referred to as the 100-year flood. The base flood is the national standard used by the NFIP and all federal agencies for the purposes of requiring the purchase of flood insurance and regulating new development (<>)

Base Flood Elevation (BFE)—The elevation of a flood having a 1 percent chance of being equaled or exceeded in any given year (FEMA, 2003)

BathymetryThe measurement and study of water depths. Traditionally bathymetry has been expressed with contours and hydrography with spot depths (Maune, 2007)

Benchmark—A permanent monument established by any federal, state, or local agency, whose elevation and description are well documented and referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) or the North American Vertical Datum of 1988 (NAVD 88) (FEMA, 2003)

Benefits—Positive effects of an action. For FEMA flood hazard mitigation projects, benefits are defined as avoided damages and losses (FEMA, 2001)

Calibration—The process of identifying and correcting for systematic errors in hardware, software, or procedures; determining the systematic errors in a measuring device by comparing its measurements with the markings or measurements of a device that is considered correct (Maune, 2007)

Catchment Area—An area of land that is occupied by a drainage system consisting of a surface stream or a body of impounded surface water, together with all tributary surface streams and bodies of impounded surface water that drains into a single outlet; also called drainage basin or watershed (<>)

Coastal FloodingFlooding that occurs along the Great Lakes, the Atlantic and Pacific Oceans, and the Gulf of Mexico (FEMA, 2003)

Confidence LevelThe probability that errors are within a range of given values (Maune, 2007)

Cooperating Technical PartnersParticipating NFIP communities, regional agencies, and state agencies that are active participants in the FEMA Flood Hazard Mapping Program (FEMA, 2003)

Cross SectionA line across a floodplain, developed from topographic data, at which a computation of flood flow has been made to establish a potential flood elevation (<>)

Datum—A common vertical or horizontal elevation reference point (<>)

• Ellipsoidal Datum—A set of constants specifying the coordinate system used for geodetic control, that is, for calculating coordinates of points on the Earth; also known as geodetic datum (<>)

• Orthometric Datum—The reference surface from which orthometric heights are measured (i.e., NAVD 88 or NGVD 29)

• Tidal Datum—A surface with a designed elevation from which heights or depths are reckoned, defined by a certain phase of the tide. A tidal datum is local, usually valid only for a restricted area about the tide gage used in defining the datum (Maune, 2007)

Design Storm—A rainfall event of specified size and return frequency that is used to calculate runoff volume. It is assumed that the design storm for a given frequency will produce a simulated runoff peak and volume having the same return frequency. Thus, a 100-year design storm should produce a 100-year runoff and volume (New York Department of Environmental Conservation, 1992)

Detailed Study, Coastal—A coastal flood hazard study that uses transects and offshore bathymetry to conduct detailed erosion, wave height, and wave runup analyses and to prepare floodplain mapping. The analysis results in the determination and publication of BFEs and designation of the coastal high-hazard areas (V zones) (FEMA, 2003)

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