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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 Model—A 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 Flood—A flood that has a 1 percent chance of being equaled or exceeded in any given year, also
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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 (<http://www.fema.gov/NFIPKeywords/>)
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)
Bathymetry—The 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 (<http://water.usgs.gov/glossaries.html>)
Coastal Flooding—Flooding that occurs along the Great Lakes, the Atlantic and Pacific Oceans, and the Gulf of Mexico (FEMA, 2003)
Confidence Level—The probability that errors are within a range of given values (Maune, 2007)
Cooperating Technical Partners—Participating NFIP communities, regional agencies, and state agencies that are active participants in the FEMA Flood Hazard Mapping Program (FEMA, 2003)
Cross Section—A line across a floodplain, developed from topographic data, at which a computation of flood flow has been made to establish a potential flood elevation (<http://www.fema.gov/media/fhm/champ/ot_chmp.htm>)
Datum—A common vertical or horizontal elevation reference point (<https://hazards.fema.gov/femaportal/>)
• 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 (<http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml>)
• 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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Detailed Study, Riverine—A riverine flood hazard study that uses topographic data, channel bathymetry, and bridge or culvert opening geometry to conduct detailed hydrologic and hydraulic analyses and floodplain mapping. The analysis results in the delineation of floodplain boundaries for the 1 percent annual chance (100-year) flood, determination of BFEs or flood depths, and normally, a regulatory floodway (FEMA, 2003)
Digital Elevation Model (DEM)—A file with terrain elevations recorded for the intersection of a fine-grained grid and organized by quadrangle as the digital equivalent of the elevation data on a topographic base map (FEMA, 2003)
Digital Flood Insurance Rate Map (DFIRM)—A Flood Insurance Rate Map that has been prepared as a digital product, which may involve converting an existing manually produced FIRM to digital format or creating a product from new digital data sources using a geographic information system (GIS) (FEMA, 2003)
Digital Terrain Model (DTM)—A land surface represented in digital form by an elevation grid or lists of three-dimensional coordinates (FEMA, 2003)
Discharge—The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second (<http://water.usgs.gov/glossaries.html>)
Drainage Area—The area upstream of a specific location, measured in a horizontal plane, that has a common outlet at the site for its surface runoff from precipitation that normally drains by gravity into a stream. Drainage areas include all closed basins, or noncontributing areas, within the area unless otherwise specified (<http://water.usgs.gov/glossaries.html>)
Elevation—The distance of a point above the specified surface of constant potential; the distance is the direction of gravity between the point and the surface (<http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml>)
Elevation Certificate—A form on which the lowest floor elevation, lowest adjacent grade, and highest adjacent grade of a building are certified relative to the base flood elevation for the location of the building. Other descriptive information is also provided to help identify the flood risk to the building surveyed (Maune, 2007)
FIRMette—A full-scale section of a Flood Insurance Rate Map created by users online by selecting the desired area from a FIRM image. It also includes the map title block, north arrow, and scale bar (<http://msc.fema.gov/webapp/wcs/stores/servlet/FemaWelcomeView?storeId=10001&catalogId=10001&langId=-1>)
Flood—A general and temporary condition of partial or complete inundation of normally dry land areas from (1) the overflow of inland or tidal waters or (2) the unusual and rapid accumulation or runoff of surface waters from any source (FEMA, 2003)
Flood Hazard Mapping Partner—Community officials; regional agency officials; state agency officials; communities, regional agencies, and state agencies participating in the FEMA Cooperating Technical Partners Program; other federal agencies; FEMA contractors; contractors of communities, regional agencies, and state agencies; community residents and property owners; other program constituents, including the U.S. Congress; insurance lending, real estate, and land development industries; and federal, state, and local disaster and emergency response officials whose combined contribution with FEMA staff obtain and maintain accurate, up-to-date flood hazard information (FEMA, 2003)
Flood Insurance Rate Map (FIRM)—The insurance and floodplain management map produced by FEMA that identifies, based on detailed or approximate analyses, the areas subject to flooding during a 1 percent annual chance (100-year) flood event in a community and flood insurance risk zones. In areas studied by detailed analyses, the FIRM shows BFEs to reflect the elevations of the 1 percent annual chance flood. For many communities, when detailed analyses are performed, the FIRM also may show areas inundated by a 0.2 percent annual chance (500-year) flood and regulatory floodway areas (FEMA, 2003)
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Flood Insurance Risk Zones—The areas, also referred to as flood insurance rate zones, shown on a FIRM that are used to determine flood insurance premium rates for properties in the community covered by the FIRM. The flood insurance risk zones include SFHAs (e.g., Zones A, A1-30, AE, V, V1-30, VE, V0) and areas outside SFHAs (e.g., Zone X) (FEMA, 2003)
Flood Insurance Study (FIS)—A compilation and presentation of flood risk data for specific watercourses, lakes, and coastal flood hazard areas within a community. When a flood study is completed for the NFIP, the information and maps are assembled into an FIS. The FIS report contains detailed flood elevation data in flood profiles and data tables (<http://www.fema.gov/plan/prevent/floodplain/nfipkeywords/fis.shtm>)
Flood Insurance Study Report—A document, prepared and issued by FEMA, that presents the results of the detailed flood hazard assessment performed for a community. The primary components of the FIS report are text, data tables, photographs, and flood profiles (FEMA, 2003)
Flood Peak—The highest value of the stage or discharge attained by a flood; thus, peak stage or peak discharge (<http://water.usgs.gov/glossaries.html>)
Flood Profile—A graph of elevation of the water surface of a river in flood, plotted as ordinate, against distance, measured in the downstream direction, plotted as abscissa (<http://water.usgs.gov/glossaries.html>)
Flood Stage—The height of a water surface above an established datum plane (FEMA, 2003)
Floodplain—Any land area that is susceptible to being inundated by water from any source (FEMA, 2003)
Floodplain Management—The operation of a program of corrective and preventative measures for reducing flood damage, including emergency preparedness plans, floodcontrol works, and floodplain management regulations (FEMA, 2003)
Floodplain Management Regulations—The zoning ordinances, subdivision regulations, building codes, health regulations, special-purpose ordinances, and other applications of enforcement used by a community to manage development in its floodplain areas (FEMA, 2003)
Floodway—The regulatory area defined as the channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the base flood discharge can be conveyed without increasing the BFEs more than a specified amount (FEMA, 2003)
Freeboard—A factor of safety usually expressed in feet above a flood level for purposes of floodplain management. Freeboard tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed (44 CFR 59.1)
Geographic Information System (GIS)—A system of computer hardware, software, and procedures designed to support the capture, management, manipulation, analysis, modeling, and display of spatially referenced data for solving complex planning and management problems (FEMA, 2003)
Geoid—The equipotential (level) surface of the Earth’s gravity field, which on average coincides with mean sea level in the open undisturbed ocean. The geoid undulates up and down with local variations in the mass and density of the Earth (Maune, 2007)
Global Positioning System (GPS)—A satellite-based navigation and positioning system that enables horizontal and vertical positions to be determined (FEMA, 2003)
Height—The distance, measured along a perpendicular, between a point and a reference surface (e.g., height of an airplane above the ground surface). The distance, measured upward along a plumb line (line of force), between a point and a reference surface of constant geopotential. Elevation is preferred if the reference surface is the geoid (Maune, 2007)
• Ellipsoid Height—The height above or below the reference ellipsoid (i.e., the distance between a
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point on the Earth’s surface and the ellipsoidal surface, as measured along the normal [perpendicular] to the ellipsoid at the point and taken positive upward from the ellipsoid) (Maune, 2007)
• Orthometric Height (Elevation)—The height above the geoid as measured along the plumbline between the geoid and a point on the Earth’s surface, taken positive upward from the geoid (Maune, 2007)
Hydraulic Analysis—An engineering analysis of a flooding source carried out to provide estimates of the elevations of floods of selected recurrence intervals (FEMA, 2003)
Hydraulic Model—A computer program that uses flood discharge values and floodplain characteristic data to simulate flow conditions and determine flood elevations (FEMA, 2003)
Hydrograph—A graph showing stage, flow, velocity, or other water properties with respect to time (FEMA, 2003)
Hydrologic Analysis—An engineering analysis of a flooding source carried out to establish peak flood discharges and their frequencies of occurrence (FEMA, 2003)
Inundation Map—A map depicting the spatial extent and depth of floodwaters in the vicinity of National Weather Service river forecast locations (<http://www.floodsafety.noaa.gov/inundation.shtml>)
Letter of Final Determination—The letter in which FEMA announces its final determination regarding flood hazard information, including (when appropriate) proposed and proposed modified BFEs presented on a new or revised FIRM, and FIS report. The letter begins the compliance period and establishes the effective date for the new or revised FIRM and/or FIS report (FEMA, 2003)
Letter of Map Change (LOMC)—A collective term used to describe official amendments and revisions to FIRMs that are accomplished by an administrative procedure and disseminated by letter (FEMA, 2003)
Leveling—The process of finding differences of elevation (Maune, 2007)
Light Detection and Ranging (lidar)—An airborne laser system that is used to acquire x, y, and z coordinates of terrain and terrain features that are both man-made and naturally occurring. LIDAR systems consist of an airborne GPS with attendant base station(s), inertial measuring unit, and light-emitting scanning laser (FEMA, 2003)
Limited Detailed Study—A flood hazard study based on fewer surveyed cross sections than detailed studies. The analysis results in the delineation of floodplain boundaries for the 1 percent annual chance (100-year) flood and often base flood elevations (FEMA, 2006a)
Map Modernization Program—A multiyear FEMA initiative (1) to provide a technology-based, cost-effective, long-term process for updating, maintaining, storing, and distributing the flood risk information portrayed on the flood maps; and (2) to use engineering tools and analysis to update the flood maps so that they reflect physical changes that have occurred since the original mapping (FEMA, 2006a)
Mitigation—A sustained action taken to reduce or eliminate long-term risk to people and property from flood hazards and their effects. Mitigation distinguishes actions that have a long-term impact from those that are more closely associated with preparedness for, immediate response to, and short-term recovery from specific events (FEMA, 2003)
Monument or control monument (also called reference mark)—A structure that marks the location of a corner or point determined by surveying; generally, any material, object, or collection of objects that indicates the ground location of a survey station or corner (<http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml>)
National Flood Insurance Program (NFIP)—The federal program under which floodprone areas are identified and flood insurance is made available to the owners of the property in participating communities (FEMA, 2003)
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Orthophoto—A photograph prepared from a perspective photograph by removing displacements of points caused by tilt, relief, and perspective (Maune, 2007)
Peak Flow—The maximum instantaneous discharge of a stream or river at a given location; usually occuring at or near the time of maximum stage (<http://water.usgs.gov/glossaries.html>)
Photogrammetry—The science of deducing the physical three-dimensional measurements of objects from measurements on stereo photographs that photograph an area from two different perspectives (Maune, 2007)
Q3 Flood Data Product—A digital representation of certain features of the FIRM that is intended for use with desktop mapping and GIS technology. The Q3 flood data product is created by scanning the effective FIRM paper maps and digitizing selected features and lines (FEMA, 2003)
Recurrence Interval—The average interval of time within which a given flood will be equaled or exceeded once; also known as the return period (FEMA, 2003)
Redelineation—A data update method that involves no new analyses, but uses effective information and new topographic data that are more up-to-date and/or detailed than those used to produce the effective FIRM to redelineate floodplain boundaries (FEMA, 2003)
Regression Equation—An experimentally determinable equation of a regression curve; that is, an approximate, generally linear relation connecting two or more quantities and derived from the correlation coefficient (FEMA, 2003)
Resolution—In the context of gridded elevation data, resolution is synonymous with the horizontal post spacing; sometimes used to state the number of points in x and y directions in a lattice (e.g., 1,201 × 1,201 mesh points in a U.S. Geological Survey [USGS] one-degree DEM) (Maune, 2007)
Restudy—A revised study of flood hazards performed for a community that already has an effective FIRM (FEMA, 2003)
Return Period—See recurrence interval
Revision—A change to an effective NFIP map based on new or revised scientific or technical data (<http://www.fema.gov/plan/prevent/floodplain/nfipkeywords/revision_maps.shtm>)
Riverine Flooding—The overbank flooding of rivers and streams (FEMA, 2003)
Runoff—That part of the precipitation that appears in surface streams (<http://water.usgs.gov/glossaries.html>)
Shallow Flooding—Unconfined flows over broad, relatively low relief areas; intermittent flows in arid regions that have not developed a system of well-defined channels; overbank flows that remain unconfined; overland flow in urban areas; and flows collecting in depressions to form ponding areas. For NFIP purposes, shallow flooding conditions are defined as flooding that is limited to 3.0 feet or less in depth where no defined channel exists (FEMA, 2003)
Special Flood Hazard Area (SFHA)—The area delineated on an NFIP map as being subject to inundation by the base flood. SFHAs are determined using statistical analyses of records of riverflow, storm tides, and rainfall; information obtained through consultation with a community; floodplain topographic surveys; and hydrologic and hydraulic analyses (FEMA, 2003)
Stillwater Flood Elevation (SWEL)—Projected elevation that floodwaters would assume, referenced to NGVD 29, NAVD 88, or other datum, in the absence of waves resulting from wind or seismic effects (FEMA, 2003)
Storm Surge—The rise in the water surface above normal water level on the open coast due to the action of wind stress and atmospheric pressure (<http://www.fema.gov/media/fhm/champ/ot_chmp.htm>)
Stream Reach—The length of a channel for which a single gage affords a satisfactory measure of the stage and discharge (<http://water.usgs.gov/glossaries.html>)
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Structure—For floodplain management purposes, a walled and roofed building, including a gas or liquid storage tank that is principally above ground, as well as a manufactured home. For flood insurance purposes, a walled and roofed building, other than a gas or liquid storage tank, that is principally above ground and affixed to a permanent site, as well as a manufactured home on a permanent foundation (FEMA, 2003)
Terrain—See topography
Topography—The form of the features of the actual surface of the Earth in a particular region, considered collectively; also called terrain (Maune, 2007)
Total Station—A tachymeter that senses angles and distances electronically. A tachymeter is a surveying instrument for the rapid determination of distance, usually together with the measurement of direction and difference of elevation (<http://www.ngs.noaa.gov/CORS-Proxy/Glossary/xml/NGS_Glossary.xml>)
Transect—Cross section taken perpendicular to the shoreline to represent a segment of coast with similar characteristics (FEMA, 2003)
Uncertainty—Degree to which an outcome is unknown or not established and is therefore in question (NRC, 2000)
• Knowledge Uncertainty—Sometimes called epistemic uncertainty—deals with a lack of understanding of events and processes, or with a lack of data from which to draw inferences; by assumption, such lack of knowledge is reducible with further information. The word epistemic is derived from the Greek “to know.” Knowledge uncertainty is also sometimes referred to as functional, internal, or subjective uncertainty.
• Natural Variability—Sometimes called aleatory uncertainty—deals with inherent variability in the physical world; by assumption, this “randomness” is irreducible. The word aleatory comes from the Latin alea, meaning a die or gambling device. In the water resources context, uncertainties related to natural variability include things such as streamflow, assumed to be a random process in time, or soil properties, assumed to be random in space. Natural variability is also sometimes referred to as external, objective, random, or stochastic uncertainty.
Watershed—See catchment area
Wave Crest—The highest point on a ridge, deformation, or undulation of the water surface (<http://www.fema.gov/media/fhm/champ/ot_chmp.htm>)
Wave Envelope—A combination of representative wave runup elevation and the wave crest profile determined by the wave results computed using the Wave Height Analysis for Flood Insurance Studies (WHAFIS) program (FEMA, 2003)
Wave Height—Vertical distance between the wave crest and the wave trough (FEMA, 2003)
Wave Runup—Rush of waves up a slope or structure (FEMA, 2003)
Wave Setup—The increase in the stillwater surface near the shoreline, due to the presence of breaking waves (FEMA, 2003)
Wind Setup—The vertical rise in the stillwater level at the face of a structure or embankment caused by wind stresses on the surface of the water (FEMA, 2004)
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