Figure III-2-9 provides an example of the variability of the monthly (net) longshore energy flux factor for

the same three locations. Figures III-2-8 and III-2-9 show that as the averaging period gets shorter, the

variability of the sediment transport rate increases due to the fact that integration is a smoothing process and

reduces the variance in the data. For design purposes, an assessment of the uncertainty in the sediment

transport climate can be addressed via simulation of the sediment transport rates. Walton and Douglass

(1985) have provided an approach to such simulation for the case of monthly sand transport rates which, for

the locations assessed, appeared to be in reasonable conformance with a normal distribution assumption.

Figure III-2-10 from Walton and Douglass (1985) shows the distribution of monthly averages of longshore

energy flux factor for a coastal location in South Carolina. The data are reasonably represented by a normal

distribution, as shown by the fit of the solid line to the data. Figure III-2-11, also from Walton and Douglass

(1985), is for the same location but with weekly averages. In this case, the weekly averages vary considerably

from a normal probability distribution.

III-2-28

Longshore Sediment Transport

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