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Floodplain Management

Flood Basics

A "floodplain" is the lowland adjacent to a river, lake or ocean. Floodplains are designated by the frequency of the flood that is large enough to cover them. For example, the 10-year floodplain will be covered by the 10-year flood and the 100-year floodplain by the 100-year flood.

Flood frequencies, such as the "100-year flood," are determined by plotting a graph of the size of all known floods for an area and determining how often floods of a particular size occur.  Another way of expressing the flood frequency is the chance of occurrence in a given year, which is the percentage of the probability of flooding each year.  For example, the 100-year flood has a 1% chance of occurring in any given year.

Dams, levees, channels, storm water projects and other protective works are designed to provide protection against some specific level of flooding. The "level of protection" is selected based on cost, desire of the community, potential damage, environmental impact, and other factors.  Engineers can design and construct levees, dams and other measures providing a very high level of protection. Communities tend to choose lower levels of protection because of the initial financial cost rather than overall costs and benefits.

The National Flood Insurance Program has established a de facto minimum standard of protection against the 100-year flood. This is a relatively low level of protection. For example, there is a 26% chance that a levee or channel designed to contain the 100-year flood will be at that design capacity at least once over a 30 year period.  All residents and businesses in areas vulnerable to flooding should have flood insurance.

Homeowner insurance policies DO NOT cover damage from rising water.

There is a wide range of measures that can be used to protect against flooding. They may be grouped in various ways, such as:

"Structural" and "nonstructural" measures.

Whether they are most suitable for protecting:

1. individual structures or
2. areas containing multiple structures and communities.

Whether their purpose is to:

1. modify the flood or
2. reduce susceptibility to flooding; and/or
3. reduce the impact of flooding.

Multiple measures are usually needed to provide protection to an area.

Most of the known floodplains in the U.S. have been mapped by the Flood Insurance Administration, one of the parts of the Federal Emergency Management Agency. These identified areas account for about 60% of flood insurance claims. The remaining 40% of the claims occur in areas not previously recognized as being vulnerable to flooding, and are generally not located near a river or other water body.
Settling Your Insurance Claim [PDF]

The National Weather Service is responsible for warning the public of the possibility of flooding. Flood predictions generally are made at the regional "River Forecast Center". There are several different warning messages that may be issued, based upon the conditions and/or probability of flooding.     

What is Floodplain Management?

Floodplain management is the operation of a community program of corrective and preventative measures for reducing flood damage. These measures take a variety of forms and generally include zoning, subdivision, or building requirements, and special-purpose floodplain ordinances (FEMA).

Federal Alliance for Safe Homes
The official site of the NFIP
Floodplain definitions
FLOOD: Understanding and Recovering from One of Nature's Worst Disasters*
Flood Protection Tips
Florida Hazards Watch - Floods
Flood Maps
FEMA Letter of Map Revision (LOMR) and Letter of Map Amendment (LOMA)
Association of State Floodplain Managers (ASFPM)
Floodplain Management Association (FMA)
National Association of Flood & Stormwater Management Agencies (NAFSMA)
Florida Floodplain Managers Association (FFMA)
Florida - Floodplain region IV (FEMA)
Community Rating System (FEMA)
National Floodplain Insurance Program (NFIP)
Florida Division of Emergency Management

        

*Reproduced with the permission of Adjusters International

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Flood Statistics

	As little as one foot of (moving) water can move most cars off the road.
	Just six inches of fast-moving flood water can sweep a person off his or her feet.
	Most flood-related deaths occur at night and are vehicular.
	Urban and small stream flash floods often occur in less than one hour.
	Tropical cyclones pose significant risk well inland due to fresh water flooding.