Sewer design

Sewer design chooses pipe sizes and slopes so that every reach carries its peak flow without surcharging, while flowing fast enough at low flows to keep solids moving. Unlike water mains, sewers normally flow partly full by gravity, as open channels inside a pipe, so Manning's equation is used with partial-flow relations for depth, velocity and discharge.

Design flows come from population, per capita sewage (commonly about 80 percent of water supply), a peak factor that is larger for small contributing populations and an allowance for groundwater infiltration. Each reach is designed for ultimate peak flow at the end of the design period, typically 30 years for pipes, and checked for present minimum flow. Indian practice designs sewers to run at most about 80 percent full at ultimate peak, keeping headroom for ventilation and surges.

Self-cleansing velocity, enough to prevent solids depositing, is required at least once a day at present peak flow; values around 0.6 m/s at design peak and 0.8 m/s at ultimate peak are typical, and minimum slopes follow from this. Maximum velocities, around 3 m/s, limit abrasion. Minimum pipe size is set so that blockages are rare. Crown levels are matched at changes of diameter so that the smaller pipe does not flow back up. Depth of cover protects against traffic loads, while very deep sewers become expensive and may justify pumping. Pipe materials include RCC, vitrified clay, DI, HDPE and GRP, chosen for corrosion resistance, strength and cost. Hydraulic calculations are tabulated reach by reach from the upstream end, accumulating flows downstream.