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Many of the tests ended after 48 hours; however, in some cases the runoff had not
completely drained through the silt fence fabrics. The relationship of the runoff level
with respect to time for the last 24 hours of the test can be approximated by a linear
function. The time for the flume to drain completely was estimated by extrapolating
linearly from the last few data points. A typical test where runoff remained in the flume
even after 48 hours and the estimated time of complete drainage is presented in Figure
5.5.
L
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Filling
Draining
0 1440 2880 4320 5760 7200 8640
Time, min
Observed
Estimated
Figure 5.5 Observed Levels During a Flume Test and the Extrapolated Time for
Complete Drainage
5.1.8 TSS Removal Efficiency
The observed TSS removal efficiency range, mean, median, and standard
deviation for each silt fence are presented in Table 5.6. Individual test results are given in
Appendix D. Removal efficiencies were based on influent and effluent suspended solids
mass loads. Calculations of these removal efficiencies also are provided in Appendix D.
The highest removal efficiencies were observed for the non-woven Mirafi fabric.
This fabric also had the lowest flow rates of the fabrics tested. Increased detention time
for the suspended solids behind the silt fence lead to increased removal efficiency.
45