flankment

Flankment is a term used to describe an earthwork or structure forming or protecting the flank, or side, of a terrain feature or built work. In civil engineering and transportation projects, flankment typically refers to the side slope or earthen wall along an embankment or cut on roads, railways, or canals. Flankment works may be natural or reinforced, employing grass or vegetation, reinforced concrete facing, retaining walls, or sheet-pile structures to stabilize the ground, control seepage, and manage drainage. They are designed to resist erosion and provide safe clearance for vehicles and trains.

In fortification, flankment denotes defensive works intended to protect the flank of a fortress or defensive line. Such works may include additional walls, outworks, or ditches that extend protection along the side and prevent attackers from turning the position. In historical contexts flankments were integrated into bastioned or trace italienne designs to maintain continuous coverage of the vulnerable flank.

The term is not common in contemporary engineering language, with embankment, side slope, or retaining wall usually preferred. When encountered, flankment generally conveys the concept of side protection or support rather than a distinct design category. Related terms include embankment, slope protection, retaining wall, and defensive outwork.