July 16, 2012 04:11

Rentention & Detention Ponds

Decorative and Utilitarian

You can find them in office complexes, municipal parks, apartment communities, commercial developments, and on just plain old properties…and in the best of all worlds they may look like typical, natural ponds.

There may be cattails along the shorelines, a jogging path around the banks, little kids tossing dried bread to ducks happily bobbing on the water, and the expanding concentric rings of rising largemouth bass that bring a smile to the mouth of any dedicated fisherman.

When those of us in the erosion control business see them, though, we know better. While there’s a chance that a pond is just a natural pond, there’s a decent chance— especially in urban and suburban areas—that the local duck pond and fishing hole is entirely manmade, and there to serve important purposes. Many are now required by local and state stormwater management laws.

Stormwater detention and retention ponds are important tools in the arsenal of anyone who has to deal with runoff; these ponds can abate the problem.

But you should know, there is a difference between detention and retention ponds.

In some cases, detention ponds are the right runoff solution. In other cases, ‘wet’ stormwater retention ponds are the way to go. This is to be determined by the design engineer.

Albuquerque uses ‘dry’ detention ponds, explains Roland Penttila, a long-time engineer in the stormwater section of Albuquerque, New Mexico’s city government. These ponds are designed to trap water for up to 96 hours, allowing it to seep into the ground, recharging the aquifer. What water does not leach into the ground is released in a controlled manner, via outlet piping, into an overall stormwater system.

‘Wet’ stormwater retention ponds are a way to gather, hold, and control runoff, while reducing and containing pollutants. Done right, these are the ponds that fool the public into thinking they’re natural features. In actuality, they are a way to gather, hold, and control runoff, while reducing and containing pollutants.

Back to Albuquerque, where it doesn’t rain, it pours. While there can be gentle, day-long drizzles and snow in winter, the city also has to deal with torrential summer downpours from thunderstorms that gather in the afternoon over the Sandia Mountains.

“When that rainfall smacks into hard surfaces like pavement and rooftops, it has to run somewhere. As it flows, it picks up all kinds of dust, dirt, and pollutants that could be deposited on those surfaces. These pollutants range from chemicals to microscopic rubber particles from auto and truck tires and much in-between,” says Penttila.

“There’s the water itself, and there are pollutants that it’s picking up and carrying along as the twin forces of gravity and water pressure usher water along after a rain event.”

Penttila urges us to start by thinking about the pollution. In the olden days, when the environment was natural, pollution wasn’t an issue. Rain would fall. That which wasn’t absorbed back into the ground to replenish the aquifer would run off naturally, and feed into streamlets, creeks, arroyos, and then rivers, lakes, bays, and finally the ocean.

Nowadays, as Penttila puts it, with real estate development and hardscape surfaces, there’s a lot more runoff and a lot more stuff for that runoff to pick up as it wends its way over parking lots and into storm drains. If those pollutants aren’t filtered out by the time they reach streamlets and rivulets, they’re going to worsen water quality downstream.

“Stormwater retention ponds are a way to control and abate that pollution,” Penttila says.

In the past, before good stormwater abatement and control systems, that pollutant-laden water would simply flow to the lowest point, and then be carried into arroyos, streamlets, streams, and the like. With a massive rain, the runoff could be devastating in different ways, both eroding banks and contaminating waterways.

Four days after the end of a rain event that fills the detention pond, that pond will be dry. Whatever water that has not leached into the ground is released into the stormwater drains. Meanwhile, many of the pollutants will either break down or settle to the bottom of the temporary pond.

Why the odd 96-hour (four-day) period? It is a response to longstanding water rights laws in New Mexico and other western states. In 1929, New Mexico signed the Interstate Water Rights Compact, which says that all lakes and streams that hold water for longer than 96 hours are subject to a complicated regulatory scheme.

In the parched Southwest, every drop matters. Albuquerque has 114 detention basins and 11 dams, all of which trap and then release runoff.

That permanent retention ponds accomplish their mission has been proved scientifically. In a paper she wrote about stormwater retention ponds in residential settings, University of South Alabama scientist Karen Jordan sampled inflow and outflow, turbidity, pH levels, and water “hardness” at two subdivision retention ponds located near Mobile, Alabama.

Her conclusion is that the ponds worked to reduce sediment pollution by up to 70 percent. “These ponds are one of the least expensive BMPs (Best Management Practices) to build, when compared to others such as infiltration trenches, basins, and sand filters .”

“Generally speaking, ‘wet’ retention ponds have one of two purposes,” Nicolas Andreyev, a Florida engineer, maintains. “Either to improve water quality, or to protect downstream flooding. A pond for water quality improvement is smaller; its intent is to catch the first flush of runoff. In contrast, a flood control retention pond is typically larger, and is intended to dampen the effect of peak discharge rate and prevent flooding downstream.”

Even as retention ponds gain in popularity, developers and engineers still make mistakes in their construction. Andreyev, whose pond-centric consulting business is based in Sanford, Florida, cautions about some of them.

“When I first started working in Florida in 1980,” he relates, “there were a lot of retention ponds that did not work as intended. The reason is that most engineers didn’t have the proper equations to deter mine the size and depth of the pond in relation to the depth of groundwater.”

Unfortunately, Andreyev sees the same common problem across the country, despite the availability of software now that can effectively model stormwater retention and infiltration rates using transient flow models.

“These models give you the right basis for pond sizing, and how deep your pond should be excavated,” said Andreyev. “Here in Florida, once regulatory agencies came to understand the utility of these models, pond failure rates dropped to a minimum.”

Along with modeling water flow—which means taking into account extraordinary rainfall events for which there might be a one-percent-per-year chance of occurrence —the developer needs to be clear on his or her assumptions as to the pond’s intended purpose. Too often, the designer sizes the pond for the wrong purpose, according to Andreyev.

Installing a stormwater retention or detention pond is not as simple as targeting a storm drain system toward a particular low lying location, bringing in a backhoe, and letting ’er rip.

Andreyev believes a high-functioning pond needs to be based on good science as well as good construction. “To make your most effective pond, you’ll want to drill soil borings—either auger borings or standard — to define the effective aquifer system below the pond. This will include defining the type of soil (sand, clay, silt, rock) and the depth to the normal seasonal high groundwater (water table).”

After the soil borings are concluded, the developer will want to conduct permeability tests on the soil, to pinpoint how quickly water will percolate down through the base of the pond and into the aquifer below. Andreyev says this can include collecting undisturbed samples and running the permeability tests in the laboratory, or installing shallow wells and running the tests in the field. Testing will also look at gradation, the percentage of silt and clay, and the percentage of organic content.

It’s only with all this information that a water retention period and plan can be modeled. This is done using a computer program that utilizes the data collected in the field and laboratory, and a computer model to simulate the effective aquifer system, runoff volume/rate, and the like.

“You want your pond to account for as many variables as possible,” Andreyev concludes. “How long will it take for the water to fully infiltrate the ground and get ready for the next storm? What happens in a drought? Where should you place your spillway, in case of catastrophic rainfall?” That’s the science. Then, there are human factors. First and foremost are safety considerations. The city of Wood River, Illinois, is all too familiar with some of these issues.

Two years ago, a four-year-old boy perished when he allegedly walked through a tunnel, climbed onto a spillway, and then fell into a municipal retention pond and drowned. His parents are suing the town, saying that the city failed to block access to the pond or warn of the dangers to kids and others.

Jonathan Jones, CEO of Wright Water Engineers in Denver, Colorado, has seen a slew of stormwa- ter retention pond safety hazards in his time. He lays out a veritable laundry list of issues. Outflow pipes may be unprotected, or their racks may have openings big enough to pose a danger. New uses on adjoining such as day care centers, may be incompatible with the pond and require the installa- tion of a separation fence or other suitable measures.

The public may find its way to a pond that the developer anticipated would be secluded, so there may be no signage about dangers, prohibitions, water depths, skating, and the like. “It can be as big as a side slope being excessively steep, or as little as a rusting bolt on a grate,” Jones says. “All of these can create public safety concerns and potential liability issues.”

A stormwater retention pond, at its best, can be an attractive amenity. There are hundreds of apartment communities that feature their “lake” in their advertising, stock them with bass and sunfish, and place picnic tables by the pondside. Ducks find their way in, birds nest nearby, and people jog around the perimeter.

It’s a little ecosystem that reduces pollution, manages stormwater, and provides recreation.

That’s a win-win-win for all involved.

Also in Soil Erosion News

In many ways, we are fortunate that, in our chosen profession, we are able to help people when certain disasters occur: the tornadoes in Missouri, Oklahoma, Louisiana and Georgia, the flooding in Louisiana, the snows in the northeastern part of the country, the rain in California, and the snow in Colorado....

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