Jan. 13, 2014 03:33

Erosion Control Products

Traditional and Green

IN THE WORLD OF EROSION CONTROL, we divide the kinds of products that are available into two types, “traditional” and “green.” But these distinctions are becoming increasingly blurred. After all, erosion control, in and of itself, is a green practice.

What you’re going to use on a particular job may already have been specified by the civil engineer who designed the project. Local, state and federal requirements also play a role. Where you do have a choice as to what best management practices (BMPs) to employ, whether they be green, traditional, or a blend of both, will depend on a number of factors.

Aesthetics are increasingly important to people, as well as managing the water flow and dealing with the steepness of the slope.

To keep the area as green space, you’ll want to use a solution that will help keep the soil in place, perhaps a turf reinforcement mat (TRM). After that, you look at which approach is most cost-effective.

Nothing is obvious when you’re doing erosion-control work. There are dozens of innovative products that work extremely well, but there’s no one-size-fits-all. All of them have their pros and cons; you’ll have to decide what to use and when.

Here’s a look at some of the erosion control products that are currently available. Many of the products that will be discussed here can be considered both traditional and green, in that they promote the growth of vegetation, are made from recycled materials, or both.

Hydroseeding and hydromulching

Both traditional and green, this is the process of spraying seed mixed with mulch, fertilizers and tackifiers, under pressure, over a disturbed area so that vegetation can eventually take over the job.

Mulch material can consist of wood chips, wood fibers, straw, paper, grass, or hay. It can be sprayed either alone or mixed with seed. Mulching aids plant growth by holding the material and topsoil in place. It also retains moisture, insulates plant roots against extreme temperatures, and prevents birds from eating seeds.

Large areas can be covered in a short period of time with low labor costs, making this method quite economical. Newly seeded areas may need to be protected by overlaying temporary biodegradable products to further protect the seed from blowing or washing away before the new plant life is established.

Silt fences

A familiar sight at construction venues, silt fences are temporary sediment barriers made of porous geotextile fabric, held up by wooden or metal posts driven into the ground.

They’re popular because they’re low cost and easy to remove. They’re also quite effective; a single 100-foot run of silt fence can hold 50 tons of sediment in place. When they don’t work, it is usually due to poor installment or maintenance. Filter socks and berms These are often used as alternatives to silt fencing. They consist of geotextile tubes, or “socks,” filled with coir, compost, sand, shredded tires, plant trimmings, biosolids, or manure. Filter socks not only function as erosion barriers but also have the ability to absorb sediment and pollutants.

They can be non-biodegradable and intended as temporary erosion control measures, meant to be removed when no longer needed. Other types are partially or completely biodegradable, intended to be left in place and vegetated. For this reason, they’re considered green.

Vegetated filter berms are normally left right where they are for long-term filtration of stormwater as a post-construction BMP. Unvegetated socks or berms are often broken down once construction is complete. The compost can then be distributed around the site as a soil amendment.

Fiber rolls or straw wattles

Similar to filter socks and berms, these tube-shaped devices are filled with or entirely composed of straw, flax, rice, coir or compost. The rolls are held together with photodegradable polypropylene netting or 100 percent biodegradable materials like burlap or jute.

These are commonly used for stabilization of long or steep slopes, and are often combined with other BMPs such as straw mulch, erosion control blankets (ECBs), hydromulch, or bound-fiber matrices.

They help to slow, filter, and spread overland flows, filter and capture sediment and minimize rill and gully development.

Inlet protection devices

These devices keep soil and debris from entering storm drain drop inlets. These measures are usually temporary and should be implemented before a site is disturbed.

Porous geotextile fabric barriers are commonly used on construction sites. They shield against sediment, soil and debris, slowing runoff, while allowing water to continue flowing into drains. They must be maintained regularly, however. If they are clogged they no longer function and can even cause erosion in unprotected areas.

Check dams

These are usually small, temporary structures constructed across swales or channels to slow the flow of water. They are typically constructed of rock, sandbags, logs or treated lumber straw bales. They are most effective when used with other erosion and sediment-control measures.

Articulated concrete block

When the going gets tough, the tough get hard armor. This is what’s needed to protect against erosion and scour in the most severe hydraulic conditions. Hard armor includes retaining walls, gabions, riprap and concrete in various forms.

Concrete products for channel, stream and shoreline applications are available from a number of different manufacturers. These products include interlocking matrices of cellular concrete blocks of uniform size, shape, and weight; concrete armor units designed to interlock into flexible, highly permeable matrices; and concrete step overlay protection systems for embankment dams and spillways. These are just a few of the concrete products available for controlling erosion.

Cable-reinforced articulated concrete block

There are several of these revetment systems available from various companies. In general, these are mattresses made up of concrete blocks poured and hardened around woven, stainless steel or poly cables, for added strength. The blocks can be made with open or closed cells; those with open cells can be vegetated. The open cells also help release hydrostatic pressure.

Riprap

Riprap consists of layers of large stones, often granite, used in areas of concentrated runoff. (Granite or other igneous rock is preferred for its durability and resistance to freeze/thaw cycles). It’s used to stabilize cut-and-fill slopes and streambanks and to line the sides and bottoms of channels. A geotextile filter material or layer of gravel is used as a base, to prevent underlying soil from moving through the riprap.

However, riprap can be unstable on very steep slopes, especially when rounded rock is used. For this reason, the U.S. Environmental Protection Agency doesn’t recommend it for use on slopes steeper than 2:1. Riprap can actually increase erosion when it’s used improperly. It can also be expensive.

Gabions

You could describe gabions as riprap in jail. Gabions are simply wire cages filled with rocks or broken hunks of concrete, and sometimes, sand and soil. They’re traditional hard armor products, used on shorelines, streambanks or slopes, usually in situations of high shear where swiftly-flowing water would rip out just about any other type of device.

They can be used as temporary flood walls, or to direct the flow of flood water around vulnerable structures. Retaining walls are sometimes built out of gabion mattresses.

While gabions are a traditional form of hard armor, they can also be green when filled with soil and vegetated. Eventually, plant material completely conceals the wire and rocks, leaving strong, stable streambanks that are resistant to erosion.

Gabion baskets, unlike loose riprap, can be stacked in various shapes. Their flexibility lets them take ground movement in stride. The most common wire material used is galvanized steel, but PVCcoated wire and stainless steel are also used.

Retaining walls

These sturdy permanent hard armor solutions are usually built from articulated concrete block, stone, or steel, and are particularly useful on steep slopes. It’s especially important that proper drainage be installed, as the main cause of retaining wall failure is a buildup of hydrostatic pressure behind the wall.

Retaining walls can be considered green when they can be fully or partly vegetated, or assist in the growth of vegetation. Living walls, made of various types of materials with growth medium inside each block, are a great example.

Foam block systems

Yes, retaining walls can even be made out of foam. At least one company makes blocks out of expanded polystyrene, a thermoplastic, closed-cell, lightweight, rigid-foam plastic. These are good in situations where weight is an issue.

Cellular confinement systems

These consist of flexible highquality, high-strength polyethylene (HDPE) strips ultrasonically welded together. When expanded, they form large honeycomb sections that then can be filled with native or imported granular materials. These systems stabilize soils and give uncohesive soils cohesion.

Applications include slope, channel and shoreline protection. They can also be used to build vegetated retaining walls and can strengthen soft soils in load support applications to improve pavement design structural numbers.

ECBs and TRMs

These two types of products are sometimes confused, or spoken of interchangeably. Erosion control blankets (ECBs) and turf reinforcement mats (TRMs) are similar in that they are both mattress-style products with some overlap in the materials used to make them.

Both provide erosion control and protect seeds and seedlings from washing away before they can germinate. The main difference between them is that ECBs are temporary, while TRMs are permanent. Both are widely used.

ECBs are used in applications where the plant material, once well established, will be able to withstand hydraulic forces without further buttressing. Thus, they are made of materials that will eventually photo- or biodegrade.

TRMs are permanent solutions that provide continuous reinforcement to the vegetation, and don’t photo- or biodegrade. They’re typically used in severe and critical applications where the hydraulic forces are such that unreinforced vegetation alone wouldn’t be able to withstand them.

Eventually, both ECBs and TRMs “disappear,” as the plant matter completely covers them. ECBs completely go away, while TRMs stay behind, providing their layers of protection quietly, behind the scenes. Both ECBs and TRMs are green, aesthetically pleasing erosion-control solutions.

There are temporary, biodegradable ECBs made of straw fiber with netting on one or both sides, expected to last approximately 12 to 18 months. These materials are good for moderate to steep slopes and midsize channels.

For steeper channels and faster flows, there are ECBs made of straw and coconut fibers, excelsior (wood) fibers and 100 percent polypropylene jute. Most are bundled with biodegradable polypropylene netting. Some ECBs last up to five years. TRMs are generally made from coir, jute, wood fibers, plastics, paper, or cotton bound by photo- or biodegradable geotextile netting.

Newer products include TRMs made from 100 percent recycled plastic bottles, with polyester fibers stitched together by two layers of UV-resistant polypropylene netting, and revetment mats made of 100 percent UV-stabilized natural rubber, with drainage holes throughout their surface.

As you can see, there are a lot of great products out there, plus more that we haven’t mentioned. Whether we use a green solution or not on a particular job probably isn’t because of a lack of products to use, but may be due to a lack of knowledge about those products. It’s up to the specifying engineer and the manufacturer to work together to educate contractors about the benefits of individual green solutions and the installation techniques involved.

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