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3.4 EFA PRECISION AND TYPICAL RESULTS
If the erosion rate is slow (less that 10 mm/hr), the error on
z
is estimated at 0.5 mm/hr. If the erosion rate is fast (more
than 100 mm/hr), the error on z is estimated at 2 mm/hr.
Therefore, the relative error on z is estimated to be less than
10%. Comparison between the c results for the sand and the
gravel tested in this study and shown on Figure 2.2 with
Shields data indicates a difference of about 10%. Therefore,
it is estimated that both z and are measured with a relative
error of about 10%.
The z versus curve is the result of a series of tests, each
of which is performed at a constant velocity. A typical series
of eight velocity tests lasts one work day. Figure 2.1 and Fig-
ure 3.4 show examples of EFA test results.
Figure 3.1. Schematic diagram and result of the Erosion
Function Apparatus (EFA).
(a) (b)
Figure 3.2. Photographs of the EFA: (a) general view, (b) close-up of the test section.
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Figure 3.3. Moody Chart (reprinted with permission from Munson et al., 1990).
Figure 3.4. Erosion function for a soil sample taken near Pier 27E of the existing Woodrow Wilson Bridge
(2.6 to 3.2 m depth): a) scour rate versus shear stress, b) scour rate versus velocity.
