Functional Floodplain Assessment as an Environmental Performance Standard: Bridge Replacements in Oregon

Abstract

Oregon (U.S.) implemented the $1.3 billion Oregon Transportation Investment Act (OTIA III) State Bridge Delivery Program to repair or replace over 300 bridges during 10 years. To meet the delivery schedule, alternative delivery mechanisms were developed including substantial use of design-build project delivery, and programmatic environmental permits conditioned upon the Bridge Program’s Comprehensive Mitigation and Conservation Strategy. One major aspect of the strategy is implementation of Environmental Performance Standards (EPS) for many aspects of the bridge design, including fluvial performance. The fluvial performance standard requires bridge design to “allow normative physical processes within the stream-floodplain corridor.” Evaluating the channel processes requires calculations for bank full width and depth, flood prone width and depth, and entrenchment ratio. The result is a deterministic compliance test for adequate bridge length to allow geomorphologic stream processes. The case studies present three bridge replacement sites in Oregon. The studies describe the field and desktop methods used to collect necessary data for applying the fluvial performance standard. An evaluation is made of the controlling factors for bridge length, distinguishing between traditional “floodway process” considerations and the more recent “channel process” requirements. Conclusions are that the fluvial EPS is a beneficial method of screening bridge sites to minimize impacts on stream function. It is most useful for identifying grossly constrictive bridges where flood management historically has not been a significant issue. Refinement and definition is needed to balance the deterministic approach with the wide variety of site conditions encountered.