1. Tunnel Sections. 2. Open Cut Sections. ─────────────────────────────────────────────────────────────────────── Type A. Type B. Type C. Type D. Where Rock Where Rock Where Rock Where Rock 16′ 6″ Where Rock is more is more is between drops below Sewer. 25′ is above than 16′ than 7′ Springing Springing Fill Springing above and less Line and 7′ Line on Line Springing than 16′ above either Line. above Springing Side. Springing Line on Line on both Sides. both Sides. Mill Creek Sewer, St. Louis, Eng. Record, Vol. 70, pp. 434, 435. [Illustration: 3. Circular Concrete Section in Soft and Hard Ground, Eng. Record, Vol. 59, p. 570. ] [Illustration: 4. Semi-Elliptical Section, Louisville, Ky., Eng. News, Vol. 62, p. 416. ] [Illustration: 5. Reinforced Concrete Sewer, Harlem Creek, St. Louis, Eng. News, Vol. 60, p. 131. ] [Illustration: 6. U-Shaped Section, San Francisco, Eng. News, Vol. 73, p. 310. ] FIG. 23. Equivalent sections are sections of the same capacity for the same slope and coefficient of roughness. They have not necessarily the same dimensions, shape, nor area. The diameter of the equivalent circular section in terms of the diameter of each special section shown is given in Table 18. The inside height of a sewer is spoken of as its diameter. For example let it be required to determine the rate of flow in a 54–inch egg-shaped sewer on a slope of 0.001 when _n_ = .015. First convert to the equivalent circle. From Table 18 the diameter of the equivalent circle is 1⁄1.295 times the diameter of the egg-shaped sewer, which becomes in this case 43 inches. From Fig. 16 the capacity of a circular sewer of this diameter with _S_ = 0.001 and _n_ = .015 is 28 cubic feet per second, which by definition is the flow in the egg-shaped sewer.