Once the capacity of the roadside farm pond is clear, the layout of the pond can be prepared. This includes the decision on the shape of the pond, its dimensions (including depth and side slopes), and the additional structures
Roadside farm ponds may normally be of three shapes: square, rectangular and circular. However, because a curved shape presents difficulties in construction, either square or rectangular ponds are normally adopted. Compared to square and rectangular ponds, structurally circular ponds are said to be more stable because there are no weak joints (edges).
Digging the pond itself or closing the surrounding area with well-compacted levees or a combination of both creates square and rectangular ponds. Ponds of this type are recommended and easily constructed, particularly in areas with flat topography. An excavated pond is often built on level terrain and its depth is achieved solely by excavation. The pond is relatively safe from flood damage, requires low maintenance, and can be built to expose a minimum water surface area in relation to volume. This is beneficial in areas of high evaporation losses and a limited amount of water supply.
Depth and side slope of farm pond
The depth of a pond is generally determined by soil depth, type of material excavated, and type of equipment used. Pond depth is the most important dimension among the three. In semiarid regions, evaporation losses can be reduced by deepening the pond for the same volume of water stored, because the area occupied by the pond is smaller. However, with increased depth, seepage losses also increase. On the other hand, when ponds are constructed using manual labor, any increase in depth beyond 3.5 to 4 m becomes uneconomical. It also becomes uneconomical and difficult for lifting devices operated by people. Therefore, a depth of 2.5 to 3.5 m may be suitable in general for ponds.
The pond’s side slope is based on the angle of repose at which the material is excavated. This angle varies with the type of soil. In most cases, side slopes of 1:1 to 1.5:1 are recommended for practical purposes. Based on practical experience, it is recommended that selected side slopes generally be no steeper than the natural angle of repose of the material. Table 10.3 presents the recommended side slopes for different soils.
If livestock will water directly from the pond, a watering ramp of ample width should be provided. The ramp should extend to the anticipated low-water elevation at a slope no steeper than three horizontal to one vertical. If water is collected for irrigation, provisions also need to be made to access the pond, such as a ramp or a platform for a pump.
Table 10.3: Suitable side slopes for different soils
|Soil type||Slope (Horizontal:Vertical)|
Source: FAO 2011
A number of ancillary features are important:
If surface water enters the pond in a natural or excavated channel, the side slope of the pond must be protected against erosion.
A spillway is an important feature of a pond. It is designed to accommodate the removal of excess runoff in a controlled manner. The spillway must be reinforced with stone pitching, concrete or, at a minimum, grasses. The spillway should be located at some distance from the road embankment so that it does not undermine the road pond embankment.
The runoff from the road embankment will carry significant quantities of sediment. These will fill the pond and end its economic life prematurely unless the sediment is either removed regularly or intercepted before it reaches the pond. The runoff is best routed across a vegetated area to intercept large part of the sediment. A silt trap will further remove sediment. It consist of a small settling basin where the sediment is trapped and then removed.
Roadside ponds may be fenced, preferably by native tree species that do not have root systems that will penetrate the pond. The fencing will help to regulate access to the pond and also provide a shelter against wind and thus reduce evaporation from the pond.
Box 10.3. Controlling sedimentation and contamination
It is important to take measures to improve the quality of the water in the pond, reduce silt loads, and avoid contamination. High, intense rainfall events cause soil erosion and the runoff carries the silt load into the farm pond. Other contaminants attach themselves to the silt. These problems can be resolved through proper soil and water conservation treatments around the pond. In order to achieve the desired depth and capacity of the proposed pond, the inflow must be reasonably free of silt from an eroding catchment. The best protection is adequate erosion control through on-site soil and moisture conservation or land-management practices in the drainage contributing area. To control sedimentation what is proposed:
- Use sediment traps. To reduce the immediate inflow of sediment into the pond the run-off is best routed through an area with vegetation – this can be grasses, wild vegetation or for instance banana trees. Sediment can also be removed from the ponds by hand or by using pits that trap the sediment.
- Land under permanent cover of trees or grasses is the most desirable drainage area. If such land is not available, consider treating the watershed with proper soil-conservation practices to control erosion before constructing the pond, or include silt traps as part of the pond design.
- Generally, the catchments must be selected in such a way that contaminated drainage from farmsteads, feedlots, sewage lines, dumps, industrial and urban sites, and similar areas does not reach the pond. Similarly, runoff from intensely used road sections—bringing hydrocarbons, rubber and oils—must be prevented from ending up in the ponds.