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Floodplain Development in Approximate A-Zones

By Bernard Wallace,2014-04-29 19:52
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    Floodplain Facts #10

    Floodplain Development in Approximate A Zones

    Approximate A Zones are those areas where floodplain boundaries have been established using approximate techniques without conducting detailed hydrologic and hydraulic studies. These

    approximate 100-year flood zones are shown on flood hazard maps as “unnumbered A zones” (the letter

    “A” with no letter or number after it).

Limited Flood Hazard Data

    Because detailed floodplain studies are expensive, the

    floodplains in many rural areas were estimated using

    approximate techniques. The resulting flood hazard maps

    delineate the boundaries for the “Special Flood Hazard

    Area” (100-year floodplain) within which development

    must comply with floodplain development standards.

    However, no information is provided regarding the Base

    Flood Elevation (BFE; anticipated depth of flooding).

    And no Regulatory Floodway (area reserved to discharge

    flood flows) is delineated. In the absence of detailed

    flood hazard information, it is still necessary to ensure

    that development is reasonably safe from flood damage.

Use of Other Flood Data

    When development is proposed in an Approximate A Zone, the municipality must “obtain, review and

    reasonably utilize any base flood elevation and floodway data available from a Federal, State, or other source...” [44 CFR 60.3 (b) (4)]. Information from other sources should be used as the basis for regulating floodplain development if it: (1) reasonably reflects flooding conditions expected during the base flood (which has a 1% probability of being equaled or exceeded in any given year, i.e. the 100-year flood), (2) is not known to be technically incorrect, and (3) represents the best available data. If authoritative BFE and floodway data are not available, historical flood heights or other information can provide guidance.

Possible sources of flood data include:

    o Flood Hazard Maps: Provisional or advisory flood hazard maps can be used for floodplain management

    prior to being finalized or adopted (unless the technical validity is being questioned). Near a municipal

    boundary, flood elevations developed for the neighboring community may be applicable. o Highway Departments: If the area is near a bridge or other highway structure, a flood study may have

    been done to size the structure.

    o NYS Department of Environmental Conservation (DEC): The DEC Regional Floodplain Management

    Coordinator may be aware of available flood data or information.

    o Other Agencies: Flood studies, reports, or other data may be available from agencies that have worked

    with the stream or watershed, such as U.S. Army Corps of Engineers, U.S. Geological Survey, USDA

    Natural Resources Conservation Service, or the County Soil and Water Conservation District. o Applicant: The applicant must provide Base Flood Elevations (developed using a “detailed method”) for

    any proposed development that is greater than either 50 lots or 5 acres if one or more structures or

    building sites are in an Approximate A Zone. For smaller projects, a BFE may be requested. o Municipality: The municipality may develop Base Flood Elevation and/or Floodway data for regulating

    development in Approximate A Zones. This is advisable if multiple applications are anticipated.

    “Simplified methods” for determining BFEs can be sufficient for floodplain management purposes and

    are less costly than the “detailed methods” required for flood hazard map revisions and insurance ratings.

    o Historical Sources: The municipality should also consult with neighbors, staff, or others who may have

    knowledge of historical flood heights at or near the proposed development.

    Prepared by Southern Tier Central Regional Planning and Development Board

    Floodplain Facts #10

    Determining the Base Flood Elevation (BFE)

    The “Base Flood Elevation” is the calculated water height that has a 1% probability of being equaled or

    exceeded in any given year (the 100-year flood). The Federal Emergency Management Agency (FEMA) provides guidance for developing BFEs using “detailed methods” (similar to those used for flood hazard map

    development) and “simplified methods” (that can be used to mange development in isolated areas). When a developer is required to provide BFEs for a large project, a “detailed method” is required.

Flood Protection Level

    In Approximate A Zones, the level to which development should be protected from flood damage (by elevation or other means appropriate to the type of development) depends on the available information: o Two or more feet above BFE required if BFE is available from a reliable source or has been developed; o Two or more feet above historic flood levels recommended if prior flood depths exceeded one foot; or

    o Three or more feet above the highest adjacent grade required if BFE is not available. If circumstances

    suggest that this may not provide adequate flood protection, the municipality can require the developer to

    determine the BFE (since the project must be consistent with the need to minimize flood damage.) Flood Insurance Consideration: If the elevation of a building in an Approximate A Zone is documented

    on an Elevation Certificate (generally required by the community to confirm compliance with

    development standards), the cost of flood insurance is rated in one of the following ways:

    o If there is no BFE, the building is rated based on its height above the highest adjacent grade.

    o If the community provides a locally-developed BFE that was determined using detailed methods, the

    building is rated based on its height relative to that BFE.

    The insurance costs will generally be lower for a building with the first floor two feet above the BFE

    (minimum standard if the BFE is available) than for a building with the first floor three feet above grade (minimum standard without a BFE). It may be more cost effective in the long run to hire an engineer to develop a BFE, than to insure a building constructed without that information.

     Proposed Development Shall Not Result in Physical Damage to Any Other Property

    Although floodway delineations are typically not available in Approximate A Zones, the concept of reserving room for flood flows still applies. A floodplain permit cannot be issued for development that would result in physical damage to any other property. If the potential for damage exists (due to increased flood heights, stream bank erosion, increased flood velocities, etc.), the municipality can require a technical analysis by a licensed professional engineer to facilitate this determination. This analysis may be warranted for any project (bridge, berm, building, fill, etc.) that encroaches on the channel or obstructs flood flows.

Locate Development Outside of the Floodplain

    Because Approximate A Zones are usually located in rural areas or along small streams, it is often feasible to locate development outside of the mapped floodplain. This is the preferred option because it protects development from flood hazards, reduces the risk of damage from streambank erosion, preserves natural floodplain functions, and protects occupants and users who might otherwise require evacuation during a flood. It may also save on the cost of a flood study (for larger developments) and the subsequent cost of flood insurance for building owners.

Additional Resources

    o Managing Floodplain Development in Approximate Zone A Areas: A Guide for Obtaining and

    Developing Base (100-Year) Flood Elevations, FEMA 265 (1995), available at http://www.fema.gov/

    library/viewRecord.do?id=2215, provides engineering guidelines for determining Base Flood Elevations

    using simplified and detailed methods.

    o QUICK-2 Version 2.0, FEMA, available at http://www.fema.gov/plan/prevent/fhm/dl_qck22.shtm,

    software and tutorial for computing flood elevations using detailed methods.

    Prepared by Southern Tier Central Regional Planning and Development Board

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