Design of the lee side cover layer is based on the extent of wave 

 overtopping, waves and surges acting directly on the lee slope, porosity 

 of the structure, and differential hydrostatic head resulting in uplift 

 forces which tend to dislodge the back slope armor units. 



If the crest elevation is established to prevent possible overtop- 

 ping, theoretically, the weight of armor units on the back slope cover 

 layer should depend on the lesser wave action on the lee side and the 

 porosity of the structure. When overtopping is anticipated, primary 

 armor units should be extended down the back slope to minimum SWL. When 

 both side slopes receive similar wave action (as with groins or jetties), 

 both sides should be of similar design. 



Lording and Scott [1971) tested an overtopped rubble-mound structure 

 that was subjected to breaking waves in water levels up to the crest ele- 

 vation. Maximum damage to the lee side armor units occurred with the 

 Stillwater level slightly below the crest and with waves breaking as 

 close as two breaker heights from the toe of the structure. This would 

 imply that waves were breaking over the structure and directly on the 

 lee slope rather than on the seaward slope. 



f. Secondary Cover Layer . The weight of armor units in the 

 secondary cover layer, between -H and -1.5H, should be greater than 

 about one-half the weight of armor units in the primary cover layer. 

 Below -1.5H, the weight requirements can be reduced to about W/15 for 

 the same slope condition. (See Figure 7-95.) When the structure is 

 located in shallow water (Fig. 7-96), that is depth d <_ 1.3H, armor 

 units in the primary cover layer should be extended down the entire 

 slope. The above ratios between the weights of armor units in the pri- 

 mary and secondary cover layers are applicable only when quarrystone 

 units are used in the entire cover layer for the same slope. When pre- 

 cast concrete lonits are used in the primary cover layer, the weight of 

 quarrystone in the other layers should be based on the equivalent weight, 

 W of quarrystone armor units. 



The secondary cover layer (Figs. 7-95 and 7-96) from -H to the 

 bottom should be as thick or thicker than the primary cover layer. Thus, 

 based on the preceding ratios between the armor weight, W in the pri- 

 mary cover layer and the quarrystone weight in the secondary cover layer, 

 if n = 2 for the primary cover layer (two-quarrystones thick), then 

 n = 2.5 for the secondary cover layer from -H to -1.5H, and n = 5 

 for that part of the secondary cover layer below -1.5H. 



g. Underlayers . The first underlayer (directly beneath the pri- 

 mary armor units) should have a minimum thickness of two quarrystones 



(n = 2), and these should weigh about one-tenth the weight of the over- 

 lying armor units (W/10). (See Figure 7-95.) This applies where (a) 

 cover layer and first underlayer are both quarrystone, (b) first under- 

 layer is quarrystone and the cover layer is concrete armor units with a 

 stability coefficient K„ < 12. When the cover layer is of armor units 

 with K£) <_ 20, the first underlayer quarrystone should weigh about W/5 



7-199 



