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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Latin America and The Caribbean> PART B. TECHNOLOGY PROFILES 1.6 Flow Diversion Structures Technical Description Diversion structures route runoff in excess of base flow to storage facilities during wet periods, for later use during dry periods. Flood diversion structures, such as dikes, are also useful methods for mitigating the adverse effect of torrential rains and at the same time capturing the excess water for later use. The following types of structures have been used to divert flood water for water supply augmentation purposes.
Transverse dikes are built in sections along a river to store excessive runoff. These dikes can be built using material dredged from the river or transported from adjacent lands. The dike material, usually clay or silt, must be highly compacted and in many cases it is advisable to place riprap on the dike to increase its strength and protect it from erosion.
Toroba are homemade diversion structures built of wooden poles, taken from trees such as the curari and cuji in Venezuela, vegetation residues, and logs. The wooden poles are 50 and 130 cm in length and are placed at intervals of 50 cm to 70 cm to define a wall of debris that will divert the runoff. This technique may also increase infiltration to the groundwater.
Water traps are used to control the deleterious effects of runoff in a river basin and to facilitate water storage and the re-charge of aquifers. They are built like an earth dam, usually 1 m to 3 m high, using local materials. The walls are compacted in 20 cm layers using the same equipment as is used to build a dam. The edges are trapezoidal with an embankment slope of 2.5:1 at high water and 2:1 at low water. The bottom width of the water trap is 2.5 m. They are normally located across a river bed, segmenting the channel into compartments. Water traps are usually designed to handle runoff produced during a 1-in-50-year rainfall. The volume of runoff captured depends upon the catchment area and the intensity of the rainfall. Extent of Use Transverse dikes have been used on rivers in the State of São Paulo and in the Serra do Mar region, Brazil. Water traps have been used in arid and semi-arid regions, particularly in the Province of Mendoza, Argentina. They have been very useful in reducing sedimentation and limiting the risk of flooding. Toroba are used in the State of Falcón, Venezuela. This technique has limited utility, but can be helpful in rural areas that lack technical resources. Operation and Maintenance The operation of these types of diversion structures is very simple. They require continual maintenance to repair damage caused by large storms and to control erosion, especially around the abutments, which can breach the dikes and water traps and significantly damage the homemade structures. Water traps require maintenance during the first few years of operation, until natural vegetation grows again in the area. When rains heavier than the design flow conditions occur, it is possible that the traps will be breached and will need to be rebuilt. All-terrain recreational vehicles used in areas at or near the water traps can cause damage that may need additional maintenance or repair.
Figure 12: A Schematic Representation of a Homemade
Structure (Toroba) in Venezuela. Level of Involvement Homemade structures can be built, operated, and maintained by local communities but may require technical assistance from government agencies and/or nongovernmental institutions and the private sector. Dikes and water traps require the participation of the government and private sector, primarily in management of the volume of water retained behind these structures and in ensuring their safe and sound construction. Costs The construction costs of dikes can range from $10 000 to millions of dollars, depending on the size of the river, the length and width of the dikes, and the scale of the project. The cost of homemade structures is minimal, since all of the materials are locally available. The cost of a small water trap in Argentina has been estimated at between $130 and $170. Effectiveness of the Technology Diversion structures are very effective in reducing sediment erosion, retaining runoff, and encouraging groundwater infiltration. Water traps have been successfully used for more than 25 years in Argentina. They have been very useful in controlling sedimentation, and reducing the risk of flooding, within river basins. Suitability Diversion structures are suitable for use in river basins where sufficient volumes of water can be diverted and stored for later use. Areas like Serra do Mar in southeastern Brazil, Falcón State in Venezuela, or the San Juan River basin in Argentina are typical areas well suited for the application of this technology. Advantages
Disadvantages
Cultural Acceptability Flow diversion structures are widely accepted among the engineering community as a method to control erosion and sedimentation, and augment water supply. Greater acceptance by local communities could yield substantial local benefits. Further Development of the Technology It is important that more data on the performance and problems of diversion structures be acquired in order to assess and suggest possible improvements. Greater educational programming on the use of this technology as a tool for river basin management should be planned and carried out. Information Sources Contacts Alberto I. J. Vich, Armando R. Pedrani, and Adriana Mariani, Programa de Investigación y Desarrollo Manejo Ecológico del Piedemonte, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales, Unidad Ecología y Manejo de Cuencas Hídricas, Casilla de Correo N° 330, 5500 Mendoza, Argentina. Tel. (64-61) 28-7029. Fax (64-61) 28-7029 / 28-7370. E-mail: ntcricyt@criba.edu.a-r. Marco Antonio Palermo, Departamento de Aguas y Energia Elétrica do Estado de São Paulo (DAEE), Rua do Riachuelo 115, sala 415, 01007-000 São Paulo, Brasil. Tel. (55-11)974-0350 / 258-4595. Fax (55-11)258-4595. Carmen Fermín, Dirección de Hidrología y Meteorología, Ministerio del Ambiente y de los Recursos Naturales Renovables (MARNR), Esquina Camejo, Edificio Camejo, 5° piso, Caracas, Venezuela. Tel. (58-02)408-1952. Fax (58-2)545-0607. E-mail: dhm@dino.conicit.ve. Bibliography Cuomo, A.R., and M.A. Palermo. 1987. Introducão as Técnicas de Correção de Cursos d'Agua Torrenciais. São Paulo, Universidade de São Paulo, Centro Tecnológico de Hidráulica da Escola Politécnica. (Boletim 6) Departamento de Águas y Energia Elétrica (DAEE). 1988. Relatório sobre a Atuação da Comissão Especial para a Restauração da Serra do Mar em Cubatao. São Paulo. Lloret, C.L., and M.A. Palermo. 1989. "Critérios para a Avaliação de Impactos Ambientais em Obras de Correção de Cursos d'Água." In Anais do VIII Simpósio Brasileiro de Hidrologia e Recursos Hídricos, Foz do Iguaçu, Brasil. São Paulo, Associação Brasileira de Recursos Hídricos. Martínez, Douglas. 1987. Manejo y Aprovechamiento del Agua a Través del Uso de un Dispositivo Antierosivo, La Toroba, Dentro de un Sistema Semiárido Zona Piloto de Mide. Barquisimeto, Venezuela, FONAIAP-FUDECO-PIDZAR.
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