fluid motion with a given set of rather complicated boundary conditions. 

 In many instances the models help the designer to better understand the 

 nature of the problem to be solved. Model studies are usually expensive 

 compared to the costs of conducting theoretical investigations and should 

 not be used when purely analytical methods will give reliable solutions. 

 However, the cost of scale-model studies is often low compared with the 

 cost of the prototype structure, including the consequences of structure 

 damage or failure that may occur as the result of underestimating the 

 wave forces imposed on the structure. Thus, because the justification 

 of many model studies is mostly an economic one, it is usually easier to 

 justify a model study if the project is large and important. Conversely, 

 smaller projects may not afford the cost of a scale model, although the 

 project may concern extremely complex phenomena that defy analysis by 

 theoretical means alone. A disadvantage of some model studies is the 

 cost and time necessary to conduct the tests. However, the importance 

 of conducting model studies of coastal structures cannot be overstated. 

 In many instances the studies are more than just a good investment; they 

 can prevent disastrous errors in prejudging the performance of the proto- 

 type structure, and they usually result in considerable savings in design 

 and construction. 



This section has emphasized the importance of model studies in deter- 

 mining the forces imposed on coastal structures so that they can be de- 

 signed to successfully withstand those forces. Model studies can also be 

 performed so that the optimum construction methods and schedules can be 

 selected. The results of stability tests on scale models can be used, in 

 conjunction with applicable economic factors, such as the economic life 

 of the structure, frequency of occurrence of the selected design wave, 

 initial cost of construction, subsequent repair costs, and the interest 

 rate, to obtain the optimum design for the structure. The results of 

 such a con5)rehensive study can be negative, indicating that construction 

 of the proposed structure would not accomplish the intended purpose, un- 

 less the structure was enlarged or strengthened to increase construction 

 costs appreciably. 



The scope of usefulness of scale models as discussed in this section 

 is very broad, and the advantages are considerable both from the technical 

 and economical viewpoints. It should be emphasized that although most 

 stability models can be designed, constructed, and operated with the re- 

 sults interpreted to provide sufficiently accurate results for the purpose 

 of the investigation, the laboratory engineer conducting the investiga- 

 tions must have considerable experience, familiarity with basic wave 

 mechanics, and engineering judgment. 



c. Examples of Model Studies Conducted . Many model studies have 

 been conducted in laboratories worldwide on the stability, wave trans- 

 mission, and overtopping characterisitics of coastal structures (rubble- 

 mound, vertical -wall, composite, and floating types). A smaller number 

 of full-scale model studies, theoretical investigations, and flume ex- 

 periments have been conducted to determine the horsepower required per 



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