ABSTRACT

A physical model relates the differences between the size-frequency distributions of sands from the zone of swash and backwash on beaches and from rivers to the environmental conditions under which they are deposited. The sweeping oscillatory motion of the breaker zone on a beach tends to separate fine-grained suspended particles from coarser-grained particles moved by saltation. Fine particles are removed seaward into deeper water and coarser particles remain as a lag deposit; their size-frequency distribution is truncated at the fine-grained end.

In a river, suspended load travels parallel with the saltation load, the fines are trapped among coarser particles or are deposited with them as the current slacks when the discharge diminishes. Coarse-grained particles deposited by rolling and sliding are commonly added to the coarse-grained end of the size-frequency distribution of river sands. Hence, river sands may include three populations; 1) particles deposited by rolling or sliding, 2) particles deposited by saltation, and 3) particles deposited from suspension, whereas beach sands are commonly deposited by saltation only. Beach sands therefore are better sorted and tend to lack the coarse and fine-grained tail fractions present in river sands, although on some beaches coarse-grained fractions may also be present. Along beache near the mouths of rivers where the supply of fines exceeds the energy to disperse them, beach sands may have fine-grained tails and the statistical parameters of their size-frequency distribution overlap those of river sands.

Previous studies have shown that correlations exist between textural parameters based on the size frequency distribution of sands and environment of deposition. The present study uses textural parameters which are sensitive to environment because they reflect differences in the transportation and deposition pattern of beach and river sands. These include fine (<62µ) and coarse-grained (first percentile) tall fractions and various moment and graphical measures, such as Third Moment (skewness), Mean Cubed Deviation, Standard Deviation, Graphic Skewness, Graphic Standard Deviation, and the Simple Skewness Measure and Simple Sorting Measure. The latter two measures are introduced here for the first time. The scatter plots presented in this study show that only between 5 and 13 pe cent of the more than 300 samples investigated fall outside the environmental boundaries to which they belong.

After 70 or more years of size frequency analyzes, we are beginning to see that this approach has merit. Despite various shortcomings, the close correlation between textural parameters and environment of deposition shows that the differences between beach and river sands are real and apparent. In geology, where a unique solution rarely can be found to resolve a problem, the approach applied in this study helps considerably in upgrading the odds in environmental interpretation.