11.3 Suitable pond side slopes for different soils

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 storing the same volume of water in a deeper pond because a smaller exposed surface area of the pond reduces losses from evaporation. Seepage losses increase with pond depth, however. When ponds are constructed using manual labor, any increase in depth beyond 3.5–4.0 meters becomes uneconomical. Lifting water from depths greater than 3.5–4.0 meters is also less economical and more difficult to do with hand-operated lifting devices. Pond depths of 2.5–3.5 meters may generally be the most suitable.

The pond’s side slope is based on the angle of repose of the material excavated. This angle varies with the type of soil. In most cases, side slopes of 1.0:1.0 to 1.5:1.0 are recommended for practical purposes. Based on practical experience, selected side slopes should generally be no steeper than the natural angle of repose of the material. Table 11.3 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 units to one vertical unit. If water is collected for irrigation, access to the pond needs to be provided, such as a ramp or a platform for a pump.

Additional features. Important ancillary features of farm ponds include the following:

• Inlet protection. If surface water enters the pond in a natural or excavated channel, the side slope of the pond must be protected from erosion. • Spillways. 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 some distance from the road embankment so that it does not undermine the road-pond embankment. • Silt traps. The runoff from the road embankment will carry significant quantities of sediment. Unless this sediment is either removed regularly or intercepted before it reaches the pond, it will fill the pond and prematurely end the pond’s economic life (box 11.3). The runoff is best routed across a vegetated area, which will intercept a large part of the sediment. A silt trap will further remove sediment. A silt trap consists of a small settling basin where the sediment is trapped and then removed. • Fencing. 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 regulate access to the pond and provide shelter from wind and thus reduce evaporation from the pond.

TABLE 11.3 Suitable pond side slopes for different soils

SOIL TYPE

Clay Clay loam Sandy loam Sandy

SLOPE (HORIZONTAL:VERTICAL)

1.0:1.0

1.5:1.0

2.0:1.0

3.0:1.0

BOX 11.3

Controlling sedimentation and contamination

Measures should be taken 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 by using proper soil and water conservation treatments around the pond. 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 catchment area. The following measures can be used to control sedimentation:

• Sediment traps. The best way to reduce the immediate inflow of sediment into the pond is to route runoff through an area with vegetation. This area can be grasses, wild vegetation, or cultivated

plants, for instance, banana trees. Sediment can also be removed from ponds by hand or by using pits that trap the sediment. • Drainage area. 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. • Removal of contamination. Generally, 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 intensively used road segments, which can carry hydrocarbons, rubber, and oils, must not be allowed to enter farm ponds.

Constructing the roadside farm pond

Ponds can be constructed using road-building equipment as part of the road construction or rehabilitation contract. Alternatively, they can be built by land users’ initiatives using earthmoving equipment or manual or animal labor.

After site selection and determination of pond shape and dimensions, the pond site should be cleared of all stones and woody vegetation. The selected site should be free of vegetation, bushes, and other obstacles. The site should be leveled so that the demarcation line of the pond area can be drawn.

Before construction of the farm pond, the proper layout should be marked on the ground. This process may use rope and lines, lime powder, or small cuts. The idea is that the demarcation lines must be visible to the equipment operator, thus enabling him or her to excavate earth from the pond area. Stakes are used to mark the limits of the excavation and spoil-placement areas. The depth of cut from the ground surface to the pond bottom should be indicated on the stakes.

The use of a bulldozer for excavation is usually appropriate for medium-sized ponds because of its inefficiency when it comes to transporting material. If the excavated material is placed near the pond, it can be used as a berm or dike. After the earth is excavated, the subgrade and banks should be compacted.

When landowners develop ponds, they often use manual labor or tractor-pulled wheeled scrapers. An alternative method for developing ponds is

to make use of animal traction with the help of scoops (photo 11.4). This method is uncommon but has great potential.

The following are the main recommendations for using animal traction for farm-pond construction:

• For animal digging, use staged ramps and construct the pond in layers. • The area to be excavated should be softened by plowing to a depth of 20–30 centimeters, which can be done by an ox-drawn plow attached to one or two oxen pairs using normal yokes. The space should be planned to enable the animals to turn easily. • The softened soil should be removed with the ox scoop. To load the scoop, the operators simply raise the handles of the device to augment the incidental angle between the soil and the scoop. The forward movement of the animals will do the rest. once the scoop is loaded, the handles are lowered again, and the scoop is pulled until the disposal area is reached. To offload the soil, the operators raise the handles until the scoop topples together with its load. These scoops are not usually available but can be made in local workshops. • excavated soil needs to be properly disposed of and is commonly used to build a berm around the pond. It is important to compact the berm to avoid erosion problems. A simple roll compactor can be built with second-hand bearings, scrap metal, and an old oil drum. The drum is filled with sand and rotates as the oxen pull it.

PHOTO 11.4

Developing a roadside pond with oxen scoop and compactor, Tanzania

Source: © MetaMeta (www.roadsforwater.com). Used with the permission of MetaMeta (www.roadsforwater.com). Further permission required for reuse.