What is silt fence used for?

A silt fence is a temporary sediment control device used on construction sites to protect water quality in nearby streams, rivers, lakes and seas from sediment (loose soil) in stormwater runoff. Silt fences are designed to retain sediment on the construction site to prevent it from leaving the site and polluting nearby water bodies. They are an important best management practice (BMP) used to control erosion and sedimentation.

What is a silt fence?

A silt fence consists of a geotextile fabric attached to wooden stakes that are driven into the ground. The bottom of the geotextile is buried in a trench so that any water flowing towards the fence will have to filter through the fabric, leaving most of the sediment behind. Silt fences are typically installed:

– Along the perimeter of a construction site to completely surround the area where soil will be disturbed. This prevents sediment from leaving the site.

– Below areas where stormwater flows off a site to slow and filter the runoff.

– Above sensitive areas like streams, lakes and wetlands to protect them from sediment-laden runoff.

– Along slopes and drainage channels to slow and retain runoff and promote infiltration.

Silt fence materials

Silt fences are constructed from the following materials:

– Geotextile fabric – Often made of polypropylene, woven monofilament polypropylene, slit film woven polypropylene or nonwoven polypropylene. The fabric allows water to pass through while trapping sediment particles.

– Wooden stakes – Typically 2×2 inch wooden stakes that the silt fence fabric is attached to. Longer stakes are used for installation on sloped or rocky areas.

– Wire mesh support – A wire mesh backing can be used to add support on slopes or areas of high runoff.

– Trench – A 6-8 inch deep and 6-8 inch wide trench is dug along the line of stakes and the silt fence fabric is buried in the trench.

Key properties

The geotextile used for silt fence should have the following properties:

– High permeability to allow water flow through the fabric. Around 0.3 gal/ft2/min.

– High UV stability to withstand sunlight exposure. 90% strength retention after 500 hours of UV exposure.

– High tensile and tear strength. At least 90 lbs grab tensile strength and 25 lbs puncture strength.

– Small enough pore size to trap sediment particles. Around 40-85 sieve size or 0.015-0.024 inch opening size.

– Durable enough to withstand machinery, weather, etc.

How does a silt fence work?

Silt fences use the combined effects of filtration, sedimentation and flow velocity reduction to control sediment in runoff water:

Filtration: As stormwater attempts to flow through the geotextile fabric, sediment particles like sand, silt and clay are filtered out and retained behind the fence. The fine pore size blocks most sediment.

Sedimentation: The flow of water is slowed down as it passes through the silt fence, allowing heavier sediment particles to settle out behind the fence by gravity.

Flow velocity reduction: The silt fence creates a small ponding area of water behind it which reduces the velocity and erosive energy of the runoff, allowing more sediment to fall out.

Silt fence installation

Silt fences must be properly installed to work effectively. The key steps are:

1. Plan the silt fence location – Outline disturbed areas and install along contours.

2. Prepare the silt fence – Unroll fabric, pre-attach to stakes if possible.

3. Dig trench – Dig 6-8 inch deep and wide trench along fence line.

4. Install stakes – Drive stakes into ground on downhill side, about 8 feet apart.

5. Attach fabric – Attach fabric to stakes, leaving 6-12 inches above ground.

6. Backfill trench – Backfill trench on uphill side to anchor fabric. Compact soil.

7. Join sections – Overlap fabric ends by 6 inches and join securely.

Maintenance

Regular inspection and maintenance is key to ensuring silt fences keep working properly:

– Inspect silt fences after each rainfall. Look for damage, deterioration and accumulated sediment.

– Repair any damage immediately. Fabric can be sewn if small holes or tears occur.

– Remove sediment deposits when they reach halfway up the fence height. Take care not to damage fabric.

– Replace deteriorating fabric sections immediately to prevent failure.

– Adjust or reinforce silt fences that become undercut by water flow.

Where should silt fences be installed?

Silt fences are installed in the following key locations on construction sites:

1. Down-slope perimeter control

– Installed along the outer downhill perimeter of the site.

– Prevents sediment from leaving the site onto neighboring properties or water bodies.

2. Internal erosion control

– Used along slopes, channels and project phases within the site.

– Protect partially stabilized areas from upslope runoff.

3. Sediment traps

– Placed perpendicular to flow paths to create small sediment traps.

– Temporary containment and settling of sediments prior to discharge.

4. Inlet protection

– Placed around inlets to prevent sediment entry into the storm drain system.

– Last line of defense prior to discharging.

5. Stream, wetland and shoreline protection

– Used along edges of streams, wetlands and water bodies.

– Protect sensitive ecological areas from sediment and turbid runoff.

Advantages of silt fences

Silt fences have numerous advantages that make them a versatile and effective sediment control device:

– Low cost
– Simple installation
– Available in standard sizes that can be joined to any required length
– Portable and movable to adapt to changing site conditions
– Effective for its low cost and simplicity
– Can be installed in almost any soil type and area
– Materials are readily available
– Temporary installation minimizes site disturbance
– Allows some water flow-through, unlike barriers that completely block flow
– Many uses across a site in combination with other controls

Limitations of silt fences

The limitations of silt fences include:

– Not suitable for areas of concentrated water flow. Can fail due to hydrostatic pressure.
– Prone to undercutting if not properly trenched and keyed into the ground.
– Not intended for permanent installation. Useful life of around 6-12 months.
– Need regular maintenance to ensure proper functioning. Can clog up over time.
– Limited sediment capture zone uphill from fence. Cannot treat large drainage areas.
– Must be removed once the site is stabilized to prevent continued ponding of water.
– Improper installation reduces effectiveness and durability.
– Can only handle limited runoff flow velocities based on fabric strength.
– Labor intensive to construct and maintain.
– Hand work required for digging trenches and compacting backfill.
– Sediment removal can be difficult in saturated conditions. May damage fabric.
– Not effective at removing fine silts and clay particles that remain suspended in runoff.

When should silt fences NOT be used?

There are some situations where silt fences should not be used as the primary sediment control method:

– Steep slopes greater than 2:1 (50%) – High failure risk and difficulty installing trenches on steep slopes. Use other erosion controls like erosion control blankets.

– Areas of concentrated flows – Silt fences cannot handle the hydrostatic pressures from concentrated flows. Use rock check dams instead.

– Live streams or channels – Not suitable for use within live streams where they may obstruct flow. Use other measures like stream buffers.

– Sites with rocky soil – Difficult to trench and install in very rocky or hard soils. Hard driving stakes.

– Sites with sandy soil – High risk of undercutting and failure in loose sandy soils with little cohesion.

– Areas of wind-driven rain – Fabric silt fences can be damaged by heavy wind-driven rain. Use rigid barriers instead.

– As a ditch liner – Not intended to line ditches or channels with high flows. Use erosion control blankets or riprap instead.

– Where ponding is undesirable – Silt fences create ponds which may flood sites or foundations.

– As standalone erosion control – Silt fences just filter sediment, they do not prevent erosion. Must be used with other practices like surface roughening and revegetation.

Recommended alternatives to silt fences

Some suitable alternatives to silt fences in situations where they are not the best option are:

– Rock check dams – More suitable for concentrated flow erosion areas. Help break up and slow down flows.

– Erosion control blankets – Better erosion protection on steep slopes. Anchor soil while vegetation establishes.

– Sand or gravel bag barriers – More rigid barriers for areas of high runoff velocity.

– Temporary sediment traps – Larger sediment basins for drainage areas greater than 1 acre.

– Wattles – Small permeable berms that can be used in ditch lines and channels. Filter flows without ponding water.

– Compost filter berms – Use compost to create berms that retain sediment but allow higher flows through voids.

– Temporary swales – Convey and slow runoff while promoting filtration and infiltration.

Key considerations for effective use of silt fences

To maximize the benefits of silt fences and avoid common failures, the following should be considered:

– Proper siting – Key factor is correct placement in suitable areas based on topography, soil, runoff volumes, etc.

– Limit drainage area – Max area draining to any one section of silt fence should be 1/4 acre or less, and on the contour.

– Reduce flows – Use diversions, berms and vegetated buffers to reduce the volume and velocity of runoff reaching silt fences.

– Extend sufficiently upslope – Fence should be long enough that the ends extend upslope enough to retain sediment.

– Proper installation – Strictly follow installation procedures like trenching, staking, fabric attachment and junctures.

– Timely inspection and maintenance – Regularly inspect for damages and accumulated sediment, make needed repairs and sediment removal immediately.

– Supplemental practices – Silt fences alone will not control erosion. Use practices like surface roughening, soil stabilizers, revegetation and mulching together.

– Removal when no longer needed – Remove silt fence once drainage area is permanently stabilized to avoid ponding issues.

Sediment control regulations for construction sites

Installation and maintenance of sediment controls like silt fences on construction sites is required by law under federal, state and local regulations.

Clean Water Act

The Clean Water Act requires construction sites over 1 acre in size to obtain stormwater discharge permits under the National Pollutant Discharge Elimination System (NPDES). Permits require the use of sediment and erosion controls to protect water quality.

State and local regulations

Many state and local governments have additional permitting programs and specific requirements for sediment controls:

– Obtaining grading, land disturbance or sediment control permits.
– Submitting erosion and sediment control plans for approval.
– Mandatory installation of silt fences and other BMPs.
– Minimum BMP design standards may be specified.
– Routine inspections and monitoring programs.
– Specified maintenance procedures and timeframes.
– Requirements for removal once permanent stabilization is achieved.

Site inspections

Periodic site inspections are conducted by regulatory agencies to ensure erosion and sediment controls like silt fences are properly installed and maintained per the approved plans. Failure to comply may result in stop work orders or fines.

Typical silt fence applications

Here are some typical applications and installation locations for using silt fences on construction sites:

Downslope perimeter control

– Installed along the downslope perimeter of the site or disturbed area.
– Intercepts storm runoff and prevents sediment from leaving the site.
– May require wire backing for support on steep slopes.

Slope protection

– Installed in rows along disturbed slopes.
– Reduce velocity of runoff and provide multiple filtration zones.
– Use a check slot to direct water for erosion protection.

Above sensitive areas

– Placed above environmentally sensitive areas like streams and wetlands.
– Provides a protective buffer to filter runoff and trap sediment.

Storm drain inlet protection

– Installed around storm drain inlets that receive runoff from disturbed areas.
– Prevents sediment from entering the storm drainage system.

Construction site plan using silt fences

Here is an example construction site erosion and sediment control plan using silt fences and other BMPs:

Key BMPs highlighted:

– Silt fence perimeter control around disturbed area (orange).
– Silt fences on contours along disturbed slopes (blue).
– Wire-backed silt fences along steep slopes (brown).
– Storm drain inlet protection (green).
– Stabilized construction entrance to prevent offsite tracking (grey).
– Earth dikes and drainage swales to direct runoff (dark green).
– Sediment trap at low point to capture sediment (purple).
– Temporary and permanent seeding to stabilize areas.

Inspection and maintenance checklist

Regular inspection and maintenance is essential to ensure silt fences continue working properly. Use the following checklist:

Inspections

– Overall condition of fabric – Rips, holes, deterioration?

– Depth of sediment accumulation behind fence

– Fence leaking, collapsing or falling over?

– Undercutting below fence, bypass occurring?

– Ends of fence properly tied into high ground?

– Fabric securely attached to stakes and fence tight?

– Stakes secure, damaged or dislodged? Need replacing?

Maintenance tasks

– Repair fabric tears immediately using sewn repairs

– Remove sediment deposits before 6 inches depth

– Replace broken stakes, install additional stakes if needed

– Reinforce undercut areas with compacted backfill

– Clear debris blocking or clogging fence

-Tighten or re-attach loose fabric to stakes

– Adjust fence alignment as needed to improve performance

Cost estimate

Here is a sample cost estimate for installing 100 linear feet of standard silt fence:

Item	Total	Cost per unit
Geotextile silt fence fabric	100 ft	$0.25/ft
2×2 inch wood stakes, 3 ft long	25 stakes	$1.50/stake
Labor – Installation	3 person-hours	$40/hr
Total cost		$175

Key variables that affect overall costs:

– Length of silt fence required
– Price of geotextile fabric
– Type and quantity of stakes
– Additional backing or support requirements
– Soil conditions affecting installation difficulty
– Level of site accessibility
– Labor costs

Key takeaways

– Silt fences are temporary sediment barriers used to retain loose soil and prevent it leaving disturbed sites to pollute nearby water bodies.

– They work by filtering runoff water through geotextile fabric, reducing flow velocity and trapping sediment through filtration and gravity settling.

– Effective siting, installation and regular maintenance are key to proper silt fence functioning. They have limitations and are not suitable for some site conditions.

– Silt fences are an important best management practice for erosion and sediment control required by regulations on most construction sites. They should be used together with other BMPs for a comprehensive control strategy.

Conclusion

Silt fences provide an inexpensive and effective way to control sediment loss from disturbed construction sites. When properly sited and maintained, they can significantly reduce the amount of eroded soil leaving a site and improve downstream water quality. Regulations require their use as part of a comprehensive erosion and sediment control plan. Careful planning, installation and maintenance are essential however to ensure silt fences work as intended. They have limitations in some situations that require consideration of additional or alternative control measures for sites with high runoff, steep slopes or poor soils. Overall, silt fences will continue to be a fundamental BMP included in most construction site water quality protection programs.