Design Calculations

Design proceeds from the most remote point of the system downward. Where a branch joins a line already designed, a new design is started at the upper end of the completed intersection. Table 6 gives the various components of sanitary sewer system design.

Columns 1-6 describe the location of sewer manholes and length of sewer lines between the manholes for a particular line.

Column 7      Reads the corresponding area increment of sewer section and their sum is entered in column 8, e.g. Tributary Area of line 3 = Tributary Area of Line 2 + Area of Line 3.

Column 9      Gives the population served by each sewer line. This is obtained by multiplying column 8 with population density (300).

Column 10    Gives the sewer flow (million litre per day) through each line, i.e. sewer flow in sewer line = 250 × 0.9 × column 9 × 10- 6

Column 11    Shows ground water infiltration for each area in million litre per day = 20,000 × 10- 6 × column 8

Column 12    Records Peak flow = column 10 × 3.5 + column 11

Column 13    Gives peak flow in litre/second = column 12 × 106/(24 × 3600).

Columns 14-15         Indicate the diameter and slope of the pipes determined from Manning's chart for each flow and maintaining a minimum flow velocity of 0.6 m/sec. A number of slopes is to be tried before a satisfactory velocity is obtained.

Columns 16-17         Provide the velocity and discharge through pipe under flowing full condition respectively. This is determined from Manning's chart for selected diameter and slope of pipe.

Column 18    Provides ratio of actual discharge to the full discharge = (Column 13)/(Column 17)

Column 19    Calculates the ratio of actual velocity to the velocity at full discharge with the help of hydraulic element curve for circular pipe given in "Manual for Sewerage and Sewage Treatment".

Column 20    Gives actual velocity of flow in metre per second = column 19 × column 16

Column 21    Provides fall for given slope and length of pipe = column 6 × column 15

Columns 22-23         Indicate the invert levels. Please note that invert elevation at lower end of line is same as that at upper end of next line except where there is a change of size or direction. For example, in manhole 7 the lower end of line 5 is upper end of line 6, above the manhole, diameter of line 1 is 200 mm. Trial indicates that a 200 mm pipe would require a slope steeper than ground to carry a flow of 27.67 lps in line 6. Since the sewer invert of line 5 is lower than dictated by minimum cover, a larger sewer pipe of 250 mm diameter is used on a lower slope. Since a change in size occurs the crown of intersecting sewers are matched. The crown of line 5 is 55.755 (Invert elevation + Diameter = 55.605 + 105); that of line 1, is at 56.145. Therefore, crown of line 6 must be at 55.755 giving an invert elevation of 55.505: Manholes number 6.0 and 5.0 are handled in a similar fashion. At manhole 4.0 there is change in direction but no change in pipe size, the invert is dropped by 30 mm across the manhole.