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15 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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16 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.