Glossary of Flooding Terms A-D

BACKWATER EFFECT – The rise in water surface elevation caused by some obstruction such as a narrow bridge opening, buildings or fill material that limits the area through which the water must flow. Also referred to as “heading up”.

BASE FLOOD – A term used in the National Flood Insurance Program to indicate the minimum size flood to be used by a community as a basis for its floodplain management regulations; presently required by regulation to be that flood which has a one-percent chance of being equaled or exceeded in any given year. Also known as a 100-year flood or one-percent chance flood. BASE

FLOODPLAIN – The floodplain that would be inundated by a 100-year (one-percent chance) flood.

BASIN – The total area from which surface runoff is carried away by a drainage system. Other comparable terms are “drainage area”, “catchment area”, and “watershed”. C.F.S. – Cubic feet per second. Used to describe the amount of flow passing a given point in a stream channel. One cubic foot per second is equivalent to approximately 7.5 gallons per second.

CHANNEL – A natural or artificial watercourse with definite bed and banks to confine and conduct flowing water.

Cross Section of a Channel and a Floodplain Image credit: Taranaki Regional Council
Cross Section of a Channel and a Floodplain
Image credit: Taranaki Regional Council

CROSS SECTION OF A CHANNEL AND FLOODPLAIN Image credit: Taranaki Regional Council

CHANNEL CAPACITY – The maximum flow which can pass through a channel without overflowing the banks.

CHANNELIZATION – The improvement of the water carrying capacity or flow characteristics of a natural or artificial channel by clearing, excavation, bank stabilization or other means. Also referred to as channel alterations.

CROSS SECTION – A graph or plot of ground elevation across a stream valley or a portion of it, usually along a line perpendicular to the stream or direction of flow.

DESIGNATED FLOODWAY – The channel of a stream and that portion of the adjoining floodplain designated by a regulatory agency to be kept free of further development to provide for unobstructed passage of flood flows.

DESIGN FLOOD – Commonly used to mean the magnitude of flood used for design and operation of flood control structures or other protective measures. It is sometimes used to denote the magnitude of flood used in floodplain regulations. Also referred to as Project Design Flood.

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