Layout of System

The sewer system layout involves the following steps :

(a)        Selection of an outlet or disposal point.

(b)       Prescribing limits to the drainage valley or zonal boundaries.

(c)        Location of trunk and main sewers.

(d)       Location of pumping stations if found necessary.

A tentative layout is prepared by drawing sewer lines along the streets or utilities easements. Arrows show the direction of flow, which is generally the direction in which ground slopes. The disposal point may be a treatment plant or a pumping station or a watercourse, a trunk or intercepting sewer. It is desirable to have discharge boundaries following the property limits. The boundaries of sub zones are selected on the basis of topography, economy or other practical consideration. Trunk and main sewers are located in the valleys.

The most common location of sanitary sewer is in the centre of the street. A single sewer serves both sides of the street with approximately same length for each house connection. In very wide streets, it may be economical to lay a sewer on each side of the street adjacent to curb or under the footpath. Manholes are provided at all sewer intersections, changes in horizontal direction, major change in slopes, change in size and at regular intervals. These manholes are numbered as per the method described in the nomenclature.

The vertical layout is dictated by the need to provide minimum cover and the desirability of minimum excavation depending upon the pipe size and expected loads. It is design practice to provide a minimum cover of 1 m at the starting point in the case of sanitary sewer network and 0.5 m for storm drainage system.

If the sewer changes direction in a manhole without change of size, a drop of usually 30 mm is provided in the manhole. If the sewer changes size, the crown of inlet and outlet sewers are set at same elevation. The vertical drop may be provided only when the difference between the elevations is more than 600 mm or more. The following invert drops are recommended by Manual on Sewerage and Sewage Treatment (CPHEEO, 1993):

(a)        For sewers less than 400 mm : Half the difference in diameter.

(b)       400 mm. to 900 mm : 2/3 the difference in diameter.

(c)        Above 900 mm : 4/5 the difference in diameter.

For storm sewer, similar procedure is adopted except lines are considered to run from inlet to inlet. Since inlets are generally at corners, the storm sewer runs normally from corner to corner. Area tributary to each inlet is noted according to ground contours in the plan.

Sewers as a design practice are not located in proximity to water supplies. When such situations are unavoidable, the sewers should be encased in sleeve pipes or encased in concrete. Tees or Yees should be provided for all house connections both for present and future locations so as to avoid breaking a hole into the side of a sewer.