Montana Flood Mapping Program



Glossary of Frequently Used Terms The following terms are frequently used in the description of many floodplain mapping products and services. Print the Glossary of Terms Fact Sheet for all of the below listed terms. # A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # 1-percent annual chance floodplain This is the boundary of the flood that has a 1-percent chance of being equaled or exceeded in any given year. Also known as the 100-year floodplain. 100-year flood The flood having a 1-percent chance of being equaled or exceeded in any given year; also known as the base flood. The 1-percent annual chance flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage. 100-year floodplain This is the boundary of the flood that has a 1-percent chance of being equaled or exceeded in any given year. Officially termed the 1-percent annual chance floodplain. 500-year floodplain This is the boundary of the flood that has a 0.2-percent chance of being equaled or exceeded in any given year. Officially termed the 0.2-percent annual chance floodplain. B Base Flood The flood having a 1-percent chance of being equaled or exceeded in any given year; also known as the 100-year flood. The base flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage. Base Flood Elevation (BFE) The elevation of a flood having a 1-percent chance of being equaled or exceeded in any given year. C Channel Bank Stations Points that identify the extreme limits of the natural stream channel. These stations are typically assigned at locations along a cross section where a relatively flat area exists outside of the channel. Cooperating Technical Partner (CTP) Program The Cooperating Technical Partners (CTP) Program is an innovative approach to creating partnerships between FEMA and participating National Flood Insurance Program (NFIP) communities, regional agencies, and State agencies that have the interest and capability to become more active participants in the FEMA Flood Hazard Mapping Program. Cross Section A line developed from topographic information across a floodplain at which a computation of flood flow has been made to establish a potential flood elevation. Cross sections are shown on the Flood Boundary Floodway Map, Flood Insurance Rate Map, and/or Flood Profiles of a Flood Insurance Study. Cubic feet per second (cfs) Typical units used to express the rate of flow of surface water in open channels. One cubic foot is approximately equal to 7.5 gallons per second. D Depth Maximum depth of water in the cross section as measured below the water-surface elevation. Detailed Study A flood hazard study that, at a minimum, results in the delineation of the floodplain boundaries for the 1-percent-annual-chance (100-year) flood and the determination of base flood elevations (BFEs) or flood depths. Digital Flood Insurance Rate Map (DFIRM) A Flood Insurance Rate Map (FIRM) 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 environment. The DFIRM product allows for the creation of interactive, multi-hazard digital maps. Linkages are built into an associated database to allow users options to access the engineering backup material used to develop the DFIRM, such as hydrologic and hydraulic models, Flood Profiles, data tables, Digital Elevation Models, and structure-specific data, such as digital elevation certificates and digital photographs of bridges and culverts. Discharge The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second (cfs). F Federal Emergency Management Agency (FEMA) The Federal Emergency Management Agency - a former independent agency that became part of the Department of Homeland Security in March 2003 - is tasked with responding to, planning for, recovering from and mitigating against disasters. The agency's mission is: to lead America to prepare for, prevent, respond to and recover from disasters with a vision of "A Nation Prepared." Flood (also Flooding) A general and temporary condition of partial or complete inundation of normally dry land areas. For flood insurance claim purposes, two or more structures must be inundated before flood damage will be covered. Flood Data Table (FDT) The Floodway Data Table is found in the Flood Insurance Study for a community and gives details regarding the floodway at cross sections of studied flooding sources in the community. The information provided includes: cross section, distance, floodway width, section area of floodway, mean velocity in the floodway, floodway surcharge and the base flood water surface elevations for the regulatory floodway, with and without floodway scenarios. 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. Flood insurance risk zones, which are used to compute actuarial flood insurance rates, also are shown. 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 preformed, the FIRM also may show areas inundated by 0.2-percent-annual-chance (500-year) flood and regulatory floodway areas. Flood Insurance Study (FIS) The examination, evaluation, and determination of flood hazards performed for a community. This report contains the information found during the study of the community's flooding sources including study methodology, source data, discharges, water surface elevations, flood profiles, and references. Flood Profile A cross-sectional drawing showing the contiguous cross sections along a stream, with ground elevations and potential flood elevations plotted. Floodplain Management The operation of a program of corrective and preventative measures for mitigating flood damage, including, but not limited to, emergency preparedness plans, flood-control works, and floodplain management regulations. Floodway Channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 100-year flood discharge can be conveyed without increasing the elevation of the 100-year flood by more than a specified amount (.5 foot in Montana). L Letter of Map Amendment (LOMA) An official determination by FEMA that a property has been inadvertently included in a Special Flood Hazard Area (SFHA) as shown on an effective 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 FIRM map designating the property as within an SFHA. Letter of Map Revision (LOMR) A letter issued by FEMA to revise the FIRM and/or FIS report for a community to change BFEs, floodplain and floodway boundary delineations. M Map Repository The location where a community's flood maps are kept; usually the local environmental health or planning office. N National Flood Insurance Program (NFIP) Federal insurance program under which flood-prone areas are identified and flood insurance is made available to residents of participating communities that agree to adopt and enforce floodplain management ordinances to reduce future flood damage. O Orthophotograph Aerial photographs that have been rectified to produce an accurate image of the Earth by removing tilt and relief displacements that occurred when the photo was taken. An orthophotograph has the same scale throughout and can be used as a base map for the DFIRM. R Redelineation This method of Special Flood Hazard Area (SFHA) identification can only be used for previously studied streams. Using this method, the SFHA boundary is redefined using the effective water surface elevations superimposed on updated topography. No changes are made to the currently effective model. This case involves situations for which new topographic data exists for a study reach or entire county. The new topographic data is more recent and of higher quality than the topographic data originally used in the effective study. When the original hydraulic model is not available digitally but the model is correct, the effective FEMA profile forms the basis of the redelineation. The appropriate vertical datum conversion is applied, and the revised flood boundaries are mapped on the new topographic source. In this case, the BFEs will be republished on the DFIRM. The deliverable will be digital flood boundaries that match best available topographic data, recreated flood profiles, and floodway data tables meeting FEMA's Guidelines and Specifications for Flood Hazard Mapping Partners. S Special Flood Hazard Area (SFHA) The area delineated on a National Flood Insurance Program 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.