EXECUTIVE SUMMARY 



St. Marys Inlet is a large jettied tidal inlet through the barrier 

 island system of Georgia and Florida. It is located approximately 30 miles 

 north of Jacksonville, Florida. The inlet is the main ocean entrance to the 

 US Navy Submarine Base at Kings Bay, Georgia. As a part of upgrading the base 

 to accommodate Trident submarines, it became necessary to improve the base 

 facilities and modify the navigation channels to the interior and exterior of 

 the inlet. 



In 1983 the Officer In Charge of Construction (OICC) , Trident, requested 

 the US Army Engineer Waterways Experiment Station (CEWES) to determine, by 

 modeling, the impact of these modifications on hydrodynamics and sedimenta- 

 tion. As a result, two studies were undertaken simultaneously. The first, 

 performed by the Hydraulics Laboratory of CEWES, used a hybrid modeling ap- 

 proach to determine the impact of the interior modifications. A report by 

 Granat, et al. (in preparation) describes the results of the investigation. 

 The second study was a numerical modeling effort by the Coastal Engineering 

 Research Center (CERC) of CEWES to determine the effect of modifications of 

 the exterior channels on coastal processes near the inlet, especially channel 

 shoaling rates. The report herein describes details of this second study. 



To accomplish the objectives of the second study, CERC employed a system 

 of numerical models called Coastal and Inlet Processes (CIP) Numerical Model- 

 ing System. The system included four separate numerical models for tides, 

 waves, wave-induced currents, and noncohesive sediment (sand) transport. The 

 system together with two computational grids — one for existing (base) condi- 

 tions and the other for plan conditions — was called Model B in contrast to 

 Model A, the hybrid model for the interior. 



To substantiate the validity of the modeling approach and to improve 

 accuracy of predicted results. Model B was first verified with available field 

 data on tides and sediment transport. The tidal model was verified by using 

 field data on tidal elevations and currents taken on 10 November 1982. Good 

 verification was obtained by matching model results with observed tidal cur- 

 rents over one tidal cycle. The wave climate for an average year in 60-ft 

 depth mean low water (mlw) at the project site was obtained from the CEWES 

 Wave Information Study based on a 20-year hindcast. This information was dis- 

 cretized into 79 different monochromatic waves. These waves were propagated 



