MORPHOLOGICAL ADJUSTMENT OF NEARSHORE PLACED DREDGED MATERIAL, ST. JOHN’S COUNTY, FLORIDA

In Florida, dredged sediments are often placed in the nearshore in water depths ranging from -3 to -45 feet. The desired outcome is for added sediments to have a beneficial effect on the littoral system. Many small inlets in Florida are dredged using the US Army Corps of Engineers (USACE) hoppers Murden and Currituck. These vessels dredge up to 500 and 300 cubic yards (CY) per load respectively, and can place material as shallow as 5-feet of water. Stakeholders commonly question if positive effects on the littoral system from nearshore placement are documented. Questions remain about morphological change following nearshore placements at small inlets on the east coast of Florida where strong littoral currents persist.

USACE Wilmington District's dredge Murden placed approximately 150,000 CY of sediment from St. Augustine Inlet and the Intracoastal Waterway in the nearshore along the St. John's County Shore Protection Project in Florida over a period of 45 days. USACE personnel and local stakeholders created a placement plan that concentrated sediment in two 1,000-ft. berms with different placement designs adjacent highly eroded sections of shoreline. Multi-beam and cross-shore surveys of the placement area were taken prior to construction.

During construction, researchers from the U.S. Army Engineer Research and Development Center (ERDC) deployed two camera arrays recording changes to the planform beach. Monitoring cameras collected time lapse video and single photos over a 180 degree swath during and post placement. The camera images were geo-rectified and analyzed for planform beach changes attributed to the nearshore placement. Multiple post-construction multi-beam and cross shore surveys were taken. Researchers applied the nearshore berm Sediment Mobility Tool developed by ERDC that uses linear and nonlinear wave theories to predict frequency of sediment mobility, onshore/offshore migration, and wave dominant axis of sediment transport in the nearshore. The project was numerically modeled with the Coastal Modeling System (CMS) for additional prediction, verification and visualization of sediment transport and morphodynamic behavior.

Researchers will be determining if nearshore placement resulted in positive littoral changes, added dry berm width and increased shore protection.