[Code of Federal Regulations]
[Title 23, Volume 1]
[Revised as of April 1, 2002]
From the U.S. Government Printing Office via GPO Access
[CITE: 23CFR650.115]

[Page 228-229]
 
                           TITLE 23--HIGHWAYS
 
 CHAPTER I--FEDERAL HIGHWAY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION
 
PART 650--BRIDGES, STRUCTURES, AND HYDRAULICS--Table of Contents
 
   Subpart A--Location and Hydraulic Design of Encroachments on Flood 
                                 Plains
 
Sec. 650.115  Design standards.

    (a) The design selected for an encroachment shall be supported by 
analyses of design alternatives with consideration given to capital 
costs and risks, and to other economic, engineering, social and 
environmental concerns.

[[Page 229]]

    (1) Consideration of capital costs and risks shall include, as 
appropriate, a risk analysis or assessment which includes:
    (i) The overtopping flood or the base flood, whichever is greater, 
or
    (ii) The greatest flood which must flow through the highway drainage 
structure(s), where overtopping is not practicable. The greatest flood 
used in the analysis is subject to state-of-the-art capability to 
estimate the exceedance probability.
    (2) The design flood for encroachments by through lanes of 
Interstate highways shall not be less than the flood with a 2-percent 
chance of being exceeded in any given year. No minimum design flood is 
specified for Interstate highway ramps and frontage roads or for other 
highways.
    (3) Freeboard shall be provided, where practicable, to protect 
bridge structures from debris- and scour-related failure.
    (4) The effect of existing flood control channels, levees, and 
reservoirs shall be considered in estimating the peak discharge and 
stage for all floods considered in the design.
    (5) The design of encroachments shall be consistent with standards 
established by the FEMA, State, and local governmental agencies for the 
administration of the National Flood Insurance Program for:
    (i) All direct Federal highway actions, unless the standards are 
demonstrably inappropriate, and
    (ii) Federal-aid highway actions where a regulatory floodway has 
been designated or where studies are underway to establish a regulatory 
floodway.
    (b) Rest area buildings and related water supply and waste treatment 
facilities shall be located outside the base flood plain, where 
practicable. Rest area buildings which are located on the base flood 
plain shall be floodproofed against damage from the base flood.
    (c) Where highway fills are to be used as dams to permanently 
impound water more than 50 acre-feet (6.17x10\4\ cubic metres) in volume 
or 25 feet (7.6 metres) deep, the hydrologic, hydraulic, and structural 
design of the fill and appurtenant spillways shall have the approval of 
the State or Federal agency responsible for the safety of dams or like 
structures within the State, prior to authorization by the Division 
Administrator to advertise for bids for construction.