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observation demonstrates that field performance can not be determined from current
parameters used to characterize the hydraulic properties of these fabrics.
The mean removal efficiency of the rock berm, when the removal efficiency of the
flume itself is subtracted, was approximately 7%. This removal efficiency is similar to
reports of sediment trapping efficiency for rock berms in the field (Reed, 1980; Weber
and Wilson, 1976). The influent flow rate for the rock berm was the highest used in the
tests and should have caused the TSS removal efficiency of the flume itself to be a
minimum; therefore, the 7 % efficiency is a minimum value.
5.1.9 Effects of Sediment Clogging and Rainfall Washing
The detention times for each test with respect to the time of testing are presented
in Figure 5.7. The rainfall data from a nearby monitoring station also are plotted. When
the series of tests of an individual silt fence are compared, subsequent tests show an
increase in the detention time. This phenomenon is likely caused by sediment
100000
80
70
Detention Time, min
10000
1000
100
60
50
40
30
20
Rainfall, mm
Rainfall
Belton woven
Exxon woven
Mirafi A non-woven
Mirafi B non-woven
Amoco woven
10
10
0
4/4/94 5/2/94 5/29/94 6/26/94 7/24/94 8/21/94 9/18/94
Date
Figure 5.7 Detention Times as a Function of Testing Schedule
accumulating in the fabric. In the field, this clogging effect may be more pronounced
because of construction and traffic debris (grass, paper, plastic, etc.). Tests on woven
47