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w
A S
h 1
Silt Fence
Figure 5.4 Volumetric Changes in Runoff Trapped Upstream of Silt Fence
runoff is equal to h 1 . The head downstream of the silt fence is assumed to be zero.
Therefore, the change in head across the fabric is equal to h 1 . At a later time, t 2 , the level
of the runoff is h 2 . The volume of runoff passing through the geotextile in this time
interval is ∆V.
∆V = (h 1 -h 2 )A S
These volumes were used to calculate a detention time, effluent suspended solids
load, and TSS removal efficiency for each test. These calculations are provided in
Appendix D. The flow-through rate, q, during this interval is the volume divided by the
time interval and submerged area of fabric (width of fabric times average level during
interval).
q= V/[(t - t )( h 1 +h 2
∆
2 1
)w]
2
Flow rates observed in the flume tests are approximately two orders of magnitude
less than the values stated by the manufacturers (Table 5.5). Recommended flow rates of
0.2 L/s . m 2 (Wyant, 1981) appear to reflect actual performance better than many used in
current practice, including the TxDOT (1992) recommendation of 27 L/s . m 2 . Much of
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