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the difference between measured and predicted flow rates is caused by sediment clogging
the fabric openings.
The woven Mirafi fabric exhibited such clogging after a series of tests with clean
water (probably from scour of the sand). In subsequent flume tests the fabric behaved as
if it were clogged from the beginning. This sample was designated Mirafi A and another
sample, Mirafi B, was placed in the flume in order to observe the performance of clean
fabric. The flow rates given for the Mirafi fabric in Table 5.5 are those of the B sample.
Runoff did not flow through approximately 2 cm of the upper portion of the silt
fence fabrics during the tests. This behavior also was noticed by Crebbin (1988) in his
experiments.
Table 5.5 Flow per Area of Silt Fence ( L/s . m 2 ) as a Function of Head
Head, m Belton w Exxon w Mirafi B nw Amoco w
0.15 2.4 0.38 0.39 5.8
0.30 5.5 0.82 NA NA
The hydraulic behavior of the rock berm in all of the flume tests was practically
identical, indicating that there was no clogging. A steady state level was maintained by
the rock berm for all five tests at the maximum possible flow rate in the flume (90
L/s . m 2 ). The average steady state level was 0.06 m. Approximately 0.4 m of rock berm
was not submerged. TxDOT (1992a) recommends a maximum flow rate of 40 L/s . m 2 ,
which is greatly exceeded in berms constructed according to the TxDOT design criteria.
5.1.7 Determining Detention Times for Flume Tests
Detention times were calculated for each test of the silt fences and rock berm and
are provided in Appendix D. An average detention time, T avg , was determined for each
∆V of runoff passing through the controls. A volume-weighted detention time, T d , for
each test was calculated by:
T
d
=
∑ Tavg∗
∆V
∑ ∆V
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