July 19, 2010 05:16

Silt Fences & Wattles


THOSE INVOLVED IN ANY ASPECT OF THE CONSTRUCTION INDUSTRY WILL LIKELY understand the importance of erosion control, but what they might not know as much about is sediment control. There are many different measures one can take to provide effective erosion control, but none of these will ever ensure 100 percent protection from erosion. Here’s where sediment control comes into play.

When soil is eroded, the loose sediment will wash away, often finding its way to the closest water source. Sediment control aims at stopping such sediment from getting into the water and polluting it. Think of sediment control devices as the last line of defense against erosion.

Among the most widely used of these devices are silt fences and wattles. They are installed in and around construction sites, with the intent to prevent sediment from escaping the area for the duration of the construction process. In other words, silt fences and wattles are both temporary sediment control measures. Nonetheless, they’re effective.

“They’re absolutely necessary to keep sediment flow out of waterways and storm drains,” says Andrew McGann, product knowledge professional for Earth Saver Erosion Control Products, Yolo County, California. “If a storm drain gets clogged up with sediment, it can overflow and flood the whole area, and no contractor wants to deal with that kind of mess.”

Part of what makes silt fences and wattles so popular is their cost-effectiveness and ease of installation. It doesn’t take a particularly thick wallet or a whole lot of know-how in order to get these sediment barriers installed. Of course, there are always a few tricks you’ll need to follow in order to get them working properly (trenching is one area some installers will falter on, especially when they’re in a hurry and don’t dig deep enough), but all in all, silt fences and wattles are as practical a sediment control method there is.

So what exactly are these devices? What differentiates a silt fence from a wat tle?

And what areas are they best suited for? To answer those questions, let’s take a look at each method.

Silt stoppers

We’ll talk first about silt fences.

These barriers are comprised of pieces of permeable, geosynthetic fabric which are trenched and staked into the ground. Once erected, they will slow the flow of water and trap the sediment, essentially creating a catch basin from which you remove the collected sediment.

“They’re the old standard product in this industry,” says Rich Quinley, CEO of World Textile & Bag, Roseville, California. “Silt fences have been around forever. They have the advantage of being a much taller barrier than wattles are. They’re easy to see, and have the ability to handle more volume of water at the point of blast.”

Silt fences are best used around the perimeter of a construction area. By establishing perimeter control, you’re preventing sediment from leaving the site. You’ll commonly find silt fences around long, spacious areas or at the bottom of slopes.

Installing silt fences is a fairly simple process that can be done by hand. First, you need to dig a trench six inches deep or more. Then you lay in the fabric, backfill the dirt, make sure it’s compacted and, finally, install posts to keep the fence in place.

It’s crucial that each of these steps be followed in exactly this order. A common mishap installers will fall into is to install the posts before having backfilled the soil. If this is done, you risk impairing the stability of the fence. After all, if you stake a fence into loosely compacted soil, it runs a fairly high risk of toppling at the first contact of high water flow.

If this process sounds too complicated, installation machines offered by companies such as Tommy Silt Fence Machine and Burchland (which will attach onto either a tractor or a skid steer), make it that much easier to install silt fences. These machines slice directly into the soil using a specially-shaped blade, simultaneously trenching, installing the fabric and backfilling all in one fell swoop.

“Our silt fence machine works twice as fast as trenching by hand,” claims Aaron Burchland, president of Burchland, Gilman, Iowa. “It lets you cut your installation time in half, and its slicing function also makes the soil much more compact.”

Jonathan Koepke, who works as the soil erosion and sediment control division manager for the Sycamore, Illinois-based landscape contracting company EnCap, says he wouldn’t install a silt fence any other way. “We use both a Burchland and a Tommy Silt Fence Machine,” Koepke says. “Not only do they help us install a better, sturdier product, but they also let us work faster. We had a job where we had to trench 1,950 feet. With a threeman crew, it took us only three-and-a-half hours to complete.

That’s a fair amount of work to get done in a short time.”

When to use wattles

Wattles are slope interruption devices made to slow the flow of water. They’re made of a series of tubes filled with a type of mulch, such as straw (the most common), wood excelsior, coir or other.

These products are great to use in areas where aesthetics are a concern. Their compact structure makes them difficult to see from a distance, and they’ll often blend right into the soil, buried in the vegetation, which acts as an added camouflage. They’re also biodegradable, meaning you never have to remove them, as they’ll disintegrate over time.

They’re generally installed on hillsides. In order to work effectively, it’s best to install multiple rows of wattles linked side-byside. Just one row of wattles isn’t nearly enough. If there’s a heavy downpour, most of the water will pass right over it. But if you install several rows of wattles on a slope, those rows will work in conjunction with one another, each of them catching as much water as they can. The grass in between the rows of wattles will work to reduce the velocity of the water and filter some of the sediment.

“Wattles can be categorized as ‘slope interruption devices,’” says Quinley, “in that they’re interrupting the flow of water and allowing for sediment to get trapped behind them.”

There are several strategies you should be aware of when installing wattles. First, you will want to make sure that water only flows over the top of the wattle; never underneath. In order to do that, make sure you smooth the soil out at the point of installation with a shovel or a pick. “Basically, we create a little groove in the soil to put the wattle in and then we stake it,” says Koepke.

You’ll also want to make sure the lowest point of the wattle is in the center of the trench. This will help prevent the water from travelling around the sides of the wattle and causing erosion.

But that’s not all

Sediment control doesn’t have to rest exclusively in the hands of silt fences and wattles. Although cost effective and easy to install, these devices can create a fair amount of maintenance in certain circumstances.

Say you’re working on a large construction site where there’s a heavy amount of traffic constantly through the area. In their hurry, your drivers fail to notice a row of wattles or a silt fence and drive right over them. These products will then need to be replaced. This means buying more materials and paying for more labor for installation. And because there’s such a heavy bustle of traffic on that site, there’s always the possibility that these devices will be accidently run over and destroyed all over again.

There are manufacturers out there who recognize this dilemma. World Textile, for example, offers a flexible sediment control barrier which they call the Heavyweight Wattle. This device is designed to rebound back into its original shape every time it compresses, so you can drive over it repeatedly. “This is very helpful in terms of accessibility,” said Quinley, “because drivers don’t have to worry about finding a way around them.”

Envirotech Biosolutions in Honolulu, Hawaii, has developed a product meant to act specifically as a silt fence substitute. Called the BioSock, this device is made up of a tube which is filled with compost. Weighing in at 10 pounds per foot, this product can also be run over repeatedly.

“Silt fences can be destroyed by all sorts of things, such as ultraviolet damage, high winds, high flows, improper installation and vehicles,” says Alan Joaquin, president of Envirotech. “One of the things our product was designed to do is withstand those kinds of things.”

In addition, BioSock will not only filter stormwater as it passes through it, but the microorganisms living inside its compost will break down pollutants such as heavy metals, pesticides, petroleum and microscopic bacteria. “Ninetynine percent of the time on these construction sites, you’re dealing with situations where you’re going to have very fine particles passing through your sediment control barriers,” says Joaquin. “Traditional wattles and silt fences can absorb fine sediment, but it can’t destroy it. This product does just that.”

There is no cure-all to sediment control. There are many different types of measures you can use to trap and control stray chunks of soil as they get carried away by stormwater. It’s your responsibility to know which types of measures work best in any given situation. Silt fences and wattles are just one piece of that puzzle. They each perform similar tasks, but in different ways. It’s only a matter of knowing how to best utilize these functions.

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