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Stormwater 101: Detention & Retention


You may be wondering what the difference is between regular water, and stormwater. The technical difference lies in the fact that stormwater originates from a precipitation event like a thunderstorm or a hurricane. Some of this water is soaked into the ground naturally, while the rest becomes surface runoff. This surface runoff must be channeled into storm sewers, or surface waterways in order to prevent flooding.

Beyond the typical methods of redirecting water into underground sewers, there also a number of stormwater detention and retention solutions that can be used to repurpose or store gathered stormwater. These two methods provide different approaches to regulating the stormwater, but both present an equally viable option.


Detention & Retention

These two categories are part of the stormwater best management practices or BMPs. This term refers to a set of concepts used by the United States and Canada to control water pollution. In regards to stormwater, there is a set of practices meant to regulate the quantity and quality of urban runoff as a result of impervious surfaces and land development.

These practices are divided into structural and non-structural options. The former refers to devices or constructed methods while the latter refers to landscaping practices.

Below we’ve outlined the differences between detention and retention which will help better illustrate their individual purposes.

1. Detention

Also known as a retention basin, this method of water management is placed on or adjacent to the area being drained. These basins are used to prevent flooding and downstream erosion by storing the water for a limited amount of time.

In most cases, they are simply meant to capture and slowly release excess stormwater back into the main source nearby. The way these functions is by landscaping the basin in such a way that it allows massive amounts of water to flow into it, but has a regulated method of controlling the outflow of water. This is typically found in the form of a small opening or pipe at the lowest point of the basin, where the water drains through at a slow, regulated rate. There are also extended detention dry basins which are known to facilitate better control of pollutants and suspended solids in the water.

Basic detention ponds are designed to empty themselves within six to twelve hours. The extended detention dry basin improves this by extending the storage time to an interval between twenty-four and forty-eight hours. This long storage time allows suspended solids to be removed, increasing the water quality.

2. Retention

Also known as a retention basin, this method of water management is used to manage stormwater runoff. It also prevents flooding, and downstream erosion. The presence of a retention pond can also improve the water quality in nearby rivers, streams, lakes, or bays. This method creates an artificial lake with vegetation around the circumference of the pond. Unlike a detention basin, these are designed to permanently hold water.

Retention ponds are used for water quality improvement, groundwater recharge, flood protection, and aesthetic improvement. They can act as a natural replacement for forests and trees, which typically soak up excess stormwater. These retention ponds are also viewed as an amenity for homeowners, adding a waterfront view to the properties nearby. The stormwater is channeled into these ponds through a series of storm drains and channels that direct the water into it.

The plant life and landscaping around these ponds includes grasses, shrubs, and wetland plants which provide stability to the banks of the pond and also add to the aesthetic value. The presence of these plants also provides benefits to the water quality because they are able to absorb soluble minerals in the water.


Stormwater Harvesting

While redirecting and managing stormwater is crucial to prevent flooding and water contamination, it is also important to make the water work for you. In this regard, stormwater harvesting is an ideal solution where other water sources are not present. The concept refers to collecting or accumulating stormwater for eventual reuse. The only real challenge to this, is removing the pollutants that accumulate in the water as it travels across impervious surfaces.

For the rural homeowner, or simply one who wishes to save money on their water costs, a ground catchment system is a great solution. The patented OcTank and cistern solutions from FreeWater Systems can be utilized to collect, filter, and reuse stormwater. Having this additional supply of water ensures that everyday needs like laundry, washing dishes, and flushing toilets won’t be hindered by a unexpected severance of supply from the city water.

Our rainwater capturing systems are sent preassembled and ready for turn-key installation making them simple and effective. Storms can be troublesome, but they can also be used to your benefit with a safe and reliable source of water at your fingertips. Visit the FreeWater Systems website today and see how you can take advantage of these systems for your home, your neighborhood, or even your city.



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