The rise in MSL during the study period, based on the averaged 

 trends at Portsmouth, Virginia, and Charleston, South Carolina (Hicks, 

 1972), was approximately 0.37 centimeter per year. The computed annual 

 rate of volumetric and excursion loss due to the rise in sea level for 

 the five beaches is given in Table 6. 



The rapid loss of beach material immediately after the placement of 

 a beach fill can be split into two components — a long-term component due 

 to the ongoing long-term processes, and an initial component due to 

 enhanced sorting by slope readjustment. The continual sorting type 

 losses are obviously compounded by beach-fill activity when sediment 

 which has a different distribution to the native beach sediment is used 

 as the fill material. Not only is the magnitude of the sorting losses 

 higher because of the generally greater mismatch between the new 

 distribution and the desired distribution, but also the rate of loss is 

 increased due to the increased exposure rate to wave activity as a 

 result of sediment movement due to slope readjustment. 



The long-term component can be represented by the slope of the line 

 of best fit through all data points after time t=t^ (Fig. 18), such 

 that at any time, t, 



l t = at (3) 



where l t is the long-term excursion loss (gain) at time t, and a is 

 the slope of the linear section of the excursion distance plot. 



Data from this study indicated that after 1 to 2 years following 

 beach-fill completion, the beach face generally eroded back during a 

 winter storm period to its approximate prefill position. Both 

 Figures 16 and 17 show this behavior and subsequent accretion of the 

 beach face during the ensuing summer period. This means that after 

 approximately 2 years most of the beach-fill material has been exposed 

 to the sorting action of wave activity and for this period on (i.e., the 

 time during which the long-term excursion rates were calculated), the 

 enhanced losses due to the sorting of beach-fill material should have 

 been minimal . 



To quantify the initial loss component, the long-term component was 

 subtracted from the excursion distances (shown by the dashline in 

 Fig. 19). The time scale was reset to zero at the time of fill (t=0), 

 and so the initial loss of beach fill after time t was S t . Values 

 of S (Fig. 19) for varying time increments up to t=t^ were plotted 

 on semilog paper. Figure 20 shows the results of these plots for the 

 MLW, MSL, and MHW excursion curves of WB15. The results from this 

 profile are typical for all profiles and indicate that the initial loss 

 component due to sorting and beach-slope adjustment can be mathe- 

 matically represented by an exponential equation of the form 



S^ = £ f.(l-10" kt ) for < t < t. (4) 



t i l 



42 



