Appendix C
Glossary

0.2 Percent Annual Chance Flood—A flood that has a 0.2 percent chance of being equaled or exceeded in any given year; also known as a 500-year flood (FEMA, 2003)

1 Percent Annual Chance Flood—A flood that has a 1 percent chance of being equaled or exceeded in any given year; also known as a 100-year flood (FEMA, 2003)

100-Year Flood—See 1 percent annual chance flood (FEMA, 2003)

500-Year Flood—See 0.2 percent annual chance flood (FEMA, 2003)

Accuracy—The degree of correctness attained in a measurement. (FEMA, 2003)

Horizontal Accuracy—The positional accuracy of a dataset with respect to a specified horizontal datum (Maune, 2007)

• Vertical Accuracy—The positional accuracy of a dataset with respect to a specified vertical datum (Maune, 2007)

Amendment—A determination by the Federal Emergency Management Agency (FEMA) that a property has inadvertently been included in a Special Flood Hazard Area (SFHA) as shown on an effective Flood Insurance Rate Map (FIRM) and is not subject to inundation by the 1 percent annual chance flood. Generally, the property is located on natural high ground at or above the BFE or on fill placed prior to the effective date of the first NFIP map designating the property as within an SFHA. Limitations of map scale and development of topographic data more accurately reflecting the existing ground elevations at the time the maps were prepared are the two most common bases for amendment requests (FEMA, 2003)

Approved ModelA numerical computer model that has been accepted by FEMA for use in performing new or revised hydrologic or hydraulic analyses for National Flood Insurance Program (NFIP) purposes. All accepted models must meet the requirements set forth in Subparagraph 65.6(a)(6) of the NFIP regulations (FEMA, 2003)

Approximate Study—A flood hazard study that uses topographic data, typically without bathymetry or bridge or culvert opening geometry, to conduct approximate hydrologic and hydraulic analyses. The analysis results in the delineation of floodplain boundaries for the 1 percent annual chance (100-year) flood, but does not include the determination of base flood elevations (BFEs) or base flood depths (FEMA, 2003)

Backwater—Water backed up or retarded in its course compared to its normal or natural condition of flow (FEMA, 2003)

Base FloodA flood that has a 1 percent chance of being equaled or exceeded in any given year, also



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Appendix C Glossary 0.2 Percent annual chance Flood—A flood that has Generally, the property is located on natural high ground at or above the BFE or on fill placed prior to a 0.2 percent chance of being equaled or exceeded in the effective date of the first NFIP map designating the any given year; also known as a 500-year flood (FEMA, property as within an SFHA. Limitations of map scale 2003) and development of topographic data more accurately 1 Percent annual chance Flood—A flood that has a reflecting the existing ground elevations at the time the maps were prepared are the two most common bases 1 percent chance of being equaled or exceeded in any for amendment requests (FEMA, 2003) given year; also known as a 100-year flood (FEMA, 2003) approved model—A numerical computer model that 100-Year Flood—See 1 percent annual chance flood has been accepted by FEMA for use in performing new or revised hydrologic or hydraulic analyses for (FEMA, 2003) National Flood Insurance Program (NFIP) purposes. 500-Year Flood—See 0.2 percent annual chance flood All accepted models must meet the requirements set forth in Subparagraph 65.6(a)(6) of the NFIP regula- (FEMA, 2003) tions (FEMA, 2003) accuracy—The degree of correctness attained in a approximate study—A flood hazard study that uses measurement. (FEMA, 2003) topographic data, typically without bathymetry or • horizontal accuracy—The positional accuracy bridge or culvert opening geometry, to conduct approx- imate hydrologic and hydraulic analyses. The analysis of a dataset with respect to a specified horizontal datum results in the delineation of floodplain boundaries for (Maune, 2007) • Vertical accuracy—The positional accuracy the 1 percent annual chance (100-year) flood, but does not include the determination of base flood elevations of a dataset with respect to a specified vertical datum (BFEs) or base flood depths (FEMA, 2003) (Maune, 2007) Backwater—Water backed up or retarded in its course amendment—A determination by the Federal Emer- compared to its normal or natural condition of flow gency Management Agency (FEMA) that a property (FEMA, 2003) has inadvertently been included in a Special Flood Hazard Area (SFHA) as shown on an effective Flood Base Flood—A flood that has a 1 percent chance Insurance Rate Map (FIRM) and is not subject to of being equaled or exceeded in any given year, also inundation by the 1 percent annual chance flood. 

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 APPENDIX C cooperating Technical Partners—Participating NFIP referred to as the 100-year flood. The base flood is the national standard used by the NFIP and all federal communities, regional agencies, and state agencies that agencies for the purposes of requiring the purchase are active participants in the FEMA Flood Hazard of flood insurance and regulating new development Mapping Program (FEMA, 2003) () cross section—A line across a floodplain, developed Base Flood elevation (BFe)—The elevation of a from topographic data, at which a computation of flood having a 1 percent chance of being equaled or flood flow has been made to establish a potential flood exceeded in any given year (FEMA, 2003) elevation () Bathymetry—The measurement and study of water datum—A common vertical or horizontal eleva- depths. Traditionally bathymetry has been expressed with contours and hydrography with spot depths tion reference point () • ellipsoidal datum—A set of constants specify- Benchmark—A permanent monument established by any federal, state, or local agency, whose elevation and ing the coordinate system used for geodetic control, that description are well documented and referenced to the is, for calculating coordinates of points on the Earth; National Geodetic Vertical Datum of 1929 (NGVD also known as geodetic datum () • orthometric datum—The reference surface Benefits—Positive effects of an action. For FEMA from which orthometric heights are measured (i.e., flood hazard mitigation projects, benefits are defined NAVD 88 or NGVD 29) • Tidal datum—A surface with a designed as avoided damages and losses (FEMA, 2001) elevation from which heights or depths are reckoned, calibration—The process of identifying and cor- defined by a certain phase of the tide. A tidal datum is recting for systematic errors in hardware, software, local, usually valid only for a restricted area about the or procedures; determining the systematic errors in tide gage used in defining the datum (Maune, 2007) a measuring device by comparing its measurements design storm—A rainfall event of specified size with the markings or measurements of a device that is considered correct (Maune, 2007) and return frequency that is used to calculate runoff volume. It is assumed that the design storm for a catchment area—An area of land that is occupied by a given frequency will produce a simulated runoff peak drainage system consisting of a surface stream or a body and volume having the same return frequency. Thus, of impounded surface water, together with all tributary a 100-year design storm should produce a 100-year surface streams and bodies of impounded surface water runoff and volume (New York Department of Envi- that drains into a single outlet; also called drainage ronmental Conservation, 1992) basin or watershed () that uses transects and offshore bathymetry to conduct coastal Flooding—Flooding that occurs along the detailed erosion, wave height, and wave runup analyses Great Lakes, the Atlantic and Pacific Oceans, and the and to prepare floodplain mapping. The analysis results Gulf of Mexico (FEMA, 2003) in the determination and publication of BFEs and designation of the coastal high-hazard areas (V zones) confidence level—The probability that errors are (FEMA, 2003) within a range of given values (Maune, 2007)

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 APPENDIX C detailed study, riverine—A riverine flood hazard cent grade of a building are certified relative to the base flood elevation for the location of the building. Other study that uses topographic data, channel bathymetry, descriptive information is also provided to help identify and bridge or culvert opening geometry to conduct the flood risk to the building surveyed (Maune, 2007) detailed hydrologic and hydraulic analyses and flood- plain mapping. The analysis results in the delineation of Firmette—A full-scale section of a Flood Insur- floodplain boundaries for the 1 percent annual chance ance Rate Map created by users online by select- (100-year) flood, determination of BFEs or flood depths, ing the desired area from a FIRM image. It also and normally, a regulatory floodway (FEMA, 2003) includes the map title block, north arrow, and scale digital elevation model (dem)—A file with terrain bar () grained grid and organized by quadrangle as the digital equivalent of the elevation data on a topographic base Flood—A general and temporary condition of partial map (FEMA, 2003) or complete inundation of normally dry land areas from digital Flood insurance rate map (dFirm)—A (1) the overflow of inland or tidal waters or (2) the unusual and rapid accumulation or runoff of surface Flood Insurance Rate Map that has been prepared waters from any source (FEMA, 2003) as a digital product, which may involve converting an existing manually produced FIRM to digital format or Flood hazard mapping Partner—Community offi- creating a product from new digital data sources using a cials; regional agency officials; state agency officials; geographic information system (GIS) (FEMA, 2003) communities, regional agencies, and state agencies digital Terrain model (dTm)—A land surface rep- participating in the FEMA Cooperating Technical Partners Program; other federal agencies; FEMA resented in digital form by an elevation grid or lists of contractors; contractors of communities, regional three-dimensional coordinates (FEMA, 2003) agencies, and state agencies; community residents and discharge—The volume of water that passes a property owners; other program constituents, including the U.S. Congress; insurance lending, real estate, and given location within a given period of time. Usually land development industries; and federal, state, and expressed in cubic feet per second () combined contribution with FEMA staff obtain and drainage area—The area upstream of a specific maintain accurate, up-to-date flood hazard information (FEMA, 2003) location, measured in a horizontal plane, that has a common outlet at the site for its surface runoff from Flood insurance rate map (Firm)—The insurance precipitation that normally drains by gravity into a and floodplain management map produced by FEMA stream. Drainage areas include all closed basins, or that identifies, based on detailed or approximate analy- noncontributing areas, within the area unless otherwise ses, the areas subject to flooding during a 1 percent specified () annual chance (100-year) flood event in a community elevation—The distance of a point above the speci- and flood insurance risk zones. In areas studied by detailed analyses, the FIRM shows BFEs to reflect fied surface of constant potential; the distance is the the elevations of the 1 percent annual chance flood. direction of gravity between the point and the surface For many communities, when detailed analyses are () by a 0.2 percent annual chance (500-year) flood and elevation certificate—A form on which the lowest regulatory floodway areas (FEMA, 2003) floor elevation, lowest adjacent grade, and highest adja-

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 APPENDIX C Flood insurance risk Zones—The areas, also referred health regulations, special-purpose ordinances, and other applications of enforcement used by a community to as flood insurance rate zones, shown on a FIRM that to manage development in its floodplain areas (FEMA, are used to determine flood insurance premium rates 2003) for properties in the community covered by the FIRM. The flood insurance risk zones include SFHAs (e.g., Floodway—The regulatory area defined as the channel Zones A, A1-30, AE, V, V1-30, VE, V0) and areas of a stream plus any adjacent floodplain areas that must outside SFHAs (e.g., Zone X) (FEMA, 2003) be kept free of encroachment so that the base flood Flood insurance study (Fis)—A compilation and discharge can be conveyed without increasing the BFEs more than a specified amount (FEMA, 2003) presentation of flood risk data for specific watercourses, lakes, and coastal flood hazard areas within a commu- Freeboard—A factor of safety usually expressed in feet nity. When a flood study is completed for the NFIP, above a flood level for purposes of floodplain manage- the information and maps are assembled into an FIS. ment. Freeboard tends to compensate for the many The FIS report contains detailed flood elevation data unknown factors that could contribute to flood heights in flood profiles and data tables () flood and floodway conditions, such as wave action, Flood insurance study report—A document, pre- bridge openings, and the hydrological effect of urban- ization of the watershed (44 CFR 59.1) pared and issued by FEMA, that presents the results of the detailed flood hazard assessment performed for a Geographic information system (Gis)—A system of community. The primary components of the FIS report computer hardware, software, and procedures designed are text, data tables, photographs, and flood profiles to support the capture, management, manipulation, (FEMA, 2003) analysis, modeling, and display of spatially referenced Flood Peak—The highest value of the stage or dis- data for solving complex planning and management problems (FEMA, 2003) charge attained by a flood; thus, peak stage or peak discharge () Geoid—The equipotential (level) surface of the Earth’s Flood Profile—A graph of elevation of the water gravity field, which on average coincides with mean sea level in the open undisturbed ocean. The geoid undu- surface of a river in flood, plotted as ordinate, against lates up and down with local variations in the mass and distance, measured in the downstream direction, plotted density of the Earth (Maune, 2007) as abscissa () Global Positioning system (GPs)—A satellite-based Flood stage—The height of a water surface above an navigation and positioning system that enables horizontal established datum plane (FEMA, 2003) and vertical positions to be determined (FEMA, 2003) Floodplain—Any land area that is susceptible to being height—The distance, measured along a perpendicu- inundated by water from any source (FEMA, 2003) lar, between a point and a reference surface (e.g., height Floodplain management—The operation of a pro- of an airplane above the ground surface). The distance, measured upward along a plumb line (line of force), gram of corrective and preventative measures for reduc- between a point and a reference surface of constant ing flood damage, including emergency preparedness geopotential. Elevation is preferred if the reference plans, floodcontrol works, and floodplain management surface is the geoid (Maune, 2007) regulations (FEMA, 2003) • ellipsoid height—The height above or below Floodplain management regulations—The zoning the reference ellipsoid (i.e., the distance between a ordinances, subdivision regulations, building codes,

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 APPENDIX C leveling—The process of finding differences of eleva- point on the Earth’s surface and the ellipsoidal surface, as measured along the normal [perpendicular] to the tion (Maune, 2007) ellipsoid at the point and taken positive upward from light detection and ranging (lidar)—An airborne the ellipsoid) (Maune, 2007) • orthometric height (elevation)—The height laser system that is used to acquire x, y, and z coordi- above the geoid as measured along the plumbline nates of terrain and terrain features that are both man- between the geoid and a point on the Earth’s surface, made and naturally occurring. LIDAR systems consist taken positive upward from the geoid (Maune, 2007) of an airborne GPS with attendant base station(s), inertial measuring unit, and light-emitting scanning hydraulic analysis—An engineering analysis of a laser (FEMA, 2003) flooding source carried out to provide estimates of the limited detailed study—A flood hazard study based elevations of floods of selected recurrence intervals (FEMA, 2003) on fewer surveyed cross sections than detailed studies. The analysis results in the delineation of floodplain hydraulic model—A computer program that uses boundaries for the 1 percent annual chance (100-year) flood discharge values and floodplain characteristic flood and often base flood elevations (FEMA, 2006a) data to simulate flow conditions and determine flood map modernization Program—A multiyear FEMA elevations (FEMA, 2003) initiative (1) to provide a technology-based, cost- hydrograph—A graph showing stage, flow, velocity, effective, long-term process for updating, maintaining, or other water properties with respect to time (FEMA, storing, and distributing the flood risk information 2003) portrayed on the flood maps; and (2) to use engineer- ing tools and analysis to update the flood maps so that hydrologic analysis—An engineering analysis of a they reflect physical changes that have occurred since flooding source carried out to establish peak flood dis- the original mapping (FEMA, 2006a) charges and their frequencies of occurrence (FEMA, mitigation—A sustained action taken to reduce or 2003) eliminate long-term risk to people and property from inundation map—A map depicting the spatial extent flood hazards and their effects. Mitigation distinguishes and depth of floodwaters in the vicinity of National actions that have a long-term impact from those that Weather Service river forecast locations () immediate response to, and short-term recovery from specific events (FEMA, 2003) letter of Final determination—The letter in which monument or control monument (also called refer- FEMA announces its final determination regarding flood hazard information, including (when appropri- ence mark)—A structure that marks the location of ate) proposed and proposed modified BFEs presented a corner or point determined by surveying; generally, on a new or revised FIRM, and FIS report. The letter any material, object, or collection of objects that indi- begins the compliance period and establishes the cates the ground location of a survey station or corner effective date for the new or revised FIRM and/or FIS () letter of map change (lomc)—A collective term National Flood insurance Program (NFiP)—The used to describe official amendments and revisions to federal program under which floodprone areas are FIRMs that are accomplished by an administrative identified and flood insurance is made available to the procedure and disseminated by letter (FEMA, 2003) owners of the property in participating communities (FEMA, 2003)

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 APPENDIX C orthophoto—A photograph prepared from a perspec- return Period—See recurrence interval tive photograph by removing displacements of points revision—A change to an effective NFIP map based caused by tilt, relief, and perspective (Maune, 2007) on new or revised scientific or technical data () or near the time of maximum stage () and streams (FEMA, 2003) Photogrammetry—The science of deducing the physi- runoff—That part of the precipitation that appears cal three-dimensional measurements of objects from measurements on stereo photographs that photograph in surface streams () Q3 Flood data Product—A digital representation of shallow Flooding—Unconfined flows over broad, certain features of the FIRM that is intended for use relatively low relief areas; intermittent flows in arid with desktop mapping and GIS technology. The Q3 regions that have not developed a system of well- flood data product is created by scanning the effective defined channels; overbank flows that remain uncon- FIRM paper maps and digitizing selected features and fined; overland flow in urban areas; and flows collect- lines (FEMA, 2003) ing in depressions to form ponding areas. For NFIP purposes, shallow flooding conditions are defined as recurrence interval—The average interval of time flooding that is limited to 3.0 feet or less in depth where within which a given flood will be equaled or exceeded no defined channel exists (FEMA, 2003) once; also known as the return period (FEMA, 2003) special Flood hazard area (sFha)—The area redelineation—A data update method that involves delineated on an NFIP map as being subject to inun- no new analyses, but uses effective information and dation by the base flood. SFHAs are determined using new topographic data that are more up-to-date and/or statistical analyses of records of riverflow, storm tides, detailed than those used to produce the effective FIRM and rainfall; information obtained through consultation to redelineate floodplain boundaries (FEMA, 2003) with a community; floodplain topographic surveys; and hydrologic and hydraulic analyses (FEMA, 2003) regression equation—An experimentally determin- stillwater Flood elevation (sWel)—Projected able equation of a regression curve; that is, an approxi- mate, generally linear relation connecting two or more elevation that floodwaters would assume, referenced to quantities and derived from the correlation coefficient NGVD 29, NAVD 88, or other datum, in the absence (FEMA, 2003) of waves resulting from wind or seismic effects (FEMA, 2003) resolution—In the context of gridded elevation data, storm surge—The rise in the water surface above resolution is synonymous with the horizontal post spac- ing; sometimes used to state the number of points in x normal water level on the open coast due to the action and y directions in a lattice (e.g., 1,201 × 1,201 mesh of wind stress and atmospheric pressure () DEM) (Maune, 2007) stream reach—The length of a channel for which restudy—A revised study of flood hazards performed a single gage affords a satisfactory measure of the for a community that already has an effective FIRM stage and discharge ()

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 APPENDIX C • Natural Variability—Sometimes called aleatory structure—For floodplain management purposes, a walled and roofed building, including a gas or liquid uncertainty—deals with inherent variability in the storage tank that is principally above ground, as well as physical world; by assumption, this “randomness” is a manufactured home. For flood insurance purposes, a irreducible. The word aleatory comes from the Latin walled and roofed building, other than a gas or liquid alea, meaning a die or gambling device. In the water storage tank, that is principally above ground and resources context, uncertainties related to natural vari- affixed to a permanent site, as well as a manufactured ability include things such as streamflow, assumed to be home on a permanent foundation (FEMA, 2003) a random process in time, or soil properties, assumed to be random in space. Natural variability is also Terrain—See topography sometimes referred to as external, objective, random, or stochastic uncertainty. Topography—The form of the features of the actual Watershed—See catchment area surface of the Earth in a particular region, considered collectively; also called terrain (Maune, 2007) Wave crest—The highest point on a ridge, deforma- Total station—A tachymeter that senses angles and tion, or undulation of the water surface () instrument for the rapid determination of distance, Wave envelope—A combination of representa- usually together with the measurement of direction and difference of elevation () determined by the wave results computed using the Wave Height Analysis for Flood Insurance Studies Transect—Cross section taken perpendicular to the (WHAFIS) program (FEMA, 2003) shoreline to represent a segment of coast with similar Wave height—Vertical distance between the wave characteristics (FEMA, 2003) crest and the wave trough (FEMA, 2003) Uncertainty—Degree to which an outcome is unknown Wave runup—Rush of waves up a slope or structure or not established and is therefore in question (NRC, 2000) (FEMA, 2003) • Knowledge Uncertainty—Sometimes called Wave setup—The increase in the stillwater surface epistemic uncertainty—deals with a lack of understand- near the shoreline, due to the presence of breaking ing of events and processes, or with a lack of data from waves (FEMA, 2003) which to draw inferences; by assumption, such lack of Wind setup—The vertical rise in the stillwater level at knowledge is reducible with further information. The word epistemic is derived from the Greek “to know.” the face of a structure or embankment caused by wind Knowledge uncertainty is also sometimes referred to as stresses on the surface of the water (FEMA, 2004) functional, internal, or subjective uncertainty.

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