6. FISH POND CONSTRUCTION

Diagram of a pond dike built using sandy soil with a clay core and cut-off trench to ensure impermeability

Factors to be considered in calculating dike heights

CH = DH � [(100 - SA) � 100]

Example If the maximum water depth in a diversion pond of medium size is 1 m and the freeboard* 0.3 m, the design height of the dike will be DH = 1 m + 0.30 m = 1.30 m. If the settlement allowance is estimated to be 15 percent, the required construction height will be CH = 1.30 m � [(100 - 15)  � 100] = 1.30 m � 0.85 = 1.53 m.		Design height and construction height WD =Water depth FB = Freeboard DH =Design height SH = Settlement height CH = Construction height
Calculating construction height (diversion pond) 1.30 m � [(100 - 15) � 100] = 1.30 m � 0.85 = 1.53 m
7. In barrage ponds with a spillway, the design height of the dike is calculated slightly differently (see Sections 11.3 and 11.4, Pond construction, 20/2), the freeboard being added above the maximum level of the water in the discharging spillway. Note: the water surface in your pond will be horizontal and therefore the top of your dike should also be horizontal, from the deepest point of the pond to its shallowest point. Determining dike thickness 8. A dike rests on its base. It should taper upward to the dike top, also called the crest or crown. The thickness of the dike thus depends on: the width of the crest; and the slope of its two sides. 9. This, together with the height of the dike, will determine the width of the dike base (see Table 27 for examples). 10. Determine the width of the crest according to the water depth and the role the dike will play for transit and/or transport.    (a) It should be at least equal to the water depth, but not less than 0.60 m in clayey soil or 1 m in somewhat sandy soil. (b) It should be even wider as the amount of sand in the soil increases.

(c) It should be safe for the transport you plan to use over it: at least 3 m for motor vehicles; for larger vehicles at least the wheel base plus 0.50 m on each side. Note: these dimensions may be slightly reduced for very small rural ponds.		Factors to be considered in determining width of dike top
		Crest width at least equals water depth

TABLE 27 Examples of dimensions of dikes Individual pond size (m2) 200 400-600 1000-2500 Quality of soil1 Good Fair Good Fair Good Fair Water depth (max. m) 0.80 1.00 1.30 Freeboard (m) 0.25 0.30 0.50 Height of dike2 (m) 1.05 1.30 1.80 Top width3 (m) 0.60 0.80 1.00 1.30 1.50 2.00 Dry side, slope (SD) 1.5:1 2:1 1.5:1 2:1 1.5:1 2.5:1 Wet side, slope (SW) 1.5:1 2:1 2:1 2.5:1 2:1 3:1 Base width4 (m) 4.53 6.04 6.36 8.19 8.92 13.66 Settlement allowance (%) 20 20 15 15 15 15 Construction height5 (m) 1.31 1.31 1.53 1.53 2.12 2.12 Cross-section area (m2) 3.3602 4.4802 5.6266 7.2560 11.0452 16.5996 Volume per linear m (m3) 1 See Soil, 6. Good soil includes clay, sandy clay, sandy clay loam, clayey loam, silty clay and silty clay loam; fair soil includes loam, sandy loam and silty loam 2 Design height, the height the dike should have after settlement 3 To be increased for use of motorized transport 4 At construction, and calculated from the construction height 5 Height at which the dike should be built taking settlement into account

Remember: the thickness of intermediate dikes can be reduced when resistance to water pressure and impermeability are less important.

TABLE 28 Expansion and settlement of pond soils Type of soil Expansion when disturbed (% undisturbed volume) Settlement allowance* (% expanded volume) Loose rock, gravel 10-15 8-10 Firm compact earth 10-20 10-15 Ordinary loose earth 5-10 15-20 Loam to light clay 15-25 20-25 Heavy clay 5-15 15-25 Compaction   Further expected settlement (% construction volume) Good: soil layered/rolled/watered   1-5 Normal: soil rolled/watered   5-10 Poor: soil rolled/not watered   10-15 Poorly compacted soils after exposure   Further expected settlement (% construction volume) One rainy season   8-12 Two rainy seasons   5-10 Three rainy seasons   2-5 * Total amount by which the expanded soil volume is expected to be reduced by either compaction plus final/small settlement, or compaction and settlement, or settlement alone

Note: if the soil to be compacted can be formed into a firm ball that sticks together, the moisture content is adequate for immediate compaction. If the soil is too wet, you should let it dry through evaporation for a while. If the soil is too dry, you should water it slightly and mix well to make it homogeneous.		Good soil material for compacting will stick together

12. Hand compacting is generally suitable for small dikes, typically 1 to 1.5 m in height and up to 1 m top width, or smaller if soils are not of good quality. Note: you can easily make a hand tamper using scrap metal fittings, a section of pipe filled with sand and a hardwood handle Various compacting tools		Note: for clays and similar soils, it may be better to use a kneading action, for example by using the heel of the foot. Compacting with the feet

Compacting soil with machinery

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).

Compactor Output m2/h Percussion tamper (frog) 30-150 Vibration plate 300-600 Roller   sheepsfoot 1000 steel-wheel 2000-5000 pneumatic 5000-15000

Compacting small areas
Vibration plate		Percussion tamper (frog)		Tracked vehicle
Compacting larger areas
Sheepsfoot roller		Vibration roller		Pneumatic roller
		Steel-wheel roller

Detail of a cut-off trench dug through a permeable layer of soil into an impermeable layer

Cut-off trench for large pond

Cut-off trench for small pond

Preparing the stream channel for a barrage pond

Cross-sections through a future dam site showing how to clean and enlarge the old stream channel

Base width = crest width + (CH x SD) + (CH x SW)

Note: use the construction height, including the settlement allowance, and not the design height of the dike (see Section 6.1).

Note: see also examples in Table 27.

CH = the construction height of the dike; SD = the slope ratio of the dry side; SW = the slope ratio of the wet side.
Example For the above 0.04-ha pond to be built in clayey soil, calculate the size of the cross-section of the dike as: area 1 = 1 m x 1.88 m = 1.88 m2; area 2 = (1.5 x 1.88 m) x (1.88 m � 2) = 2.6508 m2; area 3 = (2 x 1.88 m) x (1.88 m � 2) = 3.5344 rn2 cross-section = 1.88 m2 + 2.6508 m2 + 3.5344 m2 = 8.0652 m2.

TABLE 29 Cross-sections of dikes on horizontal ground with identical side slopes (in m2) Construction height of dike (m) Side slopes 1.5:1 Side slopes 2:1 Top width Top width 1m 2m 3m 1m 2m 3m 0.5 0.8 1.3 1.8 1.0 1.5 2.0 1.0 2.5 3.5 4.5 3.0 4.0 5.0 1.5 5.0 6.5 8.0 6.0 7.5 9.0 2.0 8.0 10.0 12.0 10.0 12.0 14.0 2.5 12.0 14.5 17.0 15.0 17.5 20.0 3.0 16.5 19.5 22.5 21.0 24.0 27.0

Calculating the cross-section of a dike on irregular ground using a scale drawing

Calculating the cross-section of a dike on irregular ground using squared paper 1 cm = 0.5 m 1 square of 0.5 m x 0.5 m = 0.25 m2 15.2 squares x 0.25 m2 = 3.8 m2

Example Using the preceeding example, the heights of A and D are estimated by drawing line XY through the base so that the areas above the line are approximately equal to those below the line. Note that the profile of the ground as drawn should represent the average height across the wall base.

Estimating dike base on rough ground

20. You will normally have to remove the topsoil before you reach soil good for construction material. Levels should therefore be taken from the base of the topsoil layer. In most cases, the sides of the excavation should be sloped to prevent them from collapsing. In many cases (ponds, channels, etc.) these will be of specified gradients.
21. For reasonably flat, level surfaces, where excavated width is at least 30 times the depth, volume of excavation can be estimated as: V = top area x depth of excavation.

V = [(top area + bottom area) � 2] x depth.

Site before construction

Check water table variations on the site for one year

(a) Mark the area to be cleared using stakes. This should include the total area of the pond to the outside limits of the pond dikes and, in addition, an area of two to three metres to serve as a work space and for walkways beyond the dikes.		(b) Clear all vegetation from the marked area (see Chapter 5). Also remove all shrubs and trees within 10 metres of the cleared area.
(c) Next, at the very centre of the cleared area, mark the pond area to the outside limits of the pond dikes using heavy string or cord. Remove the surface soil from this area and store it for later use.		(d) Now mark the inside limits of the pond bottom using heavy string or cord. Do this on the basis of the selected side slopes (see Item 13, Section 6.1). (e) When staking out the pond bottom, indicate on each stake the depth of excavation from ground surface to pond bottom.

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.
 
(h) Dig to the designed depth within the limits of the pond, cutting the sides vertically. Transport the waste soil to the planned areas.  
 
Note: usually the pond bottom in drainable ponds is given a one-percent slope between the inlet and outlet ends; in undrainable ponds, the pond bottom may be horizontal. You can calculate the volume of the material to be excavated using one of the methods in the previous section.

Disposing of waste soil material

Point Slope ratio Dam design height DH (m) Dam construction height  CH (m)* Distance from centre line to toelines (m)** B 2:1 1.10 1.29 BG = 3.58 C (wet) 1.60 1.88 CH = 4.76 D   2.10 2.47 DI = 5.94 E   1.40 1.65 EK = 4.30 B 1.5:1 1.10 1.29 BL = 2.94 C (dry) 1.60 1.88 CM = 3.82 D   2.10 2.47 DN = 4.71 E   1.40 1.65 EO = 3.48 * Calculate CH = DH� [(100- SA)�100] where SA is the settlement allowance in percent; here SA = 15 percent and CH= DH � 0.85 ** As (crest width � 2) + (CH x slope)

Stakes and lines to mark construction height

Wood templates to mark construction height

Building the first part of the dam
16. Start building the dam by laying out successive horizontal layers 15 to 25 cm thick. Work across the entire site, from one side of the valley to the new stream channel, and from the wet to the dry side of the dam. Wet the soil if necessary and compact each layer well (see Section 6.2).		Build up dam using horizontal layers

Build each layer of good quality soil

If you do not have enough good soil to build the entire dam, use what good soil you have to build a central core

A typical barrage pond built in two parts

18. To build up the dam, you can use hand labour or machinery. If you use machinery such as a bulldozer for pushing, spreading and compacting the soil material, you could proceed in the following way. (a) Build up the first part of the dam to about 1 m above the level of the foundations, progressing layer by layer. (b) Determine and stake out the centre line of the water outlet structure, perpendicular to the centre line of the dam.		Building a dam for a barrage pond
(c) Mark a parallel line on each side of this outlet centre line at a distance of about 0.5 m. (d) Dig a trench about 1 m wide, down to the planned elevation for the water outlet pipes.

(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.

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).

6.7 Constructing paddy ponds

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.
 

Staking out the base of the dikes

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).

Water control structures
Pipe inlet		Stand-pipe outlet
Earthen canal inlet		Monk outlet

Building the dikes of a paddy pond manually
12. There are several ways to build the dikes of a paddy pond. To construct them manually, you can use templates as for the barrage pond, although here only one template size is required.   Note: you can also use stakes and lines to mark construction height as shown in section 6.6, para.15.		Wood templates to mark construction height

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.

Building a dike layer by layer

Example If wet slope is 2:1 and dry slope is 1.5:1, move the inside limit by 0.20 m x 2 = 0.40 m and the outside limit by 0.20 m x 1.5 = 0.30 m. (e) Raise all the lines 0.20 m higher. (f) Build the second layer of the four dikes 0.20 m high between these new limits, as you did for the first layer. (g) For each dike, move the inside and outside limits toward the centre line by the same distances as previously. (h) Raise all the lines 0.20 m higher again. (i) Build the next layer of the four dikes 0.20 m high between these new limits. Repeat these last three steps until you reach the level of the top of the dikes as indicated on the centre line stakes. It may be that the last soil layer is less than 0.20 m thick, in which case adjust the level of the lines with the level of the top of the dike. Note: if you need to build a central core within the dikes, you should add lines setting out its width on either side of the centre line. The core is built together with the rest of the dike, using different types of soil for each 0.20-m layer.

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.

(g) Seed or plant grass to control erosion (see Section 69).

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.

I = Inlet O = Outlet

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.

17. For larger ponds it is best to ensure complete drainage through a network of shallow drains each with a slope of 0.2 percent (see Section 6.10), rather than trying to build a slope over the entire pond area.		Bottom drains (slope 0.2%)
18. For ponds where internal trenches have been dug to provide material for dikes, these trenches should be connected together and shaped so as to drain toward the water outlet.		Internal trenches (slope 0.2%)

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.  

Building a series of adjoining paddy ponds
22. When building a series of adjoining paddy ponds, remember that only the dikes forming the perimeter of the pond series should be built with the characteristics of wet/dry dikes. The intermediate dikes, which are wet on both sides, can usually be built less strongly, even without a central core if necessary. 23. First stake out the centre line of a series of perimeter dikes, for example XY for dikes ACDB. Then stake out the centre lines of the opposite perimeter dikes EGHF and the perimeter or intermediate dikes AE, CG, DH and BF. 24. Build the dikes as explained earlier in this section, either manually (see paragraphs 13 and 14) or mechanically (see paragraphs 19 to 21).

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.

5. To illustrate method one we will use a 400m2 pond (20 x 20 m along the centre lines of the dikes) with 2:1 inner slopes and 1.5:1 outer slopes, a dike construction height of 1.5 m and a crest width of 1 m. Pond area = 20 x 20 m = 400 m2 Crest width = 1 m SW = 2:1 SD = 1.5:1 Dike height = 1.5 m

METHOD 1
First trial calculations

Example

If digging depth = 1 m, dike height = 1.5 m - 1 m = 0.5 m. Using the methods described in Section 6.4 obtain:

• cross-section of dike = (1 m x 0.5 m) + [(1 m x 0.5 m) � 2] + [(0.75 m x 0.5 m) �2] = 0.50 m² + 0.25 m² + 0.1875 m² = 0.9375 m²;
• Fill = total dike volume = 0.9375 m² x 80 m = 75 m³;
• average area of cut = [(169 m² + 289 m²) � 2] = 229 m²;
• Cut = excavation volume = 229 m² x 1 m = 229 m³.

In this case, the cut volume greatly exceeds the fill volume.

METHOD 1
Second trial calculation

Example

Use higher dikes and reduce digging depth, say to 0.5 m; thus dike height = 1 m. This time, you obtain:

• Fill = total dike volume = 2.75 m² x 80 m = 220 m³
• Cut = excavation volume 197 m² x 0.5 m = 98.5 m³.

In this case, the fill volume exceeds the cut volume.
The correct digging depth lies somewhere between 1 m and 0.5 m, the two values tried above.

METHOD 1
Estimating correct digging depth

Example

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
1.50 m - 0.72 m = 0.78 m:

• total dike volume = 1.845 m² x 80 m = 147.6 m³
• excavation volume = 210.6 m² x 0.72 m = 151.6 m³.

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.

METHOD 2

(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:

• the ratio of length to width is 1:1 (the pond is square);
• both dike slope ratios are 2:1;
• pond dikes are 1.5 m high.

(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.  

Example Again using a pond 20 m square (area = 400 m2) with dikes 1.5 m high and a crest width of 1 m, from Graph 5 determine a standard cut of 0.75 m.   As the dike slope ratios differ from those of the standard pond (in this case inner slope 2:1 and outer slope 1.5:1) find, from the first table, S = -0.05 m and correct the standard cut depth as 0.75 m - 0.05 m = 0.70 m, which is the balancing cut.   If the pond had not been square, for example L = 28.5 m and W=14 m, you should have found, from the second table, P = 1.04 and corrected the cut depth as 0.70 m x 1.04 = 0.728 m.

Pond area = 20 x 20 m = 400 m2 Crest width = 1 m SW = 2:1 SD = 1.5:1 Dike height = 1.5 m

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.

REFERENCE TABLES
1. Correction factor S for dike slope ratios Inner slope Outer slope S (m) 1:1 1:1 -0.20 1.5:1 1:1 -0.15 1.5:1 1.5:1 -0.10 2:1 1.5:1 -0.05 2.5:1 1.5:1 -0.02 2:1 2:1 0 2.5:1 2:1 +0.03 3:1 2:1 +0.06 2.5:1 2.5:1 +0.08 3:1 2.5:1 +0.10		2. Correction factor P for pond shape Pond length/width ratio P 2 1.04 3 1.11 5 1.23 10 1.50		3. Correction factor D for dike height Dike height (m) D 1.0 0.55 1.2 0.74 1.4 0.90 1.5 1.00 1.6 1.10 1.8 1.30 2.0 1.50 2.2 1.80

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.

Digging ponds on sloping ground

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:

• one low shorter dike, upslope, either horizontal or varying in height;
• one high shorter dike, downslope, either horizontal or varying in height;
• two longer dikes, varying in height.

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:

• one low longer dike, upslope, either horizontal or varying in height;
• one high longer dike, downslope, either horizontal or varying in height;
• two shorter dikes, varying in height.

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.

Notes:
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.

• for small ponds it is best to use average ground level;

• for larger ponds more accurate results are obtained by calculating volumes section by section.

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:

• whether the cut-and-fill for each group will be balanced, often the case in horizontal or gently sloping ground; or
• whether a surplus is expected for a particular group (if it is on high ground level and should be lower), or a deficit needs to be made up (if it is on low ground and needs to be built up). This situation is typical in more steeply sloping ground.

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:

• if a surplus is required, add it to the dike volume quantity. The total figure is then used in the two-way graph;
• if a deficit is to be made up, add the required amount of soil to be brought into the area to the excavation volume. This total figure is then used in the two-way graph.

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.

Example If crest width of dike = 1.00 m;  pond depth = 1.50 m;  dike height = 0.78 m;  dry side slope = 1.5:1;  wet side slope = 2:1.  Then, staking-out distances at ground level are:  Z = (1.00 m�2) + (0.78 mx 1.5) = 1.67 m;  X = (1.00 m� 2) + (0.78 mx 2) = 2.06 m;  and similarly for the other three dikes.
Plan for staking dikes		Dike cross-section
31. However, as the side slopes of the dikes have to be cut below ground level, you should stake out with short pegs one additional line defining the limits of the pond bottom. Note: the pond bottom area is the same regardless of the cut depth (see Section 6.4). 32. Stake out the pond bottom to clearly indicate how deep to dig at a series of points (see Section 11.4, Topography).		Staking out pond bottom

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.

Example The pond to be built measures 25 x 15 m along centre lines. The dike is 1.40 m high with the following characteristics: wet/dry side slopes = 2:1; top width (crest) 1.00 m; the ground slope 1.5 percent; the cut-and-fill volumes are found to balance for a minimum cut = 0.86 m; the maximum cut = 1.15 m; Thus the maximum height of the dike to be built above ground level is 1.40 m - 0.86 m = 0.54 m, and the minimum height of the dike is 1.40 m - 1.15 m = 0.25 m.		Pond area= 25 x 15 m = 375 m2 Crest width = 1m  SW = 2.1  SD = 2:1 Dike height = 1.40 m
(a) Stake out the four centre lines of the dikes AB, BC, CD and DA at 25 x 15 m, the width of the pond being parallel to the contour lines.		Stake out centre lines ABCD

(b) Calculate for each pond corner the width of the dike base to be staked out on either side of its centre line as
X or Z= (crest width � 2) + (side slope x height above ground):

 

(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.

Example of dike base on ground sloping in only one  direction

Example of dike base on ground sloping in more than one direction

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.
(b) On each of the pegs mark the height to be reached by the dike crest. This forms a horizontal line.
(c) Calculate the required dike base width at each peg according to the dike to be built there above ground level.
(d) Stake out the dikes' bases all around the pond, on short perpendiculars set on the centre lines.
(e) Stake out and mark with lines the limits of the pond bottom after calculating distances Y from the centre line as:

 

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.

42. In small ponds, give the bottom a gentle slope (0.5 to 1 percent) from water inlet to water outlet.		Pond with sloping bottom (slope 0.5 - 1 %)
43. In large ponds, either give the bottom a very gentle slope (0.2 percent) or, preferably, dig a network of shallow drains with a slope of 0.2 percent over the entire bottom area (see Section 6.10). Pond with a network of shallow bottom drains (slope 0.2%) I = Inlet O = Outlet

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.

6.9 Protecting dikes against erosion by rain

Protect new dikes as soon as they are built

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.

Preparing dikes for planting or seeding grass

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.

Never plant large trees on or near dikes		Crops may be planted on top of dikes
Only small animals should graze on dikes

Selecting the grass cover

8. The best protection is obtained from perennial grasses (Gramineae) with the following characteristics:

• fast spread into a dense cover, through creeping, rooting stems (stolons*) or underground rhizomes*;
• well adapted to local climate, particularly if seasonally dry;
• easy to propagate vegetatively, for example by transplanting stolons* or rhizomes*.

9. Selected grasses recommended for the formation of a perennial grass cover on pond dikes are listed in Table 30.

6.10 Pond-bottom drains

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:

• when the bottom slope is insufficient;
• in large ponds more than 75 m long;
• in barrage ponds with an uneven bottom relief.

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:

• either radiating from the outlet if the pond shape is squarish;
• or in a fish-bone pattern if the pond is more elongated.

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.

8. Bottom drains are usually designed as trapezoidal canals with the following characteristics:

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.

6.11 First filling of the pond

1. As soon as possible and before the completion of the pond, it is advisable to put it under water:

• to check that all structures function properly such as the water intake, the canals, the pond inlet and outlet;
• to check that the new dikes are strong and impervious enough;
• to accelerate the stabilization of these dikes.

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.