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St. Johns County encompasses a 24-km beach and inlet system located in northeast Florida (United States) that includes several interconnected Federal Navigation and Shore Protection Projects that must be managed within a regional context to reduce cost, share sediment, and optimize the volume of sand within the littoral system. The objective of this study is to investigate optimal dredging volumes and intervals, and to determine the beach placement volume and renourishment interval to maintain two Shore Protection Projects. The Coastal Modeling System (CMS) was applied to analyze the morphological impact on the sediment dynamics for ebb-tidal delta mining at St. Augustine Inlet over 1.4-year simulations. Results determined that dredging scenarios under 4 Mill cu yd removed did not significantly modify the ebb-tidal delta. The CMS modeling results on sustainable dredging volumes, combined with historical infilling rates, provided constraints with which to develop dredging and beach fill scenarios for GenCade, a 1-D numerical model that predicts shoreline change. GenCade was applied to evaluate sediment management alternatives for dredging intervals of 5, 7 and 10 years, and varying beach fill volumes and placement lengths. Results indicate that imposing a 10-year dredging interval to the navigation project and ebb delta with the maximum dredging quantity of 3 million cubic yards will yield the best performance of the regional projects of St. Johns County. GenCade calculates future sediment budgets for various management scenarios, and can provide an essential benefit in determining optimal dredging periods for coordinated regional efforts to save in mobilization and demobilization costs for dredging and beach fill placement.

The region of influence of the inlet on the adjacent shoreline was determined via examination of the inlet's net sink effect. The net sink effect, or volumetric impact, was computed by adding the volume (or rate) of net sand accumulation within the inlet's channels and shoals with the cumulative volumetric losses on adjacent shorelines to conserve sediment mass after accounting for the volumes either added to adjacent beaches or removed from the ebb shoal by means of nourishment and sediment mining. Volume change of the beaches and ebb shoal complex was computed within a geospatial framework consisting of Regional Mapping and Analysis Program (RMAP), ArcGIS and the Surface-water Modeling System (SMS). Inlet-adjacent cumulative volume changes were then examined to discern the minimum distance away from the inlet along which this volumetric impact was manifest. The alongshore influence of the inlet as determined by the inlet sink method for the 1999-2010 time period was found to be 7.4 miles to the north and 5.5 miles to the south. The inlet sink effect for St. Augustine Inlet is 278,000 cu yd/year, balanced by 99,000 cu yd/yr of erosion from the north beaches and 179,000 cu yd/yr of erosion from the south beaches. If managed properly, the inlet could serve as a valuable, long-term resource for the beaches of St. Johns County within the bounds of its sink effect.

The dynamics of a dual inlet system in west-central Florida are investigated via field measurements and numerical modeling. This paper compares two commonly used numerical modeling systems, CMS and DELFT3D, in simulating the hydrodynamic and sediment transport processes. The model results are compared with various field data. Quantitatively as compared to point measurements, both models reproduced the measured nearshore wave accurately with a Willmott (1981) skill of 0.970 for CMS and 0.981for DELFT3D. Both models computed the flow through main inlet channel reasonably accurately although under-predicted the measured values. At the dominating John's Pass, the Willmott (1981) skill was 0.957 for CMS and 0.949 for DELFT3D. At the secondary Blind Pass, the Willmott skill was 0.989 for CMS and 0.938 for DELFT3D. Qualitatively, as compared to flow field measurements using a shipmounted ADCP, both models captured key flow patterns including the ebb jet and alongshore flood flow. The models also yielded reasonable longshore current and interaction between longshore current and tidal flow. These flow patterns play significant roles in the morphodynamics of the ebb shoal and adjacent beaches.