DREDGING OPTIMIZATION OF AN INLET SYSTEM FOR ADJACENT SHORE PROTECTION PROJECTS USING CMS AND GENCADE

Abstract

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.