Numerical Simulation 



Input data and model preparation . Input data requirements for shoreline 

 modeling are discussed by Kraus (1989). A straight longshore baseline was 

 drawn running down State Highway 82, and the locations of the breakwaters, 

 revetment, and shoreline positions as determined from aerial photographs and 

 profile surveys were referenced to the baseline. The revetment prevents the 

 beach from retreating landward and was represented as a seawall constraint 

 (Hanson and Kraus 1985, 1986b). Longshore model grid spacing of 4.6 m (15 

 ft) was used to provide approximately 10 calculation points behind each 

 detached breakwater. Measured shoreline positions from the 24 irregularly 

 spaced profile survey lines were transformed to the grid by using a nonlinear 

 interpolation technique. The total model reach was 1,066 m (3,500 ft). A 

 pinned boundary condition (determined from aerial photographs showing loca- 

 tions of minimal movement of shoreline position) was applied on both ends of 

 the model grid to allow sand to be freely transported into and out of the 

 calculation domain (Hanson and Kraus 1989). 



Wave height and period at 1-hr intervals were obtained from a resistance 

 wave gage on an oil platform located 140 km to the south of the study area in 

 a water depth of 12 m. Wave direction was inferred from 1-hr records of wind 

 direction measured on the same platform. These data cover the period of 

 Hurricane Bonnie and were used to provide input to GENESIS to simulate beach 

 change between the profile surveys of 1/23/86 and 7/29/86. The gage wave 

 heights are believed to be an overestimation of the waves that arrived to the 

 site because Bonnie made landfall close to the gage; also, in the modeling, 

 the waves were assumed to propagate without dissipation. Wave heights at the 

 gage were therefore halved and the data extensively censored to eliminate 

 apparent spurious extremes to give a mean wave height of 0.53 m and T = 5 sec 

 at the gage. The offshore bathymetric contours were assumed to be straight 

 and parallel, a reasonable approximation for this coast. 



Preliminary calibration . Calibration refers to adjustment of model 

 parameters to reproduce shoreline change that occurred between two surveys. 

 At the time of writing, model calibration is in progress, so that the result 

 shown should be considered as preliminary. In calibrations performed without 

 wave transmission, only two empirical coefficients are adjusted, which deter - 



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