Climatic instability in northeast Brazil has more to do with irregular rainfall than with drought. The lack of a reliable water supply to meet even basic needs is a serious hindrance to human settlement in rural areas. Like other semi-arid regions of the world, the semi-arid tropical region of Brazil has shallow, rocky soils with low water retention capacity, a low organic material content, and a high susceptibility to erosion.
There are various options for creating and tapping water reserves in this region. Surface reservoirs are the most commonly used, since geological conditions are conducive to a high degree of surface drainage. However, evaporative losses are high. Another option is to make use of groundwater. However, the underlying crystalline bedrock lacks the porous structure necessary to store a large volume of water and maintain a high rate of extraction. To overcome these shortcomings, a further option, that of creating artificial aquifers using underground dams has been devised as a means of storing large quantities of good quality water for family or community needs, for use by animals, or even for small-scale irrigation. Under semi-arid tropical conditions, alluvial pools are a widespread phenomenon. This natural pooling of water, very common in watersheds with crystalline bedrock, lends itself to the building of underground dams in the surficial alluvium. Such dams have the advantages of being able to store larger volumes of water than the natural aquifers in this area, and of being less susceptible to evaporative losses as the water is stored underground. The use of these underground dams also takes advantage of the naturally occurring alluvium (Monteiro, 1984). This technology is described in Part B, Chapter 1, "Freshwater Augmentation Technologies."
An underground dam is any structure designed to contain underground flow, from a natural aquifer or from an artificial one, built with an impermeable barrier. Two major types have been distinguished in the literature: underground or submersible dams and submerged dams (Santos and Frangipani, 1978; Silva and Rego Neto, 1992), both shown in Figure 45. Underground or submersible dams are defined as dams with walls that begin at the impermeable layer and extend above the surface of the alluvium, causing pools to form upstream during rainy periods. Water is stored both above and below the alluvial surface. The wall of a submerged dam, on the other hand, is entirely enclosed in the alluvium, and water is stored in the saturated soil. These types of dams have been built in northeast Brazil since the turn of the century to augment rural water supplies.
The dam wall, also called an impermeable plate, intercepts the flow of underground and/or surface waters, creating and/or raising the water table and pool elevation within an alluvial area. The dam wall is the main component of this technology. It extends from the bedrock or other subsurface impermeable layer up to, or beyond, the surface of the alluvial soil. It can be built of various materials, such as layers of compressed clay; packed mud; masonry; polyethylene or PVC plastic canvas; concrete; or a combination of materials.
· Construction of an Underground Dam
Site selection. The first step is site selection. Information on the soil distributions in an area is used to identify the best site. Sites with alluvial soils no more than 3 m to 4 m deep, of medium to coarse texture, and having a gradient of no more than 5% are preferred. Such sites may coincide with natural drainage routes, known as creeks, which carry large amounts of rainwater runoff in the region. In order to make optimal use of creek beds, a knowledge of the soil profile, and hence the depth to the impermeable layer, is necessary. Once a group of sites has been identified on the basis of topography, a further selection should be made on the basis of the salt content of the surface water and the average annual flow rates. Sites that have high salinities and high flow rates that could jeopardize the dam structure should be excluded from further consideration.
Topographic survey. Once a site has been selected on the basis of the topographic, salinity, and flow rate criteria, an on-site topographic survey should be performed, using 20 m x 20 m quadrants, to better determine the situation of the components. In systems that do not include a natural watercourse, this determination should include the delineation of the catchment area and location of the wall. For schemes that are being built for agricultural purposes, it is also necessary to locate the planting area to be served from the dam.
Construction of the wall. In the area chosen for the dam wall, a gutter, or cut-off trench, is dug across, or perpendicular to, the bed of the river or drainage route, down to the impermeable layer; its width depends upon the depth of the impermeable layer, the type of soil, and the material to be used in building the wall. In very dry, sandy alluvia, banks with low cohesion may constantly collapse, making excavation difficult and requiring the use of a trenching shield or other type of support to prevent slumping of the trench walls. Nevertheless, areas of sandy alluvium are desirable as dam sites because the water table is easily found there. It may be necessary to control the level of the water table by pumping so that excavation down to the impermeable layer may proceed. Some materials that can be used in constructing the wall include the following:
· Layers of clay. The clay should be deposited in the trench in uniform, 10 cm thick layers, moistened, and compressed to about half that thickness (5 cm). This is usually done by hand using wooden blocks. Multiple layers are placed and compressed, until the clay layers reach the surface of the soil.
· Packed mud. The mud, called "ambor" by farmers in the western part of Rio Grande do Norte, is a mixture of mud and water, similar to that used in rural areas to build mud huts, which is deposited evenly in the trench up to the surface of the soil.
· Masonry. A double row of bricks, joined with a cement-and-sand mortar (1:4 ratio), is used to form a vertical wall. The space between the wall and the downstream slope of the cut should be filled. The upstream side of the wall should be plastered with cement-and-sand mortar (1:3 ratio) and sealant (sica) diluted with water (1:15 ratio). The bricks in this wall must be well-baked and salt-free to minimize the risk of dam failure or seepage.
· Stone. In very rocky areas, masonry bricks can be replaced with stones joined with cement-and-sand mortar (1:4 ratio). The stones should be properly set in the mortar, leaving no crevices where seepage could occur. It is recommended that the wall be plastered with cement-and-sand mortar (1:3 ratio) and sealant diluted with water (1:15 ratio). Because the stones are less regular than bricks, use of this material normally requires more skilled manpower to ensure the integrity of the structure.
· Plastic canvas. It is also possible to use an artificial fabric core in this technology. When doing so, however, it is recommended that a mud-and-water plaster be used on the downstream side of the trench to smooth the slope cut and to prevent sharp stones, roots, etc., from puncturing the fabric. At the bottom of the trench, on the upstream side, a small gutter (20 cm x 20 cm) should be dug in the impermeable layer, and a similar gutter dug in the soil surface at the top of the trench, on the downstream side. These gutters are used to secure and seal the ends of the plastic canvas, using the same mud mortar as in the plaster. Care should be taken, when laying the canvas, to avoid stretching it; to lay it in low wind and non-extreme temperature conditions, so as to minimize expansion and contraction effects; and to protect it from sharp surfaces. If the canvas is pierced, it should be patched with a piece of plastic and an adhesive appropriate for the material.
· Management of Underground Dams
Soil and water management in underground dams has been the subject of much discussion, primarily as it relates to the potential for salination. In order to avoid salinity problems, a discharge pipe should be placed at the bottom of the dam, on top of the impermeable layer. At the upstream end, this pipe passes through the wall parallel to the trench floor and to the thalweg of the water course. At the downstream end, the pipe connects to a vertical pipe, which functions as the abstraction point as well as an overflow/outlet. Water can be pumped or drained through the vertical pipe and discharged for use or onto the soil as waste. The pipe allows an annual drawdown of the dam as a means of removing dissolved salts.
Figure 45 a: Cross-section of an Underground or Submersible Dam.
Figure 45 b: Cross-section of a Submerged Dam.
Source: J-P. dos Santos, and A. Frangipani, "Barragens Submersas - Uma Alternativa para o Nordeste Brasileira," in Congreso Brasileiro de Geología de Engenharia, vol. 2, "São Paulo," 1978, pp. 119-126 (Anais ABGE, 1).
Extent of Use
Underground dams are an option for rural areas that lack more traditional sources of water for agricultural and other uses. They are widely used in the semi-arid region of Brazil, and may be used in other semi-arid regions where similar conditions occur.
Operation and Maintenance
While an underground dam is a simple technology which does not require any particular level of training to operate or maintain, it does require some degree of care in siting and construction. Certain factors must be taken into account when building underground dams, including the average rainfall in the region, the average rates of flow of rivers/streams or drainage lines, the porosity and texture of the soil in the area, the salinity of the water, the aquifer storage capacity, and the depth of the impermeable layer.
Farmers in the western region of Brazil are generally satisfied with the operation of the underground dams. Problems that have arisen have generally done so in other areas of Brazil and primarily relate to aspects of dam construction. Some of the problems have had to do with water loss by seepage through the dam wall, which is likely to be caused by the dam wall's not extending to the impermeable layer. Other construction-related problems have to do with the drainage ditches that provide water to underground dam sites not located on natural watercourses. Where the ditches have not been adequately sized to cope with high flows, problems such as erosion and contamination of the artificial aquifer during the rainy season may result. Generally, these problems can be solved by rural extension technicians.
As with any technology, the users must be familiar with its operating principles to take full advantage of it.
Level of Involvement
Underground dams are under construction throughout the semi-arid region of Brazil, with funding from state and municipal governments and from farmers.
The costs involved in building underground dams vary depending on such factors as length of the wall, materials used, depth of the impermeable layer, and availability of manpower. An underground dam with a drainage area of 1.0 ha, built with a polyethylene plastic canvas wall, costs an average of $500.00. If 4mm PVC canvas is used for the wall instead, the dam will cost about $1 700.00.
Effectiveness of the Technology
Although simple to build, underground dams must be constructed with considerable care if they are to work effectively. For example, the dam wall should extend all the way down to the impermeable layer to prevent seepage; when plastic canvas is used for the wall, every effort should be made to prevent punctures, and, should they occur, the canvas should be patched with a piece of the same plastic and an appropriate glue. The canvas should never be left uncovered and exposed to direct sunlight, as it easily dries out and may split. A drainage ditch should also be provided as a means of managing the salinity of the impounded water.
Underground dams can be introduced throughout the semi-arid region. Given the agroecological and socioeconomic conditions that inhibit agricultural development in the area, this technology has the potential to take maximum advantage of the available water. Underground dams have been accepted throughout the semi-arid northeast region of Brazil because of their benefit to users. Their use is primarily by farmers, owing to the relatively high cost of building them.
· Underground dams are based on a simple technology, are inexpensive to build, and can make use of locally available materials and manpower.
· Once water has been stored in the alluvial soils, they have low evaporation rates compared to surface water reservoirs.
· They can be combined with other technologies, such as soil and water conservation techniques, and dug wells upstream.
· Because underground dams store water within the alluvial soil profile, their capacities are low compared with those of conventional dams.
· Given the socioeconomic circumstances of farmers in the semi-arid tropical region of Brazil, the cost of building these dams is a real obstacle to the widespread adoption of this technology.
Further Development of the Technology
In order to make the technology more acceptable for farmers and other users, certain matters must be addressed, such as the development of alternative construction materials having a lower capital cost, the provision of training programs for farmers in the proper management of soil and water resources, and the introduction of selection criteria for appropriate crops to grow with water supplied from underground dams.
Everardo Rocha Porto and Luiza Teixeira de Lima Brito, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA), Centro de Pesquisa Agropecuaria do Trópico Semi-Árido (CPATSA), BR-428 km 152, Zona Rural, Caixa Postal 23,56300-000 Petrolina, Pernambuco, Brasil. Tel. (55-81)862-1711. Fax (55-81)862-1744. E-mail: [email protected] & [email protected].
Aderaldo de Souza Silva, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA), Centro Nacional de Pesquisa de Monitoramento e Avaliação de Impacto Ambiental (CNPMA), Rodovia SP-340.km 127.5, Bairro Tanquinho Velho, Caixa Postal 69,13820-000 Jaguariuna, São Paulo, Brasil. Tel. (55-0198)67-5633. Fax (55-0198)67-5225.
Dinarde Aéda da Silva, Universidade Federal do Rio Grande do Norte (UFRN), Departamento de Agropecuária, Centro de Tecnologia, 59000-000 Natal, Rio Grande do Norte, Brasil. Tel. (55-84)231-1266, ramal 322. Fax (55-84)231-9048.
Brito, L.T. de L., et al. 1989. "Barragem Subterrânea." In Construção e Manejo. Petrolina, Brazil, EMBRAPA-CPATSA. (Boletim de Pesquisa, 36)
Monteiro, L.C. 1984. "Barragem Subterrânea: Uma Alternativa para Suprimento de Agua na Região Semi-árida." In Congreso Brasileiro de Aguas Subterrâneas, Fortaleza. vol. 3. pp. 421-430. (Anais ABAS 1)
Reboucas, A. da C., and M.E. Marinho. 1972. Hidrologia das Secas do Nordeste do Brasil. Recife, PE, Brazil, SUDENE, (Hidrologia, 40)
Santos, J.P. dos, and A. Frangipani. 1978. "Barragens Submersas - Uma Alternativa para o Nordeste Brasileiro." In Congreso Brasileiro de Geologia de Engenharia, São Paulo, vol. 2. pp. 119-126. (Anais ABGE, 1)
Silva, D.A., and J. Rego Neto. 1992. "Avaliação de Barragens Submersíveis para Fins de Exploração Agrícola no Semi-árido." In Congreso Nacional de Irrigaçao e Drenagem, Natal, vol. 9. pp. 335-361. (Anais ABID, 1)
Tigre, C.B. 1949. "Barragens Subterrâneas e Submersas como Meio Rápido e Econômico de Armazenamento d'Água," Inst. Nordeste, pp. 13-29.