The Grand Avenue Bridge is a six-span bridge with the center span a “swing” section. The five piers supporting the approach and center spans are within tidal waters. GM2 Associates was contracted to provide hydrologic and hydraulic analysis for the purpose of determining scour conditions in association with the rehabilitation of the bridge. This reach of the Quinnipiac is influenced by the tidal cycle and potential hurricane storm surges of Long Island Sound, adding to the level of analysis required to determine the worst-case scenario for maximum potential scour; unsteady hydraulic analysis was required to address not only the inland riverine flooding, but also the potential surge originating from coastal storms.