A floodwall is a freestanding, permanent, engineered structure designed to prevent encroachment of floodwaters. Floodwalls, which are typically constructed of reinforced concrete or masonry, provide a barrier againstinundation, protect structures from hydrostatic and hydrodynamic loads, and may deflect flood-borne debris and ice away from the building. Floodwalls are normally placed some distance from the building to avoid having to make structural modifications to the building. Depending on the site topography, floodwalls may protect only the low side of the site (and must tie into high ground), or they may surround the site. Floodwalls that surround a site have openings that provide access to the site.

The most common types of floodwalls are gravity, cantilever, buttress, and counterfort (see graphic).

Gravity Floodwalls: As the name implies, a gravity floodwall depends on its weight for structural stability. Structural stability is attained by effective positioning of the mass of the wall at its base, rather than by the weight of the retained materials (water or soil) on top of the wall foundations. A gravity floodwall resists overturning primarily because of the dead weight of the construction material (concrete or masonry); it is simply too heavy to be overturned by a lateral flood load.Compared to the other types of floodwalls that are discussed, gravity floodwalls are relatively easy and straightforward to construct. However, the primary disadvantage of gravity floodwalls is that they require massive amounts of material compared to the other floodwall types.

Cantilever floodwalls are the most common type of floodwall because they are economical to design and construct. They use cantilever action to retain the mass behind the wall. Cantilever floodwalls are usually constructed of reinforced concrete or concrete block with steel reinforcing bars embedded in the concrete core of the wall. Stability is achieved partially from the weight of the soil on the heel portion of the base and from the weight of the wall itself balanced by the lateral forces and overturning moments.

Buttress and Counterfort Floodwalls: The only difference between a buttress and counterfort floodwall is that the transverse support wall is on opposite sides. In a buttressed floodwall, the transverse support wall is on the heel side, opposite the retained materials, and in a counterfort wall, the transverse support wall is on the toe side. Counterfort floodwalls are more widely used than buttressed floodwalls because their transverse support walls are hidden under the retained material (water or soil), whereas buttress floodwalls occupy what could otherwise be usable space in front of the wall.

(FEMA 2013)