In assessing the data for use in predicting sand transport, it is pertinent 

 to assess the quality of the LEO data from the standpoint of consistency, both 

 spatially and temporally. Values of P^g computed from LEO stations 5714 and 

 5715 (Fig. 6, two northernmost stations) show surprisingly good agreement when 

 comparing similarly computed values within the same time frame (either current 

 or wave angle) . This correspondence also holds for station 5713 which is only 

 90 meters north of the north end of the Channel Islands offshore breakwater. 

 It is believed that station 5713 observations were strongly affected by dif- 

 fraction effects caused by the breakwater. Station 5713 data were not used in 

 any of the correlations between the longshore energy flux and sand transport 

 discussed later in this report. 



To indicate temporal consistency of the data source, average P^g values 

 (for stations 5714 and 5715) were plotted versus time (in months) for the two 

 phases of the study (Fig. 12). Comparing the two phases in the figure shows 

 that the low wave energy periods occur consistently during the summer and early 

 fall months. 



-700r- 



-600 



-500 



-400 



« -300 



«-200 



- 100 







100 



200 



Note: Each Phase is on Averoge of 

 LEO Stations 5714 and No. 5715 



I r»^ 



, 1 



J L 



Phase 1, 17 Apr. 1974 — 16 Sept. 1975 



Phase 2,28 Jon. 1976 — 30 Aug 1977 



I I I I I I I I I I I 



Figure 12, 



Net longshore energy flux versus time for LEO 

 stations 5714 and' 5715. 



Correlation between the weighted average longshore energy flux and the long- 

 shore sand transport is discussed in a later section of this report. Problems 

 arose when reversals in wave direction (and consequently the longshore energy 

 flux) occurred, due to the limited extent of the survey area in which accre- 

 tion was always dominant. As a result, there were periods in which both the 

 weighted average longshore energy flux and the volume change were positive 

 (northward-directed energy flux and accretion) . This apparent discrepancy was 

 due to the limited extent of the survey area north of the offshore breakwater 

 which was still trapping sand even though northward of the survey area, the 

 sand transport was in a northerly direction. This overtrapping effect is due 

 to wave diffraction patterns caused by the breakwater driving sand into the 

 sheltered area behind the breakwater. In these particular cases, no correlation 



26 



