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phenomenon is similar to sediment-laden runoff flowing down a slope in the field at a
high velocity. Sediment is deposited at the toe of the slope when the slope gradient and
runoff velocity decrease.
5.1.5 Description of Tested Controls
The change in efficiency over time was observed by running a series of tests
through the same control. Four types of silt fences and a rock berm were subjected to
cycles of simulated runoff events. The silt fences tested were constructed of geotextile
fabrics. Properties of the fabrics as reported by the manufacturers are summarized in
Table 5.4.
Table 5.4 Tested Silt Fence Fabric Properties
Type of Fabric AOS, # sieve Permittivity, sec -1 No. of Tests
size(µm)
Belton woven #30 (600) 0.4 9
Exxon woven #30 (600) 0.1 5
Mirafi non-woven #100 (150) 1.5 5
Amoco woven #20 (850) 0.2 5
An attempt was made to determine the percent open area (POA) of the woven
fabrics tested. The percent open area measures the percent of total fabric area that can
pass water, while apparent opening size measures the size of the openings. Information on
these characteristics would allow calculation of actual flow velocities through the fabrics.
Specimens of all the woven silt fence fabrics were scanned and copied on a
microfiche reader; however, only the Amoco 2125 fabric had well defined openings. The
magnified pore openings were measured and the area of voids calculated. Three scans of
the fabric were made on a bias across the specimen so that no one warp or weave would
appear on another scan. The average percent open area for three scans of that fabric was
3.3%.
A rock berm also was tested for sediment removal efficiency. TxDOT
specifications were followed in the construction of a Type I unreinforced rock berm,
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