2. Fill Factors . 



Krumbein and James (1965) proposed the fill factor approach discussed 

 in Chapter 5 of the SPM (U.S. Army, Corps of Engineers, Coastal Engineer- 

 ing Research Center, 1975); the other two methods were proposed by Dean 

 (1974) and James (1975). The mathematical models underlying the three 

 methods are similar, and although the ratios are not the same, each approach 

 uses many of the same assumptions: 



(a) Native sedimentary materials found on a beach are con- 

 sidered to be the most stable for the environment. 



(b) The entire volume of fill materials placed on a beach 

 is sorted by local processes to achieve a gsd similar to the 

 native material. 



(c) Sorting processes change the fill materials into native- 

 like sediments by winnowing out a minimum amount of the original 

 fill. 



(d) For calculation purposes, the native and borrow grain- 

 size composites are assumed to be normally distributed. 



These common assumptions may be questioned at times; e.g., does the 

 native composite from an eroding beach reflect the stable condition for 

 that environment? This composite usually indicates what materials are 

 being selected by the processes distinctive to that environment. However, 

 if erosion dominates in a system because not enough sediments are being 

 supplied by littoral processes, then perhaps a renourishment approach would 

 be more appropriate than using a fill factor approach. Another challenge 

 to the assumptions could be that the composite distribution for many borrow 

 sites, e.g., a finger shoal, can be expected to be normal. Often this 

 assumption is justified, considering that the composite distributions for 

 many well-sampled deposits do tend to be normal. If a site is sampled, 

 its composite characteristics can be used with these methods, and signifi- 

 cant departures from normality (e.g., some bimodal glacial deposits) should 

 be considered in the evaluation of the actual calculated values. Another 

 challenge might be, why assume that all of the fill placed will be sorted? 

 This challenge may be justified for some situations, but using the conserv- 

 ative approach is probably best if no contrary information exists. In 

 conclusion, although the assumptions seem to be legitimate and realistic 

 working postulates for many situations, they should be questioned and 

 evaluated for each problem considered. 



The three fill factor methods are also similar in that the shape of 

 the borrow gsd is mathematically altered to a shape compatible with the 

 native gsd, and the degree of difference between the original and altered 

 borrow distributions is used to calculate the volume of fill required. 

 The methods are dissimilar in the criteria used for the alterations. Fill 

 factors can be directly calculated using all three methods, but in general, 

 graphical solutions are adequate and easier. 



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