Keep Your Priorities in Order
In erosion and sediment control, prioritizing priorities is fundamental for effective management. One crucial aspect of this hierarchy involves the direct relationship between erosion control measures and sediment control. While silt fences ARE valuable tools in their specific role, it’s essential to recognize their limitations. Silt fences can only comprehensively manage some erosion- and sediment-related challenges on a work site. Instead, a multifaceted approach combining erosion control measures with sediment controls ensures a more robust and reliable strategy for minimizing environmental impact.
Erosion management, centered around reducing soil erosion through proven stabilization methods, is pivotal in the broader framework. By establishing reduced erosion as the primary goal, the demand on subsequent sediment control measures, including silt fences, is significantly reduced.
Erosion management involves minimizing the risk of soil erosion by implementing stabilization measures. At the same time, sediment control encompasses strategies to limit sediment movement. By reducing soil erosion and sediment movement through stabilization measures, subsequent steps in the framework encounter less pressure. This strategic approach establishes a foundation for more efficient, streamlined, and effective management.
Right Time, Right Place
Over time, silt fences have gained a poor reputation as ineffective and unsightly solutions to sediment control, but not everyone agrees. Most experts insist silt fences are an effective safety measure when used with other best management practices (BMPs), but only when installed properly. Most significant issues with silt fence installations often stem from errors related to placement and installation methods.
Beginning a sediment control job requires properly analyzing terrain, slope, and water flow patterns to avoid disappointment. Optimal deployment of silt fences is essential since improper positioning can render the fence useless.
- Begin by analyzing the contours of the construction site, carefully determining where fences should start, run, and end to maximize their effectiveness.
- Plan to section the fences into practical runs of no more than 100 ft, each with an independent sediment storage area. Longer runs tend to concentrate large amounts of water in a relatively small area, which can quickly overflow the fence.
- Typically, the drainage area above any 100’ of fence should not exceed a quarter of an acre. However, even if this rule of thumb has been followed, the drainage area is too large if fences are overtopping after a moderate rainfall event. Install additional fencing to avoid undercutting, washouts, and broad fence failures in this case.
- Configure silt fences in areas with high sedimentation potential in a series of J-hooks, where the curved end opens upslope. These hooks create storage areas that allow the fence to capture and retain runoff long enough for the sediment to settle out.
- A silt fence should be positioned so that the lowest point of either end of a segment is higher than the top of the fence at its center point. This arrangement prevents water from flowing around the edges and causing more erosion. Instead, water will accumulate in the center and may eventually flow over the top of the fence. That’s not an ideal situation. Still, as long as the silt fence remains standing, the water flowing over will have already dropped the bulk of its sediment.
- Silt fences should not be placed in the channel of a waterway. However, they can be deployed parallel to the watercourse to capture and filter out silt before the runoff enters the channel.
The Path of Least Resistance
Water always travels the path of least resistance, so the fundamental purpose of a silt fence is to resist. When water flows down a slope, it prefers to travel in a straight line, so when a silt fence is in the way, it will seek to go through it, under it, over it, or around it until it ultimately finds a way. When silt fences fail, the cause is often traced to water traveling under the barrier. This situation can happen when a fence has been placed over the top of the ground, when it has been buried only a few inches, or when the disturbed soil hasn’t been adequately compacted. In each case, water will flow under the fence and through any disturbed soil, taking silt and sediment with it. It’s called scouring when enough sediment is carried away to create channels. A scoured area may require additional stabilization as it becomes part of the erosion problem instead of contributing to the solution.
Static Slicing
Static slicing machines operate by pulling a narrow blade through the soil to create a slit, usually about 12” deep, and inserting the fence fabric in a single pass. The unique blade design moves the soil slightly upward and out of the slit rather than pressing it sideways and compacting the soil horizontally. This vertical disruption creates optimal conditions for compaction on both sides of the fabric, enabling a more secure anchor for the silt fence fabric. A tractor wheel is rolled along both sides of the slit several times, which ensures comparable or greater compaction than the original undisturbed soil. Vertical compaction is especially effective in this application because it reduces the air spaces between soil particles, preventing excessive infiltration. Secure staking is an easy process since the surrounding soil is left undisturbed and firm after static slicing.
Trenching
Trenching is a time-tested and well-established method for burying the bottom edge of a silt fence underground, but it has some distinct disadvantages. Trench machines generally excavate 6” deep and 6” wide. Still, they must be turned as they move to follow contours and bypass obstacles. The soil in these wider excavations where a trencher has turned cannot be compacted as effectively, leaving these locations more prone to infiltration where water is encouraged to seek pathways under the fence, ultimately leading to erosion and washing out sediment previously detained.
Conclusion
By viewing silt fences as integral components of a comprehensive erosion control strategy, construction professionals can harness their potential to shield their work sites and clients from the negative environmental impacts of development. In this pursuit, thoughtful planning, adherence to regulatory standards, and ongoing maintenance are key elements in maximizing the efficacy of silt fences. As the world focuses on the evolving demands of sustainable development, it’s clear that sediment fences, as part of a well-designed erosion control plan, represent a proactive and responsible approach to sustainable construction practices and environmental stewardship.
