by JA Nienaber and M van Zyl

All three countries covered by this new magazine, Botswana, Zimbabwe and Namibia, experience one of the most severe droughts in human history. A manual on building a dam is perhaps a little too late, but since it is never too late to plant a tree, it is also never too late to build a dam, or an emergency dam for that matter. This short article series shows how you can build your own affordable round dam in a do-it-yourself fashion.

The walls of this dam consist of welded wire mesh, with a lining of polyester cloth treated with a sealant for waterproofing. Timber posts supply the necessary support and strength to the structure. As it is important to determine the size of the dam beforehand, clear indications are given for the calculation of the diameter. A complete list of materials required is supplied for easy purchase of the components.

Step-by-step instructions with practical hints for the erection of the dam are given. At the end of the series, directives regarding the maintenance of the dam are also discussed. Collected water is a necessity on every farm – whether it is used for drinking water for man and animal, irrigation of lands or vegetable gardens, fish farming, processing or any other purpose. The cost of a dam or dams for collection of water usually constitutes a considerable part of the cost of the infrastructure of a farm.

The ARC Institute for Agricultural Engineering therefore started examining alternative and cheaper ways of collecting water. A dam was erected on the premises of the Institute for this purpose and has been evaluated for a period of three years. The dam was emptied, cleaned and refilled on a regular basis. It is entirely exposed to all weather conditions and it can be stated that the durability of the dam can be determined in the course of time.

The conventional type of dam of concrete and brick may be considered a more durable construction, but it costs considerably more and is more labour intensive to build. With careful maintenance and the prevention of mechanical damage, it is viable to consider the new structure, as it is much cheaper than commercially available dams. The cost of erecting this dam is less than half of the cost of a round concrete dam and less than a quarter of the cost of a commercial, prefabricated dam after installation.

Description of the dam:

The structure (See Figure 1) is a round dam of welded wire mesh, with a lining of polyester cloth, painted with a waterproofing substance to make it waterproof. The whole structure is secured with a number of timber poles planted into the ground, with the wire mesh secured to it. The construction of the dam is simple, and it can easily be constructed with the user’s own labour in a very short time. The materials are also easy to use, and no specialised knowledge or equipment is necessary.

Size of the dam:

The user decides on the size of the dam required according the purpose for which it will be used. The following aspects should however be kept in mind:

• How regularly will water be pumped Dams with a diameter larger than 10 m and a height exceeding 1,2 m are not discussed, because the materials place a restriction on the strength of such a dam.

List of materials:

The following materials required for the erection of this storage dam can be purchased at an agro shop or building material dealer:

• Treated timber poles, 1,5 m in length and 50 to 75 mm thick.

• Galvanised, welded wire mesh, 1,2 m wide with mesh openings of 13 x 25 mm, of which the wire is 2,0 mm thick.

• Unwoven polyester cloth (also called geotextile), grade U14, with a width of 1,32 m.

• Multi-purpose rubberised waterproofing sealant in liquid form. Ensure that the sealant does not contain any health threatening substances and that it will be stable under prolonged exposure to sun and water.

• An exterior acrylic paint of a light colour.

• Outlet pipe and 90° bend of at least into the dam?

• What will the total water consumption be during this period?

• How continuous is the water source?

• How much damage will be caused by water shortages?

After the volume of the dam has been determined, the diameter must be calculated in order to continue with the preparations for and erection of the dam. For practical reasons, the height of the dam walls is restricted to 1,2 metre. 100 mm in diameter, of PVC, polythene, and fi bre cement or galvanised steel.

• Binding wire.

Table 1 shows the ratio between the volume, diameter and radius of a round dam with a depth of 1,2 metre.

The following abbreviations are used for the units:

m = metre

m2 = square metre

m3 = cubic metre

ℓ = litre

mm = millimetre

1 m3 = 1 000 litre subsequently applies as the calculation of the volume has already been done, the materials can be purchased according to Table 2. The diameter of the dam is also known according to the particulars in Table 1 or as calculated with the aid of comparison (1). The construction can now commence.

Next month we shall look at the preparation, construction and maintenance of the dam. Published with acknowledgement to the ARC Agricultural Engineering for the use of their Affordable Round Dam for Farms Manual. Visit www.arc.agric.za for more information.