stone at the toe to fill the anticipated scour trough. The toe of a vertical 

 structure may be protected similarly against scour by the use of stone. 

 Impermeable cutoff walls at the base must be used to protect a gravity wall 

 from undermining by scour. As a general guide, the maximum depth of a scour 

 trough below the natural bed is about equal to the height of the maximum 

 unbroken wave that can be supported by the original depth of water at the toe 

 of the structure. For example, if the depth of water seaward of the face of 

 the structure is 3.0 meters (10 feet), the offshore bottom slope is 1 vertical 

 on 30 horizontal, and a design wave period of 8 seconds is assumed, the 

 maximum unbroken wave height that can be supported is 3.2 meters (10.4 feet) 

 (see Ch. 7,). Therefore, the maximum depth of scour at the toe of the 

 structure would be about 3.2 meters below the original bottom or 6.2 meters 

 (20.4 feet) below the design water level. Placement of a rock blanket with 

 adequate bedding material seaward from the toe of the structure will prevent 

 erosion at the toe and will result in a more stable structure (see Ch. 7 for 

 design methods). 



For long-term effects, it is preferable to assume that the structure would 

 have no effect on reducing the erosion of the beach seaward of the wall. This 

 erosion would continue as if the wall were not there. Since the determination 

 of scour can only be approximate, general guides are usually adopted. 



Consider the beach shown in Figure 5-2 where the solid line represents an 

 average existing profile. It is desired to place a structure at point A in 

 the figure. From prior records, either the loss of beach width per year or 

 the annual volume loss of material over the beach area, which includes the 

 profile, is known. In the latter case, the annual volume loss may be con- 

 verted to an annual loss of beach width by the general rule: loss of 8 cubic 

 meters of beach material is equivalent to loss of 1 square meter of beach area 

 on the berm ( loss of 1 cubic yard of beach material is equivalent to loss of 1 

 square foot of beach area on the berm) . '^his rule is applicable primarily at 

 the ocean front. In shallow, protected bays, the ratio of volume to area is 

 usually much less. 



o 

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8 



6 



4 



2 







-2 



-4 



-6 



-8 



-10 



-12 



-14 



-16 





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Figure 5-2. Effects of erosion. 

 5-5 



