Grading and Excavation Contractor | Sediment, Erosion, and Dust at the Job Site

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When done incorrectly, sediment and erosion control can become a very expensive burden.

By Jerald S. Fifield

When reviewing a set of plans that requires the installation of best management practices (BMPs) to control sediment and erosion while construction activities occur, do you as a contractor ever ask the following:

Why should I waste my time trying to control erosion? Isn’t it a natural process?
Is there really a difference between erosion control and sediment control?
Will straw bales take care of the problem?
If I installed the BMPs, will I have to maintain them?

The answers to all these (plus numerous other) questions are connected to the statement: “You have to do all you can to minimize damage to the environment.” It is governmental mandates that every contractor is expected to follow. Surprisingly, when done right, sediment and erosion control can actually save contractors money.

Basic Definitions
As a contractor, have you ever gotten a set of “Erosion Control” plans and found a majority of the site surrounded with silt-fence barriers? Unfortunately, silt-fence barriers represent a sediment control measure that has limited use—and many misuses. It is necessary that the difference between erosion control and sediment control be understood.

Erosion control methods minimize the disturbance of soil by rainfall, flowing water, or wind. Unfortunately, the most effective erosion control method is to maintain existing vegetation, which may be a nearly impossible task for most construction activities.
When erosion occurs (which always happens), suspension of soil particles (called sediment) in runoff waters becomes prevalent. Thus, when runoff discharges from a construction site, the transportation of significant amounts of sediment often accompanies these waters. It is when deposition of sediment (sedimentation) occurs in an uncontrolled manner that major regulatory problems are created for contractors.

A major EPA regulatory goal is to minimize the discharge of sediment from active construction sites. A review of BMPs commonly implemented on construction sites to achieve that goal found that they fall into one of the following three categories: barriers; sediment containment systems; or erosion control.

Effective use of BMPs found in the three categories allows contractors to meet the EPA’s goal while saving money during construction activities. Ironically, the most (long-term) cost-effective method contractors can implement to minimize sediment from their site is erosion control.

Sediment Control Methods
Most contractors rely upon sediment control measures on their construction sites. Unfortunately, few sediment control measures are effective in reducing suspended particles in runoff waters discharging from construction sites. More importantly, these BMPs are usually very expensive to maintain. However, often there are few other choices available to contractors until erosion protection methods are implemented.

Sediment Containment Systems
Sediment containment systems (SCSs) are also known as sediment ponds, sediment basins, or sediment traps. They are the only effective sediment control methods that remove suspended particles from runoff waters when large discharge events occur and when upstream erosion control methods are not available. Even when properly designed, an SCS will only be effective in removing larger-diameter suspended particles when contained waters discharge from the structure.

It is for this reason that removal of accumulated runoff should always occur from the top of contained waters. Increasing the effectiveness of an SCS can be realized by injecting polymers into the incoming runoff waters. However, when polymer-laden waters discharge from an SCS, they must always be carefully monitored to ensure downstream environmental impacts do not occur.

Rock Barriers
Where concentrated flows exist, rock barriers are often found on designer’s plans as check structures located within drainage channels. Installation of these structures must occur in a proper manner to ensure runoff flows through a rock depression and not around the edges. In addition, rock must consist of a mixture of different diameter material (2–6 inches, for example) to ensure the capture of sediment.

The amount of sediment captured by rock barriers in a drainage channel is dependent on the height of material at the flow line. Thus, barriers with little height will capture only small amounts of sediment and vice versa. Rock barriers can also be used in front of (or around) inlets. However, they need to be composed of smaller diameter material (1–2 inches) and installed in a manner that ensures a gap exists at the throat for overflow waters.

Rock barriers should not be installed around inlets that are on a grade where diversion of runoff waters can occur. An alternative to rock or any other type barrier in front of inlets is that of inserts, which are geosynthetic “bags” designed to capture sediment from runoff waters.

Organic Rolls and Synthetic Barriers
Where sheet and some concentrated flows exist, organic roll barriers (aka fiber rolls) can be installed. Sheet-flow conditions exist when runoff flows over the ground in a wide path. Most organic barriers placed in the path of sheet flow may remove some sediment from runoff but will be limited in effectiveness because of their height.

Organic barriers can be composed of straw, coconut, or excelsior material. When properly installed, they are placed in a 4-inch-deep trench and held in place with wooden stakes.

If organic roll barriers are to be effective in drainage channels, it is important that the diameter be at least 12 inches, with preference given to 20 inches. Anything smaller than 12 inches will not be effective in removing sediment from runoff waters where concentrated flows exist.

Synthetic barriers can also be used for sheet-flow conditions. However, their use is more prevalent with concentrated-flow conditions, such as in a drainage channel or around inlets. When placed in concentrated flows, the effectiveness of synthetic barriers to remove sediment in runoff is limited by height.

Silt-Fence Barriers
These synthetic barriers are widely used on construction sites. They can be used for sheet-flow conditions and, when properly supported, can find use where concentrated flows exist (around inlets, for example). Trenching or slicing fabric material into the ground is required for proper installation. Additional benefits can be realized when fabric material is formed into a “J-hook” to create a small sediment trap.

A common misuse of silt fence barriers is their installation in drainage channels. If placement of a barrier causes runoff to destroy the barrier, then contractors should avoid installing them in drainage channels.

Only when silt-fence barriers are properly installed so that runoff flows over the top of the material, does not undermine the fabric, and does not flow around the edges should they be used in drainage channels.

Silt-fence barriers are commonly found around area (a.k.a. catchment basins) drains. As with placing these structures in drainage channels, proper bracing of silt fence barriers must occur where concentrated flows may happen. Also, their installation around area drains should only occur where sump conditions exist to avoid downstream flooding. Thus, their use in highway medians may be limited when the roadway is on a continuous grade.

Bale Barriers
Bale barriers are one of the most expensive BMPs to use on construction sites because of their installation and extensive maintenance requirements. In addition, since 1992, bale barriers have not been recognized by the EPA as an “appropriate structural measure” to remove sediment from runoff waters.

If contractors want to use bale barriers on a construction site, installation should never occur where concentrated flows exists. Their use in drainage channels, around inlets, and for inlet or culvert protection should be avoided.

Erosion Control Methods
Without a doubt, erosion control is the best method to control the discharge of sediment from a construction site. Methods range from “staging” the removal of existing vegetation, applying an impermeable barrier, to establishing vegetation on disturbed lands. All have their benefits—and corresponding costs.

Staging
Staging the removal of existing vegetation throughout the construction process may be viewed as an inexpensive erosion control BMP. This is especially true for such linear projects as roadway and utility construction. Instead of pioneering long stretches of disturbed lands that may be subject to erosion, maintaining existing vegetation will reduce the cost of sediment control measures.
Unfortunately, staging is often not practical where massive disturbance of lands is required, as in land development for subdivision or commercial properties. Thus, other methods have to be considered.

Impermeable Cover
Covering disturbed soils with an impermeable cover, such as Visqueen, prevents the movement of soil particles by rainfall or wind. Examples include temporary stabilization of small disturbance areas, such as home sites, stockpiles, or sections of hillsides. However, impermeable covers cause other problems, such as increasing runoff when precipitation occurs. Also, installation costs may be high and increased maintenance issues will occur with damage by wind, animals, and so forth.

Establishing Sod
When maintaining vegetation is not feasible, then contractors might want to consider planting and establishing sod. However, such an alternative requires contractors to have a readily available (and inexpensive) supply of sod along with adequate quantities of water in the form of rainfall or irrigation.

If a reliable source of sod is available, then it is possible to have nearly instantaneous vegetation establishment. However, such conditions may only be pertinent where precipitation is sufficient or irrigation exists.

When placing sod, it is important that sufficient time exists for the root system to establish itself in the underlying soils. Thus, placing sod in drainage channels may not be effective if the root system is not established and the material “rolls up” when runoff conditions exist.

Planting Seed
Growing grass from seed will eventually result in effective erosion control—once the vegetation becomes established. However, planting seed does not immediately provide erosion control by itself. Planted seeds have to germinated, grow within the soil, and establish a viable root system.

In humid regions, the germination process is usually fairly rapid. It is for this reason that planting seed by hydraulic methods is regularly successful. This “one-step” process can apply seed, soil amendments, and mulch in one application.

However, in semiarid and arid environments, vegetation establishment may take two or more growing seasons. Also, it is critical that seeds planted in a dry environment have good contact with the soil, which is something that a “one-step” hydraulic planting method may not provide. A “two-step” planting method overcomes this problem by applying seed (and perhaps soil amendments) first, followed by a hydraulic application of mulch. An alternative is to plant seed by “drill” or broadcast methods, followed by an application of dry mulch that is anchored to the soil.

When planting of seed occurs, contractors should collect “seed tags” to ensure what has been specified is correct. Also, many times it will be necessary to collect soil samples to assess whether soil amendments are necessary.

Mulches
Since sufficient water is not always abundantly available throughout the United States, establishing vegetation by seed may become difficult in arid and semiarid environments. Thus, use of mulch, erosion control blankets, or some other method that temporarily stabilizes the soil against erosion is necessary. Mulches provide temporary erosion protection until establishment of vegetation occurs. They can be applied as a dry method (e.g., using straw or hay) or as a hydraulic application. When mulches cover at least 80% to 100% of disturbed soils, then erosion protection can approach what is found when dense vegetation exists.
Since wind can remove straw or hay mulches, it is important that dry mulches be anchored to the soil by “crimping” long fibers into the soil or applying a tackifier to “glue” material in place. Hydraulic mulches can overcome this problem, since the slurry is not as readily affected by wind.

Hydraulic mulch consists of wood fiber, paper, or other organic materials and can be applied as slurry on disturbed lands. The advantage of hydraulic mulch is its ability to cover disturbed ground in an expedient manner and with a minimum of effort.

Rolled Erosion Control Products Rolled erosion control products (RECPs) also provide a temporary or permanent stabilization of disturbed soils. Materials composing RECPs range from organic to inorganic materials. They are also known as erosion control blankets.

Using RECPs on hillsides requires proper installation. This entails developing a seedbed, “anchoring” the material to the slope, ensuring good contact with the soil, and using lots staples. Thus, using RECPs on hillsides can be very labor intensive. However, once these blankets are properly installed, soils are protected against erosion, vegetation establishment usually occurs, and maintenance issues are minimized.

When an RECP is processed into a mat of sufficient thickness, a turf reinforcement mat (TRM) is created for use on very steep hillsides and for drainage channels. TRMs impart immediate erosion protection and can enhance vegetation establishment within drainage ditches, steep slopes, embankments, and along shorelines. They can provide an alternative to riprap by “soft armoring” drainage channels, thereby creating short-term and long-term functionality by reinforcing vegetation during and after establishment.

As with hillside RECBs, selection and use of TRMs must include proper design and installation. Thus, good seedbed preparation, correct installation, and proper material selection must occur to ensure successful vegetation establishment.

Wind-Erosion Control
Wind erosion is one of the most difficult construction site issues for contractors to address. Once land is disturbed, soil particles are susceptible to movement by wind. With water erosion, suspended particles move downstream with runoff. However, wind erosion suspends particles in the air so that they may travel any direction the wind blows. Consequently, minimizing wind erosion is more difficult than controlling water erosion.

Studies have shown that the transportation of windborne particles occurs within 6 to 12 inches of the ground. Thus, maintaining sparse vegetation, planting such temporary crops as wheat or oats, or keeping plant debris on the site can be very effective in reducing windborne particles.

Unfortunately, maintaining vegetation and plant debris is not always feasible on construction sites. Thus, keeping disturbed soils in a roughened condition is a viable alternative when low wind velocity exist. Roughening the soil to produce ridges perpendicular to the prevailing wind can also reduce soil losses by approximately 80% when ridges are about 6 inches in height.

Why not using water for dust control? Fewer wind erosion conditions develop when soils are wet, and applying water on a limited basis—on haul roads, for example—might be a practical solution. However, applying water over a large disturbed area is not always feasible and, more importantly, can be very expensive. Also, applying water results in soil compaction and a smooth surface, which creates ideal conditions for wind erosion once drying occurs.

Finally, the most expensive method for removing suspended particles created by wind erosion is barriers. If barriers are to remove suspended, windborne particles, they must have openings that allow for passage of the airstream, which allows for material deposition. This means that silt-fence barriers provide little, if any, benefit in removing suspended, windborne particles. Besides, the force of wind usually destroys silt-fence barriers.

Summary
Contractors have to spend money to protect the environment by reducing sediment leaving their sites via runoff or wind. The least expensive method (in the long term) is to continually retain and/or implement erosion control techniques. The most expensive techniques involve those associated with trying to remove soil particles already suspended in runoff or wind. Besides, sediment control methods are not very effective—just expensive.

Jerald S. Fifield is president of HydroDynamics Inc. in Parker, CO.

GEC - February 2008

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