23 C.F.R. § 667.3
For purposes of this part:
Catastrophic failure means the sudden failure of a major element or segment of a road, highway, or bridge due to an external cause. The failure must not be primarily attributable to gradual and progressive deterioration or lack of proper maintenance.
Evaluation means an analysis that includes identification and consideration of any alternative that will mitigate, or partially or fully resolve, the root cause of the recurring damage, the costs of achieving the solution, and the likely duration of the solution. The evaluations shall consider the risk of recurring damage and cost of future repair under current and future environmental conditions. These considerations typically are a part of the planning and project development process.
Emergency event means a natural disaster or catastrophic failure resulting in an emergency declared by the Governor of the State or an emergency or disaster declared by the President of the United States.
Reasonable alternatives include options that could partially or fully achieve the following:
Repair and reconstruction means work on a road, highway, or bridge that has one or more reconstruction elements. The term includes permanent repairs such as restoring pavement surfaces, reconstructing damaged bridges and culverts, and replacing highway appurtenances, but excludes emergency repairs as defined in 23 CFR 668.103 .
Roads, highways, and bridges means a highway, as defined in 23 U.S.C. 101(a)(11) , that is open to the public and eligible for financial assistance under title 23, U.S.C.; but excludes tribally owned and federally owned roads, highways, and bridges.
23 C.F.R. §667.3