Retention Basin

Today, retention basins are one of the most commonly used stormwater treatment systems (see also stormwater management). Basically, they are ponds to store rainwater and further filter it from sediment by mimicking natural undeveloped watershed conditions (MEM 2002).

By storing water in the basin, retention ponds help to impede the negative effects of excess storm water, preventing successional flooding, and can further be used to recharge groundwater (see also surface groundwater recharge or subsurface groundwater recharge). Also, water quality increases immensely, because the standing water is filtered from solid and soluble pollutants as well as excess nutrients (see also pathogens and contaminants and sedimentation) (GDSDS 2005; MEM 2002). If not reused for groundwater recharge, it may be reused in agriculture (e.g. irrigation, fertigation or aquaculture), industry or at household level after an appropriate secondary treatment if required (e.g. free surface, vertical flow constructed wetlands or non planted filter).

Adapted from GDSDS (2005) & EPA (1999)

The design of a retention basin needs to be well fitted to its surroundings. When choosing a suitable site, the main factors to consider are the cost effectiveness of the area as well as its ability to support the pond environment.

Also, retention basins are best built where stormwater naturally flows and collects (MEM 2002). Enough space should be available, as the ponds require a minimum contributing area of five hectares. A small pool (called sediment forebay) of about 10 percent of the main pond’s size can further help to pre-treat incoming water by keeping back rough particles. The size of a retention basin is dependent on several factors such as topography, the effective contributing area, and the relationship between the amounts of incoming and discharged water.

Planting native aquatic vegetation further helps to improve the function of retention basins. Nutrients are removed more effectively because of the photosynthetic activities and bacteria attached to the plants (see also the nutrient cycle).

As retention ponds need to hold back a constant level of water, highly permeable soils can be a drawback. If this is the case, clay blankets may be used to overlay or compact the existing soil and therefore make it less absorptive.

Retention vs. Detention Basins

Retention basins are not to be confused with detention basins. Whereas retention basins hold back a constant amount of water (and are therefore also called 'wet detention ponds'), detention basins are only used for temporary storage of rainwater. To release water from the basin, an additional outlet needs to be constructed to the ground of detention ponds (TUHH n.y.).

Even though high investments are needed for the construction of retention basins, the overall marginal costs are low, because retention basins are long-lived, usually lasting for over 20 years. Once in operation, only minimal maintenance costs arise.

The United States Environmental Protection Agency (EPA) states that between $17.50 and $35 USD need to be invested per cubic metre. Further spending includes permits, design and construction, as well as maintenance (EPA 1999).

Due to higher costs of land, it is usually more expensive to retrofit retention basins to already developed areas compared to constructing one in an undeveloped region.

Adapted from UNHSC (2010)

Different concerns arise related to human and ecosystem health: For one part, standing water may be a drowning hazard. Further, ponds are a beneficial ecosystem for mosquitoes, which can also be related to increased transmittance of diseases.

Moreover, in water scarce areas or where agriculture is practised, inhabitants may reuse the water from the ponds directly without monitoring the water quality.

Furthermore, eutrophication can occur if excess nutrients are held back in the basin. Also, if the weather is exceptionally warm, the water can heat up exceedingly, impacting downstream aquatic habitats. If this is the case, gravel subdrains can help to cool down the water outlet.

Adapted from UNHSC (2010) & GDSDS (2005)

Even though only minimal maintenance is needed, it is unavoidable in order to prevent erosion and suspension of sediment, which could lead to unwanted sediment export. Required routine work includes quarterly inspections of inlets and outlets as well as sediment and trash dredging. Mowing around the margins is possible, but increases costs and further reduces possible habitat.