Detention and Retention Basins

Detention and retention basins are depressions or excavations, which can be used for temporarily storing flood water to reduce peak flow downstream. Detention basins are similar to retention basins except for the fact that the latter do not have controlled outlets. Detention basins hold the water temporarily and then slowly release it through a natural or man-made drainage channel, while water collected within retention basins slowly percolates into the ground or evaporates. According to the topography, the type and size of detention and retention basins can be different. They can be brought into operation at the desired stage of a flood wave, enabling reduction in flood peaks downstream. Often, natural depressions are also used for agricultural purposes.

Detention and retention basins may alter the fluvial ecosystem in the following way:

Flow regime: 

  • Reducing temporarily peak flood flows
  • Little impacts on natural flow regime, if the basin is designed only for storing flood water to reduce flood peaks downstream

Water quality:

  • If water is stored during low flow season or in permanently wet basins: increased temperature, decreased dissolved oxygen and possible eutrophication
  • Little impacts on river water quality if the basin is used only during flooding

Habitat, biodiversity and natural resources: 

  • By serving as an artificial wetland, the basin can help in creating habitats for many aquatic species
  • Little impact on river biodiversity if the basin is used only during flooding

Possible Mitigation Measures

  • Detention and retention basins can be designed as artificial wetlands or permanent ponds which help in creating new habitat for many aquatic and terrestrial species and which can be extended to recreational areas for human settlements in the vicinity
  • Detention basins should be designed, so as not to continuously affect the flow and sediment regimes in the main channel

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Figure adapted from: DeGroot (1982)