PART III: SCOUR PROBLEMS AT COASTAL STRUCTURES 



General 



19. One of the major problems associated with design of effective 

 coastal erosion control on navigational assistance structures is being able to 

 adquately address forces associated with wave attack and associated currents. 

 This continual attack often results in degradation of the base that supports 

 the structure. Numerous cases have been documented where structures have 

 deteriorated and failed due to such a degradation of the foundations by 

 excessive localized erosion of the base, or scour. Generally, scour is 

 defined as the deformation of a flow boundary through removal of materials by 

 a hydraulic flow. For scour to occur in coastal environments, three basic 

 elements must exist. First, there must be an erodible bottom. Second, there 

 must be sufficient energy present to cause the erodible bottom to move and be 

 carried away. Finally, there must be a structure or structural foundation 

 that is built on the erodible bottom. Problems occur when a structure is 

 placed on the seafloor, because existing "equilibrium" conditions are 

 perturbed, and responses such as increased velocities and turbulence may 

 result. Increased velocities and associated turbulence represent increased 

 ability to initiate and sustain sediment particle motion. It is clear that 

 many different conditions can result from a combination of these factors, with 

 each likely to present a unique scour potential. It is clear that unless 

 structures built in scour-prone areas are protected or designed to withstand 

 maximum scour depths, the structure is likely to be undermined and doomed to 

 some degree of failure. Because it would be impractical, if not impossible, 

 to discuss all cases, this summary will be limited to the most commonly 

 occurring coastal scour problems. 



Scour Problems at Rubble -Mound Structures 



20. For additional discussions of problems common to rubble -mound 

 structures, consult Markle (1986 and 1989), Eckert (1983), and the Shore 

 Protection Manual (SPM) (1984). In a survey of problems with rubble-mound 

 structures conducted in 1984-1986, Markle concluded that the majority of 

 failures begin with damage to the toe of these structures (Figure 6) . In 

 general, there are three major problems that occur at most rubble-mound 

 structures experiencing some degree of degradation: 



a. Improper placement and sizing of the toe buttressing stone. 



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