Local governments can use sound land use planning strategies to protect households from the hazards of natural disasters. These planning strategies include designing zoning codes and other regulations that prevent residential development in vulnerable areas, as well as strategic location of natural storm buffers and other mitigation features.
One important aspect of sound land use planning and zoning is
understanding the true risk of flooding within flood plains and how
that risk is affected by increased development over time. People often
misconceive the chance of flooding in the 100-year floodplain, which is
the standard measure used to gauge flood risk in a community.
Those not
familiar with the technical definition of this designated area often
believe it represents an area that has a chance of being flooded only
once every 100 years. However, the true probability of flooding in this
area is 1 percent annually. For a 500-year floodplain, the probability
of flooding in the respective area is 0.2 percent annually.
In addition to understanding the proper probability of 100- and
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FEMA Floodplain map (light blue) and future Community floodplain (dark gray), Charlotte-Mecklenburg County, NC -- Image courtesy of City of Charlotte and Mecklenburg County.
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500-year floods, it is also important to understand the compound risk of flooding for a residential property within a flood plain over multiple years. For example, assume a new home is built within or near a 100-year floodplain, and assume the homeowner will live there for at least 30 years, the term of most fixed-rate mortgages. Compounding the 1-percent annual risk of flood damage occurring to that home, there is roughly a 26-percent chance of that home experiencing flood damage over that 30-year period.
The 100-year flood is used by the National Flood Insurance Program (NFIP - see box below for a description) as the standard for floodplain management procedures and to determine the need for flood insurance. It is also the standard used by most state agencies to determine flood risks, develop mitigation regulations and guidelines, and conduct effective land use planning.
The National Flood Insurance Program
The National Flood Insurance Program (NFIP) is a program managed by the Federal Emergency Management Agency (FEMA) that provides flood insurance, floodplain management and flood hazard mapping to communities susceptible to flooding. The program covers almost 20,000 communities across the United States and its territories. This is a voluntary program in which communities can adopt and enforce floodplain management regulations and other measures to reduce future flood damage. In exchange, the NFIP provides flood insurance to to homeowners, renters, and business owners in these communities.
In addition to providing flood insurance and reducing flood damages through floodplain management regulations, the NFIP identifies and maps floodplains in all of its participating communities. Mapping flood hazards creates broad-based awareness of the flood hazards and provides the data needed for floodplain management programs and to actuarially rate new construction for flood insurance.
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FEMA updates floodplain designations periodically as part of a community's Flood Insurance Rate Map (FIRM), which is used to set the standards for the NFIP. However, it may be several years between map updates, and new development in and around floodplains can alter the areas at risk of flooding. Generally, as more development occurs in and around a floodplain, it creates more impervious surface in this area, which tends to expand the area at risk of flooding. Therefore, existing or new homes built near the floodplain boundaries may actually be at greater risk of flooding than is assessed in the most recent flood map.
Solutions in Action
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Mecklenburg County, North Carolina (which includes the city of Charlotte), after working with FEMA to update its Flood Insurance Rate Map in 2000 based on current land use and development conditions, conducted an analysis to determine development patterns under maximum build-out under current land use and zoning regulations. The county then analyzed how this potential development would affect its current floodplain designations.
The county compared the potential flood damages that would likely occur under the maximum build-out scenario for both the current (2000) floodplain areas and the newly projected floodplain areas (see above image). They discovered there would be an estimated $333 million in additional damages under maximum build out when building according to the current land used and flood plain designations. In response to this, the county revised its zoning code and land use regulations based on the projected change in the floodplain. [1]
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[1] Buildout Analysis: A Valuable Planning and Hazard Mitigation Tool. By David R. Godschalk. Zoning Practice. Chicago, IL: American Planning Association.
