Flood Maps/Floodplain Glossary

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1% Floodplain

Also known as the Base Flood, it is an area of land that has a 1% chance of being inundated by flood waters in a given year. 

1% Rainfall

An amount of rain that has a 1% chance of falling during a certain duration, typically 24 hours, at a given location during a given year. For Hillsborough County this amount of rainfall is just over 11 inches in 24 hours. 

100-Year Flood Plain

The flood elevation that has a 1% chance of being equaled or exceeded each year; it is not the flood that will occur once every 100 years. 

Base Flood Elevation (BFE)

This is the elevation above the average sea level that waters from a 1% flood will reach at a given point along a riverine system. These elevations are determined using hydrology and hydraulic computer models and then these elevations are mapped on the topographic data for the county to produce the 1% floodplain. 


The hydrology and hydraulic computer models used to determine the floodplains are checked using actual data from real events like Hurricanes Frances and Jeanne to make sure that the model are accurately predicting the level of flood risk for Hillsborough County citizens. 

Contour Line

A line on a map that joins points of equal elevation. 

Cooperating Technical Partnership (CTP)

Hillsborough County has entered into a CTP with FEMA which allows the County to cost share and work together on projects such as modeling the 100-year flood risk. 


All ground surface elevations are measured from a certain point called a datum. Once the revised flood maps are completed, all of the elevation data for Hillsborough County will be in NAVD 1988. 

Disaster Area

When a disaster is beyond the capabilities of state and local government to respond, the Governor must make a formal request to the President to declare the affected region a disaster area. When the presidential declaration is enacted, federal assistance is made available to public and certain non-profit entities, as well as to individuals who were adversely affected by the disaster. The assistance is available in many forms, including monetary, temporary housing, crisis counseling and even legal assistance. View more information on the Disaster Declaration process. 


The distance that any point on the ground is above a certain point called a datum. Elevations in Hillsborough County range from 0 feet along the Bay to over 100 feet near the City of Plant City. 

Elevation Certificate

An elevation certificate is a certificate issued by a professional surveyor that states that the elevation of the bottom floor of a structure on a given property in relation to the BFE for the property. These certificates are used by insurance agents to rate flood policies and illustrate compliance with minimum standards of the National Flood Insurance Program. 


Any time two or more normally dry residential or commercial lots are inundated by water. 

Flood Insurance Rate Map (FIRM)

A FIRM is a FEMA map that shows areas that have the highest probability for flooding. These maps are used to determine if flood insurance is required and what its cost will be to the buyer. 

Flood Insurance Study (FIS)

A study done by engineers to determine the level of risk citizens in a certain area have with respect to the dangers of flooding. The end result of a FIS is flood insurance rate maps that are used to determine the cost and requirements for the purchase of flood insurance. 


Rivers, tributaries, and creeks naturally come out of their banks and inundate the adjacent land from time to time. This area that is inundated is referred to as a floodplain. 

Floodplain Administrator (FPA)

Local community officials charged with enforcing NFIP regulations that regulate development in the floodplain. 

Floodplain Management

In order for a community to offer flood insurance through the NFIP, the community is required to enforce certain minimum regulations on development in the floodplain. This management of the floodplain is done to ensure that flooding problems do not increase and to work towards the reduction in the risk of flooding. This work is performed by the local communities' Floodplain Administrator. The county and its three municipalities have regulations that are more stringent than the NFIP minimum regulations. 


The area along both sides of a river, tributary, or creek including the main channel that has the strictest regulations on it because it is the area that is needed to move the 1% flood downstream and out of the homes or businesses that it may have flooded. 

Geographic Information System (GIS)

A computer program used to store different types of information and link that information to a specific location. Some examples of this information would be streets, water features, parcel data, contours, floodplains and all the data that supports this information such as names, location and much more. 

Global Positioning System (GPS)

GPS is a system that uses satellites to accurately determine the location of any point on earth, and it helps to create the most accurate floodplain maps possible. 


An engineering process used to convert a volume of water moving down a channel into a depth of water so that it can be drawn on a map of flooding areas. This process is done using a computer model such as SWMM or HEC-RAS. 


An engineering process used to convert a rainfall amount into a volume of water moving down a channel. This volume of water is then input into a hydraulic model and turned into a map of flooding areas. This process is done using a computer model called HEC-HMS. 

Letter Of Map Amendment (LOMA)

A LOMA is a report that must be submitted to FEMA to change a FIRM in order to move a piece of property out of the 1% floodplain. 

National Flood Insurance Program (NFIP)

The NFIP is a program that is part of FEMA that was developed to provide information about the risks of flooding and allow citizens to purchase flood insurance to protect them from the financial risks of flooding. 

Overland Flow (Sheet Flow) Flooding

Flooding that occurs when intense local rainfall exceeds storm sewer or roadside ditch capacity, the water can "pond" in the streets deep enough to flood residences that are not even near a primary water source.The water will seek a path to the channel by flowing overland (Sheet Flow). When residences and other structures are in that path, flooding occurs and this type of flooding is not identified on the Flood Insurance Rate Maps. 

Parcel, Panel

Hillsborough County is divided into 173 squares called panels, and these panels are further divided into approximately 370,000 parcels which is another word for properties. 

Preliminary Flood Insurance Rate Map

Term given to the new FIRMs that are being created by Hillsborough County and FEMA, so as to distinguish them from the current FIRMs, while the Preliminary FIRMs are reviewed during the appeals process. At the conclusion of the appeals process, the Preliminary FIRMs will become the official FIRM for Hillsborough County and its three municipalities. 


The process, occurring after a rainfall, when water gathers in low lying areas throughout a watershed and never makes it to a bayou or creek.


Residences, as referenced in this site, are any dwellings in which people live, including single-family houses, apartment units, mobile homes and travel trailers.

Riverine Flooding

Flooding that is the result of rivers, tributaries, or creeks leaving their banks as a result of a heavy rainfall. This is the flooding that is mapped on the Flood Insurance Rate Maps.


Run off is the water from rainfall not absorbed by the ground that flows in to the local drainage system, and ultimately, rivers, and streams.

Special Flood Hazard Area (SFHA)

An area defined on a Flood Insurance Rate Map with an associated risk of flooding from the 1% rainfall event.

Topographic Data

Topographic data is detailed information about the shape of the earth including ground elevations, and the location of roads, water features, and objects both natural and man-made.

Water Surface Elevation

The distance the water surface in a river, stream, creek or lake is above the average sea level at a given location along the river, stream, creek, or lake.


An area of land that ultimately drains to the same, specific body of water.

Zones (A , V, X )

AE, A, or AO Zone - These properties have a 1 percent chance of flooding in any year, and a 26 percent chance of flooding over the life of a 30-year mortgage.
VE or V Zone - These properties have a 1 percent chance of flooding in any year and also face hazards associated with coastal storm waves.
Shaded X Zone - These properties are outside the high-risk zones. The risk is reduced but not removed.
X Zone - These properties are in an area of overall lower risk.