Challenge. Perform hydraulics and scour analyses to ensure bridge designed to withstand storm forces and vessel impacts while meeting aggressive design and construction schedule

Solution. The New York State Thruway Authority is replacing the 3.1 mile-long Tappan Zee Bridge over the Hudson River at a cost of some $3.14 billion. In support of the design contractor, INTERA developed bridge hydraulics and cohesive scour depth estimates for the replacement bridge. We modeled both hurricane storm surge (with SWAN+ADCIRC) and river discharge (with RMA-2) and calculated long-term channel processes scour, contraction scour depths at the site, and local scour at 86 piers. INTERA’s additional responsibilities included evaluating horizontal and vertical wave and surge forces on the replacement bridges’ pile caps, calculating local scour and wave/surge forces on a proposed permanent maintenance platform that will parallel a portion of the replacement bridge. By applying a newly developed cohesive scour methodology that produced scour depths smaller than those produced by traditional sand scour equations, INTERA’s analyses are resulting in significant construction cost savings.