When bare ground is exposed during construction or other activities, the soil is prone to erosion when it rains. The erosion can have severe impacts on waterways and wetlands, creating muddy water that harms aquatic life and creates silt buildup. It's also an expensive problem in piped drainage systems. When the pipes fill with mud, there is reduced capacity to carry off storm water, which in turn can cause flooding in urban areas. In instances where the ground itself is contaminated, the contaminants can be carried into waterways through erosion.
The issue of storm water pollution was little regulated until the 1980's. Since then, the Environmental Protection Agency, under the auspices of the Clean Water Act, has acted to regulate discharges. States, counties and municipalities have enacted their own standards as well. There are watchdog groups reporting violations in many areas of the country, and fines for non-compliance are steep. In the early days of storm water erosion control, prevention was rudimentary, sometimes just some straw bales around water discharge points. It has since become sophisticated, with a multi-pronged approach to preventing muddy runoff. There are many products on the market designed to prevent erosion. The product considered to be the last line of defense when all else fails is silt fencing.
Erosion Control Basics
There are two top priorities in erosion control. One is to not allow it at all. This is possible if the ground is not left bare during rainy seasons. It can be seeded or covered in some way. Secondly, runoff water needs to be slowed down. Fast-running water carries sediment, and continues to erode the soil it is running through. Still or slow moving water cannot carry the particles, allowing them to settle out before they are carried off.
Most erosion control plans rely chiefly on these two prevention methods. There are many products and methods employed to achieve these ends. But what if it rains before the ground is protected, or the measures to slow down the water are overwhelmed? Sometimes in a downpour, there is still sediment being carried off in the water, despite efforts to prevent this. That is when silt fencing becomes most important in preventing stormwater pollution.
About Silt Fencing
A silt fence is a temporary sediment barrier. It is made from woven, synthetic materials that allow water to seep through, but do not allow larger silt particles to pass. The materials are selected to withstand the elements, and their specific properties are typically referenced in a jurisdiction's specifications. Silt fence is staked in place between the disturbed ground and the waterway or drainage system that it would flow to. Silt fence is designed largely to catch sheet flow over a fairly broad area. It is not meant to be used in high-flow situations, like across a ditch or stream.
Considerations for Design and Installation
Silt fencing should be installed crosswise across sloping ground, not up and down the hill, to slow down sheet flow. There is no reason to put a silt fence at the top of a hill or ridge.
Silt fences create ponds on the uphill side when it rains, desirable to allow the silt to settle. The fencing can be located to collect silt in a specific area by paying attention to the contours and slopes where it is placed. When it is time to remove the fencing, the silt will be where the ponds were, at low points. If silt must be removed to keep the fencing effective during the life of a project, this is easier to achieve if the silt is in limited locations.
Silt fences are not designed to be strong enough to hold back water like a dam. Care needs to be taken to not let ponding on the uphill side become too deep. Anything over a few feet could overwhelm the fencing.
If the ends of the silt fence are curved uphill to some degree, this may allow the ponding to be more effective. The water won't just run around the ends of the fence.
When it is correctly installed, the bottom of a silt fence is buried in the ground to prevent sediment-laden water from running under it. Grass or other ground covers are encouraged to grow on both sides, as vegetation is also a filter that traps sediment. It is best if there is a flat area, low area, or gentle slope on the side the runoff is coming from, to allow water to be contained long enough for it to drop the sediment particles.
When are Silt Fences Installed?
Silt fences typically go up before the ground is even disturbed, often at the perimeters of a project. It can be a jurisdictional requirement to not allow grading until some erosion control measures are installed. While earth moving is in progress, they are sometimes the only line of defense in place. This is why most mass grading operations are scheduled to occur in dry seasons. It takes time to complete the grading, then to install the other erosion control measures selected for the project.
The other measures can include wattles, which are long, straw-filled tubes staked to the ground; gravel bags, which act as filters and slow moving water; glue-like chemicals sprayed on the ground to hold particles in place; and fabrics, jute mesh, or other blankets to cover sensitive areas. Layers of straw are also often placed over bare ground to hold it in place. More extensive measures include the construction of holding ponds for runoff water, and pumping equipment designed to filter out sediments before releasing clean water back into the watershed. These methods become expensive very quickly.
Although three-foot-tall silt fencing can cost more than forty dollars a linear foot installed, it is still very economical compared to some of the other erosion control methods available.
Silt fences typically remain in place until the end, when the project is completed and permanent erosion control measures are in place. Generally, landscaping and planting are the final, permanent forms of erosion control.
Maintenance and Inspection
Silt fences are typically inspected for proper installation, then periodically to make sure they are not damaged. They need to be checked before predicted storms, during a storm, and after a storm. They can be blown over by high winds. Fast-moving water can penetrate underneath, or high water buildup can push a section over. Breached sections of silt fence can allow large amounts of sediment to pass through.
The fabrics of this fencing become clogged with silt during use, letting water pass through more slowly. A silt fence is a passive system, yet changes occur over time. These fences must be monitored during the length of a project. There are many specifications for silt fencing. Check the requirements of your local jurisdiction before purchasing and installing silt fence. The fabric, the type and spacing of stakes, and the method used to install it are likely to be governed in some manner. Know the rules where you are working.
- Photo Courtesy of Craig Garrison, Yuba City, CA