Sept. 15, 2014 06:31

Revitalizing Our Concrete Rivers

concrete rivers

There’s something majestic about a river. Volumes of lore, literature and lyrics have been inspired by the ripple and flow of mighty rivers and scenic woodland streams. Entire civilizations have grown up around these lifegiving waterways . . . the Nile, the Ganges, the Mississippi.

At times, these streams also take life, as when they swell and overflow their banks. Natural for them; inconvenient and even deadly for us. And so, to prevent this, many of our country’s rivers and streams have been hardened in place with concrete. The U.S. Army Corps of Engineers (USACE) did most of this work, starting in the early decades of the 20th century and continuing until the early 1980s.

The concretization did its job exceedingly well, speeding excess water to lakes or oceans, getting it out of the way of urban development and preventing loss of life and property. However, as so often happens when man tries to control nature, there were unforeseen, unintended ecological consequences.

“Back in the 1930s there was a perception that we were losing too much real estate to flooding,” says Brett Nathan Kordenbrock, a landscape designer at MKSK Studios in Columbus, Ohio. He wrote his Ohio State University master’s thesis in landscape architecture about the revitalization of Ohio’s Mill Creek.

“Insurance agencies wanted to recoup their losses, and cities wanted to create developable land. So how do you do that? Well, you get that real estate out of the flood plains. That means you manage floods. And at that time, that meant lining Mill Creek and other channels with concrete.”

“Fifty, sixty years ago, when a lot of real estate development was booming, the thought process was, you don’t want your development to flood,” said Gary Paradoski, P.E., CPESC (certified professional in erosion and sediment control), a green infrastructure, water and natural resources consultant, the principal at Aqua Vitae Engineering, LLC, in Arlington Heights, Illinois. “You want water to get sent away as fast as possible. The way to do that was to harden and straighten channels, make them less flowresistant.”

“They didn’t think about was what was going to happen downstream,” he continues. “Today, we know this doesn’t really work. We still want water away from our buildings, but we also know that we need to be conscientious of a watershed’s bounds. We can’t just flood out downstream, and expect good things to happen; we have to conscientiously try and slow the water down first, let it sit for awhile, and then slowly release it after a storm has passed.”

What is it that’s so damaging about fastmoving water? “A lot of species are using the shorelines, and hiding behind logs and things like that,” said Josh LaPointe, director of ecological restoration at Brodhead, Wisconsin-based Applied Ecological Services, Inc., (AES) a firm that does a great deal of stream restoration work.

“If you get too much flow, you’re ripping out all those structures, changing the whole ecology of the river system. Having the water slow down creates pockets for fish and insects; they need places to hide. With all that super-high water flow, that’s not a very good place to live; it’s like trying to survive inside a tornado.”

Now that we know better, we’re trying to do better. The Corps is, a bit ironically, deeply involved in undoing much of the work they did in the previous century. They’re helping to revitalize Nevada’s Truckee River, Utah’s Provo River, Texas’ San Antonio River, and several others.

The Kissimmee River

The Kissimmee River in Florida is another river that the Corps is helping to restore. “The river originally meandered for 103 miles from its headwaters, all the way down to Lake Okeechobee,” said Gina Ralph, South Florida section chief at USACE’s Jacksonville District Planning Division’s Environmental Branch.

“Then, between 1962 and 1971, as part of a flood control project, the Kissimmee River and its flood plain were channelized and straightened by dredging the banks, and it became a 30-footdeep central drainage canal. They built some levees, and some damlike water control structures that aided in flood risk reduction.”

“And it worked fabulously for that purpose,” continued Ralph. “However, right after it was completed, a whole bunch of negative environmental effects became apparent due to this channelization. There was a loss of wetlands and wildlife diversity, as well as water quality issues; now, we had point-source discharge into Lake Okeechobee.”

“People were outraged. So, in the early 1970s there was a grassroots effort to study those negative consequences and help effect change.”

Now, most of the concrete-andsteel dams on the river are being removed, and the side slopes of the river revegetated. “We try our best to match what existed in that location,” said Chris Ralph, civil section chief of the Engineering Division at USACE’s Jacksonville District.

“We vegetated the side slopes using native plants. We didn’t put down any turf reinforcement mats (TRMs) or hydroseed; we didn’t want to introduce any vegetation that wasn’t native to the system. Along the edges, we put in mostly wetland plantings, bulrushes and spike rush, extending six inches above and six inches below the water line.”

When people ask why the Corps isn’t resodding the Kissimmee’s banks, Chris R alph answers, “Because they were never sodded in the first place. Restoration means putting back what was there before.”

When a river is channelized, it no longer “meanders,” coursing this way and that, rippling over rocks, and flowing at a pace compatible with the maintenance of wildlife communities. Revitalizing means re-rippling and re-meandering.

“Once you’ve removed the concrete, you’ll want to recreate your meandering channel,” said LaPointe. “A normal, natural river has a ‘ripple-pool’ structure, from water running over protruding rocks, roots and other things. To recreate that rippling, you’re going to be bringing in a lot of natural stone for the river bed.”

Restoration experts don’t want to recreate this rippling just because it looks and sounds nice. “Rippling aerates the water, bringing in oxygen for habitats and for fish,” explains LaPointe. “It also cools the water down, and creates pools that fish and other living things can rest in. To do this, you’re going to want to have woody vegetation, log toes, rock toes, all kinds of different structures that would exist in a natural river system,” he says.

And remove those that don’t. “One river project we’ve been working on lately involves the removal of several old hydroelectric dams that serviced long-closed mills,” he continued. “These dams don’t have much purpose anymore, but they’re major inhibitors to fish and wildlife passage.”

“We’re using root wads to naturalize that river. We take a tree out by its roots, put the root ball into the river, stick the trunk back into the bank and tie it all down. The root wad creates microhabitats for insects and fish.”

Another technique AES uses is to create fabric-encapsulated soil lifts, what LaPointe calls “soil burritos.” A soil lift consists of a natural coir mat, infilled with soil and rolled up. It can be planted right through the coir, or seeded with native species.

“Or, you can make brush mattresses out of live stakes,” adds LaPointe. “You take willow cuttings, or some other species that will sprout from the stems, and create pads one or two layers thick.

You tie them up and place them along the banks to create microhabitats. Some of these sticks will grow, and eventually you’ll have shrubby layers at the edges of the banks.”

“Over time, any geotextiles you might have used will biodegrade, and vegetation will hold onto and stabilize the banks. But sometimes that isn’t enough, and you have to use hard armoring, riprap or gabion baskets.”

The Los Angeles River

Google the term “concrete river,” and you’ll see lots of stories about the Los Angeles River, perhaps the most famous concretized water body in the U.S. It’s been featured in movies as the scene of car chases. Long stretches of it are completely concrete-lined, and devoid of water most of the time. Many longtime residents don’t realize that it’s a real river, not just an ugly drainage ditch where old couches go to die.

Yearly flooding led to USACE lining most of it with concrete back in the 1930s. It’s been said that Los Angeles would not have become the city that it is without this flood protection.

There’s been an effort over the last two decades to return this river to a more natural state. The result was the L.A. River Revitalization Master Plan. It envisions a recreational haven with restored riparian habitat and wetlands, ringed with greenways, bike trails, and hoped-for residential and commercial development. The Corps has completed a feasibility study and has recommended approval of the $1-billion proposal. Now it’s up to Congress.

The Master Plan doesn’t encompass the river’s entire 51-mile course. Rather, it focuses on an 11- mile stretch that runs through Griffith Park.

“Sixty percent of this 11-mile stretch is soft-bottomed,” explains Dr. Josephine Axt, planning division chief at USACE’s Los Angeles District. “That habitat is what we’re using as a foundation. A primary consideration is connecting the river to significant ecological areas nearby that are still relatively unaltered by people.”

However, “We are definitely not removing all of the concrete,” she clarifies. “But we are being creative with it. If we removed all of the concrete, we’d have to take the channel, which on average is about 300 feet across, and expand it five times in order to maintain the existing flood protection. That’s our number-one constraint. We don’t want to alter the flood protection it’s always provided.”

“You have to have enough width in the channel to carry the water, or you’ll have a flood plain, meaning that the channel will overflow,” said Ira Mark Artz, P.E., divisional vice president at Pasadena, California-based Tetra Tech, Inc., a firm that does planning and environmental work worldwide. He was project manager of the consultant team that worked on the L.A. River Revitalization Master Plan. “You don’t want that, because then you’re flooding people, which is one of the reasons they concretized these channels in the first place.”

A large section of the L.A. River may remain concrete-lined forever. “There are some areas where we obviously just can’t take out the concrete, because people have built up right next to it,” admits Jeff Hutchins, principal at Mia Lehrer Associates, a Los Angeles architectural firm that also consulted on the Master Plan.

Step pools work to slow the flow of water in a stream.

Softening the banks

“The Los Angeles River is a unique system, because it’s in an almost 100-percent urban environment,” said Kerry Casey, senior hydraulic engineer at the Corps’ Los Angeles District. “Restoration in an urban environment has many, many challenges, engineering-wise.”

Because of the speed of the water, live stakes, soil lifts or root wads won’t work. “We’re dealing with such a different environment here,” says Axt, “with super-critical flows. When we do get water in the river, it can come down incredibly fast (as fast as 29 feet per second during storm events). We’re not just taking out concrete; we’re re-engineering the channel walls.”

“We’ll have to use more traditional types of revetments, such as riprap,” said Renee Vermeeren, hydrology and hydraulics branch chief at USACE’s Los Angeles District. “We’ll lay back some of the side slopes, and do some terracing there, possibly lining them with soil cement. We’ll also set back some of the levees we have in place, and open up the channel where the city has acquired the rights-of-way to the land.”

For the terracing, geotextiles and TRMs will be used. “The velocities of the water coming down will govern what we can actually use,” says Casey.

“Geotextiles and TRMs are usually used for minor flow velocities,” he explains. “We’re looking for slope stability, so we may have to use derrick stone (a large, angular type of stone), and we may have to grout it in. Whatever measures we end up using, we hope that we can bury them under several feet of earth, so that they can support the rootzones of whatever vegetation we end up placing there.”

A lot of money is being spent undoing past mistakes. As time goes on, we learn more and we change what we do accordingly. We don’t line rivers with concrete anymore, just as we don’t insulate with asbestos, or spray bugs with DDT.

“The technology we have today gives us a huge advantage,” said Tim Gysan, senior hydraulic engineer at the Corps’ Jacksonville District. “Now we have computer modeling tools that can predict the reaction of water when we do change a channel. We can input different configurations of materials and see what’ll actually occur.”

For contractors doing streambank restoration, there’s plenty more work out there to be done, and better tools to do it with. That’s as certain as if it were set in concrete.

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