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Public Works


Flood Control - Flood Terms





Base flood


The flood having a 1 percent chance of being equaled or exceeded in any given year.  Also referred to as the 100-year flood. The base flood is used by the National Flood Insurance Program as the basis for mapping, insurance rating, and regulating new construction

Base flood depth


A measurement of the base flood in feet above ground, used for shallow flooding

Base floodplain


The area of water and land inundated by the base flood



See watershed

Bench marks


Monuments on the ground that show the elevation of the spot above sea level



Defined landforms that carry water



A city, county, township, Indian tribe or authorized tribal organization, Alaska Native village or authorized native organization, or other local government with the statutory authority to adopt and enforce floodplain regulations and participate in the National Flood Insurance Program

Community Assistance Program


A FEMA program that funds state activities that help communities in the NFIP

Community Rating System


 A program that provides a flood insurance premium rate reduction based on a community's floodplain management activities

Dam breach inundation area


The area flooded by a dam failure

Damage Survey Report


A form completed by disaster assistance staff to determine the repair and reconstruction needs of public and private nonprofit facilities

Detailed studies


Flood hazard mapping studies that are done using hydrologic and hydraulic methods that produce base flood elevations, floodways, and other pertinent flood data



Any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation or drilling operations or storage of equipment and materials



The amount of water that passes a point in a given period of time. Rate of discharge is usually measured in cubic feet per second (cfs)

Dry Floodproof


To protect a building and its contents from flood damage by taking measures to seal the building so that flood waters cannot enter

Elevation Certificate


Used to provide elevation information necessary to ensure compliance with community floodplain management ordinances, to determine the proper insurance premium rate, and to support a request for a Letter of Map Amendment.

Elevation reference marks


See bench marks

Emergency Alert System


Emergency Alert System  Replaced the Emergency Broadcast system in 2002

Emergency Operations Center


A facility that houses communications equipment that is used to coordinate the response to a disaster or emergency

Enabling legislation


State laws that authorize communities to perform governmental activities, such as enacting and enforcing regulations

Encroachment review


An analysis to determine if a project will increase flood heights or cause increased flooding downstream

Federal Emergency Management Agency


Most of the National Flood Insurance Program field work and community coordination are done by the 10 FEMA Regional Offices

Federal Insurance Administration


FIA was the part of the Federal Emergency Management Agency that administered the National Flood Insurance Program. This is now the responsibility of FEMA's Mitigation Division

Flash flood


A flood in hilly and mountainous areas that can develop within minutes after a heavy rain. Can also occur in urban areas where pavements and drainage improvements speed runoff to a stream



A general and temporary condition of partial or complete inundation by water in normally dry land areas

Flood Boundary Floodway Map


An official map of a community, on which the Federal Emergency Management Agency has delineated the regulatory floodway Recent Flood Insurance Studies show the floodway on the FIRM and do not include an FBFM

Flood fringe


The portion of the floodplain lying outside of the floodway

Flood Hazard Boundary Map


An official map of a community published by FEMA that delineates the approximate boundary of the floodplain. The FHBM is generally the initial floodplain boundary map and is eventually superseded by a FIRM

Flood hazard mitigation


All actions that can be taken to reduce property damage and the threat to life and public health from flooding

Flood Insurance


Unlike a standard insurance policy, flood insurance covers losses to your property caused by flooding.

Flood Insurance Rate Map


An official map of a community on which FEMA has delineated both the Special Flood Hazard Areas and the risk premium zones applicable to the community

Flood Insurance Study


A report published by FEMA that is issued along with the community's FIRM.  The study contains such background data as the base flood discharges and water surface elevations that were used to prepare the FIRM

Flood Mitigation


The measures a community takes to correct and prevent flood risks. Efforts include zoning, rules for building in floodplains, and special-purpose floodplain ordinances

Flood Mitigation Assistance


A grant program that supports plans and projects for mitigating losses to insured buildings funded by the National Flood Insurance Program

Flood of record


The highest known flood level for the area as recorded in historical documents

Flood Zone


Land area described in terms of its risk of flooding



Any land area susceptible to being inundated by flood waters from any source



Protective measures added to or incorporated in a building that is not elevated above the base flood elevation to prevent or minimize flood damage. "Dry floodproofing" measures are designed to keep water from entering a building. "Wet floodproofing" measures minimize damage to a structure and its contents from water that is allowed into a building



The channel of a river or other watercourse and that portion of the adjacent floodplain that must remain open to permit passage of the base flood without cumulatively increasing the water surface elevation more than a designated height (usually one foot)

Geographic information system


Computer based map systems that allow the user to keep a map updated easily and to correlate geographic information with other data, such as tax records on properties

Hazard Mitigation Grant Program


A FEMA disaster assistance grant that funds mitigation projects



A computer model used to conduct a hydraulic study, which produces flood elevations, velocities and floodplain widths



A computer model used to conduct a hydraulic study, which produces flood elevations, velocities and floodplain widths

Human intervention


Actions that must be taken by one or more persons before floodwaters arrive in order for a building to be floodproofed

Hydrodynamic force


The force of moving water including the impact of debris at high velocities

Hydrologic cycle


The natural cycle that circulates water throughout the environment to maintain an overall balance between water in the air, on the surface and in the ground



The science dealing with the waters of the earth.  A flood discharge is developed by a hydrologic study

Hydrostatic pressure


The pressure put on a structure by the weight of standing water The

deeper the water, the more it weighs and the greater the hydrostatic pressure

Increased Cost of Compliance


An additional claim payment made to a flood insurance policy holder to help cover the cost of bringing a substantially damaged or repetitively damaged building into compliance with the community's floodplain management ordinance

Individual and Family Grants


A disaster assistance grant that helps people with their unmet needs (ie, needs not helped by other disaster assistance programs)

Insurance Services Office


An insurance organization that provides support to FEMA on implementation of the Community Rating System

Lateral pressure


The amount of pressure imposed sideways by standing water. Deep water exerts more lateral pressure than shallow water

Letter of Map Amendment


Documents issued by FEMA that officially remove a property and/or structure from the Special Flood Hazard Area.  To obtain a LOMA or LOMR the applicant must submit mapping and survey data for the property, much of which is available from the municipality in which the property is located. Call 702.267.3022.

Letter of Map Revision


Limited Map Maintenance Project


A small-scale restudy of a Flood Insurance Study

Manufactured home


A building that is transportable in one or more ways, which is built on a permanent chassis and is designed for use with or without a permanent foundation when attached to the required utilities It includes single- and double-wide mobile homes

Mitigation Division


The FEMA office that sets national policy for the NFIP and administers the mapping program

Movable bed streams


A type of flooding that features uncertain flow paths



See mudslide



A condition where there is a river, flow or inundation of liquid mud down a hillside

Multi-objective management


An approach to planning and funding local programs that involves a variety of local interests and concerns

National Environmental Policy Act


A Federal law that requires agencies to evaluate the environmental impact of a proposed project

National Flood Insurance Program


Established by Congress in 1968, the goals of the National Flood Insurance Program are to reduce future flood damage through floodplain management and to provide people with flood insurance

National Geodetic Vertical Datum


National Geodetic Vertical Datum of 1929.  The national datum is based on mean sea level. It was known formerly as the "Mean Sea Level Datum of 1929" (MSL)

National Oceanic and Atmospheric Administration National Weather Service


 Federal agency focused on the condition of the oceans and the atmosphere.

Non-structural flood protection measures


Administrative tools for controlling flooding and flood damage, including regulations on development, building codes, property acquisition and structure relocation, and modification of existing buildings

No-rise Certification


A certification by an engineer that a project will not cause a set increase in flood heights

National Weather Service


Short version of National Oceanic and Atmospheric Administration National Weather Service



The generic term for a law passed by a local government

Overbank flooding


Flooding that occurs when downstream channels receive more rain or snowmelt from their watershed than normal, or a channel is blocked by an ice jam or debris and excess water overloads the channels and flows out onto the floodplain

Planned development


A regulatory approach that allows a developer to design the entire area while individual requirements may be relaxed to allow for open space, mixed land uses, and other variances to traditional zoning rules

Planned Unit Development




Runoff that collects in depressions and cannot drain out, creating a temporary pond

Post-FIRM building


For insurance rating purposes, a post-FIRM building was constructed or substantially improved after December 31, 1974, or after the effective date of the initial Flood Insurance Rate Map of a community, whichever is later. For a community that participated in the NFIP when its initial FIRM was issued, post-FIRM buildings are the same as new construction and must meet the National Flood Insurance Program's minimum floodplain management standards

Potable Water


 Water fit to drink

Pre-FIRM building


For insurance rating purposes, a pre-FIRM building was constructed or substantially improved on or before December 31, 1974, or before the effective date of the initial Flood Insurance Rate Map of the community, whichever is later Most pre-FIRM buildings were constructed without taking the flood hazard into account



A statistical term having to do with the size of a flood and the odds of that size of flood occurring in any year



A graph that shows elevations of various flood events

Public/Infrastructure Assistance


A disaster assistance grant that helps public agencies and nonprofit organizations finance repairs and reconstruction of public infrastructure

Q3 Flood Data Product


A graphical representation of certain features of a FIRM in digital format 



Building a new structure on the old foundation or slab of a structure that was destroyed, damaged, purposefully demolished or razed The term also applies when an existing structure is moved to a new site

Recreational vehicle


A vehicle designed to be self propelled or permanently towable by a light duty truck that is designed for use as temporary living quarters for recreational, camping, travel, or seasonal use



An improvement made to an existing structure which does not affect its external dimensions



A new Flood Insurance Study for all or part of a community that has already had a Flood Insurance Study



Retrofitting techniques include floodproofing, elevation, construction of small levees, and other modifications made to an existing building or its yard to protect it from flood damage



 A change to a floodplain map based on new data submitted to FEMA



Of or produced by a river. Riverine floodplains have readily identifiable channels.



A measure related to ground surface conditions that reflects changes in floodwater velocity due to ground friction



Rainfall and snowmelt that reaches a stream

Sheet flow


Floodwater that spreads out over a large area and does not have defined channels at a somewhat uniform depth

Special Flood Hazard Area


Land areas at high risk for flooding. Also, the base floodplain displayed on FEMA maps.

Stafford Act


The Robert T. Stafford Disaster Relief and Emergency Assistance Act of 1988, as amended, which authorizes FEMA's current disaster assistance programs and the Hazard Mitigation Grant Program. The Disaster Mitigation Act of 2000 made extensive changes to the Stafford Act



Determining the distance along a stream

Statutory authority


The powers granted to a local government by state law

Storm drain system


The publicly owned facilities operated by the City of Henderson by which storm water is collected and/or conveyed

Storm surge


Abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the storm.

Stormwater detention


Storing stormwater runoff for release at a restricted rate after the storm subsides

Stormwater management


Efforts to reduce the impact of increased runoff that results from new development

Stormwater retention


Storing stormwater runoff for later use in irrigation or groundwater recharge, or to reduce pollution

Structural flood control


Measures that control floodwaters by construction of barriers or storage areas or by modifying or redirecting channels

Submit to rate


Process used when an insurance agent cannot complete the rate calculation for a flood insurance policy. The application is sent to the WYO Company or FEMA to be individually rated

Substantial damage


Damage of any origin sustained by a structure whereby the cost of restoring the structure to its before damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred

Substantial improvement


Any reconstruction, rehabilitation, addition or other improvement to a structure, the total cost of which equals or exceeds 50 percent of the market value of the structure before the start of construction of the improvement. The definition of "substantial improvement" includes buildings that have incurred "substantial damage" regardless of the actual repair work performed



Obtaining private property with or without compensating the owner. The term also includes reducing the value of private property to such an extent that the owner is deprived of all economic interest



The bottom of a river channel

Topographic map


Maps characterized by large-scale detail and quantitative representation of relief, usually using contour lines in modern mapping, but historically using a variety of methods.



A survey of topographic conditions used in coastal flood studies



A large wave caused by an underwater earthquake or volcano which can raise water levels as much as 15 feet 



A grant of relief by a community from the terms of a land use, zoning or building code regulation



The speed of moving water, a force that is measured in feet per second



 The dry bed of an intermittent stream



An area that drains into a lake, stream, or other body of water. Wave runup occurs when waves hit the shore and water is moving with such force that it keeps traveling inland

Wave runup


The maximum vertical extent of wave uprush on a beach or structure above the still water level.  

Wet floodproof


Protecting a building from flood damage by using flood-resistant materials below the flood level and elevating things subject to flood damage above the flood level

Write Your Own


An insurance company that has agreed to sell flood insurance policies on behalf of the NFIP

 Zone A


The SHFA (except coastal V Zones) shown on a community's FIRM. There are five types of A Zones.

Zone B


Area of moderate flood hazard, usually depicted on FIRMs as between the limits of the base and 500-year floods.  B Zones are also used to designate base floodplains of little hazard, such as those with average depths of less than one foot

Zone C


Area of minimal flood hazard, usually depicted on FlIRMs as above the 500-year flood level. B and C Zones may have flooding that does not meet the criteria to be mapped as a SFHA, especially ponding and local drainage problems

Zone D


Area of undetermined but possible flood hazard

Zone V


The SFHA subject to coastal high-hazard flooding. There are three types of V Zones.

Zone X


Newer FIRMs show Zones B and C as

Zone X