and 7-96. A logic diagram for the preliminary design of a rubble struc- 

 ture is shown in Figure 7-97. The design can be considered in three 

 phases: (1) structure geometry; (2) evaluation of construction tech- 

 nique; and (3) evaluation of design materials. A logic diagram for evalu- 

 ation of the preliminary design is shown in Figure 7-98. 



As part of the design analysis indicated in the logic diagram (Fig. 

 7-97), the following structure geometry should be investigated: 



(a) crest elevation and width, 



(b) concrete cap for rubble-mound structures, 



(c) thickness of armor layer and underlayers, and 



number of armor units, 



(d) bottom elevation of primary cover layer, 



(e) structure head and lee side cover layer, 



(f) secondary cover layer, 



(g) underlayers, and 



(h) bedding layer or filter blanket layer. 



a. Crest Elevation and Width . Overtopping of a rubble structure 

 such as a breakwater or jetty usually can be tolerated only if it does 

 not cause damaging waves behind the structure. Whether overtopping will 

 occur depends on the height of the crest of the structure relative to 

 the height of wave runup, R. Wave runup depends on wave characteristics, 

 structure slope, porosity, and roughness of the cover layer. If the 

 armor layer is chinked, or in other ways made smoother or less permeable 

 — as a graded riprap slope--the limit of maximum runup will be higher 

 than for rubble slopes. (See Section 7.21, and Figures 7-19 and 7-20.) 

 The selected crest elevation should be the lowest that provides the pro- 

 tection required. Excessive overtopping of a breakwater or jetty can 

 cause choppiness of the water surface behind the structure, and be detri- 

 mental to harbor operations. Operations such as mooring of small craft 

 and some types of commercial unloading require calm waters. Overtopping 

 of a rubble seawall or revetment can cause serious erosion behind the 

 structure and flooding of the backshore area. Overtopping of jetties 

 may be tolerated if it does not adversely affect the channel. 



7-192 



