Canal design

An irrigation canal carries water from a headworks to distributaries and field channels. Design must carry the required discharge without eroding the bed and banks and without silting up. In alluvial soils, which are erodible and carry sediment, this balance is delicate, and Indian engineers developed regime theories from long observation of stable canals in the Indus and Ganga plains.

Kennedy's theory, from the Upper Bari Doab canal, proposed a critical velocity related to depth at which a channel neither silts nor scours. Lacey later proposed that a channel in regime, with its own natural sediment, adjusts its wetted perimeter, hydraulic radius and slope to the discharge and a silt factor depending on grain size. Lacey's equations give perimeter, velocity, hydraulic radius and slope directly from discharge and silt factor. They remain useful for alluvial canals and for scour estimates at bridges.

Lined canals and those in non-erodible material are designed by Manning's equation for uniform flow, usually with trapezoidal sections and side slopes suited to the soil or lining. Freeboard, the height of bank above full supply level, protects against waves and surges. Banks need adequate top width for inspection roads. Structures along the canal include falls, regulators, outlets, cross-drainage works and escapes. Seepage losses, which can be large in unlined canals, drive the case for lining.