Stormwater harvesting works differently on a large scale. While rain barrels are perfectly appropriate for single-family homes or the back patio of an apartment, they’re merely a drop in the bucket – if you’ll pardon the pun – when it comes to a large housing development, sprawling country property, university campus, or farm.
In this case, larger-scale solutions are needed, which is where detention and retention ponds come in. And one of the most important concepts when it comes to largescale stormwater harvesting is the difference between “retention” and “detention.” Let’s take a look.
Detention Vs Retention
The difference is right there in the first letter, but it’s not necessarily obvious. The root of detention is detain, which means to hold something back for a period of time. The root of retention is retain, however, which means to hang onto something indefinitely. (Think about it: you detain someone at the border, while you retain the right to reenter your home country in most cases. One is temporary, the other is permanent.)
Thus, a detention pond is meant to hang on to water for a period of time, then move it to a more appropriate location if needed. On the other hand, a retention pond will happily keep stormwater runoff in place forever, unless you intentionally relocate it.
Typically, says Eastcoast Site Work (ESW), “Retention ponds are water-filled pools that change in reaction to rainfall and runoff. They gather water and release it slowly and steadily, preventing flooding and erosion.” However, some people do not release the water in their retention pond at all, rather allowing it to fill with every precipitation event and keeping it there. This is a popular choice for those who want to encourage wildlife to come to the area.
But “Unless there is a lot of rain, detention ponds are normally dry,” says ESW. “They’re made to temporarily store water before slowly draining it to another site.” That site is usually a streambed, river, or wetland. Often people use these to collect water which will then evaporate, popular for drier areas. Or they may simply detain water for a period of time, then allow it back into the water supply after peak runoff times.
However, you can collect stormwater from a detention or retention pond for use inside a home as well. You need a pump or gravity system to move the water from the pond to your home. You will also need a high-quality filtration device to ensure water is potable if you intend to use it as drinking water. Because stormwater contains pollutants and diseases, make sure to have an expert install this system.
Which type of pond you choose will depend on your goals for your property. If you want a pond you can turn into a wildlife haven or swim in, then a retention pond is the answer. If you’re looking for an area in which you can keep temporary stormwater runoff, that’s a detention pond.
Other Solutions
Other large-scale solutions include bioswales, which use greenery and rocks to funnel water down into soil or toward collection areas. These are an excellent contrast to the giant dykes that city planners implemented for decades in places such as Los Angeles.
It’s worth noting that while a rain barrel or two can’t do much about stormwater, a full harvesting system can do a lot more. Commercial and industrial rainwater harvesting systems have started to pop up around the world in response to the greater number and intensity of storms, helping manage stormwater as well as acquire that precious liquid gold known as water.
That said, the most common stormwater harvesting strategy on a large scale is a pond or basin, so let’s turn our attention to how that works next.
How to Build Stormwater Ponds and Basins
Building a stormwater pond requires considerable know-how. If you don’t have the expertise needed to build one on your own, the best bet is to hire someone who does.
That said, building a pond – whether for a natural habitat, a fish pond, or a stormwater basin – is a relatively simple process overall. Here are the steps you should take.
Choose a Site
First, where will you build it? If you’re constructing a stormwater basin in the middle of an apartment complex or behind an office building, this question is a little easier to answer. However, if you’re constructing your pond on a rural property, it might prove trickier. You will need to consider factors such as:
- Regulations at the local, regional, state, and federal level that specify where you can build ponds and whether permits are needed
- Where the pond will overflow if and when it releases runoff back into the environment
- How neighbors will feel about a pond that might flow onto their property
- Whether it needs to be close to your house (if you’re planning to use it as water indoors or in a garden)
Look for sites that already contain natural slopes to minimize the amount of work you’ll have to do excavating and building up the edges to hold water in. Then you will need to:
- Excavate if necessary to create the space needed
- Line your pond either with a natural substance such as clay, or with a fabricated liner that will keep your pond sound for years to come (see below for materials)
- Fill it, if needed, or leave it empty to wait for rain and runoff
- Install filters and pumps if desired
Consider Floating Islands
Floating islands are another amazing way to up your stormwater management game, especially if you have access to a large area such as a retention or detention basin.
As we have discussed on our site before, these “islands begin as a matrix of buoyant recycled material (such as shredded plastic drinking bottles) that resemble a loofah. This recycled material is then covered with a geotextile layer, soil and other materials to complete the physical structure. Plants appropriate to the growing conditions are added, and the island is either set free to drift or anchored in a convenient location.”
The benefit? These floating islands keep water way cleaner than it would otherwise be. Floating islands, because they are covered in plants, absorb nutrients in the water that would otherwise go to feeding algae. If you’ve ever seen a pond that was green and smelly with scum, then you know just how undesirable algae can be, so that’s a win for floating islands.
Plus, because they are not anchored to the bottom of the pond, their roots can grow extensively downward. This gives plants plenty of room to catch nutrients and develop biofilm, which helps them process potentially dangerous molecules. Thus, they help pull harmful nutrients out of water that might go onto crops, in public waterways where swimming occurs, or drinking water. Floating islands are potential additions to treatment plants, as well.
The International Institute for Sustainable Development terms these “floating treatment wetlands,” adding that “The unique ecosystem that develops creates the potential to capture nutrients and transform common pollutants that would otherwise plague and harm our lakes into harmless byproducts.”
If you’re curious about how to make stormwater management most ecologically beneficial on your property, floating islands are well worth your consideration.
