V. LONGSHORE SEDIMENT TRANSPORT ANALYSIS 



1 . Introduction . 



The procedure to mathematically predict the volume of sediment in 

 the littoral drift requires knowledge of the magnitude and direction of 

 the energy flux due to waves breaking along the study area beaches. To 

 determine this quantity, a wave climate representative of the annual 

 wave conditions measured or experienced in offshore waters must be 

 established. The wave climate, in this case in the form of a set of 

 wave heights with different periods and directions, must be "routed" 

 towards shore by a wave refraction model until the waves break on or 

 near the beach. Information on their breaking angles (relative to the 

 beach orientation), breaking wave heights, and wave speed at breaking 

 are determined and used to establish the longshore components of the 

 energy flux for both the northerly and southerly directions. 



The quantity of sediment carried by the littoral drift in each 

 direction is found by multiplying the magnitude of the energy flux by a 

 conversion factor (U.S. Army, Corps of Engineers, Coastal Engineering 

 Research Center, 1977). However, uncertainty exists in the exact value 

 of that factor (Vitale, 1980), and therefore, it will be recalculated 

 for this study area by comparing the known time rate of volumetric 

 change at Wrightsville Beach and Carolina Beach to the predicted values 

 of the energy flux at those beaches. The recomputed conversion factors 

 will be used to estimate the annual northerly and southerly longshore 

 transport quantities and the volume of material lost into the adjacent 

 inlets. 



2. Wave Refraction Analysis . 



(a) Wave Climate . Wave climate was determined from a joint 

 probability evaluation of wave gage data at Johnnie Mercer's Pier and 

 wave observation data from Wrightsville Beach. The directional 

 distribution of wave height and wave period, calculated from the wave 

 observation data, was assumed to hold for the Johnnie Mercer's Pier 

 data. Consequently, wave angles at the gage were statistically 

 correlated to the wave observation data observations. The SSMO and 

 Frying Pan Shoals wave data were not used due to a lack of confidence in 

 data recording (Harris, 1972). 



Under random sea conditions, the distribution of the values for wave 

 height, period, and direction is continuous. However, to perform the 

 wave refraction analysis, a representative set of wave height, period, 

 and direction conditions was needed. Consequently, the distribution of 

 wave height was divided into three ranges and the period into six groups 

 with midrange values of 3, 6.5, 8.5, 10.5, 12.5, and 16 seconds. The 

 angles of wave approach were also divided into four sectors (northeast, 



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