Although engineers realized that streams can degrade, aggrade, and change location within flood plains and that the actual construction of a bridge or highway may initiate additional morphological changes in the behavior of a stream, the existing design procedures for highway structures assumed, for the most part, that streams have fixed boundaries. Although state-of-the-art analyses were available for the mobile boundary stream condition, they were seldom used, and if assessments were accomplished at all, they were based primarily on the designer's judgment and experience. The principal reason for this situation was that available analytical procedures were difficult to use and had not been adapted to highway applications. After a thorough review of existing computer models used in the analyses of mobile boundary streams during a session of the Transportation Research Board's Second Bridge Engineering Conference at Minneapolis in September 1984, the conclusion was reached that none of the existing computer models would be totally suitable to aid either in the design of highway bridges or in determining the effects of longitudinal encroachments. For example, some existing models applied only to long stream reaches; others were not detailed enough for bridge openings and did not predict lateral erosion of streams; and many were not user-friendly.

The goal of this research project was to develop and test a practical computer model that was based on sound physical principles of flow and sediment interaction and was designed to estimate water-surface profiles, aggradation, degradation, scour, and bank widening due to bridges and longitudinal encroachments located on mobile boundary streams. The estimates were to be used to aid in the design of highway crossings or other encroachments of streams.

This research project resulted in the development of BRI-STARS (Bridge Stream Tube Model for Alluvial River Simulation), the final software product for this phase. In adition, a continuation phase was initiated (see Project 15-11A). Documentation for this portion of the research, including a BRI-STARS User's Guide, was provided to NCHRP sponsors.