The management of stormwater is an important part of modern-day site development, as more and more areas that are developed are impervious, and greatly reduce the natural method of stormwater getting absorbed into the earth, to add to the natural underground water table. Underground water retention and detention systems now are created as ponds above ground or as subsurface piping.
Ground ponds are the least expensive method of retention or detention, but it does make the use of developable land inefficient, is prone to clogging and siltation that can reduce its capacity over time, and create environments for insect breeding, growth of weeds, and also cause refuse and odor issues. Subsurface piping, on the other hand, does allow the land above to be used for gardens, parking, and other needs. Detention ponds or systems reduce the runoff of stormwater and act as a temporary store for excess water that can then be slowly discharged over time so that it does not cause floods. Retention systems or ponds can be designed in such a way that water gets absorbed into the soil gradually and add to the groundwater table.
In an underground water retention system, a pond will retain the water till it naturally seeps away. Where pipework is used for a retention system, water is still allowed to slowly discharge into the soil. This system will have large pipes or underground tanks, which retain the water until it absorbed into the installed aggregate layers and filters that are part of its structure. The main objective of a retention system for stormwater is that it helps to avoid flooding and the erosion that can be caused by excessive runoff. The filtration in the system ensures that the water that then joins the natural water sources as underground water is not contaminated and this helps in preventing the pollution of our natural streams, rivers or wetlands.
Water retention systems make for a permanent pool of water and such retention ponds are called wet ponds. Retention ponds are constructed in areas with high water tables, where the groundwater is closer to the surface being developed. The bottom of this pond or system will have to be at a level that is lower than the existing water table level. The outlet, if any, must be above the desired water level in the pond. You can decide the volume of the pond depending on the surface that will serve as a catchment for the pond and the expected maximum rainfall intensity that is expected in that geographical area.
It is not uncommon for stormwater management to opt for a combination of retention and detention, where the retention volume of water is permanently stored and the detention volume discharged slowly to absorption areas or natural watercourses. Retention ponds also act as a permanent volume of water that can be useful to remove pollutants or sediments from stormwater before it is discharged or absorbed. It can, therefore, act as a device for treatment of the water, as well as a means of flood control.