A retarding basin is a constructed empty lake, used to absorb and contain flooding in periods of high rain, the idea being that the flow of water into the basin will lessen the rate of flow further down nearby rivers or estuaries in flood or storm to a level that can be safely carried by the stormwater system. Whilst flood prevention is the primary reason for the construction of retarding basins, they can also be of great value to the community, providing both recreation opportunities and also as a natural resource or wetlands.
Retarding basins are a lot more common in urban than rural areas for a few different reasons - rural land is reasonably pervious to rainfall and the uneven surface provides natural retardation for surface flow of water. Urban land, on the other hand, contains many surfaces - road, roofs, concreting and built up areas which provide much less opportunity for the permiation of rainfall. Drainage pipelines also, ironically, contribute to the issue, by providing a quick and efficient path for stormwater through the catchment. As the flow from a storm or flood event increases, the downstream problems also increase, as the drainage system is merely a transport system, not a retarding system.
Therefore, retarding basins are constructed as an integral part of the drainage system to store a percentage of flood flows in an area for a temporary period in order to manage and maintain flow levels, and reduce flood risk/impact in downstream catchment areas.
Simple retarding basins are formed by the construction of retaining walls and earthen embankments across a watercourse. Outlet flows through the embankment can be controlled by as simple a design decision as the diameter and number of pipes placed, or complex flow control systems of gates and weirs.