Comparison and assessment of zero-rise floodplain ordinances

This paper compares and evaluates the implications of some zero-risk regulations, particularly zero-rise floodplain ordinances. An ordinance for King County, Wash., is used to illustrate the challenges of implementing such restrictions and their resulting implications for bridge design. The evaluation is performed by comparing the King County floodplain regulation to similarly restrictive regulations in other states and counties and to other zero tolerance regulations. The bridge design and construction costs associated with the ordinance since 1990 are also reviewed, as are procedures for obtaining variances from the zero-rise criterion. It was found that in King County the zero-rise criterion compounds the already existing complexities of bridge design requirements. Important differences between the King County regulations and zero-rise requirements in other locales were found to include: freedom from substantial clearance requirements, pier-in-channel prohibitions (unrelated to backwater considerations), exemptions for replacement in-kind, lenient permitting processes for rises where no damage is anticipated, and the ability of approach roads to cause flood. The zero-rise ordinance was found to increase bridge design and construction costs by about 40% on the bridges for which backwater was a constraint. Variance procedures were as impractical in other locales as in King County, but mitigation options, such as the purchase of flooding easements, were more frequently pursued.