Soil Erosion and Hydroseeding Mistakes
Soil erosion prevention is a complex field, as anyone who does this type of work for a living knows. Why else would there be so many different certifications one can get?
The alphabet soup that follows many of the experts’ names in this story reflects just how much study they had to put in (as well as the continuing education required to maintain those certifications). Most of them are CPESCs, (certified professional in erosion and sediment control). Some are also CPSWQs, (certified professional in stormwater quality), CESSWIs (certified erosion, sediment and stormwater inspector), and/or CISECs, (certified inspector of sediment and erosion control).
However, even highly credentialed professionals can make mistakes; after all, they’re still human. Soil erosion prevention errors are caused by a number of factors, most commonly, improper installation of Best Management Practices (BMPs), inadequate maintenance of same, or choosing the wrong product for a particular application.
But when mistakes happen, it’s usually the contractor who gets the blame, even if all he’s done is follow what was specified. The failure may have been inadvertently built into those specs.
“Not choosing the right solution for the problem is a critical error that I often see,” says Jennifer Hildebrand, CPESC, CPSWQ, CESSWI and CISEC, environmental compliance manager for Minneapolis-based WSB & Associates, an environmental compliance service firm. “You may have a slope area with runoff, and they’ll put a product or a solution on it that isn’t able to handle the runoff. That’s usually because of poor design or site assessment. You must provide proper project diagnostics first, before you apply a solution or BMP to fix the problem.”
Donald W. Lake Jr., P.E., CPESC, CPSWQ, has more than 40 years experience doing erosion and sediment control work. He’s an adjunct visiting professor at the State University of New York, Syracuse, in the College of Environmental Science and Forestry, and teaches CPESC certification classes as well. He’s also co-written a widely used textbook on the subject. “Oftentimes, project designers in their haste will do a cut-and-paste job, and they may not include all of the details that a certain practice or procedure requires.”
Lake hates the term ‘BMP.’ “It conveys the attitude that ‘All I need to do is put a standard detail, like a silt fence, on the drawings in order to meet that element of control. Sometimes not enough real thinking goes into the installation of all these practices. It gives the impression that whatever practice you’re using is the only possible one for that site or circumstance.”
In reality, the soil erosion or sediment control practice being used is often not the “best,” but simply the most expedient, or the cheapest. “To me, philosophically, there is no such thing as a ‘BMP,’” adds Lake. “If there were, we wouldn’t have courses out there entitled ‘How to Put the Best Back in Best Management Practices.’” Danny Ross, CPESC, CESSWI, worked for the USDA Natural Resources Conservation Service for 33 years, 20 years of which were spent as an inspector of SWPPPs (stormwater pollution prevention plan). Now he teaches CPESC certification classes, and is also an associate lecturer at Kent State University in Kent, Ohio, in both the geography and biology departments. He’s seen plenty of mistakes over the years—some of them his own, as he’ll freely admit.
For him, a little chat beforehand saves a lot of problems later. “If you don’t have a pre-construction conference, you’re doomed to failure,” he says. “That’s where you begin. You need to go over how all these things are going to go down. Look over the plans to see if they’re good enough to control erosion and sediment on that site. It needs to be a cooperative effort.” Such a conference should include the inspector, the developer, the contractor(s) and possibly the consulting engineer who designed the project.
“After we all introduce ourselves, the first thing I ask is, ‘How are we going to implement this SWPPP? Just show me on the plan how you’ll do it.’ I’ll find out fairly quick if the contractor has even looked at the plan. If he turns to the page right off, and even has it marked with some questions, such as, ‘Why is the sediment basin here instead of over there?’ or ‘Why is the silt fence on top of the hill?’ then I know we’re starting off pretty well. But if he’s flipping through, wondering which page has the plan, the conference gets everybody literally on the same page.”
“As a government inspector, I didn’t look at projects as ‘theirs’ and ‘mine,’” continues Ross. “It was our job. If they did a good job, then we all looked good. But if something was wrong and we didn’t fix it, then we all looked bad.”
The process of hydroseeding sounds pretty straightforward; just mix the seed, the fertilizer, the tackifier and the mulch, and you’re good to go. Not really. There’s plenty of opportunity for things to go sideways.
“When you’re going to get an area seeded for ultimate vegetation, there are some really important steps,” says Hildebrand.
“First, you want to prep your seedbed. And oftentimes, that’s not done properly. A seedbed has to have nutrients and proper soil loosening, so the seeds can get oxygen.
Soil needs to be tested for pH and for toxicity issues that are often relevant to construction soils. This soil testing is cheap insurance.”
“Second, you need to pick the right seed. A lot of times, people want to use seeds that are very dependent on fertilizer, and oftentimes that’s not accessible after the seed have been planted. If you use native seeds without a nurse crop, you’re going to get a lot of weed influence, and that’s going to inhibit the amount of growth you’ll get afterwards.” A nurse crop is something that grows for a year, like winter wheat or oats, to keep the weeds from taking over while the native seeds are getting established.
“Third, you want seed-to-soil contact. You don’t want to apply the seed so that it’s suspended in the mulch; it can dry out or burn up and not vegetate. You can certainly spray the seed on with the hydromulcher, but the mulch should be applied in a separate process. In summary, work the soil, test the nutrients, put the right type of feed down, and get the seed to touch the soil, and you’ll be successful.”
“Hydroseeding is usually done during the dryer months,” says Ross. “But when I first got started as an inspector, we had so much development going on that we were building all summer, and seeding and mulching in the fall. We’d go out to sites, and darn, if everything wasn’t growing.”
“Well, that looks good on a report, but I found out I’d better go back and take another look in 30 to 60 days,” he continues. “If there’s been some unbelievable rainstorm in the meantime, then instead of 90 percent seed coverage, you can find you’ve only got 30 percent, because either the wind blew all the straw mulch off, or erosion happened. You need to go back and see that you do have at least 75 to 80 percent germination. And don’t just look at it from the vehicle. Get out and walk the site.”
You also need to do a little bit of math. “One of the most important things when you’re spraying seed,” says Lake, “is to make sure that the right amount of pure, live seed is being applied.” That’s because bags of seed also contain seed shells and inert material, along with the seed. If you don’t account for that, your calculations will be off.
“Say, for example, the label on a ten-pound bag of Kentucky bluegrass seed says ‘90 percent germination, and 90 percent purity,’” says Lake. “You multiply those two numbers together, then divide by 100. Only 81 percent, or 8.1 pounds of that ten-pound bag is pure, live seed.”
“If you thought you had ten pounds of pure seed and put that in the hydroseeder, you’re not going to get the coverage you expect, because seeding rates are specified in terms of pure, live seed. You actually need 12.3 pounds of seed, so you’ll need to get 2.3 pounds from another bag to meet the actual spec.”
Choosing the right protection for the seed is just as important as seeding. “Whatever your control is, mulch, straw mulch, or a rolled erosion control product, you want to pick the right one for the conditions,” says Hildebrand. “If it’s very dry, you want to pick a control that has the ability to absorb moisture and hold it. Or maybe it’s windy. I’ve done projects where we’ve worked along the runways of major airports. The propulsion from jet engines creates a lot of wind, so you need to use a strong tackifier, or a polyacrylamide that forms a ‘crust.’” “If there’s going to be a lot of pedestrian traffic, you can’t use netting, because people are going to walk across that and wreck it, by entangling the netting. There are so many alternative netting choices on the market today; you need to make decisions based on the end land use data.”
Silt jumps the fence
A lot of foul-ups can be made with silt fencing. “Sometimes we’ll see silt fence installed diagonally, across a slope, instead of on the contour,” said Mark Cavanaugh, CPESC, regional representative at Hanes Geo Components’ Livonia, Michigan office.
“So water will come down the hill, hit the silt fence, channelize, and run parallel to the fence, causing a severe erosion problem at the base, where it accumulates and causes a really big problem. You’re taking a sediment control device and doing more damage with it than had you not used it at all.”
“When I started doing erosion control work, 20 or 30 years ago, we just wrapped sites with silt fencing,” said Ross. “Of course, that was a complete failure, because we overextended their capabilities. But it was easy to put in, looked good, and we thought we were doing a great job. We finally understood that for every 100 feet of silt fencing, there’s only supposed to be about a quarter-acre of soil going through it.”
Just as bad as using too much silt fencing is not using enough. “You want to have enough to capture the sediment, based on the drainage area,” says Raghu P.V. Namburi, CPESC, CPSWQ, erosion and sediment control program coordinator for the Oregon Department of Transportation (ODot), Highway Division. “If the land is flat, you need to have some overflow area, so that the water isn’t ponding too much. There also needs to be a release point somewhere.”
Other silt fence mistakes include installing them in waterways, not trenching them in, and neglecting maintenance. If there is a lot of silt piled up in front of a fence, it should be removed. “You don’t want to have sediment piled up more than one-third the height of the fence,” said Namburi.
Gaps in the gabion
Gaffes can happen with gabions, too. “With gabion baskets, two things come to mind,” says Cavanaugh. “We’ve seen problems with improper stone sizing. The engineers will ask for a very specific stone size. If you don’t pay attention to that, you can either end up with lumpy gabions that are pretty unsightly, or you can have leakage of stones from the baskets because the stones are too small. That causes voids, and you’ll lose the structural value of the baskets.”
“The other mistake we see is not using a geotextile separator fabric beneath or behind the stones. You should always put that in there. If you don’t, soil will wash right through the baskets, because there are lots of open spaces.”
Common mistakes when using erosion control blankets (ECBs) and turf reinforcement mats (TRMs) include not using enough staples or stakes, stretching rolled products across soils leaving air gaps underneath, overlapping the blankets or mats in the wrong direction and not trenching the blankets or mats in at the top of slopes.
If you leave air gaps under your soil erosion fabrics, you can get undermining. “If you don’t trench them in at the top, water will get underneath the matting and start eroding,” said Namburi, “And you won’t notice it until a big gully forms.”
“After you’ve trenched at the top, you don’t just toss a big roll downhill; that’s what causes the problem,” he continues. “You’ll create a lot of air pockets. The most important thing is that the matting should be in contact with the soil below.”
“After you trench it in, you should roll the product down step by step, making sure you’re tamping it down and putting your staples in as you walk it down the slope.”
ODot advises staking or stapling mats down every three feet. That means rolling down three feet of blanket, staking, then rolling down the next three feet, not rolling the whole thing down nonstop. Usually, ECBs and TRMs will have markings on them indicating where you should put the stakes or staples. Installations should be re-inspected after every rainfall.
“When we’re talking about inlet issues, a lot of times we kind of forget what the problem really is,” says Hildebrand. “So we’ll put an inlet protection device on a curb opening to manage a bunch of water, but it’ll get plugged up immediately, because we don’t have enough source controls upland from that inlet to protect the soil particles from becoming detached in the first place.”
“So the inlet protection device gets overwhelmed and doesn’t get cleaned out,” continues Hildebrand. “Then, because we have standing water, somebody punctures it. Now the inlet control is breached. It’s a perfect example of putting a cast on a broken leg, but it’s the broken arm that’s really the problem.”
To screen or not to screen?
Screened loam (soil that’s passed through a fine mesh to remove any stones, roots or other debris) is often brought in for use on disturbed sites. Butch Goodwin, operations manager of Wrentham, Massachusetts-based Groundscapes Express Erosion Control Solutions, Inc., thinks this is a mistake, because the screening process destroys the natural fungi in the soil. These fungi produce glomulin, a natural glue. “The glomulin acts like a tackifier,” he says. “It helps hold all the soil particles together.”
“When they send the soil to a screening plant, it changes it,” Goodwin adds. “I’ve seen it on job after job; the loam you get is not like the native loam anymore; it’s been processed from somebody else’s dirt and can have a lot of clay in it. That’ll act like ball bearings on a hill, helping the soil slide off.”
Screening, according to Goodwin, also destroys the natural mycelium, the mass of branching, threadlike vegetative matter produced by fungi, which also helps hang on to soil. “The mycelium can stretch 100 times further than the roots can. It’ll hold back just about any slope. We’ve used it on 1:1 slopes out here. People can’t believe it until they witness it for themselves.”
Mistakes are going to happen, no matter how careful or educated one is. After all, we’re in the business of trying to control nature, a most powerful, and at times, unpredictable beast. When we make blunders, we just have to turn them into learning experiences. Says Ross: “I’ve learned so much more from my mistakes than from the good stuff.” If we pay attention, hopefully, we won’t make the same ones twice.