Fig. 5.3 shows the common egg-shaped sewer cross-section. The calculation of areas and the wetted perimeters of these sewers at various depths of the sewage involve tedious calculation works.
Curves of the hydraulic elements of the standard forms of the egg-shapped sewers are prepared in the same way as for circular sections. The design work can be easily done by the use of these standard curves.
The hydraulic mean depth of egg-shaped sewers of equivalent section (of circular sewer) is the same of that circular sewers running full. For small depth of flow these values are more than that of equivalent circular sewer sections, because the velocity developed in egg-shaped sections is more in such conditions.
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Table 5.12 shows the difference in the proportional velocities of egg-shaped and the circular sewer sections.
Fig. 5.4 shows the proportional hydraulic elements of the egg-shaped sewers, which are used for their design.
Example 1:
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(a) The main sewer was designed to serve an area of 44 sq. km with an average population of 185 persons/hectare. The average rate of sewage flow is 340 litres/ capital/day. The maximum flow is 50% in excess of the average together with the rain fall equivalent of 12 mm in 24 hours, all of which are run off. What should be the capacity of the sewer in cu.m./sec.?
(b) Find the minimum o velocity and gradient required to transport coarse sand through a sewer 30 cm diameter with sand particles of 0.1 mm. diameter and specific gravity of 2.65. Assume k = 0.04 and f = 0.012.
Solution:
Example 2:
A 30 cm diet sewer having an invert slope of I in 150 was flowing full What would be the velocity of flow and discharge? (n = 0.013).
Is the velocity self-cleaning?
What would be the velocity and the discharge when the same is flowing 0.20 and 0.8 of its full depth?
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Solution:
Example 3:
A sewer, having diameter 1.20 m, is laid at a gradient of 1 in 400. Calculate the velocity of flow and discharge through this sewer when running one-half full. Assume N = 0.012 in Manning’s formula.
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Solution:
As per Manning’s formula:
