Application of the Pelnard-Considere (1956) solution to the design of the 

 updrift beach at a weir jetty requires that the wave climate at the site be 

 described by a single wave. It is assumed that the shoreline can be char- 

 acterized as the beach planform that results from that wave. Also, since 

 refraction is not considered, the final shoreline when t + °» is straight; 

 therefore, the shoreline at some arbitrary time must be chosen to describe the 

 beach's response to jetty construction. At best, Pelnard-Considere allows the 

 use of nearby shoreline geometry to be extrapolated or transposed to predict 

 the shoreline response near a jetty. 



The most promising method of predicting the behavior of the updrift 

 shoreline is to use a numerical model to describe the sediment balance for 

 numerous small lengths of shoreline. A number of such models have been 

 developed (Perlin, 1978; LeMehaute and Soldate, 1980) with varying levels of 

 success. The advantage of a numerical model is that the actual wave condi- 

 tions at the site can be simulated and used in the model to generate the 

 postproject shoreline. The difficulty in using these models is calibrating 

 them for a site. The simple models do not describe the complex nearshore 

 processes that determine the two-dimensional bathymetry near structures since 

 only the one-dimensional empirical relationship is usually used between 

 longshore sand transport and longshore wave energy flux. 



Numerical models, with further development, show promise for predicting 

 shoreline changes caused by jetty construction. Presently, these models are 

 qualitative; however, if calibrated using nearby structures and shoreline 

 changes, the models might be used to transpose results from adjacent sites to 

 a weir-jetty site. 



XI. APPLICATION OF HYDRAULIC MODELS TO THE DESIGN OF WEIR-JETTY SYSTEMS 



Construction of a weir-jetty system involves a sizable investment of both 

 time and money. For example, the weir-jetty system at Murrells Inlet was 

 built at a cost of $11.4 million over a period of 3 years. Project construc- 

 tion was initiated in October 1977 and completed in August 1980. Because of 

 the high cost of such projects, any design deficiencies will usually be 

 expensive to correct and small design improvements may result in significant 

 savings; therefore, the designer should use every method at his disposal to 

 ensure an adequate, optimum design which satisfies both functional and 

 structural requirements. A hydraulic model testing program for a proposed 

 weir-jetty system should be the rule rather than the exception whenever a 

 relatively large jetty project is conceived. Model tests can investigate the 

 hydraulics of inlet modification, changes in sediment transport conditions 

 caused by inlet modification, and the structural stability of proposed jetty 

 cross sections. Although the results obtained from model tests are not always 

 in total conformance with final behavior of the prototype, they can provide 

 quantitative information on project performance. At the very least, a quali- 

 tative understanding of the effect of the project can be obtained. The level 

 of confidence in model test results depends on the type of model and its 

 purpose. A detailed presentation on coastal hydraulic models is given by 

 Hudson, et al. (1979). 



1. Hydraulics of Inlet Changes. 



A number of factors in the design of a weir-jetty system can be considered 

 in a three-dimensional, fixed-bed model of the inlet and proposed improve- 

 ments. These include changes in hydraulic characteristics of the inlet-bay 



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