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Bridges & Structures

Significant Encroachments Minute Memo

To: E. Cleckley

From: S. Gordon


The following guidance is provided regarding the determination of a significant encroachment as defined in FHPM 6-7-3-2:

A significant encroachment is a Federal Action within the base flood plain (or any direct support of likely flood plain development) that would involve one of the following:

  1. significant potential for interruption or termination of a transportation facility which is needed for emergency vehicles or which provides a community's only evacuation route,
  2. a significant risk, or
  3. a significant adverse impact on natural and beneficial flood plain values.

In items 1 thru 3 above, the term significant is defined in the same manner as is used in environmental review procedures - i.e., a project with a significant encroachment must be processed with an EIS. This type of determination for items (2) and (3) above is made in a manner similar to any other type of impact and has generally been handled by FHWA and the States without problems in interpretation. Item (1), on the other hand, has caused problems in interpretation and is therefore explained in further detail.

If a planned federal action would result in a significant potential for interruption or termination of the type of transportation facility described in (1) above, it shall be termed a significant encroachment. Engineering judgment is required in evaluating the site conditions to decide whether the Federal action should be classified as a significant encroachment, taking into consideration:

  1. extent and frequency of overtopping of the facility by floodwaters,
  2. consequences of overtopping - anticipated damages and effect on the community served, and
  3. other relevant factors that may have some bearing on the case such as amount of traffic using the facility, number of people in the community, general elevation of the community as compared with the overtopping elevation, anticipated time for advance warning, etc.

The determination of a significant encroachment is based on the proposed federal action rather than on the existing transportation facility; therefore, an improvement proposed for construction on an existing alignment that alleviates or does not worsen the existing flood hazard would not normally be considered as a significant encroachment. In planning this type of project, consideration should be given to the need to minimize the flooding hazard for the entire section of the highway facility in the base flood plain. Where it is not practical to accomplish this objective, the project records should support the basis for the design selected, and there should be reasonable public notification of the proposed design.

It is our view that incorporation of the type of significant encroachment described above (resulting from a significant potential for interruption of a transportation facility) is rarely justified for Federal-aid highway projects on new location. Where the evaluation of site conditions for such facilities indicates substantial risks, we would encourage raising the transportation facility to an elevation {normally the 100-year flood or tide elevation) that would minimize the risk and avoid the significant encroachment.

Updated: 06/27/2017
Federal Highway Administration | 1200 New Jersey Avenue, SE | Washington, DC 20590 | 202-366-4000