Floodplain regulation strives to balance the benefits of reducing flood hazards to human life and property against the costs of limiting encroachment on areas that, under normal conditions, have appeal for many kinds of development. The 1990 enactment of a zero-rise ordinance in King County tightened floodplain regulations enough that agencies responsible for maintaining safe bridges and roadways are now hampered by excessive design and construction costs. Agencies responsible for maintaining transportation safety must now work in a highly restrictive regulatory environment in which bridge replacements and improvements are prohibitively expensive or impossible without variances from one or more regulations.
The general purpose of this project was to evaluate the implications of the zero-rise regulation for bridge builders in the King County Roads Division and the Washington State Department of Transportation. The study found the following:
Economic trade-off of compliance vs. noncompliance: The results of the impact analysis showed that the potential benefits of maintaining the state's infrastructure by replacing, widening, or building new bridges and roads should be considered when development is restricted within the floodplains. The additional costs for compliance with the zero-rise ordinance would be uniformly much greater than the marginal cost of damages due to additional flooding for the bridges studied.
Ability to predict, model, and measure changes: Because of the uncertainties in input parameters, it was not usually possible to predict changes in the water surface elevations to within 0.01 ft (0.003 m). The range of variation for predicted backwater elevations, given realistic levels of uncertainty in inputs, were up to an order of magnitude larger than the 0.01 ft (0.003 m) limit. A steady, one-dimensional hydraulic model, which is an economical and commonly used tool for assessing water surface profiles, was not generally accurate enough to delineate the floodplain to within 0.01 ft (0.003 m).
Alternatives to the zero-rise ordinance: An analysis of alternatives to a zero-rise ordinance suggested that, for a limitation law set to a technically enforceable tolerance, exemptions for public structures could be evaluated on the basis of economics.