1% Floodplain:
The area of land that has a 1% chance of being inundated by flood waters from a bayou, stream or creek in a given year. It is commonly referred to as the 100-year floodplain. This is a regulatory standard used to administer floodplain management programs, the National Flood Insurance Program (NFIP) and to set building requirements for new construction. Statistically, the 1% (100-year) flood has a 26% chance of occurring during a 30-year period of time – the length of many mortgages.
The 1% (100-year) floodplain is the Base Flood or Special Flood Hazard Area. It is referred to as Zones AE, AO, A, or VE for insurance purposes on Flood Insurance Rate Maps (FIRMs). Properties located in these mapped zones are required to have flood insurance if the owner has a federally backed mortgage on the property.
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 Harris County this amount of rainfall is just over 13 inches in 24 hours or just under 11 inches in 12 hours.
0.2% Floodplain:
The area of land that has a 0.2% chance of being inundated by flood waters from a bayou, stream or creek in a given year. It is commonly referred to as the 500-year floodplain. This is a regulatory standard used to administer floodplain management programs and the National Flood Insurance Program (NFIP). Statistically, the 0.2% (500-year) flood has a 6% chance of occurring during a 30-year period of time – the length of many mortgages.
The 0.2% (500-year) floodplain is referred to as Zone X for insurance purposes on Flood Insurance Rate Maps (FIRMs).
0.2% Rainfall:
An amount of rain that has a 0.2% chance of falling during a certain duration, typically 24 hours, at a given location during a given year. For Harris County this amount of rainfall is just less than 19 inches in 24 hours.
2-Foot Contour
A contour line is a line on a map that joins points of equal elevation. Each 2-foot contour line will increase or decrease in increments of 2 feet to illustrate the identified land elevation.
10-Foot Contour
A contour line is a line on a map that joins points of equal elevation. Each 10-foot contour line will increase or decrease in increments of 10 feet to illustrate the identified land elevation.
Base Flood Elevation (BFE):
This is the elevation above the average sea level that waters from a 1% (100-year) flood will reach at a given point along a creek or bayou. These elevations are determined using hydrology and hydraulic computer models. The elevations are then mapped on the topographic data for the county to produce the 1% (100-year) floodplain.
Calibrate:
The hydrology and hydraulic computer models used to determine the floodplains are checked using actual data from real events like Tropical Storm Allison to make sure that the model are accurately predicting the level of flood risk for Harris County citizens.
Contour Line:
A line on a map that joins points of equal elevation.
Cooperating Technical Partnership (CTP):
The Harris County Flood Control District has entered into a CTP with FEMA which allows the District to cost share and work together on projects such as TSARP with FEMA.
Datum:
All ground surface elevations are measured from a certain point called a datum. Once TSARP is completed all of the elevation data for Harris County will be on NAVD 1988 2001 Adjustment Datum.
Digital Elevation Model (DEM):
A computer is used to generate a grid that in the case of TSARP covers all of Harris County and results in an elevation point ever 15 feet.
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. For more on the Disaster Declaration process, go to: www.fema.gov/library/dproc.shtm
Elevation:
The distance that any point on the ground is above a certain point called a datum. Elevations in Harris County range from 0 feet in the southeast by Galveston Bay to over 300 feet in the Northwest corner of the county.
Elevation Certificate:
An Elevation Certificate is a detailed survey of a structure’s elevation to see if it is above or below the base flood elevation. An Elevation Certificate can be used to reduce the cost of flood insurance or even remove a particular structure from the 1% (100-year) floodplain.
Floodplain:
Bayous 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.
Flood:
Any time two or more normally dry residential or commercial lots are inundated by water.
FIRM Panel
FIRM stands for Flood Insurance Rate Map. In order to print the FEMA Flood Insurance Rate Maps at a scale of 1-inch = 1,000-feet, the maps are broken out into over 150 FIRM panels that cover the entire Harris County area.
Flood Insurance Rate Maps (FIRMs):
Prepared by FEMA, Flood Insurance Rate Maps, or FIRMs, show areas that have the highest probability of flooding and illustrate the extent of flood hazards in a flood-prone community. These maps are used to determine flood insurance rates for communities participating in the National Flood Insurance Program (NFIP).
Properties located in mapped zones AE, AO, A, or VE are required to have flood insurance if the owner has a federally backed mortgage on the property.
Flood Insurance Study (FIS):
A study FEMA initiates to undertake a new hydraulic and/or hydrologic analysis for streams within a community. The study itself is the documentation of the information that went into determining the flood hazard information for a community and includes publication of the Flood Insurance Rate Maps (FIRMs).
Floodplain Administrator (FPA):
Sometimes called a local floodplain manager, the Floodplain Administrator is the person identified within a local governmental jurisdiction to administer their community’s floodplain management and development regulations in accordance with the National Flood Insurance Program (NFIP). The Floodplain Administrator keeps the official FEMA Flood Insurance Rate Maps (FIRMs) that document flood hazard zones in their community. The Harris County Flood Control District is not a floodplain administrator. Check here for the floodplain administrator for your area.
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. Most of the 35 communities in Harris County, including the City of Houston and Unincorporated Harris County, have regulations that are above the NFIP minimum regulations.
Floodway:
The area along both sides of a bayou 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, bayous and channels, HCAD 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.
Hydraulics:
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 called HEC-RAS.
Hydrology:
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.
Light Detection and Ranging (LIDAR):
LIDAR is a new technology that uses lasers to measure the elevation of the ground and provides the greatest possible detail of elevations throughout all of Harris County. LIDAR provides a ground elevation point every 15 feet that is accurate to +/- 6 inches. More detailed information about LIDAR can be found at www.tsarp.org/tsarp_over/lidar.html.
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 creek of bayou. 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.
Preliminary Flood Insurance Rate Map:
Term given to the new FIRMs that are being created by TSARP, so as to distinguish them from the current FIRMs, as 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 Harris County.
Profile
A graph that shows the Base Flood Elevations along the entire length of bayou, stream or creek. See also Water Surface Elevation Profile.
Ponding:
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:
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 creeks and bayous leaving their banks as a result of a heavy rainfall. This is the flooding that is mapped on the Flood Insurance Rate Maps.
Runoff:
Run off is the water from rainfall not absorbed by the ground that flows in to the local drainage system, and ultimately, streams and bayous.
Special Flood Hazard Area (SFHA):
An area defined on a Flood Insurance Rate Map with an associated risk of flooding.
Subsidence:
Subsidence is when the elevation of the ground drops slowly over time as water is pumped out and used for commercial and domestic uses. Since 1973 most of Harris County has fallen 1 foot with some areas falling as much as 5 feet. The Harris Galveston Subsidence District was created to monitor and address this problem and the rate of subsidence has been greatly reduced throughout most of the county.
Topographic Data:
Topographic data is detailed information about the shape of the earth including ground elevations, and the location of roads, bayous, and objects both natural and man-made.
Tropical Storm Allison Recovery Project (TSARP):
The Tropical Storm Allison Recovery Project is a joint-effort project between FEMA and the District to gather information about Tropical Storm Allison and to create new flood hazard information for all of Harris County. It will result in the citizens of Harris County having more information about flooding and being better prepared for the next flood. More detailed information about TSARP can be found at www.tsarp.org/tsarp_over/index.html.
Water Surface Elevation:
The distance the water surface in a creek or bayou is above the average sea level at a given location along the creek or bayou.
Water Surface Elevation Profile
Shows the elevation above mean sea level of the 1% (100-year) or 0.2% (500-year) floodplain along all the studied stream miles in a particular watershed. See also Profile.
Watershed:
An area of land that ultimately drains to a specific body of water-mostly creeks and bayous in Harris County.
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