Minimum Measure
Construction Site Stormwater Runoff Control
Subcategory
Sediment Control
Purpose and Description
Figure 1. Silt fence retaining sediment
The purpose of a silt fence
is to retain the soil on
disturbed land (Figure 1),
such as a construction
site, until the activities
disturbing the land are
sufficiently completed to
allow revegetation and
permanent soil stabilization
to begin. Keeping the
soil on a construction site, rather than letting it be washed off
into natural water bodies (e.g., streams, rivers, ponds, lakes,
estuaries) prevents the degradation of aquatic habitats and
siltation of harbor channels. And not letting soil wash off onto
roads, which readily transport it to storm sewers, avoids having
sewers clogged with sediment. The cost of installing silt fences
on a watershed's construction sites is considerably less than
the costs associated with losing aquatic species, dredging
navigation channels, and cleaning sediment out of municipal
storm sewers.
A silt fence is a temporary sediment barrier made of porous
fabric. It's held up by wooden or metal posts driven into the
ground, so it's inexpensive and relatively easy to remove.
The fabric ponds sediment-laden stormwater runoff, causing
sediment to be retained by the settling processes. A single
100 foot (ft) run of silt fence may hold 50 tons of sediment
in place. Most construction sites today do have silt fences.
But many do not work effectively because they are not well
designed, installed, or maintained. The focus of this fact sheet
is—how to make silt fences work.
Design
The three principal aspects of silt fence design are: proper
placement of fencing, adequate amount of fencing, and
appropriate materials.
Proper Placement of Fencing
Placement is important because where a fence starts, runs,
and ends is critical to its effectiveness. Improper placement
can make the fence a complete waste of money. Analyze the
construction site's contours to determine the proper placement.
Segment the site into
manageable sediment
storage areas for using
multiple silt fence runs.
The drainage area above
any fence should usually
not exceed a quarter of an
acre. Water flowing over
the top of a fence during a
normal rainfall indicates the
drainage area is too large.
An equation for calculating
the maximum drainage area
length above a silt fence,
measured perpendicular to
the fence, is given in Fifield,
2011. Avoid long runs of
silt fence because they
concentrate the water in a
small area where it will easily
overflow the fence. The
lowest point of the fence in
Figure 4 is indicated by a
red arrow. Water is directed
to this low point by both
long runs of fence on either
side of the arrow. Most of
the water overflows the
fence at this low point and
little sediment is trapped for
Figure 2. Create manageable sediment
storage areas
Figure 3. Wafer should not flow over the
filter fabric during a normal rainfall
Figure 4. Avoid long runs of silt fence
such a long fence.
Office of Water, 4203M
www.epa.gov/npdes/pubs/siltfences.pdf
www.epa.gov/npdes/stormwater/menuofbmps
EPA833-F-11-008
April 2012
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Stormwater Best Management Practice: Silt Fences
Use J-hooks as shown in Figures 5 and 6, which have ends
turning up the slope to break up long fence runs and provide
multiple storage areas that work like mini-retention areas. If the
fence doesn't create a ponding condition, it will not work well.
The silt fence in Figure 7 doesn't pond water or retain sediment.
Stormwater will run around the fence carrying sediment to the
street, which will transport the water and its sediment load to
the storm sewer inlet.
Figure 5. Use J-hook fences to break up
long fence runs
Figure 6. J-hook silt fences
provide multiple storage areas
-i
Figure 7. This silt fence doesn't work
Water flowing around the ends of a silt fence will cause
additional erosion and defeat its purpose. The bottom of each
end of the fence should be higher than the top of the middle of
the fence (Figure 8). This insures that during an unusually heavy
rain, water will flow over the top rather than around either end of
the fence. Only fine suspended material will spill over the top,
which is not as harmful as having erosion at the ends. When
there is a long steep slope,
install one fence near the
head of the slope to reduce
the volume and velocity
of water flowing down
the slope, and another
fence 6-10 ft from the toe
of the slope to create a
sediment storage area near
the bottom. A common
misconception is that you
only have to worry about
water running off steep
slopes. However, steep
slopes may have a relatively
Small water collection area. F'9ure 9- Poor installation, water can
_. , , - flow around the ends causing additional
The total drainage area eras/on
Figure 8. Proper installation, bottom of
both ends are above the top of the middle
Figure 10. Gentle slopes may require a
silt fence
of a gentle slope, if large
(Figure 10), can be more
important than its slope in
determining sediment loss.
A silt fence should not be
placed in a channel with
continuous flow (channels
in Figures 8 and 9 don't
have a continuous flow), nor
across a narrow or steep-
sided channel. But when necessary a silt fence can be placed
parallel to the channel to retain sediment before it enters the
watercourse.
Paved streets are major conduits of Stormwater and silt, and
they drain to storm sewer inlets. The best solution is to retain as
much sediment as possible before it reaches paved surfaces.
Install a silt fence at the inlet side of a storm sewer or culvert,
rather than at the discharge where there is greater velocity and
less storage area. Streets cut in the grade, but not yet paved,
are also prime erosion conduits. If the streets are not going to
be paved right away, they need a containment barrier such as
a silt fence. Finally as a construction site's dynamics change,
the silt fence layout should be adjusted when necessary to
maintain its effectiveness.
Designers and contractors should also consider diverting
sediment-laden runoff water to a sediment detention pond. If
the site can provide a large enough area, this is usually the
most effective and economical best management practice
for retaining sediments. Silt fences are needed when there is
insufficient space for a detention pond or when roads and other
structures are in the way.
Adequate Amount of Fencing
The amount of fencing means the total linear length of the silt
fencing runs on the construction site. A reasonable rule-of
-thumb for the proper amount of silt fence is—100 ft of silt fence
per 10,000 square foot (sq ft) of disturbed area. Soil type, slope,
slope length, rainfall, and site configuration are all important
elements in determining the adequate silt fence protection
for a site, and to what extent it fits the 100 ft per 10,000 sq ft
rule-of-thumb. If the amount of fencing provides the volume of
runoff storage needed, then over-flowing the silt fence runs will
be minimized. This is the basic test; if fences are over-flowing
after a moderate rainfall event, the amount of fencing probably
needs to be increased to avoid undercutting, washouts, and
fence failures.
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Stormwater Best Management Practice: Silt Fences
Appropriate Materials
There are different types of porous fabrics available, e.g.,
woven, non-woven, mono-filament, but all types tend to clog
rapidly and don't provide lasting filtration. The support posts
and installation method are more important than the fabric type
for overall sediment retention. However, a lightweight fabric
tends to tear where it is attached to the posts. Posts must hold
the fabric up and support the horizontal load of retained water
and sediment. Hardwood posts (2" x2") are potentially strong
enough to support the loads, but are difficult to drive into the
ground more than 6-8". To hold 2 ft of sediment and water,
the posts should be driven 2 ft into the ground. Steel posts
are best because they can be driven into compacted soil to a
depth of 2 ft. The support posts should be spaced 3-4 ft apart
where water may run over the top of the fence, 5 ft in most other
areas, and 6-7 ft where there isn't a considerable horizontal
load. Improper post depth and spacing is often the cause of
sagging fabric and falling posts. Some authorities believe a
more robust wire or chain link supported silt fence is needed
to withstand heavy rain events. However, this may double the
cost of a silt fence installation and entails disposing of more
material in a landfill when the fence is removed. Installing silt
fencing having five interacting features: (1) proper placement
based on the site's contours, (2) adequate amount of fencing
without long runs, (3) heavy porous filter fabric, (4) metal posts
with proper depth and
spacing, and (5) tight soil
compaction on both sides
of the silt fence will usually
obviate the need for wire
or chain link reinforced
fencing. Prefabricated silt
fences, e.g., fabric attached
to wooden posts in a
100ft package, doesn't
provide for posting after the
ground is compacted or
allow variable post spacing.
Figure 11. Chain link supported silt fence
Silt Fence Installation
Two commonly used approaches for installing silt fences are
the static slicing method and the trenching method.
Static Slicing Method
The static slicing machine pulls a narrow blade through the
ground to create a slit 12" deep, and simultaneously inserts
the silt fence fabric into this slit behind the blade. The blade is
designed to slightly disrupt
soil upward next to the slit
and to minimize horizontal
compaction, thereby
creating an optimum
condition for compacting
the soil vertically on
both sides of the fabric.
Compaction is achieved
by rolling a tractor wheel
along both sides of the slit
in the ground 2 to 4 times to
achieve nearly the same or
greater compaction as the
original undisturbed soil.
This vertical compaction
reduces the air spaces
between soil particles,
which minimizes infiltration.
Without this compaction
infiltration can saturate
the soil, and water may
find a pathway under the
fence. When a silt fence is
holding back several tons
of accumulated water and
sediment, it needs to be
supported by posts that
are driven 2 ft into well-
compacted soil. Driving in
the posts and attaching the
fabric to them completes
the installation.
Figure 12. Static slicing machine
Figure 13. Tractor wheel compacting
the soil
Ponding height
max. 24"
Attach fabric to .
upstream side of post —
FLOW-— I
Drive over each side of \
silt fence 2 to 4 times
with device exerting
60 p.s.i. or greater !
100% compaction
^w
%^>^
POST SPACING:
7'max. on open runs
4' max. on pooling areas
POST DEPTH:
As much below ground
as fabric above ground
100% compaction
Figure 14. Silt fence installation using
the static slicing method
Trenching Method
Trenching machines have been used for over twenty-five years
to dig a trench for burying part of the filter fabric underground.
Usually the trench is about 6" wide with a 6" excavation. Its
walls are often more curved
than vertical, so they don't
provide as much support
for the posts and fabric.
Turning the trencher is
necessary to maneuver
around obstacles, follow
terrain contours or property
lines, and install upturns Figure 15 Trenchers make a wjder
OrJ-hOOkS. But trenchers excavation at turns
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Stormwater Best Management Practice: Silt Fences
Figure 16. Poor compaction has resulted
in infiltration and water flowing under
this silt fence causing retained sediment
washout
can't turn without making
a wider excavation, and
this results in poorer
soil compaction, which
allows infiltration along the
underground portion of the
fence. This infiltration leads
to water seeking pathways
under the fence, which
causes subsequent soil
erosion and retained sediment washout under the fence. The
white line on the fence in Figure 16 and red arrow both mark
the previous sediment level before the washout. Post setting
and fabric installation often precede compaction, which make
effective compaction more difficult to achieve. EPA supported
an independent technology evaluation (ASCE 2001), which
compared three progressively better variations of the trenching
method with the static slicing method. The static slicing
method performed better than the two lower performance
levels of the trenching method, and was as good or better than
the trenching method's highest performance level. The best
trenching method typically required nearly triple the time and
effort to achieve results comparable to the static slicing method.
Proper Attachment
Regardless of the installation method, proper attachment of
the fabric to the posts is critical to combining the strength of
the fabric and support posts into a unified structure. It must be
able to support 24" of sediment and water. For steel posts use
three plastic ties per post (50 Ib test strength), located in the
top 8" of the fabric, with each tie hung on a post nipple, placed
diagonally to attach as many vertical and horizontal threads as
possible. For wooden posts use several staples per post, with a
wood lath to overlay the fabric.
Perimeter Silt Fences
When silt fences are placed around the perimeter of a stock pile
or a construction site, the conventional silt fence design and
materials discussed previously may not be sufficient.
Stock pile example A
stock pile of dirt and large
rocks is shown in Figures 17
and 18 with a silt fence
protecting a portion of its
perimeter. Rocks that roll
down the pile would likely
Figure 18. Front of silt fence on part of
stock pile's perimeter
damage a conventional
silt fence. The bottom of
the porous fabric is held
firmly against both the
ground and base of precast
concrete, highway, barriers
by light-colored stones. An
alternative installation would
be having the concrete
barriers rest directly on the
bottom edge of the filter fabric, which would extend under the
barriers about 10", so the barriers' weight will press the fabric
against the ground to prevent washout. Water passing through
the silt fence (red arrow in Figure 18) flows to a storm sewer
culvert inlet, which is surrounded by a fabric silt fence (yellow
arrows in Figures 17 and 18) that reduces the runoff's velocity
and allows settling before the water is discharged to a creek.
Bridge abutment example. During the construction of a bridge
over a river between two lakes, an excavation on the river bank
was needed to pour footings for the bridge abutment. The silt
fence along the excavation's perimeter, composed of concrete
highway barriers with orange filter fabric, was designed to
prevent stormwater from
washing excavated spoil
into the river and to fend off
the river during high flows.
A portion of the orange filter
fabric that has blown away
from the concrete barriers
shows the need to overlap
and reinforce the joints
where two sections of filter
fabric are attached.
Figure 19. Silt fence for bridge abutment
excavation
Figure 17. Back of silt fence on part of
the stockpile's perimeter
Highway example. Because of the proximity of a construction
site to a highway, a concrete barrier was required by Minnesota's
DOT to protect the highway
and an underground fiber
optic cable next to the
highway from construction
activities. The concrete
barrier was used to support
a silt fence along the
perimeter of a large amount
of dirt that was stock piled
before being used for fill at
a different location.
Figure 20. S/7f fence protecting a
highway and underground fiber optics
cable
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Stormwater Best Management Practice: Silt Fences
Lake shore example.
The lake's shoreline is being
restored with plant plugs
and seeded with native
plant species. A plywood,
perimeter, silt fence is used
to trap sediment from a
construction site on the
right-side of the picture,
protect the lake shore from
boat-wake erosion, and to prevent geese from eating the seeds
and young plants. This fencing will be removed when 70%
vegetative cover is achieved.
Inspection and Maintenance
Permanent Soil Stabilization
Figure 21. S/7f fence protecting a lake
shore
Figure 22. A silt fence full of sediment
that needs maintenance
Silt fences should be
inspected routinely and
after runoff events to
determine whether they
need maintenance because
they are full (Figure 22) or
damaged by construction
equipment. The ASTM
silt fence specification
(ASTM 2003) recommends removing sediment deposits
from behind the fence when they reach half the height of the
fence or installing a second fence. However, there are several
problems associated with cleaning out silt fences. Once the
fabric is clogged with sediment, it can no longer drain slowly
and function as originally designed. The result is normally a low
volume sediment basin because the cleaning process doesn't
unclog the fabric. The soil is normally very wet behind a silt
fence, inhibiting the use of equipment needed to move it. A
back hoe is commonly used, but, if the sediment is removed,
what is to be done with it during construction? Another solution
is to leave the sediment in place where it is stable and build a
new silt fence above or below it to collect additional sediment
as shown in Figure 23. The proper maintenance may be
site specific, e.g. small
construction sites might not
have sufficient space for
another silt fence. Adequate
access to the sediment
control devices should be
provided so inspections
and maintenance can be
Figure 23. New silt fence below the old
performed. fence
When the land disturbing activities are sufficiently completed to
allow permanent soil stabilization on the site, the silt fences and
sediment basins are removed. The fabric and damaged posts
go to the landfill. Steel posts and some of the wooden posts
can be reused. Then the sediment is spread over the site to
provide fertile soil, and the area can be seeded and mulched to
support revegetation.
References
ASCE 2001. Environmental Technology Verification Report for
Installation of Silt Fence Using the Tommy Static Slicing Method.
CERF Report #40565. Washington, DC: American Society of
Civil Engineers. www.epa.gov/etv/pubs/08_vs_tommy.pdf
ASTM 2003. Standard Practice for Silt Fence Installation.
D 6462-03(2008). West Conshohocken, PA: American Society
of Testing Materials International.
www.astm.org/SEARCH/search-reskin.html?query=D6462-
03&siteType=store-standards&searchType=standards-full
Carpenter, Thomas 2000. Silt Fence That Works. Ankey, Iowa:
Thomas Carpenter, www.tommy-sfm.com/pages/resources/
Silt%20Fence%20That%20Works%20Manual.pdf
Fifield, Jerald S. 2011. Designing and Reviewing Effective
Sediment and Erosion Control Plans. 3rd Edition. Santa Barbara,
CA: Forester Press.
U.S. Environmental Protection Agency 2007. Developing Your
Stormwater Pollution Prevention Plan. EPA 833-R-06-004.
Washington: EPA. Available from EPA hardcopy 800-490-9198
orwww.epa.gov/npdes/pubs/sw_swppp_guide.pdf
Photograph Credits
Figures 1-10, 12-16, 22, 23. Thomas Carpenter, CPESC, Carpenter
Erosion Control
Figure 11. Pefe Schumann, Fairfax County, Virginia, Department of
Public Works and Environmental Services
Figure 17-21. Dwayne Stenlund, CPESC, Minnesota Department of
Transportation
Disclaimer
Please note that EPA has provided external links because they provide
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