Tidal Bridge Optimization Studies for MassDOT

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Applied Coastal conducted a series of bridge optimizations for Massachusetts Department of Transportation (MassDOT). The bridges crossed over tidal inlets and waterways and as part of the bridge redesigns were hydraulically optimized to ensure that the bridges and the associated structures did not adversely impact the estuarine systems tidal hydrodynamics.

The William H. Dalton Memorial Bridge which carries traffic over Cohasset Narrows between Bourne and Wareham was redesigned from a 5-span bridge to a 3‑span bridge. The optimization study involved collecting water surface elevation, current, and bathymetry data for calibrating and verification of the two-dimensional hydrodynamic model of Buttermilk Bay that was used to examine the bridge crossing. The goal of the hydrodynamic study was to evaluate how the bridge design influenced tidal attenuation, tidal hydrodynamics, and tidal flushing within the Buttermilk Bay.  The optimized waterway configuration reduced the number of piers within the channel, positioned the piers along the periphery of the main channel, and moved the bridge abutments further landward meeting the redesign goal of improving upon the tidal circulation within the estuary system.

To support the redesign of the John Greenleaf Whittier I-95 Bridge over the Merrimack River between Newburyport and Amesbury, Massachusetts a hydrodynamic model was developed for the lower reaches of the tidally influenced Merrimack River.  The hydrodynamic model was used to assess alterations to the local flow patterns associated with the revised bridge design which incorporated two parallel bridges, one for northbound traffic and one for southbound traffic over the river. The study evaluated how the bridge redesign would influence the riverine and tidal hydrodynamics along with evaluating the potential scour impacts associated with the foundation design for the updated bridge design. The optimized bridge configuration within the waterway reduced the number of piers within the channel along the cross-sectional axis (cross-river), while adding a second set of piers in-line along the flow channel. The piers were designed to be positioned along the periphery of the main and secondary navigation channels, and the bridge abutments were moved to a location on the river bank above the 500-year flood elevation of the active river channel. All the data collection, modeling and design work was completed as part of the Accelerated Bridge Program.

A tidal study was also completed for the three bridges crossing over Swan Pond River tidal estuary located along the south shore of Cape Cod in Dennis, MA. The objective of the study was to determine the degree to which the existing waterway openings for Lower County Road, Route 28, and Upper County Road restricted or impeded the propagation of the tidal signal from Nantucket Sound to Swan Pond and then recommend design solutions for the redesign of the bridges so that design of the replacement structures minimized impacts upon the tidal system.