51. In order to relate the variations in grain-size distributions to 

 the physical processes, a comparison of the changes in composite mean grain 

 sizes of (a) the profile mean, (b) the beach group mean, and (c) the nearshore 

 group mean were compared with the wave power and wave steepness values. Fig- 

 ure 26 shows the time series of variation in the means of these groups to the 

 wave parameters. Owing to the differences in sediment transport modes and 

 depositional environments, the beach group had the highest variation in com- 

 posite means. The nearshore group had the smallest variation. The profile 

 mean fell between these two and was composed of all the means from the dune to 

 the nearshore. All of the means reflect the trend to coarser (and more poorly 

 sorted) sediment grain-size distribution after high wave conditions. High 

 wave power values and, to a lesser extent, wave steepness values correlated 

 with times when the means become coarse. The shift to finer means occur as 

 the wave parameters decrease. 



Erosion/Accretion Calculations 



52. Most numerical modeling efforts to identify beach response and 

 cross -shore transport require some measure of grain size. However, there is 

 no standard in choosing a good representative value. In fact, most equations 

 using grain-size data use a median value (T) x ) , but the specifics of what 

 median value to use are left to the individual. With the known variability in 

 grain- size distributions across the beach as shown in this report, it is 

 important to pick a reasonable representative value to apply to model 

 equations . 



53. One example of the use of grain- size data in modeling is to predict 

 whether a beach profile of a specific sand size will erode or accrete given 

 specific data on incident wave height and period. This information is impor- 

 tant to assess the stability of a native beach or predict the stability of a 

 beach fill. The procedure for calculation is given in Kraus (1990). The 

 variables used to determine the erosion of accretion on a beach are the deep- 

 water wave height, H ; wave period, T ; and sediment fall velocity, w . 

 Sediment fall velocity is calculated from the median grain diameter, T> x , and 

 water temperature. Additional information on the calculation procedure is 

 given in Kraus (1990) and Stauble et al . (in preparation). Two nondimensional 

 ratios are used in the calculations: deepwater wave steepness, H /L„ , 



63 



