A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of time.
These basins are also called “dry ponds”, “holding ponds” or “dry detention basins” if no permanent pool of water exists. 
Detention ponds that are designed to permanently retain some volume of water at all times are called retention basins.
In its basic form, a detention basin is used to manage water quantity while having a limited effectiveness in protecting water quality, unless it includes a permanent pool feature.
Hazard Mitigation :
Flood water detention and retention basins are designed to capture and slow stormwater runoff to prevent downstream flooding.  
They may also be designed to accept and store floodwaters during highwater events, increasing the overall flood storage of a system and reducing flood damages as waters can be held long enough to reduce the peak flow downstream.
Siting Considerations :
Depending on the specific type of flooding being addressed, the primary consideration when siting a flood detention basin is identifying where current and potential future flood issues exist – whether stormwater or riverine flooding –  and securing the necessary land in that area to allow for a meaningful intervention in the flood impacts.
For floodwater detention basins intended to address riverine flooding, the basin should be sited or located near the middle of the watershed to provide the most efficient means of peak flow and volume reductions and control.  
Existing topography should be used as an advantage.  Taking advantage of existing low lying areas either in proximity to the main watercourse or in relative proximity to regularly occurring nuisance flooding areas can provide a logical location to site a basin.
Importantly, avoid areas where seasonal groundwater levels are at or near the bottom of the basin, as this will reduce the effectiveness of the basin itself and may also impact construction.
Costs :
Land often must be purchased and some earthworks is often needed. 
Therefore, constructing detention areas in areas already developed can be challenging. However, if the detention areas are developed as part of a new development project, land costs may not be significant and the earthworks can be undertaken in conjunction with the road works and other earthworks required to develop the property.
Co-Benefits of the Strategy :
Floodwater detention areas can be used for recreation, an amenity or as wildlife habitat. As dry detention basins are intended to be dry between rain events, they can be used as sports fields, or other types of open space. 
Wet detention basins function similar to a pond and hence can provide an aesthetic and social benefit to the community while increasing flood protection and improving water quality.  
Maintenance Considerations :
Maintenance measures will vary depending on the type of basin used.  
Retention basins may require periodic removal of sediment and particles which have settled at the bottom of the basin. 
The detention area may need basic vegetation management and cleanup maintenance.  
In addition, erosion repair may be needed after a flood event.  
Regular inspection of the inlet and outlet pipes in wet and dry basins may also be required to ensure they are not clogged and are functioning to full capacity.

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