that, in the absence of calibration and verification data with which the 

 transmission parameter can be determined, the transmission coefficient should 

 be calculated using the largest waves within the period of record to more 

 accurately model shoreline change. Relationships for estimating rubble-mound 

 structure transmission are presented in Chapter 4. 



(2) Cell spacing. Hanson and Kraus (1989b) and the individual 

 simulations discussed herein recommend using from 8 to 10 longshore cells 

 per breakwater segment to ensure proper resolution of shoreline change to the 

 lee of the structure. 



(3) Median grain size. As discussed previously, GENESIS uses median 

 grain size to determine the steepness of the equilibrium profile shape. Ideally, 

 a typical project profile should be used to back-calculate an effective grain 

 size that produces a similar profile shape (see equilibrium profile template 

 provided by Hanson and Kraus (1989b)). In the absence of bathymetric data, 

 a representative median grain size in the surf zone should be specified. The 

 user should check to ensure that the structure depth specified in the GENESIS 

 input file approximates the profile depth corresponding to the desired distance 

 offshore. 



(4) Wave climate. Gravens and Scott (1993) compared different hindcast 

 wave climates to measured wave gauge data for a site in Florida, and 

 evaluated the data set that best reproduced longshore sand transport rates. 

 They recommended that, if available, a two-component wave climate be used 

 in numerical modeling of longshore sediment transport. A two-component 

 wave train allows wave input from two wave sources, and more accurately 

 represents what occurs in nature. 



Physical Models 



The final design of coastal structures such as a detached breakwater system 

 is often evaluated using physical hydraulic models. These models can predict 

 the breakwater's performance in the actual (prototype) coastal location and 

 determine desired or necessary modifications to improve breakwater 

 performance. Physical model results can also be used to validate results from 

 the previously described numerical simulations. Physical model geometric 

 scales for coastal applications typically range from 1:20 to 1:500, and in 

 some cases near full-scale modeling or tracer studies are used to reproduce 

 sediment movement observed at the actual site location. 



Physical models exactly reproduce prototype conditions when geometric, 

 kinematic and dynamic similarity are attained. However, complete similarity 

 is seldom possible. Therefore, the more critical physical conditions (i.e., 

 gravity waves, water currents, friction, surface tension, sediment motion, etc.) 



Chapter 3 Tools for Prediction of Morphologic Response 



63 



