Hydraulic Resistance of Small-Diameter Helically Corrugated Metal Pipe
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Hydraulic Resistance of Small-Diameter Helically Corrugated Metal Pipe

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    The hydraulic resistance of helical corrugated metal pipe (CMP) is strongly dependent on helix angle – the angle of the corrugations relative to the longitudinal axis of the pipe. Smaller-diameter pipes have smaller helix angles. Helical corrugations with a significant downstream orientation (small helix angle) offer less resistance to flow than annular corrugations. Through this study we hope to ascertain the currently unknown Manning n values to use in the design of helical CMP. This report documents an experimental investigation of Manning n values for small diameter helical CMP with 2.67” x 0.50” corrugations. Sixty-foot lengths of 18-inch, 15-inch and 12-inch pipe were tested in the University of Kansas hydraulics laboratory. Each test pipe was assembled from three 20-foot pipe sections. The helical CMP sections were manufactured with 12 inches of annular corrugations at each end to accommodate the connector bands. The 18-inch pipe was tested on a 0.39% slope, the 15-inch pipe on a 0.85% slope, and the 12-inch pipe on a 0.68% slope. The pipes were tested flowing full and partly full at depths ranging from 30% to 90% percent of the pipe diameter. The full-flow tests yielded Manning n values of 0.0178 for the 18-inch pipe, 0.0154 for the 15-inch pipe, and 0.0142 for the 12-inch pipe. These n values are considerably lower than the accepted values for annular CMP but higher than the values recommended by the American Iron and Steel Institute for helical CMP. The Manning n values obtained for partly full flow varied with the depth of flow. These tests yielded n values in the following ranges: 0.0184-0.0206 for the 18-inch pipe, 0.0170-0.0180 for the 15-inch pipe, and 0.0160-0.0194 for the 12-inch pipe. Manning n values for partly full flow tend to be higher for shallower flow. Manning n values for nearly fully flow (80-90% full) are about 10% higher, on average, than n values for full flow. The annular corrugations at the section joints cause significant local energy losses. The Manning n values computed from the experimental data account for these local losses across the joints as well as the losses through the helical sections.
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