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
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.
· Homemade Diversion Structures (toroba)
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
Water traps are used to control the deleterious effects of runoff in a river basin and to facilitate water storage and the recharge 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 l-in-5 0-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 Sao 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 Falcon, 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.
Source: Douglas Martinez, FUDECO, Barquisimeto, 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.
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.
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, Falcon State in Venezuela, or the San Juan River basin in Argentina are typical areas well suited for the application of this technology.
· Diversion structures enable the use of water that normally would run off.
· Diversion structures provide some in-stream control of erosion and sedimentation.
· Diverted water may serve as a source for groundwater recharge.
· Water velocities in river channels are reduced.
· Soil fertility is improved by retaining water on the land surface and reducing soil loss.
· Retention of runoff may contribute to biodiversity and ecosystem restoration by reducing erosion and retaining water on the land surface.
· Construction of diversion structures may disrupt vegetation.
· Structures may be breached by storms that exceed the design flows/capacities.
· Structures may adversely affect aquatic flora and fauna by altering flow patterns and flooding regimes.
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.
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: email@example.com.
Marco Antonio Palermo, Departamento de Aguas y Energia Elétrica do Estado de São Paulo (DAEE), Rua do Riachuelo 115, sala 415,01007-000 Sao Paulo, Brasil. Tel. (55-11)974-0350/258-4595. Fax (55-11)258-4595.
Carmen Fermin, 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: firstname.lastname@example.org.
Cuomo, A.R, and M.A. Palermo. 1987. Introdução as Técnicas de Correção de Cursos d'Água Torrenciais. Sao 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. "Criterios para a Avaliacao 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. Sao Paulo, Associação Brasileira de Recursos Hídricos.
Martinez, 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.