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Pipe Flow Charts The following pipe flow charts have been derived by the U. S. Public Roads Administration, Division Two, Washington, D. C. These charts are designed to enable direct solution of the Manning formula for circular pipes flowing full and for uniform part -full flow in circular pipes. The "n" scales of 0.013 and 0.024 have been inserted to facilitate the use of these charts for storm drainage systems in Hawaii. The following examples will help to explain the use of the pipe flow charts. EXAMPLES A. Determine the depth and velocity of flow in a long 30 -inch pipe, n 0.013, on a 0.5- percent slope (So = 0.005) discharging 25 cfs. Enter the 30 -inch diameter chart at Q = 25 on n = 0.013 scale, follow up to intersection with line for slope So 0.005, and read normal depth do = 1.8 feet and normal velocity V = 6.6 fps. To find criti- cal depth, enter chart at Q = 25 on n = 0.013 scale, and read critical depth do = 1.6 feet at intersection with dotted critical curve. Also critical velocity Vc = 7.6 fps. (Note: Critical depth and velocity would be the same, regardless of pipe roughness.) B. Determine friction slope for a 30 -inch corrugated metal pipe, n = 0.024, on a slope So = 0.008 ft /ft with a discharge Q = 25 cfs. Enter the 30 -inch diameter chart at Q = 25 on n = 0.024 scale. Note that this ordinate falls to the right of the 0.008 slope line, therefore, the pipe will flow full. Read friction slope Sf = 0.012 at the line for depth equal to pipe diameter. (Note: Q = 25 x 0.024 =40 cfs on the Q -scale for n = 0.015.) 0.015 22