6.62_Sediment Fence

Practice Standards and Specifications
Definition A temporary sediment control measure consisting of fabric buried at the
bottom, stretched, and supported by posts.
Purpose To retain sediment from small disturbed areas by reducing the velocity of
sheet flows to allow sediment deposition.
C o n d i t i o n s W h e re Below small -disturbed areas that are less then 1/4 acre per 100 feet of fence.
P racti ce Ap p I i es Where runoff can be stored behind the sediment fence without damaging the
fence or the submerged area behind the fence.
Do not install sediment fences across streams, ditches, or waterways, or
other areas of concentrated flow.
Sediment fence should be placed along topographic elevation contours, where
it can intercept stormwater runoff that is in dispersed sheet flow. Sediment
fence should not be used alone below graded slopes greater than 10 feet in
height.
Planning A sediment fence is a system to retain sediment on the construction site. The
fence retains sediment primarily by retarding flow and promoting deposition.
Considerations In operation, generally the fence becomes clogged with fine particles, which
reduce the flow rate. This causes a pond to develop behind the fence. The
designer should anticipate ponding and provide sufficient storage areas and
overflow outlets to prevent flows from overtopping the fence. Since sediment
fences are not designed to withstand high water levels, locate them so that
only shallow pools can form. Tie the ends of a sediment fence into higher
ground to prevent flow around the end of the fence before the pool reaches
design level. Curling each end of the fence uphill in a "J" pattern may be
appropriate to prevent end flow. Provide stabilized outlets to protect the fence
system and release storm flows that exceed the design storm.
Deposition occurs as the storage pool forms behind the fence. The designer can
direct flows to specified deposition areas through appropriate positioning of
the fence or by providing an excavated area behind the fence. Plan deposition
areas at accessible points to promote routine cleanout and maintenance. Show
deposition areas in the erosion and sedimentation control plan. A sediment
fence acts as a diversion if placed slightly off the contour. A maximum slope
of 2 percent is recommended. This technique may be used to control shallow,
uniform flows from small disturbed areas and to deliver sediment -laden water
to deposition areas. The anchoring of the toe of the fence should be reinforced
with 12 inches of NC DOT #5 or #57 washed stone when flow will run parallel
to the toe of the fence.
Sediment fences serve no function along ridges or near drainage divides where
there is little movement of water. Confining or diverting runoff unnecessarily
with a sediment fence may create erosion and sedimentation problems that
would not otherwise occur.
Rev. 5/13 6.62.1

0
Straw barriers have only a 0-20% trapping efficiency and are inadequate. Straw
bales may not be used in place of sediment fence. Prefabricated sediment fence
with the fabric already stapled to thin wooden posts does not meet minimum
standards specified later in this section.
Anchoring of sediment fence is critical. The toe of the fabric must be anchored
in a trench backfilled with compacted earth. Mechanical compaction must be
provided in order for the fence to effectively pond runoff.
Design Criteria Ensure that drainage area is no greater than acre per 100 feet of fence. This
is the maximum drainage area when the slope is less than 2 percent. Where
all runoff is to be stored behind the fence, ensure that the maximum slope
length behind a sediment fence does not exceed the specifications shown in
Table 6.62a. The shorter slope length allowed for steeper slopes will greatly
reduce the maximum drainage area. For example, a 10-20 % slope may have
a maximum slope length of 25 feet. For a 100-foot length of sediment fence,
the drainage area would be 25ft X 100ft = 2500sq.ft., or 0.06 acres.
Table 6.62a Maximum
Slope Length and Slope for
which Sediment Fence is
Applicable
Slope Slope Length (ft) Maximum Area (ft2)
<2% 100 10,000
2 to 5% 75 7,500
5 to 10% 50 5,000
10 to 20% 25 2,500
>20% 15 1,500
Make the fence stable for the 10-year peak storm runoff.
Ensure that the depth of impounded water does not exceed 1.5 feet at any point
along the fence.
If non -erosive outlets are provided, slope length may be increased beyond
that shown in Table 6.62a, but runoff from the area should be determined and
bypass capacity and erosion potential along the fence must be checked. The
velocity of the flow at the outlet or along the fence should be in keeping with
Table 8.05d, Appendix 8.05.
Provide a riprap splash pad or other outlet protection device for any point
where flow may overtop the sediment fence, such as natural depressions or
swales. Ensure that the maximum height of the fence at a protected, reinforced
outlet does not exceed 2 feet and that support post spacing does not exceed 4
feet.
The design life of a synthetic sediment fence should be 6 months.
Construction MATERIALS
1. Use a synthetic filter fabric of at least 95% by weight of polyolefins or
Specifications polyester, which is certified by the manufacturer or supplier as conforming to
the requirements in ASTM D 6461, which is shown in part in Table 6.62b.
Synthetic filter fabric should contain ultraviolet ray inhibitors and stabilizers
to provide a minimum of 6 months of expected usable construction life at a
temperature range of 0 to 1200 F.
6.62.2 Rev. 5/13

Practice Standards and Specifications
2. Ensure that posts for sediment fences are 1.25 lb/linear ft minimum
steel with a minimum length of 5 feet. Make sure that steel posts have
projections to facilitate fastening the fabric.
3. For reinforcement of standard strength filter fabric, use wire fence with a
minimum 14 gauge and a maximum mesh spacing of 6 inches.
Table 6.62b Specifications For Sediment Fence Fabric
Temporary Silt Fence Material Property Requirements
Supported'
Un-Supported' Type of
Test Material Units Silt Fence
Silt Fence Value
Grab Strength ASTM D 4632 N (lbs)
Machine Direction 400
550 MARV
(90)
(90)
X-Machine Direction 400
450 MARV
(90)
(90)
Permittivity2 ASTM D 4491 sec-1 0.05
0.05 MARV
Apparent Opening Size ASTM D 4751 mm 0.60
0.60 Max. ARV3
(US Sieve #) (30)
(30)
70% after
Ultraviolet Stability ASTM D 4355 Retained
70% after
Typical
500h of exposure
Strength
500h of exposure
' Silt Fence support shall consist of 14 gage steel wire with a mesh spacing of 150 mm (6 inches), or prefabricated poylmer mesh of
equivalent strength.
2 These default values are based on empirical evidence with a variety of sediment. For environmentally sensitive areas, a review of
previous experience and/or site or regionally specific geotextile tests in accordance with Test Method D 5141 should be performed
by the agency to confirm suitability of these requirements.
3 As measured in accordance with Test Method D 4632.
CONSTRUCTION
1. Construct the sediment barrier of standard strength or extra strength
synthetic filter fabrics.
2. Ensure that the height of the sediment fence does not exceed 24 inches
above the ground surface. (Higher fences may impound volumes of water
sufficient to cause failure of the structure.)
3. Construct the filter fabric from a continuous roll cut to the length of the
barrier to avoid joints. When joints are necessary, securely fasten the filter
cloth only at a support post with 4 feet minimum overlap to the next post.
4. Support standard strength filter fabric by wire mesh fastened securely to
the upslope side of the posts. Extend the wire mesh support to the bottom of
the trench. Fasten the wire reinforcement, then fabric on the upslope side of
the fence post. Wire or plastic zip ties should have minimum 50 pound tensile
strength.
5. When a wire mesh support fence is used, space posts a maximum of 8 feet
apart. Support posts should be driven securely into the ground a minimum of
24 inches.
6. Extra strength filter fabric with 6 feet post spacing does not require wire
mesh support fence. Securely fasten the filter fabric directly to posts. Wire or
plastic zip ties should have minimum 50 pound tensile strength.
Rev. 5/13 6.62.3

0
7. Excavate a trench approximately 4 inches wide and 8 inches deep along
the proposed line of posts and upslope from the barrier (Figure 6.62a).
8. Place 12 inches of the fabric along the bottom and side of the trench.
9. Backfill the trench with soil placed over the filter fabric and compact.
Thorough compaction of the backfill is critical to silt fence performance.
10. Do not attach filter fabric to existing trees.
SEDIMENT FENCE INSTALLATION USING THE SLICING METHOD
Instead of excavating a trench, placing fabric and then backfilling trench,
sediment fence may be installed using specially designed equipment that
inserts the fabric into a cut sliced in the ground with a disc (Figure 6.62b).
Installation 1. The base of both end posts should be at least one foot higher than the
middle of the fence. Check with a level if necessary.
Specifications
2. Install posts 4 feet apart in critical areas and 6 feet apart on standard
applications.
3. Install posts 2 feet deep on the downstream side of the silt fence, and
as close as possible to the fabric, enabling posts to support the fabric from
upstream water pressure.
4. Install posts with the nipples facing away from the silt fabric.
5. Attach the fabric to each post with three ties, all spaced within the top 8
inches of the fabric. Attach each tie diagonally 45 degrees through the fabric,
with each puncture at least 1 inch vertically apart. Also, each tie should be
positioned to hang on a post nipple when tightened to prevent sagging.
6. Wrap approximately 6 inches of fabric around the end posts and secure
with 3 ties.
7. No more than 24 inches of a 36 inch fabric is allowed above ground
level.
8. The installation should be checked and corrected for any deviations before
compaction.
9. Compaction is vitally important for effective results. Compact the soil
immediately next to the silt fence fabric with the front wheel of the tractor,
skid steer, or roller exerting at least 60 pounds per square inch. Compact the
upstream side first, and then each side twice for a total of 4 trips.
6.62.4
Rev. 5/13

Practice Standards and Specifications
8' max. standard strength fabric with wire fence
6' max. extra strength fabric without wire fence
Steel
post
Plastic or
Iwire ties
r
Wire 18-24
fence
Natural
ground
8" down & 4"
forward along
24" the trench
Filter
,L I fabric
Wire
fence
Cross -Section
View
Filter
Steel fabric Backfill trench Natural
Post and compact ground
thoroughly
0 m 0 mmm 0
mmmmmm4,m mmmm Upslope
0 ° 0 min 0 0 °°
0 m m 0° 0 0 0
m 0 mm 0 0 0 00 8"
0 0 O
mm°° m°m ° min
24" m°°m 00 mmm°0
0 mm00m m°m00m°
0 0 m 000 0 0 0 0 m
00 0 00
m m m 0 0 0
Figure 6.62a Installation detail of a sediment fence.
6.62.5
Rev. 5113

a
The Slicing Method
Ponding height
max , 24"
Attach fabric to
upstream sidebf post
FLOW —
Drive over eachsideof silt
Pence 2 to 4 times with
device exerting 60 p.s.L or
greater
POST SPACING:
6. max :on open runs
4' r»ax on pooling areas
POST MPTH•
2 feet
compacted sail compacted soil
/
\\\ \\\ \\\ !
\`\\
No more than 24" of 36" fabric
is allowed above ground.
e_____________ ___ ---------------- Top of Fabric
Belt
top 8
Diagonal attachment
doubles strength.
ATTACHMENT DETAILS:
* Gather fabric at posts, if needed.
* Utilize three lies per post, all within top 8" of iabric.
* Position each iie diagonally, puncturing holes vertically
a minimum of 1" apart.
® Hang each tie ona post nipple and tighten securely.
Use cable lies (50lbs) or soft wire.
Roll of silt fence
Horizontal chisel point Slieina blade:
3" width 0.7"
Post
installed
after
compaction
Silt Fence
Completed Installation
Vibratory plow Is not acceptable because of horizontal compaction
Figure 6.62b Schematics for using the slicing method to install a sediment fence. Adapted from Silt Fence that Works
6.62.6
Rev. 5113

Practice Standards and Specifications
Maintenance Inspect sediment fences at least once a week and after each rainfall. Make any
required repairs immediately.
Should the fabric of a sediment fence collapse, tear, decompose or become
ineffective, replace it promptly.
Remove sediment deposits as necessary to provide adequate storage volume
for the next rain and to reduce pressure on the fence. Take care to avoid
undermining the fence during cleanout.
Remove all fencing materials and unstable sediment deposits and bring the
area to grade and stabilize it after the contributing drainage area has been
properly stabilized.
Refe renceS ASTM D 6461— 99. "Standard Specification for Silt Fence Materials" ASTM
International. For referenced ASTM standards, visit the ASTM website,
www.astm.org, or contact ASTM Customer Service at [e-mail]. For
Annual Book of ASTM Standards volume information, refer to the standard's
Document Summary page on the ASTM website.
ASTM D 6462 — 03. "Standard Practice for Silt Fence Installation" ASTM
International. For referenced ASTM standards, visit the ASTM website,
www.astm.org, or contact ASTM Customer Service at [e-mail]. For
Annual Book of ASTM Standards volume information, refer to the standard's
Document Summary page on the ASTM website.
C. Joel Sprague, PE, Silt Fence Performance Limits and Installation
Requirements. Sprague and Sprague Consulting Engineers and TRI/
Environmental, Inc.
Carpenter Erosion Control. http://www.tommy-sfm.com/
Kentucky Erosion Prevention and Sediment Control Field Manual, 2004.
Runoff Control Measures
6.20, Temporary Diversions
Outlet Protection
6.41, Outlet Stabilization Structure
Appendix
8.03, Estimating Runoff
Rev. 5/13 6.62.7