development, concrete unit forming, and difficulties in handling. A reduced 

 slope (increased cot 6 in Equation 1) will reduce the armor weight require- 

 ment but will change the runup characteristics of the seaward face in a non- 

 linear manner. Overtopping and the associated transmitted wave characteris- 

 tics are then affected, which in turn affects the required crest elevation for 

 acceptable wave attenuation. The overall volume of a roughly trapezoidal- 

 shaped breakwater (in cross section) increases as the square of the increase 

 in the crest elevation. A significant effort is therefore necessary to deter- 

 mine the most economical combination of slope, armor type, armor weight, and 

 crest elevation for every pair of functional and structural design criteria, 

 even when first cost is the only consideration. 

 Other breakwater features 



36. The constraints involved in primary armor layer design can 

 sometimes overshadow other considerations for design of the secondary armor 

 layers, underlayers, core, foundation filters, and scour protection (Fig- 

 ure 2). The terminology of the SPM (1984) refers to a secondary armor layer 

 as material placed on the face of the breakwater below the primary armor 

 layer. An underlayer is placed between the armor on the exposed face and the 

 core in the interior of the structure. Underlayers serve basically three 

 functions: to keep the core in place through filtering action, to further 

 dissipate wave energy that has penetrated through the primary armor, and to 

 act as a foundation for the primary armor. These functions also apply to 

 underlayers between the primary armor and the natural foundation (sea floor). 

 Multiple underlayers may be required to satisfactorily accomplish all these 

 functions. Material with small enough voids to hold finer core material in 

 place may be too fine to stay in place itself under the larger voids in the 

 primary armor layer. The primary armor also needs a relatively rough surface 

 under it to discourage sliding. A coarser underlayer also provides some pro- 

 tection from waves during placement of the primary armor (Hedges 1984). 



37. Filtering criteria developed for water quality or seepage control 

 purposes, such as D^c (filter) < 5Dgc (foundation) (Sowers and Sowers 1970), 

 can be restrictive, thus the size gradation should be a consideration in evalua- 

 tion of borrow areas for core material. Efficient use of quarry materials is 

 encouraged in the SPM (1984). Given the practical problems of accurate place- 

 ment of complex underlayers in the field (especially underwater), this goal 

 may not always prove as economical as relaxing gradations of the various 



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