Retention ponds are becoming increasingly more popular on both residential and commercial sites, and many states and municipalities now require them in new developments. Although they have a variety of functions, including flood control, their main purpose is a distinctly eco-green one, and that is to manage stormwater runoff.
Landscape contractors are sometimes called on to build retention ponds. So if you’re new to this type of construction, there are some things you need to know. One of those things is the difference between ‘retention’ and ‘detention’ ponds. These terms are often used interchangeably, but they don’t mean the same thing.
“A detention pond means you’re detaining water, slowing it down,” said Annette DeMaria, P.E., senior engineer at Gainesville, Florida-based Environmental Consulting & Technology, Inc. “A detention pond will have an outfall, usually a pipe at a certain level. Once the water reaches that level, it overflows into this pipe. A detention pond always has an outlet.”
“A retention pond is used when you want to retain water, keep it. The only time water leaves a retention pond is through infiltration or evaporation. There’s no outlet.” There are also temporary ponds built to control runoff during construction phases. However, in this article we’re going to concentrate on permanent installations.
What do retention ponds do? Consider a typical residential subdivision or large commercial development. Once finished, it will have many impermeable surfaces—streets, driveways, parking lots, sidewalks, patios and roofs (unless greener alternatives, such as permeable paving and green roofs, have been used).
Vehicles will be driving around and parking on those surfaces, dripping gas and oil. Landscapes will be planted and maintained, producing runoff containing fertilizers, pesticides and herbicides. There will also be heavy metals, litter, dog waste, leaves and grass clippings.
Dirt itself is a pollutant—silt and sediment clog fish gills, among other things. It doesn’t have to rain for dirt to get moving, either. Wind can do it, too.
Without something to block or filter all this gunk, when a rain event happens, it washes right into a storm sewer. From there, all of these pollutants find their way to our streams, rivers, lakes and oceans.
The retention pond will capture the runoff coming from all of the impervious surfaces. It will do two things:
one, retain the stormwater before releasing it to the storm sewer; and two, settle out the pollutants and sediment in the stormwater. It will then infiltrate the particulates into the soil at its bottom slowly.
No two ponds are alike. Where a pond goes will be determined by a site’s topography. Look for where you can maximize storage and minimize cost. Are there wetlands nearby? Utility pipelines or cables? An underlying layer of bedrock that would require an expensive blasting operation? The site must have adequate base-flow from the groundwater or from the drainage area to maintain the permanent pool.
Before you begin excavating, make sure you have all the permits you’ll need—local, state and federal. You might need a wetlands permit, water quality certifications, dam safety permits, sediment and erosion control plans, waterway permits, local grading permits, land-use approvals, and even more. Get your paper ducks in a row first.
You’ll need to determine the quality of your soil, because that is your pollution filtration medium. But as you know, different soils have different rates of absorbency. This also has to be determined before excavation starts.
“You don’t want to just dig a hole and have it not hold water,” said Shawn Howard, owner of Clearly Aquatics, a retention-pond company in Bloomfield, New York. The first thing he does is a ‘perc,’ or percolation, test to see how fast water can seep into the soil. Where clay soil is an impediment to landscape work, here it’s a good thing. “You can dig that pond, and it’ll hold the water fine,” Howard says.
If the soil is gravelly or sandy, a poly pond liner may be needed. This can get expensive, depending on how big the pond is. Liners come in 60,000- square-foot rolls. If you need a bigger one, you can heat-seal sheets together right at the site with a special machine.
Ponds can become wildlife areas, supporting geese, ducks and fish. Howard says that about 30 percent of retention ponds are also there for decorative reasons. “People want to see the birds, the fish, and all the wildlife that comes along with having a big pond.”
How deep should the pond be? Depth affects many of the pond’s removal processes. It needs to be deep enough to prevent the resuspension of trapped sediments, but not so deep that you get thermal stratification and low-oxygen conditions near the bottom. That will decrease biological activity, and kill any fish that may be in there.
For these reasons, the U.S. Environmental Protection Agency says that “permanent pool depth should not exceed 20 feet. The optimal depth ranges between three and nine feet for most regions.”
Howard likes to make his at least six to eight feet deep. However, he says that a lot of times, they end up being no more than four feet in depth. This is usually due to costs. “It costs more to make ponds deeper, so often the developers will only excavate as deep as the minimum volume of water required by the total area of the development, and no more.”
The shallower the pond, the more sunlight penetrates to the bottom. If the bottom sediment is full of phosphorus and nitrogen from fertilizer runoff, when the sun hits it, you can get an algae bloom. Algal blooms take up all the oxygen in the pond, which kills fish, too.
Part of the purpose of a retention pond is to remove sediment. It’s the sedimentation process that removes particulates, organic matter and heavy metals. Dissolved metals and nutrients are removed through biological uptake.
However, success can ultimately breed failure. Over time, sediment can build up, and after a while the pond doesn’t hold the amount of water it originally did. What then?
“You have to dredge the bottom,” said Lauren Hoffman, CFM (certified floodplain manager), a landscape ecologist who also works at Environmental Consulting & Technology, Inc.
That, too, can be expensive. To avoid having to do that, “There are certain things you can do designwise to prevent too much sediment from getting into the pond in the first place, or at least slow down the rate,” says DeMaria. What’s she’s talking about is creating a buffer zone around the pond. This buffer area also allows the heavier sediments to drop into it, letting only the lighter sediments into the pond.
So, instead of having turf going all the way to a pond’s edge, “what we prefer to see is a nice, 10- to 20-footwide strip of native aquatic vegetation that reptiles can live in,” says Hoffman. “It’ll filter the stormwater as it flows into the pond.”
The plants in the buffer zone also uptake the excess nitrogen and phosphorus that leads to the oxygen-depleting algae blooms. “This helps emulate what happens in a natural pond,” said Hoffman.
Part of this buffer zone can be in the form of shallow shelves or ledges. The ledges provide easy access for maintenance, and a way to get out of the pond should someone fall in.
Once the pond is built, it needs to be maintained. Invasive species have to be removed, and inlet protection devices cleaned out. As we said earlier, periodic dredging might be needed to remove sediment buildup.
As you can see, retention ponds aren’t just holes with water in them. There are a lot of technical factors involved in building them, such as calculating volume and area ratios.
Ponds should be able to control two- and/or ten-year storms as well as safely pass through 100-year storm events.
That’s why, if you’re new to this, and you’re not already working from an engineer’s design, you might want to consult with one.
Building, maintaining, and creating landscapes for retention ponds can be a lucrative sideline for a landscape contractor. Many more of these vital elements of green infrastructure are going to be required and requested, especially as the new-construction market continues to recover.