Jan. 15, 2015 12:07

Sediment Control

sedcontrol
MORE IMPORTANT THAN EVER

If you were walking along a certain part of North Carolina’s Neuse River this past October, you might have noticed a most unpleasant odor. The stench would have come from the approximately 200,000 dead fish laying on its shores—Atlantic menhaden, to be precise.

What caused this massive fish kill? In a word, sediment.

A large residential subdivision under construction nearby is blamed for not taking enough measures to keep sediment out of the river. “Sedimentation is the number-one pollutant in North Carolina waterways,” said Matthew Starr, the Upper Neuse Riverkeeper (someone who works as an educator/advocate for the New Bern-based Neuse Riverkeeper Foundation).

The sediment clogged the menhaden’s gills. It settled on the tiny invertebrates that live on the river bottom, suffocating them, too. Then, all that decaying biomass in the water depleted still more oxygen.

Add to that soup still more sediment from stormwater, which carried with it phosphorus and nitrogen from fertilizer. This fueled an algae bloom, another oxygen thief. The low oxygen content of the water fostered the growth of a bacterium called aphanomyces invadans, which ate away at the fish from the inside out. The poor menhaden didn’t have a chance.

No doubt about it; sediment, a.k.a. dirt, has a soiled reputation. “Sediment decreases water quality for fish, but also for people,” said Neal Orsbon, project manager for The Shelly Company, Thornville, Ohio. “As consumers of drinking water, we all need to be concerned about it. Sediment increases the difficulty in filtering water, which increases its cost.”

As a soil erosion contractor, you may not consider yourself an environmentalist, but you are. Your job is to corral, manage and trap sediment before it can get into our rivers, lakes and streams and cause environmental damage. And fortunately for you, a lot of great tools have been developed to help you do that job well.

Erosion control does not equal sediment control

So, where do you start? With erosion control, said several of the experts interviewed for this story. Erosion control and sediment control are often spoken of interchangeably, but they are not identical.

“Everybody thinks they’re the same thing,” said Sarah Haggard, CPESC (certified professional in erosion and sediment control) and a soil erosion contractor, owner of Deluge Consulting in Bakersfield, California. “But they’re not. If you first eliminate or control erosion, you eliminate the need to control sediment.”

What best management practice (BMP) is really “best” depends on the severity of the potential erosion problem. “There’s a very valuable tool called a ‘Rusle equation’ that you can run before ground is disturbed, during the height of the disturbance, and again after a project is stabilized,” said Jennifer Hildebrand, environmental compliance group manager at WSB & Associates, Inc., in St. Paul, Minnesota. The numeric values will give you a risk indicator to understand the potential soil loss.

“If it’s an area where you already have sheet flow, and you don’t want to try to concentrate any water, you could use a linear BMP such as a fiber roll, or a gravel-backed berm,” said Haggard. These will slow down the velocity of the water, filter it and disperse it evenly.

For areas of concentrated water flow, rock check dams work extremely well, Haggard says. “If you’re dealing with a slope or a channel, lining them with turf reinforcement mats (TRMs) or erosion control blankets (ECBs) with high friction coefficients will keep that water from accelerating.”

“What you want is the dropout of sediment as you go,” she says. “You don’t want it speeding up or whooshing down the channel, eroding the soil at the end of it, or causing a big blowout of the slope or channel.”

“Slopes are especially difficult,” said Brendan Mundorf, sediment control operations manager at Hanover, Maryland-based M&M and Contracting, Inc. “The more severe the slope, the harder it is to keep sediment in place. There are situations that can overflow your devices, and create washouts. Anytime the water moves quickly, it’s tough.”

Preventing erosion first is the approach Haggard was taught by her professors at Cal Poly San Luis Obispo, California, and tries to use in the field. However, “getting out into the real world, seeing how things are actually done on various projects, I’ve seen that it’s often not done that way.”

The usual reason: cost. “It’s cheaper for a developer to say, ‘We’re just going to grade the area, and then use a ‘catcher’s mitt’ to grab any sediment that happens to come off,” she says.

The “catcher’s mitts” Haggard is referring to are silt fences and straw wattles. Not that these things, properly installed, don’t work; they work extremely well. But these BMPs are often chosen just because they’re cheaper than TRMs or ECBs.

Erosion isn’t the only way to lose soil off construction sites. Trucks are a big one. They transport equipment, personnel and supplies to sites, but also dirt to waterways. “When it rains, they track mud,” said Clayton Fawcett, area manager, Armortec-west, for Contech Engineered Solutions, LLC, in West Chester, Ohio. “That gets into storm drains, and ultimately, to waterways.”

Vehicular tracking pads and rough stone driveways can help with that problem. As the trucks roll over them, the vibration beats the mud off. Washing stations help, too.

Wind is another engine of sediment migration. Haggard works on a lot of solar farm developments in breezy areas. Solar farms require that large amounts of earth be graded out. To keep that soil down on the farm, she turns to hydromulching.

Seeds in hydromulch don’t sprout overnight. Fortunately, a good hydromulch formula doesn’t wait for roots to form. She recalls one solar farm project where a new hydromulch formula was being tested.

When the wind blew sediment over the test section, “you could see that in all the places the hydromulch had been sprayed, the sediment had dropped out of the air.”

The bits of paper and straw suspended in the mulch created a textured surface, which increased the surface friction.

The sediment accumulated in the little holes on the surface of the mulch, within its matrix. The test was so successful that the developers of the solar farm hired the company that developed this hydromulch to spray the entire site.

Hydromulch also works well in another instance. When subdivisions are built, topsoil is stripped off and put in large stockpiles until it can be redistributed back to the home sites. To prevent wind or water from eroding the piles, they can be sprayed with fast-growing oats or grass seed. The roots will temporarily anchor the piles.

Polyacrylic coatings are another great way to keep dust down. There are many different products that can be sprayed on for temporary protection, and when no longer needed, will biodegrade.

Whatever is used, one shouldn’t depend too heavily on any single BMP. “I’ve traveled around the country a lot, and too often, I’ll see our products used all by themselves, to take care of everything Mother Nature throws at a site,” said Dan Cleveland, president of Powell, Ohio-based Dandy Products, a manufacturer of inlet protection devices.

Inlet protection devices, such as “witches’ hats,” are geotextile filter sacks that are installed either above or below curb inlets or grates to keep debris and dirt out of storm drains. These products are stationed at the end point of the ‘sedimental journey’ to waterways.

“We’re supposed to be the last line of defense, after the wattles and the silt fencing, and everything else,” said Cleveland. “There should be other products also in place, so that a product like ours, or any other inlet protection device, isn’t taking the full brunt of everything that’s leaving the site.”

Hard armoring

Severe conditions require serious measures. That’s where hard armor comes in.

This was the case at a dam in North Dakota that Fawcett worked on. “There was a grass spillway on the back side of this dam,” said Fawcett. “But over time, hydraulic flow conditions exceeded what the spillway could withstand. When you get flow in the neighborhood of five feet per second, it’ll remove vegetation.”

The increased water flow ripped out the vegetation on the downstream face of the spillway, and the subsequent erosion impacted water quality in the basin. More alarmingly, the water started to erode back into the structure of the dam itself. “With a more extreme event like this, that’s when we start looking at hard armor solutions,” said Fawcett.

The “extreme” event he’s referring to is known as “head cutting.” Head-cut erosion is powerful; it’s what formed the Grand Canyon, albeit over thousands of years. Head cuts propagate from downstream to upstream. Soil particles become mobilized and are shot downstream, forming a gully, or rill. “In the face of fast-moving soil particles, you have to go to hard armoring: rip rap, gabions, a concrete lining, or an articulated concrete block product,” Fawcett says. In the case of the spillway, the latter is what finally stopped the head-cutting process.

Staging

“One of the best techniques for controlling erosion and sediment isn’t talked about nearly enough,” said Danny Ross, CPESC, CESSWI (certified erosion, sediment and stormwater inspector). He has spent 33 years inspecting BMPs for the feds, and now teaches at Kent State University in Kent, Ohio.

The technique is called “staging.” “Staging means that you don’t bulldoze an entire site at once, ”heex-plains. “ You expose one part at a time, finish building that, then move on to the next.”

“Here’s a great example,” continues Ross. “Say you have a giant big-box store going up, where they’ll have to expose an awful lot of soil. You need to ask, ‘Can we put in the parking lot first, then do the foundation next, and so forth, and only expose those areas that need to be exposed at any one natural vegetation in place, to help time?’ Can we leave some of the filter the sediment?’”

“Instead, everybody talks right away about hauling out the BMPs—doing the seedings, laying out ECBs, putting in silt fences and sediment basins,” said Ross.

Reading this, you’re probably already forming the objection in developer’s call, when and how your mind. “Yes, but that’s the much to excavate. I just control the erosion and sediment.”

Ross has run into this roadblock many times. When a developer says, “I can’t afford to do it that way; it’ll take too long,” he counters that needing fewer and small- er BMPs will more than make up for the extra time spent in staging.

And if you don’t expose as much earth, you won’t need as much maintenance on those same BMPs.

Mission-critical: proper installation and maintenance 

Of course, no BMPs work well unless correctly installed. Haggard sees a lot of silt fences that aren’t. “Silt fencing should be trenched in at least six inches. Otherwise, water is going to flow directly underneath.”

She advises a lot of her clients to use fiber rolls or filter socks rather than silt fencing whenever possible, as they handle wind better, and are easier to install and maintain. However, they must also be trenched in, at least three inches— and never across contours (elevation lines, as seen on civil drawings, or topographic maps).

“That’s not effective, because then you’re giving the water a little slide to go down,” says Haggard. “Your flow is going to be absolutely perpendicular to your elevation. You’ll want to make sure that water will be intercepted perpendicular to your linear controls, whether they’re silt fences or a rolled product.”

Haggard recalls working with a revegetation company, where the crew leader was inexperienced with sediment control products. “He pointed out to me, very proudly, how his guys had spent all day installing the fiber rolls I wanted—but they’d installed them across contours.

Well, they didn’t like me very much when I told them they had to go back and reinstall everything the right way.”

Let’s go back to the Upper Neuse River. It’s not that there were no sediment control measures taken at the subdivision site; several different BMPs were employed. “The main issues stemmed from improper installation or lack of maintenance of those measures,” said Starr.

Some of the things he’s seen include check dams that filled up with sediment and weren’t cleaned out; or, if they had been, the sediment was left right next to the check dams, defeating their purpose. Silt fencing that wasn’t trenched in, but simply strung out across an area, with some dirt piled on the little bit of extra fabric at the bottom. Huge, uncovered, unseeded dirt stockpiles sitting next to retention basins, that infilled those basins as soon as it rained. And that wasn’t all.

“It all comes down to maintenance,” said Starr. “If you have a retention pond with a skimmer, and the dam overtops, but there’s nothing done to prevent that from happening again, what’s the point of having the pond in the first place?” “A silt fence gets loaded up, pretty much like a diaper, and has to be changed,” said Fawcett. “After a rainfall event, you should inspect your silt fencing to make sure there are no breaches, and that your wattles are securely in place and functioning as designed.”

Since the passage of the Clean Water Act way back in 1972, sediment control has only grown in significance. Increasingly stringent environmental regulations continue to be passed, along with stronger enforcement of those regulations. The EPA isn’t going anywhere, but the industry you work in is, onward and upward. Anyone who works in erosion and sediment control is assured of plenty of work to do in the years to come. The Earth needs you.

Hard Armor continued from page 19

hauling it. The further it has to travel, the more it’ll cost. “Stone in Arkansas is pretty cheap compared to Florida, where they have to freight it in from Georgia,” says Titus.

When you’re figuring out your rock budget, make sure you factor in the amount of rock needed, its size and the transportation cost. Don’t forget about accessibility.

Equipment may be needed to place larger stones; make sure it can reach the site.

“Every job is its own beast,” said Titus. “Some cities and counties don’t have that inner budget, and that’s why so many jobs are just dumped riprap. Or, they only get a certain amount to spend each year, so they just keep dumping more riprap.”

When Akowicz is told that the budget won’t allow for certain purchases, he searches for alternatives. For example, if a gabion structure was ruled out, “we can still provide the facing they’re looking for by using a terramesh wall. The cost may go down, because now we’re not having to use as much rock.”

A thorough analysis of all of a job’s parameters should help overcome obstacles. “There’s so much you have to investigate to ensure that you’re using the right product mix,” says Akowicz.

Don’t neglect what erosion control tools may already exist on the site. “If you can keep your natural armaments, the native trees, grass and weeds, you won’t have to do as much erosion control,” said Titus.

No matter which hard-armor solution your project ultimately employs, whether it’s riprap, gabions, concrete blocks or a combination, there will be pros and cons to every choice. If all the variables have been weighed carefully and the solutions installed correctly, long-term erosion control that is both environmentally and aesthetically desirable can be achieved.

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