TM 5-818-8/AFJMAN 32-1030
CHAPTER 6
EROSION AND SEDIMENT CONTROL
Table 6-1 gives recommended minimum strength
6-1. Erosion Control
values.
Erosion is caused by a group of physical and
(3) Cover material. The cover material (gravel,
chemical processes by which the soil or rock
rock fragments, riprap, armor stone, concrete
material is loosened, detached, and transported
blocks, etc.) is a protective covering over the
from one place to another by running water,
geotextile that minimizes or dissipates the hydrau-
waves, wind, moving ice, or other geological sheet
lic forces, protects the geotextile from extended
and bank erosion agents. Clayey soils are less
exposure to UV radiation, and keeps it in intimate
erodible than fine sands and silts. See figure 6-1.
contact with the soil. The type, size, and weight of
This chapter covers the use of geotextiles to
cover material placed over the geotextile depends
minimize erosion caused by water.
on the kinetic energy of water. Cover material
that is lightweight in comparison with the hydrau-
6-2. Bank Erosion
lic forces acting on it may be moved. By removing
Riprap is used as a liner for ditches and channels
the weight holding the geotextile down, the
subjected to high-velocity flow and for lake, reser-
ground-water pressure may be able to separate the
voir and channel banks subject to wave action.
geotextile from the soil. When no longer con-
Geotextiles are an effective and economical alter-
strained, the soil erodes. The cover material must
native to conventional graded filters under stone
be at least as permeable as the geotextile. If the
riprap. However, for aesthetic or economic reasons,
cover material is not permeable enough, a layer of
articulated concrete mattresses, gabions, and pre-
fine aggregate (sand, gravel, or crushed stone)
cast cellular blocks have also been used to cover
should be placed between it and the geotextile. An
the geotextile. The velocity of the current, the
important consideration in designing cover mate-
height and frequency of waves and the erodibility
rial is to keep the void area between stones
of the bank determine whether bank protection is
relatively small. If the void area is excessively
needed. The geotextiles used in bank protection
large, soils may move from areas weighted by
serve as a filter. Filter design is covered in chapter
stones to unweighted void areas between the
stones, causing the geotextile to balloon or eventu-
3.
a. Special Design Considerations.
ally rupture. The solution in this case is to place a
graded layer of smaller stones below the large
(1) Durability. The term includes chemical,
stones that will prevent the soil from moving. A
biological, thermal, and ultraviolet (UV) stability.
layer of aggregate may also be needed if a major
Streams and runoff may contain materials that
part of the geotextile is covered as for example by
can be harmful to the geotextile. When protected
concrete blocks. The layer will act as a pore water
from prolonged exposure to UV light, the common
dissipator.
synthetic polymers do not deteriorate or rot in
(4) Anchorage. At the toe of the streambank,
prolonged contact with moisture. All geotextile
the geotextile and cover material should be placed
specifications must include a provision for covering
along the bank to an elevation below mean low
the geotextile to limit its UV radiation exposure to
water level to minimize erosion at the toe. Place-
30 days or less.
ment to a vertical distance of 3 feet below mean
(2) Strength and abrasion resistance. The re-
low water level, or to the bottom of the streambed
quired properties will depend on the specific appli-
for streams shallower than 3 feet, is recommended.
cation-the type of the cover material to be used
At the top of the bank, the geotextile and cover
(riprap, sand bags, concrete blocks, etc.), the size,
material should either be placed along the top of
weight, and shape of the armor stone, the han-
the bank or with 2 feet vertical freeboard above
dling placement techniques (drop height), and the
expected maximum water stage. If strong water
severity of the conditions (stream velocity, wave
movements are expected, the geotextile needs to be
height, rapid changes of water level, etc.). Abra-
anchored at the crest and toe of the streambank
sion can result from movement of the cover mate-
(fig 6-2).
r i a l as a result of wave action or currents.
(5) If the geotextile must be placed below low
Strength properties generally considered of pri-
water, a material of a density greater than that of
mary importance are tensile strength, dimensional
water should be selected.
stability, tearing, puncture, and burst resistance.
6-l
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