There are two accepted alternatives for determining the design flood frequency at a specific site: (1) by policy and (2) by economic assessment. An example of an establishment of a design flood frequency by policy is the Code of Federal Regulations, which specifies that the design flood for encroachment onto through lanes of interstate highways shall not be less than the 50-year discharge. Most state and local agencies have established guidelines for policy requirements of design flood frequencies. For example, whereas bridges are designed to convey a 50-year discharge with a specified freeboard and to convey the 100-year discharge with no freeboard, California has adopted the policy that culverts may be designed for a 10-year flood without headwater or to convey the base flood without damage to the facility or adjacent property. The base flood is defined as the flood or tide having a 1 percent chance of being exceeded in any given year, which is also defined as the 100-year flood. A design flood is a flood that will not inundate the highway—that is, will not cause the through lanes to be overtopped. An overtopping flood is a flood that will overtop the roadway, culvert, or bridge.
Blind adherence to the policy guideline to determine the design storm should be avoided. As a minimum, a range of peak flows should be considered and their potential effects on the traveling public, the potential damage to upstream and downstream properties, and the possibility of loss of life should be analyzed. This preliminary assessment will indicate whether the policy determination for the design flood frequency was applicable or whether further analysis is required. Additional studies could take the form of providing the greatest flood hazard avoidance at the least total expected cost, as recommended in Federal Highway Administration (FHWA) Hydraulic Design Series (HDS) 6, “River Engineering for Highway Encroachments.”
