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Rack clogging can produce dramatic changes in channel hydraulics. Previous studies have investigated the hydraulics of trash racks for various parameters, but the methodology and the findings were not sufficiently refined. Free-surface depression has also been neglected so far. This study considers the rack blockages as impermeable and box-shaped accumulations (instead of considering their bar thicknesses or spacings) for the hydraulic conditions. Hence, flume experiments were performed to clarify the impact of the governing variables on the rack head loss and to examine the characteristics of free-surface depression (i.e., the length of free-surface depression and maximum depth of the depression) because of predefined blockage ratios. The results prove that the rack head loss and flow turbulence behind the rack mainly depend on the rack blockage and Froude number. However, the results for the blockage ratio ≤0.13 at the approach Froude number ≤0.12 has a minor effect on the resulting rack head loss; therefore, the effects are negligible. This study proposed design equations that determine the rack head loss, length of free-surface depression, and maximum depth of the depression behind the rack because of the box-shaped accumulation body that could be used by water engineers. Furthermore, the study improves upon the process understanding of rack blockages to avoid the potential hazards of open channel infrastructure.

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