breakwaters have already been mentioned. This area of transition from a 

 hopefully stable static environment (the breakwater) to the natural, often 

 dynamic, sea floor is critical to the overall stability of the structure. Toe 

 features not only protect the bottom from scouring (which can lead to under- 

 mining) but also support the weight of the armor material above. The need to 

 provide primary armor 1.5 to 2.0 wave heights below the still-water level can 

 conflict with the need to filter foundation sediments at the tow. This is 

 particularly true in high tidal ranges where low water conditions can expose 

 the toe to more extreme wave effects. The support of armor materials is most 

 reliably accomplished with a substantial berm of secondary armor or underlayer 

 material at the toe of the armor slope. This berm should have at least sev- 

 eral units or a minimum 3-ni top width. Wide differences in the size of the 

 bottom sediments and the breakwater material near the bottom may require 

 excavation of a trench along the toe to accommodate a toe berm with an ade- 

 quate filtering underlayer, as illustrated in Figure 7. Geotextiles can be 

 used also in some instances to reduce the height of toe features and the as- 

 sociated exposure to more severe wave energy. The concurrent physical mod- 

 eling of armor stability and toe scour is complicated by scale effects, but 

 model tests can reveal trends which could suggest a compromise of either the 

 filtering criteria or the extent of primary armor. One radical concept in toe 

 design is the "wave reducing berm" (Delft Hydraulics Laboratory 1983) which 

 provides artifically shallow depths for dissipation of wave energy. Sugges- 

 tions for design of more conventional toe features are discussed by Eckert 

 (1983) and Jensen (1984). 

 Construction equipment and techniques 



41. The constructibility of a rubble-mound breakwater design is an 

 extremely important and practical consideration that can control its over- 

 all feasibility. Smaller breakwaters can often be constructed with con- 

 ventional land-based construction equipment and techniques by building from 

 the shore outward. Detached breakwaters can be constructed in this fashion 

 only if a temporary causeway to the permanent portion is constructed and later 

 removed. Larger or more exposed breakwaters often include features which make 

 construction exclusively with land-based equipment difficult. For example, 

 placement of large armor units in relatively deep water near the toe of a 

 shallow slope (perhaps at the head) may be too far to reach for a mobile crane 

 on the breakwater crest. Another example is the occasional need to build up 



26 



