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emained in suspension and were able to pass through the silt fence because the diameters
were smaller than the apparent opening size (AOS) of the fabric.
4.2.4 Turbidity Reduction Efficiency of Silt Fences
The amount of turbidity reduction caused by silt fences was determined by
comparing concentration in the pond created by the silt fence and concentrations in
samples collected below the silt fence. The calculated removal efficiency for silt fences
is:
Turbidity reduction % =
Upstream Turbidity - Downstream Turbidity
Upstream Turbidity
x 100
The median removal for all samples was only 2%, with a standard deviation of
±10%. Removals range in magnitude from -32% to 49%. The computed values for all
samples are presented in Table B4 in Appendix B. Increases in turbidity below the
temporary control fence probably are the result of the same sources of error that resulted
in negative removals for TSS. Since turbidity is a function of the number of small
particles in a sample, these results are consistent with the finding that all of the particles
remaining in suspension above the fence are smaller that the AOS of the fabric and
consequently no reduction should be expected except for the particles which become
attached to the fabric.
4.2.5 Observations of Silt Fence Performance
Comments of construction project supervisors indicate that maintenance of
temporary controls was not a consideration. Controls are removed or replaced frequently
because of changing conditions on the construction site so that maintenance seldom is
needed. However, various installation and maintenance deficiencies were noted during
the duration of the study.
These silt fence installations are not designed as hydraulic structures to
accommodate runoff from a rainfall event of a particular frequency, and failures caused
by volumes of runoff that exceed the capacity are common. A single release around the
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