Glossary of Flood Control Terms. Compiled by Barbara Lucas, League of Women Voters, from information furnished by Leo White, Multnomah County Drainage District No. 1, and by Jerry Schmunk and Ray Jaren, Corps of Engineers

Berm – A horizontal ledge cut into or at the top or bottom of an earth bank or cutting, to ensure the safety of a long slope.

Blister – A cover of impermeable soil that rises from water pressure. If punctured, a blister can become a boil.

Boil – A concentration of seepage in one spot, usually caused by pressure from the river on a strata of coarse sand or gravel.

Dike – Somewhat the same as a levee. The Portland District of the Corps of Engineers prefers to use the term “levee” for flood protection projects, since dikes are also used in some parts of the world for coastal protection, and since the Corps also constructs pile dikes. The latter consists of timber piling extending out into a river which are built for river control and to reduce shoaling.

Levee – As defined by the Corps of Engineers, a levee is a compacted embankment built alongside a river for the purpose of preventing high water from flooding the adjoining land.

100-Year Flood – A flood which on the average will be equaled or exceeded once in every 100 years. Computations are based on an examination of past records and probability. Most streams experienced 100-year floods during the flooding in December 1964 and January 1965.

Overtopping – When Water reaches greater height than the levee.

Ponding – A type of drainage management which allows natural depressions to serve as temporary water-holding areas until the pumping system can pump the water away. If these ponding areas are improved, they should be called storage areas or reservoirs.

Relief Well – A well drilled into the landward side of a levee to relieve ground pressure. This is done usually to protect a structure from floating or from ground movement.

Revetment – A facing of stone, concrete, or even such materials as tires, placed on a riverbank or levee to protect them from erosion.

Riprap – Material, usually rock, placed on slope or bank to prevent erosion.

Scour – Erosion caused by rapid flow of water.

Tide Box – A hinged plate that allows water to flow one way through a structure.

Toe – Bottom of levee slope.

Toe Drain – Either a pipe or open ditch along the foot of the landward side of a levee to carry off seepage. This prevents saturation of the levee.

Wave Wash – Erosion caused by waves.