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Note: This was partically rescinded by Backwater Criterion for Hydraulic Design of Structures on 04/21/1992

Subject: Design Standards for Highways in National Flood Insurance Program Mapped Floodplains Date: APR 2 1986
From: Associate Administrator for Engineering and Program Development Reply to Attn. of: HIBT-20
To: Regional Federal Highway Administrators
Regions 1-10

The FHWA recognizes the National Flood Insurance Program (NFIP) standard that provides for up to a 1-foot increase in flood stages when (1) designating a floodway or (2) evaluating an encroachment where no floodway is designated. This standard is established as the Federal standard under Executive Order 11988, Floodplain Management, and is to be used in designing highways in NFIP mapped floodplains.

The FHWA floodplain regulations were issued in November 1979 (23 CFR 650, subpart A). These regulations established standards for the cost-effective design of highways in floodplains and for consistency with the NFIP. Guidance for complying with the NFIP part of the regulations was provided to the field by Mr H. D. Morgan's memorandum of June 25, 1982, and was titled "Procedures for Coordinating Highway Encroachments on Floodplains with the Federal Emergency Management Agency (FEMA)".

The coordination procedures established that NFIP standards are to be used in designing a highway in a NFIP mapped floodplain. Highways can normally be designed to be consistent with the NFIP standards because the standards provide for a 1-foot rise in the water surface elevation of the 100-year flood. This increase is included in most State and local floodplain regulations. Development, including highways, is permitted that does not cause backwater in excess of this increase.

The courts have generally ruled that the 1-foot rise of water onto affected property is not a taking and therefore does not require compensation. In addition, the property owner is eligible for damage protection under the NFIP. The property owner is further protected from loss in that the flood insurance rates will not be increased for a rise of 1-foot or less.

Some States have elected to adopt more restrictive standards than the NFIP standards. By more restrictive, it is meant that less than 1-foot of increase is permitted. In a few States, no increase is permitted at all. Permitting little or no increase has the effect of limiting floodplain development. Limiting development in this way is a State/local option under the FEMA regulations. The FEMA encourages restrictive standards because the liability of the NFIP is reduced.

However, there may be a cost to others. Application of these more restrictive standards can also result in larger, more costly highway structures and/or right-of-way costs than would have been required under the NFIP standards. These increased costs to meet standards more restrictive than NFIP standards are the responsibility of the State.

Therefore, Federal-aid highway funds should not be used either (1) for payments to property owners or (2) for more costly highway facilities if, in either case, the costs are incurred to meet State standards that require less than a 1-foot rise in water surface elevation for the 100-year flood. Exceptions for designs that limit the water surface to less than a foot may be granted on a case by case basis where the cost effectiveness of such designs can be demonstrated by an economic analysis.

If it is cost effective to exceed the 1-foot increase, FHWA will participate in right-of-way costs for insurable buildings in order to limit flood damage in­creased for which the State or NFIP might be responsible. Attached are options which should be considered if the 1-foot increase is to be exceeded.

/S/ Original signed by
Rex C. Leathers


Participation Options for Limiting Flood Damage

If a Federal-aid highway project will cause a 100-year flood elevation which is more than 1-foot higher than the base flood elevation shown on a NFIP map at either (1) a National Flood Insurance Program (NFIP) insurable building or (2) an unimproved property with a substantially changed highest or best use, participation may be provided with Federal-aid highway funds by the following option which best fits the State's property management plan:

  1. 1. Purchase property
    1. a. Relocate owners/tenants
    2. b. Buildings should be:
      1. (1) Destroyed,
      2. (2) Relocated out of the floodplain
      3. (3) Resold with commitment of buyer to:
        1. (a) Relocate out of the floodplain, or
        2. (b) Elevate the building above the new 100-year flood level or accomplish other acceptable floodproofing techniques
  2. 2. Reimburse Owner for Floodproofing Buildings
    1. a. Buildings should be elevated to or above the new 100-year flood level or other acceptable floodproofing techniques applied
    2. b. Owner retains title
    3. c. Owner arranges for floodproofing
    4. d. Owner to sign instrument to hold State harmless from future flood damage
    5. e. Reimbursement for floodproofing is limited to the cost to purchase property and relocate residents.
  3. 3. Purchase Permanent Easement
    1. a. For unimproved property
    2. b. Reimbursement is based on before and after appraisal

Design Standards for Highways in National Flood Insurance Program Mapped Floodplains


  • The FHWA and FEMA have developed coordinating procedures that are being successfully used by the States to locate highways in floodplains within the National Flood Insurance Program (NFIP). These procedures were distributed to the field by Mr. Morgan's memo of June 25, 1982 (included in background tab).
  • The FEMA standard for the NFIP is to permit up to a 1.0-foot rise in water surface elevation for the 100-year flood. Development in this floodplain (including highways) is. permitted that does not cause backwater in excess of 1.0 foot.
  • Most States use the FEMA standard of 1.0 foot, and FHWA adopted this standard for NFIP floodplains in the FHWA/FEMA guidelines.
  • A few States (about seven) have adopted more restrictive standards than the 1.0 foot FEMA standard. Maryland, for example, has adopted a standard of a "zero increase" in water surface elevations.
  • A need for a clarification of FHWA policy was perceived as a result of recent design reviews in Maryland.


  • Region 3 has been advised that Headquarters will provide policy guidance to cover the issue of the restrictive Maryland floodplain regulations.
  • This policy guidance will also serve in the other States that apply standards more restrictive than the FEMA standards.
  • The policy guidance has been reviewed and found acceptable by:
    • Office of ROW
    • Legal Counsel (see attached memorandum from HCC-40)
    • Federal-aid
    • Region 3
    • FEMA


  • The FHWA will continue to use the existing FHWA floodplain regulation (23 CFR 650/FHPM 6-7-3-2) and the FHWA/FEMA guidelines as the basis for FHWA policy regarding floodplains in the NFIP.
  • Our policy is to utilize up to the 1 foot available for development in NFIP floodplains. Designs based on backwater of less than 1 foot will be eligible for full Federal participation when the need and/or cost effectiveness of such designs can be supported by the State as per FHWA's floodplain regulation.
  • FHWA may limit participation with Federal-aid highway funds for bridges or highway sections in floodplains designed to meet standards of less than 1 foot where there is inadequate justification to support the State's design criteria.
  • FHWA will not normally participate in ROW easements in NFIP floodplains for backwater of less than 1 foot for the 100-year flood unless a "taking" is determined to exist.


Updated: 06/27/2017
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