Gravity sewers: from population to pipe diameter

In a gravity sewer network, water moves only thanks to the gradient. This makes design an exercise in balance: too little gradient and the network fills up and stagnates; too much and the excavation depth grows until the cost gets out of control.

Flow is not water consumption

The starting point is the population served and the consumption rate, but the calculated flow must include the peak factor — because the network has to cope with the busiest hour, not the daily average. On top of this comes infiltration, groundwater that enters through joints and manholes. In old networks infiltration can be a considerable part of the real flow.

Two limits pulling in opposite directions

The upper limit is the filling ratio: the pipe is not designed to run full, but up to a set ratio, to leave ventilation and a reserve. The lower limit is the minimum self-cleansing velocity, usually around 0.6 m/s, below which solids settle and the pipe blocks over time. That is why choosing a larger diameter "to be safe" often makes the situation worse: the same flow in a larger section gives a lower velocity.

Manholes and depth

Manholes are not only access points; they are the places where direction, gradient or diameter changes. The depth must cover the connections from properties and protect the pipe from loads, but every extra metre significantly raises the cost of excavation and earth support.

How AlbGIS helps

• Sewer Network — modelling on a map with flow accumulation by population, filling and velocity using Manning, and diameter suggestions.
• Combined Network — when the same collector carries both wastewater and rainwater.
• Septic Tanks — for areas without a network, sizing according to EN 12566.