6. FISH POND CONSTRUCTION
1. When the construction site has been prepared, the fish pond and its water control structures can be built. This chapter shows you how to construct the fish pond, while the next chapters of this manual, deal with water control structures.
2. Dikes are the most important part of a fish pond, as they keep the necessary volume of water impounded and form the actual pond; their design and construction is particularly important. You will learn more about pond dikes and earthwork calculations in the next three sections, before learning how to stake out and construct the four main types of pond.
3. You will find it useful to have a notebook in which to make any calculations required and, if available, some squared or graph paper for sketching out and measuring pond and dike shapes.
Note: an intermediate dike separating two ponds may not need to be as strong as a perimeter dike, so long as the water pressure is more or less the same on both sides. If one pond needs to be drained while the adjacent one remains filled however, water pressures will be similar to those in perimeter dikes, and the dike should be stronger.
3. Impermeability of the dike can be ensured by:
Note: a dike entirely built of good soil is said to be impervious when the upper limit of its wetted zone, the saturation line*, progresses through the dike so as to remain inside. The better the soil for dike construction, the more the saturation line is deflected downward, and the thinner the dike can be. The slope of this saturation line, the hydraulic gradient*, usually varies from 4:1 (clayey soil) to 8:1 (sandy soil). As shown, a clay core will affect this hydraulic gradient.
4. To calculate the height of the dike to be built, take into account:
6. You can determine the construction height (CH in m) simply from the design height (DH in m) and the settlement allowance (SA in percent) as follows:
11. Not all the dikes of your fish farm are to be used by vehicles (see Section 1.8). But additional dike width may be required at turning points, based on the diameter of the turning circle of the vehicle used:
12. In individual ponds, dikes have two faces, the wet side inside the pond and the dry side or external side. These two sides should taper from the base to the top at an angle that is usually expressed as a ratio defining the change in horizontal distance (z in m) per metre of vertical distance as, for example, 2:1 or 1.5:1.
14.Usually side slopes of dikes vary from 1.5:1 to 3:1 depending on local conditions. The slope of the dry side can be made steeper than the slope of the wet side. (see Table 27 for various sizes of fish ponds and two groups of soil.)
15. In some cases, you may wish to change the slope, for example:
16. However, you may have to spend more time in maintaining these dikes.
6. The primary objectives of dike compaction are to start the settling of freshly placed earth, to reduce water permeability, and to strengthen the dike to keep any part of it from sliding away.
Determining the potential for compaction
7. You can estimate the bulking of any earth material and determine its potential for compaction by measuring out a known volume of material on the site to be excavated, digging down to the intended excavation depth if possible. You can then either measure the earth volume (for example with buckets, boxes, etc.) or fill the earth back into the test hole and measure the surplus. You should then be able to compact at least 80 percent of this surplus back into the hole by ramming or stamping the soil.
Using a 0.30 x 1 m trench, dig to a 1 m depth. The original earth
volume = 0.30 m3 .
(a) Estimate bulking
(b) You should expect to be able to compact at least 80 percent of the surplus (the difference between the expanded volume and the original undisturbed volume): 0.06 m3 x 0.80 = 0.05 m3 . The compaction potential is calculated as: (0.05 m3� expanded volume) x 100 = (0.05 m3� 0.36 m3) x 100 = 13.9 percent of the expanded volume.
8. If the site soil was initially loose, you may be able to compact it into a smaller volume than the original one. To define the compaction potential, you can then measure the loose earth required to fill the hole back up to the original level.
A 0.30-m3 trench is dug and the soil is stamped back, requiring 0.06 m3 of loose fill to make it level. The compaction potential of the original soil equals (0.06 m3 � 0.30 m3) x 100 = 20 percent
9. Make note of the basis on which the above calculations are made, that is, whether the compaction potential is related to the expanded soil volume or the original soil volume. Be certain to understand the relationships between undisturbed, expanded, construction and design volumes, as explained earlier.
10. To compact successfully, air and water are expelled from the soil so that its mineral particles can settle very tightly together. For best results, you therefore should always:
11. To compact thin layers of soil by hand, you can use simple tools such as:
13. As the dikes and the area to be compacted increase in size, it is better to compact mechanically.
14. For relatively small compaction jobs, you can use vibration plates and percussion tampers called frogs. For bigger jobs, it is usually sufficient to use construction equipment such as tractors and trucks to compact the earth fill by running over it repeatedly. Special compaction equipment such as sheepsfoot rollers, steel-wheel rollers and pneumatic rollers can also be used, if available, under competent supervision. Average output per working hour (m2/h per 25-cm layers) for various compactors are as follows:
Note: the compaction of non-cohesive soils such as sand requires heavy pressure (weight) and, if possible, vibration. On the other hand, the compaction of cohesive soils such as silt and clay requires a kneading action. Thus to compact a clayey soil you cannot use a normal steelwheel roller which would compact a surface layer only, but you would need a sheepsfoot or pneumatic roller (see Section 10.2 and Table 26, Soil, 6).
6.3 Preparing the foundations of the dike
1. After clearing the site, removing the surface soil and marking out the position of the dike, the foundations of the dike should be prepared. This may include:
2. The surface of the dike's foundations needs to be well compacted, so that the dike can be solidly attached without any risk of it sliding away.
3. If the foundation soil does not contain an adequate layer of impervious material at the surface, you should build a cut-off trench (sometimes called a puddle trench) within the dike's foundations. Its main purpose is to reduce water seepage under the dike. It will also help to anchor the dike solidly to its foundations.
4. The size of the cut-off trench increases with the size of the dike. You can use the following guidelines:
5. To build the cut-off trench, proceed as follows.
(a) Clearly mark the centre line of the dike base, for example with stakes and string.
(b) On each side of this centre line, clearly mark the limit of the cut-off trench to be built.
(c) Dig the trench to the depth, width and side slopes required, placing the removed soil material over the foundations of the dike in one-third of the area toward the dry side of the dike. Be careful not to include roots, organic materials or large stones.
(d) Spread this soil material in thin layers and compact it well.
(e) Make sure the trench is dry.
(f) Backfill the cut-off trench to the natural ground surface with soil material of the same quality as for a dike core (see Section 12.2, Soil, 6). Place the backfill material in thin layers, wet it if necessary, and compact well. If clay soil is used, "heel" it into place, or use suitable mechanical equipment.
Backfilling a stream channel
6. If a stream channel crosses the foundations of the dam, such as in the case of a barrage pond, you must prepare the channel of the stream where the dam will be built. If the water is flowing when you want to work on the channel, you will first have to divert the stream.
7. Dig the diversion ditch around the site of the future dam as shown. That way, you will be able to use the same diversion ditch when you build the dam (see Section 6.6, paragraph 9 and following). Then, proceed as follows:
(a) Deepen and widen the channel as necessary to remove all stones, gravel, sand, sediment, stumps, roots and organic matter.
(b) Dig at least 30 cm below the original channel bed or until you reach rock. Make the side slopes of the new channel no steeper than 1:1.
Note: if the soil base below the channel is permeable, it is best to make a cut-off trench.
1. Before starting the construction of your pond, you should calculate how much soil you will need to build its dikes. Then, you will need to estimate the excavation volume necessary to provide such soil volume. According to the topography of the construction site and the type of pond to be built, you should select the best method to be used. You should estimate expanded and compacted volumes (see Section 6.2), and you should also use standard settlement allowances (see Table 28).
2. Multiply excavation volume by the expansion factor (Table 28) to obtain the expanded volume. This expanded volume is then used in the construction volume of the dike. After compaction and settlement, as estimated by the compaction potential, it should reach the design volume required.
Calculating the width of the dike base
3. Having determined the characteristics of your dikes, determine the width of the dike base (in m) by adding:
A 0.04-ha pond (400 m2 ) has to be built in clayey soil with dikes 1.50 m high and 1 m wide at the top, according to the design. If SD = 1.5:1 and SW = 2:1, calculate the base width of the dikes.
(a) From Table 28, estimate the settlement allowance of the expanded clay volume (20 percent for medium clay soils).
(b) Consider the design height = (100% - 20%) = 80 percent of construction height.
(c) Obtain the construction height = 1.50 m � 0.80 = 1.88 m.
(d) Calculate dike base width = 1 m + (1.88 m x 1.5) + (1.88 m
x 2) = 1 m + 2.82 m + 3.76 m = 7.58 m.
Calculating the cross-section of a dike on horizontal ground
4. The size of the cross-section of a dike on horizontal ground (ABCD in m2) (see diagram) is obtained by adding:
5. To calculate the cross-section of a dike on horizontal ground with identical side slopes, you can also use Table 29.
Calculating the cross-section of a dike on sloping ground
6. The cross-section of a dike on sloping ground can be calculated most easily using a scale drawing.
(a) Draw a horizontal line from D, meeting AE at E'.
Note: for ground slopes of less than 10 percent, and where the dike side slopes are the same on each side, you can use the method given for horizontal ground.
(a) Draw a straight line D'E'F'C', approximating the shape of the ground, then use the procedure given for sloping ground.
(b) Alternatively, mark the shape out on squared paper, and using the scale, count the squares to obtain the area.
Calculating the volume of dikes on horizontal and regular ground
8. To estimate how much soil will be needed for the construction of a dike, you need to know its volume. The calculation method depends on the site topography and on the type of pond to be built.
9. If the topography of the construction site is reasonably flat (less than 0.30 m difference in average site levels) and regular, you can calculate the volume of the dike (in m3) by multiplying the cross-section of the dike(in m2 and halfway along the dike for an average area) by its length measured along the centre line (in m).
For the previous example, Graph 3a shows a standard volume of 720 m3. As the side slopes are 2:1 (inner) and 1.5:1 (outer), this is multiplied by S = 0.9 to give 720 m3 x 0.9 = 648 m3. (Compare this result with the previous example in which you calculated 653 m3.)
11. If you decide to change the crest width to 0.51 m, from Graph 3b you find C = 0.8. The volume will now be 648 m3 x 0.8 = 518.4 m3
Calculating the volume of dikes on sloping or irregular ground
13. If the topography of the site is more steeply sloping or more irregular, you cannot calculate the volume of the pond dikes just by using one cross-section. There are several possible methods, depending on the type of ground and the accuracy you require.
A 400-m2 (20 x 20 m) pond is to be constructed with
wall heights of 0.5 m at corner A, 0.3 m at corner B, 1.1 m at corner
C and 1.5 m at corner D. Crest width is 1 m and side slope 2:1 on both
sides. The cross-section areas at each corner are:
Average area for wall AB = (1.5 m2 + 0.48 m2)
� 2 = 0.99 m2 and volume for wall
Consequently, total volume of dikes = 19.8 m3 + 40 m3 + 95.2 m3 + 75 m3 = 230 m3.
16. You can also use the graphical method explained earlier (see paragraph 10), using an average height for the four walls of the dike, although this method is less accurate.
Using the graphical method, the average wall height is (0.5 m + 0.3 m + 1.1 m + 1.5 m) � 4 = 0.85 m, and the standard volume, which needs no further correction, is about 180 m3 . This is about 80 percent of the previous figure (see paragraph 14).
17. For a more accurate measurement of dike volume on rough ground, you should apply the following formula, known as Simpson's rule, where: V = (d � 3) x [A1 + An + 4(A2 + A4 + ... An-1) + 2(A3 + A5 + ... An-2)]. Proceed as follows:
(a) Divide the length of the dike into an odd number n of cross-sections at equal intervals of d metres.
(b) Calculate the area A of each cross-section as explained earlier.
(c) Introduce these values into the above formula.
Calculating volumes of excavated material
19. You will need to know excavation volumes for:
22. Where the width is less than 30 times the depth, you should correct for side slopes as follows:
A 400 m2 (40 x 10 m) area is to be excavated, 1 m deep, with side slopes 2:1. As the width (10 m) is less than 30 times the depth (30 x 1 m), the first method is not accurate (estimated volume would be 400 m2 x 1 m = 400 m3).
(a) Calculate the average cross-section of the excavation.
(b) Multiply by the average length of the excavation.
24. On more steeply sloping ground (steeper than 10 percent in any direction), you can use the same method as above; however, the lengths of the base and the corresponding cross-sections, as calculated in the previous method, will not be sufficiently accurate. To obtain a reasonable estimate proceed in the following way.
where A is the area of each grid square in m2.
In the case shown, relative elevations are marked on a grid made of 10 x 10 m squares so that the area of grid squares A = 10 x 10 m = 100 m2. According to the formula:
Volume = (100 m2 � 4) x [(3.1 m + 2.0 + 2.6 + 2.0 + 3.1) + 2(2.6 m + 3.5 + 3.0 + 2.0 + 3.5 + 2.5 + 1.8 + 2.0) + 3(2.8 m) + 4(3.1 m + 2.1 + 2.5)] = (100 m2� 4) x [(12.8 m) + 2(20.9 m) + 3(2.8 m) + 4(7.7 m)] = (100 m2 �4) x (93.8 m) = 2 345 m3.
Note: you will normally have to correct this volume for the side slopes.It is usually easier to make these adjustments outside the grid, calculating the additional volume either square by square or by averaging along each side of the grid.
Remember: with any calculations for building and excavation, do not use methods that are more precise than you require. Because of the difficulty of predicting expansion and compaction accurately, volume estimates in practice are generally only accurate to within 10 percent. There is usually therefore little point in being more precise than this, and so there is no need to allow for every little irregularity or slight change in slope.
6.5 Constructing dug-out ponds
1. Dug-out ponds, entirely obtained through soil excavation, are the simplest to build. There are two main types of dug-out pond depending on the water supply (see Section 1.4):
Selecting soil for dug-out ponds
2. To build a rain-fed dug-out pond, it is essential to have enough impervious soil at the site to avoid excess seepage losses. The best sites are those where fine-textured clays and silty clays extend well below the proposed pond depth. Sandy clays extending to adequate depths are satisfactory. Avoid sites with porous soils, either at the surface or at the depths through which the pond would be cut.
3. To build a seepage dug-out pond, look for soils where the waterbearing layer is thick enough and permeable enough to provide the required water. It is best to observe the site during a complete annual cycle to check on the possible variations of the water table elevation with the season.
Building a dug-out pond
4. Begin building the dug-out pond by preparing the site as follows.
(f) There are two easy ways to dispose of waste soil material and to prevent it from tumbling or eroding back into the dug-out pond:
Note: you can use a pile of soil as a windbreak or for the cultivation of a crop (see also Section 5.6).
(g) Clearly mark the limits of the areas where the excavated material
will be spread or piled.
(i) Shape the sides of the pond to the desired slope and finish the pond bottom and the horizontal dike tops. Remove any excess soil.
(j) Bring back the surface soil to cover the waste material and the dike tops. Then plant or sow grass all around the pond to prevent erosion (see Section 6.9).
Note: water control structures such as a feeding canal, inlet pipe, outlet sluice, spillway or drainage canal may be provided for dug-out ponds (see later).
1. Barrage ponds are embankment ponds formed by a dam, which is built across a narrow valley to impound water behind it (see Sections 1.3 and 1.4).
Note: in this manual, for barrage ponds you will learn how to build small dams only, their height being limited to 2.50 m. If you need to build a higher dam, you should consult a specialized engineer.
2. As the height of the dam increases, the need for sound foundations becomes essential. The best foundations consist of a thick layer of relatively impermeable consolidated clay or sandy clay, at shallow depth. Never build a dam over rock or sand. Whenever in doubt, ask for advice.
Obtaining the soil material for construction
4. In order to reduce the distance the soil needs to be transported, try to dig the soil necessary for the construction of the dam from an area close by, for example:
5. An area from which soil is taken is called a borrow pit. Be careful to keep the limit of any borrow pit at least 10 m from the wet toeline of the dam. Plan the drainage of this area to be included within the barrage pond, for example using a trench toward the water outlet.
6. Clearly mark the centre line of the dam at ground level with tall stakes and a line. It is usually perpendicular to the main axis of the stream in the valley to be flooded.
7. Calculate distances from the centre line to the two toelines on a series of perpendiculars set out at regular intervals as:
Note: the design height of the dam at each point along the centre line is obtained from your topographical survey of the valley cross-section at this point. From these design heights, calculate the construction heights (see Section 6.1).
You plan to build a dam with maximum design height DH = 2.10 m, crest width = 2 m, wet slope 2:1 and dry slope 1.5:1.
Soil settlement allowance is estimated at 15 percent. The valley cross-section along the centre line of the dam can be sketched as shown, giving the design heights DH(A), DH(B) ... at points A, B ... at 10-m intervals along the centre line.
Calculate the distances from centre line AF to toelines GHIK and LMNO as follows:
8. Stake out on the ground points G, H, I, K on the wet side and L, M, N, O on the dry side of centre line AF. These points show you what the outside limits of the dam base should be.
Preparing for the construction of the dam
9. Divert the stream to a site as close as possible to one of the valley sides and well away from the original stream bed (see section 6.3, paragraph 6). This task will be much easier if you have scheduled the construction work to coincide with the dry season.
10. To prepare the foundation of the dam, clear the base area, remove the surface soil and treat the surface of the foundations, giving particular attention to the old stream channel (see Section 6.3) and to the sides of the valley, according to the quality of the foundations' soil:
11. If the soil is impermeable, dig an anchoring trench (about 1 m wide and 0.4 m deep) along the centre line of the dike base to anchor the dike to its foundations. Refill this trench with good clayey soil and compact it well. Extend the trench sideways, well into the sides of the valley;
12. If the soil is permeable, build a cut-off trench (at least 1.5 m wide) along the centre line of the dam (see Section 6.3), which will also assist in anchoring the dam to its foundations. Extend the trench sideways, well into the sides of the valley
13. Build the water outlet structure(s), as necessary (see later chapters). Preferably, place it (or them) out of the stream bed, at a point to be dug lower than the lowest point in the pond.
Note: if the dam is to be constructed by machine, for example a bulldozer, the outlet structure could be built later.
14. Clearly mark the construction height of the dam and the crest width (refer to centre line) with stakes and lines, on the basis of planned dam characteristics (see Section 6.1). Maximum height is at the lowest point of the valley. Check the limits of the future pond upstream.
15. Set out the dam earthwork using templates at intervals of 25 m or less and clearly showing the slopes of the sides. You can also use strings. If you use machinery, it is best to establish an auxiliary base line outside the radius of operation of the machinery, based on topographical survey bench-marks.
17. According to the availability of clayey soil, you will use either homogeneous soil layers as wide as the dam or heterogeneous soil layers, each kind of soil covering only part of the dike width. Clearly mark the limits to be followed with stakes and lines.
(a) If there is enough good soil from which to build the entire dam, proceed by placing layers to cover the full base width.
(b) If the supply of good soil is limited, use it only to build a central core with the following characteristics:
Note: this core should be continuous with either the cut-off trench or the anchoring trench built in the foundations of the dam (see earlier) and should be properly placed and compacted.
Note: do not place heaps of soil next to each other without spreading them into a continuous layer before compaction
(c) If you have to use various types of soil to build the dam, use the most impermeable material as a central core. Place the most permeable material on the dry side of the dam. Place the intermediate quality material on the wet side of the dam. Adjust each side slope to the particular kind of material used.
(d) If you do have relatively permeable materials on the dry side of the dam, it is useful to place larger grades (such as medium to coarse gravel or small rocks) at the dry toeline. This acts as a filter and prevents seepage water from washing out the finer dike material.
Beware: you should pay particular attention to the compaction of the soil placed around the water outlet structures. Use good soil, the right moisture content, thin soil layers and thorough, strong tamping.
(e) Build the water outlet structure (see chapter 10), being careful to make proper reinforcements around the areas where the water flows in and out of the structure.
(f) Refill the trench and compact it well, rebuilding the dam section as it was before. Pay particular attention to the central core if you have built one. Check carefully the quality of compaction around the water pipes. If possible use antiseepage collars.
(g) Proceed with the building of the dam as before. The water pipes are now well protected by 1 m of earth, and they will not collapse under the weight of the bulldozer.
19. When you reach the planned contruction height for the dam, carefully
begin to form the two side slopes. Use slope gauges to assist you in
cutting each slope at its planned angle.
Note: it may be necessary to build a spillway and an emergency spillway (see Sections 11.3 and 11.4).
20. When the first part of the dam is completed, let the stream run back into its old channel and through the water outlet structure. You are now ready to finish your barrage pond.
Note: if the dam is to be built using machinery, aim to make slopes that are slightly steeper than the planned slope, as mechanical grading usually flattens off the slope.
Finishing the barrage pond
21. Repeat the previous operations for the second part of the dam in the area where the stream was diverted temporarily, but first fill the bed of the diversion ditch within the pond area.
(a) Prepare good foundations, extending them sideways well into the side of the valley. Be particularly careful to make a good foundation in the bed of the stream diversion.
(b) Set out the dam earthwork properly.
(c) Build up the second part of the dam, being particularly careful to connect it strongly to the first part of the dam and well into the side of the valley.
(d) Form the two side slopes.
22. Work on the bottom of the pond to make sure the pond is completely drainable.
(a) Tidy up and shape the course of the old stream bed.
(b) Build a regular slope toward the water outlet and dig bottom drainers (see Section 6.10).
(c) If there are depressions, dig a draining trench toward a lower part of the pond bottom. This is important if you have borrowed the soil from within the pond area.
(d) If necessary, fill any undrainable depressions.
23. Finish your barrage pond by taking back some surface soil, spreading it on the dam and planting grass (see Section 6.9).
1. Paddy ponds are embankment ponds built over flat ground. They have four dikes of approximately equal height. The size of the dikes, and thus the volume of earthwork, is usually limited to the minimum because of the need to import the soil material or to find it near the site.
Note: whenever the soil to build the dikes is taken by reducing the level across the whole pond floor, the resulting pond will be considered a cut-and-fill pond built on horizontal ground (see Section 6.8).
2. In some cases earth to build paddy ponds can be taken from areas
next to the dikes, either inside or outside the pond, thus reducing
construction costs. Trenches for dike material should not be cut with
side slopes steeper than the dike itself.
3. Clearly mark the centre line of each of the four dikes; the shape of the pond is usually either square (minimum earthwork) or rectangular, and the four centre lines will meet at right angles (see Section 3.6, Topography).
4. On each of the centre line stakes, indicate the level corresponding to the construction height CH of the dike to be built. Determine the level using one of the levelling methods described in Topography.
5. According to the characteristics of your dikes, calculate the width at each part of the dike base on either side of the centre line, as equal to: (crest width � 2) + (CH x side slope)
Preparing for the construction of the dikes
7. When the inner and outer limits of the pond have been staked, clear any remaining vegetation from the area.
8. Remove the surface soil only from the area of the dike bases, as staked out above, and store it close by (see Section 5.6).
9. Treat the surface of the foundations of the dikes (see Section 6.3).
10. According to local soil quality, build either an anchoring trench or a cut-off trench (see Section 6.3) along the centre lines of the dikes.
11. Build the water control structures, as necessary (see later chapters ). Place the outlet entrance at an elevation low enough to ensure complete drainage of the pond along the sloping bottom (see paragraph 14 of this section).
13. Another way to build the dikes of a paddy pond is given here.
(a) Lay out a line to join and clearly mark the stakes setting out the inside limits of the dike's base. Attach this line at a height of about 0.20 m above the level of the surface of the dike's foundations.
(b) Similarly lay out a line at the same level, joining the stakes setting out the outside limits of the dike's base.
(c) Build the first layer of the four dikes 0.20 m high, bringing in good soil, placing it between the two strings all around the pond area, spreading it well, wetting and mixing it if necessary and tamping it thoroughly, especially around the outlet structure.
(d) For each dike, move the inside limits of the dike base (stakes and lines) toward the centre line of the dike by a distance equal to 0.20 m x side slope; similarly, move the outside limits by a distance equal to 0.20 m x dry slope.
Finishing the dikes
14. Now the dikes are built with sides that look like staircases. To reform these dikes with smooth side slopes and to finish their construction, proceed in the following way.
(a) On the top of each dike, set out the planned dike crest width, measuring half of its value on either side of the centre line and marking the limits with wooden pegs and lines.
(b) Starting from the top of the dike, obliquely cut the end of each soil layer on the wet side of the dikes following a slope that joins the limit of the dike crest to the bottom limit of the layers, until reaching the staked-out limit of the dike base.
(c) Repeat this cutting on the dry side of the dikes.
(d) Transport the soil removed, as necessary.
(e) Remove all stakes and lines.
(f) Bring back some of the surface soil on the top of the dikes and on the dry sides.
Building bottom slopes and drains for paddy ponds
15. The bottom of the pond now has to be finished; which is done using a levelling survey (see Section 11.4, Topography, 16).
16. For smaller ponds, give the bottom of the pond a gentle slope (0.5 to 1 percent) from the water inlet to the water outlet to ensure easy and complete drainage of the pond.
Note: you should always ensure that the entrance of the water outlet structure is at an elevation slightly lower than the lowest point of the bottom of the pond.
Building the dikes of a paddy pond using machinery
19. When using machinery to build the dikes, a method similar to the one for barrage ponds can be used (see Section 6.5), except that four dikes are progressively built up instead of only one.
20. It is best to establish an auxiliary base line with temporary bench-marks from which to set out the earthwork. This base line should be established outside the operation radius of the machinery.
21. If the water outlet structure is built first, all pipes should be protected by a layer of earth at least 0.60 m thick to keep it from collapsing under the weight of the machinery.
Note: if a central core, an anchor or a cut-off trench is required, adapt the size(s) to the size of the dike.
6.8 Constructing cut-and-fill ponds
1. In cut-and-fill ponds, at least some part of the pond dikes is made up by the natural ground, cut according to the planned side slope. Normally, a certain volume of soil necessary to build the dikes above ground level is obtained by digging a similar earth volume from inside the pond area, so as to obtain a pond with the planned depth (see below cross-sections showing examples of earthwork in cut-and-fill ponds). The height of the dikes to be built is no longer equal to the depth of the pond, as in paddy ponds.
Note: cut-and-fill ponds are generally of the diversion type, water-fed either from a natural water body or from pumped groundwater.
Balancing cut-and-fill on horizontal ground
2. During the planning stage, you must calculate how deep the pond should be dug to have enough soil material to build the four dikes and to create a pond with the planned depth. In good soils, this is usually done by matching the quantity of earth dug to the quantity needed to build the dikes. This is called balancing cut-and-fill. On horizontal ground, two methods can be used for determining the balance of cut-and-fill.
3. In method one, excavation (cut) and dike (fill) volumes are calculated and approximately balanced by trial and error using a graph, as shown in the examples below. In method two, the balancing cut is determined using a graph and tables. Corresponding balancing volumes are then calculated, as described and shown in the example.
4. In practice there is no need for great accuracy, as it is not practical to control depths, heights and slopes with great precision. There will also be additional small volumes to allow for in making pond shapes and allowing space for inlets and outlets, access points, etc.
If digging depth = 1 m, dike height = 1.5 m - 1 m = 0.5 m. Using the methods described in Section 6.4 obtain:
In this case, the cut volume greatly exceeds the fill volume.
Use higher dikes and reduce digging depth, say to 0.5 m; thus dike height = 1 m. This time, you obtain:
In this case, the fill volume exceeds the cut volume.
To estimate the correct digging depth, use a simple two-way graph (see Graph 4). Plot the excavation (cut) and dike (fill) volumes for trial 1 (points A and B) and trial 2 (D and C) respectively. Join AD and BC. The intersection E gives the digging depth required (0.72 m) and the approximate balancing volume (155 m3).
You can check these results with a third set of calculations,
where digging depth = 0.72 m and dike height
6. Now let us look at method two, determined by using a graph and three reference tables.
7. This method is quick, but it is less accurate than method one. In addition, method two does not directly calculate the balancing volumes, although these can easily be calculated once the balancing cut is known. Proceed as follows.
(a) In Graph 5 enter the area of the pond (in m2) . According to the width of the dike crest (in m) find the balancing cut depth (in m) for a standard pond where:
(b) If the side slopes of your dike are not 2:1, correct the standard cut depth by S (in m), according to the first table given below.
(c) If the shape of your pond is not square, multiply the cut depth by P, using the second table given below.
(d) If the height of the dikes is not 1.5 m, multiply the cut depth by D, using the third table given below.
If the height of the dikes had been 2 m for example, you should have found, from the third table, D = 1.5 and corrected the cut depth further as 0.728 m x 1.5 = 1.092 m.
Balancing cut-and-fill on sloping ground
8. On regular sloping ground, the material needed for the dikes is also obtained from inside the pond area, but here both the height of the dikes above ground level and the digging depth vary according to the slope angle. This usually determines the position of the pond and hence the balancing depth.
9. If the ground slope is 0.5 percent at the most, the site can be considered horizontal. If the pond is built with its length perpendicular to the contour lines and if dug to the same depth all round, the bottom of the pond will naturally have a 0.5 percent slope at the most.
10. If the ground slope is from 0.5 to 1.5 percent, the pond should also be built with its length running across the contour lines, but the height above ground level of the two longer dikes will vary from one end to the other. Similarly, the width of these dike bases also varies. The downslope dike will be the highest above ground level, and the upslope dike will be the lowest above ground level. Digging depth is the reverse: it is greatest at the upslope end, least at the downslope end.
11. It the ground slope is greater than 1.5 percent, the pond should be built with its length running along the contour lines. The height above ground level of the two shorter dikes will vary from one end to the other. Similarly, the dike base width also varies. The longer dike downslope will be the highest above ground level. The longer dike upslope will be the lowest above ground level. Digging depth is the reverse: greatest in the upslope part of the pond, least in the downslope part.
12. To obtain a rapid estimate with any slope values, you can use either of the previous two methods.
(a) Method 1, the trial-and-error method, uses the volume calculations for horizontal ground with average ground level and average dike height figures.
(b) Method 2, for horizontal ground, uses average ground level and average dike height figures.
Note: these methods are accurate enough when the slope is less than 0.5 percent.
13. To obtain a better estimate of the balancing depth of cut in more steeply sloping ground (more than 0.5 percent) you should use Method 1 together with the methods for calculating excavation and dike volumes on sloping ground.
14. The details of this procedure will vary according to the ground slope.
15. On gentle slopes (0.5 to 1.5 percent), you will have different types of dike:
Notes: In ponds A and C, all of the dikes vary in height. In pond B, the short top and bottom dikes are level and the long side dikes vary in height
16. Apply Method 1 in the following way.
(a) Select a first minimum depth of cut measured at the lower end of the pond; calculate excavation volume using the method described in Section 6.4, paragraph 23.
(b) Calculate the corresponding dike volume using the method described in Section 6.4, paragraph 14.
(c) Plot these values on the two-way graph (see Graph 4).
(d) Select a second minimum depth of cut and calculate excavation and dike volumes similarly.
(e) Again, plot these values on the two-way graph (see Graph 4).
(f ) Join the points A to D and C to B and mark the intersection point E. This will determine the balancing minimum cut and the corresponding dike volumes.
17. For ground slopes greater than 1.5 percent, you will have:
18. Select minimum depth of cut and calculate dike volumes accordingly. Complete the trial-and-error procedure using Method 1 for sloping ground as just described.
In ponds A and B, all of the dikes vary in height
In pond C, the long top and bottom dikes are level and the short side dikes vary in height
The lower ends of ponds A and B face in opposite directions, because of their angled position on the contour
Estimating the cut-and-fill volumes on irregular ground
19. If the construction site of your pond is characterized by irregular slopes and uneven ground, the problem of balancing cut-and-fill earthwork volumes becomes much more complicated. According to the extent of the earthwork involved, rough estimates may be obtained by using Method 1 with volumes calculated as described in Section 6.4.
20. Alternatively, Method 2 using average ground level values can be applied. This method is faster, but less accurate.
Cut-and-fill volumes for groups of ponds
21. Frequently, for larger sites, several ponds and their water supply and drainage canals have to be built at the same time, so that the earthwork balances out across the entire project. This is clearly more complex, and will often require the assistance of a qualified engineer. There are, however, a few ways to estimate your requirements and to help guide your decisions, as you will learn below.
22. Water supply and drainage canals are usually more or less fixed at a specified level necessary for them to function properly. The volumes of earth involved, either excavated, built up or a combination of cut-and-fill, can be calculated according to the size of the canals. Any deficit or surplus soil must be found or used in the rest of the project.
23. The site area can be split up into main groups of ponds, depending on location, pond type or size, or method of operation. You can then decide:
24. For balancing groups of ponds, you could arrange for all of the ponds to be at the same level. This solution is appropriate for horizontal or gently sloping ground. Calculate the cut-and-fill balance using Method 1. Use average ground and dike levels for the group of ponds as a whole with the perimeter and intermediate dikes. Add in the extra cut or fill required for the canals.
Note: if the intermediate dikes are small, disregard them altogether and treat the whole group of ponds as a single large pond.
25. Alternatively you can arrange for ponds to be at different levels. This solution is appropriate for more steeply sloping ground. Here each pond is calculated individually. A simple short cut is to calculate one pond at the top end of the site and another at the bottom. The intermediate ponds will then be set at intermediate levels between these.
26. Where a surplus or a deficit is involved, you should include it in the trial-and-error balance:
27. In several areas certain groups of ponds may have preset levels defined, for example, by water supply or drainage. In this case, cut-and-fill calculations will define the surplus soil produced or the deficit needed to be made up from elsewhere.
28. Another useful method is to draw one or more cross-sections through the site or through groups of ponds. You can quite simply adjust levels graphically to obtain an approximate balance of cut-and-fill.
Staking out a cut-and-fill pond on horizontal ground
29. When the ground slope is less than 0.5 percent, the first steps of the method for staking out the centre lines of the dikes and the limits of the bases of the dikes are similar to the method described earlier for paddy ponds (see Section 6.7). The top of all the centre line stakes should be at the same level, indicating the dike's construction height.
30. In most cases, you will have made all the necessary calculations for staking out when calculating earth volumes.
Staking out a cut-and-fill pond on regular sloping ground
33. When the ground slope is steeper than 0.5 percent, you have already learned that the parts of the dikes to be built above ground level do not have the same height on every corner of the pond. The top of the dikes needs to be level, but as the base of the dikes is on a variable level, the width of the dikes at the base varies from one pond corner to another.
34. Once you have calculated how deep you need to dig at each pond corner to balance cut-and-fill volumes, the characteristics of the dikes are fully defined. In particular, their height above ground is determined for each pond corner, and therefore their corresponding base width is known. Now it remains to clearly mark these measurements on the ground before starting the construction. Proceed as shown in the example.
(b) Calculate for each pond corner the width of the dike base
to be staked out on either side of its centre line as
(c) Stake out these distances X and Z on either side of the centre lines at each pond corner and in two perpendicular directions to obtain four new points at each corner.
(d) Join these new points to set out the four dike bases at ground level. Notice that the limits of the side walls are not parallel, owing to the difference in elevation along the pond length.
(e) The pond bottom is set out in the same way as previously described.
Note: it the ground slopes in more than one direction, as when the pond is set at an angle across the slope, the same method can be used. But in this case, each of the pond corners is at a different height and therefore none of the pond walls are parallel.
Staking out a cut-and-fill pond on a very irregular slope
35. If the ground slope is very irregular, it is best to proceed slightly differently.
(a) Stake out the centre lines with a series of pegs.
Building the dikes manually
36. Start digging within the area staked out as the pond bottom, cutting vertically along the edges of this area.
37. Throw this earth into the area staked out as the dike base. Spread it over the entire area into a layer about 0.20 m thick, wet it if necessary, and compact it well (see Section 6.2).
38. Build up and shape the dikes to ground level as described for paddy ponds (see Section 6.7), checking the pond bottom level from time to time.
39. Finish the dikes by cutting the earth left between the lines of stakes marking the inside limits of the base of the dikes at ground level and at pond bottom level. This also completes the wet side slopes.
40. Remove all stakes and lines, put back the surface soil on the dikes
and plant or seed grass (see Section 6.9).
Completing the cut-and-fill pond
41. Clean the pond bottom.
Building the dikes using machinery
44. When using machinery to build the dikes of a cut-and-fill pond, it is most important to check on the digging progress closely and regularly, to avoid cutting the pond too deep. Usually the bulldozer transports the soil by pushing, spreading it into a thin layer over the dikes' area and compacting it. Compaction in particular should be thorough, after wetting if necessary.
1. Newly built dikes should be protected against erosion by planting or seeding a grass cover on the crest of the dikes, on their dry side and on their wet side down to the normal water level of the pond.
2. To form a grass cover with the minimum of delay, proceed as follows:
(a) Spread a 10- to 15-cm layer of topsoil over the area to be planted. This topsoil is either brought back to the pond area from which it was earlier removed or is obtained from a nearby source.
(b) If possible, mix in some compound chemical fertilizer, for example a 13-13-13 mixture (NPK)1, at the rate of 50 to 100 g per m2 surface area or 400-800 g per m3 of topsoil.
1N = nitrogen; P = phosphorus; K = potassium
(c) Plant either cuttings or pieces of turf of the selected grass, see Table 30, at relatively short intervals.
(d) Water well immediately after planting and afterwards at regular intervals.
(e) After the sod is formed, cut it short regularly to encourage it to spread all over the area. If possible, apply about 0.1 g of actual nitrogen per m2 to accelerate the spreading.
3. For further advice, contact agricultural extension workers.
4. If the weather is dry, you should plan for regular watering of the newly planted grass. Use mulching* to reduce soil evaporation.
5. When it rains heavily, use temporary protection, such as hay or other suitable materials, to avoid severe erosion of the dikes until a grass cover is formed.
6. Never plant large trees on or near dikes because their roots would weaken the dikes. In some areas, vegetable crops and forage bushes can be grown, but care should be taken to select plants with a good ground cover and with roots that do not weaken the dike by penetrating too deeply or disturbing the soil.
7. Care should be taken to keep the dikes in good condition, and only small animals should be allowed to graze or browse on them.
Selecting the grass cover
8. The best protection is obtained from perennial grasses (Gramineae) with the following characteristics:
9. Selected grasses recommended for the formation of a perennial grass cover on pond dikes are listed in Table 30.
1. Pond-bottom drains are ditches that are dug on the bottom of the pond to help the water flow out and to direct the fish toward the pond outlet when harvesting.
2. You do not always need bottom drains for your pond, for example in small ponds with a sloping bottom. However, it is better to build bottom drains:
Designing the network of drains
3. Bottom drains can be designed in different patterns according to the pond bottom topography and shape.
4. If the bottom topography is fairly even, it is better to build a regular network of drains, for example:
5. If the bottom topography is very uneven, it becomes necessary to build an irregular network of drains, connecting the various depressions and ensuring their complete drainage for harvesting.
6. In certain barrage ponds where the upper shallow areas are swampy, it is better to dig a 2- to 3-m wide sloping drain along their perimeter. A central drain may also be added to increase the water volume available to fish.
7. In paddy ponds, if soil is cut around the inside edge of the pond to form dikes, the trenches created should be linked in with the outlet drain.
9. The distance between drains should vary from 4 to 8 m in small ponds, to 30 to 50 m in very large ponds. The number of drains should be kept to the minimum required to completely drain the water, as they will have to be regularly cleaned by hand.
1. As soon as possible and before the completion of the pond, it is advisable to put it under water:
2. For maximum security and efficiency, proceed in the following way.
(a) Fill the pond with water very slowly and up to a maximum depth of 0.40 m at the outlet.
(b) Close the water supply and keep water in the pond for a few days. During this period, check the dikes carefully. Repair crevices and collapsed sections, compacting well.
(c) Drain the water completely and leave the pond dry for a few days. Keep checking the dikes and repair them as necessary.
(d) Fill the pond again very slowly and up to a maximum level about 0.40 m higher than the previous time.
(e) Close the water supply. Check the dikes and repair them as necessary. After a few days, drain the pond completely.
(f) Repeat this process of filling/drying until the water level in the pond reaches the designed maximum level.
(g) Check and repair the dikes as necessary.
3. If there is a limited water supply, it will not be possible to follow the above procedure. In such a case, fill the pond very slowly and gradually, closing the water supply at regular intervals and checking on the dikes carefully.