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Glossary of Flooding Terms

Number of Glossary 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 Z | Complete List

100-Year (or Base) Flood:
A flood event that statistically has a 1 out of 100 (or one percent) chance of being equaled or exceeded on a specific watercourse in any given year. A flood event of this magnitude is often used to determine if flood insurance is either advisable or required on a property.

The volume of water necessary to cover an acre of land to a depth of one foot. It equals 43,560 cubic feet or 325,851 gallons.

Agency Users:
Users employed by federal, state or local government agencies who generally have some technical knowledge in the fields of hydrology or meteorology.

A progressive buildup or raising of the channel bed due to sediment deposition. Permanent or continuous aggradation is an indicator that a change in the stream’s discharge and sediment characteristics is taking place.

Criteria can be set in the ALERT computer to notify operators when a particular threshold has been reached, such as a rainfall rate or water elevation. When the threshold is reached, the computer executes an alarm action. This may be a flashing box on the computer screen, turning on a light on a map, or a text message sent to a pager.

An acronym for Automated Local Evaluation in Real Time. ALERT was developed in the late 1970s as a format for data transmission and for the manufacture of compatible hardware and software. ALERT systems are used primarily as flood warning

ALERT Database:
The ALERT database is a collection of 10 files stored on two of the ALERT computers. The files contain the actual ALERT data, plus indexes, calibrations, tables, alarms, alarm settings, headers and sensor types. It is often referred to as the “live” database to distinguish it from archived data.

Alluvial Fan:
A geomorphologic feature characterized by a cone or fan-shaped deposit of boulders, gravel and fine sediments that have been eroded from mountain slopes, transported by flood flows and then deposited in the valley floors and which is subject to flash flooding, high velocity flows, debris flows, erosion, sediment movement and deposition and channel migration.

Archive Files:
Archive files contain ALERT data and calibrations separate from the live database. Each file contains the data for one sensor for a one-month period of time.

Area Drainage Master Plan (ADMP):
A plan which identifies the preferred alternatives of those identified in an ADMS. An ADMP provides minimum criteria and standards for flood control and drainage relating to land use and development.

Area Drainage Master Study (ADMS):
A study to develop hydrology for a watershed, to define watercourses, identify potential flood problem areas, drainage problems and recommend solutions and standards for sound floodplain and stormwater management. The ADMS will identify alternative solutions to a given flooding or drainage problem.

Surfacing of channel bed, banks, or embankment slope to resist erosion

As Built Plans:
A community may require submission of "as-built" plans to certify that a project was built in accordance with the permit. A registered professional architect or engineer certifies the actual construction.

Average Wind Speed:
ALERT wind sensors sample the wind run past the station for a length of 1 kilometer. The average wind speed is calculated knowing the time it takes for that kilometer of wind to travel by. Instantaneous wind speed may vary considerably during the time that the average wind speed is measured.

The placement of fill material within a specified depression, hole or excavation pit below the surrounding adjacent ground level, as a means of improving flood water conveyance, or to restore the land to the natural contours existing prior to excavation.

Bankfull Stage:
The point at which the water level in a stream overtops the banks and spreads out onto the floodplain.

Barometric Pressure:
The pressure exerted by a column of air from the sensor to the top of the atmosphere. It is most commonly measured in millibars or inches of mercury (Hg).

Base Flood Elevation:
A base flood elevation (BFE) is the height of the base flood, usually in feet, in relation to the National Geodetic Vertical Datum of 1929, the North American Vertical Datum of 1988, or other datum referenced in the Flood Insurance Study report, or the depth of the base flood, usually in feet, above the ground surface.

Braided Stream:
A stream whose flow is divided at normal stage by small islands.

The volume of water stored by a dam at the emergency spillway elevation, usually expressed in acre-feet. It differs from storage, which is the volume of water stored at any specific elevation.

Catch Basin:
A chamber or well, usually built at the curb line of a street, for the admission of surface water to a storm sewer or sub-drain

The measuring unit of cubic feet per second (cfs), which is used to quantify the amount of flow in a wash. A cubic foot is equivalent to 7.5 gallons of water. Thus, 1 cfs is 7.5 gallons of water passing by you every second.

An open conveyance of surface stormwater having a bottom and sides in a linear configuration. Channels can be natural or man-made. Channels have levees or dikes along their sides to build up their depth. Constructed channels can be plain earth, landscaped, or lined with concrete, stone, or any other hard surface to resist erosion and scour.

Channel Failure:
Sudden collapse of a channel due to an unstable condition.

Community Rating System:
A program administered by the Federal Emergency Management Agency (FEMA) that recognizes and rewards communities working to reduce flood damages through a variety of approved floodplain management and flood awareness activities. Through the program, a community can reduce the flood insurance premiums that floodprone property owners pay.

Crest Gage:
A gage that measures the peak stage of a rising stream or impoundment. Our crest gages consist of a length of 2” galvanized pipe with a wooden stick inside. The bottom of the pipe is perforated to allow water inside. As water enters and rises, it carries with it powdered cork which adheres to the stick at the highest point. At a later time the cork level on the stick is measured and compared with the fixed datum in order to calculate the peak stage.

A hydraulically short conduit which conveys surface water runoff through a roadway embankment or through some other type of flow obstruction.

Defining the physical boundaries of a stream, floodplain, jurisdictional wash, etc.

something dropped or left behind by moving water, as sand or mud.

Design Discharge:
The nth-year storm for which it is expected that the structure or facility is designed to accommodate.

Detention Basin:
A basin or reservoir where water is stored for regulating a flood. It has outlets for releasing the flows during the floods

A man-made change to property, such as buildings or other structures, mining, dredging, filling, grading, paving, excavation, or drilling operations.

The temperature at which water vapor condenses into droplets. When the dewpoint is at or above the surface air temperature, relative humidity is 100% and dew or fog can form. When the dewpoint is below the surface air temperature, relative humidity will be less that 100%, and the base of any clouds will be at an elevation where the dewpoint and air temperatures are equal.

The amount of water that passes a specific point on a watercourse over a given period of time. Rates of discharge are usually measured in cubic feet per second (cfs).

Display Period:
In a statistical report, the display period is the time between each generated statistic. The report period is the time between the first and last display period. For example, if viewing a report of 24- one hour values, the display period is 1 hour and the report period is 1 day.

Drainage Basin:
A geographical area which contributes surface water runoff to a particular point. The terms “drainage basin,” “tributary area,” and “watershed” can be used interchangeably.

Drainage Clearance:
The approval by the Maricopa County Drainage Administrator of a grading and drainage plan to develop a site. This plan may be a site plan or an engineered grading and drainage plan.

Dry Well:
A deep hole, covered and designed to hold drainage water until it seeps into the ground.

A water-level expressed in terms of mean sea level. It differs from stage, which is a water-level in terms of some local datum.

Elevation Certificate:
The Elevation Certificate is an important administrative tool of the National Flood Insurance Program (NFIP). It is to be 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 or Revision (LOMA or LOMR-F).
Download the Elevation Certificate Instructions or Form from FEMA.
Download the Enhanced Elevation Certificate from FCD.

A man-made earth structure constructed for the purpose of impounding water.

Emergency Spillway:
An outflow from a detention/retention facility that provides for the safe overflow of floodwaters for large storms that exceed the design capacity of the outlet or in the event of a malfunction. The emergency spillway prevents the water from overtopping the facility.

The result of placing a building, fence, berm or other structure in a floodplain in a manner that obstructs or increases the depth (or velocity) of flow on a watercourse.

The wearing away of land by the flow of water.

Erosion Hazard Zone:
Land adjacent to a watercourse regulated by Maricopa County that is subject to flood-related erosion losses.

Evapotranspiration is the sum of water lost to the air via transpiration by plants and evaporation from water surfaces.

Acronym for Flood Control District of Maricopa County.

Federally-Mapped Floodplain:
A floodprone area that has been mapped and accepted by FEMA as the result of a flood insurance study (FIS) for a watercourse and surrounding areas. Mapped floodplains are used for flood insurance needs and for other regulatory purposes.

FEMA (Federal Emergency Management Agency):
An independent federal agency established to respond to major emergencies that state and local agencies don't have the resources to handle. FEMA seeks to reduce the loss of life and protect property against all types of hazards through a comprehensive, risk-based emergency management program. FEMA web site.

Fill Material:
any material used for the primary purpose of replacing an aquatic area with dry land or for changing the bottom elevation of a waterbody. This includes both natural materials (silt, sand, gravel, rock, and wood) and manufactured materials (concrete, plastic, steel, treated wood).

Flood Control:
Various activities and regulations that help reduce or prevent damages caused by flooding. Typical flood control activities include: structural flood control works (such as bank stabilization, levees, and drainage channels), acquisition of floodprone land, flood insurance programs and studies, river and basin management plans, public education programs, and flood warning and emergency preparedness activities.

Flood Flow Frequency:
A statistically derived table of discharge vs. return period for a particular point on a stream or within a flood storage facility.

Flood Insurance Rate Map (FIRM):
Issued by FEMA, these maps show special hazard areas, including the 100-year floodplain. They also show flood insurance risk zones and other flood-related information applicable to a community.

Flood Insurance Study (FIS):
Hydrologic and Hydraulic studies that identify a flood hazard area, flood insurance risk zones and other flood data such as flood depths and velocities.

Flood Proofing:
Any combination of changes to a structure or property using berms, flood walls, closures or sealants, which reduces or eliminates flood damage to buildings or property.

Flood Response Plan:
A plan developed for a particular waterway, watershed or jurisdiction that identifies flood hazards and defines methods for avoiding them and for minimizing losses to property.

Flood Stage:
The point at which the water level in a stream begins to cause damage to structures. It may be below bankfull stage if structures are located in a floodway.

A temporary condition caused by the accumulation of runoff from any source, which exceeds the capacity of a natural or man-made drainage system and results in inundation of normally dry land areas.

The area adjoining a watercourse that may be covered by floodwater during a flood.

Floodplain Management:
A program that uses corrective and preventative measures to reduce flood and erosion damage and preserve natural habitat and wildlife resources in floodprone areas. Some of these measures include: adopting and administering Floodplain Regulations, resolving drainage complaints, protecting riparian habitat communities, and assuring effective maintenance and operation of flood control works.

Floodplain Regulations:
Adopted policies, codes, ordinances, and regulations pertaining to the use and development of lands that lie within a regulatory floodplain.
View the Floodplain Regulations

Floodplain Use Permit:
An official document which authorizes specific activities within a regulatory floodplain or erosion hazard area.
Learn more about the Flood Plain Use Permit

The channel of a watercourse and portion of the adjacent floodplain that is needed to convey the base or 100-year flood event without increasing flood levels by more than one foot of floodwater.

Floodway Fringe:
The areas of a delineated floodplain adjacent to the Floodway where encroachment may be permitted.

Flowage Easement:
Legal right to allow water to flow across someone’s property

Forecast Point:
A point along a watercourse or at an impoundment structure for which a flood forecast is generated. Current and future conditions for upstream areas only are considered in the forecast.

An acronym for Flood Retarding Structure – most commonly used to describe earthen dams built by the Soil Conservation Service between 1950 and 1985 to protect agricultural lands.

Full Weather Station:
An ALERT station that contains at a minimum a rain gage, temperature/humidity sensor, and wind speed and direction sensors. A station with a rain gage and a temperature/humidity sensor only is not considered “full”.

An instrument that measures some property in the environment, like temperature, wind speed or precipitation. It is used interchangeably with “sensor”. We spell it g-a-g-e because that’s how the USGS spells it.

Gage Record:
The period of time for which data is collected at a gage. Gage records may have gaps in the record when no data is collected – these should be documented.

Grade Control Structure:
A structure used across a stream channel placed bank to bank to control bed elevation, velocity, pressure, etc.

Disturbance of existing land contours

Water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table.

Habitat Mitigation:
The compensation for the removal of natural vegetation during the construction of a flood control project by establishing new vegetation elsewhere.

Historic Data:
Historic data is generally more than 30 days old and has been quality checked. It may exist in the live ALERT database or in archive files.

Hydraulic Structures:
The facilities used to impound, accommodate, convey, or control the flow of water, such as dams, intakes, culverts, channels, and bridges.

A field of study dealing with the flow pattern and rate of water movement based on the principles of fluid mechanics.

A field of study concerned with the distribution and circulation of surface water, as well as water dynamics below the ground and in the atmosphere.

A graph of rainfall over time.

Floodwater stored in a basin or behind a dam. It can be described in terms of a water depth (ft) or a volume (acre-ft).

When applied to rainfall, intensity is the depth of rain in a specified time. Examples are 1 inch per hour or ½ inch in 20 minutes.

Isohyets are lines on a map connecting points of equal precipitation amounts. Colors are often used to shade areas between isohyetal lines.

Lateral Stream Migration:
Change in position of a channel by lateral erosion of one bank and simultaneous deposition on the opposite bank.

A man-made structure, usually an earthen embankment often reinforced with soil cement, that is designed to contain or divert the flow of water.

LOMA (Letter of Map Amendment):
An official amendment of a current Flood Insurance Rate Map (FIRM) accepted by FEMA for a property or a structure. The LOMA verifies that the structure or portions of the property have been removed from a designated-floodplain area.

LOMR (Letter of Map Revision):
An official revision of a current Flood Insurance Rate Map (FIRM) accepted by FEMA, which reflects changes in mapped areas for flood zones, floodplain areas, floodways and flood elevations.

Low Flow Channel:
A channel within a larger channel which typically carries low and/or normal flows

Map Repository:
An agency or entity designated to maintain official FEMA flood insurance rate maps for the community as well as LOMAs and LOMRs to those maps.

Metadata can be thought of as “data about data”. Metadata can answer questions about a sensor such as “when was it installed”, “where is it located” and “who owns it”.

Multi-Use Facility:
A detention or retention basin that provides additional benefits to its primary function of flood control. Such benefits include recreation, parking, visual buffers, or water harvesting.

National Flood Insurance Act of 1968:
An Act passed by Congress that established the National Flood Insurance Program as a means of mitigating flood damages. The Act makes flood insurance available to communities that adopt and enforce measures to reduce flood losses. Prior to the Act, property owners in floodprone areas typically were not able to obtain this coverage through private insurance companies.

National Flood Insurance Program (NFIP):
A federal program that allows property owners to purchase insurance protection against losses due to flooding. In order to participate in this program, local communities must agree to implement and enforce measures that reduce future flood risks in special flood hazard areas.

Acronym for National Oceanic and Atmospheric Administration. NOAA Web site.

NOAA Atlas 14:
From the NOAA Atlas 14 documentation series: “NOAA Atlas14 contains precipitation frequency estimates with associated confidence limits for the United States and is accompanied by additional information such as temporal distributions and seasonality. The Atlas is divided into volumes based on geographic sections of the country. The Atlas is intended as the official documentation of precipitation frequency estimates and associated information for the United States. It includes discussion of the development methodology and intermediate results. The Precipitation Frequency Data Server (PFDS) was developed and published in tandem with this Atlas to allow delivery of the results and supporting information in multiple forms via the Internet. NOAA Atlas 14 Volume 1 contains precipitation frequency estimates for Arizona, Nevada, New Mexico, Utah, and southeastern California (Imperial, Inyo, Eastern Kern, Eastern Los Angeles, Riverside, San Bernardino and Eastern San Diego counties). These areas were addressed together in a single project focused on the semiarid southwestern United States. The Atlas supersedes information contained in Technical Paper No. 49 “Two- to ten-day precipitation for return periods of 2 to 100 years in the contiguous United States” (Miller et al., 1964), NOAA Atlas 2 “Precipitation-Frequency Atlas of the Western United States” (Miller et al., 1973), “Short Duration Rainfall Frequency Relations for California” (Frederick and Miller, 1979) and “Short Duration Rainfall Relations for the Western United States” (Arkell and Richards, 1986). The updates are based on more recent and extended data sets, currently accepted statistical approaches, and improved spatial interpolation and mapping techniques. The work was performed by the Hydrometeorological Design Studies Center within the Office of Hydrologic Development of the National Oceanic and Atmospheric Administration’s National Weather Service”.

Outlet Structure:
A hydraulic structure placed at the outlet of a channel, spillway, pipe, etc., for the purpose of dissipating energy and providing a transition to the channel or pipe downstream.

Peak Flow:
The maximum rate of flow through a watercourse for a given storm

Peak Wind:
Our wind sensors sample the wind speed every 3 seconds and store that data for a 15-minute period. At the end of that period the on-board computer determines the maximum wind speed stored in that stack and transmits it as the peak wind value.

The movement of water through the subsurface soil layers, usually continuing downward to the groundwater or water table reservoirs.

Physical Weathering:
Breaking down of rock into bits and pieces by exposure to temperature and changes and the physical action of moving ice and water, growing roots, and human activities such as farming and construction.

All forms of water that fall to the earth’s surface - including rain, snow, sleet and hail.

Probable Maximum Flood:
The flood runoff that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region.

Rating Curve:
A mathematical relationship between two values expressed as a continuous line. The most common ratings we use are stage versus discharge for streams and stage versus volume for reservoirs.

Rating Table:
A mathematical relationship between two values expressed as a table. The most common ratings we use are stage versus discharge for streams and stage versus volume for reservoirs.

Raw Data:
Data collected from ALERT sensors that has not been altered by statistical analysis. Raw data consists of a date, time and value. The value can be the original integer delivered by the sensor or a value calibrated in engineering units.

A term used to describe a specific length of a stream or watercourse. For example, the term can be used to describe a section of a stream or watercourse between two bridges.

Real-time Data:
Real-time data is generally less than 30 days old and has not been quality checked. It exists only in the live ALERT database.

Subject to the control of or required to follow rules set forth by a governmental agency. With respect to washes or streams it refers to those areas where the federal government restricts the use or development of areas it has deemed to be “Waters of the U.S.” These regulations are part of the Clean Water Act.

Regulatory Flood Elevation:
The elevation which is one foot above the base flood elevation for a watercourse. Where a floodway has been delineated, the base flood elevation is the higher of either the natural or encroached water surface elevation of the 100-year flow.

Regulatory Floodplain:
A portion of the geologic floodplain that may be inundated by the base flood where the peak discharge is 100 cubic feet per second (cfs) or greater. Regulatory floodplains also include areas which are subject to sheet flooding, or areas on existing recorded subdivision plats mapped as being floodprone.

Relative Humidity:
The amount of water in a volume of air divided by the amount of water that volume of air could hold in a vapor state at a given temperature. It is expressed as a percentage from 0 to 100.

Report Period:
In a statistical report, the report period is the time between the first and last display period. The display period is the time between each generated statistic. For example, if viewing a report of 24- one hour values, the display period is 1 hour and the report period is 1 day.

Retention Basin:
A basin or reservoir where water is stored for regulating a flood. Unlike a detention basin, it does not have outlets for releasing the flows, the water must be disposed by draining into the soil, evaporation, or pumping systems.

Riparian Habitat:
Plant communities that occur in association with any spring, cienega, lake, watercourse, river, stream, creek, wash, arroyo, or other body of water. Riparian habitats can be supported by either surface or subsurface water sources.

Riparian Zone:
A stream and all the vegetation on its banks.

The portion of precipitation on land that ultimately reaches streams, especially water from rain or melted snow that flows over ground surface.

Soil particles, sand, and minerals washed from the land into aquatic systems as a result of natural and human activities.

A large scale water treatment process where heavy solids settle out to the bottom of the treatment tank after flocculation.

The minimum distance required between a man-made structure and a watercourse. This distance is measured from the top edge of the highest channel bank or the edge of the 100-year flood water surface elevation.

Sheet Flooding:
A condition where stormwater runoff forms a sheet of water to a depth of six inches or more. Sheet flooding is often found in areas where there are no clearly defined channels.

Sheet Flow:
Very shallow overland discharge.

Soil Erosion:
The processes by which soil is removed from one place by forces such as wind, water, waves, glaciers, and construction activity and eventually deposited at some new place.

Solar Radiation:
Our solar radiation sensors measure global radiation, which is the total radiation from the sun and reflected from the sky. The reported units are watts/square meter.

An outlet pipe or channel serving to discharge water from a dam, ditch, gutter, or basin.

Staff Gage:
A fixed pole, staff or structure upon which graduated measurements are painted or affixed for the purpose of visually determining water depth.

A water-level expressed in terms of some local datum. It differs from elevation, which is a water-level in terms of mean sea level.

An ALERT station is a local collection of sensors at a common geographic point. Stations have an ID number corresponding to the precipitation sensor if there is one, or to the water-level sensor at stage-only stations.

Statistical Data:
Raw ALERT data that is altered in form by a statistical or graphical program.

The volume of water stored in a basin or behind a dam – usually expressed in acre-feet. It differs from capacity, which is the volume of water stored at the emergency spillway elevation.

Storm Drainage System:
A drainage system for collecting runoff of stormwater on highways and removing it to appropriate outlets. The system includes inlets, catch basins, storm sewers, drains, reservoirs, pump stations, and detention basins.

Precipitation from rain or snow that accumulates in a natural or man-made watercourse or conveyance system.

Surface Water:
Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.

The water surface elevation in the channel downstream of a hydraulic structure

The line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration.

Tipping Bucket:
A sensor for measuring precipitation. Two “buckets” tip on an axle as they fill with water. One bucket empties as the other one fills. Each “tip” represents a calibrated depth of water over the collection area, such as 0.01 inches or 1 millimeter.

A metal bar or grate located at the outlet structure of a detention or retention basin which is designed to prevent blockage of the structure by debris.

A stream that contributes its water to another stream or body of water.

Legal permission to build a structure in a manner that would otherwise be prohibited by an ordinance.
Download the Floodplain Variance Application.

Virgin flow:
The streamflow which exists or would exist if man had not modified the conditions on or along the stream or in the drainage basin.

Water quality standards:
Laws or regulations, promulgated under Section 303 of the Clean Water Act, that consist of the designated use or uses of a waterbody or a segment of a waterbody and the water quality criteria that are necessary to protect the use or uses of that particular waterbody. Water quality standards also contain an antidegradation statement. Every State is required to develop water quality criteria standards applicable to the various waterbodies within the State and revise them every 3 years.

Water table:
Level below the earth's surface at which the ground becomes saturated with water. The surface of an unconfined aquifer which fluctuates due to seasonal precipitation.

The 12-month period from October 1st through September 30th. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. For example, the year ending September 30, 2002 is called the "2002 water-year".

Any minor or major lake, river, creek, stream, wash, arroyo, channel or other topographic feature on or over which waters flow at least periodically. Watercourse includes specifically designated areas in which substantial flood damage may occur.

Watercourse Master Plan (WCMP):
A hydraulic plan for a watercourse that examines the cumulative impacts of existing development and future encroachment in the floodplain and future development in the watershed on potential flood damages, and establishes technical criteria for subsequent development so as to minimize potential flood damages for all flood events up to and including the one hundred-year flood.

Waters of the U.S.:
All waters which are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerce.

An area from which water drains into a lake, stream or other body of water. A watershed is also often referred to as a basin, with the basin boundary defined by a high ridge or divide, and with a lake or river located at a lower point.

Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.

Wind Direction:
Wind direction is measured from true north either by compass direction (NE, SW, etc.) or by 0-359 degrees azimuth. Wind direction is described by the direction from which the wind blows, i.e. wind blowing from the northeast would have a measurement of 45º or NE.

Zone A (unnumbered):
Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. Because detailed analyses are not performed for such areas; no depths or base flood elevations are shown within these zones. Mandatory flood insurance requirements apply.

Zone AE and A1-30:
Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. In most instances, base flood elevations derived from detailed analyses are shown at selected intervals within these zones. Mandatory flood insurance requirements apply.

Zone AH:
Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within these zones. Mandatory flood insurance requirements apply.

Zone AO:
River or stream flood hazard areas, and areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Average flood depths derived from detailed analyses are shown within these zones. Mandatory flood insurance requirements apply.

Zone B, C and X:
Areas outside the 1-percent annual chance floodplain, areas of 1% annual chance sheet flow flooding where average depths are less than 1 foot, areas of 1% annual chance stream flooding where the contributing drainage area is less than 1 square mile, or areas protected from the 1% annual chance flood by levees. No Base Flood Elevations or depths are shown within this zone. Insurance purchase is not required in these zones.

Zone D:
Areas with possible but undetermined flood hazards. No flood hazard analysis has been conducted. Flood insurance rates are commensurate with the uncertainty of the flood risk.

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