Nor aqilah bt othman 55102215008

RETENTION BASIN

Retention basin•manage storm water runoff to prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay.• act as a replacement for the natural absorption of a forest or other natural process that was lost when an area is developed. • to contain surge and release it slowly. This slow release mitigates the size and intensity of storm-induced flooding on downstream receiving waters.

In Out

Precipitation, Freshwater, Freshwater, Treated Water

In this illustration of a typical storm water system, as the water rises, sediment and pollutants — such as bacteria and heavy metals — picked up on the way to the pond sink to the bottom. The outflow structure usually has a small pipe or opening that sits just above the normal water level. When the water level reaches the outflow structure, the treated water will begin flowing out of the pipe. Not all systems work exactly in this way, but this is a standard design for wet detention ponds.

Working PrincipleSurface run-off is stored permanently in ponds, allowing increased storm water management and quality enhancement through natural processes.

Capacity/Adequacy Retention basins require a large area of land and are only applicable to areas with high rainfall and enough free space.

PerformanceRetention basins filter pollutants such as metals, nutrients, sediments, or organics by sedimentation. Further removal of pollutants is achieved through algal and wetland plant uptake as well as bacterial decomposition(EPA 1999)

Costs High costs arise for the construction of retention basins and the value of the land. Once in operation, only minimal maintenance costs occur.

Self-help Compatibility Construction requires expert design; operation, monitoring and maintenance require skilled staff.

O&M Basins have to be cleared from excess sediment and trash. Also, quarterly inspections of water in- and outlets are needed.

Reliability Reliability persistence of retention basins is very high if well maintained, with a life span of usually more than 20 years.Main strength Floodwater can be controlled and water quality is enhanced.

Main weakness Large land areas need to be available to build ponds; risk of uncontrolled reuse and mosquito breading.