Jul 27, 2006

Effective & Environmentally Friendly

Erosion control and sediment control go hand-in-hand. Consequently, many erosion control professionals are also involved in controlling sediment, which has become a major challenge at construction sites. As enforcement of the regulations for strict sediment control practices has increased, the demand for sediment control devices has risen. Various types of sediment control devices are now available, and more new and improved sediment control devices are introduced each year.

Standards, specifications and test procedures for erosion control products and applications have advanced significantly in recent years. However, because the sediment control industry is fairly new, such practices for sediment control products and applications have not yet reached comparable levels. As it was with erosion control products, sediment control products will go through phases of industry acceptance before well-established standards, specifications and test procedures are developed.

Typically, when there is no field performance or test data to support a new erosion or sediment control product, a specifier, contractor or project owner has to rely on information provided by the marketing literature. Often, the most suitable product for an application is identified through trial and error.

Fiber roving systems (FRS) for controlling erosion in channels and slopes offer an example from the erosion control industry. FRS consists of spraying asphalt emulsion on the soil surface and then spraying synthetic fibers on top. FRS was introduced over 10 years ago with a strong marketing program. Initially, thousands of square yards of FRS were applied. Over time, alternative products were developed and end users began favoring them over FRS. As a result, usage of FRS declines over the years.

Tables 1 and 2 (S18) show annual erosion control product usage for Georgia Department of Transportation (GDOT) construction projects for the past six years. The data show significant decrease in FRS (bituminous treated roving) usage during the past five years. On the other hand, data show significant increases in erosion control mats and wood fiber blankets usage. Within the GDOT, sprayed on mats or bonded fiber matrix product is referred to as wood fiber blankets. If end users were properly educated about the advantages and disadvantages of FRS at the beginning, most probably would not have used it in the first place.

In addition, data on the tables shows that use of TRM also experienced a significant decrease in the GDOT construction projects. Once again, this may be due to end-user education on erosion control products and identifying that there are many erosion control applications where successful erosion control can be achieved by using organic products, where in the past permanent products such as TRMs were used. Similarly, with time, end users of sediment control products will identify those best suited for their applications.

Important features

There is a major difference between erosion control products and sediment control products. Erosion control products are used to control erosion in either the short term or long term, depending on the project. For example, degradable blankets control erosion until vegetation can become established to control erosion permanently. Permanent turf reinforcement mats (TRMs) control erosion after installation and assist vegetation in controlling erosion permanently once vegetation is established. Erosion control products are not required to be removed at the end of the project.

On the other hand, most sediment control products in construction applications are used on a temporary basis. The function of a sediment control product is to prevent sediment from washing off a construction site and into nearby water bodies. At the end of the construction project, permanent measures are installed to prevent erosion. This eliminates dislodging of sediment that can wash off the site to pollute water bodies. It is important to recognize this fundamental difference—the temporary use of sediment control products and the permanent use of most erosion control products—in the development and application of these two types of products.

Selecting a device

Efficiency of a sediment control device relates to how well it blocks or traps sediment while allowing sediment-free water to pass through, and the length of its functional life. Failure to account for functional life of a sediment control device could lead to serious problems. If a sediment control device performs well for a short time and gets clogged with accumulating sediment, it can quickly lose its filtering capability and fail. This failure can be flooding or collapse of the device or both.

On the other hand, if the device fails to trap any sediment it’s of no value. If the sediment control device is designed only to reduce the flow velocity and not to filter sediment through it, flow leaving the device should create no downstream or down slope erosion. Until proper test methods and specifications are developed to measure the efficiency of sediment control devices, it is important to have some general idea of how the device traps sediment.

Eco-friendliness of a sediment control device pertains to how well it blends with the environment during and after its use. Since most sediment control devices are designed for temporary use, they are required to be removed and disposed of at the end of the project.

In terms of eco-friendliness, it makes sense to use biodegradable sediment control products if possible. Most sediment control devices used as check dams, continuous barriers and diversion dikes get buried as they accumulate sediment. In general, all buried sediment control devices are required to be removed unless they are 100% biodegradable.

Removal of these devices is expensive, and disposal takes up limited space at landfills. Also, removal of a sediment control device disturbs the sediment deposited on and around the device. This disturbed sediment can be easily washed away from a rain event. In addition, most of the biodegradable sediment control products create an aesthetically pleasing appearance in their applications .

Cost of a sediment control device can be divided into direct cost and hidden cost. Direct costs of a sediment control device include costs of the actual product, installation, maintenance and reusability. Material costs of some traditional sediment control products, such as hay bales, are less than many manufactured products. But the installation and maintenance costs of hay bales are fairly high.

Past experience shows that products with low material cost and high maintenance cost frequently fail in the field. Each time a sediment control product fails, it produces irreversible environmental damage by allowing sediment to move away from the site and, in some instances, into nearby water bodies. Such harm to the environment can be identified as a hidden cost of the failed sediment control product.

Similarly, sediment control products that require disposal in a landfill entail a hidden cost in the form of the space they occupy in the landfill. Other hidden costs can include the extra work involved with removing and/or transporting products to the landfill. Usually, these hidden costs are passed on to the taxpayers.

Eco-friendly sediment control

Based on the development and use of degradable products in the erosion control industry and recent trends, eco-friendly products are likely to attract much more attention from end users in the future. Proof for this can be seen in the data presented on Table 1 and 2.

In the GDOT, construction projects square yardage of erosion control products usage increased 448% (Table 2) compared to usage in 1999. At the same time, usage of non-degradable products went down and natural fiber product usage for erosion control recorded significant increases. Organic fiber erosion control blanket usage increased to 182% and BFM increased to 337% (Table 1).

Therefore, it is sensible for sediment control product manufacturers to consider eco-friendly raw material in developing their products. The availability of efficient, eco-friendly and cost -effective sediment control products will encourage greater use throughout the industry.

Currently, eco-friendly sediment control products are attracting increasing interest by end users. The raw materials used in these eco-friendly sediment control products are mainly natural fibers, such as straw, wood, jute and coir (coconut fiber). The type of raw material used in these products is directly related to functional life of the products. For example, those made of straw have shorter functional life than coir-based products.

Reusability is also related to functional life. Most coir-based sediment control products are reusable while straw-based products are a one-time use item. On the other hand, straw products cost less than those made of coir. Therefore, end users should identify what is best for their needs. Products manufactured from natural raw materials, like straw and coir, also provide a market for abundant byproducts.

Check dams in construction sites

Every development project involves disturbing soil during the construction phase. There are various check dams used in these sites to trap and retain the sediment move with water during rain events. Some of the common check dams are silt fences, hay bales and rock.

Numerous deficiencies exist in these check dams in performance. In Figure 1, silt-fence check dam failure is due to water ponding behind it and water flowing over the top of it, which eroded the downstream soil. Hay-bale check dam in Figure 2 washed away from the flow of water where failure was initiated with undercutting. Figure 3 is a failed application of a wood fiber log in a check dam application. As it appears, water flowed underneath the fiber log and washed soil under it.

Fiber logs and wattles are good in sediment control on slopes where no concentrated flows exist. In field applications it is very difficult to install fiber logs or wattles as a check dam in a flow channel in order to resist the flow. This is because water ponding behind the fiber log or wattle finds its way to flow under it in between anchors since anchoring of the log or wattle is not continuous. Therefore, fiber-log check dams in concentrated flows such as channels have a potential for failure due to under cutting. In addition, they also tend to have severe downstream erosion from the over-the-top flow, which ultimately assists undercutting by removing downstream soil.

All of the above described failures can be eliminated with a check dam that allows water to pass through at a faster rate while trapping sediment; protects downstream soil from water passing through and over the top of the check dam; has a means to avoid undercutting by the water ponding in the upstream; and is one single elongated unit instead of a number of smaller units butted together. Furthermore, if such a check dam is made of durable yet biodegradable natural material, it delivers added advantages relating to environmental friendliness and may not be required to remove at the end of the project.

Preventing sediment from entering water resources such as lakes and streams in construction projects is critical for protecting water quality. However, to stop sediment moving out of construction site effectively, a check dam must have features as described previously.

For example, the check dam in Figure 2 is made of 100% coir. It lasts a few years in the field applications. It has a means to anchor, which also stops possible under cutting from the water ponding in the upstream. It has a means to protect downstream soil surface from water flowing over the top as well as through. It actively allows water to pass through its body while trapping sediment, reducing the potential for ponding.

Figure 3 shows an application of the feature coir check dam in a channel where the ultimate goal is to develop a vegetated waterway. Figure 4 is an application of feature coir check dam in a construction site where it performed as needed by accumulating sediment during rain events. In both of these applications, the check dams can be left at the site for natural degradation.

Figure 5 shows cross sections of how the feature coir check dam can be installed in a construction site. As illustrated in the figure, upstream apron is used for anchoring, and it also eliminates undercutting from water ponding in the upstream. Other apron in the downstream protects erosion due to water flowing over the top and through the device. When high flow volumes are expected, use of longer downstream apron extends the protected length immediately after the check dam.

This feature coir check dam was developed to address the main deficiencies that exist in the majority of the check dams in the construction sites. It will improve the quality of construction through better sediment control with minimal negative effects to our environment.

About the author

Santha is CEO of RoLanka Intl., Inc. She can be reached at 770/506-8211 or by e-mail at [email protected].

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