Bridges located within complex coastal systems are highly susceptible to scour-related damage resulting from complex flow patterns and extreme flood conditions. The SR A1A Matanzas River bridges serve as typical examples of bridges affected by multiple tidal inlets. Several physiographic characteristics of the area create complex and potentially extreme flood conditions. These include large marsh expanses, dune overtopping, and proximity to the Matanzas and St. Augustine Inlets. The bridges' proximity to Matanzas Inlet and dune overtopping allow simultaneous propagation of hurricane surges toward the bridges. To guard against scour-related damage, the Federal Highway Administration (FHWA) and the Florida Department of Transportation (FDOT) require a detailed analysis to assess the vulnerability of bridges to flow-induced scour and to develop countermeasures, if necessary. For the complex hydrodynamic system involved with this study area, a two-dimensional RMA2 hydrodynamic model describing the entire interconnected system provided predictions of flow conditions necessary to estimate scour during extreme (50-, 100- and 500-year) flood events at the bridges. FDOT surge hydrographs provided offshore model boundary conditions.

The hydraulic analyses evaluated peak flow conditions at the bridges with and without dune breaching. A comparison of the maximum velocities and maximum stages indicated the breached dune scenario provides greater maximum velocity and maximum stage (and consequently greater scour) at both bridges. The CHAMP wave model provided estimates of wave height and period at the bridges. FDOT and HEC-18 provided guidance in estimation of long-term aggradation and degradation, contraction scour, and local scour. The recommended abutment protection from currents and waves follows FHWA, HEC-18, FDOT Cross Drain Manual, and the FDOT Drainage Manual.