Tutorial 8

The objective of this assignment is to design the sewer system on IIT Kanpur campus.  A map of IIT Kanpur campus (Figure 1), proposed alignment of sewers in IITK campus (Figure 2), and contour map of IIT K campus (Figure 3) are provided for this purpose.

Please go through the accompanying handout on sewer design before attempting this assignment.  As specified in this handout, the required calculations must be done in a tabular form.  A sample table for this purpose is attached (Table 1).  A separate table must be created for each trunk or main sewer designed.  Design of branch sewers is not required.

Following points should be remembered during the design:

1.      Minimum diameter of a sewer is 150 mm.  Other available diameters are 200mm and higher at increments of 100 mm.  Take Manning’s coefficient to be 0.013.

2.      Check that the sewers are not more than 0.8 full at ultimate peak flow in year 2020.

3.      The velocity in the sewer should be at least 0.6 m/s at the present peak flow.  This is required to ensure that particles are not deposited in sewers on a permanent basis.

4.      Maximum velocity in the sewer at no time should be more than 3 m/s.

5.      Manholes should be built at every change of alignment, gradient or sewer diameter.

6.      For straight sewers of uniform diameter and constant slope, cleaning manholes should be provided every 100m.

7.      Main sewers should be connected to trunk sewers (or, branch sewers should connect main sewers), at angles ranging from 30 to 90 degrees.  The center-line of the smaller sewer should be at least 0.4 m above the center-line of the larger sewer at the manhole.  This will ensure dissipation of excess energy of water in the smaller sewer.

8.      Manholes are connected over the center-line of the sewer.  A minimum drop of 0.03m is provided between center-line of sewers entering and exiting manholes.

9.      The diameter of a sewer exiting a manhole must always be greater than or equal to diameter of the sewer entering the manhole.

10.  The top of a sewer entering a manhole must never be at a lower elevation than that of a sewer exiting the manhole.

11.  Typical slopes in sewers vary from 1 per 1000 to 10 per 1000, with larger diameter sewers having less slope.  Remember to put the least slope possible, while ensuring that the minimum velocity criterion is satisfied, such that sewers do not go too far underground.

12.  Design of branch sewers is not required.  However, assume them to have a uniform slope of 8 in 1000.  This is required to fix the levels of the main and trunk sewers.

13.  A sewer (branch, main or trunk) should be at least 1 m below the ground surface.  This should be kept in mind when fixing the depth of the main or the trunk sewers.

Table 1.      Tabular Calculations for Sewer Design

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Manhole Length, m Average Sewage Flow, (2020) Litre/s Peak Flow, (2020) Litre/s Diameter, mm Slope Discharge, (2020)  Litre/s Velocity, (2020) m/s Present Peak Flow, Year 2020 Litre/s Self-Clensing  Velocity (2000) m/s Total Fall, m Invert Elevation, m From To Full Actual Full Actual Upper Lower

·        A separate table is required for each trunk and main sewer

·        Locate manholes on a particular sewer, giving each an identification number (columns 1-2)

·        Determine the length of each section (between two manholes) (column 3)

·        Determine the average sewage flow in each section, i.e., at the starting manhole in year 2020 (column 4)

·        Determine the peak flow in each section by multiplying by the peaking factor (column 5)

·        Determine the diameter and slope such that the sewer flows 0.8 full at ultimate peak flow (columns 6-7)

·        Remember that a minimum diameter of 150 mm must be provided

·        Calculate full discharge for the pipe diameter selected and compare with actual full discharge (col. 8-9)

·        Calculate corresponding velocities and check that they are not less than 0.6m/s, or more than 3.0 m/s (columns 10-11)

·        Calculate the present peak flow (Year 2000) in each sewer section (column 12)

·        Calculate the velocity corresponding to present peak flow, and ensure that this is more than 0.6 m/s (column 13)

·        Calculate total fall for each section (column 14)

·        Calculate the invert level, i.e., level at the bottom of the sewer, at the upper end (starting manhole) and lower end (ending manhole) of the sewer section (columns 15-16)

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Figure 1.   Map of IIT Kanpur Campus

 Branch Sewers

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