Sewerage : Design philosophy

Sewerage : Design philosophy

1 Design philosophy

A sewer system is a network of pipes used to convey storm runoff and/or wastewater in an area.

The design of sewer system involves the determination of

Diameters,

Slopes, and

Crown or invert elevations for each pipe in the system

2Constraints and assumptions

Free surface f low exits for the design discharges; that is, the sewer system is designed for

'gravity of low';

pumping stations and pressurized sewers should be avoided as much as possible (are not considered here)

The sewers are of commercially available circular sizes

The design diameter is the smallest commercially available pipe having f low capacity equal to or greater than the design discharge and satisfying all the appropriate constraints

Sewers must be placed at a depth such that they

Will not be susceptible to frost,

Will be able to drain basements, and

Will have sufficient cushioning to prevent breakage due to ground surface loading.

To these ends, minimum cover depths must be specified.

The sewers are joined at junctions such that the crown elevation of the upstream sewer is no lowerothe downstream sewer

To prevent or reduce excessive deposition of solid material in the sewers, a minimum permissible flow velocity at design discharge or at barely full-pipe gravity flow I specified To prevent scour and other undesirable effects of high- velocity f low, a maximum permissible flow velocity is also specified

At any junction or manhole, the downstream sewer cannot be smaller than any of the upstream sewers at that junction

The sewer system is a dendritic, or branching, network converging in the downstream direction without closed loops