To overcome these problems, i.e., to remove the rapid fluctuations 

 without significantly altering the longer term trends, a nine-point 

 running filter was applied to the results of the energy flux computa- 

 tions. The running filter averages the values from nine points (in this 

 case, nine 250-meter points are equivalent to averaging over a 

 2-kilometer stretch of beach) and assigns that average to the middle 

 point. The filter is then moved to the next (middle) point and averages 

 its value with the four values on either side, etc. 



Figure 37 shows the filtered results of the northerly and southerly 

 components of the annual longshore energy flux; Figure 38 combines both 

 components and shows the net annual longshore energy flux acting along 

 the study area. 



4. Longshore Sediment Transport Model. 



The accepted practice for computing the longshore sediment transport 

 rate has been to use an empirical relationship between the longshore 

 component of the energy flux entering the surf zone and the volume of 

 sand moved. This dimensional relationship is given in the Shore Pro- 

 tection Manual (SPM) (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1977) and can be expressed as 



M 

 yr 



= 1,288 



M -s 



N-yr 



Pi, 



N-M 



s-M 



(10a) 



yd' 



yr 



= 7,500 



.3 



yd- s 



lb-yr 



P l. 



ft- lb 



ft-s 



(10b) 



where P-^ is the energy flux factor and is the longshore sediment 

 transport rate. This equation was developed from field observations in 

 which wave height characteristics were represented by only one value — 

 the significant wave height. 



In this study, actual longshore energy flux components were cal- 

 culated for a set of wave types which were subsequently summed together 

 according to their percent occurrence. Consequently, this calculation 

 of the longshore energy flux is not compatible with equation (10) above; 

 hence, the dimensional constants given in the SPM cannot be directly 

 applied or compared. Jarrett (1977) performed a refraction analysis 

 similar to that performed in this study and found a value for the 

 constant by correlating measured volumetric changes along Wrightsville 

 Beach to computed energy flux values at each end of the beach. 

 Jarrett' s successful results showed that the same type of relationship 

 which is given in the SPM exists between the computed values of the 

 longshore energy flux and the sediment transport rates. Therefore, that 

 relationship is used in this study and is expressed by 



"0 



.1 

 i=l 



(Pl;P,-)b 



LF 

 TL 



(11) 



73 



