Runoff water can be successfully stored in artificial reservoirs, or intercepted and impounded by small dams. An extensive body of literature on the design of local impoundments and dams exists, since this technology has been used extensively throughout the world. In applying this technology in developing countries, both the lack of materials and skilled labor in certain regions and the cost must be taken into consideration.
Local impoundments are storage pools dug into the ground to store surface water runoff for use at a later date. Dams are designed to increase the storage capacity of rivers or streams, intercepting runoff and keeping it in storage for later use. The main difference is that dams are built where flowing water already exists, while local impoundments are essentially for harvesting and storing local rainfall runoff. These impoundments can dry out during drought periods; reservoirs behind dams usually do not.
General features of dams. Earth or rockfill dams consist of a foundation, which is either earth or rock; an embankment, resisting both the vertical and the horizontal loads; an impervious core; and a shell. The purpose of the core (membrane) is to hold back water. Depending on the structural requirements of the dam, the core can be located at the center, upstream from the center, or, in the case of certain rockfill dams, on the upstream face. When the foundation is not capable of resisting underseepage, it is necessary to extend the core down into the foundation to a depth where impervious materials are reached. Such an extension of the core is termed a cutoff.
The purpose of the shell is to provide structural support for the core and to distribute the loads over the foundation. An internal drain is an essential feature of all but the smallest dams, where the downstream shell may be so pervious that it can act as a drain. Riprap is required to cover the upstream face to prevent erosion or the washout of fine particles from the shell by wave action. Ordinarily, the riprap extends from above the maximum waterline to just below the minimum waterline. If the downstream face is subject to inundation, it also requires riprap protection.
Three types of dams are commonly used:
· Earth dams, which are constructed of compacted dirt or earth fill with flat side slopes.
· Rockfill dams, with relatively loose, open embankments of natural rock, with dimensions suitable for stability.
· Concrete arch dams, which have a concrete wall built in the form of a horizontal arch curved upstream, and anchored into the bedrock by abutments on both sides of the valley.
Impoundment description and components. Artificial impoundments are often dug below the ground surface in a soil which is naturally impervious or treated to become impervious. The shape of the impoundment may be rectangular, square, circular, or quasi-circular, depending on the desired depth and capacity of the pool. The side slopes may range from 2:3 to 1:2 (vertical:horizontal) depending on the types and angle of repose of the soils. Impoundments may be dug by hand or machine. The capacities of typical impoundments of this type range between 500 m3 and one million m3 depending on the availability of runoff and the demand for water. A filtration plant or chlorination unit may be added if the water from is to be used for domestic consumption.
Criteria for construction sites. Criteria for a good dam and impoundment site include the following:
· Topography which permits the enclosure of a large volume of stored water.
· Strong and impervious rock formations and soils which permit a sound foundation.
· No existing roads or buildings.
· Availability of construction and fill materials near or within the site.
· Short distances between the reservoir and the agricultural lands to be irrigated or other potential points of use.
Extent of Use
Dams and impoundments are extensively used in Latin American countries and in some Caribbean islands. For example, both Argentina and Aruba use such facilities to collect and store runoff. This is one of the most productive freshwater augmentation technologies. In the northeastern region of Brazil, for example, dams and local impoundments have been built for water supply and irrigation purposes, as shown in Figures 10 and 11. In Panama, this technology has been applied on a regional basis in the provinces of Hen-era, Los Santos, and Coclé. In Suriname, an artificial lake. Lake Brokopondo, was built after the construction of the Apolaka Dam on the Suriname River in 1964. The lake is used as a source of water for hydroelectric power generation. In Venezuela, this technology has been applied to augment water supplies from the Monón River. In Costa Rica, it has been used in the Chorotega region for hydroelectric power generation and for irrigation supply purposes along the Arenal River. In Argentina, impoundments have been constructed on several rivers for hydroelectric power generation and irrigation supply purposes. In Ecuador, reservoirs have been used extensively for water supply and flood control purposes. On Aruba, there are 32 possible catchment areas which are suitable for reservoirs, dams, or storage tanks. Underground barriers were built in Brazil to confine and better utilize surficial aquifers (see case study in Part C, Chapter 5).
Operation and Maintenance
The collection area should be highly impermeable to reduce infiltration losses, and the impoundment should be provided with an overflow device to avoid flooding of adjacent lands during heavy rains. A sedimentation basin at the inlet of the impoundment is also recommended.
In general, most of the construction work is done with local materials, which facilitates maintenance. Dams and reservoir facilities should be inspected at least once a year. Operation of the dam and related facilities, such as pumping stations, hydroelectric power generators, or sluice gates, should be by trained personnel.
In cases where the impounded water is used for hydroelectric production, as in Lake Brokopondo, Suriname, the reservoir level needs to be managed within a predetermined range of elevations. Excessive growths of water hyacinth or other aquatic plants may occur in some lakes and local impoundments, such as Lake Brokopondo, and in extreme cases may interfere with reservoir operations, clogging mechanical devices and increasing local evapotranspiration rates. Further, thermal stratification, which is common in warm water lakes, may lead to deoxygenation in the hypolimnion or bottom waters of the lake. Use of this water can create corrosion problems in the hydroelectric power plants.
Where the user community is not immediately adjacent to the reservoir or dam and a distribution system is required, proper operation and maintenance of the system are essential to avoid leaks, stoppages, and/or other water losses.
In Brazil, large and complex systems, like the one operated by the State of Sao Paulo (to collect and store water and then redistribute it for multiple uses), are equipped with highly sophisticated hydro-meteorological and telemetry systems to provide real-time information to operators on the status of the system, water levels, and water flows. The operation and maintenance of these systems require highly trained personnel.
Figure 10: Dam or Reservoir System used for Irrigation Purposes in Northeastern Brazil.
Source: L. de L. Brito, et. al., "Barragem Subterranea. I. Construção e Manejo," EMBRAPA-CPATSA Boletim, Pesquisa 36, 1989.
Figure 11: A Schematic Representation of an Underground Barrier in Brazil.
Source: L. de L. Brito, et. al., "Barragem Subterranea. I. Construção e Manejo," EMBRAPA-CPATSA Boletim de Pesquisa 36,1989.
Level of Involvement
Government participation is essential in the construction phase of reservoir and dam systems. In some cases, private companies involved in hydroelectric power generation and large agricultural enterprises are also capable of building these systems. Small systems can be built by local communities or individuals, usually with government assistance to ensure the integrity of the dam structure and management of the water resource. Operation and maintenance can be performed at the community level. The university community in some countries, such as Ecuador, has also provided technical guidelines in the design and construction of local impoundments.
The construction cost per cubic meter of water varies considerably depending on the region and the size and type of project. In Ecuador, the average cost was estimated at $0.93/m3 of water, but the range was from $0.10 to $2.00/m3.
A reservoir and dam system in northeastern Brazil, with a storage capacity of 3 000 m3, in a drainage area of 3.8 ha, was built at a cost of $2 500, including soil preparation for cultivation of 1.5 ha of corn. The construction cost of an underground barrier to facilitate utilization of 1.0 ha of surficial aquifer in Brazil was estimated at $500.
In Costa Rica, water in excess of base flows from the Arenal River is stored in a reservoir and then used for hydroelectric power generation and in an irrigation system in the Tempisque River basin, where precipitation is considerably less than in the Arenal basin. This 6 000 ha reservoir and irrigation project cost $19.8 million to develop. A second phase of the irrigation project, providing water to 11 600 ha, is estimated to cost $45.4 million. The annual operation and maintenance cost is estimated at $55/ha.
A small-scale, 1 600 m3 impoundment in Costa Rica cost $1 800.
The cost of reservoirs built in the western region of Argentina ranges between $0.60 and $1.20/m3 of storage capacity. The operation and maintenance costs range between $0.01 and $0.03/m3 of storage capacity.
Effectiveness of the Technology
The effectiveness of this technology can be measured by the amount of water that can be stored in the reservoirs or dams, but it is usually measured as a function of the benefits obtained by the utilization of the additional water. For example, in Mendoza, Argentina, irrigation efficiency increased between 8% and 15% following the construction of a reservoir. In Brazil, water stored in the Sao Paulo area and transferred to the Santista basin supplies 100% of the water demand. Previously, the natural water in the Santista basin was able to supply only 10% of the industrial demand. In the region of Llazhatar, Ecuador, the availability of water for domestic and agricultural use has increased four times, from 6 l/s to 25 l/s. In Suriname, because of the construction of the Afobaka Dam, the minimum discharge to Lake Brokopondo increased ten times, from 20 n^/sec to 224 in/sec. Also, the salinity intrusion in the Suriname River moved 30 km downstream after construction of the Dam. Increased irrigation efficiencies of up to 55% were reported in Costa Rica after the construction of the reservoir in the Arenal River. Judging by these experiences, the use of dams and impoundments is a highly effective technology.
These methods are applicable in regions where the time and spatial distribution of rainfall are highly variable and storage is required to meet specific demands, such as water supply for irrigation and hydroelectric power generation. Their suitability depends on favorable topography, geology, and economic conditions.
· Impoundments provide water for agricultural production and domestic water use in arid and semi-arid regions.
· Impoundments provide water for hydroelectric power generation and other, non-consumptive uses.
· The flora and fauna of a region, and particularly the fisheries, may be enhanced, although large dams develop a lacustrine fauna over time that gradually replaces the pre-existing riverine fishes.
· The degree of water pollution may be decreased by dilution of contaminants.
· The perennial flows from impoundments could reduce saltwater intrusion in certain rivers by increasing minimum flows and levels.
· Impoundments are ideal for multiple-use water projects.
· Reservoirs can be used as recreational areas.
· Impoundments require the availability of land with the proper topography, and generally consume valuable agricultural land when the lake basins are filled.
· To minimize seepage losses, impoundments need impermeable soils (soil with less than 15% content of clay).
· Impoundments can lose an average of 50% of the total volume of water stored in the reservoir to evaporation and infiltration in arid and semi-arid areas.
· Construction costs are relatively high.
· There is a risk of possible failure.
· Impoundments can flood adjacent lands during wet periods.
· Impoundments can produce environmental impacts and exacerbate public health and other problems as people and animals are attracted to the lake shores.
Dams, reservoirs, and impoundments are widely accepted as a water supply augmentation method for developed and developing countries. Both the engineering and the local communities have used this technology in small-scale (e.g., farm dams) and large-scale projects.
Further Development of the Technology
Research has improved the design of local impoundments and small-scale dams, making them more efficient in retaining water, preventing failures, and reducing evaporative losses. Improvements in operation can be very beneficial. Methods to further reduce evaporation should be developed. Impermeable, low cost materials to line the local impoundments and reduce infiltration should also be developed.
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