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PART C - CASE STUDIES
5.3 Underground Dams in Brasil
Climatic instability in northeast Brasil 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 Brasil has shallow, rocky soils with low water retention
capacity, a low organic material content, and a high susceptibility to
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
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). 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 Brasil 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.
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
- 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.
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
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
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
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 4 mm PVC canvas is used for the wall instead, the
dam will cost about $1 700.
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 Brasil, 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 Agropecuária (EMBRAPA), Centro de
Pesquisa Agropecuária 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:
firstname.lastname@example.org & email@example.com.
Aderaldo de Souza Silva, Empresa Brasileira de Pesquisa
Agropecuária (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, Sao Paulo, Brasil. Tel. (55-0198)67-5633. Fax
Dinarde Aeda da Silva, Universidade Federal do Rio
Grande do Norte (UFRN), Departamento de Agropecuaria, 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., EMBRAPA-CPATSA Boletim de Pesquisa, 36, Petrolina, Brazil.
Monteiro, L.C. 1984. Barragem Subterrânea: Uma Alternativa para
Suprimento de Agua na Região Semi-árida. In:Congreso
Brasileiro de Aguas Subterraneas, Fortaleza. Anais ABAS 1 vol. 3. pp.
Reboucas, A. da C., and M.E. Marinho 1972. Hidrologia das Secas do
Nordeste do Brasil. , SUDENE (Hidrologia, 40) Recife, PE, Brazil.
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. Anais ABGE 1 vol. 2. pp.
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. Anais ABID 1 vol. 9. pp. 335-361.
Tigre, C.B. 1949. Barragens Subterrâneas e Submersas como Meio Rápido
e Econômico de Armazenamento d'Água, Inst. Nordeste,