One feature of an offshore breakwater is that it tends to "overtrap" 

 material, especially when waves approach in a direction nearly perpendicular 

 to the shoreline. This effect can be demonstrated by noting that even for waves 

 propagating directly toward shore, the effects of diffraction will cause depo- 

 sition in the form of a salient or tombolo behind the breakwater. If this 

 deposited material is removed such that an equilibrium topography does not 

 exists, more material will be transported behind the breakwater, etc. Unfor- 

 tunately, the degree of overtrapping for different wave characteristics is 

 currently unknown. A related question of whether the impounded volumes should 

 be correlated with the net or downdrift component of longshore energy flux is 

 discussed in Section II. 



For more detailed analyses of shoreline changes and historical bypassing 

 operations in this area, see Beach Erosion Board (1953), Savage (1957), and 

 Herron and Harris (1966) . 



4. Past Longshore Sand Transport Relationships and Studies . 



The relationship between immersed weight transport rate of sediment, I, 

 and longshore energy flux factor at breaking, P£s' ^^ generally presented in 

 terms of a dimensionless constant, K (e.g., Vitale, 1980). 



I = ^Zs (1) 



where for a single wave train of breaking height, H^, and breaking direc- 

 tion, a^ relative to a normal to the beach, P£g can be expressed as 



P^g = — g— C sin a^j cos a^, (2) 



in which y is specific weight of water, and C the wave group speed at 

 breaking. A more useful form of equation (1) is obtained by noting that the 

 immersed weight transport rate, I, and bulk longshore sediment transport, 

 Q, are related by 



^ = /Ps \ ^^^ 



p(/- ijgd -P) 



in which p and Pg are the mass densities of water and sediment, respec- 

 tively, g the gravitational constant, and P the inplace porosity of the 

 sediment. Combining equations (1) and (3) 



Q = —77^ ^ Po„ = K*Pj^g (4) 



p(^ - l)8(l - P) 



The above equations for longshore sediment transport do not account for 

 sediment size, beach slope, porosity, etc.; therefore, it is very unlikely 

 that K is a true constant. Tests conducted to date have not provided the 

 ranges of independent variables (e.g., sediment size) or accuracy necessary 

 to distinguish the effects of these variables, although models which account 

 for the effects of these variables have been postulated (Dean, 1973; Walton, 

 1979). 



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