38?. The principal latural force to be resisted by s cietached 

 breakwater ifi wave attack. This is of primary ijaportarice in determining 

 the type of- structure. Heavy viave action would require one of the more 

 massive types of structure, vjhereas sheet piling might suffice where 

 wave attack is light. In some few instances involving submerged break- 

 waters to retain a sand bea^h, the earth pressures also must be taken 

 into cons idv: ration. 



388. The type of foundation may be the governing f?!ctor in the 

 selection of type. For instance a rock bottom would not permit me of 

 one of the sheet pile types. Also, with a readily erodible bottom, some 

 fl_exible type of structure such as a random stone mound would probably 

 prove desirable. Timber crib structures are moderately flexible to 

 uneven settlettient. Obviously, undermining of the structufe by scour of 

 an erodible foundation would be most detrimental to a caisson or masonry 

 wal]_ type breakwater. 



389. 'vail ability of mpterials may also dictate the type of 

 structure by its effect on cost. Lack of stone within economic hauling 

 distance may make mandatory use of some other material. In many in- 

 stances steel sheet pile caniiot be obtained due to shortages, htnce 

 concrete or timber must be used. 



390. The desired life of the structure also dictates its type. 

 Cbvicusly, an untreated timber structure could not be installed on a 

 sea coast where a structure is desired to last 50 years. Conversely, 

 a permanent stone structure would not be constructed if the need for 

 protection was of a temporary nature, 



391. Finally, the choice of structure would depend on either its 

 first cost or its annual cost. All of the factors discussed affect the 

 cost in one way or another. Generally the structure selected would be 

 that one which would accomplish the desired purpose at the lowest 

 annual cost. Under some circumstances a structure with a somewhat 

 higher annual cost might be selected to secure a substantial reduction 

 in the first cost. 



TYPE CF PRCBLQiS 



392, Caisson Ty pe.-:i- - By application 01 laethods for forecasting 

 and determining the characteristics of the design wave, described in 

 the section on -ave action, the following data have been found: 



H ■= 13 feet = design wave height at the structure. 

 L = 1^0 feet » ivave length at the structure . 



190 



-"-From Corps of Engineers, Engineering Hanual for Civil a.orks Construction, 

 "Design of Miscellaneous Structures," "Brealcwaters and Jetties", Part 

 -CXXIX, Chapter U, June 1952. 



