TM 5-818-8/AFJMAN 32-1030
Figure 6-3. Geotextile Placement for Currents Acting Parallel to Bank or for Wave Attack on the Bank.
6-5. Sediment Control
support component may be a wire or plastic mesh
support fence attached to support posts or in some
Silt fences and silt curtains are sediment control
cases may be support posts only. The designer has
systems using geotextiles.
to determine the minimum height of silt fences,
a. Silt Fence. A silt fence is a temporary vertical
and consider the geotextile properties (tensile
barrier composed of a sheet of geotextile supported
strength, permeability) and external factors (the
by fencing or simply by posts, as illustrated in
slope of the surface, the volume of water and
figure 6-5. The lower end of the geotextile is
suspended particles which are delivered to the silt
buried in a trench cut into the ground so that
fence, and the size distribution of the suspended
runoff will not flow beneath the fence. The purpose
particles). Referring to figure 6-7, the total height
of the permeable geotextile silt fence is to inter-
of the silt fence must be greater than
cept and detain sediment from unprotected areas
; where is the height of geotextile necessary to
before it leaves the construction site. Silt fence are
allow water flowing into the basin to flow through
sometimes located around the entire downslope
portion or perimeter of urban construction sites.
the geotextile, considering the permeability of the
Short fences are often placed across small drainage
geotextile;
is the height of water necessary to
ditches (permanent or temporary) constructed on
overcome the threshold gradient of the geotextile
the site. Both applications are intended to function
and to initiate flow. For most expected conditions,
for one or two construction seasons or until grass
is about 6 inches or less. The silt fence
sod is established. The fence reduces water veloc-
accomplishes its purpose by creating a pond of
ity allowing the sediment to settle out of suspen-
relatively still water which serves as a sedimenta-
sion.
tion basin and collects the suspended solids from
(1) Design concepts. A silt fence consists of a
the runoff. The useful life of the silt fence is the
sheet of geotextile and a support component. The
time required to fill the triangular area of height
6-4
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