38 



Review of Empirical Methods 



A desktop study using empirical relationships is recommended as the first 

 step in the design of a detached breakwater system. Empirical relationships 

 are somewhat limited due to their inherent simplicity; however, they can be 

 used as reasonable methods prior to detailed studies to quickly assess 

 prototype response and/or project costs for several design alternatives and as a 

 means of assessing model results. 



Numerous laboratory, numerical, and prototype studies have focussed on 

 detached breakwaters with the objective of developing and improving 

 functional design guidance. As a result, a number of empirical relationships 

 for the design and prediction of beach response to single or segmented 

 detached breakwater systems have been developed. Most investigations 

 present information on when tombolos will form and when minimal beach 

 response can be expected. Table 3 presents a summary of studies whose 

 empirical methods have been used to design both U.S. and foreign detached 

 breakwater projects. Detailed information on various empirical relationships 

 is presented in Rosati (1990) and summarized in CETN 111-43 (Coastal 

 Engineering Research Center 1984) and EM 1110-2-1617. It is recommended 

 that Rosati (1990) and/or the original reference be reviewed prior to using any 

 of these empirical methods for prototype design. 



EM 1110-2-1617 presents conditions for the three types of beach response 

 as predicted by the various relationships described in Table 3. This 

 information is summarized here, and is presented in terms of a dimensionless 

 breakwater length L s /y, where L s is the breakwater segment length, and v is 

 the distance from the average shoreline. Tables 4, 5, and 6 present conditions 

 for tombolo development, salient development, and limited response, 

 respectively. 



Evaluation of empirical methods. Rosati (1990) conducted an evaluation of 

 empirical design methods that consisted of compiling data from five U.S. 

 breakwater projects, and comparing the prototype response with empirical 

 predictions where possible. These projects encompass a range of structural 

 and site parameters and beach response, from salient formation (Lakeview 

 Park, Lorain, Ohio, and Redington Shores, Florida), to no sinuosity 

 (Lakeshore Park, Ohio), to periodic tombolo formation (Colonial Beach, 

 Central and Castlewood Park Sections, Virginia). 



The majority of relationships are of the type that predict a limited, salient, 

 or tombolo response as a function of structural parameters. Rosati (1990) 

 conducted an evaluation of these relationships as presented in Figure 24. 



In general, the simplicity of the empirical methods evaluated and prototype 

 data limitations resulted in widely varying predictions for most design 

 relationships. However, several of the evaluated relationships proved to 

 predict prototype response, although careful consideration must be given to 



Chapter 2 Functional Design Guidance 



