and in front of seawalls, is not discussed in this section. However, 

 scour is sometimes a problem in the stability of coastal structures, 

 and when model studies (conducted in accordance with the scale-model 

 techniques eind transference equations discussed in Section V) or field 

 experience indicate that scour due to wave action is likely to endanger 

 the stability of these structures, the area of scour may be covered with 

 a riprap blanket. The stability of the blankets can be determined by 

 model studies of the type used to test the stability of rubble break- 

 waters, jetties, and wave absorbers. 



2. Similitude Relations . 



a. Rubble-Mound Structures . When a rubble-mound breakwater, jetty, 

 or wave absorber with a given geometry (crest width, crest height above 

 or below the Stillwater level, number of layers and size of stones in the 

 underlayer system, height of the impervious core material above or below 

 the Stillwater level, and the depth below Stillwater level to which the 

 armor units in the protective cover layer extend) are subjected to the 

 attack of gravity water waves, the stability of the armor units is a 

 function of the following variables: 



d = depth of water in which the structure is situated, 

 measured at the seaside toe 



D = percent damage to the cover layer caused by waves of 

 a given height, as measured by the number of armor 

 units displaced compared with the total number of 

 armor units in the cover layer 



g = acceleration due to gravity 



H = height of waves that attack the structure 



(Jie) = characteristic linear dimension of the armor units 



V^^ = velocity of water that impinges on and flows around 

 individual armor units in the area where damage to 

 the cover layer occurs 



a = angle of seaside slope, measured from the horizontal 



3 = angle of incidence of wave attack 



A = shape of armor units 



9 = angle of bottom slope seaward of the structure, 

 measured from the horizontal 



\ = wavelength 



y = dynamic viscosity of water in which the structure is 

 situated 



316 



