stone weighing about one-thirtieth the weight of primary cover layer armor 



(W/30) was stable as cover for a core-stone apron in water depths of more 



than one but less than two wave heights. The width of the tested aprons was 



four to six of the aprons' cover layer stones, and so could be calculated 



using equation (7-120) with n = 4 to 6 and W = -rrr . 



Hales (1980) describes jetties, small breakwaters, and revetments with 

 slopes of 1 on 3 or steeper and toes exposed to intense wave action in shallow 

 water that have their aprons protected by a one-stone-thick layer of primary 

 cover layer quarrystone. The aprons were at least three to four cover stones 

 wide; i.e., if equation (7-120) were used, n = 3 to 4 and W = w^j . In 

 Hawaii, the sediment beneath the toes of such structures was excavated down to 

 coral; or, if the sand was too deep, the toe apron was placed in a trench 0.6 

 to 2.0 meters deep. 



(4) Materials . The quarrystone of the structure underlayers, 

 secondary cover layer, toe mound for cover layer stability, or the primary 

 cover layer itself can be extended over a toe apron as protection, the size of 

 which depends on the water depth, toe apron thickness, and wave height. 

 Eckert (1983) recommended that, in the absence of better guidance, the weight 

 of cover for a submerged toe exposed to waves in shallow water be chosen using 

 the curve in Figure 7-120 for a rubble-mound foundation beneath a vertical 

 structure and equation (7-125) as a guide. The design wave height H to be 

 used in equation (7-125) is the maximum expected unbroken wave that occurs at 

 the structure during an extreme event, and the design water depth is the 

 minimum that occurs with the design wave height. Since scour aprons generally 

 are placed on very flat slopes, quarrystone of the size in an upper secondary 

 cover layer w^/2 probably will be the heaviest required unless the apron is 

 exposed above the water surface during wave action. Quarrystone of primary 

 cover layer size may be extended over the toe apron if the stone will be 

 exposed in the troughs of waves, especially breaking waves. The minimum 

 thickness of cover over the toe apron should be two quarrystones, unless 

 primary cover layer stone is used. 



(5) Shallow-Water Structures . The width of the apron for shallow- 

 water structures with reflection coefficients equalling or greater than 0.25 

 can be planned from the structure slope and the expected scour depth. As 

 discussed in Chapter 5, the maximum depth of a scour trough due to wave action 

 below the natural bed is about equal to the maximum expected unbroken wave at 

 the site. To protect the stability of the face, the toe soil must be kept in 

 place beneath a surface defined by an extension of the face surface into the 

 bottom to the maximum depth of scour. This can be accomplished by burying the 

 toe, where construction conditions permit, thereby extending the face into an 

 excavated trench the depth of the expected scour. Where an apron must be 

 placed on the existing bottom or only can be partially buried, its width can 

 be made equal to that of a buried toe; i.e., equal to the product of the 

 expected scour depth and the cotangent of the face slope angle. 



(6) Current Scour . Toe protection against currents may require 

 smaller protective stone, but wider aprons. Stone size can be estimated from 

 Section IV below. The current velocity used for selecting stone size, the 

 scour depth to be expected, and the resulting toe apron width required can be 



7-246 



