The most economical combination of rock and wire for streambank protection is simply laying
wire mesh over stone. The major problem with this approach is keeping the mesh in place.
One successful solution has been to bend pipe or rebar into the shape of a staple and then
drive it through the mesh into the bank. The major drawback is that a rock and wire mattress
generally costs more to place than a comparable riprap blanket.
6.6.4 Gabions
Gabions are patented rectangular wire boxes (or baskets) filled with relatively small-size stone,
usually less than 200 mm (8 in.) in diameter. Where flow velocities are such that small stone
would not be stable if used in a riprap blanket, the wire boxes provide an effective restraint.
Limiting recommended maximum velocity for use of gabions ranges from 2.4 to 4.6 m/s (8 to
15 ft/s), depending on the manufacturer. Gabions are used primarily for revetment-type
structures, but have also been used for dikes and sills. HEC-11 (Brown and Clyde 1989)
provide design guidelines for gabion revetment.
Gabions act as a large heavy porous mass having some flexibility. The baskets are
commercially available in a range of standard sizes and are made of heavy galvanized wire
(coated when used in a corrosive environment). They are supplied at a job site folded flat and
are assembled manually, using noncorrosive wire. The baskets are normally 0.5 m deep by 1
m by 2 m (1.6 x 3.3 x 6.5 ft) and are set on a graded bank for revetments. A filter blanket or
synthetic filter fabric is used, where required, to prevent leaching of base material and
undermining of the baskets.
6.6.5 Sacks
Burlap sacks filled with soil or sand-cement mixtures have long been used for emergency work
along levees and streambanks during floods (Figure 6.21). In recent years commercially
manufactured sacks (burlap, paper, plastics, etc.) have been used to protect streambanks in
areas where riprap of suitable size and quality is not available at a reasonable cost. Although
most types of sacks are easily damaged and will eventually deteriorate, those sacks filled with
sand-cement mixtures can provide long-term protection if the mixture has set up properly.
Sand-cement sack revetment construction is not economically competitive in areas where good
stone is available. However, if quality riprap must be transported over long distances, this type
of sack revetment can often be placed on an eroding streambank at a lesser cost than riprap.
If a permanent revetment is to be constructed, the sacks should be filled with a mixture of 15
percent cement (minimum) and 85 percent dry sand (by weight). The filled sacks should be
placed in horizontal rows like common house brick beginning at an elevation below any toe
scour (alternatively, riprap can be placed at the toe to prevent undermining of the bank slope).
The successive rows should be stepped back approximately 1/2-bag width to a height on the
bank above which no protection is needed. The slope steepness of the completed revetment
should be no more than 1:1. After the sacks have been placed on the bank, they can be hosed
down for a quick set or the sand-cement mixture can be allowed to set up naturally through
rainfall, seepage or condensation. If cement leaches through the sack material, a bond will
form between the sacks and prevent free drainage. For this reason weepholes should be
included in the revetment design. The installation of weepholes will allow drainage of
groundwater from behind the revetment thus helping to prevent pressure buildup that could
cause revetment failure. Detailed design guidelines for commercially available articulating
grout filled mattresses are provided in HEC-23 (Lagasse et al. 2001).
6.39
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