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10. POND OUTLET STRUCTURES

10.0 Outlet structures

1. Outlet structures are built for two main reasons:

  • to keep the water surface in the pond at its optimum level , which usually coincides with the maximum water level designed for the pond;
  • to allow for the complete draining of the pond and harvesting of the fish whenever necessary.

2. In addition to these major functions, a good outlet should also ensure as far as possible that:

  • the amount of time necessary to drain the pond completely is reasonable;
  • the flow of the draining wate r is as uniform as possible to avoid disturbing the fish excessively;
  • there is no loss of fish , especially during the draining period;
  • water can be drained from the top, bottom or intermediate levels of the pond;
  • any reasonable excess of water is carried away;
  • the outlet can be easily cleaned and serviced;
  • the construction cost and maintenance are relatively low.

3. In most cases, outlets have three main elements :

  • a collecting area on the inside of the pond, from which the water drains and into which the stock is collected for harvest;
  • the water control itself , including any drain plugs, valves, control boards, screens and gates;
  • a means for getting the water to the outside of the pond such as a pipe or a cut through the wall, and/or an overflow structure. In both cases, a protected area on the outside of the wall must prevent the drain water from scouring the walls or drainage channel.

4. Pond outlets can be built in various ways, using different materials such as bamboo, wood, bricks, cement blocks or concrete. There are four main types , which will be discussed in turn:

  • simple cuts through the dike (see Section 10.2);
  • simple pipelines and siphons (see Sections 10.2 and 10.3);
  • sluices (see Section 10.4); and
  • monks (see Sections 10.5 to 10.9).

5. The main principles of design, including pipes and overflows, are discussed first in the next section.

10.1 Important points when designing outlets

1. When designing and constructing an outlet structure, you should pay particular attention to the following points:

(a) Design a water discharge capacity large enough so that the pond can be drained within a reasonable amount of time, from a few hours for a small pond to one to two days for a large pond. (To select the right size of pipes see later in this section.)

(b) Design the elevation of the bottom of the outlet structure low enough to ensure complete drainage of the pond, so that:

  • on the pond side, it is at least 10 cm lower than the lowest point in the pond;
  • it slopes away from the pond, preferably with a slope equal to or greater than 1 percent;
  • at its end, it is at least 20 cm higher than the bottom of the drainage canal.

(c) Design its total length so that the water will be discharged well away from the outside toe of the dike to avoid damaging it by erosion.

(d) For a barrage pond , the outlet should be built away from the streambed if possible and dug in lower than the lowest point of the pond bottom.

(e) Plan for the outlet to be built before or right after the beginning of the dike construction, depending on the type of pond. (see Table 50, section 12.6)

(f) If the outlet structure is heavy, be sure to build it on very well compacted soil only (see Section 6.2). You may also need to support it with simple piling.

(g) If there is an outlet pipe through the dike , it is always best to build at least one antiseepage concrete collar around it:

  • place it so that it will be integrated with the dike;
  • build it perpendicular to the pipe;
  • extend it at least 15 cm from all sides of the pipe;
  • make it at least 10 cm thick.

(h) When building the dike above the pipes , be sure to compact the earth well around them.

(i) Remember that small pipes can become easily blocked , particularly inside the pond. Therefore:

  • avoid using pipes with a very small diameter;
  • protect the entrance of all pipes with a screen, and keep this screen clean through regular checks.

Note: you should try to standardize the type of outlet to be built on your fish farm, to make them easier and more economical to use.

2. To assist you in selecting the right type of outlet, consult Table 46 . You should also take into account how much you can invest and which kind of materials are locally available.

Fill the form with a mixture of one part cement
to two parts sand and some water

Place the pipe in the form

Remove the form after the concrete
has completely dried

Sizing outlet pipes

3. The inside diameter of outlet pipes will determine the water discharge capacity of the outlet structure.

TABLE 47
Sizes of outlet pipes for diversion ponds

5. The water carrying capacity of the selected pipes can be estimated from Graph 1, Tables 13 and 14, or mathematical formulas (see Section 3.8).

Note: for barrage ponds fed directly by a stream, you might need larger pipes than usual if the excess water inflow has to be discharged continuously through the pond outlet. In most cases, it will be safer and more economical to build a lateral overflow structure (see Sections 11.3 and 11.4).

Placing and fixing outlet pipes

6. In general, in stable soil with well-constructed dikes and smaller pipes, no special precautions are required, apart from ensuring that pipe trenches are level and that the pipe will not be damaged .

Additional overflow structures

8. For security reasons, you should always ensure that the water level in the pond does not rise above the designed maximum level and flow over the top of any dike (see Section 11.1 on the discharge of excess water). This circumstance might result not only in the loss of most of your fish but also in heavy repairs before you could use your pond again.

9. In a diversion pond , where most of the excess inflow water is discharged at the inlet diversion structure, a pond outlet such as an open stand-pipe, a monk or a sluice should discharge any excess automatically. But you have to ensure that all screens are kept clean .

10. For a barrage pond directly fed by a stream , however, you need additional emergency such as a mechanical spillway for the continuous discharge of the excess water and possibly an additional emergency spillway for the occasional discharge of flood water (see Sections 11.3 and 11.4).

11. If you do not use an outlet or if you use one which does not regulate the pond water level (a closed pipe, for example), you will definitely need a protection structure to discharge any occasional excess water (see Chapter 11).

10.2 Simple outlets for small ponds

Using a cut in the dike

1. Very small rural ponds can be harvested by cutting the dike open at one of the deepest points of the pond. It is rebuilt when the pond has to be filled again. In such a case:

  • the dike next to the cut can be damaged, especially if the water in the pond is relatively deep and the water current too strong ;
  • repairing the dike creates additional work;
  • the quality of the dike can be impaired and the risk of it breaking away increased.

2. If for some reason, you cannot build an outlet for your pond, you can limit the damage to your dike :

(a) Do not build your pond too deep , and make the cut in stages, draining off the upper levels of water first.

(b) When draining the water, use a piece of canvas .

(c) When building the dike, include two rows of strong wooden poles in it . Between them, close the gap by carefully compacting clay soil up to the maximum water level. Protect the top of this gap with stones or gravel, so that any excess water can flow from the pond over it without damage. Each time you drain the pond, cut open the gap between the two rows of poles and rebuild it to fill the pond with water.

Using a siphon as a pond outlet

3. Small ponds can be drained either partly or fully, using a siphon (see Section 8.9 for use of siphons). One of the limitations of this method is that the outlet of the siphon has to be at least 20 cm below the level to which you want to drain. using a siphon (see Section 8.9 for use of siphons). One of the limitations of this method is that the outlet of the siphon has to be at least 20 cm below the level to which you want to drain.

5. To start a siphon flowing, proceed as follows, with the help of another person:

(c) Fill the siphon with water from the other end; when it is full, block this end either with your hand, or better, with a second plug.

(d) Bring down the end of the pipe slowly and submerge it fully into the pond, keeping it blocked all the time.

(e) Ask your assistant to open the other end of the siphon, ensuring that it remains at its lowest level.

(f) At the same time, open your end of the siphon, ensuring that it remains well under water. The water should start flowing through the siphon.

(g) As the water keeps flowing continuously and its level drops, ensure that the pond end of the siphon remains under water and that the outer end is kept below the inner water level.

6. To estimate how long it will take you to drain your pond , you can use either Graphs 11 and 12 or Table 45 , together with the pond water volume (see Water, 4 ).

Note: in some cases, it may be useful to make a filling point at the top of the pipe . Proceed as follows:

(a) Plug both ends of the pipe.
(b) Fill the pipe completely
(c) Plug the filling point.
(d) Making sure the outlet end is below the draining level and that the inlet is well submerged, open both end plugs.

10.3 Simple pipe outlets

Using a simple pipe and stopper

1. For small-size and shallow ponds , a straight pipeline with a small diameter can be used as a water outlet. It is important that the pipes be laid down at the lowest point of the pond before the dike is built. You can select one of the following pipes according to availability and cost:

2. The pipeline should be closed at one end before you start filling the pond with water. You should preferably close the end of the pipe that is under water so that it does not become blocked by debris or even a fish. You can use, for example:

  • a simple wooden plug;
  • a screwing cap fitting the galvanized or plastic pipe; or
  • a mechanical valve fitted to these pipes.

3. In some cases, you can also use a stand-pipe , which can be adjusted or renewed as required. This alternative is discussed in detail next in this section.

4. When you wish to drain the pond , proceed as follows:

(a) Remove the plug or cap, or open the valve; if you are using a plug, it may be useful to fix to it a metal or cord loop, which you can pull with a long-handled hook. You can even attach a pulling cord if you affix it securely.

(b) Immediately put a screen on top of the pipe end so that it does not get blocked.

(c) Keep cleaning the screen as necessary.

(d) When the pond is drained, either remove the screen and close the pipe immediately or remove the screen later, when you start filling the pond.

The flexible tube stand-pipe

5. This type of outlet is made of two parts:

  • a slightly sloping rigid pipe such as a bamboo or a plastic pipe, running through the base of the dike; and
  • a vertical flexible tube connected inside the pond to the rigid pipe and reaching up to maximum water level.

7. To drain the pond , detach the top end of the vertical tube from the pole. Lower it progressively as the water level drops. Remember to keep the screen over the end of the tube at all times.

Note: you can also use this type of outlet for regular overflow. Remember to keep a screen securely fitted.

8. You can also arrange this system with the flexible tube on the outside . In this case you need a screen at the inside end of the rigid pipe. Otherwise the principle of its use is the same.

The turn-down stand-pipe

9. Similar to the flexible stand-pipe, this pond outlet is made of three rigid plastic parts:

  • a slightly sloping base pipeline , made for example of one or more PVC pipes running through the dike-,
  • a vertical pipe , which reaches up to the maximum water level;
  • a 90°-elbow , which connects these two pipes. It can be glued to the vertical pipe with plastic cement, but need not be unless the fit is very loose. The connection to the base pipe is unglued , but can be greased with a suitable material such as mineral grease, lard or palm soap.

10. To ensure that the 90°-elbow , does not become separate from the horizontal pipe , either by accident or for poaching, protect it as follows:

(a) Drive a treated wooden stake about 3 to 5 cm thick and 50 to 60 cm long well into the pond bottom, right in front of the vertical pipe . Choose a water-resistant wood (see Section 3.1). Drive the stake firmly into the pond base, making sure it is long enough to reach up above the top edge of the upright elbow pipe.

(b) Right at the back of this stake and at its centre, drive a steel rod or a small diameter steel pipe into the pond bottom to a depth of about 1 m and attach it to the stake. It should be long enough to reach up to the top of the vertical pipe.

(c) Weld a chain or tie a rope to the steel post, about 10 cm from its top end.

(d) Near the top of the vertical plastic pipe, strongly fix a small device for attaching this chain or rope. You can use, for example, a steel hook, a swivel eye or a small screw-shackle.

11. This type of outlet can be set up either:

  • inside the pond , in front of the dike; or
  • outside the pond , at the back of the dike, in which case you need a screen at the inner end of the base pipe.

12. It is usually best to have the vertical pipe inside the pond to reduce the risk of blocking the horizontal pipe and to control leakage.

13. When using such a turn-down pipe, remember the following points:

(a) If possible, design the opening of the horizontal pipe to be at least 10 cm below the lowest point in the pond.

(b) For additional protection, you can cover with concrete the section of the horizontal pipe that sticks out in front of the dike inside the pond.

(c) Always secure the pipe well to the steel post in front of it with the rope or chain.

(d) Tightly fit a screen on top of the vertical pipe.

14. To regulate the water level in the pond , set the pipe at the required angle by turning it up or down. Fix it in the set position with the chain or rope.

15. To drain the pond , turn the vertical pipe down progressively, iollowing the water level as it drops. When it has reached the horizontal position, remove the elbow pipe from the end of the horizontal pipe to corroete the draif-4ng and harvest the fish.

Note: keep the screen on the vertical pipe until you detach the elbow from the horizontal pipe. At that moment, immediately transfer the screen to the front end of this pipe.

16. As with the flexible pipe, you can use this system for handling normal overflow water , because any surplus in the pond above the selected pipe level will automatically drain.

10.4 The sluice gate

1. A sluice gate consists of a protected opening in the pond dike that can be easily closed with wooden boards to regulate water level and can be screened to avoid fish losses. Whenever required, it can discharge excess water continuously.

2. As the sluice gate is an open outlet structure , it does not require a base pipe through the dike. It has the same functions as a monk (see Section 10.5), but has some advantages :

  • a sluice gate is easier to build;
  • less water leaks within the dike;
  • it frightens the fish less and makes their harvesting easier;
  • its water discharge capacity for a given size is usually greater.

3. However, its major disadvantage is that it is more expensive to build :

  • it is built of bricks or cement blocks, concrete or reinforced concrete because, to last for a long time, it requires very good quality masonry;
  • because more material is needed, this difference in cost rapidly increases as the size of the dike increases, as shown in the illustration.

4. Therefore, sluice gates are usually preferred to monks for small- to medium-size ponds only , when the dike to be crossed is relatively narrow.

Amount of concrete(m 3 )

5. A sluice gate essentially consists of:

  • a horizontal floor ;
  • two vertical walls parallel to each other;
  • grooves to fix wooden boards and screen; and
  • side wings that can be added to reinforce the construction and to reduce the risk of water leaks along the side walls.

6. It is best to limit the inside width of the sluice gate to 0.80 m at the most. More than that and it becomes difficult to handle the wooden boards, particularly when the pond is full.

7. The number of grooves can vary according to the size of the pond and the method used for harvesting the fish.

8. If the pond is small , or if the harvesting is done in a catch basin built behind the sluice gate, one set of grooves built into the centre of the sluice gate may be sufficient.

9. If the pond is larger and the harvesting is done in the sluice gate itself, two sets of grooves will make harvesting easier. Build them as follows:

  • a set of two pairs of grooves in the front of the central part for one set of boards and one screen, for pond operation;
  • a set of two pairs of grooves in the rear part of the gate for one set of boards and/or screens, for drainage of the pond and harvesting.

10. It is useful to build a simple bridge over the top of the sluice gate to allow the passage of people and light carts or barrows. It can easily be made by assembling wooden planks together, or by making a simple reinforced slab top, if you are using concrete.

11. To find out how much water you should be able to discharge through typical sluice gates, use Graph 6 and Tables 32 and 33 in Chapter 7.

12. Sluice gates can be built of various materials such as wood, bricks, cement blocks or reinforced concrete. You will now learn how to use each of these materials to build a simple sluice gate.

Building a wooden sluice gate

14. Determine the size of the sluice and the pieces of wood required according to the size of your dike. If possible, build the sluice before constructing the dike or leave enough space for the sluice, plus its anti-seep boards, to be installed in one piece. In this case you may be able to assemble and prepare the sluice in good working conditions away from the pond , and then fix it firmly into place on site.

15. If the sluice is too large you will need to build it on site .

16. First excavate and clear out the required area. Pack the base area firmly with good pond soil. If pond soils are very soft, either increase the length of the vertical posts or use more of them. Alternatively you can use piling (see Section 10.7).

17. Mark out the positions for the main vertical posts. Make up the vertical posts into square frames by bolting, screwing or nailing (one point per corner) the horizontal pieces.

18. Fix the frames into position, and drive them well in. Line the frames up, making sure the horizontal pieces are level, and that they line up along the length of the sluice.

Building a sluice gate with bricks or concrete blocks

21. If you are using bricks or blocks , the structure is considerably heavier and needs good foundations , usually of concrete or reinforced concrete. You also need good masonry skills. You should proceed as follows (see also Section 10.7):

(a) First prepare, mark and level the site. Dig out the foundation area to the required level. If necessary, fix and level foundation piles and lay and level the bedding material.

(b) Mark out and position simple wooden forms for the concrete base and fix them well. Prepare the surfaces, for example using old motor oil. Fix any reinforcement required. If possible, place some wooden blocks 2 to 5 cm below the upper base level, in the position of the walls. Pour the concrete. Use a lean to medium mix containing 195 to 250 kg cement/ m 3 (see Section 3.4).

(c) Once the concrete is well set and cured, prepare and position guide markers for the sluice walls. If you have used wooden blocks, remove these to obtain a “key” for the walls. Otherwise, make a key for the walls using a hammer and chisel to cut the foundation. Build the walls, taking particular care to ensure that inner surfaces are smoothly and cleanly finished off. If you wish, fix in attachment bolts for sluice board guides. For the best finish, it is preferable to plaster the inner walls with a medium mortar.

(d) Once the walls are completed, fix the sluice board guides, using pre-placed attachment bolts or masonry bolts and mortar. Make sure they are parallel.

(e) Carefully place and pack the dike material around the sluice structure.

Building a sluice gate with reinforced concrete

22. You can make a stronger structure, of similar weight, by using reinforced concrete . You should proceed as follows (see also Section 10.8):

(a) Prepare the foundation area as for the brick sluice, fix the base reinforcement, and tie in any wall reinforcement that has to connect into the foundation. Take care that all reinforcement is in position. You may need to make temporary holding forms to support the vertical bars.

(b) Pour the foundation concrete and allow it to set and cure.

(c) Mark out positions, place and fix the forms or shutters for the sluice walls (note that if you are making several sluices it may be worth using steel forms that can be re-used repeatedly). Fix in any additional wall reinforcement needed, and if required, fix in attachment bolts for sluice board guides. Make sure the shutters are well secured at the base.

(d) Pour the wall concrete, tamp it well down and allow it to set and cure. Use medium-rich concrete (250-350 kg cement/m 3 ) . Remove all the shuttering, and clean, pick out and finish off all surfaces.

(e) If sluice board guides are to be added, fix these, using pre-placed attachment bolts or masonry bolts and mortar.

Operating the sluice gate

23. Before filling the pond , insert two rows of wooden boards into the set of grooves until you reach a little lower than the maximum water level. Fill the space between these rows with earth or sawdust, compacting well. Place the screen on top of the front row of boards. Fill the pond with water and check that there are no leaks at the sluice gate. If necessary, repeat the compaction process.

24. To drain the pond , remove one pair of boards and the compacted material at a time, keeping the screen on top of the front row of boards while the water flows out. When there are only two to three pairs of boards left at the bottom of the sluice gate, remove all the compacting material and the second row of boards. Then finish in one of the following two ways, according to the harvesting method:

(a) Lower the water further and harvest the fish inside the sluice gate . If there are two sets of grooves, harvest the fish in front of the screen placed in one of the rear grooves. If you need to remove the screen to clean it, first place another screen in the other rear groove.

(b) Alternately let part of the water, together with fish, flow into the catch basin where the latter can be easily harvested; repeat this procedure until all the water and fish have been drained from the pond.

10.5 The monk outlet

1. The monk is one of the oldest and most common pond draining structures. It consists of a vertical tower closed with wooden boards to regulate the water level. The water is discharged through a pipeline buried under the dike. A screen keeps the farmed fish from leaving the pond.

2. A monk has advantages similar to those of the sluice gate. The pond water level is easily controlled and adjusted. It can function as an overflow. It simplifies the fish harvest. In addition, a monk is more easily protected than a sluice gate, and it is more economical to build if the pond dike is large. However, it has the disadvantage of not being very simple to construct , particularly if it is built with bricks or concrete.

Locating the monk

5. The monk can be built either into the dike or freestanding some distance into the pond:

  • If the monk is built into the dike , water infiltration through the dike will be more common and access to the outlet will be easier for poachers. To prevent soil from entering the monk, you will have to build an additional protective wing on both sides, but servicing the monk will be easier;
  • If the monk is built on the pond bottom in front of the inside toe of the dike, you will need a longer pipeline, but access to the monk will be through a removable catwalk and tampering with it will be much more difficult.

How much water should a monk discharge

6. The water discharge capacity of a monk depends on the inside diameter of the pipeline (see Section 3.8 and Section 10.1).

7. To estimate quickly how much water you should be able to discharge from a typical monk, use the pipe flow Tables 12, 13 and 14 from Section 3.8.

8. The cross-section of the monk increases as the diameter of the pipeline increases . Remember that:

  • the internal width of the tower should be equal to the pipe’s diameter plus 5 to 10 cm on each side;
  • the space in front of the first groove should be about 8 to 10 cm;
  • the gap between the two rows of boards should be at least 8 to 10 cm;
  • the distance from the last row of boards to the back wall of the tower should increase as the water discharge capacity increases, up to a maximum value of 35 to 40 cm.

9. To be able to move the boards easily, try to limit the internal width of a monk to 50 cm at the most .

Example
Inside dimensions of monks according to pipeline size (in cm)

Calculating dimensions for a monk

  • WIDTH (W) = diameter of pipe + 2 x (5 to 10 cm)
  • LENGTH (L) = (1) + (2) + (3) + (grooves)

(1) = 8 to 10 cm
(2) = 8 to 10 cm
(3) = maximum 35 to 40 cm
(grooves) = 4 cm each

10. The height of the monk is related to the maximum water depth in the pond. The monk should be at least 20 cm higher than this depth. Usually, the monk has the same height as the outlet dike . Unless specially designed, the height should not exceed 2.5 m .

Materials to build a monk

11. Monks can be built in wood, bricks or concrete depending mainly on the availability of materials, their cost, the local technical expertise and the size of the structure.

12. The most difficult type of monk to build is the brick monk . It requires a very skilled mason to make it so that it is leak-proof. If not done properly, the mortar surfacing will have to be redone frequently, increasing maintenance costs. Generally, wooden and concrete monks are cheaper and easier to build. You will learn how to build these in the next sections.

Note: the following are some points to remember when you build a monk .

(a) The pipeline should be laid down before building the dike and the monk tower.

(b) Build a solid foundation to avoid future problems.

(c) Pay particular attention to :

  • the junction of the monk tower to its foundation;
  • the junction of the pipeline to the back of the monk tower;
  • the finishing of the monk’s grooves.

(d) Give a reasonable slope to the pipeline, preferably 1.5 to 2 percent.

(e) If you have to build several monks on your fish farm:

  • try to standardize their type and size as much as possible;
  • for concrete monks, prepare strong forms and re-use them if possible (see Section 10.9).

(f) Provide a separate overflow wherever there is danger of uncontrolled entry of flood water into the pond (see Section 11.1).

10.6 Wooden monk outlets

1. A simple monk outlet can be built entirely of wood. It is the easiest and cheapest type of monk to construct, although you need to be careful to ensure its watertightness and its durability. The height of a wooden monk should be limited to 2 m .

Choosing the wood

2. To build a wooden monk, select a heavy, durable wood , which is resistant to water such as iroko or mukulungu (see Table 6 ). To improve its durability, you can treat it with a wood preservative or use discarded motor oil. Remember to wash away surplus preservative before putting your fish in the pond.

3. Use wooden boards without knots , 3 to 5 cm thick. For example, for a 2-m-high monk tower, you will require about 0.4 m 3 of wood.

Building the wooden pipeline

4. Instead of using standard plastic or cement pipes, you can build a pipeline entirely of wood. Simply nail or screw four boards together in the shape of a box. Fix the structure well over compacted soil, and bury it underneath the outlet dike. Pay particular attention to the compaction of the dike soil around the pipeline (see Section 6.2).

5. In most cases, a foundation is not needed, but in less stable soils such as certain heavy clays, it may be useful to use simple wooden stake piling.

Building the wooden monk tower

6. Wooden monks require hardly any foundation , as they are very light. It is normally quite sufficient to use light foundation such as paving slabs or simple wood piling, or larger boards placed flat on the pond floor.

7. Both the small- and the medium-size monks are nailed or screwed together so that the side toward the pond is open. Depending on the overall dimensions and on the width of the boards available, assemble the tower as shown in the drawings.

Note : this monk is built using boards 2 to 3 cm thick

8. It is best to screw an anchoring post on each side of the tower. First drive these two posts well into the bottom of the pond and then screw them on to the monk.

9. For a stronger structure, you can add an oblique brace to each side, supporting the top part of the tower against the pipeline.

Medium-size wooden monk

Note: this monk is built using boards 2 to 3 cm thick

10.7 Small brick, concrete block and concrete monks

1. Monks of up to 1.5 m in height , fixed to pipelines up to 25 to 30 cm in diameter, can be built using single- thickness brick and mortar . Although taller and wider monks can be built, they require a double-width base and good bracing for stability and strength, and so become too heavy and expensive for most purposes.

2. Suggested dimensions for such monks are given in the first part of Table 48 .

3. Small monks can also be built with concrete blocks and with reinforced concrete . In general, the principles of construction are similar (see paragraph 4 onwards), with the following exceptions:

  • brick and block monks should be well finished internally, using a plaster coat;
  • as mentioned earlier, a skilled mason should be used; the quality of workmanship should be very high to ensure a durable structure.

TABLE 48
Suggested dimensions for brick, block and concrete monks

Constructing monk pipelines

5. To prepare the foundation for a monk pipeline , you can proceed as follows:

(a) Excavate and compact well the area along which you plan to build the pipeline. The level of this area should be at least 10 cm lower than the lowest point in the pond.

(b) Stake out with a marker line the centre line of the pipeline , usually perpendicular to the centre line of the outlet dike (see Section 3.6, Topography ). The line should extend to 50 cm beyond the pipeline length at each end.

(c) On each side of this centre line measure a distance equal to half the outside diameter of the pipeline plus 10 to 15 cm. Stake it with marker lines, starting at the back wall of the monk tower. These are the foundation lines .

(d) Remove the central stakes and line, and between the foundation strings, dig a trench whose depth should be:

  • in hard, undisturbed soil, 10 to 15 cm depending on the pipe size;
  • in soft soil, 20 to 25 cm depending on the pipe size.

(f) Drive in stakes down the centre of the trench and adjust their height to the designed thickness of the foundation and the 1.5 to 2 percent slope.

(g) Prepare a lean concrete (175 kg cement/m 3 ).

(h) Place the concrete in the prepared trench. Adjust its surface level to the level of the top of the stakes. Tamp it well, protect it, and let it cure for two days. (For information on cement concrete, see Section 3.4).

Laying the pipeline

6. When the foundation is ready, lay the pipeline as follows, according to the type of pipe you are using:

(a) If you are using cement or ceramic pipes :

  • lay the head pipe, its female end starting about 1 m from the rear wall of the monk tower, and position it carefully with stones or concrete/ mortar;
  • lay down the next pipes one by one, connecting the pipes close together, until you reach about 0.50 m beyond the outside end of the foundation, and position these pipes in the same way;
  • check the alignment of the pipes and make sure they are well fitted together – it may be useful to fix holding boards and wedges at each end of the pipeline to ensure a good fit;
  • fix the pipeline well into its final position with mortar;
  • make the joints of the pipes, using a slightly liquid mortar (see Section 3.3).using a slightly liquid mortar (see Section 3.3).

(b) If you are using plastic pipes , there are two main types of connection:

  • push-fit types , which use simple rubber seals at the joints. These are usually used for low-pressure drainage and can be dismantled later. They are quick and easy to assemble, but because of the possible risk of leakage may be less reliable inside pond dikes;
  • glued (solvent-welded) types , which are commonly the thicker-walled pressure pipes, and once assembled cannot be dismantled. They require more care and are usually more expensive, but will provide a reliable and long-lasting job.

7. For both types, the overall laying procedure is similar to that for cement pipes, although there is less need for joint anchoring, because the pipe joints are typically 3, 6 or even 9 m apart.

8. In the case of solvent-welded pipes , the joints are as strong as the pipe itself, and so the pipe needs less protection from movement. Also, as plastic pipes are flexible and smooth inside, they can deform slightly but still drain well. Therefore it is not necessary to use stiff foundations. It is possible in firm soils to eliminate them altogether, but in soft soils, 5 to 15 cm foundation should be sufficient.

9. For push-fit pipes , pay particular attention to the following:

(a) Make sure joint ends are absolutely clean and free from rough edges (e.g. that come from sawing the pipe), and that the rubber seal ring is not twisted, crushed or broken, and is properly seated.

(b) Use a suitable lubricant on the pipe. Silicon grease is best, but even soapy water will do. Push the pipe in until it reaches the end of the socket. Do not use too much force, as you may split the socket.

10. For solvent-welded pipes , proceed as follows:

(a) Clean the pipe, preferably with the recommended cleaning solution, and apply the cement as instructed. Make sure it is spread all around the pipe.

(b) Push the pipe into the socket. Do not twist it as this may cause “channelling” in the joint, leading to leaks.

(c) Ideally a thin ring of, solvent should appear all around the end of the socket; if this is the case, you should have a good weld.

Note: solvent cements are commonly of two types:

  • PVC cement is for PVC pipes only;
  • ABS solvent can be used both for PVC and ABS pipes.

11. Check the pipes and the solvent tin carefully to make sure they match.

Using a concrete culvert

Building the monk tower foundation

13. As discussed before, the monk tower foundation should be built at the same time as the pipeline foundation. However it is generally thicker than the latter. Proceed as follows:

(a) Starting from the pipeline axis, stake out the tower foundation and mark with lines. The tower foundation usually has a square shape. Its dimensions should be larger than those of the tower :

  • for hard, undisturbed soil, by at least 20 cm on all sides;
  • for soft soil, by at least 30 cm on all sides.

(b) Between the marker lines, dig a hole whose depth will vary according to the quality of the soil:

  • in hard, undisturbed soil, 30 cm;
  • in soft soil, 60 cm;
  • if the soil is particularly soft, use a piled foundation (see next page).

(c) Remove all stakes and lines.

(d) Level the bottom of the hole properly.

(e) Fill this hole with foundation materials :

  • in the bottom half, use rocks and gravel, filling the gaps with a slightly liquid mortar; or use a soft lean concrete (175 kg cement/m 3 ) (see Sections 3.3 and 3.4);
  • in the top half, use ordinary concrete (250 kg cement/m 3 );
  • if possible, place in the foundation some pieces of wood of approximately the same width as the monk tower walls, in the position where the walls are to be built. This will help to anchor the walls firmly in place. The pieces of wood should be set about 5 cm below the surface of the foundation.

(f) Tamp these materials well and adjust the surface level , as required by the design, so that it is at least 10 cm below the lowest point in the pond.

(g) Protect the surface of the concrete, keep it moist, and let it cure for at least two days.

(h) Remove the wood blocks, leaving the foundation ready for the walls.

Remember: the solidity of a monk tower depends mainly on the stability and strength of its foundation . You should prepare a good foundation for your monk.

Place a length of pipe from front end of pipeline
to inside of monk

Preparing a piled foundation

14. In very soft soils, for example those with significant plastic clay content, the use of simple piling will improve the strength of the foundation, In most cases, wood or bamboo can be used, although it is preferable to use durable woods (see Section 3.1). They are typically driven several metres into the ground using a hammer. It is simplest preferable to use durable woods (see Section 3.1). They are typically driven several metres into the ground using a hammer. It is simplest to test first , using 2 to 3 m of pile, 6 to 10 cm in diameter, hammering it in until it will go no further. Cut the remainder of the pile, leaving a short stub above the surface. With this as a guide, you can select a suitable standard length of pile. If any pile fails to drive in completely, cut it to leave a stub, as with the test pile. These simple piles are set at approximately 30 to 50 cm from centre to centre. The normal foundation is then built on top of the piles.

Note: to increase stability, some piles can be driven in at an angle, which has the effect of increasing the base area and tying in the foundation more securely.

Building the concrete monk tower

15. To build the monk tower in concrete, you need a wooden form in which to pour the concrete. If you have to build several monk towers of the same dimensions, you can use the wooden form several times, saving time and money. You can also borrow it from or lend it to neighbours and share its cost among a group of people. You will learn more about wooden forms in Section 10.9. (Refer to Section 3.4, for more details about concrete preparation and placing.) You will learn more about wooden forms in Section 10.9. (Refer to Section 3.4, for more details about concrete preparation and placing.)

16. When you have the wooden form ready on site , proceed as follows:

(a) With two stakes and a line, mark out the longitudinal centre line of the monk foundation . It extends directly from the pipeline centre line.

(b) With nails and a line, mark out on the concrete foundation the exact position of the three tower walls to be built, at similar distances from the longitudinal centre line.

(c) If wooden blocks have been used to make the shape of the wall footings, the next stage is quite simple; if not, you will have some more work. Using a hammer and chisel, make deep lines in the concrete surface, about 2 cm from the strings and outside the position of the walls.

(d) Break the concrete surface between these lines to a depth of about 5 cm. Clean away the broken concrete. This small gully will be used to reinforce the junction of the tower and its foundation.

(e) Clean the elements of the wooden form well, removing any dry concrete. Apply used motor oil to their inside walls so that they can be easily removed from the concrete after it has cured.

(f) Get a piece of straight pipe long enough to run from the pipeline through to the inside wall of the monk. It should have the same diameter as that of the pipeline.

(g) Place it at the back of the tower, on the centre line of the pipeline, making sure that it overlaps the inside wall slightly . Block it well in place.

(h) Assemble together the elements of the form; check that the form is well centred, both around the piece of pipe in its back walls and in the gully prepared in the foundation.

(i) Brace the assembled form strongly so that it will not move while the concrete is being placed.

(j) Prepare a relatively soft rich concrete (350 kg cement/m 3 ), (see Section 3.4). The quantity required to fill the form can be readily estimated from the dimensions chosen for the tower:

The monk tower is 1.30 m high. The thickness of the walls is 12 cm. Internal width is 33 cm and internal length is 44 cm. The volume of concrete required is obtained as:

  • volume for the back wall: 0.12 m x 0.57 m x 1.30 m = 0.08892 m 3
  • volume for the two side walls: (0.12 m x 0.44 m x 1.30 m) x 2 = 0.13728 m 3
  • total for the monk tower: 0.08892 m 3 + 0.13728 m 3 = 0.2262 m 3 or about 0.25 m 3 of concrete .

Refer to Table 9 (Section 3.4) for concrete at 350 kg cement/m 3 . Mix 90 kg cement with 113 l sand, 200 l gravel and 50 l water.

(k) Place the concrete into the form to fill it progressively by layers, tamping each layer well before pouring the next one.

(l) Protect the top of the fresh concrete and let it cure for at least 24 hours before removing the form . Be very careful not to break any part of the concrete, particularly next to the vertical grooves.

(m) Join the pipe piece , which has been placed in the tower, to the pipeline with mortar.

(n) Using ordinary mortar , complete the connection of the tower base with the foundation, both outside and inside. Finish the foundation inside the tower smoothly.

(o) If necessary, finish the grooves smoothly . Their quality can be improved by cementing 4-cm-wide U-irons into the concrete grooves.

Note: if you choose to use U-irons as grooves, enlarge the grooves made by the concrete forms slightly to about 5 cm.

10.8 Reinforced concrete monks

1. Larger monks, higher than 1.5 m and including a pipeline with an inside diameter greater than 25 cm, should be built of reinforced concrete.

2. If you use the dimensions shown in Table 48 , for monks with concrete reinforcement, the result will be a monk tower roughly square in shape (72 x 74 cm or 80 x 80 cm, for example), depending on the size of the pipeline.

Building the pipeline foundation

3. Larger monks are usually equipped with commercially available pipes, either of asbestos cement or concrete, laid down on a good foundation. Such a foundation should be built together with the monk tower foundation . Proceed as described in Section 10.7 with the exception of some dimensions, which should be increased as follows:

(a) The width of the foundation should be at least 30 cm wider than the outside diameter of the pipeline.

(b) The thickness of the foundation should be:

  • in hard, undisturbed soil, 15 to 20 cm depending on the pipe’s size;
  • in soft soil, 25 to 30 cm depending on the pipe’s size;
  • in very soft soil, with pile foundations, as described earlier.

4. To build this foundation you can use rock and mortar for the lower half and lean concrete for the upper half.

5. You can build a thinner foundation (about 15 cm thick) if you use reinforced concrete .

6. One method is as follows :

(a) Fill the bottom half of the trench with concrete.

(b) Tamp it well and level properly.

(c) Place the reinforcement (6-8 mm diameter steel bars at about 10 cm intervals, with 6-8 mm cross-bars at about 50 cm intervals tied to these) on top of the wet concrete.

(d) The reinforcing should run the whole length of the pipeline foundation and extend enough to tie in with the monk foundation.

(e) Then, when the concrete below is still wet, cover the reinforcing with more concrete to the top of the trench.

(f) Tamp it well and level properly.

7. An alternative method is as follows :

(a) Hang the reinforcing at the midpoint of the trench using lengths of wood and wire hangers.

(b) Fill the whole trench with concrete and at the same time tamp it well.

(c) When the trench is full to the top, level it properly.

Laying the pipeline

8. When the foundation is ready, lay down the pipeline as explained above for smaller monks (see Section 10.7).

9. If you use 30-cm diameter asbestos cement pipes , you can greatly improve their durability by coating them with 10 cm of soft lean concrete . Do this after finishing all the joints of the pipeline and before building the monk tower.

Building the monk tower foundation

10. The monk tower foundation is to be built at the same time as the pipeline foundation , as described earlier for smaller monks (see Section 10.7, paragraph 13) except for the following:

(a) The foundation size should be larger than the tower base:

  • for hard, undisturbed soil, by at least 30 cm on all sides;
  • for soft soil, by at least 50 cm on all sides;
  • for very soft soil, at least 50 cm, with suitable piles.

(b) The foundation thickness should be:

  • in hard, undisturbed soil, 50 cm;
  • in soft soil, 70 to 90 cm;
  • in very soft soil, 70 to 90 cm, plus piles.

(c) When placing concrete in the top half of the foundation :

  • stop when you reach a level 8 cm below the foundation surface level;
  • place the reinforcement of the monk tower well on top of the concrete layer (see next paragraphs), with its opening toward the pond, making sure that there is no steel bar in the prolongation of the proposed pipeline;
  • pour the last 8 cm layer of concrete without displacing the reinforcement.

Preparing the steel reinforcement

11. You have learned earlier (see Section 4.5) about steel bar reinforcement for concrete. In this section, you will learn more about this through two specific examples of monks.

12. If you want to build a reinforced monk tower 1.50 m high , with a draining pipeline of 25-cm inside diameter ( Table 48 , type 4), prepare the reinforcement as follows:

(a) Get about 20 m of 6-mm diameter steel bar reinforcement; cut the following lengths and shape them as indicated :

  • 3 sections of 3.40 m each to shape into a long U;
  • 3 sections of 1.50 m each to shape into a short U;
  • 2 sections of 1.95 m each to shape into an L;

(b) Assemble them as shown in the drawing , attaching them together at their intersections with 1-mm diameter soft annealed wire :

  • the three long U shapes are set vertically to reinforce the bottom and the side walls;
  • the two L shapes are also set vertically to reinforce the bottom and the back wall;
  • the three short U shapes are set horizontally to reinforce the side and back walls, as well as to join together their previous reinforcement.

13. If you want to build a reinforced monk tower 2 m high with a pipeline of 30-cm inside diameter ( Table 48 , type 5), prepare the reinforcement similarly to the above, as follows:

(a) Get about 25 m of 6-mm diameter steel bar reinforcement; cut the following lengths and shape them as indicated :

  • 3 sections of 4.45 m each to shape into a narrow U;
  • 3 sections of 1.55 m each to shape into a wide U;
  • 2 sections of 2.5 m each to shape into an L.

(b) Assemble them as shown in the drawing , attaching them together at their intersections with 1-mm diameter soft annealed wire .

Note: see section 10.9 for details on how to build two kinds of wooden forms for pouring concrete monks

Building the reinforced concrete tower

14. As explained earlier, you need a wooden form in which to pour the concrete (see Section 10.9). To build the tower, proceed as described in Section 10.7, paying particular attention to the following points:

(a) To improve the junction of the foundation and the tower , break the foundation surface where the tower base will sit and all around the vertical reinforcement bars.

(b) Secure the small section of pipe well in the prolongation of the existing pipeline, at the right level in the prepared gully, and within the steel reinforcement.

(c) Assemble the elements of the wooden form together without displacing the steel reinforcement.

(d) Make sure that this reinforcement rises evenly between the sides of the wooden form – it should not run too close to either side. Secure it well there and brace the form so that it will not move when placing the concrete.

(e) Compact the concrete well (see Sections 3.4 and 3.5) without disturbing the position of the reinforcement.

(f) Remove the form very carefully.

(g) Finish the junction between the foundation and the tower with ordinary mortar.

10.9 Wooden forms for concrete monks

1. In Section 3.4 we dicussed general points on preparing wooden forms for concreting. In this section, you will learn more about the specific design of these forms for building concrete monks.

Preparing a plywood form for small monks

2. For the construction of small monks about 1 m high at the most, you can use a form made of wood strips and plywood 1.5 cm thick . The drawings show you how to build this kind of form.

Note: if you want a three-groove monk, add one more groove strip to this design

Preparing a plank form for larger monks.

(Adapted from a design by J. Miller, FAO Expert in Rural Fish Farming Development)

3. You can build a simple form made of 3- to 4- cm thick planks . The next drawings show you how to build this kind of form.

10.10 Water control for sluice or monk

Grooves and boards

1. Normally, a sluice or a monk is equipped with two pairs of grooves , in which boards are inserted up to the desired water level. To prevent water seepage, the gap between the rows of boards is filled with compressed material such as clayey soil or sawdust.

2. While the pond is in operation, a screen should be fitted on the top of the front row of boards to keep the fish from getting out if the water level should rise. a screen should be fitted on the top of the front row of boards to keep the fish from getting out if the water level should rise.

3. A very small sluice or monk , however, may have only a single row of grooves and boards. In this case, you can prevent seepage by packing strips of jute burlap between the grooves and the boards. The joints between the boards can also be sealed using polythene or rubber flaps.

4. A medium-size sluice or monk can have two pairs of grooves to be used as follows:

Selecting the boards to use

7. Size varies according to the dimensions of the structure but, in general, the following rules should be adopted:

  • thickness : 2.5 to 3 cm and 0.5 to 1 cm thinner than groove width;
  • height : 15 to 20 cm (maximum 30 cm);
  • length : 1.5 to 2 cm shorter than distance between opposite groove ends.

8. It is important that the wooden boards do not fit too tightly into the grooves after the wood has swelled under water. If this happens, it will be very difficult to remove the boards when necessary.

9. You can improve the fitting of the wooden boards on top of each other and reduce water losses by:

  • planing the four sides of the boards well;
  • shaping their top and bottom edges as shown;
  • avoiding using timber with knots.

10. If your monk is rather high and wide, it will be easier to remove the boards from the top of the monk using a handle with a T welded at its end. You can easily have one made by a blacksmith. At the back of each board, secure two steel bolts or hooks to lift it.

11. Another way to reduce water leakage is to use old inner-tube rubber to make a simple seal :

  • as a flap below each board, to overlap the previous board and cover the gap between them;
  • to put in the groove to help to seal the board and minimize leakage.

10. POND OUTLET STRUCTURES 10.0 Outlet structures 1. Outlet structures are built for two main reasons: to keep the water surface in the pond at its optimum level , which usually ]]>