Hanson and Kraus (1990) extended these sensitivity tests by applying 

 GENESIS in a generalized manner for the case of a single detached 

 breakwater. They developed a nomograph predicting morphologic response as 

 a function of several dimensionless design parameters (deepwater wave height 

 over depth at structure, breakwater length over wave length at the structure, 

 and structure transmission), which compared favorably with six prototype 

 detached breakwater projects. Rosati, Gravens, and Chasten (1992) continued 

 this work to develop nomographs for single and segmented detached break- 

 waters. However, since these studies were conducted, a limitation within 

 GENESIS was identified which indicated that the nomographs may tend to 

 overpredict tombolo formation 1 . The nomographs presented by Hanson and 

 Kraus (1990) and Rosati, Gravens, and Chasten (1992) may be useful in 

 indicating dependencies on controlling dimensionless parameters. However, 

 they are not recommended for application to project design in their present 

 form. 



Site-specific examples. Application of GENESIS to two detached 

 breakwater projects is summarized from existing literature. Discussion of 

 these studies herein is directed towards providing the engineer steps involved 

 in numerical modeling of detached breakwater systems. For details about 

 each application, the referenced publications should be consulted. In addition, 

 Appendix B discusses the application of GENESIS at the Bay Ridge, 

 Maryland, detached breakwater project. 



(1) Holly Beach, Louisiana. Hanson, Kraus, and Nakashima (1989) 

 demonstrated use of the breakwater transmission capabilities of GENESIS 

 through preliminary calibration results with the Holly Beach, Louisiana, 

 detached breakwater project. The project consists of six detached breakwater 

 segments, each with a different cross-sectional design. The structures are 

 constructed of various quantities and arrangements of timber piles, used tires, 

 and riprap, which result in varying degrees of wave transmission. 



The first step in the modeling project was to gather and evaluate all 

 relevant data sets and previous studies. Ten grid cells are recommended 

 behind each detached breakwater, thereby requiring a cell spacing of 4.6 m. 

 From available shoreline change data, it was determined that there were 

 locations of minimal movement outside the project area. Therefore, the 

 "pinned beach" boundary condition (see Hanson and Kraus (1989b); Gravens, 

 Kraus, and Hanson (1991) for details) was applied at the ends of the project 

 reach, to allow sand transport in and out of the calculation domain. Based on 

 field dye studies of structure permeability, the structure transmission 

 coefficients were qualitatively known to generally decrease from east to west. 

 The western-most segment was riprap, and showed little wave transmission, 

 whereas the eastern-most segment consisted of tires mounted on one row of 

 timber piles, and had the greatest observed dye transmission (Nakashima et al. 



58 



1 Personal Communication, 1992, Mr. Mark B. Gravens, U.S. Army Engineer Waterways 

 Experiment Station, Coastal Engineering Research Center, Vicksburg, MS. 



Chapter 3 Tools for Prediction of Morphologic Response 



