Subhorizontal gravity drains or horizontal drains (HDs) are commonly used drainage measures for groundwater regime management in landslide mitigation. The effectiveness of an HD depends on several factors, such as the hydraulic properties of the slope and the location, orientation, length, spacing, perforation arrangement, and envelope material of the drainpipe. The effect of perforations on the performance of an HD is quantified by the entrance resistance (ER), which depends on the physical properties of the pipe. HDs are considered ideal drains in current design practice of landslide mitigation works, and the effect of the ER on their hydraulic performance has not been adequately evaluated. This study is conducted to evaluate the impact of perforation arrangement on the hydraulic performance of HDs. Among the three perforation types examined in the experimental study, the HDs with circular and longitudinal perforations have the highest and lowest performance, respectively, and the performance of that with circumferential perforation lies in between. For the same pore pressure, the discharge of the circular holes is more than twice that of the longitudinal slots. In addition, the importance of considering the entrance head loss on the hydraulic performance and design of HDs is highlighted.
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