Two-Dimensional Modeling of Sediment Transport and Bed Morphology to Identify Shoaling Reduction Alternatives near Matanzas Inlet in St. Johns County, Florida

This study describes the field measurements and MIKE21 numerical modeling of tides, waves, sediment transport, and bed morphological changes in the Intracoastal Waterway (ICWW) near Matanzas Inlet in St. Johns County, Florida. The model results generally agree well with measured water levels, current speeds, current direction, and observed morphological patterns in the area. The models applications provided the means to (1) understand the transport of sediments and formation of the shoals in the ICCW, (2) identify various alternatives that could reduce ICCW shoaling rates, and (3) evaluate the performace and the associated cost and frequency of maintenance dredging for alternatives that show potential to reduce the ICCW shoaling. Model results show flood slows as it flows south around the northern tip of Rattlesnake Island. This rapid decrease in flow velocity explains the observed sediment deposition (shoaling) in the ICWW. Model results show basins located in the north arm of Matanzas River and north of Fort Matanzas will likely reduce shoaling in the ICWW and reduce maintenance dredging from the current once every three years to once every four years. A dune breach that opened another outlet to the ocean during the period of this study likely had minimal impact on shoaling at the ICWW.