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are characterized by lower permeabilities, longer holding times, and a greater tendency
for particle interception than comparable thin fabrics (Crebbin, 1988). The fabric must
not be susceptible to elongation in order to maintain the integrity and prevent deformation
of fabric openings.
Other fabric characteristics that influence the operation of the system include
tensile, puncture, burst, and tear strength. Resistance to climatic conditions are partially
responsible for fabric longevity and the ability to circumvent failures. Tears, unraveling,
or rotting severely reduce the efficiency of solids removal by the fabric.
Particle retention on the upstream face or within the width (non-woven fabric
only) of the fabric and subsequent pore size reduction may result from high filtration
efficiencies. This phenomenon accelerates the percentage of particle capture and causes
the permeability of the fabric to be reduced from the bottom to the top. Retention times
increase as the permeability declines and/or the surface area for flow is restricted.
Kouwen (1990) states that extensive theoretical modeling and testing to predict flow rates
and behavior has been accomplished by Bell and Hicks (1984) and by Koerner (1984,
1985).
Experimental results indicate that the sedimentation pattern is a delta formation
process. Initially, large solids settle near the point of velocity reduction. Additional
deposition occurs sequentially at the existing delta face until the delta reaches the
obstruction created by the fence. The surface area accessible for filtration is diminished
and flow through the fabric is reduced as the delta blocks the flow to a portion of the
fence (Kouwen, 1990). The volume capacity is diminished over time by the deposition of
sediments through settling, and may be limited by the strength of the structural support
system. Uncontrolled release of suspended solids may occur by over-topping or an end
run.
2.4 Silt Fences
Silt fences are temporary, vertical structures of wood or steel supports, wire mesh
reinforcement, and a suitable permeable filter fabric (Goldman et al., 1986). Silt fences
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