Floodplain zoning: Implications of hydrologic and legal uncertainty

Implementation of floodplain regulations in the United States has been very slow. There is a danger that regulation will not be widely adopted before floodplains become largely developed. Important among the reasons for failure to regulate floodplains appear to be the misconceptions that precise floodplain delineation is (1) hydrologically possible and (2) legally required. Our analysis shows that wide confidence intervals are unavoidably associated with even the most elaborate and expensive delineation techniques. Application of the ‘fairly debatable rule’ legally supports regulation based on expedient techniques, as long as they are fairly applied, and issues of constitutional purpose and taking of property are properly considered. Thus there is hydrologic and legal justification for floodplain regulation based on expedient delineation methods, and it is imperative that such methods be used if regulation is to be an effective tool for flood damage reduction.