometimes less suitable for fishponds.

(c) Relationship to amount and cost of construction work. There is a direct relationship between vegetation and the amount and cost of fishpond construction work. Areas where mangrove trees with dense rooting systems like Rhizophora, Nypa and Melaleuca are more difficult to excavate. At the same time, densely vegetated areas takes longer time to clear of stumps. As a result, cost of fishpond construction is higher.

Vegetative density is dependent upon the kind, size and quantity per unit area. Total vegetation from the site can be estimated by obtaining at least five random samples regardless of area. Then the vegetation is classified according to kind, size (3 cm trunk diameter and above only) and number. Total vegetation is computed using the following formula as suggested by BFAR-UNDP/FAO, 1982.

2.1.6 Climatic and watershed conditions around the site

The climatic factors largely affecting site selection and fishpond design are wind and rainfall. The direction of prevailing wind is reckoned with in designing the layout of fishponds as it generates erosive wave action against the dikes. The wind energy also causes natural water circulation and aeration in the pond. For rainfall information, maximum intensity, duration, frequency and annual distribution within the watershed are important as these are associated with flooding that would affect the site.

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The land area that surrounds or lies above the site is referred to as a watershed. This drains its collected surface runoff through a river, river system or body of water causing flood water which could affect the site. The volume of flood water or runoff are affected by the rainfall and soil characteristics, type of vegetative cover, topography, and area of the watershed. Much higher rate of runoff can be expected in a watershed that has high rainfall intensity and duration, clay or heavy-textured soil covered with less vegetation or grassland, high slopes and covering large area than a watershed characterized with low rainfall intensity and short duration, loamy or coarse-textured soil that is forested with flat or gently rolling slope and small area.

The pattern and recurrence of maximum height of flood waters in the site must be determined. These information can be obtained from the rainfall or flood records of appropriate agency for each country. It is common, however, that the sites are located in remote areas where such records may not be available. In this situation, rough information on flood may be obtained from knowledgeable residents who have seen the recurrence of floods for years in the area.

The design of fish farm should be based on a flood height with recurrence interval of 10–15 years. Longer recurrence interval of 25 or 50 years means much higher flood level and hence, much bigger dikes and structures which makes the design uneconomical or impractical. It would be more economical to repair damaged fishponds (designed for 10–15 years flood) during the sudden occurrence of say 25 years or longer term floods than to design the entire farm for such floods.

2.1.7 Other criteria

There are other factors which are significant in fishpond site selection. These are equally as important as those previously mentioned and likewise require the same careful evaluation during the survey.

a. Accessibility. This is important for the transport of construction equipment and material, and for production inputs required for daily operations. Transporting costs can considerably increase if materials are manually carried through long distances. It is better if the site is accessible throughout the year by means of land and water.

b. Availability of labour. The cheapest sources of labour are those which can be provided by the local residents, or people living within or near the area. It is important that the customs and tradition of local labourers are known. The pattern of labour distribution and utilization should be considered as this is important in preparing the calendar of activities. In the Philippines, it is generally difficult to obtain enough labour during the rice planting and harvesting season or during milling season for sugarcane.

c. Availability and cost of material. In fishpond production, it is important that critical production inputs such as fishseeds, fertilizers, pesticides and other related materials are readily available when needed. For some inputs, especially inorganic fertilizers, the supply is restricted and the cost is uncontrolled for non-agricultural uses. Other inputs like organic manures are difficult to obtain, or may be available only at certain times of the year. If purchase in bulk is necessary, then storage space must be available. If material is to be imported, restrictions and corresponding costs must be known.

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d. Availability of marketing outlets and prices. Aquaculture products are highly perishable. Immediately upon harvest, products must be disposed of to maintain good quality and for better prices. If marketing outlets are located at a distance, larger quantities must be harvested and transported requiring some post-harvest marketing practices. If so, then the required support facilities especially ice-making plants must be available.

e. Availability of credit and technical assistance. Fishpond operations require high initial capital investment. In this respect, credit at reasonable terms play a major role in providing the needed cash outlays.

Technical assistance may be obtained from government extension services, public or private university research stations and lending institutions. The services rendered by these agencies are important especially in cases of emergency.

f. Pattern of land and water use. It is important to assess the pattern of land and water use in the area to determine the impact of this on the project. Activities such as navigation, fishing, industries, public utilities, recreation and nursery areas must be included in the overall assessment. It is best that a complementary rather than competitive relationship between these various uses and the project be established.

g. Peace and order situation. Good peace and order conditions at site are favourable for both public and private interests.

2.2 Making the decision

The success of coastal fishpond operations depends not only on the general site suitability for fish production but also on other related factors such as input and labour availability, accessibility, marketing considerations and others. Site selection not only involves the determination of desirable physical, chemical and biological factors. It is also important in providing valuable information in the preparation of the overall design and layout of the facility, engineering modifications to be made and the choice of management practices appropriate for the given site.

It is important to remember that there is no site that possesses all the desirable characteristics for fishpond operation. Moreover, no two sites are exactly identical with one another. Hence, the degree of suitability of various sites evaluated vary from one area to another.

2.2.1 Method of evaluation

The evaluation of the suitability of fish farm sites involves a detailed survey of both technical and non-technical aspects, and the processing of information gathered in order to make the final selection.

a. The survey. If possible, the survey should be comprehesive to cover aquacultural, ecological, engineering, socio-economic, management and financial aspects. As a standard practice at least two surveys should be conducted during the year—one each

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during the dry and rainy seasons. It is important to evaluate the prevailing conditions for different seasons because there are factors that change at different times of the year. A wrong judgement could be made in the selection if only one survey is conducted.

Conducting the survey can be a costly, laborious and time-consuming exercise if the proper steps, procedures and preparation are taken for granted. Adisukresno (1982) recommends the following to save money, time and effort:

i. Sufficient and complete questionnaires or forms should be prepared before leaving for the survey;

ii. Questionnaries must be properly and completely accomplished during the survey;

iii. An itinerary or travel schedule should be prepared;

iv. Only trained or capable personnel should conduct the survey;

v. Needed field equipment should always be put together and brought along;

vi. A list of people to be the met and purpose of meeting should be prepared ahead of time.

b. Processing of information. Data and information collected during the survey are combination of quantitative and qualitative; thus, it is very difficult to arrive at a general decision. The most commonly used method of deciding the suitability of sites from among a number of prospective sites surveyed is the point and ranking system.

c. Applying the point and ranking system. Under this system, all data and information are transferred into numbers or assigned numerical points or scores. The scoring could be from 1 to 10 or from 1 to 100. A site with the most desirable characteristic for a certain criterion is assigned the highest score, the lower the value, the less ideal or desirable site becomes and vice-versa. For example, in evaluating the accessibility, if a road leads right into the central area, a score of 10 can be given to this site; if reached within walking distance, 9; and if reached along across a river or creek, 8 or less. With regard to the type of vegetation, presence of Nypa sp. can be ranked lowest; mangrove with Avicennia sp. can be ranked highest, and so on.

There are two ways of assigning points for the different items in the criteria: (i) one may consider every criterion to have equal degree of importance; and (ii) the other way recognizes the varying degree of importance of each criterion by assigning weights or multiplying factors. The latter (ii) is considered better than the former. Essentially it is just an added step from the first (i) in order to get the weighted score. Jamandre and Rabanal (1975) suggest the following relative weight multiplier for various criteria under conditions in Peninsular Malaysia.

  Criterion Relative weight (multiplier)

(a) Accessibility 1

(b) Socio-economic impact 3

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(c) Water supply system 2

(d) Available area 2

(e) Water quality 3

(f) Soil quality 3

(g) Kind of vegetation 0.5

(h) Density of vegetation 0.5

(i) Elevation 3

(j) Possibility of mechanization 1

(k) Protection from winds, waves, currents, etc. 1

The points earned under each criterion using either the unweighted or weighted method are added together. Final selection from among the sites is done by setting a cut-off score. Sites that obtained total points above the cut-off score are qualified for selection. However, the sites that qualify must be ranked according to the total scores earned. Sites with the highest scores are the most desirable. Rabanal and Jamandre (1975) suggest an evaluation range for scores earned under Malaysian conditions as follows:

Range of scores (%) Evaluation

80 to 100 Excellent site for development

60 to 79 Very good

40 to 59 Good

Below 40 Not worth considering

2.2.2 Illustration of the point system

To illustrate the system described, two examples (2.1 and 2.2) are hereto presented. Example 2.1 is modified after Adisukresno (1975) and Example 2.2 deals with surveyed sites in West Malaysia taken from Jamandre and Rabanal (1975).

In Example 2.1 (Table 2.6), four sites (A, B, C and D) were evaluated. Considering the relatively high scores, all four sites can be considered for fishpond development. Based upon the ranking, area D is the top priority and area B is second.

However, it must be noted that all the criteria used have been assigned equal weights. If factors are used to convert the individual scores into weighted scores based on the criteria are given emphasis, the ranking of the evaluated sites is likely to change.

In Example 2.2 (Table 2.7), the different criteria used are assigned relative weights with maximum point equivalents totalling 100. Evaluation scores for each criterion is assigned with

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the maximum point or less depending on the results of the survey. Evaluated sites with scores below the cut-off of 40 percent are not considered for fishpond development.

Example 2.1 Application of unweighted point system

Table 2.6Characteristics and points earned by four sites (modified after Adisukresno, 1982)

CriteriaLocation

A B C D

Soil quality:        

Texture Sandy loam Sandy loam Sandy Sandy loam

  (30% sand) (35% sand) (60% sand) (35% sand)

Depth of top soil (cm) 20 25 10 20

pH 4.5 5.0 5.0 4.8

Water quality:        

pH 7.8 7.9 8.0 7.9

Salinity (ppt) 20 22 24 20

Environmental and climatic factors:

       

Land elevation (m b.s.l.) 1–2 1–2 1–3 1–3

Tidal fluctuation (m) 1–2 1.5–2.5 0.5–2.5 1–2

Water flow capacity (cu.m/sec) 3 2.4 6 5

Rainfall (days/year) 150 130 120 100

Annual precipitation (mm) 1 800 1 500 1 500 2 000

Vegetation Rare Dense Rare Dense

Other factors:        

Distance to source of supplies (km)

25 20 10 5

Distance to nearest industrial area (km)

15 15 10 5

Presence of stream from the industrial area

No No Yes -

Presence of stream from ricefield

No No No No

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Accessibility Good Good Good Good

Distance to source of fishseed (km)

50 70 30 60

Distance to nearest source of labour (km)

5 3 2 2

Distance to nearest market outlet (km)

50 50 25 30

Availability of area Yes Yes Yes Yes

Occurrence of typhoon in the area

None None None None

Occurrence of monsoon toward the area

East (Jul-Aug)West (Dec-Feb)

West (Dec-Feb)

West (Dec-Feb)

Beach abrasion by monsoon No No No No

Flood hazardYes, during heavy rain

No No No

Criteria

Score

A B C D

Soil quality:        

Texture 8 7 4 7

Depth of top soil 7 8 6 7

pH 6 8 8 7

Water quality:        

pH 7 8 9 8

Salinity 8 9 10 8

Environment and climatic factors:        

Land elevation 8 8 6 6

Tidal fluctuation 8 10 6 8

Water flow capacity 6 5 8 7

Rainfall 8 7 6 9

Annual precipitation 9 7 6 10

Vegetation 9 6 9 6

Other factors:        

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Distance to source of supplies 6 7 9 8

Distance to nearest industrial area 9 9 8 7

Presence of stream from the industrial area 10 10 0 10

Presence of stream from ricefield 10 10 10 10

Accessibility 10 10 10 10

Distance to source of fishseed 8 6 10 7

Distance to nearest source of labour 7 9 10 10

Distance to nearest market outlet 5 5 7 6

Availability of area 10 10 10 10

Occurrence of typhoons in the area 10 10 10 10

Occurrence of monsoon toward the area 8 7 7 7

Beach abrasion by monsoon 10 10 10 10

Flood hazard 5 10 10 10

Total points 192 196 189 198

Rating (%)* 80 82 79 83

* Maximum total points is 240, maximum score per item is 10.

Example 2.2 The weighted point system

Table 2.7Evaluation of suitability for fishpond development of various swampland sites surveyed in

West Malaysia (Adopted Jamandre and Rabanal, 1975)

Location of site

Accessibility (× 1)

Socio-

economic impact (×

3)

Water supply system (× 2)

Available area (× 2)

Water quality (× 3)

Soil quality (× 3)

Kind of

vegetation

(× 0.5)

Vegetation density (× 0.5)

Elevation (× 3)

Mechanization (× 1)

Protection (wind

-flood) (× 1)

Weighted total (%)

Remarks

Kuala Perlis-Utara, Perlis

6 6 3 1 4 6 8 8 7 7 4 50 Coop has M$5 000 intended

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for seabass, crabs

Pulau Langkawi, Perlis Tanjong Dawai (Left bank due south), Kedah

3 8

2 8 6 7 9 10 8 8 5 8 3 4 6 6 4 5 5 7 8 6 54 74

On an island Good demonstration project site, 4 ha pond under construction (planned for shrimp)

Tanjong Dawai (Right bank due south), Kedah

8 8 6 10 8 8 4 6 6 6 6 74

Should be supervised and encouraged or make into demonstration project

Kuala Moda, Penang

7 6 2 2 3 4 7 5 5 7 6 44 Fair

Peatal Acheh, Penang

6 4 3 4 6 5 6 5 6 6 7 51

North end of mud flat, fair

Kuala Jalan Baharu Penang

7 5 5 5 4 5 6 5 7 4 7 53 Good

Pulau Betong Penang

8 6 6 4 5 7 6 5 5 5 7 57 Maybe good for Chanos

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demonstration project

Kuala Kurau, Kuala Gula: Telok Rubian, Perak

2 2 4 5 5 7 7 5 3 6 4 44 Fair

Larut Matang Selingsing Island, Perak

5 7 6 6 7 7 4 5 7 4 5 63 Good

Jabong (roadside), Perak

9 4 4 4 5 4 2 5 4 6 5 45 Fair

Larut Matang: Sungei Kechil, Perak

7 7 5 8 7 6 4 5 4 4 5 59 Good

Bagan Pancor: Sungai Tinggi; Passir Hitam; Sungai Kerang; Jaron Mas, Perak

7 7 7 10 8 6 5 4 3 6 5 64

Can be good for state demonstration pond at Bagan Pancor (8 to 15 ha recommended)

Kuala Selangor, Selangor

6 6 3 4 4 6 5 4 2 8 4 45 Fair

Sungai Lukut, Negeri Sembilan

5 4 5 3 5 3 4 4 2 8 4 40Not recommended

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Sungai Linggi Negeri, Sembilan

4 3 5 5 5 3 3 4 2 8 4 39Not recommended

Sungai Rembau, Negeri Sembilan

5 3 3 4 4 3 3 4 2 8 5 36Not recommended

Gelang Patah, Johore

6 5 6 5 3 6 5 4 3 8 5 48

Project site salinity and water quality poor when it rains

Sungai Chukoh Serkat, Johore

6 3 5 4 5 5 6 4 3 7 5 45 Fair

Kampong Celo Dawa. Johore

8 4 7 1 6 6 5 5 4 8 5 51

Existing and profitable crab fattening ponds

Plentong Tebrau. Johore

7 5 8 4 6 5 6 6 5 8 5 57 Good

Kampong Kuala Penor, Penang

7 1 2 0 5 2 3 4 1 6 5 26

Very poor not recommended

Kudatan Riverside. Pahang

5 2 7 1 8 5 6 4 5 5 5 48Very fair

Kampong Cherating. Pahang

8 2 7 4 7 4 4 4 8 6 5 54 Good

Sungai 7 7 5 6 6 5 5 6 6 6 5 59 Fairly

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Chukai, Trengganu

good

Sungai Kemaman, Trengganu

7 7 5 7 3 4 6 5 5 6 4 52 Good

Kuala Kerteh, Trengganu

8 7 6 6 7 6 5 6 6 7 5 64

Good demonstration site for east coast

Sungai Datu, Trengganu

7 7 6 4 3 5 5 5 5 6 5 52 Good

Kampong Pulau Krengga Merchang. Trengganu

9 3 5 1 6 4 8 9 8 8 6 53

Recommended for experimental pond only

Kuala Ibai, Trengganu

7 5 4 1 4 2 6 4 7 6 5 44 Fair

Kampong Penarek, Trengganu

Negligible swamps

- - - - - - - - -

Not worth considering

Kampong Fikri, Trengganu

6 5 5 2 5 3 2 3 5 7 5 44

Not worth considering

Sabak, Kelantan

Very marginal mangrove

- - - - - - - -

Not worth considering

Tumpat, Kelantan

7 6 6 3 1 2 6 7 6 7 1 42 Flooded

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CHAPTER 3DETAILED ENGINEERING AND ECOLOGICAL SURVEYS IN POTENTIAL/SELECTED SITES

3. DETAILED SURVEYS AFTER SITE SELECTION

Detailed engineering and ecological/environmental surveys of the fish farm site follows after site selection. Actual site surveys is done in connection with the full-scale planning and design layout and construction of the farm.

The engineering surveys may include measurements or verification of distances, directions, and areas, and topographic mapping. Ordinarily, existing topographic maps which include probable sites cover large areas such that the scale used is small and unsuitable for planning designing process. For purposes of fish farm project development, a more suitable or updated topographic map has to be drawn out. A topographic map shows the nature of the ground relief or its characteristics, such as differences in elevation, location and measurement of boundaries or fences, physical facilities (buildings, roads, rivers, canals, bridges, land use-tilled, swamps, woods) and other features. This map is of paramount importance because it gives the basic horizontal (areal) and vertical (elevation) controls in the planning/design of the farm. It provides the relationship of the site with the tidal fluctuation, determines direction of water movement, guides in locating water control structures and in estimating quantity of earthwork, and other factors which are closely tied-up with development costs.

The ecological/environmental survey may verify or provide more in-depth information about the physico-chemical and biological make-up of the environment, in addition to what has been known during the site selection process.

This topic, therefore, covers the survey procedures involved in the measurement of distances, areas, levelling contour mapping, including the ecological concerns and their application to fishpond design and construction.

3.1 Engineering survey equipment

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There are a variety of equipment available for field survey work. The principal equipment are composed of the engineer's transit, levels, magnetic compass, surveying tape, levelling rod, and range poles. Added to these are minor tools such as hatchet, mallet, bolos, stakes, etc. Other equipment for actual field mapping work is place table with alidade.

(a) Engineer's transit. The cost depends on the model available which range from the simplest kind to the most sophisticated model. It is a versatile piece of equipment which is used for measuring vertical and horizontal distances; vertical and horizontal angles, for prolonging lines, for levelling operations, and others.

(b) Levels. Just like the transit, levels vary from simple or less accurate models of hand levels up to the sophisticated and precision models such as the self-levelling level. These are used mainly for measuring vertical and horizontal distances in levelling operations. Some models are equipped with horizontal circle to measure horizontal angle.

(c) Magnetic compass. The essential features of a surveyor's compass are: (i) a compass box with circle graduated from 0° to 90° in both directions from the N and S points and usually having the E and W points interchanged: (ii) a line of sight in the direction of the SN points of the compass box: and (iii) a magnetic needle supported freely on a pivot. The whole compass can be attached to a tripod by a ball and socket joint.

(d) Levelling rod. Also called target rod, this is usually made of wood graduated either in English or metric units for measuring vertical distances in conjunction with the transit or level. This comes in lengths of 2 to 4 m.

(e) Range poles. These are slender round poles usually made of metal or wood painted with alternate bands of red and white. These are stuck along the line of survey in order to establish a straight line of sight.

3.2 Measurement of distances

Distances in survey work are measured in either vertical or horizontal plane. Vertical distances or differences in elevation in fish farm planning are usually determined by the use of level instruments and level rods. Horizontal distances are determined in various ways depending on the accuracy desired. Among the available methods, the common and practical ones in use are pacing, taping, and the stadia method.

3.2.1 Pacing

Distances may be roughly calculated by pacing when the desired accuracy is not greater than 0.6 m in 30 m. A pace is the normal length of a step or stride of an individual. The length of pace of an individual should be checked with an accurately measured distance in order to determine the so-called Pace Factor. Pace Factor (P.F.) is defined as the ratio of the measured distance in the number of paces made by an individual to cover the measured distance or:

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In determining the P.F., the measured distance is at least 200 m or more. The 200 m distance is walked at normal pace, counting the number of paces to cover it. This is done at least three times. The average number of paces is used as the divisor in determining the P.F.

A person who has a P.F. of 0.70 means that the normal length of his step is 0.70 m. If the same person has walked 3 000 paces, the rough estimate of the distance covered is:

Distance

= P.F. × Number of paces

= 0.70 × 3 000 = 2 100 m

In pacing, one should be aware of the factors which vary the length of pace. Some of these are: (i) when walking through short or tall vegetation; (ii) when going up or down hill; (iii) when walking on wet or dry ground; on plowed or firm soil; and (iv) when crossing fences.

3.2.2 Taping

Tapes (Fig. 3.1) are used for direct measurements of horizontal distances. Commercially sold are made of steel, metallic cloth or fiberglass materials. These tapes are graduated in English, metric or combination of the two systems of units and come in various lengths of 50 ft or 15 m. 100 ft or 30 m. and as much as 100 m. The first and last foot or meter of the tapes are usually fully graduated, as small as tenth of a foot or in mm. The middle graduations are in full feet or meter graduation. There are, however, exceptions to this, Accurate taping requires skill in the use of the tape, marking stakes or pins, plum bob and range poles or flags. For accurate taping, the following should be observed:

(a) Pull tape tight enough avoiding too much sag especially when long lengths of the tape is suspended. Break the tape or use only a portion of a tape length when measuring horizontal distances on slopping ground; use also plumb bob.

(b) On the other hand, avoid too much stretching as in the case of fiberglass tape.

(c) Error due to expansion (in the case of steel pipe) during hot days.

(d) Alignment of tape during measurement. Use range poles or flags as guide in having straight line of sight. Rear tapeman should always align with the head of the tapeman during taping.

(e) Count number of tape lengths carefully. Taping pins should be used (Fig. 3.1b).

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Fig. 3.1 Equipment for measuruing horizontal and vertical distances

(f) Inspect the full tape length always before using. When damaged or short tape is used, apply correction properly.

(g) Be proficient in measuring distance less than a tape length, and in reading graduation in tape.

3.2.3 The stadia method

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A quick way of measuring distance is by the stadia method. The measurement of distance by stadia uses the transit or level instruments, having telescope provided with stadia hairs (Fig. 3.2) and levelling rod. The stadia hairs are equidistant from the horizontal cross-hair. The procedure of measuring distance on level ground is illustrated as follows (Fig. 3.2). Let us say that distance OA is to be measured.

From the position of the instrument, distance OA can be determined by getting the upper and lower rod readings which are intersected by the upper and lower stadia hairs of the telescope, respectively. The general formula is as follows:

Distance OA = (Upper rod reading - Lower rod reading) (100) + Stadia Constant

Old model transits and levels have stadia constants indicated in their box but the modern ones have this value as zero. With stadia constant equal to zero, the formula becomes:

Distance OA = (Upper rod reading - Lower rod reading)(100)

The difference between upper and lower rod readings is called stadia interval. The unit of the distance follows the unit of the rod used.

As an example, if upper and lower rod readings are 2.75 and 0.25 m, respectively, the distance is:

Distance OA = (2.75 - 0.25) (100) = 250

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Fig. 3.2 Illustration of stadia method

The horizontal cross-hair is used as a check in the correctness of the distance. Since the horizontal cross-hair is located at the middle of the two upper and lower stadia hairs, the distance measure using either of the two stadia hairs and the horizontal cross-hair is one-half of the distance OA. Hence, the check is as follows:

Distance OA = (Upper or lower rod reading - Middle rod reading) (2) (100)

If the line of sight is on sloping ground, it is necessary to apply a correction in order to obtain the true horizontal distance. This is done by the use of tables which indicate corrections for various angles of slope. However, in the fish farm survey, corrections are usually ignored as slope encountered rarely exceeds 5 percent and such percentage does not require corrections.

3.3 Measurement of angles and directions

The direction of any line is measured in terms of angle between the line and some reference line — usually the North-South line in the compass. The instruments used to measure angles are compass, transit, tapes, plane-table, alidade, and sextant.

In general, accomplishment of survey work revolves in the properly organized measurements of distances and angles. Angles just like distances, are also measured along the horizontal and vertical planes are called horizontal and vertical angles, respectively.

3.3.1 Methods of expressing angles and directions

Angles and directions may be expressed in different ways, namely: (i) bearing; (ii) azimuth; (iii) interior angles; (iv) deflection angles; and (v) angles to the right. Among these, the first two are commonly used in fish farm survey. The method using interior angles is useful in checking or adjusting the plotted sides of an area (based on the field data gathered to have a closed survey.

(a) Bearing. It is the angle that is referred from the North and South, whichever is nearest with the added designation of east or west, whichever applies (Fig. 3.3). A bearing can never be greater than 90°. Examples of bearing are: N 37° E. N 45° 50' W, S 54° 15'30" W, S 89° 45' E, N 90° E or due East.

(b) Azimuth. The azimuth of a line is a clock wise angle measured from a reference direction usually North. The South end of the North-South line is also being used as reference direction for azimuth in geodetic surveys. Azimuths based from the North are called North azimuth; those referred from the South are South azimuth (Fig. 3.4).

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Fig. 3.3 Sketch of example bearing of line

Fig. 3.4 Illustration of north and south azimuth of a line

Some examples of equivalent azimuth and bearings of a line are as follows:

Azimuth Bearing

North South  

120° 300° S 60° E

200° 30 20° 30' S 20° 30'W

290° 110° N 70° W

30° 45' 30'' 210° 45' 30'' N 30° 45' 30''E

Determining equivalent bearings and azimuths can best be done by figuring out in which quadrant the angle lies.

(c) Deflection angle. This refers to the angle between a line and the prolongation of the preceding line. Deflection angles are identified as right or left. Right deflection if the angle measured lies to the right (clockwise) of the extension of the preceding line. Left deflection if the angle lies to the left (counter clockwise) of the extension of the preceding line (Fig. 3.5).

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Fig. 3.5 Definition sketch of deflection angles

In the case of deflection angles, the reference lines for lines (or sides of a field) BC and CD, is the prolongation or extension of the preceding lines AB and CD, respectively. However, those reference lines must be tied up with the reference line of AB which could be expressed either in bearing or azimuth.

Deflection angles may have values between 0° and 180° but they are not usually used for angles greater than 90°. In any closed polygon (or sides of a given fish farm site), the algebrate sum of the deflection angles (considering right deflection as plus (+) and left deflection as minus (-) is 360°.

(d) Angles to right. Angles may also be determined by clockwise measurements from the preceding line to the following line (Fig. 3.6). Such angles are referred to as angles to right.

Fig. 3.6 Angles to right

(e) Interior angles. In any closed polygon, the angles inside the figure between adjacent lines are called interior angles (Fig. 3.7). The sum of the interior angles in a closed polygon is equal to (N-2)(180°), where N is the number of sides. For a five sided field, the sum of the interior angles is 540°. Actual value of every interior angle is computed from the field data on directions, such as bearings, azimuths or the other methods. An error is incurred if the total interior angles obtained from survey data is more or less than the value determined by the formula.

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Fig. 3.7 Illustration of interior angles

As an example, the interior angles in A and B (Fig. 3.7) can be computed as follows:

(i) For interior angle A, example bearings of line AB and EA of N 80°E, and N 11°E, respectively, will be needed in the computation including the cross-directional lines N-S and E-W. Hence:

Interior Angle A = 10° + 90° + 11° = 111°

(ii) For B, the bearings of lines AB and BC, N 80°E and S 95°E, respectively, will be needed. Therefore,

Interior Angle B

= (90° - 10°) + 85°

= 165°

or = 180° - 10° - 5° = 165°

3.3.2 Methods of determining angles and directions

Only the common or simple methods are presented herein.

(a) Measurement of angles by tape. As an example, the angle between two sides of a field is to be determined as in Figure 3.8. The procedure is as follows:

44

From the figure, measure and mark with stakes a convenient distance, say 50-m each along line OA and OB. The points at O, A and B are marked by range poles in order to have a straight line of sight. Measure the distance between CD and locate the mid-point E. The angle is then determined by using the sine function and trigonometric table or calculator.

Let us say, distance CE is 25 m; then,

Locate the angle with Sin = 0.5 from the table or by calculator, which is 30°

therefore, angle 0 = 30° × 2 = 60°

Fig. 3.8 Angle measurement by taping

Another situation is how to establish a perpendicular line to either of the two lines OA and AB to make a right triangle (Fig. 3.9). The angle is determined by taking the necessary measurements in order to use the sine or tangent function. However, the above method is slightly quicker than this.

45

Fig. 3.9 Angle by taping, right triangle method

(b) Measurement of angles and directions by compass and transit

(i) Bearing of a line. To determine the bearing by compass, set the instrument over some point on the line. Level the instrument and lower the magnetic needle of the compass to make it swing freely — next, sight along the line the bearing of which is sought. The bearing is then read on the graduated circle at the point of the needle which will be less than 90° and either in the West or East of the North or South. Example, if the needle stands 45° east of north, then the bearing is N 45°E (Fig. 3.10). Take note that the magnetic needle of the compass always aligns itself towards the magnetic North. During reading the North and South end of the needle is distinguished from each other by the counterweight, it being located in the South end.

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Fig. 3.10 Measurement of line bearing

The engineer's transit is also provided with compass which is mounted on its upper or vernier plate. In taking bearing, the instrument is set-up on the line and properly levelled. Then the telescope line of sight, the 0° mark of the horizontal circle, as well as its vernier, are all locked and all in one line directed towards the magnetic North to coincide with the released free-swinging needle of its compass. After attaining a steady position, the horizontal circle is unlocked to make it rotate with the telescope. The telescope is then rotated about its vertical axis and directed along the line until the vertical cross hair bisects the marker stake or range pole. The angle read on the horizontal circle and the vernier corresponds to the bearing of the line. The directional N-S and E-W are taken from the compass.

(ii) Azimuth of a line by transit. Let us say that the azimuth of the line BC is desired (Fig. 3.11). Set the instrument at B. Backsight point A with the vernier set to read the azimuth of the line BA. When the telescope or line of sight is rotated to C, the vernier reading will be the azimuth of line BC.

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Fig. 3.11 Measurement of azimuth

(iii) Deflection angle. Referring to Fig. 3.12, the deflection angle at B can be determined as follows: A backsight is taken at A with the vernier set at 0°. Then the telescope or line of sight is rotated vertically or plunged in altitude to point in the direction BD. Then the line of sight is rotated until it sights C. The vernier reads the deflection angle DBC.

Fig. 3.12 Measurement of deflection angle

3.4 Laying out perpendicular and parallel lines

This is usually encountered in the actual layout of pond dike. The job is easily done with transit but in its absence the use of tape is also convenient.

3.4.1 Laying out perpendicular lines

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For example, it is desired to layout the centreline of dike CD (Fig. 3.13) perpendicular to dike AB at point D.

Fig. 3.13 The 3-4-5 method in laying out perpendicular lines

(a) The 3-4-5 method. It is a common knowledge that a right triangle is one whose sides are in the proportion of triangle, with shorter sides 3 and 4 perpendicular to each other while the longest side 5 is the hypotenuse. To lay out the perpendicular lines AB and CD using the same principle, the procedure is as follows:

(i) One tape length of 100 m is convenient to use such that the 0, 15, 35, and 60-m graduation marks can be held as a loop in one set-up.

(ii) Three men have to do the work. First man holds the zero and 60-m graduation of the tape, the second man, the 15-m and the third, the 35-m mark.

(iii) The tape is held tight enough, and the first and second man are aligned along AB while the third man adjusts himself as necessary to keep the tape stretched.

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(iv) The points D and C are then marked and extended.

(v) This can be checked by using larger proportions of distance such as 30, 40 and 50 m.

(b) Intersection method

Fig. 3.14 Intersection method

(i) From Fig. 3.14 measure equal distance of 30 m from both sides of point D.

(ii) While one man holds the tape at 0, another man describes an arc using, say a full tape length of 50 m.

(iii) The procedure in (ii) is repeated at point 0'.

(iv) Point C is located by the two intersecting lines. Line CD is then perpendicular to AB.

The intersection method applies on relatively clear ground where the described are can be marked or seen. An alternative quicker procedure is to use one tape length of 50 m and an equivalent length of rope or two ropes of equal length. Two men hold one end of the tape/rope and each of them stays at point 0 and 0'. Another man holds the other end of the two ropes pulled tight and point C is located.

3.4.2 Laying out parallel lines

Suppose in Fig. 3.15, dike CD is to be laid parallel to and at 65 m distance from dike AB. From AB erect perpendicular lines EF and GH in the same way described in the previous topic. Extend the line from points E and G until it exceeds 65 m from AB. Measure equal distances of 65 m along EF and GH from AB. The line CD formed passing through points F and H is the desired parallel.

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Fig. 3.15 Laying out parallel lines

3.5 Measurement of areas

In ordinary land surveying, the area of a tract of land is taken as its projection upon a horizontal plane and not the actual area of the surface of the land.

Available methods used in computing areas are the: (i) planimeter method — where boundaries of the farm are plotted to scale and area is determined by the use of planimeter; (ii) double-meridian-distance (DMD) method — where area is calculated from the coordinates of the farm; (iii) trapezoidal rule and Simpson's ⅓ rule for calculating areas of land bounded by irregular curves; and (iv) by plotting the boundaries to scale and dividing the tract into regular geometric figures (such as triangles, rectangles, or trapezoids), scaling the dimensions of these figures and computing their areas mathematically. Likewise, the tract of land may also be actually divided into regular figures and all necessary measurements of sides are taken.

Among these methods, the trapezoidal rule and the last method of subdividing into regular geometric figures are easily understood. Moreover, with the advent of precision pocket calculators, direct computation of areas is now convenient to do. The principles and procedures for two methods are illustrated in Appendix D.

3.6 Topographic survey

This kind of survey requires technical know-how and skill in levelling operations. The ultimate objective in doing this survey is to reflect on map the relief or changes in elevation of the fish farm site including other relevant ground features.

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3.6.1 Levelling

This is a basic operation in engineering survey that leads to the production of a topographic map. Direct levelling is commonly used among the methods in levelling available and it is the process by which differences in elevation in the site is determined with the use of a level or transit instruments (Fig. 3.16) together with a level or stadia rod. When it is necessary to locate or fix the ground points as in the case of full-scale topographic survey, additional information on directions (angles) and distances are obtained.

There are two kinds of direct levelling — differential and profile levelling. Differential levelling is the operation which determines the difference in elevation of two points which are distance apart. Profile levelling is the operation that determines the differences in elevation of points along a prescribed line and at measured intervals.

The following terms and their definitions are useful in understanding the principle of levelling.

(a) Elevation — refers to the vertical distance of a ground point from the reference datum plane (MLLW).

(b) Bench mark (BM) — it is a station or point on the ground of known elevation and of a permanent nature. BM provides the reference elevation from which relative elevations for other stations are calculated. A BM may be established on permanent objects/structure on wooden or bamboo stakes driven firmly near a construction project.

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Fig. 3.16 Level instruments

(c) Station (Sta) — any point where a rod reading is taken and is generally along the line being run.

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(d) Backsight or plus sight (BS) — a rod reading taken on point of known elevation. This is used for obtaining the level line of sight or HI. Also known as plus sight since it is always added.

(e) Foresight or minus sight (FS) — a rod reading taken on any point of unknown elevation. Also known as minus sight since it is always subtracted.

(f) Turning point (TP) — it is generally impossible to take all the readings along the direction of survey without moving the instrument. The TP is an intermediate station or reference point whenever the instrument is moved from one set-up to another. A point which is no longer needed after the necessary readings have been taken.

(g) Height of the instrument (HI) — is the relative elevation of the line of sight of the instrument as referred to the elevation of the datum plane, bench mark or turning point.

(h) Ground profile — a graph of the ground surface which shows change in elevation (along vertical y-axis) with distance (along horizontal x-axis).

3.6.2 Differential levelling

The principle involved, keeping of differential level note, the arithmetic check, and acceptable degree of accuracy are given below.

(a) Principle of differential levelling

Case 1: Two points visible. From Fig. 3.17, the difference in elevation, H = Ha - Hb; where Ha and Hb, are rod readings at points A and B, respectively.

Fig. 3.17 Case of two points visible from the instrument

To determine the elevation of point B, the elevation of point A must be known. Assuming that the elevation at A is ELA, the elevation of point B, ELB is:

ELB = ELA + Ha - Hb Note that ELA + Ha = HI; and since the elevation at A

54

is known, then Ha is a BS; Hb is a FS since the elevation of B is unknown.

= ELA + BS - FS

ELB = HI - FS

Case 2: The objective points are not visible from each other or far apart. To determine the difference in elevation between points A and B (Fig. 3.18), a series of differential levelling is done. This situation occurs when another bench mark is to be established in the fish farm project. Instrument is set approximately midway of turning points. The corresponding differential level note as obtained in the field for Fig. 3.18 is presented in Table 3.1.

(b) Arithmetic check. Completion of the level note for HI's and elevations follow after the field work. To check for the accuracy of addition and subtraction, the difference in the sums of the backsights and foresights must be equal to the numerical difference in elevation between BM2 and BM1, or:

Sum BS - Sum FS = Elev. BM2 - Elev. BM1

From the given example of a complete differential level note (Table 3.1), the corresponding arithmetic check is as follows:

Sum BS = 9.53  Elev. BM2 =

4.32

Sum FS = 6.71  Elev. BM1 =

1.50

  2.82 m       Check       2.82 m

(c) Allowable error of closure. The arithmetic check determines only the correctness of the addition and subtraction done in completing the table. It does not tell the degree of error incurred in the conduct of the field survey. Depending on the nature of the job, the accuracy of work must be within the allowable limit prescribed. In fish farm site survey, the allowable error is largely dictated by the range of tidal fluctuations. The allowable error of closure for rough and ordinary levelling works are as follows (Davis, et al., 1966):

Type of work Allowable error, in feet (0.305 m)

Rough levelling  

(For reconnaisance or preliminary survey)

+ (0.4) ( / M ): Distance of sights is about 300 meters

Ordinary levelling(Most engineering work)

+ (0.1) ( / M ): Distance of sights is about 150 meters

Where M = length of traverse or level circuit in mile (1.609 km).

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Fig. 3.18 Levelling procedure when objective points are not visible in single instrument set-up

Table 3.1Differential level note for Figure 3.18

STA BS HI FSELEVATION,

mREMARKS

BM1 1.50 3.00 - 1.50 BM, located on an undisturbed or permanent structure

TP1 1.71 3.71 1.00 2.00

TP2 1.85 4.41 1.15 2.56

TP3 1.67 4.83 1.25 3.16

TP4 1.45 5.15 1.13 3.70

TP5 1.35 5.38 1.12 4.03

BM2 - - 1.06 4.32

The actual error incurred during the field survey must not exceed the allowable error. Actual error is determined by completing the level survey loop or circuit. In other words, the levelling operation from point A to B (Fig. 3.18) is continued back (B to A), thus completing the loop and taking into consideration its length. An example of a level circuit is shown in Fig. 3.19.

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Fig. 3.19 An example of levelling circuit

The actual error is the difference in values of elevation at A at the beginning of survey and computed elevation (also at A) at the end of the survey.

3.6.3 Profile levelling

An important aspect in pond design and construction is devoted to control of water movement. Water must be conveyed in desired direction and at controlled velocities. To accomplish this, it is necessary to measure accurately differences in elevation along a definite line. Such line may be the centreline for a water supply canal or drainage ditch. The procedure in conducting profile levelling is illustrated in Fig. 3.20. The figure is the centreline of a freshwater supply canal for regulating salinity or other purposes in a brackishwater fish farm.

Profile levelling begins by setting up the instrument at a convenient location where several stations are visible. As BS is taken from the BM, then FS readings are taken as many as the instrument man can clearly read. When no more FS can be read, the level instrument has to be transferred. Before transferring, an FS is taken at the TP. At the new location of the level, a BS is taken from the TP, then additional foresights (FS) are again taken. The whole process is repeared until the work is finished. The column for elevations in the level note is completed by computations (Table 3.2).

There are only two HI's under the illustration. To compute for the elevation of the stations, the foresights from Station 0 + 00 up to 1 + 00 shall be subtracted from HI1. At TP1, a new HI must be determined. Elevations of remaining stations (1 + 25 to 2 + 00) are determined by subtracting the foresights from the HI2 of TP1.

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Fig. 3.20 Illustration of profile levelling procedure

Table 3.2The profile level note for Figure 3.20

STA, m BS HI FS Elevation Remarks

BM1 1.37 2.87 — 1.50

Beginning of canal on top of dike

End of canal

0 + 00 — — 1.53 1.34

+ 25 — — 1.67 1.20

+ 50 — — 1.73 1.14

+ 65 — — 1.90 0.97

+ 75 — — 2.05 0.82

1 + 00 — — 2.22 0.65

TP1 1.80 3.07 1.60 1.27

1 + 25 — — 2.27 0.80

1 + 50 — — 2.37 0.70

1 + 75 — — 2.57 0.50

2 + 00 — — 2.77 0.30

2 + 30 — — 3.00 0.70

From the completed profile level notes, the ground profile is drawn using a cross-section paper (Fig. 3.21). The elevation is plotted on the vertical axis, while the stations or distances represent the horizontal axis. The scale for elevations are usually made larger than the horizontal scale to emphasize differences in elevation. Example of information obtained from the profile are

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available ground slope for determining possible velocity in canal, and location and depth of cuts or fills.

Fig. 3.21 Profile of centerline of supply canal

3.6.4 Contour mapping

Contour lines show the configuration or changes in elevation of the ground in a topographic map. Each contour line represents points of same elevation and are spaced according to the difference in elevation between two adjacent lines. Hence, contour line (C.L.) 0.50 means that every point on that line has elevation of 0.50 m above the reference datum. If the next contour lines lower and above C.L.0.50 are 0.25 and 0.75, respectively, the contour interval is 0.25.

Topographic or contour survey is commonly done with three methods — the laying — out-square, random shot, and by sounding methods. These methods use combined knowledge of differential and profile levelling.

Accurate measurements of lengths and directions of the farm boundaries are done prior to taking rod readings during contour survey.

(a) Laying-out-square method. This method is done by literally setting up squares in the area with the use of either tape or transit. Each intersection or corner in the square is marked with stake and represents ground point where rod reading is to be taken. Proper identification of ground points must be done for proper note keeping and plotting during mapping. Identification is usually done using numbers in one direction and letters in the other direction (refer to Fig. 3.22). The format for data recording is given in Table 3.3

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Fig. 3.22 Laying-out square method

Table 3.3Format for the level note for laying-out-square method

Station BS HI FS Elevation Remarks

BM          

A1          

A2          

A3          

An          

B1          

B2          

60

B3          

Bn          

C1          

etc.          

Note: “n” refers to the last number of observation

The above menthod is appropriate for small areas with less vegetation. With this method plotting of points on the scaled map is also easy. For large areas, however, laying out squares in the field becomes laborious and impractical.

(b) Random shot method. This method has more advantage than the preceding method when large areas are involved. Field survey is done faster, but plotting of ground points on the scaled map may take longer time since it involves measurement of angles and distances. A transit equipped with stadia hairs is best to use in this method. However, frequent check of the transit telescope level need to be done to minimize error. A plane table with telescopic alidade can also be employed in this method.

The sequence of field survey activities in the random shot method is as follows:

(i) The first step consists in measuring the boundaries of the field to be surveyed, determining the length, directions and angles of all sides.

(ii) The instrument stations have to be established in the area. The locations of the instrument position on the ground must be fixed so that it can be accurately plotted on the map. The instrument stations can be established either separately and ahead of the actual taking of rod readings or simultaneously as the work progresses. However, when the work is quite large, it is better to establish the instrument stations separately together with additional turning points or bench marks as necessary.

(iii) The instrument is set at Instrument Station 1 (or simply Station 1). Before random shots are taken, the location of Station 1 should be fixed or tied-up with at least two concrete corner points or monument of the land boundary. This is necessary in locating the positions of the instruments during map construction.

The location of point on the ground is fixed if measurements are made of: (a) its direction and distance from a known point: (b) its direction from two known points: (c) its distance from two known points; and (d) its direction from one and distance from another known point.

The position of the instrument at Station 1 in Fig. 3.23 is fixed by the two lines connecting Station to boundary monuments at points A and B.

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(iv) With the instrument set at Station 1, a reference line for the radiating random shots is selected. This reference line can either be the North-South line of the instruments' compass or the line connecting two adjacent instrument stations. The illustration in Fig. 3.23 uses the line between Stations 1 and 6 as the reference line or the zero-degree (0°) line. To take random shots (rod readings) the transit telescope deflects incremental angles in clockwise direction from the 0° — line. Incremental angles of 3°, 5°, 10°, etc. may be used depending on the situation and convenience. Along the line of sight of these incremental angles, random rod readings are taken as many as necessary. Each rod reading, however, must be accompanied with corresponding angular reading and distance of the ground point from the instrument measured by the stadia method. Once the 360° - turn is completed, the instrument is transferred to Station 2. Before going to Station 2, a FS is taken from the selected TP1. Once the instrument is set at Station 2, a BS is taken from the TP1. At Station 2, the ground points not covered in Station 1 are now taken using the same procedure. The survey proceeds until the whole area is covered with radiating shots. Fig. 3.23 illustrates the general procedure in the random shot method.

Fig. 3.23 Illustration of instrument stations within an area and the random shots for each angle

(v) Once the field survey is completed, the elevation of all ground points are computed to complete the suggested format for level notes (Table 3.4).

(vi) Construction of the topographic map follows. A suitable scale to be used is selected based on the size of the paper and purpose of the map. Then the boundaries of the land is drawn to scale, followed by the loop of the instrument stations. Radial lines corresponding to the incremental angles at each station is drawn by using a protractor; distances along each line corresponding to the ground points are also plotted to scale.

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The computed elevations are then noted on their respective ground points. Each building, road, creek, river, etc. should also be located with the proper symbols. Contour lines are then drawn out in order to show the topography of the land.

(c) Contouring. The contour interval (C.I.) to be used must be decided on before drawing the contour lines. The C.I. depends on the desired accuracy and purpose of the contour map. Generally, however, wider C.I. is used in area with rugged topography and closer such as 0.15, 0.25, 0.5 m for flat or gently sloping area as in the case of most fish farm sites. Examination of Fig. 3.24 provides better understanding on how to construct a contour map. The first contour line to be drawn may be based either from the lowest or highest ground elevation. Take note that a contour line always passes between ground points with values lower and higher than the value or number of the given contour line. Elevations denoting each contour line must be indicated, otherwise it does not mean anything.

Fig. 3.24 A contour map

Table 3.4Format for the level notes for random-shot method

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StationBS HI FS

Station distanceElevation Remarks

Upper Lower Distance

BM                

Instrument 1                

0°1                

    2                

    3                

    n                

5° 1                

    2                

    3                

    n                

10° 1                 

    2                

etc.                

The location of each contour line as it passes between points on the map can be determined by inspection or eyeballing and more accurately by interpolation method.

(d) Characteristics of contour lines. Certain knowledge on characteristics, of contour lines will facilitate contouring as well as its interpretation. Some of these characteristics are as follows:

i. All points on a contour line are of the same elevation.ii. Contour lines always close to itself within or outside the confines of the map. Where they

close outside the boundaries of the map, they are ended at the boundaries.

iii. Contour lines that close within the confines of the map represent either a rise or a depression.

iv. Contour lines regularly spaced on the map represents uniform slope.

v. Contour lines never intersect nor cross each other except in the case of an overhanging cliff.

vi. Contour lines never branch nor split.

vii. Contour in crossing a valley run up the valley at one side, cross the stream, and run back on the other side.

viii. The steepest slope is always at right angles to the contours.

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3.6.5 Topographic survey by sounding

It is a common situation in coastal fish farm site to conduct topographic survey on an area that is under shallow tidal water. The usual method used is by sounding. The following describes the procedure on how the survey is done by using magnetic (surveyor's) compass, stadia/levelling (sounding) rod, tape and two small boats. One boat is for the instrument (compass) man and the other for the rod man.

(a) The predicted tide curve for the day the survey is conducted is prepared in advance. This curve provides tide level information at any time of the day.

(b) The surveying party establishes the boundaries of area under survey as well as fixing location of some landmarks or ground points with reference to corner points of boundary lines.

(c) The instrument man at point A (refer to Fig. 3.25) locates his position on the map. Whenever appropriate, the instrument man chooses to tie-up directly from the corner points of the boundary or from fixed landmarks such as trees, in order to locate his position on the map. His position is located by taking down the bearings of at least two of his “tie lines” (i.e., the corner points or landmarks). The bearings obtained when plotted will intersect at one point which defines the position of the instrument man. The actual distance of the boat from the “tie lines” may be measured by tape to check. It is assumed that the water is shallow enough to permit actual measurements by tape. Otherwise, distance is measured approximately while on boat.

(d) Then the instrument man signals to the rodman that he is ready to sight him (the rod). The rodman holds the rod straight down to the ground surface (under water) at ground point (GP) 1 and the instrument man gets the bearing of GP 1. Rodman records the water surface level or depth of water at GP1 and the corresponding time. The process is complete after taking the distance by tape between points A and GP1.

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Fig.3.25 Illustration of topographic survey by soundings

At this point, the ground surface elevation at GP, can already be computed as follows:

Elevation of GP1=

Height of tide (from predicted tide curve) at the time rod reading is taken minus the rod reading at GP1.

The location of GP1 is located on the map by plotting its bearing and distance with reference to point A.

(e) The rod man at GP1 then moves to another ground point, GP2 and the instrument man at A reads the bearing, the rodman records the depth and time; and the tape men measure the distance from A to GP2. This process is repeared until such time the rodman has covered the area that is visible from the instrument man.

(f) The instrument man moves to another position B. The instrument man now at point B backsights points A to get the back bearing of BA. Similarly, the tapeman measure the distance between AB. Hence, this locates point B on the map. Elevations of other ground points not yet covered at point A are subsequently taken using the same procedure. A sample of field survey data is presented in Table 3.5.

3.7 Ecological (environmental) survey

After a site has been selected, there are more information (in addition to those already gathered during the site evaluation) needed which are useful in the project development. It should be remembered that the site may have some strong and weak points during the evaluation. During the detailed planning and design, it is therefore necessary to identify the positive and especially the negative factors that would affect productivity of the cultured species, in order to make the necessary modifications in the engineering design and cultural management practices.

Among others, the following need more in-depth attention and evaluation:

3.7.1 Water quality

More information on the physico-chemical and microbiological characteristics of the water should be obtained. The water temperature, pH, dissolved oxygen, biological oxygen demand (BOD), nutrients (NO3, P2O5, etc.), salinity, pollution and other information such as COD and measures of productivity must be looked into in detail. These values, aside from being used in design process, would also help in evaluating the environmental changes once the site has been cleared and the project established.

There should be at least two samplings done. Once at minimum and once at maximum tidal range. If possible, this sampling should also be done at different seasons. This allows calculation of data for intermediate ranges. Water samples or measurements should be obtained at least from

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the surface, mid-length and near the bottom of the sampling site. On each sampling occasion, observation may be done at each depth at hourly interval over a tidal cycle.

3.7.2 Salinity in rivers and canals

The normal salinity during high and low tide at different seasons of the year should also be known. The following are of particular importance and should be known: (i) the rate of flow and velocity in the rivers or canals during each season in relation to saltwater intrusion and formation of saltwater-freshwater wedges; (ii) the length of wedge from the river mouth, the depth of saltwater and freshwater layers within the wedge, and salinity values along the depth with wedge region; (iii) along with these, the frequency and duration of freshwater flooding. This is important to know if there is a need to induce mixing or destroy the wedge to alter the salinity; to minimize siltation along the river or canal, or in selecting the site for gate construction and others.

Table 3.5Sample field data on survey by soundings

Station

BearingDistance

(m)

Time(hr.min.)

Height of tide

tide (m)

Elevation

(m)

RemarksInstrument ground point

Water depth

(m)

A   1 0.92 N35°E 20 10.00 1.07 0.15  

    2 0.70 N60°E 33 10.15 1.05 0.35  

    3 0.39 S45°E 28 10.35 0.99 0.60  

  BA   - N70°W 87 10.46 0.94 -  

B   1 0.40 N50°W 25 10.50 0.90 0.50  

    2 0.43 N45°E 35 10.55 0.85 0.42  

    3 0.09 S52°E 20 11.10 0.77 0.68  

    4 0.05 S65°W 30 11.15 0.74 0.69  

3.7.3 Tidal range, currents and prevailing directions

Verification of tidal fluctuations and currents is desirable. Particular attention should be given to changing winds and current patterns at different times (wet and dry seasons) of the year.

The added information on prevailing currents are useful in the planning and designing of erosion control measures to protect dikes or gates; finding out possibility of siltation or sedimentation in water control structures and how to solve the problem such as building of sediment trap within the water supply system and so on.

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3.7.4 Biological

(a) Natural food. Abundance of food organisms in aquatic environment are indicators of productivity and considered the most valuable resource for extensive fish farming. The composition of plankton as well as some macrophytes are the major groups of food organisms that are of interest.

Plankton tows and netting should be done at some selected survey stations. Three net sizes, with mesh openings of 0.063 mm, 0.21 mm and 1.0 mm and towing speeds of 1.5 to 3, 3 to 5, and 6 to 8 km per hour will retain the majority of phytoplankton and the smaller and larger mesoplankton. All samples taken during plankton tows and netting should be preserved for subsequent identification and analysis.

(b) Bio-fouling. Any structure constantly immersed in water are susceptible to marine fouling and boring organisms. The rate and severity of attack depends on the kind of materials, location of the structure in relation to depth and hydrographic conditions and biological conditions in the area.

The foulers are grouped as microfoulers and macrofoulers. The latter should be given more importance than the former because macrofoulers include most of the “borers” such as shipworms (Teredo), bivalves (Martesia), gribble (Limnoria), and such encrustations as tubeworms, barnacles, bryozoans, algae, hydroids, mussels and oysters.

Among the borers, wood boring organisms is a factor in deciding the type of material to use in sluice gates. Examination of the kind of organisms should be done and evaluated if it is a problem. Old pieces of wood stuck in the ground, or wooden boats of residents in the area should be examined to determine which group of borers cause the damage.

(c) Seed resources. Continuous supply of fish seed (fry) is imperative in fish farming operation. In addition to ascertaining the supply from hatcheries or dealers, and assessment of local resource, evaluation of the availability and seasonality of cultivable species other than the identified species for culture should be surveyed. Assessment should also be done on the impact of clearing the mangrove on the local seed resource.

(d) Predators and competitors. Predators prey upon other animals while competitors may compete for food and space in the pond. Hence, these two are important factors affecting water and cultural management practices. The kind of screenings and pre-stocking procedures must be effective to control predators and competitors.

Examples of some predators that must be checked and evaluated are given below. Aside from the finfishes, benthic community in the site must also be examined. The quantified data on the benthos is desirable.

Some known predators and competitors of milkfish in the region are: Mozambique tilapia (Tilapia mossambica); tarpon (Megalops cyprinoides); ten pounder (Elops hawaiiensis); apahap

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(Lates calcarifer); erong-erong (Therapon jarbua; Therapon theraps); Indian white shrimp (Penaeus indicus); green tiger shrimp (Penaeus semisulcatus): yellow shrimp (Metapenaeus brevicornis).

(e) Vegetation. This is well discussed in Chapter 2.

CHAPTER 4LAYOUT DESIGNS FOR COASTAL FISH FARMS

4. COMPONENTS AND LAYOUT PLAN OF POND SYSTEM

Fish farm layouts that are properly engineered should strike a balance considering economy, functionality and aesthetics. Within a prescribed production management scheme, the layout must be economical. The basic principle is to minimize the number of gates, and the size and length of the main secondary and tertiary dikes and canals. But this should not sacrifice the biological requirements for suitable environment of the cultured species.

Fishponds should be planned in such a way that the length of the pond is positioned parallel to the prevailing wind direction. The wind direction in Southeast Asia is shown in Fig. 4.1 and the suggested orientation of the ponds is seen in Fig. 4.2. As such, the length of dike exposed to wave action is lessened, thus, the cost of repairs also less. The position also takes advantage of the wind energy in effecting good water aeration through mixing and circulation. However, in areas where very strong winds are prevalent, wind breakers are included in the design and layout of ponds.

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Fig. 4.1 Wind direction in Southeast Asia(After Tiensongrusmee, 1982)

Fig. 4.2 Layout of pond compartments oriented to the prevailing wind direction(After Tiensongrusmee, 1982)

4.1 Components of a fish farm

In general, the fish farm is an establishment which is composed of pond system and support facilities. The pond system usually consists of various compartments with specific uses such as nursery or fry pond, transition or holding stunting pond, production or rearing ponds and other features (catching, desilting food-growing ponds, etc.). Also a part of the system are the water

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control structure or gates, pipes or culverts and water supply or drainage canals. Each of these units should be properly located and fitted in the system in order to have ease in water management and manipulation of cultured stock.

On the other hand, support facilities consists of farm buildings, farm roads and road dikes, bridges, fish tanks, storage shed (for feed and equipment), chilling tanks, and other ancillary structures. Efficient organization of support facilities in relation to the pond system is of paramount significance in the overall developmental planning and operation of the farm.

Fish farms are located at convenient distance from the sea or river. In the Philippines, a sanctioned buffer zone of at leat 100 m from the sea to the main perimeter dike and 20 m along river banks is spared for ecological consideration as well as physical protection against flooding and wave action. The required buffer zone along the shoreline in Indonesia is 400 m.

4.2 Types of pond compartments

Existing fishpond layouts, especially for a milkfish farm may have all or just a number of the following compartments depending on the layout requirements as dictated by the management scheme and cultured species.

4.2.1 Fry acclimatization pond

Sometimes called fry box this is the smallest unit in a pond system usually 4 to 8 m2. Fry are first stocked in this pond for 1 to 4 days and then allowed passage to the nursery pond proper by just cutting open the small dike partition (Djajadiredja and Daulay, 1982).

4.2.2 Nursery pond

The nursery pond is small in size, about 1 to 4 percent of total production area and usually square or rectangular in shape. It may be a single pond unit or made up of two, four, six, etc. sub-compartments which form the whole nursery unit. A manageable area ranges from 500 to 10 000 m2 per compartment, although 1 000 to 5 000 m2 is preferred (BFAR-UNDP/FAO, 1982).

The nursery is used for rearing the fry for at least 30 days (in the case of milkfish) before transferring into another larger pond. Rearing the fish in small area is more convenient and safer as it can be watched more closely and taken cared of more adequately.

Nursery pond should be located in elevated portion of the farm in the central or near the corner of a rearing pond compartment (Djajadiredja and Daulay, 1982). The most suitable place is where it can be easily supplied with new unpolluted water at all times when necessary and at elevation where it can readily be drained even during ordinary low tides (Alcantara, 1982).

Avoid locating nursery ponds directly adjacent to perimeter dikes. Crab holes and leaks that might occur during the rearing period will serve as exits of fry from the nursery pond to the river.

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These can also serve as entrance for predators and unwanted species into the nursery pond, causing further loss of stock.

From the nursery pond the fry is moved into the transition pond or directly into the rearing or production ponds.

4.2.3 Transition pond

The transition, holding or stunting pond is located adjacent to the nursery pond in order to have efficient and quick transfer of fingerlings. Depending on the management scheme, close to 10 percent of the total production area is usually allocated for this purpose. The fingerlings or post-fingerlings are reared here for varying periods before finally stocking them in the production or rearing ponds. The fish can be retained in the transition pond longer or up to a few months especially when the number of fry stock is sufficient for several cropping within the year. A manageable area for transition ponds ranges from 1 000–20 000 m2 per compartment but 5 000–15 000 m2 is preferred (BFAR-UNDP FAO, 1981).

4.2.4 Production or rearing pond

This is also called growout pond. It is the largest compartment in the pond system occupying about 80 percent of the total farm area.

The bottom elevation of the rearing pond should be about 0.2 m lower than that of the transition pond but slightly higher than the Mean Lower Low Water (MLLW) or zero tidal datum. The pond bottom slopes toward the catching pond or water supply canal to facilitate harvesting of marketable-sized fish. A manageable size ranges from 1.0 to 10 ha per compartment although 2.0 to 5.0 ha is preferred. Production ponds for milkfish of 15 to 20 ha per compartment is common in the Philippines.

4.2.5 Catching pond

This pond serves as a concentration area or basin for the fish during harvest. It is constructed adjacent to the gate inside a bigger pond compartment. Catching ponds may be provided also for nursery ponds, transition ponds, and rearing ponds. The catching pond for the nursery and transition ponds is usually about 2 percent of the respective compartments' water surface area; for rearing pond, it is usually 1–1.5 percent.

4.2.6 Food growing pond

This pond is optional and may be built, if deemed necessary. Named “kitchen pond”, it is a compartment set aside for growing live food organisms at high density. this is a recent innovation and is intended to augment the availability of food in fishpond areas where natural food organisms does not grow well or in farm set-up where high density stocking of cultured fish is used.

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4.3 Layout of pond system

The simplest form of pond layout is that of a single compartment. More recently, improved layouts consisting of multiple combination of compartments have come to general use. Through the years of experience in pond fish production the pond operators have evolved and developed the arrangement and relative proportion of the various pond compartments that would fit into the system together with the appropriate production management scheme.

4.3.1 Suitability of layout for cultured species

Pond layouts may be grouped into: (i) conventional; (ii) radiating; (iii) modular or progression; and (iv) multiple stock/harvest pond system (BFAR-UNDP/FAO, 1981 and Denila, 1976). Examples of these layouts are shown in Figs. 4.3 to 4.7 and Figs. 4.17 and 4.18. All of these, however, are intended for milkfish production and in general maintain shallow water that is required by fish food called “lab-lab” (a complex community of micro-benthic biota closely associated with pond bottom). However, combination of deep-water for plankton production and shallow water for lab-lab production is also being practiced. The basic characteristics or differences of these layouts are shown in Table 4.1.

Table 4.1Comparision of various layouts of milkfish ponds

 Layout scheme

(production: kg/year)

Nursery pond Transition pond Rearing pond

1.Conventional(1 000–2 000)

1 percent of total production area

9 percent of total production area

80 percent1 of total production area

2. RadiatingSame as above

Same as above Same as above

3.Modular(1 800–3 000)

4 percent 6 percent

80 percent1; there are three production process stage; each stage follows a ratio 1:2:4 or 1:3:9 (Figs. 4.6, 4.17 and 4.18)

4.Multiple stock/harvest(1 000–2 000)

6 percent

No transition pond; instead holding canal for fingerlings is allocated for each rearing pond; 19 percent of every rearing pond

 

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1 Some 10 percent is used for canals, catching ponds and dikes.

Fig. 4.3 A conventional pond system with catching pond (CP), nursery pond (NP), transition pond (TP), feed pond (FP) and rearing pond (RP) (After Alcantara, 1982)

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Fig. 4.4 Radiating type layout showing transition pond (TP) and rearing pond (RP) (After Denila, 1976)

Fig. 4.5 Radiating layout of Taman and Porong types of milkfish farm with division pond (D); rearing ponds (A,B & C); fry pond (E) and canals (After Djajadiredja and Daulay, 1982)

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Fig. 4.6 A modular pond system in the Philippines showing rearing pond stages (RPS) with ratio of 1:2:4 and 1:3:9 (After Alcantara, 1982)

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Fig. 4.7 Layout of a farm by multiple stock/harvest system showing fish holding canal (FHC) as added feature (After Alcantara, 1982)

The difference between the conventional and radiating type of layout is the presence of much longer canal and secondary dikes in the former than the latter. The short supply canal of the radiating layout is desirable from the viewpoint of economy in dike construction. It also serves as catching pond. However in the case of Indonesia, a division pond (D) precedes the production or rearing ponds (Fig. 4.5) instead of supply canal.

For most of the layouts, the space occupied by the partition and canal dikes is approximately 10 percent; this is exceeded when large dikes are constructed.

Thailand concentrates more in shrimp culture with practically no milkfish culture. Pond layout for shrimp in this country vary depending on the levels of inputs and rearing methods as traditional, semi-intensive and intensive shrimp ponds. Although inner canals are occasionally found in milkfish farm layout (as in Indonesia), shrimp ponds are characterized by the presence of extensive inner canal system. Transition pond is generally absent unlike in milkfish farm (Fig. 4.8).

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Fig. 4.8 Layout of the traditional shrimp pond in Thailand (After Chalayondeja, Thornbuppa and Sikga, 1982)

The traditional shrimp pond usually has shallow depth of water of 70 to 90 cm with one inlet water gate at one end and one outlet gate in the other end. The production is usually 25 to 90 kg/ha/year (Fig. 4.9). This traditional pond is modified by constructing larger ditches, higher dikes and increasing water depth to 100 to 150 cm, and hence, the size of pump (Fig. 4.10). By doing so, production has increased by 200 to 300 kg/ha/year.

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Fig. 4.9 Layout of a modified traditional shrimp pond; N,nursery and gates (inlet, G1 and outlet, G2) (After Chalayondeja, Thornbuppa and Sikga, 1982)

The semi-intensive pond has a removable nursery pen in the rearing pond where post larvae are kept for one month, then released into the pond.

The intensive ponds also have nursery ponds or pens which are also constructed within the rearing pond. Supplementary feeding and aeration are necessary for this type of rearing. Intensive ponds are generally small in area. Production from an intensive pond of 1 to 6 ha with ditch of 8 to 10 m wide and 1.5 m deep and with a water level above the berm of approximately 75 cm is 1 000 to 5 000 kg/ha/year (Figs. 4.11 and 4.12).

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Fig. 4.10 Layout of an intensive shrimp pond with nursery pens (N), inlet gate (G1), and outlet gate (G2)(After Chalayondeja, Thornbuppa and Sikga, 1982)

Fig. 4.11 Layout of an intensive shrimp pond of 3 ha. consisting of three rearing ponds (R) and three nursery ponds(N), and provided with separate intake and discharge gates(G)(After Chalayondeja, Thornbuppa and Sikga, 1982)

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Fig. 4.12 Indicative layout for a 5-ha shrimp monoculture project(After Esquerra, undated)

4.3.2 Layout appropriate for prescribed management method

The requirements of the cultured species is always the basis for planning the layout and formulation of management scheme. Within limits, however, management techniques can be manipulated to enhance production without affecting the normal growth of species.

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The modular or progression system is a typical example of a layout wherein the management scheme involving fish movement in the various compartments are prescribed. In this system, specified number of milkfish fingerlings from the transition pond are stocked in the smallest production pond, then moved to the next bigger then to the largest pond. Movement of fish in each production pond stage can vary but is usually done in about 30 to 45 days. When inputs and conditions for normal growth exist, by this time the weight of fish stock has at least double, hence, movement to an area twice as large than where the fishes are, is logical. This enables the fish farmer to make four to six harvests per year with food growing period of 2 to 4 weeks between crops.

The multiple stock harvest system involves stocking of two to four different size groups of fish at different times in the pond. After 20 to 45 days, the large ones are harvested by gillnet or by netting selectively the fish swimming against the current during water inflow known as “pasubang” method in the Philippines. Another batch of small fish replaces the harvested ones. Repeated harvests, thereafter, is done every 30 to 50 days.

Because of this prescribed management method, fish holding canal (FHC) for each rearing pond is added in the layout, instead of transition pond. This is to insure availability of designated size(s) of fish for the rearing of ponds (Fig. 4.7).

Another example is a flow through system of shrimp culture. The rate of water exchange is regulated depending on the density of stocking. Water must be available any time irrespective of the tide cycle; hence, a combination of pump and reservoir system (using a headpond) or just a pump system should be provided (Fig. 4.10 and 4.11). Gates and canals are also strategically located to effect good movement and circulation of water.

4.4 Location of gates and water supply/drainage canals

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Fig. 4.13 Indicative layout for a 5-ha shrimp monoculture project(After H.R. Rabanal, Personal communications, 1983)

When necessary such as in intensive shrimp monoculture farms, each pond compartment should have individual water supply and drainage outlets to make them independent from each other (Fig. 4.13). The location of water control gates depends primarily on the water management

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scheme. In general, main gates and secondary gates are positioned where entrance and circulation of water could be most efficient. Ordinarily, a single gate per pond connected to a canal provides passage for tidal inflow and outflow. In a flow through system, two gates located in opposite ends of the ponds are required. Although slightly expensive, narrow rectangular slope is desirable from the viewpoint of effective water exchange. Likewise, separate canals that accommodate inflow and outflow of water from the gates are provided (Figs. 4.9 to 4.14). Canals should be located where it could connect or serve the most number of pond compartments. The lengths of canals should be minimized without sacrificing the functionality of the pond and intended management scheme.

Water control gates and canals that are properly located provides ease in water management and reduces operational costs.

4.5 Other facilities/features in pond system

4.5.1 Peripheral, central or diagonal ditch

The desirable temperature for milkfish and shrimp ranges from 27 to 32°C and 28 to 30°C, respectively. During the dry season, the water temperature may increase, especially in the shallower part of the pond. Providing canals inside pond compartments deeper than the general pond bottom remedies the situation and serves as a hiding place for shrimp during critical pond condition. These canals are also suitable in milkfish ponds with generally shallow water, where polyculture with shrimp is desired. The ditches can vary from 0.5 to 1.0 m in depth (Fig. 4.15).

4.5.2 Division pond

The use of division pond is popular in Indonesia. this compartment distributes the tidal inflow to the various ponds and provides independence in the operation of individual pond compartment. It is a common feature for rearing ponds (Fig. 4.5) and even in nursery farm systems (Fig. 4.16).

4.5.3 Reservoir pond

This is appropriate for the flow through system in shrimp culture. The pump raises the level of water in the reservoir even during low tide so that gravity flow through in the rearing pond of shrimp can be effected.

4.5.4 Sedimentation basin

This may be located near the water source before incoming tide enters the ponds. It is intended to settle suspended solids carried by the inflowing water.

4.5.5 Chilling tank

Wooden or concrete tanks with capacity of 1 to 5 tons are usually constructed near the catching pond. Newly harvested milkfish are dumped and immediately covered with crushed ice to chill

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them to preserve their quality and freshness. This serves also to wash the fish and reduce bacterial growth.

4.5.6 Road system

It is advisable to have road system which should reach at least the main gate and catching ponds for easy and cheap transportation. This can reduce marketing cost.

4.5.7 Housing site

some space is to be set aside for houses of persons employed and as storehouses for feeds, equipment and other fish farm materials.

Fig. 4.14 Indicative layout for a 5-ha shrimp monoculture project(After H.R. Rabanal, Personal communication, 1983)

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Fig. 4.15 Indicative layout for a 10-ha milkfish/shrimp polyculture fish farm (After H.R. Rabanal, Personal communication, 1983)

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Fig. 4.16 Layout of Jakarta and Kamal types of milkfish nursery with division pond (dp); fry pond (fp); transition pond (tp); and canal (c)

(After Djajadiredja and Daulay, 1982)

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Fig. 4.17 Indicative layout for a 10-ha milkfish monoculture grow-out project(After H.R.Rabanal, Personal communication, 1983)

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Fig. 4.18 Indicative layout for a 10-ha milkfish monoculture grow-out project (After H.R.Rabanal, Personal communication, 1983)

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CHAPTER 5DESIGN OF FISH FARM PHYSICAL STRUCTURES

5. DESIGN OF FISH FARM GATES AND POND SYSTEM

5.1 Design tide curve and elevation of pond bottom

The local pattern of tide curve upon which fishpond design is based can be obtained from an analysis of the tide record. It is also referred to as design tide curve. The design tide curve is drawn along the zero datum level which is usually the Mean Lower Low Water (MLLW). Adoption of inappropriate design tide curve can cause problems in the management of the pond after construction is completed.

The bottom elevation of ponds is the primary consideration in the design and is determined based on the design tide curve (Figs. 5.1 and 5.2). Primary consideration should be given to both the biological needs of the cultured species and construction aspect such as the minimum and maximum water level to be maintained in the pond and adequate flow of water into the ponds. On the economic side of construction, the elevation of pond bottom should strike a balance between the excavation or filling work and the tidal range. For a site with relatively high ground elevation, it is possible in a specific instance that pumping water to the ponds can prove to be more economical than excavating the soil to the desired elevation (Gedney, Shang and Cook, 1983).

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Fig. 5.1 Relation of tide curves to design elevations of a fish farm at the Sungai Merbok estuary, Malaysia

(After Hechanova and Tiensongrusmee, 1980)

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Fig. 5.2 Relation of tide curves to the different pond elevations (Modified after Denilo, 1976)

The elevation of other structures such as gates, canals and dikes are also based on the design tide curve and these should fit properly to the water management and operational requirements of the ponds. An example of representative tide curve which applies to the Ban Merbok estuary in Malaysia is given (Fig. 5.3).

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Fig. 5.3 Representative tide curve (Mean High Water Spring) referred at Tanjong Dawai, Secondary Port, Ban Merbok Estuary, Malaysia(After Hechanova and Tiensongrusmee, 1980)

5.2 Design, specifications and components of main water control gate

5.2.1 Components of water control gates (Fig. 5.4)

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Fig. 5.4 Parts of a main gate (double opening) made of reinforced concrete (Modified after Jamandre and Rabanal, 1975)

(a) Floor. The floor serves as the foundation of the structure and its elevation for main gates must be lower than the pond bottom elevation and as low or slightly lower than the lowest tide in the site. If this is observed, the main gate which rests on a prepared foundation support will not be exposed even during extreme low tides.

(b) Apron. This is the broadened and extended part of the floor and also generally rests on the foundation piles, which are made of seasoned bamboo or wood driven at 0.3 m intervals into the

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soft soil with the butt end up. The apron serves as protection to scouring and future seepage of water at the gate's sides.

(c) Cut-off-walls. Cut-off-walls are provided at both ends of the gate's floor to prevent seepage and undercutting of water within the gate's foundation. They extend down into the soil at a minimum of 0.60 m and are an integral part of the gate's foundation. Wooden sheet piles may be used as an extension of concrete cut-off-walls in order to reach deeper depths at reduced cost.

(d) Side or breast walls. Side walls define the sluice way in addition to their being retaining wall for the dike fill. Grooves or double cleats for flashboards and screens are built on these walls. The top of these walls are as high as the top of the dike.

(e) Buttress. This is built against the side walls to support or reinforce it. It also helps in reducing seepage flow along the side walls.

(f) Wing walls. Wing walls provide the transition from the sluice way into the main canal in addition to retaining the earth at both sides of the gate. This transition improves the condition of the flow by providing a control on flow velocities from one bed material to another.

(g) Bridges or catwalks. These are reinforced concrete slabs or thick wooden planks that span the side walls. At least three catwalks are provided, two at each end and one at centre near the flashboard grooves.

(h) Flashboards. Slabs or flashboards are generally wooden planks, 2.5 to 5 cm thick and 30 cm wide inserted into grooves or double cleats. They are used to control the amount of water flowing through the gate.

(i) Screens. Screens are usually made of bamboo strips or of fine polyethylene meshes attached to a wooden rectangular frame that fit into the grooves. The screens are used to prevent the exit of the cultured fish and the entry of predators into the ponds.

(j) Pillars. In wooden gates, these are vertical supports where wooden walls are nailed. They are placed at regular intervals so that they form a framework for the gate itself.

(k) Braces. In wooden gates, these wooden members hold or fasten two or more pillars together or in place. They keep the opening of a gate rigid.

5.2.2 Main gate

The main gate links the pond system to the source of water. It regulates the exchange of water between the pond system and the tidal stream or sea. Instead of wood, it should be made of concrete for effective control and to last longer. The main gate is usually situated at the central side of the proposed fishpond facing the source of water. The following provides some information needed in the design of the main gate.

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(a) The floor elevation of the main gate should be lower than the lowest pond bottom elevation desired inside the pond system. It should also be as low or slightly lower than extreme low tides (Fig. 5.5). The front end or riverside elevation of the floor or apron may be made lower if a pump is to be installed (Fig. 5.5).

(b) The height of the main gate depends upon the highest tide and flood and should be the same as the elevation of the main dike which is also dependent upon the tidal fluctuations, floods and other factors in the area.

Fig. 5.5 Detail of main gate with pump sump (Reinforcement not shown)(After Hechanova and Tiensongrusmee, 1980)

(c) Main gates may have single, double, or triple or even quadruple or more openings (Figs. 5.6 to 5.8). The opening of the main gate depends upon the area to be flooded. Gates should not have too wide opening so that they would be difficult to manage. One to 1.2 m wide per opening has been found appropriate for easy handling of wooden slabs and screens. Wider opening than these may require lifting mechanisms in the operation of the gate. Experience in the Philippines shows that a single

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opening of 1.0 m for a concrete main gate could flood a 10 to 15 ha pond system in a milkfish farm in two or three successive high tides. This opening, however, does not necessarily apply to shrimp farm because of the difference in water depth requirement between shrimp and milkfish. Based on the computations done (Appendix B), a gate with three openings and 1.2 m wide each would fill an 11.35 ha shrimp farm to a minimum depth of one meter in two consecutive tide cycle. For much larger areas and deeper ponds, a double or triple opening-gate of proper width may be constructed at one or more spots along the perimeter dike.

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Fig. 5.6 Main concrete gate single-opening

(After Lijauco, 1977)

Fig. 5.7 Main concrete gate double-opening (After Lijauco, 1977)

Fig.5.8 Sample of a triple-opening concrete main gate

(d) There must be a separtate groove for the slabs and screens. It may be necessary to have four pairs of grooves; two pairs for slabs and two pairs for screens (one at each end) depending upon their use.

(e) The wings should be properly designed to provide easy current flow. The best angle of flare should be 45° towards the outside. This angle may not be necessarily the same for both ends of the gate.

(f) The gate foundation must be rigid and stable. It must be able to carry the whole weight when the gate is fully constructed. There are two designs of gate foundations in use — one has the floor and apron of gate resting on a combination of piles and layers of boulders and gravel. The other one uses piles alone to stengthen the foundation that supports the structure. The former design is common in the Philippines and Indonesia while the latter is found in Malaysia (Fig. 5.9 to 5.11).

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(g) Cut-off-walls and aprons must be provided. They must be wide enough to include portions susceptible to scouring and under-cutting of water.

(h) Adequate reinforcements against sidewise pressure must be provided. Spacing of steel bars should not exceed 40 cm centre to centre. The size of vertical bars should be 12 to 13 mm and 10 mm for horizontal bars.

Fig. 5.9 Foundation support and piling scheme (Philippines) (Portion of flooring only)

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Fig. 5.10 Foundation support plan and piling scheme (Malaysia) (Portion of flooring only) (After Khoo, pers. comm., 1982)

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Fig. 5.11 Foundation and elevation plan of concrete main gate (single opening) (After Denila, 1976)

5.3 Design of secondary and tertiary gates and other water control structures

5.3.1 Secondary and tertiary gates

These provide the control of water to and from the main canal and into the different pond components such as catching ponds, rearing ponds and nursery ponds. These structures are usually made of wood and can be treated with coal tar for durability. Single or double opening made of reinforced concrete or hollow blocks can also be used but it is sometimes too expensive (Figs. 5.12 to 5.14). Considerations in the planning and designing of secondary and tertiary gates are the same as those of the main gate except that their respective elevations are dependent upon the elevation of the canal bed where they are being constructed. The usual elevation of the flooring of these gates above the canal is 0.15 m. The flooring elevation of the farthest gate from the main gate should be checked against the design tide curve to insure that it still is capable of filling the pond within the prescribed time. The width of opening may vary from 0.6 to 1.0 m. Wing walls can be provided (Figs. 5.15 and 5.16) but some existing designs, especially the wooden gates, do not have these structures (Figs. 5.17 and 5.18). Anti-seep boards at the side of the gate is also a good feature (Fig. 5.16).

5.3.2 Culverts or pipes

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These structures convey water across dikes, roads, and similar embankments. A recent innovation for a smaller and less expensive gate is the use of culverts or pipes made of concrete hollow blocks or asbestos cement. Culverts or pipes gates may or may not have wing walls (Figs. 5.19 and 5.20) and are likewise provided with slabs and screens and are even more effective for water control in a fishpond, the conduit section may be circular or square in shape (Figs. 5.19 to 5.21). For low-cost design wooden culvert may be used (Fig. 5.22).

Fig. 5.12 Detail of a single opening secondary concrete gate(After Hechanova and Tiensongrusmee, 1980)

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Fig. 5.13 Details of secondary gates with double opening (After Hechanova and Tiensongrosmee, 1980)

103

Fig. 5.14 Concrete hollow blocks sluice gate (single opening - Indonesia)(After Djajadiredja and Daulay, 1982)

104

Fig. 5.15 Wooden sluice gate - with wing wall in two ends side braces and cat walk (After Djajadiredja and Daulay, 1982)

105

Fig. 5.16 Wooden main sluice gate (Top view) - with middle anti-seep board and wing wall in pond side (Modified after Jamandre and Rabanal, 1975)

106

Fig. 5.17 Secondary wooden gate (No wing wall)(After Jamandre and Rabanal, 1975)

107

Fig. 5.18 Detail of wooden tertiary gate, for nursery/rearing ponds(After Hechanova and Tiensongrusmee, 1980)

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Fig. 5.19 Cut-out diagram of concrete culvert as secondary gate (with wing wall)(After Jamandre and Rabanal, 1975)

Fig. 5.20 Concrete culvert (No wing wall)(After Djajadiredja and Daulay, 1982)

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Fig. 5.21 Design of a square culvert gate(After BFAR - UNDP/FAO, 1981)

110

Fig. 5.22 Wooden square culvert(After Djadjadiredja and Daulay, 1982)

5.4 Design and specifications of main or perimeter dike

The function of perimeter dikes is to retain water for use in the fish farming operation as well as to protect the farm ponds, fish crops and other farm facilities from destruction by floods and tidal inundation. Design of these embankments must be based on sound engineering principles and economic feasibility. The design of perimeter dikes are two types: (i) for exposed areas; and (ii) for protected areas (Figs. 5.23 and 5.24).

5.4.1 Location of perimeter dike

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Fig. 5.23 Sample designs of perimeter dike within the reach of coastal waves

Fig. 5.24 Sample designs of perimeter dike located along river or protected area

The perimeter dikes of a coastal fish farm is usually built along the river banks, on the seaward side or in certain spots that are vulnerable to flooding. In locating the dike, the Philippines require a belt of mangroves of 20 m from a river bank, and 100 m wide from seashore to be left for the purpose of protecting the dikes against waves and currents, and absorption of wave energy and, to some extent, for flood control and conservation of the environment. Indonesia requires 400 m of mangrove as green belt of trees along the shore.

The path of the dike is determined by survey to avoid (a) crossing of streams or creeks that have substantial rate of flow; (b) areas of extremely poor soil which result in high construction cost; and (c) locating the dike near an actively eroding line of rivers or coasts.

5.4.2 Cross-section of main dike

The cross-section of dikes is described by the crown or top width, height, side slope and the bottom width or base (Fig. 5.25). Modifications are made by providing berm and core or puddle trench (Fig. 2.6). Cross-section of the perimeter dike should be designed to: (a) prevent over-tapping at high tide combined with a maximum flood height from the river system; and (b) prevent failure due to slips and seepage.

Due to poor and soft foundation soil in coastal swamps, the embankment requires a process of consolidation before it becomes stable. Consolidation is a natural phenomenon and it occurs as a counterbalance between the settlement and bearing capacity of the foundation. Slip and collapse of an embankment constructed on poor ground occurs when irregularities or unbalanced conditions develop in the foundation.

(a) Determination of height

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The height of the dike should be above the highest tide and flood that occur in the site. The design flood level is based on the maximum flood water that was observed in the locality to recur within 10 to 15 years (in Chapter 2).

Fig. 5.25 Steepness of side of dike for different values of side slope (Horizontal and vertical ratio)

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Fig. 5.26 Design of different dikes(After BFAR-UNDP/FAO,1981)

The design height of dike should be provided with a freeboard after shrinkage and settlement of 0.3 to 1.0 m above the highest water level. Given below are the recommended allowance for shrinkage and settlement:

  ConditionAllowance for structure and

settlement (%)

1.Poor material and poor methods and practices in construction

15–30

2. Soil exceptionally high in organic matter 40 or more

3. Compacted by construction equipment 5–10

The total height of the main dike above the ground level can be computed by the following formula (see Fig. 5.27).

114

Where:

Hm =height of the main dike

  Hat =highest astronomical tide

  Gs =elevation of the ground surface

  Mf =maximum flood level

  F =allowance for freeboard

  %S =percent shrinkage and settlement

Fig. 5.27 Cross-section of perimeter (main) dike and basis for determining height

Example 5.1

It is required to determine the height of a main dike with the given conditions in the site as follows:

a. Highest astronomical tide — 2.04 mb. Elevation of the land — 0.91

c. Flood allowance — 0.30 m

d. Freeboard — 0.30 m

e. Settlement/shrinkage — 15%

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Solution:

Hm

=   2.04 m

(b) Determination of side slopes, crown and base

The dike should also be of adequate width so that it could hold water inside and prevent flooding from outside with appropriate side slope of 1:1 for clay soil and height up to 3.0 m. Side slope of 2:1 (horizontal to vertical) is used for height greater than 4.0 m and even flatter if located along seashore and being subject against wave action. If available soil permits, provision for a berm (single or both sides) is desirable for additional stability. The berm should slope towards the dike wall to trap eroded soil particles during rains. It also serves as small ditch that conveys runoffs towards the outlet gate especially when acidity of exposed dike is a management problem.

The top width or crown of the dike should be designed so as to serve its purpose. For dikes used as roadways, top width of 3.5 m to 4.0 m can be used but preferably 4.0 m. A 0.6 m wide allowance should be provided on each side of a roadway dike to prevent ravelling. In the Philippines, the desirable minimum crown for milkfish culture is at least 2.0 m for main dike.

Dikes subjected to wave action should have a minimum crown of approximately equal to the height of the maximum wave.

The base (without a berm) is computed in accordance with the width of crown and side slope as follows (Fig. 5.28)

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Fig. 5.28 Simple pond dike

  b = T + 2(zd)

Where: b = width of base, m

  T = width of crown, m

  d = height of dike, m

  z = horizontal value of side slope

Example 5.2

Referring to Figure 5.28, determine the width of base if the dike should have a top width of crown of 2.5 m, height of 1.3 m and a side slope of 1.5:1 (or z = 1.5).

Solution: Using the formula above:

b = 2.5 + 2(1.5) (1.3)

= 6.4 m

(c) Cross-sectional area and volume of dike

The cross-sectional area is estimated by adding the width of crown and base, divide the sum by two, and multiplied by the height. The height should be the estimated height for main, secondary or tertiary dike which includes allowance for shrinkage. The volume of soil required to construct the dike is computed by multiplying the cross-sectional area by the length of dike. In symbol,

 

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Where A = cross-sectional area of dike, m2

V = volume of dike, m3

L = length of dike, m

Example 5.3

From Example 5.2, the cross-sectional area and volume of a dike that will be constructed 1 250 m long are:

5.4.3 Leakage and seepage

To maintain a watertight pond, leakage should be given due attention in dike design and proper construction procedure followed. After the dike has been constructed, leakage is usually caused by the damage of the dike due to crustacean burrows, particularly the species Thalassina anomala (Tang, 1982). They make burrows in the soft mud under the dike thereby causing “piping” by which sand and silt particles are moved by seepage flow. Effective measures for preventing leakage include:

a. Minimizing the amount of seepage flow through proper compaction, core trenching, embedding vertical plastic membrane inside dike, covering dike wall with concrete bricks, riprapings, etc.

b. Minimizing destruction by crustaceans by desalinizing and drying out the embankment soils.

The rate of seepage through a dike can be estimated if the following factors are known: (i) the permeability coefficient of the dike; (ii) height of water level in the pond or canal; (iii) effective width of the dike; and (iv) the nature of dike foundation whether permeable or impermeable. Under normal conditions, the passage of water through a dike with impermeable foundation is only confined within the dike soil. When the foundation is permeable, the rate of seepage flow is the sum of the seepage passing through the dike itself and the quantity passing through the thickness of the dike soil foundation. In brackishwater fish farm, dike foundation is mostly, if not all, permeable. The appropriate formula for estimating seepage flow is given in Appendix E.

Since the permeability of alluvial clay is relatively constant, the increase or decrease of seepage flow in a coastal fish farm depends mainly upon the levels of tidal fluctuation and effective width of dike. The effective width of dike can be calculated by constructing the seepage line (Appendix E). For a value of permeability of the embankment and foundation soil of 1.32 cm/hour, water level in the pond maintained at 3.3 m, and tidal level of 4.8 m and 0.0 (datum level), estimates of seepage inflow and outflow through the dikes with three effective widths is given by Tang, 1976

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(Table 5.1). The table shows that as the effective width of dike becomes narrower, more seepage flow occurs. Hence placing linings of impermeable material is more needed in small dikes.

5.5 Cross-section of secondary and tertiary dikes

The secondary and tertiary dikes are smaller than the main dikes. Secondary dikes are usually provided on both sides of the canals and should be able to contain the mean high water springs. Tertiary dikes are partition dikes that separate the ponds and should be able to contain the desired water levels in the ponds.

Table 5.1Seepage flow through dikes with three effective widths

Dimension of dikesMinimum seepage inflow in

rising tidesMinimum seepage outflow

in ebb tides

Top width (m)

Height (m)

SlopeEffective

dike width (m)

Rate of seepage flow

(cm3/hr)

Effective dike width

(m)

Rate of seepage flow

(cm3/hr)

2.0 2.0 3:1 11.0 0.13 14 0.28

2.0 2.0 2:1 8.0 0.18 10 0.32

2.0 2.0 1:1 5.0 0.29 6 0.69

5.5.1 Determination of height

The equation below can be used for the determination of dike heights:

(a) Secondary dikes (Fig. 5.29)

 

Where: Hs = height of the secondary dikes above the ground

surface

Hst = mean high spring tide

Mr = maximum rainfall within 24 hours

Example 5.3

If the mean high spring tide above the zero datum is 1.35 m, the ground elevation at the side of dike is 1.00 m, and maximum 24-hour rainfall in the locality for 15-year interval is 12 cm.

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Determine the height of secondary dike needed. Provide 25% allowance for shrinkage and 30 cm freeboard.

Solution: (Refer to Fig. 5.29)

= 1.30 m

(b) Tertiary dikes (Fig. 5.30)

 

Where: Ht = height of the tertiary dike above datum

Dwl = desired water level above datum

Fig. 5.29 Typical cross section of secondary dike showing ground elevation, water levels and basis for determination of height

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Fig.5.30 Typical cross section of tertiary dike showing ground and water levels and basis for determining height

Example 5.4

Determine the height of a tertiary dike if the desired water level in the pond is 1.35 m from the zero datum and the ground elevation is 1.00 m. The maximum 24-hour rainfall is 12 cm, allowance of shrinkage is 20% and freeboard is 30 cm.

Solution: (Refer to Fig. 5.30)

= 0.96 or rounded to 1.0 m

5.5.2 Side slope, crown and base

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The top width of secondary and tertiary dikes are narrower than the perimeter or main dike. Top width of 1 to 2 m are common for the secondary dike and even less than one meter for the tertiary dike.

The side slope is usually 1:1. Side berms in secondary dike may be provided if there is excess soil in order to reduce the cost of hauling. Puddle trench in the dike base is provided when necessary.

The computation of the width of base is done in the same way as in the perimeter or main dike. Table 5.2 gives values of base for different values of dike height, crown and side slopes.

5.6 Design of various types of ponds and pond bottom

Fishponds are designed to have the best environment for the cultured species — through efficient water management, ease in the cultivation of food organisms and manipulation of stock for good growth and production. This objective could be attained when the arrangement of the pond compartments, water control structures and all other facilities mutually complement each other.

Production ponds are designed independent of each other by providing each with individual water supply and drainage gates. Within the compartment, pond bottoms are designed to further fit the environmental requirement of cultured species. The whole bottom should slope towards the drainage gate to facilitate removal of water. This sloping bottom can be modified and improved by providing bottom ditch within the pond running along and close to the base of the dike. This ditch collects and leads the water to the catching pond where the drainage gate is also located. In this scheme, a slope divide is provided at the centre of the pond (Fig. 5.31). For much larger compartments, a middle ditch connecting the peripheral ditch may be provided (Fig. 5.32).

Ponds designed purposely for shrimp culture usually have two separate gates — supply (inlet) and drainage (outlet) gates. Peripheral canals are provided mainly to serve as shelter for the shrimp; to increase the pond bottom surface area; and to have better water circulation. Facilitating drainage is only secondary in the purpose. Hence, more canals or bottom platforms are sometimes provided (Fig. 5.33) and some Figures in Chapter 4).

Table 5.2Relationship among the top width, bottom width and height of dike with given side slopes

Height (m) Top width or crown (m) Bottom width, m at given side slop

1:1 1:5:1 2:1

1.5 2 5 6.5 8

2 2 6 8 10

3 2 8 11 14

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Fig. 5.31 Peripheral ditch or canal in bottom of shrimp pond(Not drawn to scale)

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Fig. 5.32 A central ditch in addition to peripheral ditch in shrimp pond(Not drawn to scale)

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Fig. 5.33 A flow-through type of pond bottom design for shrimp pond(After Pinij, pers.comm., 1982)

(Not drawn to scale)

Pond bottom for shrimp culture may or may not be cleared of tree stumps depending on harvesting method. Stumps may just be cut short well below the pond water level (Fig. 5.34). Milkfish ponds, however, require that the pond be totally cleared of stumps to facilitate harvesting by gillnet seine. This method of harvest, however, is not common in the Philippines, because it removes some of the scales, thus reducing the quality of fish.

5.7 Design of water canals or channels

Water from the outer sea is drawn into the fishpond at the specified rate and time through the canal and discharged into the outer sea also through the same canal. In the design of the canal, it is necessary to give consideration on the following criteria:

Fig. 5.34 Dike-canal type pond (Modified after Cook, 1976)

a. The cross-section is determined to effect the flow of required amount of water in rational way. That is, the most effective section in terms of hydraulics; within the possible minimum time.

b. If the canal is to be used for other purposes than water conveyance, it should be designed to fulfill such purpose.

5.7.1 Kinds of water channel

Canals in fish farms are usually made of soil material. They may only vary in size depending on its location and purpose. A canal may serve the purpose of supplying and draining water to and from the ponds.

a. Main water supply canal. — This starts from the main gate and usually traverses the central portion of the fish farm. The size of the main canal should consider the emergency

125

discharge of water from the entire fish farm and surrounding area, if any, during heavy rain.

b. Secondary water supply canal. — This serves the portions where main canal cannot reach. It starts from the main canal and traverses the inner portion of the fishpond. It is usually constructed in large fishpond areas and is smaller than the main canal.

c. Tertiary canal. — This is the canal that usually supply water in the nursery and transition ponds. Because of the small size, it is sometimes said to be a part of the nursery pond system. The tertiary canal may be modified to serve as catching pond. Usually the bed width is 1.0 to 1.5 m.

d. Diversion canal. — The purpose of this canal is to protect the farm from being flooded with runoff water coming from the watershed. It should have the capacity to carry at least the peak runoff from the contributing watershed for a ten-year frequency storm. The slope of the diversion canal should be such that the water flows toward the drainage area or around the fish farm to a convenient and prepared outlet.

e. Drainage canal. — A separate drainage canal is recommended in intensive culture, especially of shrimps, in order to effect flow-through system. This is usually located at the other side of the pond, opposite and parallel to the supply canal, if provided.

5.7.2 Cross-section of canal bed

The cross-section of the canal is generally of trapezoidal shape with side slope of 1:1 for the alluvial clay soil (Fig. 5.35). The depth of the main canal ranges from the level of mean higher high water (excluding the height of freeboard) for mixed tide or mean high water (for diurnal), to the mean lower low water (the datum plane); and the secondary canals from the designed pond water level to the mean tide level. The lower limit of the water canals depends on the range of tide. Generally, a smaller tide range requires a lower canal bed.

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Fig.5.35 Design of different canals (After BFAR-FAO/UNDP,1981)

For the most efficient cross-section, canal bed should be.

 

Where: b = canal bed

d = depth of canal excluding freeboard

z = horizontal value of side slope

Example 5.5

Using the above formula, if the water depth inside a canal that has side slope of 1:1 is 1.0 m, the bottom width would be 0.83 m. Values of bottom width for other depths and side slopes are given in Table 5.3. The table also includes the cross-sectional area.

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5.7.3 Design velocity

Velocity of water on canals should be designed properly to avoid soil erosion and siltation on the canal bed. Velocity should be slow to prevent excessive erosion but not too slow to prevent siltation. In most soils, design velocity of water in canals should not be lower than 0.3 m/sec to avoid silting. The usual velocities in canals are within 0.5 to 0.7 m/sec.

The nature of velocity of flow in a canal during tidal inflow is different compared to the velocity during drainage of pond water at ebb tide. The former is an unsteady flow and water is flowing against the direction of canal slope. The latter is a flow of water in the same direction of slope. It is still an unsteady flow or flow that changes with time because of the falling depth of water in the ponds during drainage.

The opposite of unsteady flow is steady flow. This flow does not fluctuate or change with time. An example of a steady flow that exists in brackishwater fish farm is when pumping water from a river or a well and discharging in a canal that delivers the water to the ponds. The flow in the canal is further called uniform flow because the depth of water does not change and flows by gravity or in the direction of the slope.

Table 5.3Cross-sectional area, A of trapezoidal earthen canal at given side slope, z:1; water depth, d;

and bottom width, b

  Side slope 1:1 1.5:1 1.75:1 2:1

dm

z = 1 z = 1.5 z = 1.75 z = 2

bm

Am2

bm

Am2

bm

Am2

bm

Am2

0.25 0.207 0.114 0.152 0.132 0.133 0.142 0.118 0.155

0.50 0.414 0.457 0.302 0.526 0.265 0.570 0.236 0.618

0.75 0.621 1.028 0.453 1.184 0.397 1.284 0.354 1.391

1.00 0.828 1.828 0.604 2.104 0.530 2.280 0.472 2.472

1.25 1.035 2.856 0.755 3.288 0.662 3.562 0.590 3.863

1.50 1.242 4.113 0.906 4.734 0.795 5.130 0.708 5.562

1.75 1.449 5.598 1.057 6.444 0.927 6.981 0.826 7.571

2.00 1.656 7.312 1.208 8.416 1.060 9.120 0.944 9.888

2.25 1.863 9.254 1.359 10.652 1.192 11.541 1.062 12.515

2.50 2.070 11.425 1.510 13.150 1.325 14.250 1.180 15.450

2.75 2.277 13.824 1.661 15.912 1.457 17.241 1.298 18.695

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3.00 2.484 16.452 1.812 18.936 1.590 20.520 1.416 22.248

Under the condition of steady and uniform flow, the velocity of water in the canal can be calculated by using Manning's formula:

 

Where: V =

velocity, m/sec

R =

hydraulic radius, A/P, m

S = canal bed slope, m/m

n = roughness coefficient of the canal wall

A =

cross-sectional area of the canal

P =wetted perimeter or cross-sectional length of side wall of canal that is in contact with the water, m

The design discharge or capacity of a canal for steady and uniform flow is calculated by using the continuity equation: Q = AV, where Q = discharge or rate of flow in m3 sec.

Example 5.6

Determine the bottom width and capacity of an earthen canal with water depth of 0.75 m and side slope of 1.5:1. The canal has a bed slope of 0.0010 or 1 1 000.

Solution:

From Table 5.3, for d = 0.75, and side slope of 1.5:1, the bottom width of the canal for an efficient cross-section is 0.453 m. The velocity of flow at bed slope of 1 1 000 is 0.44 m/sec (Table 5.4). In Table 5.3, the corresponding area for d = 0.75 and b = 0.453 m is 1.184 m2. Therefore, the capacity of the canal is 1.184 m2 × 0.44 m/sec = 0.521 m3 sec.

5.7.4 Design requirement for multiple use

When canals are used for temporary holding of fish, the size of the canal will be calculated by the quantity of fish to be held. Under normal conditions, the maximum quantity of milkfish that can be held by tidal water is about 1.3 to 1.7 kg/m3. If the length and depth of canal are fixed, then the width should adjust in order to satisfy the required volume of water for a given quantity of fish to be held. The bottom of the canal if intended for temporary holding of fish should be 30 cm lower than the pond bottom or secondary gate.

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Table 5.4Velocity of water (m/sec) in trapezoidal earthen canal in clay soil at given side slope;

roughness coefficient, n = 0.025; depth, d; and bottom width, bSide slop = 1:1

C A N A L   S L O P E

Depth (m)

Bottom width (m)

0.25 0.207 0.45 0.37 0.32 0.28 0.26 0.24 0.22 0.21 0.20 0.19 0.18

0.50 0.414 0.71 0.58 0.50 0.45 0.41 0.38 0.35 0.33 0.32 0.30 0.29

0.75 0.621 0.93 0.76 0.66 0.59 0.54 0.50 0.46 0.44 0.42 0.40 0.38

1.00 0.828 1.13 0.92 0.80 0.71 0.65 0.60 0.56 0.53 0.50 0.48 0.46

1.25 1.035 1.31 1.07 0.92 0.83 0.75 0.70 0.65 0.62 0.58 0.56 0.53

1.50 1.242 1.48 1.21 1.05 0.94 0.86 0.79 0.74 0.70 0.66 0.63 0.60

1.75 1.449 1.64 1.34 1.16 1.03 0.94 0.87 0.82 0.77 0.73 0.70 0.69

2.00 1.656 1.79 1.46 1.26 1.13 1.03 0.96 0.89 0.84 0.80 0.76 0.73

2.25 1.863 1.94 1.58 1.37 1.22 1.12 1.03 0.97 0.91 0.87 0.83 0.79

2.50 2.070 2.08 1.69 1.47 1.31 1.20 1.11 1.04 0.98 0.93 0.88 0.85

2.75 2.277 2.21 1.81 1.56 1.40 1.28 1.18 1.11 1.04 0.99 0.94 0.90

3.00 2.484 2.34 1.91 1.66 1.48 1.35 1.25 1.17 1.10 1.05 1.00 0.96

Side slope = 1.5:1

0.25 0.152 0.45 0.37 0.32 0.28 0.26 0.24 0.22 0.21 0.20 0.19 0.18

0.50 0.302 0.47 0.39 0.34 0.30 0.27 0.25 0.24 0.22 0.21 0.20 0.19

0.75 0.453 0.62 0.51 0.44 0.39 0.36 0.33 0.31 0.29 0.28 0.26 0.25

1.00 0.604 0.67 0.55 0.48 0.42 0.39 0.36 0.34 0.32 0.30 0.29 0.27

1.25 0.755 0.87 0.71 0.62 0.55 0.50 0.47 0.44 0.41 0.39 0.37 0.36

1.50 0.906 0.99 0.81 0.70 0.62 0.57 0.53 0.49 0.47 0.44 0.42 0.40

1.75 1.057 1.09 0.89 0.77 0.69 0.63 0.58 0.55 0.52 0.49 0.47 0.45

2.00 1.208 1.20 0.98 0.85 0.76 0.69 0.64 0.60 0.56 0.54 0.51 0.49

2.25 1.359 1.29 1.06 0.92 0.82 0.75 0.69 0.65 0.61 0.58 0.55 0.53

2.50 1.510 1.39 1.13 0.98 0.88 0.80 0.74 0.69 0.65 0.62 0.59 0.57

2.75 1.661 1.48 1.21 1.05 0.94 0.85 0.79 0.74 0.70 0.66 0.63 0.60

3.00 1.812 1.51 1.28 1.11 0.99 0.90 0.84 0.78 0.74 0.70 0.67 0.64

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Side slope = 1.75:1

0.25 0.133 0.45 0.37 0.32 0.28 0.26 0.24 0.22 0.21 0.20 0.19 0.18

0.50 0.265 0.71 0.58 0.50 0.45 0.41 0.38 0.35 0.33 0.32 0.30 0.29

0.75 0.397 0.93 0.76 0.66 0.59 0.54 0.50 0.46 0.44 0.42 0.40 0.38

1.00 0.530 1.13 0.92 0.80 0.71 0.65 0.60 0.56 0.53 0.50 0.48 0.46

1.25 0.662 1.31 1.07 0.92 0.83 0.75 0.70 0.65 0.62 0.58 0.56 0.53

1.50 0.795 1.48 1.21 1.04 0.93 0.85 0.79 0.74 0.70 0.66 0.63 0.60

1.75 0.927 1.64 1.34 1.16 1.03 0.94 0.87 0.82 0.77 0.73 0.70 0.67

2.00 1.060 1.79 1.46 1.26 1.13 1.03 0.96 0.89 0.84 0.80 0.76 0.73

2.25 1.192 1.93 1.58 1.37 1.22 1.12 1.03 0.97 0.91 0.86 0.82 0.79

2.50 1.325 2.08 1.69 1.47 1.31 1.20 1.11 1.04 0.98 0.93 0.88 0.85

2.75 1.457 2.21 1.81 1.56 1.40 1.28 1.18 1.11 1.04 0.99 0.94 0.90

3.00 1.590 2.34 1.91 1.66 1.48 1.35 1.25 1.17 1.10 1.05 1.00 0.96

Side slope = 2:1

0.25 0.118 0.48 0.39 0.34 0.30 0.28 0.26 0.24 0.23 0.21 0.20 0.20

0.50 0.236 0.71 0.58 0.50 0.45 0.41 0.38 0.35 0.33 0.32 0.30 0.29

0.75 0.354 0.93 0.76 0.66 0.59 0.54 0.50 0.46 0.44 0.42 0.40 0.38

1.00 0.472 1.13 0.92 0.80 0.71 0.65 0.60 0.56 0.53 0.50 0.48 0.46

1.25 0.590 1.31 1.07 0.92 0.83 0.75 0.70 0.65 0.62 0.58 0.56 0.53

1.50 0.708 1.48 1.21 1.04 0.93 0.85 0.79 0.74 0.70 0.66 0.63 0.60

1.75 0.826 1.64 1.34 1.16 1.03 0.94 0.87 0.82 0.77 0.73 0.70 0.67

2.00 0.944 1.79 1.46 1.26 1.13 1.03 0.96 0.89 0.84 0.80 0.76 0.73

2.25 1.062 1.93 1.58 1.37 1.22 1.12 1.03 0.97 0.91 0.86 0.82 0.79

2.50 1.180 2.08 1.69 1.47 1.31 1.20 1.11 1.04 0.98 0.93 0.88 0.85

2.75 1.298 2.21 1.81 1.56 1.40 1.28 1.18 1.11 1.04 0.99 0.94 0.90

3.00 1.416 2.34 1.91 1.66 1.48 1.35 1.25 1.17 1.10 1.05 1.00 0.96

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CHAPTER 6CONSTRUCTION OF A FISH FARM

6. CONSTRUCTION ACTIVITIES, EQUIPMENT AND METHODS

6.1 Pre-construction activities

6.1.1 Programming of activity and staffing of the project

The purpose of project programming is to have a clear flow on how the project will be implemented, the starting and completion time for a given amount of work, and labour force. This is done by estimating the amount of labour force available and their daily output in order to determine the number of days a piece of work can be finished.

The preparation of the development programme/schedule requires careful evaluation and realistic calculations that would result in an efficient and economical implementation of job activities. Probable constraints that would hamper the smooth implementation of the project should be anticipated and solutions presented.

In assessing the availability and quality of manpower in the vicinity of project site, considerations are given to the quantity and experience of skilled workers, time of availability (year round or seasonal), rate and condition of payment, and working arrangements prevailing in the locality.

In the event of manpower shortage, importation of workers from adjacent places becomes likely and the same assessment survey should be done.

As an example, the following is a simplified proposed programme of work for a 30-ha milkfish farm.

Activities Work arrangemen

t

Daily labour

requiremen

Duration (days)

Equipment/Tools

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t

1. Site clearing Daily basis 100 75 1. Chain saw

  a)Under brushing; removal of tree trunks and debris;

     2. Bolo

3. Ax

  b) Path of perimeter dike       4. Bamboo raft

2. Construction of gatesContract work

45 130All tools provided by contractor, such as concrete mixer or mixing form, digging tools, boat, water container, carpentry tools, masonry tools, steel saw

  a)Construction of temporary dike

     

  b)Construction of foundation

     

    - Gate site excavation;      

   - Bamboo piling, boulders and gravel filling

     

  c)

Construction/installation of wooden forms and reinforcing bars

       

  d) Concreting        

  e)

Removal of forms, plastering, soil backfilling, removal of temporary dike

       

  f)Gate curing, conditioning and construction of screens

       

  g)

Construction and installation of secondary and tertiary wooden or concrete gates and pipes

Daily basis 30 30

As in a-f and 3

3. Earthwork (manual) Daily basis 150 160 1. Digging blade

  a) Construction of dikes       2. Flatboat

  b) Construction of canals       3. Flashboard

  c) Uprooting stumps       4. Stakes, plastic

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hose, side slope model

  d)Levelling pond bottom (cut and fill)

     

4.Construction of caretaker's hut, storage and working shed, chilling tank

Contract work

25 65

1.Concrete mixer or mixing form

2. Digging tools

3. Masonry tools

4. Plumbing tools

5. Road dikeContract work

4 20

1. Dump truck

2. Payloader

3. Grader

Based on the programme of work, a schedule of construction activities is also prepared. Development schedule should consider nature of funding and time elements. If funding is obtained through loan from the bank, the time of payment for the interest should be included in the development strategy. Banks usually charge interest payment on fish farm development loan two years after the first release. It is likely that during the second year, production would not be enough to pay for the interest. Chances are that unforeseen shortage of workers exists during construction or there may be a delay in pond conditioning especially if it is acidic. Under this situation, pond development should proceed ensuring that: (i) the ponds are secured against flood and other destructive forces of nature; and (ii) it must produce at the start of the second year, enough to pay for the loan interest and other financial requirements. Thus, schedule of construction of pond compartments should be phased such that one or more rearing ponds are already complete and functional, so that by the end of the year, production can start while the other portions are still being completed.

An example of general development schedule for a 30-ha farm is as follows:

Activities Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

1.Preparation of feasibility study

xxxx

xx                    

2.Acquisition of bank loan

 xxxx

xxxx

xxxx

               

3.Relocation survey

      xxx                

4. Canvassing of materials and

    xxxx

                 

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labourers

5.

Procurement and stock piling of materials

      xx xxxx              

6.

Construction of worker's temporary shelter

        xxxx              

7. Site clearing         xxxxxxxxx

xx          

8.Establishment of markers

        xx              

9.

Construction of main gate and temporary enclosure dike

       xxxxx

xx            

10.Construction of main dike

         xxxxx

xxxx

xxxx

xxxxx

     

11.Construction of partition dikes

             xxxx

xxxxxxxx

   

12.Construction of secondary gates

                 xxxx

xxxx

xxxx

13.Construction of tertiary gates

                 xxxx

   

14.Construction of supply canal

             xxxx

xxxx      

15.Pond excavation and levelling

                 xxxx

xxxx

xxxx

16.

Construction of caretaker's hut and storage shed

          xxxxxxxx

         

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17.Road dike and others

                     xxxx

6.1.2 Procurement/stockpiling of materials

The purchase and stockpiling of materials should also be accomplished according to the construction schedule. Costs should be weighed against availability and transport of materials under different climatic conditions. Some materials like gravel, sand, cement, lumber, and bamboo poles should be purchased and transported to the pond site during dry weather. Transporting them at this time is easier and cheaper. Bamboos have to be purchased during dry season to get good quality poles. Some equipment like the cement mixer, hollow blocks machines, vibrators, steel cutters, and water pumps should be acquired or leased at the proper time. All these will require a cash flow which should be indicated in the construction schedule.

Experience in Malaysia indicates that since water control structures are generally small and widely scattered over the pond site, it has been expensive to transport material and construction plant to the various locations. In many cases, transport cost of material have exceeded the actual cost of material itself (Khoo and Santhanaraj, 1982).

6.1.3 Site clearing

Initial clearing begins where the main dike and main gate are to be located. Full scale clearing then continues as the construction of main dike and main gate proceeds. The entire area of the fishpond site should be cleared of all grasses, trees, roots and stumps. All cleared materials should be thoroughly removed from the site of work.

Site clearing can be accomplished by any or combination of the following methods:

a. Underbrushing. In underbrushing, vegetation including nipa trees and shrubs, of less than 10 cm in diameter are cut with the use of bolo. This operation must be done systematically in blocks and be completed immediately before the perimeter dike and the main gate are completed. This is to prepare the pond for the next process. Underbrushing is done by manual labour and the work begins as soon as the foundations of the main dike and the main gate have been established.

b. Withering (Optional). Withering is to kill the trees by filling up the pond with water. It has been found that mangrove trees, specially the group of Rhizophora, usually die out when their trunks are constantly soaked with water at a depth of more than 0.5 m for a period of 4 to 6 months. The fallen leaves serve as fertilizer.

c. Falling. Falling is simply cutting down big trees left after underbrushing. The falling operation should commence when the tree bark begins to peel, but before the leaves and branches fall. The prerequisite of falling is to dry and harden the ground, which can be done by keeping the water table of the entire pond area at 0.3 to 0.5 m below the surface for a period of 1 to 3 months, depending on the weather conditions. Either manual or

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mechanical method, or a combination of both, can be employed for falling. A chain saw is effective in falling big trees and cutting logs. It is a fast method and economical to use.

d. Uprooting of stumps. Complete removal of tree stumps and root system embedded in the soil is done by manual labour or by the use of small machines. Manual labour is effective and economical in areas having small stumps and roots, but for big stumps and larger areas, it is more economical to use a winch and pulley block (Fig. 6.1) or a mechanical tree puller (Fig. 6.2).

The mechanical puller consists of a winch and its accessories. The winch is powered by 8 to 16 Hp gasoline engine for lightness and portability. The winch and the engine are set on a common steel base provided with an anchor, holdfast or a log-deadman (Fig. 6.3). The steel cable, provided with a quick-detach shackle, is then tied securely to the stump. When the engine is started the winch will pull out the stump. For extra large trees, roots opposite the winch may have to be cut if the winch cannot pull the tree out entirely.

The mechanical puller is moved from place to place, until all the stumps have been uprooted. When pond is flooded with tide water, the uprooted stumps will partially float and can thus be removed from the pond.

6.2 Construction equipment

6.2.1 Equipment for manual construction

(a) Digging blade. Manual construction of dike is usually done by piling soil blocks that have been cut by a digging blade. A soil block measures approximately 30 cm × 30 cm × 60 cm which is cut by a hand tool locally called “tagad” in the Philippines (Visayas) and “sarap” in Indonesia (Fig. 6.4). In Thailand, they also use this type of digging blade, with a slightly different construction. There are also other hand tools for digging soil found in the region and these are shown in Fig. 6.5.

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Fig.6.1 Sketch showing how to use a winch and a pulley block for manual destumping (After Maar and Mortimer, 1966)

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Fig.6.2 A mechanical tree puller (After Jamandre and Rabanal, 1966)

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Fig.6.3 Types of anchor for manual destumping (After Hechanova and Tiensongrusmee, 1980)

140

Fig. 6.4 Types of digging implements for fishpond construction(After Jamandre and Rabanal, 1975)

141

Fig. 6.5 Additional digging tools in Indonesia for fishpond construction and repairs (After Ranoemihardjo, 1982)

(b) Bamboo raft, dugout boat, flatboat. The soil blocks are transported from the digging site to the path of the dike by a bamboo raft, dugout boat or a flatboat. The bamboo raft, however (Fig.

142

6.6), has a much smaller load capacity than the flatboat (Fig. 6.7). The flatboat is presently considered the best method of hauling soil blocks because of the following advantages:

i. Does not require much effort to load;ii. Construction is simple;

iii. Maneuverable, easy to tilt and dump the soil (Fig. 6.8); and

iv. Requires minimal maintenance — only periodic tar coating.

Fig.6.6 Bamboo raft for transporting soil blocks in diking(After Denila, 1977)

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Fig. 6.7 Flatboat for transporting soil blocks (After Denila, 1977)

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Fig. 6.8 Two ways of unloading flat boat (After Jamandre and Rabanal, 1975)

(c) Wooden mallet or tamping device. This tool is used to compact the soil blocks on top and sides of dikes. This is made of a short log about 30 cm long and 30 cm in diameter. The end that gets in contact with the soil is cut into a wedge of 45° angle and provided with a handle about 1.8 m long. The device is manually raised and made to fall against the dike surface repeatedly. Proper soil moisture condition is needed for using the mallet effectively.

6.2.2 Heavy equipment for construction

(a) Dragline. Singapore and Malaysia generally use dragline. The operations consist of bringing to site a track-mounted crane. With the crane are many 0.1 × 0.3 × 4.0 to 5.0 m planks which are self-laid by the crane for its own base. As work progresses, the planks are self-shifted by the crane so that it is resting at all times on a series of platform planks preventing it from bogging down in mud.

This particular equipment is particularly good for dike construction, canal digging and deepening (Fig. 6.9). Hence, it is suited for excavating shrimp trapping ponds in order to have deeper water. However, it is not practical for large-scale within-compartment-levelling because of its being too

145

slow and unwieldy. Its use is also limited where mass hauling of earth in some areas for dike building is involved. It is by design, an equipment for in-place working. There is now in the market a crane mounted on LGP (low ground pressure) tracks. This makes the crane more maneuverable in swampy areas and cuts its non-productive plank-transferring time by half. This crane can be equipped with clam shell buckets.

Fig. 6.9 Range diagram and limits of work of drag line as defined for the construction of the perimeter dike

(After Hechanava and Tiensongrusmee, 1980)

The cost of using draglines vary with the kind of soil material and its size. The effective working range depends on the length of boom (Fig. 6.9).

(b) Dozer-crawlers (Fig. 6.10). This particular equipment is good for levelling provided the load-bearing capacity of the soil can support the equipment weight. Recent designs of these

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equipment are provided with mechanisms which improves speed and maneuverability, thus cutting down construction time. The effective working range of most dozers in dry soil conditions vary from 20 to 40 m.

Fig. 6.10 Some heavy equipment for fish farm construction(After Tarnchalanukit, 1982)

Most crawler manufacturers have now incorporated LGP tracks to their line of crawlers to enable it to work on wet soils. On LGP, the ground pressure usually is around 0.28 kg/cm2. The LGP crawlers come in a variety of sizes, ranging from 20 to 200 BHp. One has therefore a good choice to suit the area to be developed or the transport situation to the work area. In some instances, small LGP dozers are chosen over a large one for convenience in transporting the equipment to the job site by raft or passing on bridges which have limited load capacity.

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(c) Hydraulic excavators, backhoe, shovels, cranes, pay loaders (Fig. 6.10). These are equipment that can also be used to advantage in fishpond projects. One advantage the hydraulic equipment have over cable cranes are their fast action and flexibility to adapt to a variety of situations. These can also be adapted with clam shells and grapplers and can be used for uprooting small tree stumps.

(d) Scrapers, dump trucks, wheel-type loaders (Fig. 6.10). These may also be used effectively where soil conditions permit.

(e) Suggestions on mechanization. Much waste has been observed in the use of mechanized equipment. In order to be able to adopt this method with success, one must know the following: (i) capacity; (ii) limitations; (iii) limit of maximum operating range; (iv) ground pressure; (v) working conditions; (vi) amount of work to be done; (vii) type of work required; and (viii) cost per unit of work accomplished.

There are equipment now especially fabricated for reclamation and swamp work. These types can be used in mangroves effectively. However, one should be conversant with the different types of equipment and be able to judge critically the type of equipment needed after seeing the project area. There are some large equipment which can be taken apart into several components and reassembled at job-site. Some cranes and hydraulic lifters are also designed to be barge-mounted with their tracks on while on the shore.

In addition, the economical or effective range of the equipment should also be known. At what distance should one consider a loader and a dump truck combination over a crawler? It would be uneconomical beyond about 35 m due to its track wear and long cycle time. Beyond 35 m on volume of soil movement, one might want to employ combination pay loaders with several dump trucks. The economics of this operation should be worked out. Scrapers (self-loading and crawler-assisted) are ideal for large projects. Dredges can be used effectively too. Under special situations, jetting pumps can effectively facilitate uprooting of stumps. All of the above require a thorough knowledge of the project site peculiarities as well as the different equipment available, their specifications, capacities and limitations. An expert's advise in this field is usually necessary.

6.3 Construction methods

6.3.1 Construction of reinforced concrete or hollow block main and secondary gates

The main gate is constructed ahead of the main or perimeter dike to allow time for curing of concrete and have it used while the main dike is being completed. All the materials needed should be in the site prior to construction.

During construction, the design specifications must be followed particularly in the construction of gate foundation; the provisions against undercutting of water, the spacing and size of reinforcements against side and bottom pressures; and the proper mixture and curing of the concrete or brick (hollow block).

148

Construction begins by preparing the site. The exact location is measured and staked out. Enough working space of 1 to 2 m around the gate foundation should be provided.

A reference point for checking the elevations of gate flooring, heights, soil excavations and others must be established.

A temporary but strong dike capable of withstanding tidal water pressure must be constructed to enclose the site and working space. Entrapped water inside the site is removed manually or by pumping.

(a) The gate foundation. Construction is guided by following either of the two illustrations in Figures 5.10 and 5.11. In Figure 5.10, the site is excavated to 0.6 m deep from the reference point. Removal of roots, stumps and mud or soft soil is done, if any. Excavation should include the portion where the toes of the gate will be constructed. Excavation is done manually or by dragline or a backhoe on tracks in mechanized construction.

The spacing and lengths of bamboo base, mangrove or wooden piles that support the structure should be observed. A common practice by fish farmers is to drive two lengths of piles vertically: the 3-m length is driven at one-meter interval while the shorter (one to two-meter) length is driven 25 to 30 cm apart within the longer piles. Leave at least 25 cm of the pile head above the soil surface. This exposed ends should be in level.

In some designs, wooden planks with pointed ends measuring 5 cm × 15 cm × 180 cm are also used in addition to bamboo piles. These planks are driven side by side along the centreline of side and wing walls and both ends of aprons. These planks extend the depth of concrete cut-off or toe walls and further help in preventing undercutting of water.

Boulders are laid about 20 cm thick between the piling to form a floor. Gravel layer of 5 cm thick is spread on top of the boulders, and then compacted. The exposed ends of the piles should be level with the surface of the gravel layer.

(b) Forms and reinforcing bars. After the foundation, the forms for the slab or flooring and toes are constructed. The reinforcing bars are laid as per plan. The elevation of flooring should be checked if still within the prescribed value.

Initial pouring of mixed concrete along the footing is done to keep in place the vertical reinforcements for side and wing walls including collars. Then the horizontal bars are tied with No. 16 wires with the vertical bars as per plan. In addition, reinforcements for catwalk or bridges are also installed. While installing the reinforcements, the forms for walls, bridges and collars are being prepared.

The forms are properly set and should be rigid to stand the weight of poured concrete and to avoid bulging of sides. The reinforcement bars should be centred within the forms. The forms are spaced apart to have a finished concrete wall of at least 15 cm.

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(c) Concrete mixture, pouring and curing. Concrete mixture should be in proportion of 1:2.4 (cement:sand:gravel) for all concrete works. If concrete hollow blocks are used (for small main and secondary gates), the mixture should be 1:7 (cement:sand).

Prior to pouring of concrete mixture, the water that have seeped into the gate construction site should be drained out. Then pouring of mixture follows continuously until completed. During pouring, concrete vibrator or a piece of iron or firm wood should be used to compact the mixture to prevent hollow or void spaces and to insure smooth concrete surface upon removal of the forms. Saltwater should not come in contact with the concrete while still wet.

Allow the concrete to set in and harden for 2 to 4 days before removing the forms. Plastering the surface may be done as necessary. Plastering mortar of 1:3 (cement:sand) ratio should be applied at a thickness of 6 to 10 mm.

Soil backfilling of excavated areas is done to have a finished ground surface around the structure. Proper compaction between walls of the structure and adjacent soils should be observed.

Continuous curing of the concrete structure should be done for 28 days. Curing is done by covering it with jute sack or similar materials and sprinkling freshwater to make it moist throughout the day for the whole curing period.

The temporary dike may be removed after 30 days. Condition the structure by letting tidal water to come in and out of the gate.

6.3.2 Construction of main secondary and tertiary wooden gates and pipes

Wooden gates have much shorter life span than concrete. This is preferred for reasons of economy or when initial capital is limited the use of wooden gate is resorted to until such time that the owner decides to replace it with concrete.

The parts, shape, height and inner dimensions of wooden gates are also similar with concrete. They are however, easier and faster to construct. Construction is carried out as follows:

The materials used are either regular lumber, stripped/ whole coconut or palm trunks and bamboo. These materials are selected for quality; they must be adequately shade dried for 2 to 3 weeks before construction.

The wood are planed to have smooth edge and surfaces. The materials are then cut as per plans and specifications of the gate. To prolong the life of the wood, thick coating of coal tar is applied. Creosote or other similar wood preservatives may also be applied. Some gate builders mix coal tar and cement and the mixture is painted in two coatings to the materials, then left under the sun for drying.

While drying the wood materials under the shade, the usual reference mark for checking the elevation of the gate should be established as well as preparation of the foundation.

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The foundation may not be as strong as in concrete gate. Excavate the site according to desired elevation. Bamboo base or mangrove piles are driven to support the structure if needed (Figs. 6.11 and 6.12).

Fig. 6.11 Wooden gate construction(No middle anti-seep board)

(After Jamandre and Rabanal, 1975)

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Fig. 6.12 Wooden gate frontal view, no wing wall but with anti-seep board (Modified after Jamandre and Rabanal, 1975)

The various parts of the gate such as the walls, flooring, cut-off walls, anti-seepboard (Fig. 6.12) are separately nailed to the respective braces, pillars or supports. This is done outside the site. The component units (side walls, flooring, cut-off wall, etc.) are then assembled together by using galvanized nails, or bronze nails, if available, to form the gate after the preparation of the foundation (Figs. 6.11 to 6.13). In some cases, the whole gate unit is already assembled outside the site and just lifted and placed properly on top of the site, but sometimes the the finished gate is quite heavy.

The walls and flooring of the gate are tightly nailed side by side. Water tightness improves the moment the wood is soaked in water and expands. However, there are also water sealant compounds that further insure water tightness.

In Indonesia, some secondary gates are designed and constructed without nailing the flooring, the side and wing walls. The wallings are vertically piled one on top of the other and tightly sandwiched between the vertical supports or pillars and the dike soil. Even the cleats or grooves

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for the screens are not nailed permanently. The major advantage of this design is convenience in repair or replacement of decayed wood member. The member to be replaced is just pulled out easily and then the replacement is immediately inserted.

Fig. 6.13 Wooden gate construction, preferably preservative-treated wood and copper or galvanized iron nails

(After Jamandre and Rabanal, 1975)

After installing the gate, the space between the excavations and walls are mud-packed by soil blocks arranged in layers. Then the soil is allowed to dry and harden.

Conditioning the gate is done by allowing tidal water in and out of the pond. Checking for seepage is also being done at this time.

6.3.3 Construction of perimeter or main dike

The most important component of the fishpond system is the dike enclosing the entire pond area. The perimeter dike is the first dike to be constructed to free the area from the danger of floods. The utility of the pond system will depend on the strength and lifetime of the perimeter dike.

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Construction may be done by manual method with light implements and by using heavy equipment.

(a) Manual construction. Main dike passing across rivers, creeks and low areas should be constructed first. Construction should be done by arranging soil blocks properly in between staggeredly driven bamboo or mangrove piles as reinforcement. The arrangement of soil blocks and piles are shown in Figure 6.14.

Fig. 6.14 Closing of river or creek (After Denila, 1977)Note: This is not advisable but done only if necessary and possible adverse effects compensated

As the first step, construction of all dikes is preceded by thorough clearing of the path of dikes and their immediate vicinity. The width of clearing is slightly wider than the base of the dike to be constructed. Trees, roots, stumps and undecomposed organic matter should be removed out of these path. Clearing is guided by staking out the centreline of the dike. After the clearing, the base and top width of the dike are properly marked with stakes on the ground. Construction of the core or puddle trench along the main dike follows in order to provide a good key between the dike soil and the foundation as well as minimizing seepage through the dike. The puddle trench is constructed by excavating a trench measuring at least 0.5 m wide by 0.5–1.0 m deep along the centre path of the main dike. The excavated trench is then backfilled to the same ground level with a new soil which is wet enough to be puddled by feet or compacted by a wooden mallet or tamping device. The importance of puddle trench is well recognized but sparingly practiced

154

probably due to the added cost. Although costly, it cuts a lot of water management problems in the future.

Construction of main dike proper follows. The stakes set for the size of dike guide the proper arrangement or piling of soil blocks which are taken from higher grounds and are being transported by flatboats or rafts. Other methods of transporting soil blocks are by the line system and sliding system. In the line system, workers form a single line (Fig. 6.15). Each worker is positioned at 1 to 2 m apart. The line extends from the source of soil to the dike construction site. Soil blocks are relayed to each man until it reaches the piler. The sliding system applies when the source of blocks is close to the site (Fig. 6.16). The worker throws the block on the board letting the soil slide down to the base of the dike. Among these methods, the use of flatboat is considered the best.

Fig. 6.15 Five workers working in group in pond with water(After Denila, 1977)

The piler of blocks sees to it that they are tightly placed end to end. Compacting each layer of soil blocks by feet or tamping device is recommended. The proper placement of soil are shown in Figure 6.17. The recommended finished form of dike is shown in Figure 6.18.

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Fig. 6.16 The sliding board method of moving soil blocks (After Denila, 1977)

Fig. 6.17 Proper way of piling soil blocks for dike (After Denila, 1977)

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Fig 6.18 Arrangement of soil blocks and proper form of dike in manual method of construction (After Denila, 1977)

The proper side slope must also be observed in the piling of blocks. The base and top width stakes as well as a side slope model (Fig. 6.19) serves as guide in checking the correctness of side slope.

The height or elevation of the top of dike should also be checked, if done according to specifications. It is important to have uniform elevation of top of dike in every compartment. To accurately measure this, a 12-mm transparent plastic hose, 25 to 30 m long is filled with water.

157

One end is held by a man at the starting point while the other end is held by another man. The water level at the two ends of the hose must be the same. This level is properly marked and is checked against the finished elevation of the top of dike. The same procedure is done in subsequent stretch or station of 20 to 25 m until the entire length of dike is covered.

The plastic hose with water is also used in laying out the bed slope of canals of a fish farm.

(b) Team work versus individual work in dike construction. Some construction workers group themselves as a team in working. The team is usually composed of four members. each with distinct functions — the soil piler, the soil block digger, raft or flatboat pusher, and the carrier. The carrier receives the soil blocks transported and unloaded by the boat pusher at the site, and passes them to the piler.

Fig. 6.19 Side slope model made for dike slope of 1:1(After Hechanova and Tiensongrusmee, 1980)

Other workers prefer to work individually. Each worker is provided with flatboat and does the digging, transporting and piling (Fig. 6.20).

It is claimed that one skilled worker with flatboat can finish a dike with size of 6 to 7 cu.m in 8 to 9 hours, compared to 3.5 cu.m only for each member in the team work (Denila, 1977).

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Fig. 6.20 One man - one flat boat operation (After Denila, 1977)

6.3.4 Construction of secondary and tertiary dikes

Construction of secondary and tertiary dikes follows the same procedure as in construction of perimeter dikes. Puddle trench is also recommended to be included in the construction. The dikes may have berm to accommodate excess soil from the pond and to decrease soil erosion and water turbidity after a heavy rainfall. This berm is also a good working space during repair of dike leaks or seepage rather than doing repairs by staying on the crown.

6.3.5 Construction of water canals

Water canals are constructed following the same procedure as in dike construction. Canal bottom is, however, excavated deeper than the pond bottom and secondary gates if the channel or canal is purposely designed for filling and draining the fishpond.

6.3.6 Pond levelling

Pond levelling is the final step in fishpond construction. Some operators pay little attention to pond levelling and think that construction of fishpond is finished after the main and the secondary dikes have been constructed. One of the major reasons for low pond productivity is due to rough or poorly levelled pond bottom.

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Levelling the pond just after the dikes are constructed is quite expensive. It is advisable to wait for two to three years until the root systems of the trees have partially decayed before levelling is started. This will lessen the capital outlay. Partial levelling may be done just after enclosure, but excavation should be limited only to portions where there are no trees. The soil excavated should be dump in low portions that cannot be drained. After two to three years, final levelling can be completed.

After a topographic survey has been made, the pond bottom elevations should be determined. Likewise, the volume of soil to be cut and the portions to be filled should be marked out by stakes. In the Philippines, a simple method of pond levelling is done by using the tidal water. The procedure for this is as follows:

a. Bring the water down to the desired pond elevation and place a bench mark to identify it (Fig. 6.21). Beside the bench mark, place another stake about 7.5 cm wide, 2.5 cm thick and 2 m long, marked from 0 to 100 cm (Fig. 6.22). The zero mark of the gauge should be level with the bench mark. The gauge indicates the depth of water and serves as a levelling guide during filling of low spots and in cutting soil from high places.

Fig. 6.21 Illustration of procedure in determining depth of soil above O tidal datum using water level, staff gauge and depth gauge

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Fig. 6.22 Staff gauge as benchmark(After delos Santos, 1980)

b. Establish stations by marking principal locations of high grounds or above the zero elevation mark as well as the low areas to be filled.

c. Allow water in to a depth of about 30 cm so that the flatboats are afloat and start excavation. All vegetative cover is stripped and loaded to the flatboats and dumped outside the pond. The standard length for one handle length of the digging blade is 30 cm so that if the handle is placed in the water and the ground level is 15 cm above the zero mark, 50 percent of the handle length should be excavated. In areas of extensive excavation, the soil can be loaded directly on flatboats and placed in the dikes, in low spots or areas that are below zero elevation. The remaining soil on either side can be levelled easily using spades (Fig. 6.23). One to two workers should be at the receiving end to supervise the dumping of soil in the deep portions. The receivers check the depths by a depth gauge (Fig. 6.24). Each receiver can supervise 20 flatboats.

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d. When all marked portions have been excavated, bring down the water to 0 elevation. With the use of a spade the final levelling can be done.

e. The process is repeated for other stations or locations until all other low portions are filled.

f. During the final levelling, with the water at elevation 0, all water pockets should be connected by small canals.

In levelling work consisting of excavations and filling of low areas, a worker can have a finished job output as follows:

Working range (from excavation to filling area)(m)

Levelled/filled area(m2)

(a) 10–100 40–50

(b) 101–300 30–40

(c) 301–500 20–30

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Fig. 6.23 Stripping method of levelling (Cut and fill)(After Jamandre and Rabanal, 1975)

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Fig. 6.24 A simple depth gauge (After Jamandre and Rabanal, 1975)

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CHAPTER 7EQUIPMENT AND FACILITIES FOR FISH FARM OPERATION AND MANAGEMENTCoastal fishponds require some essential equipment and facilities which are used for varied purposes. Such equipment may be used for maintenance and repairs, harvesting the crop, monitoring and maintaining water quality, excluding predators and pests, and other miscellaneous facilities for maximizing the use of various inputs.

7.1 Equipment for maintenance and repairs

7.1.1 Digging tools

Most of the maintenance and repair works in fish farms are devoted to dikes. Practically, the same implements in manual construction work are also being used in making repairs of dikes. Digging tools, flatboats, wooden dugouts and rafts as previously mentioned (Figures 6.4 to 6.8 in Chapter 6) are most appropriate for work like digging out trenches and backfilling them with puddled soil to repair leakage/seepage, and for deepening canals and other similar jobs.

Maintenance of digging tools are easily done by cleaning and rubbing with oil or grease to prevent them from rusting. Boats should be protected against the sun by putting them under the shed. Dugout boats, when not used, may also be filled with water to prevent the walls from cracking when exposed under the sun.

7.1.2 Levelling tools

Practical and simple equipment have also been devised for levelling fishpond bottoms. The simple manual mud rake is a good example (Fig. 7.1), while in Indonesia, a levelling board manned by four or more men is also used (Fig. 7.2).

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Fig. 7.1 A wooden mud rake(After Rabanal, 1951)

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Fig. 7.2 Pond mud bottom levelling board used in Indonesia

Fig. 7.3 Fingerling seine (A) and operational view (B) (After Motoh, 1980)

7.1.3 Desilting equipment

For manual removal of silt in fishpond bottoms or canals, a simple metallic or wooden shovel has been devised and used in Thailand and Indonesia (Fig. 6.5). If the area to be desilted is extensive, a type of silt or sludge pump can be used. This can be a small unit so that it can be portable and can be transferred to the various areas of the pond system.

7.2 Nets and traps

7.2.1 Fingerling seine

The industry has designed various types of nets for use in fishpond operations. The fingerling seine, which is used for catching milkfish fingerlings and shrimps juveniles, is a fine-meshed rectangular net, about two to four meters long by one meter wide. It is supported by two poles at both ends with floats of wood, rubber or synthetic material on the upper side and sinkers of lead on the opposite side (Fig. 7.3).

7.2.2 Fingerling suspension net

A fingerling suspension net is usually a standard equipment in brackishwater fish farms. This is a rectangular or square net 2 to 3 meters wide by 3 to 5 meters long. Double line of coarse twine line the margins, the net has meshes of 0.5 to 1.0 cm square mesh. It is used to hold fingerlings during counting or before transport (Fig. 7.4).

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7.2.3 Gillnet seine

This is a harvesting net of about 1.5 to 2.0 m wide by 30 to 50 m long (Fig. 7.5). It should have a float line at one of the long sides and lead line on the opposite side. It is made of coarse thread of nylon with mesh of 4 to 5 cm square mesh. To catch fish, this seine is dragged over the pond from one end to the other. The fish are gilled on the net but some jump over while small ones pass through the net meshes so that this net is usually used for partial harvesting.

Fig. 7.4 Sample of fingerling suspension net

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Fig. 7.5 Part of a gill net for partial harvesting

7.2.4 Screens on water control structures

Fine-meshed nylon or Manila hemp cloth are used as screens on frames on gates. These screens are usually framed with the wooden frame set on the gates to reinforce the bamboo screens on the frame (Figs. 7.6a and 7.6b).

For pipes, a fitting bamboo screen basket is used for milkfish nurseries in the Philippines (Fig. 7.7a). During the early stages of fry rearing, this is further coated with nylon mesh or Manila hemp cloth. Sometimes, only a fine-meshed net bag is tied to the water control pipe to screen-off unwanted organisms (Fig. 7.7b).

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Fig. 7.6 Soil-sealed gates with screens(After delos Santos, 1978)

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Fig. 7.7 Netting screens in water gates and pipes

7.2.5 Harvesting bagnet on gates

Bagnets so constructed so that their openings could fit the wooden frame for screens on gates are made for use in harvesting. These are installed on the frames during low tides when water is drained from the ponds which were previously fully flooded during the previous high tide. With the force of the current, the stock of shrimp from the ponds are led into the bagnet where they are

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collected. These harvesting bags for shrimp are used in the shrimp trapping ponds in Malaysia and in milkfish/shrimp polyculture ponds in the Philippines (Fig. 7.8).

Fig. 7.8 Harvesting net with lazy line-arrows indicate water flow(After ASEAN National Coordinating Agency of the Philippines, 1978)

7.2.6 Cast net

The net is a versatile net for fishermen as well as for fish farmers for small-scale individual catching or sampling. In fish farm, this net can be used for sampling stock of fish or shrimp to monitor growth or for partial harvesting when required (Fig. 7.9).

7.2.7 Bamboo screen trap

Bamboo screen traps have been devised for partial harvesting in coastal fishponds, especially for penaeid shrimps. In Indonesia, the shrimp fyke (bubu udang) is widely used (Fig. 7.10a). This consists of a fabricated catching fyke and an antechamber and a leader of bamboo screen set perpendicular to the pond dike. Series of the shrimp fykes may be set in the pond during harvest. In the Philippines, a similar trap is used but the catching end is formed in place rather than a pre-devised catching fyke (Fig. 7.10b).

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Fig. 7.9 A cast net for sampling or partial harvest

173

Fig. 7.10 Traps for use in shrimp ponds

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7.3 Equipment for monitoring and maintenance of water quality

7.3.1 Water circulation and aeration

Portable water pump is usually used to effect water movement whenever needed in the pond system. This is often necessary when water circulation is needed and the tide condition is not conducive for this operation such as in alleviating stagnation or lack of oxygen.

Fig. 7.11 The Thai-made paddlewheel aerator towed by tractor(After Menasveta and Leeviriyaphanda, 1982)

Paddle wheels set in the ponds can remedy critical oxygen condition. These devices can be powered by electricity or by small portable engines (Fig. 7.11). Better aeration of the pond water may also be accomplished through the gates with the use of the closure slabs (Fig. 7.12).

Fig. 7.12 Aeration by manipulation of closure slabs(After delos Santos, 1978)

7.3.2 Analysis kit

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Water and soil analysis kits are now available in the market. For coastal fish farms a set that could monitor dissolved oxygen, salinity, pH, are most essential. Additional useful observations involve nutrient-content (N-P-K), depth of visibility (turbidity), etc. Simple visibility observations can be done using the Secchi disc (Fig. 7.13). Direct salinity readings using refractometers is easy to do but the equipment is quite expensive (Fig. 7.14). Improvised hydrometers may be used after standardizing them with a salinometer (hydrometer) or refractometer (Fig. 7.15).

Fig. 7.13 Measurement of depth of visibility by Secchi disc.

176

Fig. 7.14 Hydrometer and refractometer for measurement of salinity

177

Fig. 7.15 An improvised salinometer (After IFP, 1974)

7.4 Other facilities

Chilling chamber or box has become a standard facility in coastal fish farms. These are made of concrete, wood or galvanized iron sheets forming shallow tanks, square or rectangular in form scarcely 0.5 m deep set within the fishpond premises. It may be 2 to 3 m wide by 3 to 5 m long by 0.4 m deep. During harvest, this is filled with clean water and some crushed ice. The harvested fish or shrimp are placed in this box to wash and chill before packing them for the market (Fig. 7.16).

Fertilizer platforms (Fig. 7.17) where the sacks of inorganic fertilizers are placed so that the nutrient substances dissolve slowly into the water instead of chemically reacting with pond soil are useful facilities of the fish farm.

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Fig. 7.16 Chilling tank for newly harvested milkfish

179

Fig. 7.17 Fertilizer platform (Source: Anonymous, 1976)

There are a number of devices designed to exclude predators and pests. The crab hook (Fig. 7.18a) is used to catch mangrove crabs that may have made burrows into the fishpond dike. Eel hooks (Fig. 7.18b) are used on moist pond bottoms to catch the mud eels that may persist to stay in the pond bottom after harvest. For predatory birds, various scaring devices are used. Lines of white twines are usually set over milkfish nurseries. Scarecrows may be set or other scares such as those using mirrors (Fig. 7.19), noise scares, moving scares, etc. A trap has also been devised for the mound-forming mud lobster, Thallasina anomala (Fig. 7.20).

180

181

Fig. 7.18 Devices used to get rid of pests

Fig. 7.19 Bird scaring device (From Cook, 1977)

Fig. 7.20 Bamboo trap for mound-building mud lobster

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CHAPTER 8WATER PUMPS FOR THE MANAGEMENT OF COASTAL FISH FARMS

8. PUMP SELECTION AND INSTALLATION FOR AQUACULTURE

The present use of pumps in aquaculture are:

a. As a total or supplementary means of obtaining water for the purpose of maximizing production per unit area or volume, say in ponds or tanks.

b. As aerators, water circulation device or for effecting continuous flow system in intensive culture where water quality deteriorates rapidly and becomes a limiting factor.

c. For lifting water in sites where the elevation is beyond the ample reach of tidal fluctuation; where the source is ground-water, whether saline or fresh; or where the cost of excavation is more expensive than the cost of pump and its operation.

8.1 Terminology used in pumps

A number of technical terms can be helpful in understanding the selection, installation and operation of pumps for coastal aquaculture.

a. Suction head. Refers to the vertical distance from the surface of water (including drawdown, if any) to centreline of the pump impeller.

b. Discharge head. Is the vertical distance from the centreline of the impeller to point of discharge.

c. Total Dynamic Head (TDH). Is the sum of the suction head, discharge head, hydraulic head losses and the velocity head.

d. Drawdown. Is the lowering of water surface below the static level during pumping.

e. Static level. Is the water level before pumping begins.

f. Hydraulic loss. Is loss due to pipe wall friction, elbow design, joints, gate valves, sudden reduction or enlargement of pipe size. This is expressed in its equivalent height or head of water loss.

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g. Discharge or capacity. Refers to the rate of flow or the volume of water pumped per unit time such as gallons per minute; cubic feet per second; cubic meters per minute; liters per second; etc.

h. Performance curves. Is the variation of head with capacity at a constant impeller speed. It also includes efficiency and brake horsepower curves.

8.2 Types of pumps for aquaculture

Coastal aquaculture operations may require both freshwater and brackishwater. Freshwater may be needed for maintaining salinity of water during dry months due to rapid evaporation or for staff use or for domestic animals in an integrated farming set-up. Hence, pumps suitable for freshwater and brackishwater are discussed.

Pumps may be differentiated in how the water is forced from the intake to the discharged side, as well as the height of water lift and corresponding discharge. Under this differentiation, there are three main types, namely: (i) centrifugal: (ii) deep-well turbine; and (iii) propeller.

8.2.1 Centrifugal pump

This pump is characterized by operating at low head and low discharge. For best performance, the pump should be set close to the water level with a total suction lift usually not more than 6 m.

The pumps operates on the principle of centrifugal action. A motor or driver rotates an impeller with vanes immersed in water and enclosed in a casing. Water that enters the case is immediately engaged in by the rapidly rotating impeller. This rotation causes a flow from the centre of the impeller to the rim or outside of the case where pressure head is rapidly built up. To relieve the pressure, the water escapes through the discharge pipe. The centrifugal pump will only operate if the case is entirely full of water or primed and air-tight. The kinds under this category are the volute centrifugal pumps which include self-priming models (Fig. 8.1 and 8.2).

8.2.2 Deep-well turbine pump

This is capable of operating at high head and low to high discharge. It is used in cased wells or situations where the water lift is below the practical limits of a centrifugal pump. Successful installations have been made for lifts up to 300 m and capacities up to 7 000 gpm or 441 liters per sec. Deep-well turbines are much more expensive than centrifugal pumps and are more difficult to inspect and repair.

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Fig. 8.1 Horizontal centrifugal pump cross section

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Fig. 8.2 Self-priming volute pumps

The turbine has three main parts: (i) head; (ii) pump bowl; and (iii) discharge column (Fig. 8.3). The pump bowl is always placed beneath the water surface of the well. Fluctuation in the water table is determined prior to installing the pump so that the bowls of the turbine can be placed below the farthest drawdown point (Fig. 8.4.). The depth at which the bowls are located is called depth of setting. Since well diameters are relatively small, it is often necessary to use more than one impeller or one-stage pump. The head or height of lift produced by a multi-stage pump is proportional to the number of stages or bowls.

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Fig. 8.3 Deep-well turbine pump

187

Fig. 8.4 Turbine pump installation

8.2.3 Propeller pump

This pump has the characteristic of operating at low head but delivering large volume of flow. In almost all brackishwater aquaculture farms, ponds are constructed close to the water or within the tidal range. This makes the total dynamic head (TDH) to be as low as possible within the range of pumps designed for low head and large discharge.

Single-stage propeller pumps are limited to pumping against heads of around 3 m. By adding additional stages or bowls, 9 to 12 m heads are obtainable.

There are three basic designs of propeller pumps namely: (i) radial-flow; (ii) mixed-flow; and (iii) axial flow. All these three pumps have shaft to which impeller bowls are attached and submerged with the pump operating at proper submergence depth. A brief characteristic of the three pumps are given in Table 8.1.

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Table 8.1Characteristics of different types of propeller pumps

Propeller pump

Differentiating characteristics

Radial flow(Fig. 8.5)

1.Water enters the pump and thrown at 90° angle towards the wall of the bell.

2. Energy or force imparted to the liquid is all centrifugal.

3.Delivers flow at higher heads than the other two but less volume for the same power.

4.Normally operates at speeds up to 3 600 rpm, generally higher than the two.

Mixed-flow(Fig. 8.6)

1.Water entering the pump is thrown to the bell wall at an angle of 40° to 80° of the shaft.

2.Force imparted is combination of centrifugal and displacement energy.

3. Available at capacities over 30 000 gpm.

4.Normally operates at speed of 1 760 rpm. The usual speed of electric motor; hence, suitable to install where electricity is available.

Axial-flow(Fig. 8.7)

1.Water enters the pump parallel to the shaft and is lifted also parallel with the shaft; hence, all force imparted is displacement energy.

2. Hydraulic head range is up to 6 m per stage.

3. Available at capacities up to 500 000 gpm.

4. Operates at speed of about 1 160 rpm and higher.

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Fig. 8.5 Radial flow propeller pumps

190

Fig. 8.6 Mixed flow propeller pumps

191

Fig. 8.7 Axial flow propeller pumps

Among the three propeller pumps, the axial-flow with TDH of up to 7.5 m per stage falls within the usual range of operation necessary in brackishwater fishponds. One stage is only needed as the head required seldom exceeds 3 m TDH, because tidal fluctuation are slightly greater than 2 m only.

8.2.4 Special types of pumps

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There are two main types of special pumps developed in Thailand—the so-called dragon-wheel pump and the push pump. Both pumps are being used in shrimp ponds and suitable for low lifts of water such as from tidal water.

(a) Dragon-wheel pump. This is a simple type of pump which delivers water into the pond by using a wooden trough. Inside the trough is a series of blades connected by flexible joints and being moved by an axle which is being turned by a wheel. The wheel is connected by belt to the driving engine or windmill. The lower end of the pump is always submerged in water. As the series of blades moves along the trough, water is trapped and lifed to the pond (Fig. 8.8). The specifications of existing wheel pumps as given by Tharnbuppa (1982), are as follows:

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Fig. 8.8 Dragon wheel pump run by engine and windmill (After Tamiyavanich, 1977)

  Parts of pump Dimension/size

1. Engine (diesel, low rpm) 3.5; 8–10 hp

2. Wheel diameter 114–127 cm

3. Length of axle 3 m

4. Wooden trough:length 5–6 m

width 17.8–30 cm

5. Blade width 15.2–27.9 cm

(b) Push pump. This type is suitable for lifting water at an inclination of not more than 20°. This type has been used in Thailand within the last 10 years and some data on existing installation and area capacity are shown in Table 8.2. The water is being pushed up by means of a propeller through a tube or pipe such as asbestos, to a water conveyor in large volume. The propellers are made to rotate at a third or fourth of the engine rpm (300 to 500 rpm) (Jamandre, 1977). It was estimated that the rate of flow using a 120-Hp engine is about 5 196 m3 per hour (Tharnbuppa, 1982).

The pump unit consists of five main parts and accessories as follows: (i) diesel engine; (ii) propeller shaft with length of 6 to 8 m; (iii) propeller, 2–3 blades; (iv) pipe, concrete or asbestos; and (v) propeller shaft joints (Fig. 8.9).

Table 8.2Some data on push pump installation in Thailand (after Tharnbuppa, 1982)

Size of engine (hp)

Fuel consumption

(1/hr)

Diameter of shaft casing

in. (cm)

Shaft diameter in. (cm)

Propeller diameter in. (cm)

Pipe column

diameter in. (cm)

Area capacity

(ha)

40–75 6 2–3 (5–8) 1 (2.5) 12 (30.5) 16 (40.6) 4

120–150

10–12 3 (7.6) 1.5 (3.8) 16 (40.6) 20 (50.8) 8

180 to 220or more

10–12 3 (7.6) 2 (5) 20 (50.8) 24 (60.9) 16

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Fig. 8.9 Push pump and installation (After Tharnbuppa, 1982)

The sizes of engine for push pumps are somewhat oversized because they are reconditioned automotive diesel engines of trucks that are repaired and bought cheap. Where electricity is available, electric motors are used instead of diesel engine. At least 20 Hp motor should be used for the pump.

Where the engine is over-sized for the driven push pump, the extra power may be used for another purpose such as pumping underground water for mixing with seawater to reduce pond water salinity and for household use.

An exmple set-up is to add a unit of air compressor in the engine-push pump assembly. In this set-up, the engine will operate the air-compressor while also operating the push pump. From the compressor the high-pressured air is introduced into the pipe casing of a well above the water surface. The air pressure will then push the water into another pipe in the casing which is the water supply type (Fig. 8.10). This follows the principle of operation of an airlift pump.

Push pump is also to gather shrimp fry from the tidal canal for the pond aside from being a supplementary water source for the pond system. The shrimp seeds are drawn through the pump column and mortalities are estimated to be less than 20 percent due to the low impeller rpm. The gathered fry are then treated with saponin which selectively kills finfish species but not the shrimp fry. The fry are then allowed to enter the pond for culturing.

8.3 Selection of pump

The above discussion on pump provides a general basis in the selection of the type of unit. The final selection of pump is enhanced if one has knowledge of the characteristic performance curves of a particular pump which is usually available from the manufacturer. Examples of pump performance curves for the turbine propeller pumps are shown in Figs. 8.11 and 8.12.

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Fig. 8.10 Combination pump (After Tharnbuppa, 1982)

The characteristic performance curves provide a guide in the proper operation of a pump and indicate what could be expected of it or what it can do for different capacities at various speeds. The curve has two-fold purposes: (i) selection of a pump that will give maximum efficiency under any local condition; and (ii) adapt the pump to the best operating condition at the lowest possible cost or best efficiency.

For the pump installation in Fig. 8.4 and pump characteristic performance curves in Fig. 8.12, a verification whether given data are accurate may be made as follows:

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Fig. 8.11 Performance curves for propeller pumps (After Jamandre, 1982)

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Fig. 8.12 Pump characteristic performance curves for deepwell turbine pump

1. The discharge Q = 1 200 gpm.2. The total dynamic head (TDH) as determined from the pump installation can be verified from

the data below.

    Feet m

a) Suction head 55 (16.77)

b) Discharge head 25 (7.62)

c)

Friction head in discharge and suction pipeline, 375 (320 + 55) of 8" pipe at 1 200 gpm = 375 x 2.2/100 ft/ft-(from Table 8.3) 8.25 + (2.51)

8.25 (2.51)

d)

Friction head in fittings — equivalent length of two 45° angle fitting, 8" diameter (Table 8.4) is 10 × 2 = 20 ft; loss is 20 x 2.2/100 ft/ft

0.44 (0.13)

e)

Velocity head at end of discharge, 8" diameter and 1 200 gpm, velocity is about 7 ft/sec;

  0.76   (0.23)

89.45 (27.26)

An analysis of the pump characteristic curves (Fig. 8.12) shows the following: At Q = 1 200 gpm, head capacity = 90 ft (27.4 m) and brake horsepower = 33, the efficiency is 82 percent.

The comparison shows that at the given requirements of the installation, the pump has the necessary head capacity and is about at peak efficiency.

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8.4 Components of a pumping plant

The components of a pumping plant in coastal fish farm are as follows:

a. Pump and prime mover foundation. The bearing capacity of the concrete foundation must be sufficient to carry the weight of the pump and engine or motor driver. Considerations in the layout and elevation of pump and the prime mover must be given to the: (i) suction lift limitation; (ii) highest flood level; and (iii) accessibility and economy.

b. Suction sump. A sump is a basin provided at the foot of the pump column suction end. This protects the system against excessive debris, floatsam and also minimizes silting.

c. Distribution canals. These consist of main and secondary canals of the fishponds including a stilling basin or pool to which the pump directly discharges.

8.5 Design of suction sump

The design of suction sump should consider: (i) strainers and trash rack; (ii) spacing between a number of pump units; (iii) sump intake or flow pattern; (iv) submergence; and (v) clearance from floor and walls. Correct and incorrect location and spacing of suction ends are illustrated in Figures 8.13 and 8.14.

Proper depth of submergence of suction bell is to be observed in order to avoid cavitation and vortices in suction sump. The lower edge of the suction bell must have a depth of submergence of at least 1.5 m for usual velocity of water in pipes of about 8 ft per sec (2.4 m/sec.). For other velocities, Figure 8.15 may be used. The minimum allowable should be equal to the diameter of the suction bell. The suction bell, on the other hand, should not be less than twice the impeller hub in order to keep the pump self-priming during operation time (Jamandre, 1977). When vortices appear, baffles may be provided in the sump to avoid it. Figure 8.16 provides some idea on the location or design of baffles for some arrangement of suction pipe.

Adequate floor and wall clearances between the suction bell and the sump should be provided. This clearance should be equal to the diameter of the bell itself. Figure 8.17 illustrates the flow pattern or distribution at the entrance of a suction bell in relation to its distance from the suction floor and wall.

Table 8.3Friction loss of water, in feet per 100 ft of clean wrought-iron or steel pipe*

Flow, gpm

Nominal diameter of pipe, in.

1 1¼ 1½ 2 2½ 3 4 5 6 8 10 12

5 1.93 0.51                    

10 8.86 1.77 0.83 0.25 0.11              

14 12.8 3.28 1.53 0.45 0.19              

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20 25.1 6.34 2.94 0.87 0.36 0.13            

24 35.6 8.92 4.14 1.20 0.50 0.17            

30 54.6 13.6 6.26 1.82 0.75 0.26 0.07          

40   23.5 10.79 3.10 1.28 0.44 0.12          

50   36.0 16.4 4.67 1.94 0.66 0.18 0.06        

75     35.8 10.1 4.13 1.39 0.28 0.12        

100     62.2 17.4 8.51 2.39 0.62 0.20 0.08      

120       24.7 10.0 3.37 0.88 0.20 0.12      

150       38.0 15.4 5.14 1.32 0.33 0.17      

170       48.4 19.6 6.53 1.67 0.54 0.22      

200       66.3 26.7 8.90 2.27 0.74 0.30 0.08    

220         32.2 10.7 2.72 0.88 0.36 0.09    

260         44.5 14.7 3.24 1.20 0.49 0.13    

280         51.3 16.9 4.30 1.38 0.56 0.14    

300           19.2 4.89 1.58 0.64 0.16    

340           24.8 6.19 2.00 0.81 0.21    

400           33.9 8.47 2.72 1.09 0.28 0.09  

500           52.5 13.0 4.16 1.66 0.42 0.14 0.06

600             18.6 5.88 2.34 0.60 0.19 0.08

700             25.0 7.93 3.13 0.80 0.26 0.11

800             32.4 10.22 4.03 1.02 0.33 0.14

900             40.8 12.9 5.05 1.27 0.41 0.17

1 000             50.2 15.8 6.17 1.56 0.50 0.21

1 100               19.0 7.41 1.87 0.59 0.25

1 200               22.5 8.76 2.20 0.70 0.30

1 300                 10.2 2.56 0.82 0.34

1 400                 11.8 2.95 0.94 0.40

1 500                 13.5 3.37 1.07 0.45

2 000                 23.8 5.86 1.84 0.78

3 000                   12.8 4.00 1.68

4 000                   22.6 6.99 2.92

200

5 000                     10.80 4.47

* Reprinted from “Tentative Standards of Hydraulic Institute, Pipe Friction,” Copyright 1948 by the Hydraulic Institute, 122 E. 42d St.,New York, New York, 10017.

Fig. 8.13 Correct and incorrect sump designs for minimum entrained air into suction line(After Jamandre, 1982)

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Table 8.4Length of steel pipe, in feet, equivalent to fittings and valves*

Nominal size, in

Item 1 1¼ 1½ 2 2½ 3 4 5 6 8 10 12

90° elbow 2.8 3.7 4.3 5.5 6.4 6.2 11.0 13.5 10.0 21.0 26.0 32.0

45° elbow 1.3 1.7 2.0 2.6 3.0 3.8 5.0 6.2 7.5 10.0 13.0 15.0

Too, side outlet 5.6 7.5 9.1 12.0 13.5 17.0 22.0 27.5 33.0 43.5 55.0 66.0

Close return band

6.3 8.4 0.2 13.0 15.0 18.5 24.0 31.0 37.0 48.0 62.0 73.0

Gate valve 0.6 0.8 0.9 1.2 1.4 1.7 2.5 3.0 3.5 4.5 5.7 6.8

Globe valve 27.0 37.0 43.0 55.0 66.0 82.0 115.0 135.0 105.0 215.0 280.0 338.0

Check valve 10.5 13.2 15.8 21.1 26.4 31.7 42.3 52.8 63.0 81.0 105.0 125.0

Foot valve 24.0 33.0 38.0 46.0 55.0 64.0 75.0 70.0 70.0 70.0 70.0 70.0

* Courtesy the Gormon-Hupp Company

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Fig. 8.14 Section sump design showing proper spacing(After Jamandre, 1977)

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Fig. 8.15 Minimum suction pipe submergence for various pipe flow velocity(Source: Goulds pumps manual)

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Fig. 8.16 Baffle arrangement for vortex prevention(After Jamandre, 1977)

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Fig. 8.17 Floor and wall clearances between sump and suction bell(After Jamandre, 1977)

8.6 Power requirement

The capacity or discharge of a pump, its efficiency, and the total dynamic head are the necessary information in determining power requirement. The pump discharge is determined from the flow requirement of the fish farm, the efficiency at a given discharge rate and head from the manufacturer's pump characteristic performance curves (for different kinds of pumps), and the total dynamic head by obtaining the necessary measurement as implied in the example problem and Fig. 8.12.

The brake horsepower is computed by the formula:

 

Where: Bhp =the brake horsepower that must be supplied by the prime mover to the pump to operate it at the required capacity and given efficiency.

  E = pump efficiency

  Q = discharge of pump in gallons per minute

 TDH =

total dynamic head in feet

 3 960 =

a constant of conversion

When the prime mover is an engine, it should be operated at 75 percent of its full load capacity. The required engine horsepower is therefore:

8.7 Selection of prime mover

Available prime movers of pump to choose from are:

(a) Engine. Internal combustion engine are run either by gasoline or diesel fuel. In deciding which to use between the two, consideration should be given to the initial engine cost, fuel cost, cost and availability of spareparts, and availability of repair mechanic in the area. Since diesel engine has higher initial cost than the gasoline, it is advisable to use it more hours per season than the latter in order to be economical.

The brake and engine Hp in the preceding problem will be:

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(b) Electric motor. This is preferable if electricity is cheap and no frequent power interruptions occur. The advantage of electric motor is its long life, dependability, low maintenance cost, quiet and easy to operate and it is usually taken as 100 percent efficient.

If an electric motor is required to drive the pump in the preceding example problem, the needed Hp will be only 33 Hp.

8.8 Accessories and other devices

There are some accessories and devices that are important in the operation of pumps. These are as follows:

a. Foot valves. Centrifugal pumps usually need foot valves in order to hold water during priming. This valve is not necessary in propeller, turbine pumps and self-priming pumps.

b. Gear drive. One of the accessories in propeller pumps is the “gear drive”. This device does three things (Fig. 8.18): (i) change the direction of drive from vertical to horizontal for attachment of other prime movers; (ii) change input rpm to the desired or designed pump rpm; and (iii) provides alternative horizontal drive where there is already a vertical electric motor driving the pump. The gear drive, however, may cost as much as the pump itself.

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Fig. 8.18 Illustration of the function of a gear drive(After Jamandre, 1977)

c. Cross joints and shafts. These may be used instead of the gear drive. This is done by installing pump in a slanting position (Fig. 8.19).

d. Hydraulic driven pump. This is a system where the prime mover drives a hydraulic pump and the high pressure transmitted through hydraulic hoses drives a hydraulic motor attached to the impeller. One advantage of this system is that it becomes flexible as it eliminates the need for long drive shafts that need careful alignment (Fig. 8.20). It also eliminates the shaft as an obstruction in the pump column.

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Fig. 8.19 Cross joint and shaft assembly(After Jamandre, 1977)

e. Pump columns. Careful consideration on the kind of material for pump column in brackishwater is important to avoid or minimize corrosion. Pump columns are usually made of cast iron and not steel because it is less affected by saltwater. There are pumps, however, that have columns made of stainless steel; some have thick-coating of zinc making it durable and rust-resistant. Fiberglass columns, wood and concrete are also available in some pumps.

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Fig. 8.20 High discharge hydraulic driven pump(After Jamandre, 1977)

8.9 Pump installations in certain conditions

a. Vertical and slanting installations. These are illustrated in Figs. 8.19, 8.20 and 8.21.

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Fig. 8.21 Types of propeller pump installations (After Jamandre, 1977)

b. Installation where pond may be filled or drained irrespective of tidal conditions. Jamandre (1977) suggested and designed propeller pump and open channel combination, and system of gates valves to flood and drain ponds at will.

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Figure 8.22 illustrates the pump installation and elevation of bottom of two adjacent channels provided with system of gates for checking the passage or entry of tidal water. The discharge pipe can discharge water in either direction in the channel through the manipulation of the gates.

In Fig. 8.23 the pump can water the pond with valves 1 and 3 close, and 2 and 4 open. With valves 1 and 3 open, and 2 and 4 closed, the pump can drain the pond.

Fig. 8.22 Set-up for filling and draining pond water irrespective of tidal level (After Jamandre, 1977)

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Fig. 8.23 Gate valve system for filling and draining fishpond regardless of tide level(After Jamandre, 1977)

Being able to fill and drain the pond at will offers several advantages:

a. Enables harvesting of crop when prices are good while other pond owners have to wait for appropriate tidal condition.

b. Drain low oxygen water and replenish with fresh and highly oxygenated water.

c. Enables greater stocking densities or intensive culture in ponds.

8.10 Economics of pump use

The use of pumps in coastal aquaculture as an alternative solution to some of the problems associated with tidal fishponds is becoming popular. Although some of these problems could be remedied by proper pond construction and efficient management, they are not entirely eliminated and the costs involved significantly affect the financial viability of the fishpond enterprise.

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Probably, the use of pumps is a better alternative. However, before a decision is made or whether to use pumps or not, a close examination of the costs associated with their use should be made.

Information regarding the use of pumps in brackishwater fishpond culture is very scarce. One such study made in Malaysia (Gedney, Shang and Cook, 1982) offers significant information. Designs for both tidal and pumped-operated pond culture systems were prepared and a comparative cost analysis of expense items which are different between the two systems were made. These items are interest and principal payment of pond construction and pumping, maintenance and land. Results of this particular study showed that a pump-operated system is more economical than a tide-operated system because of the savings in costs of construction and operation.

The specific preliminary conclusions of the study identified the following advantages of pump-operated system over tide-operated system:

a. Less construction cost because of smaller dikes and reduced time of construction.b. Better land utilization due to greater effective water area and use of lots otherwise not

feasible under a tidal fishpond system.

c. More efficient management which allows for flexibility in filling, draining and harvesting and easier pond maintenance.

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CHAPTER 9DEVELOPMENT COSTS OF COASTAL FISH FARMS

9. COST ITEMS IN THE DEVELOPMENT OF A FISH FARM

One of the major constraints that impede the development of coastal aquaculture not only in the region but also throughout the world is the high initial cost involved in site selection, planning/design activities, and actual construction of fishpond facilities. The initial cost of fishpond development comes from two sources, namely: cost of land and the cost of feasibility planning and designing activities. Subsequently, the major engineering cost consists of the actual construction.

9.1 Cost of land

The cost or value of land depends primarily on its earning capacity or in this case specifically on its suitability as a fishpond site. Normally, land value is a function of the amortized net returns of the products that can be produced from the land.

Land for fishpond development can be acquired by purchase from a private owner or through lease of public land. The acquisition of land by purchase is an important decision to make and an unwise judgement can lead to disastrous results. Prospective land buyers should determine, regardless of the market value, whether the production potential of the land justifies the purchase.

In most countries in the region, potential areas for coastal aquaculture are public or government property. Their acquisition by leasehold contracts are governed by the country's respective leasehold policies and regulations. In the Philippines, for example, the Bureau of Forest Development and Bureau of Lands in cooperation with the Bureau of Fisheries and Aquatic Resources (BFAR), all under the Ministry of Natural Resources (MNR) are in charge of the disposition of public lands for fishpond development. Applicants must comply with certain requirements and must adhere to established terms and conditions before the lease contract is issued. The annual lease rental is P30/ha/year. Maximum area that can be leased is 50 ha for

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individuals and 500 ha for associations or corporations. Fishpond sites can be leased for 25 years and renewable.

There are no general recommendations as to whether it is better to buy or lease land. The decision depends on the assets of the developer, the availability of land for sale and for lease, rates and preferences.

9.2 Feasibility planning and designing cost

The lack or absence of a proper plan and workable design for a coastal fishpond has often resulted in waste of money, time and effort. Despite recent advances made in coastal aquaculture development, especially in the field of culture management practices, there have been increasing problems in the planning and implementation of construction of projects. In order to come up with an integrated approach in the design and engineering of fish farm, the ideas of biologists, engineers, and economists should be considered.

9.2.1 Pre-construction evaluation work

Suitability of a particular area for fish culture is site specific and depends upon numerous factors. Sites are to be pro-rated in terms of their technical suitability and likelihood of economic viability and freedom from possible social constraints. The pre-construction evaluation is made up of the above analyses.

9.2.2 Costs

Feasibility planning and design of a small-scale fish farm is usually done by the proponent himself. However, large fish farms require the services of skilled and trained personnel. There are groups of specialists which specialize in this kind of work for which a fee is paid for their services.

There are no standard rates of charge for this kind of work. The cost generally depends on the extent of work that is required but in most cases, a fee of about 10 percent of the overall development cost is charged.

9.3 Construction cost

The major cost in coastal fishpond development is the cost of construction which is made up of three components, namely: cost of land clearing and grubbing, expenses for earthwork, and cost of construction and installation of water control gates and other structures. These works comprise about 50–70 percent of the total development cost.

In carrying out the construction work, a detailed plan and budget is required. It is also important that a construction schedule is prepared. Punctual execution of work activities according to the specified timetable will be necessary.

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9.3.1 Land clearing and grubbing

Cost estimates for this operation depend largely on the type and density of vegetation in the area. Factors that affect work efficiency such as number, size, wood density and rooting system of trees and other vegetation are taken into consideration. One estimate of the total manual labour requirement for clearing a typical tidal forest according to specifications for fish farm development is 140 man-days/ha (Tang, 1977). The rate per day can vary for different areas, and if heavy equipment is used the costing will be different.

Generally, cost estimates are made on a per unit area basis (per ha or per acre). Sometimes, costs are computed in a per individual tree basis (e.g., 7 Philippine Pesos per tree of 20 cm or more trunk diameter in Aklan province, Philippines). Approximately, 10–12 percent of total construction cost is spent for this aspect of the construction work.

9.3.2 Earthwork

Earthwork for coastal fishpond development includes the construction of dikes, water supply and drainage canals and pond excavation and levelling. These operations constitute the major actual construction cost and the bulk of the construction work (estimated at about 50 percent or more).

Dikes are measured by volume. The volume for each kind of dike (main, secondary and tertiary) are estimated separately following the example given in Chapter 5. When the canals of the pond system have their own separate dikes or are not parts of the pond dikes, their volumes are also added to the total estimate. In outline form, the estimate for total volume is done as follows:

  Type of dike Cross-sectional area (m2) Length (m) Volume (m3)

1. Main perimeter      

2. Secondary      

3. Tertiary      

4. Canal (if separate unit)      

  Total volume, m3      

Pond excavation and levelling is done after the dikes have been constructed. The job includes a cut-and-fill method where certain areas are cut excavated and filled and dumped into the low areas.

Determination of the volume of soil to be moved can be determined from a prepared topographic map. However, an alternative method is being used which is done by manipulating the water level with the help of an ordinary wooden depth gauge (Fig. 6.24). The procedure is as follows:

The water level at the staff gauge or benchmark (Figs. 6.21 and 6.22) is lowered to the desired level of pond bottom. At this point, the waterline of all portions of the proposed pond that are exposes are staked out. The area of the exposed ground must be determined: this represents the

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excess elevation of the pond bottom and hence, must be removed by excavation. After staking out the exposed area, the water level is again raised up to the highest portion of the ground. Random measurement of water depths by the depth gauge within the limits of the exposed area follows in order to determine the average depth. The data obtained can be used in determining the average thickness of soil to be excavated and the volume of excavation.

The earthwork cost is estimated based on the needed time to finish the job or on the volume of soil or earth needed for the construction of dikes, supply and drainage canals and excavation/levelling according to the required engineering specifications. In manual construction, the total cost is usually calculated based on the actual earthmoving expenses for labour including construction tools. If machinery is used, the cost is estimated based on volume of earth moved, per unit area or per unit of time including equipment rent, operating costs and a profit margin for the owner.

9.3.3 Cost of water control gates and other structures/ facilities

The cost of construction and installation of water gates and other structures rank next to earthwork as the major expense items in the actual construction of coastal fishponds. The cost estimates for these items depend on the design and specifications proposed for the area. Approximately, 20 percent of the total development cost is spent for these items.

There are three types of gates commonly constructed in tidal fishponds. These are the main, secondary, and tertiary gates. Construction materials to be used can either be concrete, wood, or combination of both.

The cost of water control gates can be calculated based on the design (size, volume and type of construction materials) and the labour for construction and installation. Estimating the cost for water control gates can be done following the suggested method by the BFAR-UNDP/FAO, 1981.

(a) For concrete gate

(i) Use the following formula to calculate the area and volume of the walls, wings, floor, bridges, toes, aprons and cut-off-walls.

  A = L × W

  V = A × t

  VT = Sum V = V1 + V2 + V3 + … + Vn

Where:

A = area

V = volume

VT = total volume

L = length

W = width

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t = thickness

(ii) Use the following to determine the number of bags of cement, and volume of gravel and sand plus 10 percent allowance for wastage. This is based on class A mixture which has a proportion of one part cement, two parts sand, and four parts gravel.

No. of bags of cement

= (VT × 7.85) 1.10

Volume of gravel = (VT × 0.88) 1.10

Volume of sand = (VT × 0.44) 1.10

(iii) Use the following to estimate the number of reinforcement bars using a standard length of 20 ft (6 m) per bar. Bars of 0.25 inch (0.6 cm) or 0.5-inch (1.3 cm) diameters are usually used, based on engineer's choice.

For the floor and toes:

No. of bars = (Af + At) 1.5

Where Af = area of floor

At = area of toes

For the walls, wings, etc.:

Where:

Aw = area of walls

Ax = area of wings

An = other areas on gate not previously included

(iv) Use the following to calculate the weight (kg) of tie wire (No. 16) required, multiply the total area of gate from (i) by the factor 0.3.

Weight of tie wire (kg) = AT × 0.3Where: AT = total area of gate

(v) The volume of boulders needed is calculated by multiplying the floor area with the thickness of fill.

(vi) Form lumber is calculated by multiplying the area of walls, wings and bridges by 2. Plywood can be used as form while 2" × 3" (5 × 8 cm) lumber can be used as form support or braces.

(vii) Bamboo trunks (base) as pilings are calculated based on the floor area. In general, about 20 pieces of bamboo per m2 are staked at about 0.25 m intervals.

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(viii) Screens and wooden slabs are calculated based on the design of the gate.

(ix) Assorted nails needed for the construction is calculated based on the thickness of the form lumber used.

(x) Labour cost is estimated based on 35 to 40 percent of total cost of materials.

(xi) A contingency cost of 10 percent of the total cost of material is also included in the total cost estimate for the construction of a concrete gate.

(b) Wooden gate

(i) Determine size and number of lumber for the sidings and flooring based on the plan of the wooden gate. Compute for the total board feet (applicable in the Philippines where lumber dealers still use this unit) using the following formula:

Where:

L = length of lumber in feet

W = width of lumber in inches

T = thickness of lumber in inches

(ii) Compute for the total board feet required for the pillars and braces based on the design and specification of the gate.

(iii) Determine size and number of lumber needed for slabs and screen frames and compute the total board feet.

(iv) Calculate the assorted nails (bronze) needed based on the lumber used.

(v) Calculate the coal tar requirement in liters or number of containers based on their capacity.

(vi) Estimate the cost of nylon and bamboo screens needed.

(vii) Estimate labour cost at 30 to 40 percent of total material cost.

Examples of cost estimates for the construction of a concrete main gate, secondary wooden gate, and tertiary wooden gate are presented in Table 9.1.

(c) Other structures and facilities. A fish farm includes not only the pond system but also other support facilities. These include a caretaker's house, working shed, storage space, chilling tanks and others. To be able to make an accurate estimate, there must be a plan for these various facilities. Cost estimates depend on the floor area and type of construction material to be used. Cost of other structures and facilities comprise about 3 to 4 percent of the total development cost.

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9.3.4 Other costs

In addition to the major expense items in coastal fishpond construction, there are other costs that are incurred in the development stage. Among these are the following:

Table 9.1Example of estimate of material and labour requirement for water control gates

(after BFAR-UNDP/FAO, 1981)

1.Double opening main concrete gate

2.Secondary wooden gate

3. Tertiary wooden gate

  Materials Quantity   Materials Quantity   Materials Quantity

  1. Cement 140 bags   1. Plywood -     1.Plywood,1" × 12" × 10'

20 pc

  2. Sand 10 cu.m.    a) 1" × 10" × 14'

34 pc   2.Slabs,1" × 12" × 10'

3 pc

  3. Gravel 20 cu.m.    b) 1" × 10" × 8'

3 pc   3.

Pillars and braces,2" × 3" × 10'

14 pc

  4. Boulders 8 cu.m.   2.Slabs, 1" × 12" × 14'

2 pc   4.

Grooves and screen frames,1.5" × 2" × 8'

18 pc

  5.Reinforcement bar

      3.Pillars and braces

      5.Nails (assorted)

6 kg

    a) 0 ½ x 20' 80 pc    a) 2" × 3" × 10'

4 pc   6. Coal tar 1 can

    b) 0 3/8; x 20' 35 pc    b) 2" × 3" × 8'

7 pc          

  6.Plywood form,(¼" × 4' × 8')

49 pc    c) 2" × 3" × 14'

2 pc          

             d) 3" × 4" × 10'

12 pc          

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  7. Lumber (S4S)     4.Screen frames

             

   a) 2" × 2" × 12'

30 pc    a) 2" × 3" × 16'

2 pc          

   b) 2" × 3" × 12'

16 pc    b) 1.5" × 2" × 7'

1 pc          

   c) 1" × 2" × 12'

10 pc   5.Groove, 1.5" × 2" × 10'

16 pc          

   d) 1" × 12" × 12'

6 pc   6.

Nylon screen, mesh size No. 16

8sq. m.

         

  8. Assorted nails 10 kg   7.

Bamboo screen, 3 m long (whole)

6 pc          

  9.G.I. Wire No. 16

20 kg   8.Nails (assorted)

8 kg          

  10. Bamboo base 400 pc   9. Coal tar 2 cans          

 

Labour (40% of material cost)Contingencies (10% of material cost)

 Labour (30% of material cost)

 Labour (30% of material cost)

(a) Purchase of equipment. Equipment are necessary in the proper operation and management of a fishpond. Cost for these items are normally included in the initial development cost. Among the important equipment required are fish nets, digging blades, shovels, scoop nets, cutting equipment, carpentry tools, and others. The required equipment should be listed down and their corresponding costs are estimated. Additional details on these equipment are given in Chapter 7.

(b) Contractor's tax, labour and profit. In some cases, fishpond investors hire the services of a private contractor to do the overall development of their fishpond projects. Under these circumstances, a fee is agreed beforehand between the owner or investor and the contracting party. The fee is composed of three major components. These are the commensurate payment of labour of the contractor plus a reasonable amount of profit. Since the fee represents the contractor's earning, it is a taxable income. It is common practice that the equivalent tax is borne by the investor and included in the total payment for the contractor's services.

(c) Contingencies. A contingency fund is normally set aside specifically intended for unexpected additional expenses.

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For example, a certain amount must be allocated to cover inceases in prices of materials, cost of labour and for other expense items not included in the original cost estimate. At least 10 percent of the total development cost is assumed for contingencies.

9.4 Estimating development cost (Examples 9.1 and 9.2)

Presented in the following outlines are examples in estimating construction and development costs of coastal fishponds. The outline includes the sources of expenditures under each major cost item, bases for computing costs and estimated percentage cost composition.

Cost estimates for the improvement/renovation of an existing fishpond depend on existing physical conditions/ development and the extent of additional work/improvement to be done. Pre-development feasibility analysis may or may not be undertaken. Thus, the major expense items are expenses for construction and other costs.

The following are the possible sources of expenditures for a typical improvement/renovation work for an existing fishpond. Additional details on pond renovation are also given in Chapter 7.

Example 9.1. Development of a new brackishwater fish farm

Cost category/source of expenditurePercent of total

cost

I. Pre-development   10

  1. Feasibility study (6–10)  

  2. Construction of temporary shelter for labourers (1)  

  3. Construction of flatboats and dugout canal (1)  

  4. Others (representation and transportation expenses, etc.) (1)  

II. Construction cost   80

  1. Land clearing and grubbing (10)  

    a)Cutting, chopping, burning and removal of trees (based on per unit area)

   

    b)Uprooting and destumping (based on per unit area or per individual tree)

   

  2. Earthwork (55)  

    a)Construction of dikes (based on volume of earth deposited in place)

   

    b)Construction of water supply and drainage canals (based on volume of earth moved)

   

    c)Excavating and levelling (based on per unit area or volume of earth moved)

   

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  3.Water gates and other structures/facilities (based on type of construction materials used)

(15)  

III. Other costs   10

  1. Equipment (nets, digging blades, containers, tools, etc.)    

  2. Contingencies    

Total   100

Example 9.2. Improvement/Renovation of an existing fishpond

Cost category/source of expenditure Percent of total cost

I. Construction cost 80–85

  a) Uprooting and destumping  

  b) Earthwork  

    (i) Raising and widening of dikes  

    (ii) Repair of water supply and drainage canals  

    (iii) Excavation and levelling  

    (iv) Construction or repair of water control gates  

II. Other costs 15–20

  a) Equipment  

  b) Contingencies  

Total 100

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