Keeping Retention Ponds Healthy
If you’ve ever lived where the weather gets snowy, or there are frequent, intense rainstorms, you’re probably familiar with the concept of the “mud room.” It’s the place where you shed your snow and rain boots and other gear to keep from tracking muck into the rest of the house. One mud room per dwelling is usually plenty.
One couple in Mobile, Alabama, however, was unlucky enough to have every room in their home turned into a mud room.
It happened last May. A big rainstorm sent a wall of water, mud and debris rushing across their property. This “mudnami” was so power- ful that it knocked the house off its foundation and flooded its interior with sludge. It even washed the couple’s above-ground swimming pool half a mile away into the woods behind the house. All of this occurred because a retention pond at a nearby construction site failed.
The engineer and the contractor involved with designing and building the retention pond are disputing the exact cause of the disaster, and the matter is currently in litigation. It’s possible, however, that a concrete outfall device, called a weir, may have had a temporary plug in it so that it didn’t release the pond’s excess water as it was designed to do.
While this is, admittedly, an extreme example, it serves to illustrate what can happen when retention ponds don’t work properly.
-Most of the time, they do, and serve their purposes well. In fact, welldesigned, well-built and well-maintained retention ponds usually prevent disasters such as the one that happened in Mobile.
Let’s define some terms. The words “retention” and “detention” can be thrown around interchangeably, depending on whom you’re talking to. Some people use the term “retention pond” to mean a more or less permanent installation that stays wet, and “detention pond” to mean a temporary one that dries up between storm events.
These are also called “wet” ponds and “dry” ponds. Temporary retention ponds are often used to control stormwater on construction sites, while permanent installations are usually found at large land developments.
“A retention pond, as the name suggests, retains water,” said Tiffany Clark, P.E., a land development review coordinator for the Southeast Metro Stormwater Authority in Centennial, Colorado. She’s also a CFM (Certified Floodplain Manager). “There’s no outlet structure; it retains the water as it generally percolates down into the soil.”
Detention ponds are more common in arid regions, where they serve as flood-control devices. They are usually dry except after storm events. Their mission is to slow down water just enough so that it can be absorbed safely. In urban settings, they are used to reduce peak flows which can lead to flooding.
A detention pond detains the water for a certain amount of time. It has an outlet structure, or weir. The weir is designed to release the entire volume of the pond at its historic rate over a certain number of days or hours.
An “historic” rate is the speed at which rainfall left the site back when it was in its natural, undisturbed, vegetated condition. It’s a much slower rate of flow than the one that comes after that, when the site has been developed, and concrete, asphalt, buildings and other impervious structures have been built on it.
“Don’t get caught up in “detention” or “retention”—they’re old terms,” said Christopher B. Burke, P.E., PhD, D.WRE (Diplomate, Water Resources Engineers), Dist. M. ASCE (Distinguished Member of the American Society of Civil Engineers), and president of Christopher B. Burke Engineering, Ltd. in Rosemont, Illinois.
“It’s a storage volume that is required to attenuate flows to a prescribed maximum. That’s as complicated as it gets. You need to attenuate the flows; otherwise, you’re going to flood people in the downstream areas.”
Whatever you call a retention pond, it needs to be kept healthy. That means regular monitoring and maintenance.
Pollution control system
Retention ponds have a dual purpose. Besides keeping stormwater from causing floods and erosion, they keep pollutants from finding their way to, and contaminating, lakes and streams. A healthy retention pond keeps other bodies of water healthy.
When a rainstorm dumps a lot of water on an impervious surface, such as a large, asphalt-covered parking lot, some of the water will pool and eventually evaporate. The rest of it will leave the site.
“If you don’t have a retention pond or some other capture system, you’re going to have debris, motor oil, bio waste, and toxic chemical runoff from all the vehicles sitting in that parking lot,” said Joshua Manning, a stormwater specialist at Richmond, Virginia-based ACF Environmental.
“The impervious surface is going to funnel all of that straight into a stream.”
Rather than go right to the storm sewer, heavy metals and other toxic particulates enter the pond and settle out. The pond then infiltrates that dirty water into the ground slowly, and the soil filters the pollutants out.
Soil works great as a pollutant filtration medium. Municipalities around the highly sensitive Chesapeake Bay watershed have used soil and sand filtration for years to treat their stormwater runoff.
However, different soils have different rates of absorbency. This 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 in Bloomfield, New York. His company “does everything with retention ponds except dig the holes,” i.e., planting and maintaining the landscapes around them and keeping them clean and healthy.
Before he designs a pond, he does a “perc (percolation) test” to see how fast water seeps into the soil. If he’s designing a half-acre retention pond eight feet deep, he first digs two smaller eight-foot-deep holes, and looks at the material that comes out.
“If it’s all heavy clay, then there’s no problem,” said Howard. “You can dig that pond, and it’ll hold the water fine.”
If the soil that comes out of the perc test is gravelly or sandy, a poly pond liner may be needed. This can add quite a bit to the cost, depending on the size of the pond. A very large liner can be expensive.
“Most of the time when we’re using a liner, we’re putting a pond where it doesn’t want to be,” said Howard. “Either it’s way up on a hill where there’s no watershed, or on soil that won’t hold water.”
Pond liner can be obtained in 60,000-square-foot rolls. Should a liner bigger than 60,000 square feet be required, sheets can be heat-sealed together onsite with a special machine.
Good pond health through maintenance
Regular maintenance is the key to a healthy retention pond. Fortunately, most of them get it, at least according to Manning. “I’ve never seen or heard of a retention pond that didn’t have maintenance done on it regularly,” he said. “For the most part, folks are pretty on top of it.”
Retention pond maintenance isn’t hard, says Clark. “A lot of it is just cleaning and some landscape-type work, such as mowing ponds when they’re dry. It’s sediment and trash removal, checking and cleaning of inlet and outlet structures, and repairing access driveways.”
Ponds are a magnet for windblown debris, leaves, mulch and grass clippings. Retention ponds can overtop when their inflow or outflow devices get clogged with this junk. Manning says the best way to maintain retention ponds is to prevent potential cloggers from getting into them in the first place.
This could be something as simple as an inlet protection device such as a “witch’s hat” over a pipe inlet, preventing mulch, large leaves or pieces of trash from getting into the system. Many catch-basin-type devices are made of fabrics that soak up the oils and petrochemicals found in runoff. There are also fine filters that can screen out the small stuff, particulates and microbial pollutants.
“You’re preventing the need for a lot of pond maintenance tasks that way,” says Manning. “If you keep them from getting into the pond in the first place, it makes the job a lot easier.” It’s much simpler to clean out a pond’s inlet protection devices than it is to clean out the entire pond.
And the inlet protection devices do need frequent cleaning. If this is ignored, their purpose is defeated. They can overflow, and all of the gunk they’ve collected gets released. This is one of the things that causes retention ponds to overtop and flood.
There’s another reason for not letting all those leaves and clippings simply settle out. “All of that good decaying organic matter at the bottom is like good garden soil. It just grows weeds,” said Howard.
Eurasian milfoil, hydrilla, bulrush, lily pads and cattail often grow prolifically in retention ponds. They can be cleared with electrically-powered sickle-type mowers and rakes that can be attached to small boats.
“They’re a mechanical means of getting rid of weeds, without using herbicides,” says Annamarie Gervais, owner of Jenson Technologies in San Marcos, Texas, a company that makes these products. “You can specifically target the areas where you want to get rid of weeds, and not treat the areas where the weeds provide a habitat for fish.”
A settlement for sediment
One of the purposes of a retention pond is to let sediment settle out so it doesn’t get into streams and lakes. Over time, a lot of sediment can accumulate.
Howard says that retention ponds naturally want to fill in. “Eventually, if left alone, they get more cattail-like and woody, and can become more like a wetland.”
“Wetlands are good for water quality enhancement, but you probably don’t want that in an onsite facility,” adds Clark.
A wetlands-like condition will diminish the effectiveness of soil infiltration, Clark says. “You can get soggy, boggy conditions in the bottom of a pond where water is standing all the time, and it just won’t function the way it’s supposed to.” You can prevent this by dredging the pond bottom periodically.
Retention pond as ecosystem
The easiest way to keep a retention pond healthy, says Howard, is to aerate it using an onshore compressor. “We’re talking about bottom aeration—pushing air into the bottom of the pond. This keeps it from having different levels of oxygen in it. It turns the whole pond over so it stays a lot more balanced.”
The compressor pushes air into plastic tubing, then into diffusers spread around to different areas of the pond. The bigger the pond, the more diffusers you’ll need. Howard says it’s really no different than the bubbler systems you see in home fish aquariums, just on a larger scale. There are pond aeration pumps designed just for this purpose that are concealed under fake rocks.
“All the good bacteria in the pond helps break down the decomposing weeds, algae and leaves that are there,” says Howard. “It speeds those processes up, so the retention pond stays a lot younger, instead of just filling right in.”
Most retention ponds are dug on new developments to help fulfill SWPPPs (Stormwater Pollution Prevention Plans). However, Howard says that about 30 percent of retention ponds are also there for decorative reasons. “People want to see the birds, the fish—all the wildlife that comes along with having a big pond.”
Maintaining the health of a scenic retention pond includes keeping the fish, birds and other wildlife around by keeping the pond biologically in balance. You’re actually maintaining a little ecosystem.
Howard likes this sort of retention pond to be at least six- to eight-feet deep. However, he says that a lot of times, they end up being under the six-foot mark, even as shallow as four feet.
“It’s tough to balance a really shallow pond, unless there’s always good water coming in and out of it,” says Howard. “But in a retention pond, water only comes in when there’s a storm event.”
Here’s the problem: the shallower the pond, the more sunlight penetrates to the bottom. As we stated, all that settled siltation—full of phosphorus and nitrogen from fertilizer runoff—makes for a very rich growth medium. When the sunlight hits that, it encourages algae and weeds to grow like crazy.
The more vegetation in a pond, the less oxygen there is for the fish. And when the fish die off, the wildlife that eats that fish doesn’t stick around very long.
Keeping ponds balanced is hard enough, even when they’re nice and deep. But shallow ponds are common, says Howard, the result of shallow budgets. “Because it costs more money to make ponds deeper, often the developers will typically only excavate as deep as the minimum volume of water required by the total area of the development, and no more,” claims Howard.
You may be thinking, why go to all this effort if there’s no fish in the pond? As we’ve explained, aeration is good for ponds even when they don’t have fish in them.
Howard says that even ponds that people insist don’t have fish, tend to have fish. Neighborhood kids catch them, bring them home to show Mom and Dad, and end up dumping them in the local pond. Birds transport minnow eggs from lakes to ponds. Most ponds end up having some sort of fish life.
“What we have here is a failure to communicate.”
You may recall that as the oftquoted line from the classic film, “Cool Hand Luke.” Bad communication between contractors, engineers and people in the field can lead to the failure of a retention pond like the one in Mobile.
The permitting process that any stormwater control measure goes through should mitigate that. For instance, any proposed new developments—either residential or commercial—in Clark’s agency’s jurisdiction come through her desk first. She personally checks the plans for compliance with regulations.
When she does catch something in the later stages, it’s rare. “We have plenty of meetings before they submit their plans for review. We usually catch really big problems then, because we’ve discussed them thoroughly.”
Manning says that a lot of problems with retention ponds can be headed off by simply reading the directions. He says that almost daily, he’s urging people on pond construction sites to follow the manufacturer’s details and instructions to a “T.”
“You read things, and sign things, and then, once you get on the job site, things can be different,” he says. “Stuff can get blurred. I’ve had people drive over structures that weren’t stable yet. Before you know it, everybody’s pointing fingers, and there’s no real way to know what actually happened.”
“I would just say to everyone, take your time and do it correctly,” adds Manning. “Nobody wants a failure—not the engineer, not the contractor, not the distributor, and certainly not the client.” You might have to pay for a couple of extra man-hours, but it’s cheaper than having to remove a retention pond system and put in a brand new one.
Manning also stressed that it never hurts to get a subcontractor. If there’s something you’re not experienced enough to do yourself, it’s smart business to find someone who does.
Retention ponds, whether simply functional or filled with waterfowl and fish, remain an effective means of controlling stormwater. If they’re kept healthy, they can keep doing their job for decades to come.